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One- and two-dimensional high-resolution solid-state NMR investigation of zeolite structures Feng, Yi 1991

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ONE- A N D T W O - D I M E N S I O N A L H I G H - R E S O L U T I O N SOLID-STATE N M R I N V E S T I G A T I O N O F Z E O L I T E STRUCTURES By YI F E N G M . S c , Nanjing University, P. R. China, 1982 A THESIS S U B M I T T E D I N P A R T I A L F U L F I L L M E N T O F T H E REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in T H E F A C U L T Y O F G R A D U A T E STUDIES (Department of Chemistry)  W e accept this thesis as conforming to the required standard  T H E U N I V E R S I T Y O F BRITISH C O L U M B I A July 1991 © Y i Feng, 1991  In  presenting  degree freely  this  at the  thesis  in  partial  fulfilment  of  University  of  British  Columbia,  I agree  available for  copying  of  department publication  this or of  reference  thesis by  this  for  his thesis  and study. scholarly  or for  her  of  The University of British Columbia Vancouver, Canada  DE-6 (2/88)  purposes  representatives:  requirements that  agree  may  be  It  is  financial gain shall not  permission.  Department  I further  the  that  the  Library  an  advanced  shall make it  permission for  granted  by  understood be  for  allowed  the that  without  extensive  head  of  my  copying  or  my  written  A B S T R A C T The work reported i n this thesis describes for the first time the application of two-dimensional  2 9  S i high-resolution solid state N M R experiments to the  investigation of the three-dimensional Si-O-Si bonding connectivities in zeolites. 2D C O S Y type, I N A D E Q U A T E  type and spin-diffusion experiments  are  discussed and evaluated, the I N A D E Q U A T E experiments being particularly successful in this work. By preparing highly crystalline, highly siliceous samples of zeolites and careful optimization of all experimental parameters, it is possible to directly observe S i - 0 - S i J couplings i n these experiments. 29  29  The three-dimensional lattice connectivities obtained from this work for 2 9  S i enriched zeolite ZSM-39 and natural abundance ZSM-12 and ZSM-22 are in  excellent agreement with the lattice structures determined by X R D techniques. In the case of a S i enriched sample of zeolite DD3R, the S i 2D N M R results 2 9  2 9  indicate that the structure is of lower symmetry than has been postulated from diffraction studies. Zeolite ZSM-5, which has the most complex three-dimensional framework of all the k n o w n zeolites, was extensively studied in its room temperature phase by 2D N M R spectroscopy.  In addition, the effects of temperature and the  presence of sorbed p-xylene and p-dichlorobenzene on the phase behavior of ZSM-5 were also investigated. The  2 9  S i 2D N M R data on ZSM-11 at high temperature are in good  agreement with the k n o w n structure, I4m2. L o w temperature 2D experiments on  ii  ZSM-11 gave the assignment of space group symmetry 14" to the structure which was previous unknown. Finally, ^ S i 2 D N M R  results on ZSM-23 reveal that there are 12  independent T-sites i n the structure which is not consistent with the space groups proposed i n the literature which have 7 crystallographically inequivalent T-sites.  T A B L E O F  C O N T E N T S  ABSTRACT  ii  TABLE OF CONTENTS  iv  LIST O F T A B L E S  xiii  LIST O F FIGURES  xvi  SYMBOLS A N D ABBREVIATIONS  xxiv  ACKNOWLEDGEMENTS  xxvi  CHAPTER ONE  I N T R O D U C T I O N A.  1  ZEOLITES A N D T H E METHODS FOR THE INVESTIGATION OF THEIR STUCTURES  1  I.  ZEOLITE STRUCTURES  1  H.  A P P L I C A T I O N S O F ZEOLITES  6  m.  M E T H O D S FOR T H E C H A R A C T E R I Z A T I O N OF ZEOLITE LATTICE STRUCTURES  11  a) Developments i n Powder Diffraction Methods  11  iv  b) High-Resolution Solid-State Nuclear Magnetic Resonance Spectroscopy  B.  12  c) Electron Microscopy  13  d) Computer-Modeling Techniques  13  HIGH-RESOLUTION SOLID STATE N M R  14  I.  H.  N U C L E A R SPIN INTERACTIONS IN T H E SOLID STATE  14  a) Direct Dipole-Dipole Interaction  15  b) Chemical Shift Interaction  18  c) Quadrupolar Interactions  18  EXPERIMENTAL TECHNIQUES USED TO OBTAIN H I G H - R E S O L U T I O N N M R S P E C T R A O F SOLIDS . . . . 20  C.  a) H i g h Power Decoupling of Protons  20  b) M a g i c Angle Spinning (MAS)  21  c) Cross Polarization (CP)  24  HIGH RESOLUTION  2 9  S I SOLID STATE N M R STUDIES  OF ZEOLITE STRUCTURES  29  I.  INTRODUCTION  29  H.  STRUCTURAL INFORMATION AVAILABLE 29  SI A N D  2 7  A L N M R STUDIES  FROM 30  a) Determination of the Composition of the Aluminosilicate Framework  30  b) Coordination Number of A l  32  c) H i g h l y Siliceous Zeolites  35  v  CHAPTER TWO  T W O - D E V D E N T I O N A LS O L I D S T A T E N U C L E A R M A G N E T I C R E S O N A N C ES P E C T R O S C O P Y A.  40  T W O - D I M E N T I O N A L (2D) N M R S P E C T R O S C O P Y  40  I.  BASIC C O N C E P T S  40  H.  D A T A REPRESENTATION  45  a) White-Washed Stacked Plots  45  b) Contour Plots  46  c) Projections  46  d) Cross Sections  46  m.  C L A S S I F I C A T I O N O F 2D S O L U T I O N NMR EXPERIMENTS  48  IV. H O M O N U C L E A R C H E M I C A L SHIFT C O R R E L A T I O N SPECTROSCOPY  50  a) Introduction  50  b) C O S Y (chemical shift COrelation SpectroscopY) Experiments  53  c) I N A D E Q U A T E (Incredible Natural Abundance DoublE Q U A n t u m Transfer Experiment) Experiments B.  56  A P P L I C A T I O N S O F 2D H O M O N U C L E A R C O R R E L A T I O N E X P E R I M E N T S TO ZEOLITES  59  I.  GENERAL CONCEPTS  59  E.  BACKGROUND INFORMATION  60  m.  OUTLINE OF PROPOSED RESEARCH  61  vi  C.  EXPERIMENTAL CONSIDERATIONS FOR OBTAINING 2D SOLID STATE N M R SPECTRA I.  H.  m.  63  P R E P A R A T I O N O F H I G H L Y SILICEOUS ZEOLITES  63  a) Zeolite Synthesis  63  b) Dealumination  64  OPTIMIZATION OF THE N M R EXPERIMENT  65  a) 2D Data Acquisition Parameters  69  b) Data Processing  70  M E A S U R E M E N T O F R E L A X A T I O N TIMES  74  a) Introduction  74  b) Experimental  74  c) Results and Discussion  75  CHAPTER THREE  A R O Z  P P L I C A T I O N E S O L U T I O N F T H E S I L I E O L I T E A.  O F T W O - D I M E N S I O N A L S I H I G H S O L E D S T A T E N M R T O T H E I N V E S T I G A T I O N C A T EL A T T I C E S O F S I - E N R I C H E D SZ S M - 3 9A N D D D 3 R 2  2  TWO-DIMENSIONAL  2 9  9  9  79  S I HIGH-RESOLUTION SOLID  STATE N M R INVESTIGATION OF THE LATTICE S T R U C T U R E O F S I - E N R I C H E D Z E O L I T E ZSM-39 . . .  79  I.  INTRODUCTION  79  n.  EXPERIMENTAL  82  2 9  ffl. RESULTS A N D DISCUSSION  vii  84  B.  a) I D Experiments  84  b) Spin-Diffusion Experiments  84  c) C O S Y Experiments  91  TWO-DIMENSIONAL  2 9  S I HIGH-RESOLUTION SOLID  STATE N M R INVESTIGATION OF THE LATTICE STRUCTURE OF  2 9  SI-ENRICHED  D E C A - D O D E C A S I L 3R (DD3R)  ZEOLITE  ..  98  I.  INTRODUCTION  98  II.  EXPERIMENTAL  100  m.  RESULTS A N D DISCUSSION  102  a) I D Experiments  102  b) 2D C O S Y Experiments  102  c) 2D I N A D E Q U A T E Experiments  106  CHAPTER FOUR  N A T U R A L - A B U N D A N C E T W O - D I M E N S I O N A L S O L I D S T A T E S IN M R I N V E S T I G A T I O N SO F T H E L A T T I C E C O N N E C T I V I T I E SI NZ E O L I T E SZSM-12 A N DZSM-22 2  9  A.  INTRODUCTION  109  B.  NATURAL-ABUNDANCE TWO-DIMENSIONAL  2 9  SI  HIGH-RESOLUTION SOLID STATE N M R INVESTIGATION O F T H E L A T T I C E S T R U C T U R E O F Z E O L I T E ZSM-12 . . 110 I.  INTRODUCTION  110  H.  EXPERIMENTAL  112  viii  m.  RESULTS A N D DISCUSSION  113  a) C O S Y Experiments  113  b) Direct Observation of S i - O- S i C o u p l i n g 29  117  2 9  c) 2D I N A D E Q U A T E Experiments  121  d) Comparison of I N A D E Q U A T E and C O S Y Experiments  124  C. NATURAL-ABUNDANCE TWO-DIMENSIONAL  2 9  SI  HIGH-RESOLUTION SOLID STATE N M R INVESTIGATION O F T H E L A T T I C E S T R U C T U R E O F Z E O L I T E ZSM-22 . . 128 I.  INTRODUCTION  128  H.  EXPERIMENTAL  130  m.  RESULTS A N D DISCUSSION  130  a) C O S Y Experiments  130  b) I N A D E Q U A T E Experiments  133  CHAPTER FIVE N A T U R A L - A B U N D A N S IN M R I N V E S T I B O N D I N G C O N N E C T F O R M SO F Z E O L 2  9  C E G A T I V I I T  T W O - D I M E N S I O N A L I O N SO F T H E T H R E E T I E S I N T H E D I F F E EC A T A L Y S TZ S M  S O L I D S T A T E - D I M E N S I O N A L R E N T - 5  A.  INTRODUCTION  B.  I N V E S T I G A T I O N O F T H E H I G H - L O A D E D F O R M O F P-  136 136  X Y L E N E W I T H Z E O L I T E ZSM-5 B Y  HIGH-RESOLUTION  29SI S O L I D S T A T E N M R S P E C T R O S C O P Y  ix  140  I.  INTRODUCTION  140  E.  EXPERIMENTAL  141  m. RESULTS A N D DISCUSSION C.  141  NATURAL-ABUNDANCE TWO-DIMENSIONAL  2 9  SI  HIGH-RESOLUTION SOLID STATE N M R INVESTIGATION OF T H E K N O W N LATTICE  D.  S T R U C T U R E S O F Z E O L I T E ZSM-5  148  I.  INTRODUCTION  148  H.  R E S U L T S A N D DISCUSSION  154  a) Orthorhombic Phase (12 T-sites)  154  b) M o n o d i n i c Phase (24 T-sites)  164  c) Orthorhombic Phase (24 T-sites)  172  TWO-DIMENSIONAL HIGH-RESOLUTION  2 9  S I SOLID S T A T E  N M R INVESTIGATION OF T H E LATTICE STRUCTURES OF Z E O L I T E ZSM-5 L O A D E D W I T H P - D I C H L O R O B E N Z E N E  E.  . 178  I.  INTRODUCTION  178  H.  RESULTS A N D DISCUSSION  179  a) I D M A S N M R Expriments  179  b) 2D I N A D E Q U A T E Experiments  182  C O R R E L A T I O N STUDIES B E T W E E N  2 9  SI M A S N M R  C H E M I C A L SHIFTS A N D X-RAY D I F F R A C T I O N D A T A F O R H I G H L Y SILICEOUS ZEOLITES  192  I.  INTRODUCTION  192  n.  DISCUSSION  194  x  C H A P T E R SIX  A P P L I C A T I O N O F T W O - D I M E N S I O N A L S IM A S N M R T E C H N I Q U E S T O T H E S T R U C T U R A L I N V E S T I G A T I O N O F L E S SW E L LC H A R A C T E R I Z E DZ E O L I T E S 2  A.  9  N A T U R A L A B U N D A N C E TWO-DIMENSIONAL  2 9  204  SI  MAS N M R INVESTIGATION OF THE STRUCTURES O F T H E H I G H - A N D LOW- T E M P E R A T U R E  FORMS  O F Z E O L I T E ZSM-11  204  I.  INTRODUCTION  204  H.  EXPERIMENTAL  208  m.  RESULTS A N D DISCUSSION  209  a) I D Experiments on Zeolite ZSM-11  209  b) 2D Experiments on ZSM-11 at H i g h Temperature and the n-Octane Loaded Form  216  c) Investigation of the Low-Temperature Lattice Structure of ZSM-11 B.  221  N A T U R A L A B U N D A N C E TWO-DIMENSIONAL M A S N M R INVESTIGATION OF T H E  2 9  SI  THREE-DIMENSIONAL  B O N D I N G C O N N E C T I V I T I E S O F Z E O L I T E ZSM-23  230  I.  INTRODUCTION  230  n.  RESULTS A N D DISCUSSION  231  a) I D Experiments  231  b) 2D I N A D E Q U A T E Experiments  233  xi  CHAPTER SEVEN  C O N C L U S I O N SA N D S U G G E S T I O N SF O RF U T A.  CONCLUSIONS .  235  B.  SUGGESTIONS FOR FUTURE W O R K  237  LIST O F R E F E R E N C E S  . . .  240  LIST OF T A B L E S  Table 1  Classification of Some Zeolites  Table 2  Free Dimensions of Some Planar n-Ring Apertures Found  Table 3 Table 4  5  i n Zeolites  6  Some Commercial Processes U s i n g Zeolite Catalysts . . .  10  1 3  C Nuclear Spin Interactions i n a 4.7 Tesla Field  15  Table 5  Classification of Some 2D N M R Experiments  49  Table 6  Important Transformations of Product Operators  52  Table 7  The Pulse Sequences for Tj and  Table 8  T-sites, Their Occupancies, and Connectivities for the  Measurements  ....  Asymmetric U n i t in Zeolite ZSM-39 Table 9  95  T-sites, Their Occupancies, and Connectivities for the Asymmetric U n i t in Zeolite D D 3 R  Table 11  106  T-sites, Their Occupancies, and Connectivities for the Asymmetric Unit in Zeolite ZSM-12  Table 13  100  Connectivities Related to T-sites 4,2 and Resonances C , D / E of Zeolite D D 3 R  Table 12  80  The Results of Variable Fixed Delay Experiments on ZSM-39  Table 10  75  T-sites, Their Occupancies, and Connectivities for the  xiii  112  AsyrrvmetricUrut in Zeolite ZSM-22  128  Table 14  T w o Possible Assignments of the Spectrum of ZSM-22 . . 131  Table 15  Description of the Four ZSM-5 Samples Investigated  Table 16  Connectivities for the Asymmetric Unit i n the Orthorhombic Phase (12 T-sites) of ZSM-5  Table 17  . . . 149  ; . 151  Connectivities for the Asymmetric Unit i n the M o n o d i n i c Phase of Zeolite ZSM-5  Table 18  Connectivities for the Asymmetric Unit i n the Orthorhombic Phase (24 T-sites) of ZSM-5  Table 19  162  Connectivities of the Four Membered Ring T-Sites i n the M o n o d i n i c Phase of ZSM-5 at 300 K  Table 21  200  Connectivities for the Asymmetric Unit i n Zeolite ZSM-11 at H i g h Temperature  Table 25  216  Chemical Shifts of the Resonances i n T w o Cases of ZSM-11 W i t h Symmetry I4m2  Table 26  188  Linear Regression Analysis of Chemical shift vs Geometric Parameters  Table 24  174  Calculated Si-H distances (< 4 A ) for ZSM-5 Loaded W i t h 8 p-Xylene Per U n i t Cell  Table 23  167  Connectivities Related to Resonance W and T-site 1 i n the H i g h Loaded P-xylene Form of ZSM-5  Table 22  153  T w o Possible Assignments of the Resonances for Zeolite ZSM-5 at 403 K  Table 20  152  219  Connection Scheme of the Resonances of ZSM-11 in the L o w Temperature Form from the 2D N M R Data  xiv  223  Table 27  Connectivities of T-sites W i t h i n the Asymmetric U n i t of the L o w Temperature Form of ZSM-11  Table 28  224  Complete Connectivities of T-sites i n the L o w Temperature Form of ZSM-11  xv  227  L I S T O F  F I G U R E S  Figure 1  The Framework Structures of Selected Zeolites  Figure 2  Secondary Building Units C o m m o n l y Occurring i n Zeolite Frameworks  2  4  Figure 3  Powder Pattern Arising from Dipolar C o u p l i n g Effects . 17  Figure 4  Schematic Representation of the Chemical Shift Anisotropy  Figure 5  19  Schematic Representation of the Geometric Arrangement for Mechanical Sample Spinning  23  Figure 6  Pulse Sequence Used for Cross Polarization  25  Figure 7  The Carbon-13 Spectra of Bisphenol A  28  Figure 8  2 9  S i Chemcal Shift Ranges of the Five Possible Local  Silicon Environments in Aluminosilicates Figure 9  2 9  S i M A S N M R Spectra of a Series of Faujasite Zeolites  W i t h the S i / A l Ratios Indicated Figure 10  27  Figure 11 Figure 12  31  33  A 1 M A S N M R Spectrum of Zeolite Y  34  2 9  S i M A S N M R Spectra of Zeolite ZSM-22  38  2 9  S i M A S N M R Spectra of Zeolite ZSM-5 Loaded  w i t h (A) p-Xylene; (B) p-Chlorotoluene; and (C) p-Dichlorobenzene  39  xvi  Figure 13  (A) T i m i n g Sequence for a One Dimensional N M R Experiment; (B) The Inversion-Recovery Pulse Sequence  Figure 14  2 9  S i M A S N M R Spectra of ZSM-12 from a r  42 2  Measurement Figure 15  43  (A) T i m i n g Sequence for a T w o Dimensional N M R Experiment; (B) Schematic Representation of the Steps Involved i n Obtaining a 2D N M R Spectrum  44  Figure 16  The represnetation of a C O S Y Experiment on ZSM-39 . . 47  Figure 17  (A) The Pulse Sequence Used for C O S Y Experiments; (B) Schematic Contour Plot of a C O S Y Experiment . . . .  Figure 18  55  (A) Pulse Sequence Used for I N A D E Q U A T E Experiments (B) Schematic Contour Plot of an I N A D E Q U A T E Experiment  Figure 19  58  A Schematic Representation of the Mechanism of Hydrothermal Dealumination of the Zeolite Framework  Figure 20  2 9  S i C P M A S N M R Spectrum of Q g M g  . 65 68  Figure 21  Comparison of Some Time-Domain W i n d o w Functions . 72  Figure 22  The Contour Plot of a 2D S i C O S Y Experiment 2 9  73  Figure 23  2 9  S i T j Relaxation Times in Some Zeolites  77  Figure 24  2 9  S i T Relaxation Times i n Some Zeolites  78  2  Figure 25.  Schematic Representation of the Structure of ZSM-39 . . . 81  Figure 26.  Schematic Representation of the Pulse Sequences Used i n 2D C P M A S N M R Experiments  xvii  83  Figure 27.  I D S i C P M A S N M R Spectra of Zeolite ZSM-39  Figure 28  I D S i Spin Diffusion Experiments on Zeolite  2 9  2 9  ZSM-39 (from T Resonance) 3  Figure 29  ID  2 9  87  S i Spin Diffusion Experiments of Zeolite  ZSM-39 (from T Resonance) 2  Figure 30  85  88  Contour Plot of a 2D Spin-Diffusion Experiment on ZSM-39  90  Figure 31  Contour Plot of a C O S Y Experiment on ZSM-39  93  Figure 32  Contour and Stacked Plots of a C O S Y Experiment on ZSM-39 at 373 K  94  Figure 33  Contour Plot of a C O S Y Experiment on ZSM-39 at 298 K  96  Figure 34  Contour Plot of a D Q F C O S Y Experiment on ZSM-39 at 298 K  Figure 35  97  Schematic Representation of the Zeolite D D 3 R Lattice Framework  Figure 36.  99  Schematic Representation of the Pulse Sequences Used i n the 2D M A S N M R Experiments  Figure 37  2 9  101  S i M A S N M R Spectrum of Zeolite D D 3 R at 300 K  and its Deconvolution  103  Figure 38  Contour Plot of a C O S Y Experiment for Zeolite D D 3 R . . 105  Figure 39  Contour Plot of an I N A D E Q U A T E Experiment on Zeolite D D 3 R  Figure 40  108  Schematic Representation of the Lattice Structure of Zeolite ZSM-12  Figure 41  ID  2 9  Ill  S i M A S N M R Spectrum of Zeolite ZSM-12  xviii  114  Figure 42  Contour Plot of a C O S Y Experiment on Zeolite .ZSM-12 . 115  Figure 43  Contour plot of a C O S Y Experiment on Zeolite ZSM-12 w i t h Better Resolution i n F  119  2  Figure 44  Cross Sections Plotted from Figure 43  Figure 45  Contour Plot of an I N A D E Q U A T E Experiment  120  on Zeolite ZSM-12  122  Figure 46  Cross Sections Plotted from Figure 45  123  Figure 47  Contour Plot of a Symmetrical I N A D E Q U A T E Experiment on Zeolite ZSM-12  Figure 48  125  Schematic Representation of the Zeolite ZSM-22 Lattice Framework  129  Figure 49  ID  131  Figure 50  Contour Plot of a C O S Y Experiment on Zeolite ZSM-22 . 132  Figure 51  Contour Plot of the Same Experiment as Figure 50  2 9  S i M A S N M R Spectrum of Zeolite ZSM-22  w i t h Better Resolution in F Figure 52  134  2  Contour Plot of an I N A D E Q U A T E Experiment on Zeolite ZSM-22  Figure 53  135  Schematic Representation of the Pentasil Chain-Type Building Block  Figure 54  137  Schematic Representations of a Pentasil Layer and the Channel Systems in ZSM-5  Figure 55  2 9  S i C P M A S N M R Spectra of ZSM-5 w i t h  Increasing Concentrations of p-Xylene Figure 56  138  142  The Effect of p-Xylene Loading o n the Proportion of High-Loaded Form in the Samples  xix  144  Figure 57 Figure 58  H M A S N M R Spectra of ZSM-5 w i t h p-Xylene  l  2 9  S i M A S N M R Spectrum and Deconvolutions for a  ZSM-5 Sample i n the H i g h Loaded Form Figure 59  ID  2 9  150  S i M A S N M R spectra of ZSM-5 i n its  Various Forms . . Figure 61  155  Contour Plot of a C O S Y 45 Experiment on ZSM-5 with 2 Molecules of/^Xylene per Unit Cell  Figure 62  159  Contour Plot of an I N A D E Q U A T E Experiment on ZSM-5 at 403 K  Figure 64  157  Contour Plot of an I N A D E Q U A T E Experiment on ZSM-5 w i t h 2 Molecules p-Xylene per U n i t Cell  Figure 63  147  Schematic Representations of the Asymmetric Units of ZSM-5 i n its Various Forms  Figure 60  146  160  Plots of the S i Chemical Shifts as Functions of the 2 9  Average T-T Distances for ZSM-5 at H i g h Temperature . 163 Figure 65  Contour Plot of an I N A D E Q U A T E Experiment on ZSM-5 at 300 K  Figure 66  165  A Graphical Representation of the Variation of Chemical Shift w i t h Temperature for Zeolite ZSM-5  Figure 67  166  Plots of the S i Chemical Shifts as Functions of the 2 9  Average T-T Distances for ZSM-5 at Room Temperature . 170 Figure 68  Relationship Between the Resonances of the Room and H i g h Temperature Forms  Figure 69  171  Contour Plot of a C P - I N A D E Q U A T E Experiment on ZSM-5 w i t h 8 Molecules ofp-Xylene per Unit Cell  xx  173  Figure 70  Plots of the S i Chemical Shifts as Functions of the 2 9  M e a n T-T Distances for the H i g h Loaded Form of ZSM-5 Figure 71  ID  2 9  177  S i M A S N M R Spectra of ZSM-5 w i t h Increasing  Concentrations of p-Dichlorobenzene Figure 72  2 9  S i M A S N M R Spectrum and its Deconvolution for ZSM-5  Loaded w i t h 8 Molecules p-Dichlorobenzene Figure 73  180  181  Variable Temperature S i C P M A S N M R Spectra of 2 9  ZSM-5 Loaded w i t h 8 Molecules p-Dichlorobenzene . . . 183 Figure 74  Contour Plot of an I N A D E Q U A T E Experiment on ZSM-5 Loaded w i t h 2 Molecules of p-Dichlorobenzene  Figure 75  184  Contour Plot of a C P - I N A D E Q U A T E Experiment on ZSM-5 Loaded w i t h 8 Molecules of p-Dichlorobenzene . . 185  Figure 76  Schematic Representation of the Locations of P-Xylene Molecules in the Channels of ZSM-5  Figure 77  Variable Contact Time S i C P M A S N M R Spectra of ZSM-5 2 9  Loaded w i t h 8 Molecules of p-Dichlorobenzene Figure 78  187  190  Intensities of Selected Resonances of the ZSM-5 Spectrum as a Fuction of the Contact Time  191  Figure 79  N M R and X R D Correlation Diagrams for ZSM-12  195  Figure 80  N M R and X R D Correlation Diagrams for ZSM-22  196  Figure 81  N M R and X R D Correlation Diagrams for ZSM-5 (part I)  . 198  Figure 82  N M R and X R D Correlation Diagrams for ZSM-5 (part II)  199  Figure 83  N M R and X R D Correlation Diagrams for the Three Forms of ZSM-5  202  xxi  Figure 84  N M R and X R D Correlation Diagrams for A l l Available Data Sets  203  Figure 85  Stacking Sequence and Channel Systems i n ZSM-11 . . . . 205  Figure 86  Schematic Representation of the ZSM-11 Lattice Framework  Figure 87  2 9  207  S i M A S N M R Spectra of Zeolite ZSM-11 Before and  After Sodium H y d r o x i d e Treatment Figure 88  Variable Temperature S i M A S N M R Experiments 2 9  (273-318 K) on ZSM-11 Figure 89  212  Variable Temperature S i M A S N M R Experiments 2 9  (298-342 K) on ZSM-11 Figure 90  2 9  213  S i M A S N M R Spectrum of ZSM-11 at 302 K  and its Deconvolution Figure 91  210  2 9  214  S i M A S N M R Spectrum of ZSM-11 at 342 K  and its Deconvolution Figure 92  215  Schematic Representation of the Asymmetric U n i t of ZSM-11  Figure 93  217  Contour Plot of an I N A D E Q U A T E experiment on ZSM-11 at 340 K  Figure 94  218  Contour Plot of an I N A D E Q U A T E Experiment on ZSM-11 Loaded W i t h n-Octane at 300 K  Figure 95  Contour Plot of an I N A D E Q U A T E Experiment on ZSM-11 at303K  Figure 96  220  221  Graphical Representation of the Variation of Chemical Shift w i t h Temperature for Zeolite ZSM-11  xxii  226  Figure 97 Figure 98  Projection of the Zeolite ZSM-23 Lattice Framework 2 9  230  S i M A S N M R Spectrum and its Deconvolution for  ZSM-23 Figure 99  ...  232  Contour Plot of an I N A D E Q U A T E Experiment on ZSM-23  234  xxiii  SYMBOLS A N D ABBREVIATIONS  a,b,c  unit cell edges vectors parallel to the x, y, and z axes, respectively  A  Angstrom unit; 1 A = 1 0 " m  BQ  strength of staticmagnetic field  B|  strength of the radio-frequency field during a pulse  COSY  chemical shift correlation spectroscopy  CP  cross polarization  DANTE  delays alternating w i t h nutation for tailored excitation  DQFCOSY  double quantum filtered C O S Y  ESD  estimated standard deviation  FD  fixed delay  FID  free induction decay  Fj, F h  10  2  frequency dimensions corresponding to t| and t  2  (i) hour (ii) Plank constant  H  Hamiltonian operator, subscripts indicate the nature of the operator  HPD  high power decoupling  Hz  hertz  INADEQUATE incredible natural abundance double quantum transfer experiment J  nuclear spin-spin coupling constant through n bonds (in H z )  MAS  magic angle spinning  n  xxiv  ms  milli-second  Mg  equilibrium macroscopic magnetization of a spin sysytem in the presence of B 0  M x , M y , M z components of macroscopic magnetization NMR  nuclear magnetic resonance  POF  product operator formalism  ppm  parts per million  QgMg  cubic octamer silicic acid trimethylsilyl ester  rf  radio frequency  s  second  TMS  tetramethylsilane T-site X in the structures of zeolites  Tj  longitudinal relaxation time  T  transverse relaxation time  2  if  T2  decay constant describing inhomogeneity and transverse relaxation  t|  length of the evolution period  t2  running time during the detection period  XRD  X- ray diffraction  XRF  X- ray fluorescence  I D , 2D  one-dimensional, two-dimensional  a  T - O- T angle  'Yx  magnetogyric ratio of uncleus X  S  chemical shift, usually in p p m  Avj/2  full w i d t h of a resonance line at half-height  Vx  Larmor precession frequency of uncleus X (in Hz)  xxv  ACKNOWLEDGEMENTS  I w o u l d like to sincerely thank Dr. C. A . Fyfe for his guidance and encouragement during the course of the research programme and throughout the preparation of this thesis. I am indebted to numerous friends and colleagues for their collaboration, advice, encouragement and discussions on several aspects of this work.  In  particular, I wish to thank Dr. H. Grondey, Dr. H . Gies, Dr. G. T. Kokotailo, Dr. N . Burlinson, Dr. B. Fahie and Dr. L. Randall. I also wish to thank Dr. C o x , Brookhaven National Laboratory U S A , for kindly providing the computing program for Rietveld refinement of crystal structures using powder X R D data. I am thankful for permission to use the V A X computer system i n the Electrical Engineering Department, and wish to thank M r . R. Rose for his assistance i n this regard. I gratefully acknowledge the University of British Columbia for financial support i n the form of graduate fellowship. Finally, I wish to sincerely thank m y husband, Changshi, and m y son, Lei, for their patience and understanding over the past four years.  xxvi  CHAPTER ONE  INTRODUCTION  A.  ZEOLITES A N D THE METHODS FOR THE INVESTIGATION OF THEIR STRUCTURES  I.  ZEOLITE STRUCTURES  The name "zeolite" was coined by Cronstedt^) in 1756 from the Greek words for 'to boil' and 'stone' to describe the behavior of the newly discovered mineral stilbite, which loses water rapidly o n heating and thus seems to boil. Zeolites have been extensively studied during the last 30 years, leading to the synthesis of novel structures and to a great number of applications (as ionexchangers, molecular sieves, catalysts and so on). Zeolites are framework aluminosilicates composed of corner- and edgesharing S i O ^  and A l O ^ " tetrahedra and containing regular systems of  intracrystalline cavities and channels of molecular dimensions (Fig. 1)( "6). 2  The general oxide formula of a zeolite is given by Equation [1]: M  x / n  (A10 ) (Si0 )y •m H 0 2  x  2  2  1  [1]  zeolite  pentasil  Figure 1  Y  zeolite  The framework structures of selected zeolites together with the apertures of their respective channels, (ref. 10)  2  where M represents exchangeable cations of valence n, which neutralize the net negative charge of the zeolite framework arising from the A l O ^ " tetrahedra, and m H 2 0 represents the water molecules of hydration. The silicon to aluminum ratio, y / x , is invariably found to be equal to or greater than one and can approach infinity for completely 'aluminum free' frameworks. A s stated earlier, a zeolite framework consists of tetrahedral T-atoms (Si or A l atoms tetrahedrally coordinated to four oxygens) linked through common oxygen atoms to form a three-dimensional structure. Zeolite frameworks can also be thought to consist of finite or infinite (i.e. chain- or layer-like) component units. The recurring finite units are called secondary building units (SBU).  One  simple way to classify zeolite structures is based on the SBU's, which describes all k n o w n zeolite frameworks as arrangements linking the 16 possible SBU's. The commonly occurring SBU's are shown in Figure 2. In the SBU, a tetrahedral atom (Si,Al) is present at each corner or termination, but the oxygen atoms are not shown. These bridging oxygen atoms are located approximately half-way between the tetrahedral atoms but not usually on the line joining them. Table 1 lists some k n o w n zeolite structures classified by (a) their SBU content, (b) structure type ( I U P A C nomenclature),^) (c) their common names.  3  Figure 2  Secondary building units commonly occurring in zeolite frameworks, (a) single four ring (S4R), (b) single sue ring (S6R), (c) single eight ring (S8R), (d) double four ring (D4R), (e) double six ring (D6R), (f) complex 4-1, (g) complex 5-1 and (h) complex 4-4-1. (ref.7)  4  Table 1  Classification of Some Zeolites (Ref. 8)  Secondary Building Unites  Structure type (IUPAC nomenclature)  Name  S4R  ANA GIS PHI  Analcime Gismondine Phillipsite  S6R  ERI LTL OFF MAZ  Erionite Zeolite L Offretite Zeolite omega  S8R  Occurs in many structures but with other SBU'S. (i.e. chabazite, zeolite A)  D4R  LTA  Zeolite A  D6R  CHA FAU FAU GME KFI  Chabazite Faujasite Zeolite X Gmelinite Zeolite ZK-5  4-1  NAT NAT THO  Natrolite Scolecite Thomsonite  5-1  MOR DAC MF1 MEL  Mordenite Dachiardite Zeolite ZSM-5 Zeolite ZSM-11  4-4-1  HEU STI  Heulandite Stilbite  5  n.  A P P L I C A T I O N S O F ZEOLITES To be interested in zeolites as ion-exchangers, molecular sieves and  catalysts is to be become involved i n many aspects of science and technology, such as petrochemistry and o i l processing, organic synthesis of intermediates and fine chemicals, and nuclear waste treatment^ "'*' 2  9 /  ^\  M a n y of these  applications depend o n the arrangement and internal dimensions of the intracrystalline channel structures. The accessibility of the intracrystalline pores is governed by the size of the apertures i n the following way. The apertures are of such a size as to be able to selectively take up some molecules into their porous structure, whilst rejecting others on the basis of their larger effective molecular dimensions. The free dimensions of some n-ring apertures are given i n Table 2 by assuming 2.8 A as the diameter of the oxygens lining the inner surfaces. Table 2  Free Dimensions of Some Planar n-Ring Apertures Found in Zeolites (Ref.4) Ring Size  Free Dimensions  (n T-atoms)  of Aperture (A)  Sodalite  6  2.1  Zeolite A Erionite Offretite  8  4.1 5.2 / 3.6 5.2 / 3.6  Ferrierite  10  Zeolite  ZSM-5  Mordenite Fauiasite Zeolite Y  5.5 / 4.3 5.6 / 5.4 7.0 / 6.7 7.4 7.4  12  6  The ion exchange property of zeolites which allows the replacement of cations held in their framework by ions present in an external solution has been intensively studied for two main reasons. One is their industrial importance in acting as ion exchangers. The other is the interest i n modification of the catalytic or molecular sieving actions of the parent zeolite. Some of the uses of zeolites as ion exchangers are: - Components i n commercial detergent compositions for exchanging Ca  2 +  for N a to soften water. +  - Treatment of liquid nuclear effluents. - Waste-water treatment.  The applications of zeolites as molecular sieves depend on the dimensions of their framework apertures as discussed above. It is well k n o w n that Linde Molecular Sieve 4 A (zeolite N a A , having the pore dimensions of approximately 4 A ) is used as a gas or solvent drying agent i n almost every chemical laboratory i n the w o r l d . Other important industrial applications are: - Gas separation, e.g. n-paraffins are accepted by Molecular Sieve 5 A (CaA) while i-paraffins are excluded by virtue of their larger effective kinetic diameters. - Gas purification. For example, 13X zeolite.(Na faujasite) is used to remove sulphur- and nitrogen- containing molecules from gaseous environments.  The characteristic properties of zeolites as catalysts are acidity, shapeselectivity and thermal stability, making them unique among catalytically active  7  materials and sharing many of the characteristics of enzymes ^ \  The catalytic  activity of zeolites is based upon the production of acidic Bronsted sites arising from the creation of l i y d r o x y l s ' within the zeolite pore structure.  These  l i y d r o x y l s ' are usually formed by ammonium cation exchange followed by a calcination step which involves heating i n air at - 500° as in Equations [2] and [3], where ' Z ' represents the zeolite structure.  NaZ(s)+NH NH Z(s)  4  ( a q . ) o N H Z ( s ) + Na (aq.)  +  4  calcine  [2]  +  > N H ( g ) T + HZ(s)  4  [3]  3  500°c The 'protonated' form contains protons associated with negatively charged framework oxygens linked into alumina tetrahedra, i.e. acidic Bronsted sites are created:  /  O  \ /  Si  /  O  \  \ /  /  Al \  O  O O O ./ \ / \ ./ Si Al Si / \ / \ / \ \  A t 550°C protons can be lost i n the form of water with the consequent formation of Lewis sites shown i n Equation [4].  \ /  Si  / \  O  H \  / Al / \  H  +  O \  / Si / \  O \ /  / Al \  +  o  O  o  / \ / \ / \ /  Bronsted site  Lewis site  8  p \  o  v  /  \  The acid strength and number of the acid centers (both Bronsted and Lewis acid centers) can be adjusted i n a controlled manner during synthesis and/or by subsequent treatments. The shape selectivity means that only molecules smaller than the aperture of the zeolite can react with the zeolite catalyst. In addition, only those molecules whose transition state geometry is smaller than the cavity and/or pore diameter can be formed and released. The thermal stability of the zeolites permits them to be used above 150° C , and they are therefore very useful for  reactions  temperatures.  in  which  the  thermodynamic  Some major commercial  catalysts are listed i n Table 3.  9  equilibrium requires  processes  making  use of  high zeolite  Table 3  Some Commercial Processes using Zeolite Catalysts (Ref. 8)  Process  Catalyst  Advantage  Catalytic cracking  REY ( R E X , R E H Y W REMgY)*  Selectivity and high conversion rates  Hydrocracking  X, Y, Mordenite loaded with Co,Mo, W^alsoHYCa MgY and H-ZSM-5  High conversion rates  Selectoforming  N i clinoptilolite/ erionite,Ni erionite  Hydroisomerization  Pt mordenite  Converts low octane and hexane feeds to higher octane yields  Dewaxing  Pt mordenite  remove long-chain paraffins  Benzene alkylation  ZSM-5  Ethylbenzene and styrene production with low byproduct yield  Xylene isomerization  ZSM-5  Increase in p-xylene yield with low by-product yield  Methanol to gasoline conversion  ZSM-5  High gasoline yield with high octane rating  NOx reduction  H-mordenite  Effluent clean-up in nitric acid and nuclear reprocessing plants  '  Increase in octane number  * REY stands for zeolite Y materials with rare earth metal cations  10  m.  M E T H O D S FOR THE C H A R A C T E R I Z A T I O N O F ZEOLITE LATTICE STRUCTURES  In principle it should be possible to solve all zeolite structures by the use of modern single-crystal X-ray diffraction techniques and indeed a number of naturally occurring species have been studied i n this way.  In the case of  synthetic materials, however, very few of them are available as large enough single crystals for conventional X R D measurements. These materials are highly crystalline, but w i t h dimensions of the individual crystals of the order of a few microns or less.  Even i n the case where good quality crystals are available,  distinguishing structural A l from Si is difficult because of the similarity in the scattering factors of these two elements and the fact that they are usually disordered over the available T- sites. W h e n single-crystal X-ray crystallographic methods are inapplicable, other techniques have usually to be employed. These mainly involve the following four distinct approaches. a) Developments in Powder Diffraction M e t h o d s ^ ^'^) Conventional powder X-ray diffraction has been  the primary tool for the  determination of the structures of zeolite materials. Recently, improvements i n X-ray fluxes using synchrotron sources and developments i n data analysis techniques based on Rietveld methods^ ^M)  have had a considerable impact i n  the area of zeolite structure d e t e r m i n a t i o n ^ ) ,  x-rays from these sources are  very intense, polarized and sharply focused, and give therefore a improvement  i n the resolution of a powder  diffraction experiment  great and  dramatically increase the amount of structural information, while the Rietveld method predicts likely X R D patterns from simulated structures and presents  11  data outputs of the closeness of fit between experimental and computed patterns. Furthermore, the 'brightness' of the synchrotron source, i n principle, also enables conventional single-crystal diffraction measurements of very small crystals. Although little has been published i n this latter area, preliminary r e s u l t s ^ ) a r  e  described as promising.  b) High-Resolution Solid-State Nuclear Magnetic Resonance Spectroscopy^' ) 7  In the study of zeolite structures, high-resolution solid-state N M R spectroscopy has emerged as an important complementary technique to X-ray diffraction measurements.  The former probes short range ordering and  structure, while the latter is sensitive to long range ordering and periodicities. A s is well k n o w n , the usefulness of N M R i n chemistry i n general rests on the fact that the chemical shifts of the nuclear magnetic resonance signals depend i n a sensitive manner o n the local chemical environments of the nuclei, while their intensities relate directly to the numbers of nuclei i n the different environments. The contributions of S i and A 1 solid-state N M R i n the study of 2 9  27  the framework of zeolites can be summarized as follows. - Detennining the composition of l o w S i / A l ratio framework i n terms  aluminosilicate  of the local silicon environments, i.e. Si[4Al],  Si[3Al,Si], Si[2Al,2Si] Si[Al,3Si] and Si[4Si].  - Resolving crystallographically nonequivalent tetrahedral sites. - Monitoring the effect of adsorbed organic species on the zeolite structure.  12  - Distinguishing tetrahedral from octahedral A l and framework from nonframework A l .  c) Electron Microscopy^**) The technique of Scanning Electron Microscopy (SEM) is widely used i n the characterization of the crystal morphologies of zeolites.  It is useful i n  synthesis and quality control for the detection of new phases and mixed zeolite phases.  High-Resolution Electron Microscopy ( H R E M ) can yield structural  information i n 'real space'  at the subnanometer  level.  Under  suitable  circumstances, even the pore openings can be clearly observed, giving the most direct structural information.  It is the most appropriate way of  examining  defects and mixed phases in zeolite systems^ ). 9  d) Computer-Modeling T e c h n i q u e s ^ ) D u r i n g the last decade, computer-modeling techniques have developed and  are n o w considered by some to constitute a viable procedure for  investigating the properties of perfect and defective materials.  This process  entails predicting the m i n i m u m energy configuration of a crystal structure, studies of zeolite structures using this techniques have been of two types - Zeolite structures and relative stabilities^ *). 2  - Structure and energetics of sorbed species^ ' ^). 22  2  The limiting factor i n this area at present is the reliability of the available potential energy functions needed for the calculations.  13  B.  HIGH-RESOLUTION  SOLID  STATE  N M R (24-26)  I.  N U C L E A R SPIN INTERACTIONS I N T H E SOLID STATE  The nuclear spin interactions are generally anisotropic (orientation dependent). The main interactions which can occur for a nucleus are: - The Zeeman interaction with the magnetic field; - Direct dipole-dipole interactions with other nuclei; - Magnetic shielding by the surrounding electrons giving rise to chemical shift effects; - Spin-spin or J couplings to other nuclei; - Quadrupolar interactions which w i l l be present for nuclei with spin > 1/2.only. In the liquid state these interactions apart from the Zeeman interaction are averaged b y fast molecular motion, whereas they have a strong effect on solid state N M R spectra, because molecules are much less mobile i n the solid state than i n the liquid state. In general, the Hamiltonian which describes the total nuclear spin interaction can be written as the s u m of the Hamiltonians representing the individual interactions, as i n Equation [5]:  H =H  Z  +H  D  +H C  S  + HJ + H Q  [5]  Table 4 shows the approximate magnitudes of the various interactions for 13c  nuclei both i n solution and solid state. In liquids, these terms are either averaged to isotropic values or vanish, while i n solid state these Hamiltonians 14  must be represented as being proportional to the product of the appropriate vectors and a second rank tensor which characterizes the three-dimensional nature of the interactions.  Table 4  1 3  C Nuclear Spin Interactions i n a 4.7 Tesla Field (Ref. 33)  Interaction  Hamiltonian  Magnitude i n solid  Magnitude i n solution  Z  50 M H z  50 M H z  D  -15 k H z  0 Hz  Zeeman  H  Dipolar  H  Chemical Shift  HCS  up to 10 k H z  J  -200 H z  -200 H z  HQ  up to 1 M H z  0 Hz  Scalar C o u p l i n g ( C- H) 13  l  Dipolar coupling of Quadrupolar nuclei  H  a  iso  For solid state N M R spectroscopy, the three most important anisotropic interactions which dominate the N M R spectra are: a) Direct Dipole-Dipole Interaction The dipolar interaction H  D  arises from the direct dipole-dipole interaction  through space between two nuclei. In the heteronuclear case, i.e. one with spin i and the other with spin j , the dipolar interaction for two isolated spins may be written as in Equation [6]  A where  is the internuclear distance;  is the dipolar coupling tensor; Yi> Yj are  the gyromagnetic ratios of the two nuclei respectively and I, J are the spin operators of the two nuclei respectively.  For a single crystal with only one  orientation for i j j , there are two lines in the spectrum of each nucleus with the resonance frequencies given by Equation [7]^^.  3  YiYj  V = v ± ——  ( 1 - 3 cos e )  [7]  2  0  t1  2TI 4 r 3 4TC  J  t J  '  where 0y is the angle between the internuclear vector and the magnetic field, |1Q is the magnetic permeability constant, and V  0  the resonance frequency in the  absence of dipolar interactions for the appropriate case.  Typical values for  homonuclear dipolar interaction of protons are of the order of 40 k H z , while * H - 13c heteronuclear dipolar interactions are of the order of 15 k H z < ) . 28  In a  polycrystalline sample, where there are random orientations, the spectrum shows a dipolar powder pattern, as shown i n Figure 3. The dotted curve gives the powder pattern for an isolated pair of nuclei, which is called a T a k e doublef  ( 2 7 )  .  The full curve shows the effect of neighboring nuclei on the  isolated system. Important features of the interaction are: (1) dipolar interactions have a dependence on the magnitudes of the magnetic moments and hence they w i l l be more important for spin 1/2 nuclei with large magnetic moments; (2) There is a very strong inverse dependence (1/r ) o n the internuclear distance; thus only the 3  nearest atomic neighbors w i l l experience a strong effect; and (3) dipolar interactions are independent of the applied magnetic field strength.  16  CHEMICAL  Figure 3  SHIFT  A powder pattern arising from Hipolar coupling effects for a two-spin system. The dotted curve represents the spectrum for an isolated pair of nuclei and the full trace shows the effect of neighboring nuclei on the isolated system.(ref. 26)  17  b) Chemical Shift Interaction Since the chemical shift interaction involves the surrounding electrons, it is very sensitive to the geometry and to the identities of the other atoms surrounding the nucleus being examined and is usually the most chemically diagnostic measurable i n N M R studies. This interaction can be represented by Equation [8]: Hcs = Y l U . a . B  0  ,  [8]  A  where a is the chemical shielding tensor. It is clear that the interaction is linear with the applied field B  Q  and w i l l be more important at higher magnetic  field  strengths. In a single crystal, an isolated nucleus w i l l give rise to a sharp signal whose frequency is dependent u p o n the orientation of the crystal w i t h respect to the applied magnetic field.  However, the spectrum of a single nucleus in a  poly crystalline material w i l l be a broad line whose exact shape depends on the principal elements of the shielding tensor. Figure 4 illustrates this orientation dependence of the chemical shift interaction. c) Ouadrupolar Interaction The nuclear electric quadrupolar moment eQ interacts w i t h the nonspherical field gradient around the nucleus.  It is field independent and is  described for a single spin I > 1/2 by Equation [9]  HQ = I . Q . I ,  18  [9]  A  i  o  i  i  i  CHEMICAL  Figure 4  i  i  SHIFT  A schematic representation the chemical shift anisotropy: (A) A single crystal with two different orientations of the carbonyl function with respect to the magnetic field vector produces two different C resonances. (B) A polycrystalline sample results in the superposition of peaks resulting from all possible orientations. (C) A solution shows only the isotropic average as a result of rapid molecular morion, (ref. 26) 13  19  A  where Q is the quadrupolar coupling tensor characterizing the three-dimensional nature of the interaction. The magnitude of the interaction is such that it usually dominates the spectra of most nuclei which have quadrupole moments. The critical difference between solution and solid state N M R is that the rapid and random molecular motions i n the liquid state produce an isotropic average of the interactions equal to 1/3 of the sum of the trace diagonalized matrix of their corresponding second rank tensors.  of  the  Since the  tensors describing the dipolar coupling and the quadrupolar interactions are traceless, their isotropic values are exactly zero. In the case of the chemical shift interaction, the traces are non-zero, resulting in discrete isotropic values for the shifts. In the solid state, however, the fundamental spin interactions lead to broad and featureless lines i n the N M R spectra with linewidths of the order of a k H z or more as indicated in Table 4.  H.  E X P E R I M E N T A L TECHNIQUES USED TO O B T A I N HIGHR E S O L U T I O N N M R S P E C T R A O F SOLIDS  a) H i g h Power Decoupling of Protons For most dilute spin systems, i n which the magnetically active nuclei of interest are present i n l o w concentrations, the major line broadening interaction for the dilute spins is the heteronuclear dipolar coupling with the abundant spin system (usually protons). The local field, B  loc  , at a nucleus i i n the dilute spin  system is altered by a nucleus j i n the abundant spin system, as described by Equation [10]  20  Bi  [10]  = B ±u r -3(3cos e -l) 2  o c  0  1  ij  ij  where B is the external magnetic field, |ij is the magnetic moment of a nucleus j , 0  r/jj is the internuclear distance, Gy is the angle between the internuclear axis and the static field, and the plus and minus signs arise because the spins which modulate the local field may be orientated with or against the applied field. It is possible to eUminate this interaction by irradiating the abundant spin system w i t h a strong rf field at its Larmor frequency.  The effect of this  decoupling irradiation is to induce rapid transitions in the abundant nuclei which cause their contribution to the effective local field to become zero o n the N M R time scale. Since the interaction may be of the order of tens of k H z in the solid state, the decoupling power level has to be much higher (~ l k W ) than the relatively low decoupling power of ~5 W or less commonly used i n solution NMR.  b) Magic Angle Spinning Magic  angle  spinning  (MAS)  was  independently by Lowe, i n 1959 (29,30)  first  used  by  Andrew,  and  M A S subjects the solid to a motion  which produces to a first approximation, the same net averaging effect as a rapid isotropic molecular tumbling i n solution. The basis of these experiments is the observation that most of the spin interactions have a spatial dependence of the form of (3 cos 9 -1) where 9 is the angle between some vector r and the magnetic 2  field BQ. In the case of dipolar interaction, the vector r is the internuclear vector Tjj, and i n the chemical shift interaction the vector r represents the principal axes  21  of the shielding tensor. If a sample is made to rotate about an axis R which is inclined to BQ at an angle a and to the vector r by an angle of P (Figure 5). The average of (3 cos 9 - 1 ) about the conical path indicated for the vector r is 2  given i n Equation  [11]:^)  (3 COS © - l ) 2  a v g  = (1/2) (3 COS p -1) (3 COS cc -1) 2  2  [11]  where the extremes of the angle 0 are cc+P and a-p. Fortunately the angle a is under the control of the experimentalist. When a  =54.7°,  then cos a = 1/^3 and  3 COS a - 1 = 0, so that (3 cos 8 - 1) = 0 for all orientations (i.e. all values of 0). 2  2  Therefore, the magic angle spinning technique reduces or eliminates both homonuclear and heteronuclear dipolar interactions, quadrupolar interactions to first order and yields the isotropic values for chemical shifts. A critical feature of magic angle spinning is that the rate of rotation required to average the anisotropic interactions properly has to be greater than the static bandwidth expressed i n H z . Such speeds cannot always be achieved i n practice. In this case, spinning sidebands w i l l be present, located on each side of the isotropic chemical shift position and separated b y distances equal to the spinning frequency, and broadening due to residual dipolar interactions may also be observed.  22  Figure 5  Schematic representation of the geometric arrangement for mechanical sample spinning: The solid sample is rotated with an angular velocity of CO,, about R which is inclined to the magnetic field by the angle a. A typical vector r is inclined at the angle {$ to the rotation axis. Its inclination to BQ varies periodically with time, (ref. 28)  23  C) Cross Polarization (CP) The line-narrowing techniques of dipolar decoupling and magic-angle spinning provide the resolution necessary to obtain chemical and structural information on individual dilute spins i n solids.  However, by their nature,  dilute spin systems are of l o w sensitivity and some may also have extremely long relaxation times.  Thus, direct one-pulse N M R experiments on dilute spin  systems can be inefficient. Both disadvantages can be remedied with the help of the  cross-polarization technique, first  introduced by  Pines,  Gibby  and  W a u g h ^ * ' ) , by which spin polarization and thus net magnetization is 3 2  transferred from the abundant spins to the dilute spins i n the system via the dipolar interaction, providing both signal enhancement and much shorter recycle delays. The magnetization transfer is accomplished by using the pulse sequence illustrated i n Figure 6 A . In this example, protons are taken as the abundant spins and  1 3  C the dilute spins. The first step is to apply an on-resonance 90° pulse to  the protons, which rotates the proton magnetization along the Y ' axis in the rotating frame. Then the * H magnetization is spin-locked by an on-resonance B^H along Y ' . A t this point, a rf field, BJQ, is applied to the amplitude  of  the  B  1 C  adjusted  so  that the  1 3  C nuclei, with the  Hartmann-Hahn  matching  c o n d i t i o n ^ ) , equation [12], is fulfilled during the contact time period.  [12]  A  90; (SPIN LOCK).  DECOUPLE  ALLOW PROTONS TO RE-EQUILIBRATE U  ,  CONTACT  TIME  Figure 6  (A) Pulse sequence used for cross polarization to a dilute nucleus (in this case " O from the abundant spin system(in this case (B) Schematic representation of and precessing around the spin locking fields Bjpj and Biq respectively when the Hartmann- Hahn match is achieved, (ref. 36)  25  This 'matching' condition means that, i n their respective rotating frames, the protons and the  1 3  C nuclei both precess at equal rates in the spin locking  fields and the energy required for spin flips between a and p states is identical for both spins,  allowing a rapid transfer of magnetization (Figure 6B).  Therefore, this 'spin-contacf results i n the growth of C-nucleus magnetization, 13  since the protons are much more abundant. A t the end of the contact time, the B|  C  field is switched off and the free induction decay (FID) of the  recorded, while the B  1 H  1 3  C signal is  field is maintained during this period for proton  decoupling. The entire sequence is repeated many times until a suitable signal to noise ratio is achieved. The use of the cross-polarization technique increases the sensitivity of the dilute nuclei i n two ways.  Firstly, there is a maximum enhancement of the X  nuclear magnetization equal to the ratio of the magnetogyric ratios of the abundant and rare spins, Y H / Y X ( YIH/YI3C ~ 4 ) . Secondly, since the X nucleus signal is generated from the proton magnetization, the rate at which the experiment may be repeated is determined only by the spin-lattice relaxation rate of the * H nuclei rather than the generally much longer X nuclei relaxation time. Thus, the recycle time can be much shorter than for a simple 90° pulse experiment on the X nuclei, resulting i n a much better signal-to-noise ratio i n a given time period. Schaefer and Stejskal< > were the first to combine M A S , C P and high35  power decoupling techniques to obtain high-resolution solid- state spectra. To illustrate the effects of these different techniques, variety  of  1 3  C N M R experiments under a  conditions on bisphenol-A (4, 4'- dihydroxydiphenyl- 2, 2'-  dimethylpropane) are presented i n Figure 7. Figure 7a shows the spectrum of a  26  static sample obtained with a simple 90° pulse experiment. It is broad (ca. 15 k H z at the baseline), but shows some structure because of the large difference in chemical shift between aromatic and aliphatic carbons. In 7b proton decoupling and cross polarization are applied but without sample spinning. The aliphatic carbons show a discernible axially symmetric anisotropy patten, but the aromatic carbons give rise to a very broad, l o w intensity resonance. Figure 7c shows the spectrum obtained w i t h magic angle spinning but without decoupling or cross polarization. The chemical shift anisotropy has been removed completely, but the dipolar broadening only partially. Figure 7d presents the high-resolution spectrum obtained by simultaneously using all three techniques. The small peaks labeled 'sb' are due to spinning sidebands.  27  (a)  250  200  150  100  50  0  -50  -100  ppm FROM TMS  Figure 7  The 50.3 MHz C solid state NMR spectra of bisphenol A. (a) Nonspinning and with no proton decoupling and cross polarization, Cb) nonspinning but with proton dipolar decoupling and cross polarization, (c) with magic angle spinning but without decoupling and cross polarization, (d) with magic angle spinning, dipolar decoupling, and cross polarization. (ref.36) 13  28  c.  HIGH RESOLUTION  2 9  S I SOLID STATE N M R STUDIES OF  ZEOLITE STRUCTURES  I.  INTRODUCTION  The importance of zeolites as catalysts, molecular sieves and ionexchangers has been discussed in Section A .  The properties of a particular  zeolite are mainly dependent on the topology of its framework and the size of its free channels. Therefore, detailed structural information is critical i n order to understand the sorptive and catalytic properties of zeolites^ "^. 2  Nuclear  magnetic resonance spectroscopy is well suited as a technique to investigate the structure of zeolites as they are composed of elements which have N M R active isotopes such as  2 9  Si,  27  A 1 and  1 7  0.  The use of various line-narrowing  techniques, as discussed i n Section B make high-resolution solid state N M R an important complementary technique to diffraction studies for the investigation of the structures of zeolites. Since there are no framework hydrogen atoms i n pure zeolites, high power proton decoupling is not needed and the C P cannot be used. The experiment is thus reduced to a simple 90° pulse sequence with M A S and  may  be  easily  performed  using  a  conventional  high  resolution  spectrometer^). In the case of zeolites with sorbed molecules or templates, C P and high power decoupling can sometimes be used to discriminate between phases w i t h mobile and immobile components. sorbates,  2 9  For zeolites with immobile  S i C P M A S N M R experiments can be very efficient (37).  29  H.  STRUCTURAL INFORMATION AVAILABLE FROM  2 9  SI A N D  2 7  AL  N M R STUDIES  a) Determination of the Composition of the L o w S i / A l Aluminosilicate Frameworks The first application of high resolution  2 9  S i N M R spectroscopy to the  investigation of zeolites was made by Lippmaa and Engelhardt/ **) and they 3  s h o w e d ^ ) that up to five peaks should be observed for S i spectra of zeolites, 2 9  corresponding to the five possible Si environments: SiCOAl)^  SKOAl^OSi);  S i ( O A l ) ( O S i ) ; Si(OAl)(OSi) and SKOSi)^ based on S i M A S N M R studies of 2  2  2 9  3  minerals of known structure.  The characteristic ranges of these isotropic  chemical shifts could be defined as shown i n Figure 8. A particularly important application is that the S i / A l ratio of the lattice can be calculated directly from the 2 9  S i spectra. W h e n (1) a S i spectrum is correctly interpreted in terms of Si(nAl) 2 9  units, (2) assuming Loewenstein's Rule ^ \ which postulates that no Al-O-Al linkages are present, and (3) there is no appreciable shift dispersion due to crystallographically inequivalent sites, it is possible to calculate the S i / A l ratio of the sample from the  2 9  S i spectrum alone . In the absence of Al-O-Al linkages,  the environment of every A l atom is Al(4Si). Each Si-O-Al linkage in a Si(nAl) unit therefore incorporates 1/4 A l atom, and the whole unit n/4 A l atoms. The S i / A l ratio i n the aluminosilicate framework is given by Equation [13] 4  Si  2 i Si(nAI) n=0  Al  4 2 0 . 2 5 n l Si(nAI) n =0  30  [13]  Al AlOSiOAl 0 Al  Al 0 AlOSiOSi 0 Al  Al 0 AlOSiOSi 0 Si  Al 0 SiOSiOSi 0 SI  Si 0 SiOSiOSi O Si  SK4AI)  Si(3AI)  SK2AI)  SiOAl)  Si(OAI)  4:0  3:1  2:2,  1:3  0:4  O  I  Si(OAI)  SiOAl)  [ I SK2A!) LZZZZD  I I  - 8 0  Si(3AI)  I  SK4AI) '  l  - 9 0  l  '  I  - 1 0 0  L_  - 1 1 0  ppm f r o m T M S  Figure 8  S i chemical shift ranges of the five possible local silicon environments i n aluminosilicates. (ref. 42)  2 9  31  where I si(nAl) *  s  m  e  intensity of the N M R signal attributable to Si(nAl) units.  Equation [13] is valid for all zeolites provided the assumptions made i n its derivation are justified.  Figure 9 shows  2 9  S i N M R spectrum of a series of  faujasite zeolites w i t h identical structures but different S i / A l ratios determined by the X-ray fluorescence (XRF) technique. The numbers above the resonances indicate the numbers of A l atoms connected to the silicon atom of the resonance. The spectra have been simulated by a computer program using Lorentzian peak shapes and are shown to the right of the spectra. The silicon to aluminum ratios calculated using Equation [13] are also shown i n the figure and are in good agreement w i t h those measured by XRF. A s the S i / A l ratio increases, there is a corresponding increase in the relative intensities of the high-field peaks.  A  particular advantage of this method of calculating the S i / A l ratio compared w i t h chemical or X R F analysis is that it only detects framework A l atoms, whereas the others w i l l include both framework A l and also A l occluded in the cavities and channels and even outside the particles. b) Coordination Number of A l 2 7  A l M A S N M R is capable of quantitatively distinguishing between  tetrahedrally and octahedrally coordinated aluminum whose resonances are clearly separated w i t h chemical shift ranges of about +50 to +80 p p m for AIO4 and about -10 to +20 p p m for A 1 0 w i t h respect to A 1 ( H 0 ) 6  2  6  3 +  .  The  2 7  Al NMR  spectrum of zeolite Y after dealumination is presented in Figure 10. The resonance at 60 p p m corresponds to the tetrahedrally coordinated A l of the framework.  The other signal at about 0 p p m belongs to octahedral A l species  extracted from the lattice by dealumination processes. Thus, A 1 M A S 27  NMR  spectra can be used to monitor dealumination and also realumination processes.  32  observed  -SO  -90  -ico  deconvoluted Si/Al (by XRF)  Si/Al (by NMR)  1.19  1.14  1.35  1.39.  1.59  1.57  1.67  1.71  2.61  2.56  -no  -60  ppm from TMS  Figure 9  -90 -100 -110  ppm from TMS  Observed and deconvoluted S i M A S N M R spectra of a series of faujasite zeolites with various Si/Al ratios obtained from both XRF and XRD techniques. (ref.40) 29  33  tetrahedral  Al  octahedral A l  30  100 Figure 10  PPM  A 1 M A S N M R spectra of Zeolite Y samples with the Si/Al ratios indicated, (ref. 43)  27  34  c) H i g h l y siliceous zeolites  In the S i M A S N M R spectra of highly siliceous zeolites, where the A l 2 9  content is so l o w that it does not affect the S i spectrum, sharp resonances are 2 9  observed whose numbers and relative intensities reflect the numbers and relative populations of the crystallographically ^equivalent sites i n the unit cell and whose frequencies reflect the local geometries of the T-sites. A great deal of information can be obtained from high resolution S i M A S N M R spectra of 29  highly siliceous z e o l i t e s ^ ) , which relate directly to the results of X-ray diffraction experiments. These experiments may be used to: i)  Solve lattice structure problems i n terms of determining the correct  space groups by combining the N M R data with X R D information. U n t i l recently, conventional powder X-ray diffraction has been  the primary tool for the  determination of the structures of zeolites. However, small changes i n lattice symmetry related to subgroup-supergroup relationships are often difficult to observe using powder X R D techniques because of the adverse affects of small crystallite size o n X-ray peak widths, which can correspond to large atom positional errors i n a refined crystal structure. In contrast, S i solid state N M R 2 9  is very sensitive to local environments and can act as a very sensitive probe of the unit cell contents at the atomic level. Thus, high resolution S i M A S N M R is 2 9  an ideal technique to examine the correctness of the lattice structures proposed by X R D studies^ ). For example, the S i M A S N M R spectrum of zeolite Z S M 45  2 9  12 (see Chapter Four) shows seven narrow resonances of exactly equal intensity indicating seven inequivalent T-sites i n the asymmetric unit.  35  This is i n  agreement with the structure proposed by L a Pierre et al (*6\  However,  synchrotron X-ray data collected recently indicate a doubling of the c- cell dimension parameter from 12.16A to 24.33A, the others being unaffected. The systematic extinctions led to two possible space groups, Cc and C2/c, which have respectively  14 and 7 symmetrically inequivalent T-sites of  equal  occupancies. The S i M A S N M R spectrum of the same sample that was used for 2 9  the X R D experiment shows seven resonances of equal intensity, unambiguously leading to the space group symmetry C2/c. The structure refinement based on this space group was successfully carried to an R factor of ~5.4% with all lengths and angles within reasonable limits (47). ii)  Study the effect of sorbates and elevated  structures of zeolites^ ** ^) 4  -4  temperatures on the  The effect of temperature and the presence of  sorbed organic species can induce phase transformations i n some zeolites, e.g. ZSM-5 and ZSM-11, which result in changes in pore geometries, distributions of T-atoms and catalytic properties. iii) Act as a 'fingerprinf to identify zeolites. This is based on the fact that 2 9  S i N M R spectra of a zeolite from different sources are usually similar and  characteristic. Thus, it has been demonstrated by Fyfe and co-workers  that  the series of zeolites KZ-2, ZSM-22, 8-1 and NU-10, which are prepared under different hydrothermal conditions from different reaction mixtures and using quite different templates, all have the same structure (Figure 11). iv) Y i e l d information o n the nature of the interactions between the host zeolite framework and sorbed organic molecules. The S i M A S N M R spectra of 2 9  ZSM-5 loaded with p-xylene, p-chlorotoluene and p-dichlorobenzene are almost  36  identical (Figure 12),  which indicates that, at least for hydrocarbons i n this  system, the molecule-lattice interactions are mainly based on  size and shape  alone (51).  The amount of information which can be obtained from these spectra depends ultimately on the resolution of the spectra.  In turn, the resolution  depends o n the degree of crystallinity and perfection of local ordering of the completely siliceous sample examined and care in setting up and running the N M R experiments, as w i l l be discussed i n Chapter Two.  37  OBSERVED  DECONVOLUTION  PPM  Figure 11  FROM  TMS  Observed and deconvoluted S i M A S N M R spectra of: (A) KZ-2; (B) Zeolite ZSM-22; (C) NU-10; (D) Theta-1. (ref. 50) 2 9  38  39  CHAPTER  TWO  TWO-DIMENSIONAL SOLID STATE NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY A.  T W O - D I M E N S I O N A L (2D) N M R  I.  BASIC C O N C E P T S  SPECTROSCOPY^ " ) 5 2  5 6  Two-dimensional N M R was first proposed conceptually i n 1971 by Jeener (57),  and there has been a very rapid growth i n recent years i n high-resolution  N M R applications of these techniques i n solution since Ernst and co-workers discussed and illustrated various possibilities for their application (58-60)  jhe  common feature of one-dimensional N M R experiments is the timing sequence, "preparation - evolution - detection", as shown i n Figure 13A.  D u r i n g the  preparation period, the spins are first allowed to come to equilibrium, and then are perturbed by one or more rf pulses at the end of this period to initiate the N M R experiment. D u r i n g the evolution time, t^, the x, y, and z components of the spins evolve under all the forces acting on the nuclei. A signal (FID) is then detected as a function of time, S(t). converts  The process of Fourier transformation  this time-dependent function into a frequency-dependent  one, S(F).  In  the most common one-dimensional N M R experiment, the evolution time t^ is zero and  acquisition immediately  follows the  single pulse  preparation.  However, there are a number of other experiments where a finite evolution  40  period is inserted. The inversion-recovery experiment for measuring spin-lattice relaxation time evolution period.  can be used as an example to demonstrate the function of the The pulse sequence and vector diagrams  for  the  magnetization of a nucleus are shown i n Figure 13B. The z magnetization M Q (Fig 13B, a) is inverted by a 180° pulse (b). This magnetization then begins to grow back through zero toward its equilibrium value due to the Zeeman interaction, as shown at (c). To sample this regrowth of the z magnetization, the magnetization vector is turned by a 90° pulse to the y axis (d) at the end of the 7  evolution time t j and detected. Figure 14 displays a series of S i M A S spectra of 2 9  zeolite ZSM-12 obtained using this pulse sequence. For each experiment, the evolution time t| is indicated i n the figure. W i t h 2D N M R spectroscopy, the evolution time, t y is also a variable. The timing sequence is shown i n Figure 15A. The mixing period can consist of pulses and delays with fixed lengths or may be absent. The other three periods have the same meanings as i n the I D experiment. The evolution time in 2D experiments is linearly incremented by a constant amount in a series of experiments keeping all other parameters constant. Thus the received signal becomes dependent on the time period tj as well as on t2, and the data can be arranged in a two dimensional matrix, S(tj, t^-  This data matrix is composed of n rows and k  columns, where k is the number of data points collected during the acquisition time and n is the number of experiments performed with different values of tj. Then, all rows or FID'S are Fourier transformed, yielding a S(tj, F ) data matrix, 2  which is transposed to S(F2, tj) matrix. Each r o w of S(F2, t^) data matrix gives the time evolution for the corresponding point i n F  2  or t , referred to as an 2  'interferogram'. The second set of Fourier transformations leads to a S(F , Fj) 2  41  Evolution  Preparation  Acquisition  B 180%  (a)  Figure 13  90  (b)  (c)  x  (d)  (A) Time sequence of one dimensional N M R experiment, (ref. 53) (B) The inversion-recovery pulse sequence for measurement of T l and vector representation, (ref. 61)  42  —I  -106  Figure 14  '  AA A  Aaa, AAA,  K  A A A .  AA.  1  1  1  -108  '  -110  16  A  '4  A,  1  -112  1  1 —  -114  Vertical stacked plot of S i M A S N M R spectra of zeolite ZSM-12 from an inversion-recovery experiment for the measurement of T j . Spectra were obtained at 795 M H z with 28 scans for each experiment, using a 120 second recycle delay. The corresponding values of tj in seconds are indicated. 29  43  Preparation  B  PREPARATION 90°,  Evolution  EVOLUTION  /  90°,  DETEC-  ,°  TI  Mixing Acquisition  ,,,,, ^  _  n  _  1,(2)  N  ^  -ti.  4 • FT*  TRANSPOSE  Q  FT • FT  -1  V T ^  CONTOUR PLOT  4 •  FT  Figure 15  "4  (A) Timing sequence of two dimensional N M R experiment, (ref. 53) (B) Schematic representation of the steps involved i n obtaining a 2D N M R spectrum, a) A number of FIDs are recorded with incremented values of the evolution time t j . b) Each of the FIDs is subjected to Fourier transformation and a Sttj, F2) data matrix is obtained, c) The data S(tj, F2) is transposed to give a data set of S0?2' ty- d) A second series of Fourier transformation is carried out and the final data is represented as contour plot. (ref. 36)  44  data set and is followed by a transposition, to obtain the final data matrix S(F|, F2). The formation of a 2 D spectrum is schematically represented i n Figure 15B using the C O S Y (Correlation SpectroscopY) experiment as an example. pulse sequence w i l l be discussed i n Section IV.  This  In a C O S Y experiment, both  frequency domains contain the information on chemical shift. The number of rows and columns are usually made to be equal by adequate zero filling, forming a square data matrix. For a single resonance as the case in Figure 15B, the plot shows only one signal o n the diagonal. In weakly-coupled spin systems, the 2 D C O S Y experiment plots w i l l show both diagonal and cross peaks, which display the J-coupled network of the spin system (see Section IV). 2 D N M R spectroscopy is usually possible if a systematic variation in the evolution period results i n a periodic change of phase and/or amplitude of the spin system at the end of the evolution time.  Each peak i n the chemical shift  spectra formed i n the first Fourier transformation component  modulation frequencies  or  none  at  may have one or more all.  Then  the  second  transformation process can determine the frequencies of the modulations, resulting i n a 2 D spectrum displaying intensity as the third dimension.  The  spreading of the N M R spectrum i n a second orthogonal direction gives increased spectral resolution and provides additional information. II.  D A T A REPRESENTATION a) White-Washed Stacked Plots A white-washed stacked plot of a C O S Y experiment on Zeolite ZSM-39 is  shown i n Figure 16A. In this form, each successive spectral trace is plotted by keeping track of the vertical deflection of each point i n previous traces.  45  This  k i n d of plot gives a good impression of the three-dimensional nature of the data, and particularly the relative peak intensities, but it is not readily interpreted when some peaks are obscured by others. b) Contour Plots Contour lines are drawn through points of equal intensity and define the shape of resonances.  Intensity information is indirectly portrayed in the  numbers of contour levels. Figure 16B shows a contour plot from the same data set as Figure 16A. Obviously, the relationships in the frequency information are displayed more clearly. Thus, contour plots are often the preferred method of 2D N M R presentation. c) Projections Projections are one-dimensional 'spectra' created by recording the highest intensity level of each data point along a given axis and provide a useful means of portraying the information of a contour plot. The corresponding projections on the two frequency axes are shown flanking the contour plot in Figure 16B. d) Cross Sections In practice, the peaks with the crucial information lie on a limited number of cross sections i n the 2D spectrum. The cross sections required are parallel to one of the main frequency axes, and it is often very useful to plot these different traces as separate one- dimensional "spectra". Figure 16C shows three spectra which correspond to the cross sections indicated i n Figure 16B.  46  IM  Figure 16  -«oi  -no  -in  -in  -n«  -111  -no  -m  -u<  The representation of a 2D COSY experiment on zeolite ZSM-39 (see Chapter Three). (A) A white-washed stacked plot. (B) A contour plot with the projections in both dimensions. (C) Three cross sections in the positions indicated in the contour plot.  47  m.  C L A S S I F I C A T I O N O F 2D S O L U T I O N N M R E X P E R I M E N T S T w o main categories of 2D N M R experiments which can be distinguished  by their objectives have emerged from solution N M R studies: a) Chemical Shift Correlation N M R Spectroscopy This class of experiments uses interactions between groups of nuclei to establish correlations between them. Both frequency axes represent the chemical shift. The most common interactions used are: - scalar coupling - chemical exchange - dipolar coupling (cross relaxation) -multiple quantum coherence: 2D N M R  experiments  facilitate the  detection and enlarge the applicability of multiple quantum transitions, which i n general are not detected i n simple I D N M R experiments. b) T-Resolved N M R Spectroscopy In two-dimensional J-resolved spectra, the two parameters, coupling constant J and chemical shift 6, are separated along the two frequency axes. Therefore, multiplets which overlap i n the conventional I D  spectrum are  resolved i n the 2D J-spectrum allowing chemical shifts and coupling constants to be measured readily and independently. In all of these different experiments, the resultant spectrum is a map of interactions among spins. The position of a signal on the two dimensional map shows both the chemical shift of that signal, and an additional piece of information which depends on the nature of the interaction shown i n F j . Table 5 shows some examples of these two classes of experiments.  48  Table5  Classification of Some 2D N M R Experiments (Ref. 53)  2D N M R Experiment  Interaction  Name  Variable Fl  Shift Correlation  Homonuclear  Heteronuclear  2  J-Coupling Chemical exchange Dipolar coupling Double quantum  COSY NOESY NOESY INADEQUATE  800*  800 800 8(X)  8(X) 8(X) 8(X) DQF  J-Coupling  HETCOR  S^H)  800 8(Y)  Dipolar Coupling  J-resolved  F  Homonuclear  J-Coupling  JCVH)  S^H)  Heteronuclear  J-Coupling  KY^H)  8(Y)  * Nucleus X can be * H or any other nucleus Y. ** DQF stands for double quantum frequency.  49  ± ±  IV.  H O M O N U C L E A R C H E M I C A L SHIFT C O R R E L A T I O N S P E C T R O S C O P Y a) Introduction The main goal of the present work is to exploit the application of  homonuclear  chemical  shift  correlation  experiments  to  determine  the  connectivities of T-atoms i n zeolites in order to investigate the structures of their frameworks. For understanding the information derived from these experiments and performing the experiments efficiently, a brief description of the pulse sequences w i l l be presented i n this section. There are three approaches commonly used to explain I D and 2D N M R pulse experiments: i) Classical or semiclassical vector models (see Figure 13B)< ). 53  This  approach is satisfactory to describe many experiments, including spinechoes and Tj measurements.  It is simple and provides a pictorial  explanation, but has severe limitations for describing more sophisticated techniques, for example, COSY-type and multiple quantum coherence experiments. ii)  Density matrix formalism (62,63)  This quantum mechanical approach  deals w i t h the whole state of the spin system rather than the observable magnetization i n the case of vector models.  It provides a complete  understanding of the pulse sequences, yet tends to be tedious for more than two coupled spins. iii) Product operator formalism (POF) (64/65^ -j^g  m e t n 0 (  j follows a middle  course, w h i c h is founded o n density operator theory but retains the intuitive concepts of the vector models. This approach w i l l be adopted to analyze some homonuclear correlation experiments.  50  In density matrix theory, the state of a spin system is expressed by the density operator a(t).  If relaxation is disregarded, the time evolution of the  density operator is described by the Liouville-von Neumann equation: 8a — = -i[H(t),a(t)],  [14]  at  where H represents the Hamiltonian, including chemical shift terms, coupling terms and the interaction with the external rf field. The time evolution can be expressed by Equation [15]: a(t)= expWHt} a(0) exp{+/Ht}  [15]  The observable magnetization components can be evaluated from the trace relationship: M (t)ocTr{I o-(t)}, y  where  [16]  y  M y is observable magnetization ( suppose that the signal is detected  along the Y axis) a n d l y is the observable operator. For the evaluation of Equation [16] The density operator a may be expressed as a linear combination of base operators  B: g  c(t) = I b ( t ) B , s  s  [17]  where b (t) is the coefficient, so that the time dependence of a(t) is expressed in s  the coefficient b (t). The complexity of such calculations greatly depends on the s  choice of the base operators B . g  In product operator formalism, the product  operators are used as base operators, as shown in Equation [18].  51  B = *<H>  ft  s  (  J  k  [  v  1  8  ]  k=l  where I is a single spin operator, N is the total number of spin-1/2 nuclei i n the spin system, k is the index of nucleus, v stands for the spatial coordinates, x, y or z, and q is the number of single spin operators used i n the product; a is equal to 1 for q nuclei and 0 for the N-q remaining nuclei. This choice greatly simplifies the interpretation of pulse experiments, because the destiny of individual operator terms can be followed throughout the experiment.  Thus, P O F deals with the time evolution of individual product  operator terms instead of the state of the spin system.  The important  transformations of product operators can be obtained b y using equation [15], under the conditions of a weakly-coupled two spin system a n d ignoring relaxation processes, a n d are presented i n Table 6.  Table 6  Important Transformations of Product Operators (Ref. 65)  Pulses y,x' z -»I  l  Chemical Shifts nt/ z >*z Qt/  p  r  z  cos p ± I  z  sin P  v  x  l  Z  ->'x,y -»I y  x,y  J  X  cos P +  Iy  >  X  I sinp  COS  fit 1 Iy  X  S\Vi Q.t  z  Scalar C o u p l i n g * z  z  l  7  ^kxfy 7  > 2 I  kx,ly  I  >J^ cosicJt±2I y  jtjt2/ / kz  2 r  kx ly  kx4y lz /  k y / l x  J sinjcJt l z  l2  * ^ k x ^ l z « » *Jt ± k y , l x 7  s i n  *J*  * Here P represents the tip angle and J the coupling constant between spin k and 1.  52  b) C O S Y (chemical shift Correlation Spectroscopy) experiments The basic C O S Y experiment consists of two 90° pulses separated by a time tp as shown in Figure 17A. Considering a spin k weakly coupled to another spin 1, the sequence of events occurring to I  k  under the pulse sequence can be  described by a cascade of transformations.  J  90°  x  >I  k 0  c^tjlja k l  ^ 1 2 / ^ 90° > •  x  > /k3  ™  The indices of the operator 7 refer to the numbers on the time axis i n Figure k  17A. Equations 20-23 show the development of spin operators i n various stages of the sequence using the information given i n Table 6, starting at the equilibrium state.  'k0 = ' k z  [20]  I  [21]  k l  =-/  k y  1-^2 = - t f  k v  cos n]ti -21^ I\2 sin 7tjt|] cos Q t j k  +[1]^ cos 7cjt| +2/ y I\z sin 7tjt|] sin £2 tj k  *k3 ~^kz =  c  o  s  "1*1 ^ k x hy +  k  s  m  ^Jtikos ^ k * l  +[1]^ cosrejt|- 2 1 ^ I\y sin 7tjti] sin Q t | k  53  [22]  [23]  The first term i n Equation [23] including longitudinal magnetization (7^) and double quantum coherence (Z^x hy*  w  o  u  ld  n o t  b  e  detected i n the following t  2  period. The in-phase term, 7 ^ , w i l l evolve during the detection time, t , w i t h 2  the same chemical shift  as that i n t^ , which means that the signal detected  w i l l be located on the diagonal i n the resulting 2D contour plot. It is noted that the anti-phase term of spin 7 , i. e. 2 I y 7 ^ , i n Equation 22 is transferred to the k  k  anti-phase term of spin 7i, i.e. 2 7 ^ 7jy , by the second 90° pulse (or mixing pulse). This coherence transfer between the two spins, 2 / ^ 1 ^ -» 2 7 ^ 7 ^ , is the origin of the cross peak. This anti-phase term of spin 7i, 2 7 ^ 7 ^ , w i l l develop to an observable 7jy w i t h the coefficient of -sin £2vt sin Q^t^ sin 7tjt| sin 7tjt , and 2  2  other terms which do not contain any observable magnetization. Thus, this signal with the chemical shift of Qi i n F is modulated w i t h a chemical shift of 2  Qj, during t j , which results i n the cross peak located at the coordinate of (Qj, Q ) k  i n the contour plot, assuming that the J-splittings are not resolved. The peak intensities are proportional to the amplitudes of coherence transfer. Similarly, spin 7i w i l l show two signals. One is on the diagonal and the other is a cross peak located at ( Q , Qi), as shown i n Figure 17B. k  54  mixing  preparation  evolution  90:  I I  detection  90j <  ti  • \  0 1  <  l  2  1\  2 3  B  Figure 17  (A) Schematic representation of the pulse sequence used for COSY experiments.  (B) Schematic contour plot of COSY experiment on a weakly coupled two spin system.  55  c) I N A D E Q U A T E (Incredible Natural Abundance DoublE O U A n t u m Transfer Experiment) experiments The  INADEQUATE  experiment was first suggested by Bax a n d  Freeman^**), and uses the pulse sequence given i n Figure 18A. The excitation of double quantum coherence is achieved b y a pulse 'sandwich' at the end of the preparation period.  This double quantum coherence develops during the  evolution time and is then converted into observables during the detection period. U s i n g a similar P O F treatment as described for the C O S Y experiment above, the evolution of product operators, Z and Zj, at the various times k  indicated i n the figure can be obtained, and explained as follows.  J  k 0 10 = k z l z  J  +7  7  ™  + J  k l hi = -V iy +  7  [ 2 5 ]  During the 2% period, where x= 1/4 J , only the J coupling interaction has to be considered, because the spin echo sequence refocuses the effect of chemical shift.  Ac2 12 k y +7  = 7  c o s  ^J  "^kx^lz  21  s u l  ^J  21  hy  +  -2(Wlz+Wkz> 7  k3  + J  B =2(Wly  +  I  c o s  ~^bJkz  s u l  ^J  21  ™ &7]  lxV  Therefore the double quantum coherences, Zj^Z^y and Z^Z^y, are created at the end of the preparation period.  Their intensities reach the m a x i m u m when  x=l/4J. The double quantum coherence does not develop under the interaction of J coupling according to Table 6. Thus only the chemical shift term is  56  considered during the evolution time. The double quantum coherence evolves at the double quantum frequency Q + Qy as indicated in Equation 28. k  [28]  k5 15 = - <Wlz + Wlx) cosCQk+Q^t!  2  +;  2  [29]  + <Wlz " Wlx> sinCQk+^t! 2  The last 90° pulse converts the double quantum coherence into the anti-phase coherences of Ik and  l\,  i.e., JkxAz  ^kz^lx respectively. Then they evolve to be  detectable i n t with chemical shifts 0,^ and Qi respectively, if the couplings are 2  not resolved. It is clear n o w that a coupled spin pair shows two signals i n the I N A D E Q U A T E experiment which are present at chemical shifts of  ^1 i n  the F dimension respectively, while they appear at the chemical shift, Qk ^1' +  2  m  F|, the two signals occurring equally-spaced on both sides of the diagonal of the plot, as shown i n Figure 18B. The frequency i n F j , Qk ^1' * referred to as the +  s  'double quantum frequency . 7  A s discussed above, a series of cross-peaks i n the plots of C O S Y and I N A D E Q U A T E experiments reflects the bonding interactions, and makes it possible to deduce the connection patterns both i n molecular structures and lattice frameworks.  57  preparation  A 0  wK  <  e  3  >u c  I  1/4J  180 X  a  90; <  evolution  I  detection 90 0  1/4J  < —  t  2  ^ /  i  :J  <%  5  B  + C!  Figure 18  (A) Schematic representation of the INADEQUATE pulse sequence. (B) Schematic contour plot of an INADEQUATE experiment on a weaklycoupled two spin system.  58  B.  A P P L I C A T I O N S O F 2D H O M O N U C L E A R  CORRELATION  E X P E R I M E N T S TO ZEOLITES  I.  GENERAL CONCEPTS In the past decade, a number of 2D N M R  experiments have been  introduced i n high resolution solid state N M R studies. There are two kinds of 2D experiments i n the solid state.  One is a straightforward analogue of 2D  experiments in solution. For example, Szeverenyi^T) investigated the chemical exchange process, V e g a ^ ) has demonstrated a method to establish  2 9  Si/*H  heteronuclear chemical shift correlations i n silicas and zeolites and Benn and coworkers^^  have  recently  established  1 3  C/  1 3  C  connectivities  using  the  I N A D E Q U A T E sequence for the plastic crystal camphor and have used the C O S Y sequence to establish  2 9  S i / S i connectivities i n the reference molecule 2 9  Q g M g (cubic octamer silicic acid trimethylsilyl ester).  Although identical or  slightly modified versions of the pulse sequences used i n solution N M R were used with the addition of the resolution and sensitivity enhancement techniques discussed i n Chapter One, the situation in solids is often much more complicated than i n solution due to the anisotropic nature of the spin interactions. A second type of 2D solid state experiments has no analogue i n solution N M R .  These  experiments are used to study some special interaction i n solids, and the 2D spectra obtained usually present isotropic chemical shifts i n the F and some spatial interactions i n the F|  dimension.  2  dimension  For example, such  experiments have been used to retrieve chemical shift a n i s o t r o p i c s ^ ) , dipolar c o u p l i n g s ^ " ^ , and to probe spin-diffusion p r o c e s s e s ^ ) .  59  The 2D N M R experiments used to investigate the structures of zeolites i n this thesis belong to the first category. Due to the nature of the spin interactions i n solids, it is usually impossible to predict the feasibility of the application of a specific experiment to a given system. This was true i n the case of zeolites before this work was begun although it was felt that 2D N M R techniques could well be used to establish connectivities i n zeolites.  H.  BACKGROUND INFORMATION Harris and c o - w o r k e r s ^ ) have reported a series of studies of the  2 9  Si  N M R spectra of aqueous silicate solutions. The combined use of S i isotopic 29  enrichment and S i -{ Si} homonuclear decoupling made it possible to deduce 2 9  29  the structures of the silicate anions present and to measure the Jsi-0-Si 2  couplings, which appear to be i n the range of 3-10 H z and dependent on ring size. Furthermore, this group has presented 2D J-resolved and shift- correlated 2 9  S i N M R of silicate solutions^ **). The Jsi-osi values derived from these 2D 2  7  experiments are consistent with those from the I D experiments. In related work, Knight^ ) has described I D and 2D S i N M R of germanosilicate solutions. The 77  2 9  Jsi-OSi couplings are 7.5 H z i n the double four-membered ring system and 4.3  2  H z i n the double three-membered  ring  system.  In solid silicates a n d  aluminosilicates, only one example of J coupling has been reported so far, that is a Jsi-OAl ^ 2  v a  u e  °*  deduced from a variable frequency M A S N M R study of  9  the mineral Albite( >. 78  ID  2 9  S i M A S N M R techniques have been widely used to investigate the  structure and properties of zeolites, as discussed in Chapter One. However, the matching of an N M R resonance w i t h a particular silicon atom i n the crystal  60  structure is difficult even i n the highly siliceous cases unless the peak intensities of the resonances and related population parameters are unique. The assignment of resonances using geometric information from X R D experiments has been reported recently^  but these methods are empirical and very ambiguous  when the chemical shift differences are small. In addition, very few accurate X R D studies of zeolites are available and it becomes attractive to consider alternative N M R methods of obtaining the information, which in turn w i l l also give detailed structural information on the systems. OUTLINE OF PROPOSED RESEARCH  m.  The work presented i n this thesis, for most part, is directed towards the application of S i I D and 2D N M R spectroscopy to the investigation of zeolite 2 9  structures. The rest of this chapter is concerned with the experimental aspects of both sample preparation and the N M R experiments. Chapter Three describes i n detail the application of 2D C O S Y techniques to isotopically enriched, relatively simple zeolites of k n o w n structure, ZSM-39 and zeolite DD3R. Successful results from these experiments showed the potential of this approach for establishing the S i - 0 - S i connectivities i n zeolites. Further work extended these studies to 29  29  the natural abundance samples, ZSM-12 and ZSM-22, and the results are presented i n Chapter Four.  Both 2D C O S Y and I N A D E Q U A T E experiments  were successfully performed on these zeolites. Chapter Five deals with the most complex zeolite structure k n o w n , i n terms of the large asymmetric unit in its unit cell, zeolite ZSM-5, both i n its low-temperature monodinic phase and the orthorhombic forms to which it is converted by the action of temperature and/or p-xylene. The 24 T-sites i n the asymmetric unit and 48 connectivities of the  61  structures present a real challenge to the 2D N M R techniques.  Some poorly-  characterized zeolites, ZSM-11 and ZSM-23, are treated i n Chapter Six. These successful experiments demonstrate the considerable potential of 2D  NMR  experiments for solving structural problems i n zeolites when combined with X R D techniques.  62  c.  E X P E R I M E N T A L CONSIDERATIONS FOR OBTAINING 2D SOLID STATE N M R SPECTRA  N M R spectroscopy has developed, through the introduction of twodimensional methods, into the most important method for the investigation of the structure and dynamics of molecules in solution. The general application of 2 D techniques to solids is considerably more difficult, principally because of dipolar interactions and chemical shift anisotropics which produce the broad lines typical of solid state N M R and make the interactions of spins in the solid more complex than i n solution. In addition, the short T2 relaxation times restrict the use of long evolution times which are required i n some experiments. In the case of zeolites, the experiments are also insensitive, because of the l o w natural abundance of  2 9  S i , and the porous nature of the structures.  Thus, the 2 D  experiments are very demanding and time consuming, both i n terms of sample preparation and the spectroscopy involved.  The important factors i n the  experiments w i l l be briefly discussed.  I.  P R E P A R A T I O N O F H I G H L Y SILICEOUS ZEOLITES  a) Zeolite Synthesis A l t h o u g h zeolites were first discovered in natural form as minerals, it was the production of synthetic zeolites with novel framework structures which eventually  led to their widespread  application.  Zeolites  are  hydrated  aluminosilicates usually synthesized under hydrothermal conditions' ' ^). The 4  7  term 'hydrothermal' is used i n a broad sense and includes the crystallization of  63  zeolites from aqueous systems, often at elevated temperatures.  Generally a  zeolite synthesis is achieved by crystallization from an inhomogeneous gel, created from a silica source, an alumina source and various cations combined w i t h water under high p H conditions. The cations are usually considered to act as structure-directing agents, called templates. H o w all the parameters, e.g. the ratios of the components, temperature and templates, can be manipulated to create different zeolites is a complex problem, not yet understood in any detail. The choice of parameters to obtain a particular lattice structure is largely a matter of trial and error. Each as-synthesized material used in the present work was examined by powder X R D techniques, and the very best ones according to X R D results were calcined i n air at 550°C to drive off templates and water molecules included i n the zeolite lattice, and also to heal any possible defects in the structure. b) Dealumination Although the synthesis of materials used i n this work were carried out i n the absence of any A l sources, there are still traces of A l as impurities introduced with the reactants used. It was desirable to effect complete dealumination to yield an 'aluminum free' sample. Essentially there are two major methods of dealumination: i) hydrothermal treatment of the ammonium or hydrogenexchanged form of the zeolite; ii) chemical treatment of zeolites with suitable reagents (e.g. acid, chelating agents)^ ). 80  The hydrothermal treatment adopted i n this work was first demonstrated by M c D a n i e l and M a h e r ^ ^ . A calcined sample is ammonium-exchanged with a 8  1  M aqueous solution of N H 4 F and then this ammonium exchanged sample is  64  subjected to the steaming treatment, by passing water vapor over the sample at ~750°C for several days. A schematic representation of a possible dealumination mechanism is presented in Figure 19. The first step is the 'deammoniation' to form an acidic H- form, then hydrolytic splitting of Si- O A l bonds occurs, A l is released from the framework as (intermediate) Al(OH>3 and four S i O H groups are formed at each vacant site of the framework. The mechanism of Step 3 can be explained by migration of the 'vacancies' within and out of the framework by exchanging places w i t h neighbouring Si- sites (**2) The whole process of preparing highly siliceous zeolite samples was monitored by X R D and N M R measurements, and only those materials giving very narrow N M R lines were used for the 2D N M R experiments.  H.  OPTIMIZATION OF THE N M R EXPERIMENT To obtain well-resolved spectra w i t h a good signal to noise ratio in the  shortest possible measurement time, some practical aspects in performing the N M R experiments have to be considered. First, the proper setting of the "Magic Angle" is especially important to obtain high-resolution solid-state N M R spectra. This was done by observing the  7 9  B r resonance of a sample of K B r (83)  jh  e  number and intensities of the sidebands in the B r resonance are very sensitive 7 9  to the angle of the spinning axis and maxima are reached when the angle is set to exactly 54.7°. G o o d homogeneity of the static field is also important factor for  obtaining good resolution in the resulting spectra. In order to obtain the m a x i m u m S / N , the probeheads must be precisely tuned to resonance.  The  stability of the whole system during the experiment is crucial, since fluctuations  65  -Si-  I  o  I  -Si-  -Si—O—Si-  I  0 -Al-  I  I  0  I  -Si-  O  -Si-  I  +I  NH 0 A  -si-  -Si-  I  0  I  I  -Si-  0  o  I  -si-  I I  -Si-  -Si—O—Si-  I  0 NH, -Al-  I  O  I  -Si-  +I  NH - ZEOLITE  0  4  I  -Si-  I  S T E P 1:  o  Deammoniation [-NH]  I  3  -Si-  —  H - ZEOLITE STEP 2:  Hydrolysis [-A1(0H)  < —  3  1  UNSTABLE INTERMEDIATE  STEP 3:  <—  1  DEALUMINATED ZEOLITE  Figure 19  A schematic representation for a possible mechanism of hydrothermal dealumination of the zeolite framework, (ref. 17)  66  ]  i n temperature, sample spinning and field strength can considerably decrease the resolution and sensitivity and could generate artifacts. It is good practice to check the system performance using a standard sample i n standard conditions, whenever the spectrometer (Bruker MSL-400 i n the current work) is  reset to ^ S i M A S N M R experiments from other probes or  nuclei. A sample of Q g M g (cubic octamer silicic acid trimethylsilyl ester) was used as a standard. There are methyl groups attached to some of the silicon atoms i n Q g M g , so it is a suitable sample for setting the Hartmann-Hahn condition for C P experiments and the spectum is also a measure of field homogeneity. It gives sharp, well separated resonances and a reasonable S / N ratio for easy and quick data accumulation. Figure 20 shows S i C P M A S N M R 2 9  spectrum of QgMg. The S / N ratio for the strongest resonance and the linewidth of the highest field resonance indicate the sensitivity and resolution of the current performance of the spectrometer. The major factors affecting linewidth are sWmming, decoupling power and setting of the magic angle. The factors affecting the S / N ratio are probehead tuning, amplifier tuning, setting of the Hartmann-Hahn condition, the cleanliness of the coil and any defects i n the preamplifier or later stages of electronics. A l l S i chemical shifts indicated i n 2 9  this thesis are indexed with respect to T M S , using Q g M g as intermediate standard and taking the highest field resonance i n the S i spectrum to be -109.7 2 9  p p m (17).  67  MQ  MO-  M  »QM  OM  OM  MQ  -109.7  ppm  Hz  h • 12 PPM  i  20  Figure 20  >  r  -20  1  108  10  '40  PPM  •60  PPM  -110  -80  •100  -120  S i C P M A S N M R spectrum of QgMg ( M : trimethybilyl silicons; Q : silicons of the silicate backbone). The spectrum was obtained at 795 M H z , using 4K data points and zero filling to 8K with the spectral width of 12.8 kHz. There are 4 scans with the contact time of 20 ms and recycle time of 10 s.  2 9  68  a) 2D Data Acquisition Parameters A s discussed in section A , each interferogram obtained from the first Fourier transformation can be considered and treated i n the same way as a I D FID. This means that parameters i n the F j dimension such as acquisition time, spectral w i d t h , and digital resolution can be controlled in analogous ways as in the conventional F dimension. 2  -Digital resolution i n the F j domain The digital resolution required i n F j has a crucial bearing on the time requirement of a 2D experiment. If very high-resolution information is desired, a large number of spectra w o u l d be required, demanding in terms of total experimental time and also disk storage space. In the case of S i 2D correlation 2 9  experiments on zeolites, a great number of transients are needed to achieve a good enough S / N ratio because of the l o w natural abundance of  29  Si.  In a  compromise between these two factors, a digital resolution of -40 H z / p o i n t (before zero filling) i n F-i was found to be generally acceptable i n the present studies. -Pulse Calibration Pulse calibration is important i n that without accurately calibrated pulses even the simplest N M R experiments requiring a specific flip angle cannot be performed properly.  In particular, pulse sequences using 180° pulses for  inversion or refocusing are very sensitive to pulse imperfections or missetting. Furthermore, in some 2D N M R experiments, the variation of length of a specific pulse can be used to suppress unwanted signals or to enhance the information of interest. For example, the second pulse i n the C O S Y experiment (Figure 17) is set to 45° to make the cross peaks close to diagonal more easily observed, and the  69  fourth pulse length i n I N A D E Q U A T E (Figure 18) is adjusted to 135° in order to provide quadrature detection in the F j domain (see Chapter Three). The method for pulse calibration was the determination of the 180° pulse length using a simple one pulse sequence. W h e n the 180° condition is achieved, zero amplitude should be observed. -Optimization of cross polarization experiments Cross polarization (CP) is an important sensitivity enhancement technique i n high-resolution solid state N M R when it can be used (see Chapter One). The critical part of the C P experiment is the setting of the Hartmann-Hahn condition, w h i c h controls the magnetization transfer from abundant to rare spins.  A  reference sample of Q g M g was used for this purpose, since the low sensitivity of 2 9  S i precludes setting this condition o n the zeolites themselves even with  templates or sorbed molecules. The optimization was carried out by observing the  2 9  S i FID using a C P pulse sequence (see Figure 6) w i t h the power of the  proton channel fixed. The FID of a single scan can be seen clearly for Q g M g and the power of the S i rf field, B| , was carefully adjusted until a m a x i m u m FID 2 9  x  was obtained. A cross check can be obtained by measuring the 90° pulse lengths of both nuclei, which should be very close i n magnitude to each other. b) Data Processing After a 2D experiment has been completed, a disk file is stored w i t h a set of n FIDs, each composed of k data points. The basic steps in processing 2D data involve zero filling, w i n d o w multiplication, Fourier transformation, computation of magnitude or power mode spectra and finally symmetrization when desired. Instrumentation time constraints dictate that most 2D spectra are acquired w i t h coarse digital resolution i n F j .  In addition, 2D spectra are normally  70  presented in either power- or magnitude mode. In these cases, the absorption and dispersion parts of the transformed spectrum become intermingled. The resulting line-shape has a very wide base, which dramatically reduces the baseline separation between individual signals. There are two ways to improve the resolution after acquiring the data:  One is by 'zero filling' and the other is  ' w i n d o w multiplication'. If the genuine data points are m, then 2  n  data points  can be obtained by adding proper points, each containing only a zero, to the end of the FID before Fourier transformation. This is called zero filling. In this work, the total data points (after zero filling) are i n most cases 512 and 256 i n the F2 and F l dimensions respectively, which is 2 to 8 times the number of genuine data points. W i n d o w multiplication is the important part of the 2D data processing. The time-domain data are multiplied by a w i n d o w function for the purposes of enhancing resolution or of optimizing sensitivity.  Figure 21 shows some  examples of the w i n d o w functions commonly used.  Taking the sine-bell  w i n d o w (see the right curve i n Figure 19C) as an example, this w i n d o w starts and finishes close to zero and is symmetrical about the middle-point.  The  application of the w i n d o w multiplication is to produce the desirable absorptive line-shape i n a magnitude s p e c t r u m ^ ) . The spectra w i t h and without sine-bell w i n d o w treatments are given in Figure 22 taking a C O S Y experiment of zeolite ZSM-39 as an example. The sine-bell w i n d o w is commonly used i n 2D data processing and is optionally phase shifted and/or squared ( Figure 19C and D) according to the degree of line narrowing required  and  the  degree of  acceptable degradation of S / N ratio. The particular parameters used for both  71  data acquisition and processing are presented in the figure captions for all of the 2D experiments reported in the thesis.  0.1 +  Figure 21  0.3 0.5 + •  Comparison of some time-domain window functions. The time domain is 0256 s. (A) Exponential multiplication with LB=1,3,5 Hz. (B) Gauss function with LB= -5 Hz and GB= 0.1,03,05 times the time domain. (C) Sine-bell shifted by 0, ti/2, Jt/4, ji/8 (labeled at the max. with 0,2,4,8, respectively). (D) Sine-bell squared shifted by 0,7t/2, n/4, jt/8. (ref. 84)  72  A  Figure 22  B  Contour plots of a 2D S i COSY experiment (see Chapter Three). (A) Without any window function treatment (B) With sine-bell window multiplication in F dimensions. (C) With sine-bell window multiplication in Fj dimensions. (D) With sine-bell window multiplication in both dimensions. 29  2  73  V.  M E A S U R E M E N T O F R E L A X A T I O N TIMES a) Introduction In all two dimensional N M R sequences, it is necessary to allow the spin  system to relax back towards equilibrium between the acquisition of one FID and the start of the next pulse train.  If this is not done, then at best the signal  intensity w i l l be less than optimum, and at worst, artifacts w i l l appear.  In  addition, multipulse sequences for 2D N M R have a finite length of evolution and fixed delays during which transverse magnetization decays due to spin-spin relaxation (T2). In the case of the spin diffusion experiments, (which w i l l be discussed in Chapter Three) the perturbed longitudinal magnetization grows back toward its equilibrium value at the rate of l / T j during the mixing period. Therefore, the amount of transverse magnetization available for detection could be significantly reduced by the relaxation process, perhaps even making it impossible to carry out the experiments. Thus, it is desirable to measure  the  relaxation times for each sample before performing 2D experiments. Throughout this thesis, the italic forms Tj and T2 w i l l be used to denote the spin-spin and spin-lattice relaxation times respectively, to avoid confusion w i t h the indexing used for the T sites of zeolites. b) Experimental 2 9  S i and * H relaxation time (Tj and T2) measurements were performed on  a Bruker MSL-400 spectrometer at 79.49 M H z , using the pulse sequences listed i n Table 7.  74  Table 7  The Pulse Sequences Used for T% and T  relaxation time  T2  name of the sequence  pulse sequence  equation for calculation  inversion-recovery (Ref. 61)  180°- x- 90° (FID)  ln[S(<»)-S(t)]=ln2+lnS(oo)- t / T j  CPMG (Ref. 85 A)  90 -[x-180° -x] (FTD) o x  Tjof^ with CP T!0f Si with CP 2 9  Measurements  2  y  lnS(t)-lnS(0)= -t/T  n  2  *H: 180°-c-90°-CP-HPD ln[S(«) -S(t)]= ln2+ lnS(~) - x/Ti Si: CP(FID)  29  (Ref. 85 B)  iH:90°-CP—x HPD ^S i: CP90°-T-90°(FID) phase cycle:180° shift of *H pulse  lnS(t)= ln2+ lnS(O) - x/T  1  S(t) represents the intensity of a resonance at the time of t.  c) Results and discussion The results of Tj measurements o n some representative siliceous zeolites are given i n Figure 23. The T j values at ambient temperature of the T-sites i n pure highly siliceous zeolites (in comparison with the forms with templates or organic molecules in the cavities and channels) are mostly i n the range of 3 -11 s. It is generally assumed that the dominant spin-lattice relaxation mechanism i n synthetic zeolites is the direct interaction between the  2 9  S i nucleus and the  electron spin of atmospheric paramagnetic d i o x y g e n ^ ) . This mechanism is a satisfactory explanation of the present results. For example, in the case of Z S M 1 2 , the second highest field resonance has the longest T j among the seven resonances, because this T site is the only one i n the system which is not part of a channel surface (see Chapter Four) and thus w i l l not be i n direct contact with adsorbed oxygen. A t elevated temperatures, the amount of absorbed oxygen  75  w i l l be decreased, and as a consequence, the Tj values should be longer. This has been observed i n the case of ZSM-5 at 403 K. When the internal voids of a zeolite are filled with templates or organic molecules, the Tj can be lengthened to 70 s or more due to the displacement of oxygen from the intracrystalline space by the guest molecules. ZSM-5 loaded with 8 molecules p-xylene per unit cell is an example, as shown in Figure 23. Figure 24 shows the results of a series T2 experiments on the same zeolites. The T2 values fall i n the range of 100 ms -1 s except for the very long T2 values of ZSM-12. The effect of T2 on the 2D experiments used i n current work w i l l be discussed i n Chapter Three.  76  ZSM-5 (RT)  x—i  1  i  1  -110  1  1  1  p  -115  1  i  1 r  ZSM-5 (HT)  -i  1  -lia  ZSM-5 (8 mol. p-xylene)  i  i 1  1 1 1 1 1  -110  -115  1 1 1 1 1  1 r  -120  ZSM-22 —i -108  1  1  -110  1  1  -112  10 11 12  1  1  -114  10 11  1  1  -116  1  r  23 11  ZSM-12 ~i  Figure 23  '  I  -108  1  1  1  -110 PPM  1  -112  «  1  -114  S i spin-lattice relaxation times Tj (in seconds) of some of the T-sites in some highly siliceous zeolites.  2 9  77  0.29 0.18  0.18 0.20 0.20 0.16  ZSM-5 (2 mol. p-xylene)  -i  1  1  1  -i—|  r  -110  1  1  1  1  1  1  r  -115 0.67  0.67  ZSM-22 -.  1  1  -108  1  1  -110  1  -112  3.6 4.1 4.3  1  1  -114 3.6 4 3  .  r  -116 5.5 3.6  ZSM-12  l  Figure 24  1  1  -108  .  1  -110 PPM  1  1  -112  .  1 —  -114  S i spin-spin relaxation times T (in seconds) of some of the T-sites in some highly siliceous zeolites.  2 9  2  78  CHAPTER THREE  APPLICATION OF TWO-DIMENSIONAL SI HIGHRESOLUTION SOLID STATE NMR TO INVESTIGATION OF THE SILICATE LATTICES OF THE SI-ENRICHED ZEOLITES ZSM-39 AND DD3R 29  29  A.  TWO-DIMENSIONAL  2 9  S I HIGH-RESOLUTION SOLID STATE  N M R INVESTIGATION OF THE LATTICE STRUCTURE OF  29  SI-  E N R I C H E D Z E O L I T E ZSM-39  I.  INTRODUCTION  In order to develop the relationship between high-resolution solid-state N M R data and the three-dimensional structures of zeolites, a sample of k n o w n structure, zeolite ZSM-39, was chosen as a probe material. The structure of Z S M 39 is well characterized and relatively simple. To facilitate the application of 2D experiments,  the sample  was prepared  from  2 9  Si  enriched  sources  (approximately 8 0 % i n S i ) and used i n the as-synthesized form containing 29  template so that the C P technique (see Chapter One) could be applied. The term ' Z S M ^ w i l l be used i n the following text instead of 'ZSM-39 with piperidine template' for reasons of simplicity, but in all instances template was present i n the sample.  79  Zeolite ZSM-39 (dathrasil dodecasil-3C) is a highly siliceous tectosilicate first synthesized by. Jenkins and Dwyer (87) determined by Schlenker et  al(88)  crystal structure was  xhe  and refined i n detail by  Gies<89) and consists  of layers of face-sharing pentagonal dodecahedra (5 ).  The space group  12  symmetry of the high-temperature form of the compound is Fd3, and a schematic representation of its lattice framework is given i n Figure 25. There are 136 T-atoms i n the unit cell distributed over inequivalent sites Ty  three crystallographically  T 2 and T 3 of relative proportions 1:4:12.  The room  temperature form of the as-synthesized material is tetragonal and deviates from cubic symmetry by the absence of the 3-fold axis^O). Therefore, there are a total of five T-sites: 8TJ, 3 2 T , 32T ', 3 2 T " and 32T '". The connectivities of the T2  3  3  3  sites are given i n Table 8 for the ideal cubic form. Table 8 T-sites, Their Occupancies, and Connectivities for the Asymmetric Unit i n Zeolite ZSM-39 (Ref. 89) connectivity  T-site  occupancy  Ti  l  4T  4  1T 3T  12  1T :3T  T T  2  3  2  1:  2  80  3  3  Figure 25.  Schematic representation of the zeolite ZSM-39 lattice framework in the high temperature form. The three crystallographically inequivalent tetrahedral lattice sites are indicated by T|, T , and T3 (inside circles), and in each case the identities of the four nearest neighbors are shown, (ref. 95) 2  81  II.  EXPERIMENTAL 2 9  S i C P M A S N M R spectra were obtained at 79.49 M H z on a Bruker MSL-  400 spectrometer using the techniques previously described.  2D  COSY  experiments were performed using a modified version of the sequence used in solution w i t h the initial 90° pulse replaced by the cross-polarization pulse scheme. Besides the use of the C P technique, a fixed delay (FD) was introduced before and after the second 90° pulse to emphasize the effect of small couplings (52), as shown i n Figure 26C. The value of F D was optimized by trial and error. 2D spin diffusion experiments were carried out as described by Maciel and coworkers^)  using a standard * H to  sequence (Figure 26B).  ID  2 9  S i cross-polarization to initiate the  Spin diffusion experiments from individual  resonances were performed as described by VanderHart( > using a D A N T E 97  (Delays Alternating with Nutation for Tailored Excitation) sequence to invert the selected resonance^ ), as shown in Figure 26A. Since magic angle spinning is 8  sufficient to suppress the effects of  1  H - S i dipolar interactions in these 29  experiments, no heteronuclear decoupling needed to be used during the detection periods.  Nevertheless,  this interaction remains  active  in  the  magnetization- transfer process. A small quantity of a highly siliceous sample of ZSM-39 was synthesized hydrothermally by Dr. Hermann Gies, U. Bochum, Germany, i n a sealed silica glass tube in 8 days at 200°C using piperidine as template. The silica source was enriched to approximately 80 % i n S i . 2 9  82  901 'H  CP  -90 29  Si  il  90°  CP  FD  n DANTE 180°  B  90° •H  CP  -90° 29 Si  CP  n  90°  x  FD  n  90° CP  il  90°,  29,  CP  FD  FD  t  2  (AQ) //  Figure 26.  Schematic representation of the pulse sequences relevant to the 2D C P M A S N M R experiments. (A) one-dimensional spin-diffusion pulse sequence with selective inversion of one resonance using a D A N T E sequence. (B) two-dimensional spin-diffusion pulse sequence. (C) two-dimensional modified COSY pulse sequence.  83  JH.  R E S U L T S A N D DISCUSSION a) I D Experiments The I D  2 9  S i C P M A S N M R spectra of ZSM-39 which are presented in  Figure 27 show good resolution and are similar to those previously reported 94). The three T sites are clearly resolved, and the structure of the T 3 resonance reflects the absence of a 3-fold symmetry axis. The resonances are quite narrow, indicating that the system is both highly siliceous and highly crystalline. b) Spin-Diffusion Experiments Spin diffusion can be understood as an energy-conserving 'flip-flop' process between two dipolar coupled nuclear spins.  This phenomenon is  responsible for the fact that a uniform spin temperature is obtained for abundant spins throughout a solid^ ). Due to its dipolar origin, the spin diffusion rate is 54  proportional to r"*\ Thus, the spin diffusion experiments provide information o n the spatial proximity of nuclei. Since S i - 0 - S i distances are all approximately 29  29  3A, while Si-0-Si-0- Si distances are around 5.5A in these systems, it was 29  29  hoped that the two w o u l d be clearly differentiated. The pulse sequences used are shown i n Figures 26A and B. D u r i n g the mixing period (FD), spin diffusion takes place. The m a x i m u m mixing time (FD) w i l l be limited by the spin-lattice relaxation time Tj of the S i nuclei, usually the F D < T j . In the present instance, 2 9  is of the order of 650 s and imposes no limitation o n the experiments. However, the recycle time between pulse sequences is determined only by the spin-lattice relaxation time T j of the protons due to the use of the cross polarization technique. The proton Tj i n this sample is ~4 s, which makes the experiments very efficient.  84  T T  3  2  K  373  35 Hz  T, 40 Hz —i  -io*  Figure 27.  (A)  (B)  2 9  2 9  1  1 -iM  r—i  -no  J  •—i—'->  -iia  r—'  -ii4  1  -ue  —I  -it«  •  r—•  -uo  1  -na  '  I -a*  S i CP M A S N M R spectrum of zeolite ZSM-39 at 373K.  S i CP M A S N M R spectrum of zeolite ZSM-39 at 298K.  85  One-dimensional spin diffusion experiments were carried out using a D A N T E sequence to selectively excite a specific resonance (Figure 26A). In this experiment, the magnetization of the S i nuclei is generated i n the xy plane by 2 9  the C P technique, and stored back along z axis by a 90° pulse. Then one of the individual magnetizations is inverted by a D A N T E sequence. After a mixing time (FD),  where spin-diffusion takes place, application of a 90°  pulse  reestablishes the magnetization i n the xy plane where the FID is recorded. The results of I D spin diffusion experiments inverting T 3 and T  2  respectively are  shown in Figure 28 and 29 w i t h mixing times of Is and 5s. Curve a represents the spectrum using the pulse sequence i n Figure 26A. Curve b is the spectrum using the same sequence as curve a except that the transmitter of the  29  Si  channel was gated off during the D A N T E period, and curve c is the difference between b and a , which reflects the progress of the spin-diffusion process. A s can be seen from the figures, there is relatively rapid spin diffusion between T  2  and T 3 and between T | and T , while the diffusion process between T | and T 3 is 2  very much slower, in agreement w i t h the k n o w n connectivities i n the structure. The  spin-diffusion experiment can also be performed i n a  two-  dimensional format using the sequence shown i n Figure 26B. Figure 30 shows the result of a S i 2D N M R experiment with a mixing time of 10 s, i n which the 2 9  expected connectivities T | T  2  and T T 3 are clearly observed while that between 2  T | and T 3 is not observed under these experimental conditions. It is k n o w n that the motion generated by the M A S  technique decreases the dipole-dipole  interactions of the spins and therefore the efficiency of the spin-diffusion process increases as the spinning rate is lowered. A t either longer spin-diffusion periods  86  One-dimensional experiments using the pulse sequence of Figure 26A illustrating spin diffusion from the T3 resonance, fixed delay 1 s. 8 scans were taken in each experiment with a contact time of 20 ms.  87  Figure 29  One-dimensional experiments using the pulse sequence of Figure 26A illustrating spin diffusion from the T resonance, fixed delay 5 s. 8 scans were taken in each experiment with a contact time of 20 ms. 2  88  (25 s at 2 k H z spinning frequency) or shorter times at lower spinning rates (5-1 Os at 1 k H z rotor frequency), spin diffusion between  and T 3 is eventually  observed, as w o u l d be expected. Although the spin-diffusion rate is strongly dependent on the internuclear distance, it is also related to the chemical shift anisotropics and the isotropic shift differences of the interacting nudei^9/100) Since the atoms involved all have tetrahedral co-ordinations and the chemical shifts are well separated i n this case, these non-distance dependent effects could well be minimal.  89  J,  -1*4  . -in  . -tie  -112  Figure 30  Contour plot of a 2D spin-diffusion experiment on ZSM-39 using the pulse sequence of figure 26B, with 128 experiments, 8 scans in each experiment, sweep width of 5 kHz, and 256 data points collected during acquisition. The fixed delay during which spin diffusion occurs was 10 s, the spinning rate 2 kHz, and the total time for the experiment approximately 6 h. Sine bell squared apodization was used, and the plot has been symmetrized.  90  c) C O S Y Experiments A n alternative and more reliable and unambiguous method is the C O S Y experiment.  The pulse sequence used is shown in Figure 26C.  The total  evolution time is the sum of t | and F D due to the introduction of the fixed delay. A preliminary series of 2D C O S Y experiments were first carried out at room temperature. It was difficult to optimize the experimental conditions even i n this simple system, because the lack of information on the magnitudes of the J couplings, and the values from solution N M R are not directly transferable to the solid state (see on). Either none or a single pair of cross peaks were observed i n this series of experiments. Figure 31 shows the best results obtained in these preliminary experiments. A clear connectivity is established between T but the expected interaction between T | and T  2  and T 3  is not observed. Although the  2  intensity of the T^ resonance is considerably less than the others and the number of interactions is 3 times lower than for T T 3 , the S / N of the 2D plot is such that 2  the T | T  2  cross-peak should have been observable if it had had a similar growth  profile. The major factors effecting the profile could be the J coupling and the T * relaxation time. That is, the amount of magnetization transfer detected is 2  proportional to exp(-tj/T *)-sin Tcjt^ (55)  j  2  t  m a x  tr e a c  h e s a m a x i m u m at the time of  = (l/TCptan^TcJT/. For small J ( JT *< 1), t 2  m a x  = ~T *. Thus an encoding 2  time = 2 T * is needed. In solution, the major contributions to the line-width are 2  field inhomogeneity and the natural spin-spin relaxation time.  However the  present situation is more complicated, and there are several other factors which w i l l broaden the line: i) dispersion of isotropic chemical shifts due to structural disorder including residual aluminum and the disorder of templates;  ii)  unaveraged dipolar interactions of the S i nucleus w i t h * H ; iii) the presence of 2 9  91  paramagnetic impurities i n the sample; iv) overlapped multiplet splittings due to the S i - 0 - S i couplings i n this enriched sample. Therefore, the best encoding 29  29  time is expected to be longer than 272*, ^  ^  u t  n o t  attainable i n practice.  Inspection of Figure 27B reveals that the w i d t h of the T | resonance is approximately twice that of the other signals (~80 vs ~45 H z ) , which could affect the detection of the cross peaks involving this resonance.  In an attempt to  narrow the resonances, the sample temperature was raised^0/93) -phe linewidth of the Ti resonance at 373 K was reduced to one half of that at room temperature (Figure 27), which w o u l d make the C O S Y experiments more efficient. Therefore a second series of experiments were performed at 373 K and the results of the best one under the conditions indicated i n figure caption are shown i n Figure 3 2 . In addition to the T T 2  3  cross peaks previously observed, T j T  n o w clearly visible. The doubling of the T T 2  resolution of the T  3  3  2  connectivities are  cross peak is caused by the partial  resonances due to the absence of cubic symmetry.  These  results are i n exact agreement with the k n o w n connectivities of the structure.  In  an attempt to optimize the encoding times, variable fixed delay experiments were carried out, keeping all other parameters constant. The results are listed i n Table 9, in which the intensities of the T j T the T  2  2  cross peak are presented relative to  diagonal peak intensities . W h e n the encoding time is increased from 31-  46 ms the intensities of the T j T cross peak decease dramatically, confirming the 2  importance of T * i n the solid state experiments. The T^ resonance has a 2  linewidth of -40 H z and thus T2 equation: T2* = l/bt&Vyj). -31 ms, being around  is approximately 8 ms calculated from the  The best encoding time from these experiments is  4r *, i n agreement 2  with expectations.  92  Figure 31  Contour Plot of COSY experiment on ZSM-39 carried out at ambient temperature with 128 experiments, 48 scans in each experiment, sweep width of 5 kHz, and 256 data points collected during the acquisition. The fixed delay was 15 ms and the total experimental time approximately 17 h. Sine bell squared apodization was used and the plot symmetrized.  93  -io«  Figure 32  -10*  -no  -112  -ii4  -in  -in  -120  -122  -124  rr*  Contour and stacked plots of a COSY experiment on ZSM-39 carried out at 373k, with 128 experiments, 64 scans in each experiment, sweep width of 5 kHz, and 256 data points collected during the acquisition. The fixed delay was 5 ms and the total experimental time approximately 23 h. Sine bell apodization was used, and the data are presented without symmetrication.  94  Table 9  The results of variable fixed delay experiments. fixed delay (ms)  total encoding time(ms)  cross peaks visible (pair)  relative intensity ratio* ofTjTj/Tz  20 15 10 5 2  46 41 36 31 28  1 2 2 2 2  0 0.15 0.5 1  -  1  ,  * The intensity ratio of T1T2/T2 at 5 ms fixed delay is taken as unity.  W i t h the experience i n the experiments at 373K and the knowledge of the importance of the T * , an attempt of performing C O S Y experiments at ambient 2  temperature was made again with the number of experiments reduced to 64, to reduce the encoding time to one half. The best result is shown i n Figure 33 and was obtained at the total encoding time of 15 ms, which is again ~ 4 T . Both 2  T|T  2  and T T 3 correlations are clearly observed over a range of total encoding 2  times of 13-18 ms. The resolution and sensitivity of these experiments are worse than those at 373 K. The degradation i n the quality of the correlations at ambient temperature may be recovered to some extent by the use of double quantum filtered (DQF) C O S Y experiments. A conventional DQF-COSY pulse sequence was used except that a C P sequence was used to excite the S i magnetization. A 2 9  suitable choice of the phase cycle is used to separate out single quantum signals from  the signals w h i c h  come  from coupled pairs.  Figure 34  shows the results of  a D Q F C O S Y experiment where the quality of the connectivities is almost completely recovered. It is even possible to observe both connectivities when the number of frequency encoding experiments is reduced to 32 for this system.  95  -126 .  -124  122  .  -120  ue 116  .  -114  .  -112  110  108  .  -106  104  -104  Figure 33  -106  -108  -110  -112  -114  -US  -118  -120  -122  -124  -126  Contour plot of a COSY experiment on ZSM-39 carried out at 298k, with 64 experiments, 80 scans in each experiment, sweep width of 5 kHz, and 256 data points collected during the acquisition. The fixed delay was 2 ms and the total experimental time approximately 14 h. Sine bell apodization was used, and the data were symmetrized and a smoothing function applied.  96  Figure 34  Contour and stacked plots of a DQF COSY experiment on ZSM-39 carried out at 298k, with 64 experiments, 128 scans in each experiment, sweep width of 5 kHz, and 256 data points collected during the acquisition. The fixed delay was 2 ms and the total experimental time approximately 23 h. Sine bell apodization was used, and the data are presented without symmetrization.  97  B.  TWO-DIMENSIONAL  2 9  S I H I G H - R E S O L U S I O N SOLID STATE  N M R INVESTIGATION O F T H E LATTICE STRUCTURE O F ^SIE N R I C H E D Z E O L I T E D E C A - D O D E C A S I L 3 R (DD3R)  I.  INTRODUCTION  Deca-dodecasil 3 R (DD3R) was first synthesized b y Gies< ) i n 1986, b y 101  crystallization from an alkaline silicate solution with 1-aminoadamantane as template. The sample used i n the present work was prepared from S i enriched 2 9  sources again to increase the number of S i - 0 - S i interactions. The silica host 29  29  framework of D D 3 R is related to the framework types of the dodecasil series 0-02)^  o  n  e  example of which is ZSM-39 (see section A). In ZSM-39, layers of  pentagonal dodecahedra are directly stacked i n an ABCABC sequence. However, i n DD3R, the same layers are stacked i n an ABCABC sequence, but interconnected by additional S1O4 tetrahedra.  The space group of D D 3 R was determined by  as R3m from a single crystal diffraction study of template-containing material. The schematic representation of the D D 3 R lattice framework is shown i n Figure 35A. The T-site 5 cannot be shown i n this figure because it is located between the layers and acts as a linkage. Figure 35B shows the [100] projection of the structure and i n this projection T-site 5 is clearly visible. The structure has a unit cell of 120 T-atoms distributed over seven sites i n the relative proportions 6 : 3 : 3 : 3 : 3 : 1 : 1 . The expected connectivities for the T-sites are presented i n Table 10. Compared to ZSM-39, 2D correlation experiments on D D 3 R w i l l be a more demanding test of the techniques because of the larger number of signals and more complex connection pattern expected.  98  Figure 35  (A) Schematic representation of the zeolite DD3R lattice framework viewed along c axis. (B) The projection along the a axis of the structure. The seven crystallographically inequivalent tetrahedral T-sites are indicated by Sil etc. (ref. 95)  99  Table 10  T-sites, Their Occupancies, and Connectivities for the Asymmetric U n i t i n Zeolite D D 3 R (Ref. 101) T-site  occupancy  Tl  6  lTj: 1T :1T :1T  T  2  3  2Tj: 1 T : 1 T  3  3  1T :2T :1T  4  3  2T lT :1T  3  2Tj: 2T  6  1  3T :1T  7  1  3T :1T  T T T T T  H.  5  connectivity 2  3  2  i :  3  3  4  2  4  5  7  4  6  5  7  6  EXPERIMENTAL 2 9  S i M A S N M R spectra were obtained at 79.49 M H z on a Bruker MSL-400  spectrometer  using the  techniques  previously  described.  2D  COSY-45  experiments were performed using the pulse sequence shown i n Figure 36A. The replacement of the second 90° pulse (Figure 17A) with a 45° pulse simplifies the appearance of the spectrum around the diagonal to make the cross peaks close to the diagonal more easily observed. The fixed delay is used to enhance small couplings. The sample as-synthesized showed broad lines and had to be investigated in its calcined form and thus C P and proton decoupling techniques could not be used. 2D I N A D E Q U A T E experiments were performed using the pulse sequence shown i n Figure 36B.  The replacement of the last 90° pulse  (Figure 18A) w i t h a 135° pulse provides quadrature detection i n the double quantum frequency domain (103).  100  A 45  90: t1  FD  FD  t  2  (AO)  B 1/4J  Figure 36.  135:  90:  180)  90j  1/4J  ti  t  2  (AQ)  Schematic representation of the pulse sequences used in the 2D M A S N M R experiments: (A) 2D COSY-45 pulse sequence, (B) 2D INADEQUATE pulse sequence.  101  A highly siliceous sample of D D 3 R was synthesized hydrothermally by Dr. Hermann Gies, U. Bochum, Germany, i n a sealed silica glass tube i n 14 days at 160°C using 1- aminoadamantane as template, and the silica source was enriched to - 8 0 % i n S i . 2 9  m.  RESULTS A N D DISCUSSION  a) I D Experiments The I D  2 9  S i M A S N M R spectrum is shown in Figure 37A, i n which the  assignments of the T-sites come from the 2D studies (see on). Figure 37B shows the computer simulation of the spectrum as seven peaks. The relative intensities of the peaks are 1:3:3:3:6:3:1 as expected. The linewidths are around 40 H z , which is of the same order as those i n ZSM-39 and the previous results were used as a guide for the choice of parameters in the present work. b) 2D C O S Y Experiments According to the results obtained from the experiments on ZSM-39, the total encoding time was chosen to be ~ 4 T  2  because both samples were 29si  enriched and the sources of line broadening should be similar. Figure 38 shows the results of a C O S Y experiment carried out using the experimental parameters given in the figure caption. To facilitate the discussion, the seven resonances are denoted as A-G from high to l o w field, as indicated in the F  2  projection.  Although a number of cross-peaks are clearly observed, it is difficult to make an assignment of the spectrum from this data alone because the postulated structure is not completely correct for this sample (see on) and the l o w field resonances are  102  T;+VT  5  A  103  overlapped. The assignment may be obtained by combining the N M R results with X R D data i n such way that the T- sites with longer mean T- T distances w i l l be associated with the resonances at the lower f i e l d ^ ^ ) (  see  Section E, Chapter  Five). The resonances A and G of unit intensity can be assigned to Tg and T7, which are connected to each other as expected. Resonance A can be assigned as T 5 due to the longer average T-T distances than that of Ty^^\  It should be  noted that there are very substantial errors i n the X R D derived parameters. However i n the present instance the chemical shift difference is thought to be large enough that the assignment is reliable. From Figure 38 and Table 10, Tg is connected to T 4 as well as T7, thus assigning G ->Ty and C - ^ 4 . T 7 is connected to T  2  as well as Tg and therefore the T  resonance is within D/E.  2  In order to  develop the connectivity network, Table 11 lists the connectivities of T4, T  2  and  C, D/E. The assignment of B — ^ 3 can be readily made. Thus some complexity is removed by the fact that two resonances, F and one in D/E  are associated to  T-j. N o connectivity between T and T | is visible, which can be understood since 2  their resonances are too close to be resolved and the cross peaks w i l l be buried in the strong diagonal signals. This experiment was repeated a number of times to confirm the 'doublet* structure of Tfli>  A possible explanation is that the  symmetry of the structure is lower and there are two inequivalent T | sites. The only resonance unassigned until n o w /Tij, must be within D/E, and the complete assignment is shown i n Figures 37 and 38. Thus, it is deduced that there are 8 resonances i n the N M R spectrum, which is not clear from the I D data.  This  illustrates the advantages of 2D experiments i n determining the contents of the asymmetric unit of an u n k n o w n sample.  104  Figure 38  Contour plot of a COSY experiment with the projection on the top, carried out at 300K with 64 experiments, 992 scans in each experiment, a sweepwidth of 2500 H z and 256 data points collected during the acquisition. The fixed delay was 2 ms and the total encoding time was 28 ms. Sine-bell squared apodization was used and the plot is unsymmetrized.  105  i  Table 11  Connectivities Related to T-sites 4,2 and Resonances C, D/E in the D D 3 R Structure  T  4  C  T  l  T  D/E  l F  T  3 B  T  Tl  6  T B  D/E  A  Tl  3  T G  7  C  c) 2D I N A D E Q U A T E experiments Figure 39 shows the results of a 2D I N A D E Q U A T E experiment on the same sample. 2D I N A D E Q U A T E experiments w i l l be discussed in more detail in Chapter Four, but this one example is included here for completeness of the discussion of zeolite D D 3 R structure.  A l l of the connectivities previously  observed i n the C O S Y experiment are clearly seen as indicated by assignments i n the figure. The existence of the T T 2  7  the  connection is confirmed. In  addition, the T-iT coupling pair within the l o w field signals can be observed. 2  The 'doublef structure of the T 1 T 4 pair is very clearly and unambiguously seen confirming the results of the C O S Y experiment. The same observation in two different types of experiments indicates that the existence of two inequivalent T-i's is real. N M R spectroscopy is k n o w n to be more sensitive to local environments than diffraction techniques and can be used as a subtle probe of the contents of asymmetric units. A s a w o r k i n g hypothesis, it is postulated that the T | sites are split into two types T j and T j ' due to the loss of some symmetric element or elements. F r o m subgroup-supergroup relationships, R3 is the only space group  106  which can explain the splitting of the T | position. It should be noted that the X R D study of the single crystal of D D 3 R was performed on its uncalcined form containing template, while the N M R measurements were carried out on a calcined sample. There are some k n o w n examples where calcined samples have lower symmetry, eg. zeolite ZSM-5. The templates are trapped inside channels and cavities of zeolites during preparation and the total energy of the system could be lower because of the interaction between templates and frame work, and the symmetry could be h i g h e r ' ! ^ .  A recent refinement (105)  o  n  m  e  calcined sample prompted by these results supports the lower space group symmetry in agreement w i t h the present N M R data.  107  Ti*VT TI  5  y \  ,''T T 4  ••  o  ®  T z T 3  T , ,  T,T ,' *  3  T T£ Q 7  y  4  ^^^^ *  -112.0  -114.0  -116.0  -118.0  -120.0  -122.0  PPN  Figure 39  Contour plot of an INADEQUATE experiment on DD3R at 300 K with 44 experiments, 448 scans per experiment, 1285 H z sweepwidth, a fixed delay of 20 ms, and 128 data points collected during the acquisition. Trapezoidal and shifted sine bell apodization were used for F j and F dimensions, respectively. 2  108  CHAPTER FOUR  NATURAL-ABUNDANCE TWO-DIMENSIONAL SOLID STATE SI NMR INVESTIGATION OF THE THREE-DIMENSIONAL LATTICE CONNECTIVITIES IN ZEOLITES ZSM-12 29  AND ZSM-22 A.  INTRODUCTION  Our initial 2D N M R studies of zeolites ZSM-39 and D D 3 R (see Chapter Three)  have  including  demonstrated  that  COSY, I N A D E Q U A T E  homonuclear  correlation  spectroscopies  and spin-diffusion experiments yield the  correct (known) connectivities of the lattice structures.  The results of these  experiments thus lay the groundwork for subsequent investigations. However, these two samples were prepared by using an 8 0 % S i enriched silica source to 2 9  increase the signal to noise ratio and enhance the connectivities due to the S i 29  0 - S i interactions. Such high degrees of isotopic enrichment are too expensive 29  and difficult for routine use. In order that these techniques can be widely used to investigate the lattice structures of zeolites it must be possible to perform them o n natural abundant samples. Sensitivity could be considered to be the most serious problem i n the 2D w o r k because of the l o w  2 9  S i natural abundance of  4.7%. The probability of a S i nucleus being involved in a Si-0- Si pair w i l l 2 9  29  29  be - 1 9 % . Therefore, ~ 8 1 % among the 4.7% S i nuclei are not coupled, and 1 9 % 2 9  w i l l be i n coupled pairs. Fortunately, there are some advantages in using natural  109  abundance samples. Firstly, only the very best samples need be chosen for 2D N M R work due to the lower expense i n synthesis. They are usually very highly crystalline and 'aluminum-free', giving very sharp peaks i n their N M R spectra, usually with linewidths of 10- 30 H z . Thus the resolution w i l l be better and the intensity left after the encoding time may be improved due to the longer T . 2  Secondly, the interactions involved between  2 9  S i nuclei w i l l be simpler than  enriched samples since they are dilute. The zeolites chosen for starting this study were zeolite ZSM-12 and ZSM-22, whose structures are well defined and which were considered reasonably typical of these systems in terms of complexity.  B.  NATURAL-ABUNDANCE  TWO-DIMENSIONAL  2 9  S I HIGH-  RESOLUTION SOLID STATE N M R INVESTIGATION OF THE L A T T I C E S T R U C T U R E O F Z E O L I T E ZSM-12 I.  INTRODUCTION Zeolite ZSM-12 was first synthesized by Rosinski and R u b i n *  general lattice structure was proposed by L a P i e r r e ^ ) .  106  ).  The  Fyfe and Gies (47) have  recently determined the space group of ZSM-12 as C2/c by a combination of synchrotron powder X-ray data w i t h solid state N M R measurements Chapter One).  (see  ZSM-12 is high silica zeolite with 12-membered ring channels  along the c axis. A schematic representation of the lattice framework is shown i n Figure 40.  The  asymmetric unit consists of seven crystallographically  inequivalent T sites as indicated in the figure. The connectivity scheme of ZSM12 is given in Table 12. There are a total of ten connectivities, of w h i c h nine are  110  Figure 40  Schematic representation of the lattice structure of zeolite ZSM-12. The seven crystallographically ^equivalent T-sites are indicated by S i l , Si2,...,Si7. (ref. 108)  111  expected to be observable i n 2 D experiments. The link between the equivalent silicons at the site 7 can obviously not be detected directly in N M R experiments.  Table 12  T-sites, Their Occupancies, and Connectivities for the Asymmetric Unit i n Zeolite ZSM-12 (Ref. 47) T-site  occupancy  Tl  1  2T : 2 T  3  T  2  1  2Ty. 2 T  4  3  1  2T  4  1  2T :1T :1T  6  'I  1T :1T :2T  6  6  1  1T : 2 T : 1 T  7  1  1T :1T :2T  T T T T T  H.  5  connectivity 2  i:  2  3  4  3  1T :1T 5  5  4  5  6  7  7  7  EXPERIMENTAL 2 9  S i M A S N M R spectra were obtained at 79.49 M H z on a Bruker MSL-400  spectrometer  using the techniques previously  described.  In the C O S Y  experiments, samples were spun at frequencies equal to, or a multiple of, the spectral sweep width. The pulse sequences used are shown i n Figure 36. Since the linewidths of ~9 H z (Figure 41) are of the order of those obtained i n solution spectra a n d the coupled pairs are isolated, it is considered that  field  inhomogeneity is the major source of the linewidths. Thus ~2T2 value of 58 ms was chosen for the total encoding time i n C O S Y experiments. A highly siliceous sample of ZSM-12 was synthesized hydrothermally b y Gwyneth Barlow, U . of Guelph, using methyltriethyl ammonium chloride as a template according to the l i t e r a t u r e ^ ) .  112  m.  RESULTS AND DISCUSSION a) C O S Y experiments The I D  2 9  S i M A S N M R spectrum shown in Figure 41 is composed of  seven well-resolved resonances w i t h ~9 H z linewidths.  The number and  intensities of the resonances are i n agreement with the proposed structure and previously published spectra^?). 2D C O S Y 45 experiments were carried out first, and Figure 42 shows contour plots from a typical one. A s can be seen, all 9 expected connectivities between the resonances are clearly observed i n the plot. The assignments of the resonances can be deduced unambiguously from the connectivities given i n Table 12 in several different ways.  The most important thing is to identify a  starting point for tracing the coupling patterns.  One starting point is the  observation that three T sites have only two connectivities which can be obtained i n a C O S Y experiment, i.e. T j , T  2  and T7. By drawing vertical lines through  every peak in the projection, three lines hit cross-peaks twice and the others three times. The three resonances showing two connectivities are A , C and D, which are thus associated w i t h these three T sites. From the fact that T j and T  2  are  connected to each other and T j and T 7 are simultaneously connected to T3, the following associations can be made: A-> T , D-> Ty C-> T 7 and G-» T3. T 2  2  has  double connectivities to both T 4 and T j , and since T j is k n o w n , the resonance F must be T4. Similarly, the assignments of B—» T 5 and E—> Tg can be made. The complete assignment is shown i n the figure. A n alternative starting point comes from the differences i n spin-lattice relaxation times, T j , of the seven nuclei.  113  I  Figure 41  I  -10B  I  -109  I  1  -110  PPH  -111  1  -112  1 —  -113  S i M A S N M R spectrum of zeolite ZSM-12. The spectrum were obtained at room temperature with 128 scans and 1 K data points before zero-filling.  29  114  •  '  •  -108.0  Figure 42  *  *  *  *  •  -109.0  •  *  •  1  •  •  *  -110.0  •  1  *  •  -111.0  •  •  1  •  *  -112.0  •  •  *  •  •  -113.0  Contour plot of a COSY experiment on zeolite ZSM-12 with a projection in the F2 dimension carried out at 300 K with 64 experiments, 160 scans i n each experiment, sweep width of 1200 H z , and a fixed delay of 5 ms. 256 data points in F before zero-filling, sine bell squared apodization and magnitude calculation were used for data processing. 2  115  A l l Tj values of  2 9  S i nuclei i n ZSM-12 are ~ 10 s except that of the nucleus  associated with resonance B (see Figure 23), which is almost double this value. A s discussed i n Chapter Two, T 5 site is the only one that is not in the surface of the channel and, therefore, w i l l not be in direct contact with adsorbed oxygen which provides the mechanism for the spin- lattice relaxation of S i nuclei. In 2 9  the N M R experiments, the resonance B is always of low intensity if the repeat delay for the pulse sequence is not long enough for complete recovery of the magnetization. Obviously, this C O S Y experiment with a delay time of 15 s is the case and the intensity of resonance B is much less than the others, as shown in the projection. From the starting point of B-» T5, resonances of E, F and G could be associated with T3, T 4 and Tg. A m o n g them, T 4 and Tg are connected to each other, and T 3 and Tg are linked to T  simultaneously.  7  Therefore, the  assignments of E-» Tg, F-> T4, G-» T 3 and C-> T can be made. The remaining 7  resonances are easily assigned, and the whole assignment is in exact agreement w i t h the previous result.  Another possible starting point is the relative  intensities of the cross-peaks. There are four double connectivities in Table 12, i.e. T j T , T1T3, T T 2  2  4  and T5T5, whose intensities, in general, are expected to be  stronger than those from single connectivities. The four strongest cross-peaks in both the upper left and lower right parts i n the contour plot (Figure 42) are A D , A F , BE and D G . It is easy to see that A and D are associated to T | and T , B, E to 2  T , Tg, and F, G to T3, T . The only one left, C , must be T . Both T and T 5  4  7  5  6  have  a connection to T4, thus F-» T4, G - ^ 3 , A - » T and D-»Tj can be made. A l l these 2  assignments are self-consistent. These methods plus those discussed i n the case of zeolite D D 3 R provide the basic 'techniques' for making assignments, and were used i n subsequent studies.  116  b) Direct observation of ^ S i - O- ^ S i couplings In the 2D C O S Y experiments discussed in the previous section, 512 data points were collected during detection period for each experiment.  However,  only 256 points were used i n the Fourier transformation i n order to obtain a better S / N ratio i n the 2D spectrum (Figure 42). When the number of points in F  2  is increased to 450 before zero filling, the 2D plot (Figure 43) shows the same  connectivities, w i t h the signal to noise ratio being traded for extra resolution. A power calculation was used to compensate, to some degree, for the loss of sensitivity. Doublet splittings are observed for almost all the cross peaks i n the F  2  dimension, as shown in the figure.  In principle, four peaks should be  observed i n each 'cross peak', but the real digital resolution in F| without zero filling is only -40 H z / p o i n t and is not sufficient to resolve the splittings i n this dimension. The magnitudes of the apparent couplings are in the range of 10-16 H z , and they are symmetrical about the diagonal as expected. The structure of the cross-peaks can be clearly observed i n plots of cross sections from the 2D spectrum as shown i n Figure 44.  These values must be considered as only  approximate due to the limited real digital resolution of -5 H z per point before zero filling. O n the other hand, the experiments are not phase sensitive, which w i l l tend to increase the observed s p l i t t i n g s ^ ) . The mechanism for the splitting is thought to be scalar coupling. However, possible dipolar interactions within each S i - 0 - S i pair must also be considered since a similar connectivity pattern 29  29  to that from scalar couplings could be produced. The expression for the dipolar splitting, R, i n a powder sample is given i n Equation 27@&\  117  "ft Yl Y2 HO  R  =  — - T —  [27]  4rc r3 4TC  where )Xq is permeability constant; r the internudear distance and Yi= T2 Y ( Si) =  29  the magnetogyric ratio. In the case of zeolites, the value of R for S i - 0 - S i is 29  29  calculated to be around 170 H z b y assuming r= 3 A . The spinning rates used i n these experiments are ~2.4 k H z and are rapid enough to eliminate the dipolar interactions. However, an experiment was carried out at rotor frequency of 1.2 k H z to make sure that the splitting is not due to a dipolar interaction, since the magnitude of any observed coupling from the dipolar interaction w i l l depend inversely on the spinning rate.  N o differences i n the magnitudes of the  couplings were observed, which ruled out the dipolar interaction being the source of the splitting of the cross peaks. In addition, i n C O S Y experiments, scalar couplings must be present.  The observed splittings are of the correct  magnitude when compared with solution data of 3-10 H z * ^) (see Chapter Two). 7  Most silicate species in solution contain a substantial number of three and four membered rings, while in zeolites five and six membered rings are dominant. The couplings are very dependent on ring size, and the larger the size of the rings, the greater the J values for the silicons. W i t h the knowledge of these J couplings, the I D spectrum (Figure 41) could be reexamined. The line shapes could not be fitted by standard Lorentzian curves, there being "bumps' o n both sides of the base of each peak, w h i c h could be the satellites of those coupled pairs. Thus the main intensity of each peak comes from the uncoupled S i and 2 9  the satellites separated by 10-16 H z are buried within both side wings, resulting i n "distorted' line-shapes.  118  . -113.0  . -118.0  . -110.0  . -109.0  . -108.0  -108.0  Figure 43  -109.0  -110.0  PPM  -111.0  -112.0  -113.0  Contour plot with projection in F2 of a COSY experiment on ZSM-12 obtained under the same conditions as in Figure 42, except that there are 80 scans in each experiments and 450 data points and power calculation were used in the data processing.  119  -108.0  -108.0  -110.0  PPM  -111.0  -112.0  -113.0  Cross section plots from Figure 43. The sections correspond to those indicated Figure 43.  120  c) 2D I N A D E Q U A T E experiments The I N A D E Q U A T E pulse sequence was proposed by Bax, Freeman and Kempsell* ) in 1980 to directly determine the carbon skeletons of organic 66  molecules through J-couplings. T h e I N A D E Q U A T E experiments of  1 3  C are  considered to be of l o w sensitivity due to the - 1 % natural abundance. Although it seems a little easier to perform this k i n d of experiment on  2 9  S i of 4.7%  abundance, no work regarding this has been reported i n either solution or solid state N M R to date.  W i t h the knowledge of the J coupling values of 10-16 H z ,  obtained from the C O S Y experiment, a fixed delay of -16 ms ( l / 4 p was used i n I N A D E Q U A T E experiments to optimize the intensities of J-coupled signals (see Chapter Two). A one-dimension I N A D E Q U A T E experiment was first performed on ZSM-12. The spectrum was not easy to interpret because the residual main signals and the satellites are overlapped due to the similar magnitudes of the linewidths of the residual main signals and the separations of satellites, but substantial signal intensity was observed. Considering the advantages of two dimensional experiments, a 2D I N A D E Q U A T E experiment was  undertaken  (Figure 45). A l l of the connectivities found i n the previous C O S Y experiments were confirmed, including that between T 4 and T^, which is clearly resolved here but was somewhat ambiguous i n the C O S Y experiment due to the close proximity of these cross-peaks to the diagonal. A l l of the assignments made from the C O S Y experiments could have been made from the I N A D E Q U A T E experiment as well.  The doublet structures are clearly observed in this  experiment, again confirming the previous results.  Figure 46 shows plots of  slices from the 2D contour plot corresponding to the maxima of the signals, where the fine structure is clearly observable.  121  Figure 45  Contour plot of an INADEQUATE experiment on 2SM-12 at 300 K with the ID M A S N M R spectrum shown above , with 52 experiments carried out, 64 scans per experiment, 800 H z sweep width, fixed delay of 20 ms 450 data points collected during the acquisition, sine bell apodization and power calculation were used in the data processing.  122  5  A  6  S5 . M TT  T 4  '2/l.  TsT5  T 6  /I  T  5 n  KZ  k  /\A /lsT T  6 5  4  S4  T  V  T.T .. . . 5  T  T  3 s T  If  7  X  2  X2  5  S3 T T '3 h  x  2  T T, 3  i -108  Figure 46  -109  TT  4 6 141  x  J  -110 PPM  I  -111  I  -112  ^  L  -113  Cross section plots from Figure 45. The numbers of the rows correspond to those indicated in Figure 45.  1 2 3  A symmetrical I N A D E Q U A T E experiment^ ®®\ a simple modification of the standard I N A D E Q U A T E procedure was also performed. By applying a 90° pulse in the middle of the evolution period i n the normal sequence (see Figure 36B), the resulting correlation spectrum has a COSY-like appearance, but without intense diagonal signals.  Figure 47 shows the results of a symmetrical  I N A D E Q U A T E experiment w i t h a projection in the F all the connectivities are clearly observed.  2  dimension, where again  Compared with the results of the  corresponding C O S Y experiment (Figure 42), this spectrum is neater unambiguous.  and  Since the correlations are n o w symmetrical relative to the  diagonal, the symmetrization routine could be used as for C O S Y experimets to remove some artifacts and increase the signal to noise ratio. d) Comparison of 2D I N A D E Q U A T E and C O S Y experiments Both I N A D E Q U A T E and C O S Y experiments contain essentially the same information, i.e.  homonuclear  connectivities  via J-coupling.  The  major  advantage of I N A D E Q U A T E is that the spectrum lacks the diagonal peaks due to the single quantum resonances from uncoupled nuclei. Thus, for complex systems, it may be possible to trace correlations in the 2D I N A D E Q U A T E spectrum which w o u l d be lost close to the diagonal of the C O S Y experiment. The uncoupled  2 9  S i nuclei could produce strong single quantum coherences  during the detection period i n both experiments, but i n I N A D E Q U A T E , these unwanted conventional signals are filtered out based on the different phase properties between the single and double-quantum signals ^ \  If the phase of  the read pulse (see Figure 36) is cycled in the x-direction i n the first step and in the y-direction i n the second, the phase of the transverse signal generated from single quantum coherence w i l l cycle in the same sense. In contrast, the signal  124  —  ,  -108.8  ,  ,  ,  ,  ,  -110.19  ,  ,  1  -112.0  PPM Figure 47  Contour plot of a symmetrical INADEQUATE experiment on ZSM-12 at 300 K with the projection in the F dimension, with 26 experiments carried out, 64 scans per experiment, 522 Hz sweep width, fixed delay of 20 ms 256 data points collected during the acquisition, sine bell apodization and power calculation were used in the data processing. 2  125  generated from double quantum coherence shifts by 270° for each 90° step in the read pulse, i.e. i n the x-direction i n the first step and i n the -y direction in the second.  The receive phase can be adjusted such that the weak signals from  coupled pairs are accumulated and the single quantum signals as well as the spinning sidebands from these signals are not present i n the final spectrum. Unfortunately, i n the case of C O S Y experiments, both diagonal and cross peaks come from single quantum coherences, and there is no simple way discrirninate between them.  to  In addition, the suppression of the spinning  sidebands is very important i n solid state 2D experiments, because the presence of spinning sidebands and their fold-back signals makes the spectrum difficult to interpret. In order to eliminate the interference from these unwanted peaks and other artifacts i n C O S Y experiments, all resonances are put within either the left or right half of the spectral range, resulting i n half the possible digital resolution i n both dimensions compared to the corresponding I N A D E Q U A T E experiments. Furthermore, the samples must be spun at frequencies equal to, or a multiple of, the spectral sweep w i d t h i n C O S Y experiments so that any spinning sidebands in F j are exactly coincident with the main signals. The lack of intense single quantum signals i n I N A D E Q U A T E experiments gives a better dynamic range for the connectivities, and the weak coupled signals can be readily detected. Another advantage of I N A D E Q U A T E is that a better S / N may anticipated.  The generation of double quantum coherences  during the  preparation period is optimized for each t^ incremented experiment.  It is  obvious that each experiment makes an efficient contribution to the final results. In addition, since the final signal is more constant i n time, a much efficient w i n d o w function can be used i n the data manipulation. However, in cases of P-  126  type selection C O S Y experiments used i n this thesis work, the echoes shift substantially during the experiment, which makes it difficult to apply a single w i n d o w function which is efficient for every t j incremented experiment. The  major  disadvantage  of I N A D E Q U A T E  experiments  is that  a  reasonable estimate of the J coupling must be made to choose an appropriate fixed delay, during which the double quantum coherence is generated. If the J coupling is not k n o w n , it is hard to make the experiments work properly, whereas  the  corresponding C O S Y  experiment  will  usually  yield  some  information . In addition, the I N A D E Q U A T E pulse sequence involves a long preparation period, and the situation may arise where the values of the relaxation times and couplings are such that it is not possible to carry out this k i n d of experiment due to the restriction of short T or T *. In this work, the J 2  2  coupling doublets are directly observed in the C O S Y experiments, and the J coupling values are measured to be within a narrow range of 10-16 H z because all silicon atoms are i n similar chemical environments in the highly siliceous samples.  Hence, the I N A D E Q U A T E experiments are very efficient.  The  linewidths of the resonances are i n range of 9-30 H z for highly siliceous zeolite samples. Consequently, there are no severe restraints on the experiments. Thus, the I N A D E Q U A T E experiment can be carried out very efficiently, w i t h no interferences from signals originating from the majority of uncoupled 2 9  S i nuclei and their sidebands and with improved S / N .  This technique w i l l  usually be the method of choice i n the investigation of zeolite structures.  127  C.  NATURAL-ABUNDANCE  TWO-DIMENSIONAL  RESOLUTION SOLID STATE  NMR  2 9  SI  HIGH-  INVESTIGATION OF  THE  L A T T I C E S T R U C T U R E O F Z E O L I T E ZSM-22  I.  INTRODUCTION  Zeolite ZSM-22 was synthesized quite independently by different groups using different reaction conditions and templates and is also described as KZ-2, Theta-1 and NV-10 (H0~H4)  A crystal structure of space group C m c 2 j was  proposed based o n powder X R D studies^!5,116) There are 24 silicons in a unit cell distributed over four crystallographically inequivalent sites, and there is a one-dimensional 10-membered ring channel system running along the c axis (Figure 48). A single crystal structural refinement of a completely siliceous ZSM22 sample has been presented by M a r l e r ^ ) , giving better geometric parameters for the structure. Table 13 presents the connection scheme of ZSM-22. connectivities between the pairs T j T , T1T4, T T 2  2  3  Four  and T 3 T 4 are expected to be  observed i n 2D N M R experiments. Table 13  T-sites, Their Occupancies, and Connectivities for the Asymmetric U n i t i n Zeolite ZSM-22 (Ref. 115) connectivity  T-site  occupancy  Ti  l  2T : 2 T  T  1  2Ty. 2 T  2  1T :2T :1T  2  1TJ: 1 T : 2 T  T T  2  3  4  2  2  4  3  3  128  3  4  4  Figure 48  Schematic representation of the zeolite ZSM-22 lattice framework, the four crystallographically inequivalent T-sites indicated by S i l , Si2, Si3 and Si4. (ref.108)  129  n.  EXPERIMENTAL A highly siliceous sample of ZSM-22 (GEB 100) was previously prepared  by Gwyneth Barlow, U . Guelph, using diemylamine as a template according to reference 112. The high-resolution S i M A S N M R spectra were acquired at 79.5 2 9  M H z using the equipment and techniques previously described. m.  RESULTS A N D DISCUSSION a) 2D C O S Y experiments The I D  Figure 49.  2 9  S i M A S N M R spectrum of highly siliceous ZSM-22 is shown i n  The four resonances of relative intensities 2:1:1:2 are i n excellent  agreement w i t h the diffraction-determined structure and previously published N M R spectra* ). 50  2D C O S Y experiments were performed i n a similar w a y to those discussed for ZSM-12. The results are shown i n Figure 50, and three cross-peak pairs are clearly observed. The resonances may be divided into two groups A , D and B, C on the basis of their relative intensities i n the I D spectrum. The resonances A and D with double intensities are assigned to the pair T3 and T4, while B and C to the pair T | and T . A connectivity between T | and T is expected, but the 2  2  cross-peaks are too close to the diagonal to be resolved. Because of the intrinsic symmetric nature of the structure of ZSM-22, there are two possible assignments starting from B -» T-j and B -¥ T  2  respectively (Table 14), both of which can  satisfy the connectivity pattern indicated in Table 13. There are two ways to solve the problem. One is based o n the difference of T j relaxation times. It can be seen from the structure of ZSM-22 (Figure 48) that the 10-membered ring is formed b y 2 T j atoms, 4 T3 and 4 T atoms. Therefore, T is not on the surface of 4  2  130  Table 1 4  T w o Possible Assignments of the Spectrum of ZSM-22 Assignment I  Assignment II  Resonance  A  B  D  A  B  T-sites  4  2  1 3  3  1 2  ^109  r  i 10 J  Mil  C  M12  ^iH  I  H4  =115  C  D 4  -116  PPM  Figure 49  ID ^ S i M A S N M R spectrum of zeolite ZSM-22 obtained at room temperature with 280 scans and 512 data points.  131  -110.0  Figure 50  -111.0  -112.0  -113.0 PPM  -114.0  -115.0  Contour plot with projection in the F2 dimension of a COSY experiment on zeolite ZSM-22 carried out at 300 K, 64 experiments, 592 scans per experiment, 1200 H z sweep width, fixed delay of 0.5 ms. 256 data points were used during the data processing and sine-bell-squared apodizahon and magnitude calculation employed.  132  the channels, which results in it having a longer Tj relaxation time than any other nuclei. The I D spectrum in Figure 50 shows that the intensity of resonance B is smaller than A if the repeat time is < 5 T j . Thus a assignment of B-» T  2  can  be made and a complete assignment is obtained as indicated in Figure 50. The other way is by combining the N M R results with X-ray structural data, i. e. from the correlation between average T-T distances and general, the resonances of located to lower field  2 9  2 9  S i chemical shifts.  In  S i associated T site w i t h shorter T-T distances are In the present case, the average T-T distances of T3  and T4 are 3.08 and 3.11 A respectively  Thus, the resonances of A and D  may be assigned such that the lowest held resonance D is associated with T3 and the resonance A is T4. resonances makes  The large difference in chemical shift between the  the assignment reliable, and yields exactly the  same  assignments as the Tj method. W h e n 512 data points are used i n F  2  dimension before zero-filling, the  doublet structure is observed i n the cross-peaks as in the case of ZSM-22. The results is shown in Figure 51. The values of the splittings are again in the range of 10-15 H z . b) 2D I N A D E Q U A T E experiments Figure 52 shows the results of the I N A D E Q U A T E experiment on zeolite ZSM-22. A g a i n the connectivities previously observed are confirmed, and the T | T coupling not observed at all in the C O S Y experiments is n o w clearly seen. 2  The S / N ratio is better than i n C O S Y experiment although less experimental time was used. A l l of the signals exhibit a doublet structure, and the interactions have values similar to those previously observed i n the C O S Y experiments.  133  V  -115.0  -114.0  -113.0  -112.0  •111.0  -110.0  -109.0 PPM -110.0  Figure 51  -111.0  -112.0 -113.0 PPM  -114.0  -115.0  Contour plot of a COSY experiment on zeolite ZSM-22 with a projection in the F2 dimension. Conditions were the same as employed in Figure 50 except that 512 data points and sine bell apodization were used in the data processing.  134  7  1  C  -111.0  -112.0  PPM  -113.0  X  -111.0  Figure 52  -112.0  PPM  T i  -113.0  -114.0  T4  \  S3  -114.0  Contour plot of INADEQUATE experiment on zeolite ZSM-22 with three individual rows shown as indicated. The experiments were carried at 300 K, 16 experiments, 256 scans for each experiment, 750 H z sweep width, and fixed delay of 20 ms. 200 data points before zero-filling and sine bell and trapezoidal apodization for F and F j dimensions respectively were used i n the data processing. 2  135  C H A P T E R FIVE  NATURAL-ABUNDANCE TWO-DIMENSIONAL SOLID STATE SI NMR INVESTIGATIONS OF THE THREE-DIMENSIONAL BONDING CONNECTIVITIES LN THE DIFFERENT STRUCTURAL FORMS OF THE ZEOLITE CATALYST ZSM-5  29  A.  INTRODUCTION  Zeolite ZSM-5 has been of particular interest i n recent years because of its high catalytic activity and extreme size and shape selective adsorption properties. quality  Examples of its use include the conversion of methanol to high-  gasoline, paraffin  cracking, olefin interconversion,  ethylbenzene  synthesis, xylene isomerization and toluene d i s p r o p o r t i o n a t i o n ^ l ^ ! ^ ) . -  Zeolite ZSM-5 is the best k n o w n of a whole family of zeolites called "pentasils", which are characterized by closely related structures^20,121) pentasil framework can be constructed from a secondary building unit (SBU) of the 5-1 type shown in Figure 53A.  Pairs of 5-1 units are joined to form a  building unit of the framework (Figure 53B), which is the asymmetric unit of the phase with the space group Prima or half of the asymmetric unit of the other phases (see on). These units can be linked to form chains (Figure 53C), and such chains interconnected to form a layer, as shown i n Figure 54A. Different ways of linking the sheets form different members of the pentasil family.  W h e n the  sheets are connected such that neighboring pairs are related by an inversion  136  center, the ZSM-5 framework with 96 T-sites per unit cell (u.c.) is formed (Figure 54B).  The structure of ZSM-5 is characterized by two interconnected channel  systems (Figure 54C). There is a straight channel parallel to the b axis with a nearly circular cross-section varying from approximately 5.3 to 5.6 A i n diameter , and a zigzag channel along the a axis w i t h an elliptical cross-section of approximately 5.1 X 5.5 A.  The shape and size of the three-dimensional pore  system together with the high thermal stability and unique catalytic and adsorption properties make ZSM-5 a commercially significant zeolite.  Figure 53  (A) A secondary building unit of the 5-1 type. (B) A n asymmetric unit of orthorhombic form with the space group Pnma. (C) Chain-type building block, (ref. 121)  137  Figure 54  (A) Skeletal diagram of a pentasil layer linked by the chain-type building blocks. 03) Stacking sequence of layers in ZSM-5 (layers shaded). (C) The channel system in ZSM-5. (ref. 120)  138  The structure of ZSM-5 was first deduced from powder diffraction studies by Kokotailo a n d co-workers^O)  m  ^  "as-synthesized form" containing  templates and has orthorhombic symmetry arid space group Pnma. Removal of the template molecules by calcination produces a reduction of the symmetry of the framework to monoclinic, space group P Z j / n ^ l ) .  Recently, several single  crystal studies of the ZSM-5 system have been reported by van Koningsveld and co-workers, including the as-synthesized formU22>, the calcined forms at both room temperature and high temperature (104,123)^  m  adsorption of eight p-xylene molecules per unit cell (124)  e  phase induced by Synchrotron X-ray  powder studies of highly siliceous samples at both l o w and high temperatures have also been carried o u t ^ ^ W ^ . H i g h resolution S i M A S N M R studies have also demonstrated that the 2 9  room temperature form of the completely siliceous ZSM-5 is monoclinic with 24 T-sites and that a structural change to the orthorhombic form w i t h 12 T-sites is induced by increasing the temperature or by the addition of two molecules of pxylene or p - d i c h l o r o b e n z e n e ^ ' l ^ " ! ^ ! ) . In the sorbate-induced case, the change is gradual, both monoclinic and orthorhombic forms being crystalline and coexisting at the intermediate state, while for the thermally-induced change, the monoclinic 24 to orthorhombic 12 T-site turnover occurs over a very small temperature range. These phase transitions are reversible. Recently, Fyfe and co-workers have presented an extensive study of these conversions ^9), In this chapter, the investigation by one-dimensional N M R techniques of a  phase transition of ZSM-5 induced by more than two molecules p-xylene per unit cell w i l l be reported. Then the application of 2D correlation experiments to the phases whose structures are k n o w n w i l l be described and finally, some less well- definded structures of ZSM-5 w i l l be examined.  139  B.  INVESTIGATION OF THE  HIGH-LOADED F O R M OF  ZEOLITE  ZSM-5 W I T H P - X Y L E N E B Y H I G H - R E S O L U T I O N S O L I D S T A T E 2 9  I.  SI NMR SPECTROSCOPY  INTRODUCTION  The adsorption properties of ZSM-5 have attracted a number of studies, especially, the adsorption of p-xylene due to its industrial importance.  These  studies have indicated that four molecules of p-xylene can readly sorb into a unit cell of ZSM-5 and up to eight molecules of p-xylene per unit cell may be incorporated into the ZSM-5 lattice under certain conditions (121,132-133)  p  o r  average sorbate loadings greater than four molecules per unit cell a sudden increase of the adsorption from 4 to ~7 molecules per unit cell is observed and a phase transition occurs. A X-ray powder diffraction analysis of the location of adsorbed p-xylene in the high-loaded form was carried out by assuming orthorhombic Pnma symmetry ^34)_ ^ detailed single crystal X-ray diffraction study was recently reported by van Koningsveld and co-workers (*24)^  a n (  j  a  structure of orthorhombic symmetry P 2 2 2 with an asymmetric unit cell of 24 1  1  1  T-sites was found for this "high-loaded" form of ZSM-5. Previous  NMR  studies of  this degree  of p-xylene  loading  were  characterized by broad and featureless resonances with no clear indications of the presence of a new phase (17/135)  stimulated by the new reported structure,  experiments to investigate this phase using high resolution N M R were carried out.  ,  140  techniques  n.  EXPERIMENTAL 2 9  S i M A S N M R spectra were obtained at 79.49 M H z o n a Bruker MSL-400  spectrometer using the conditions discribed i n Chapter T w o . siliceous samples of ZSM-5 used i n the present w o r k synthesized  using  tetrapropyrammonium  ion  as  a  The highly  were previously  template  ^)  and  dealuminated by G w y n e t h Barlow, U . G u e l p h . The p-xylene loaded samples were prepared by activating highly siliceous zeolite ZSM-5 at 500°C for four hours. After the sample was cooled to room temperature, various amounts of l i q u i d p-xylene were added to weighed amounts of ZSM-5 i n a glass vial.  The  samples were cooled to l i q u i d nitrogen temperature and then fire-sealed under vacuum. They were then kept in an oven at 100°C for one day to ensure an equilibrium distribution of the sorbate through the sample. m.  RESULTS A N D DISCUSSION A series of single pulse M A S experiments were carried out without proton  decoupling. Figure 55A shows the  2 9  S i M A S spectra of ZSM-5 loaded with p-  xylene from 2 to 8 molecules per unit cell. The spectrum at 2 molecules per u.c. is identical to that in the literature and indicates clearly that the asymmetric unit contains 12 T-sites ^ \  A s more p-xylene is added, there is a gradual  broadening of the spectra, most noticeable in the bases of the resonances, as previously reported (17,135)  A l t h o u g h it can be seen that the position of the  resonance w i t h the highest intensity shifts when the loading is increased from 2 to 8 molecules, there is no clear indication of a phase transition due to the severe line-broadening. Possible reasons for the broadening of the resonances could be: a) The crystallites are partially destroyed due to adsorption, resulting i n a lower  141  Figure 55  (A) S i M A S N M R spectra of ZSM-5 with increasing concentrations of p-xylene. The numbers indicate the numbers of p-xylene molecules sorbed per u. c. (B) S i M A S N M R spectra of the same samples with proton decoupling during acquisition. A 350 s delay time between pulses ensures the spectra are quantitative. (C) S i CP M A S N M R spectra of the same samples with a 20 ms contact time and 5 s delay time. 2 9  2 9  2 9  142 A  142 B  degree of  CTystallinity;  b) The distribution of sorbates i n the lattice is not  uniform, creating a distribution of various local chemical environments; c) The dipolar interactions between  2 9  S i and the H nuclei of the sorbed molecules 1  become significant when the amount of p-xylene is increased. To investigate this further, a second series of single pulse experiments with proton decoupling during acquisition were performed on the same series of samples. There is a substantial narrowing of the resonances, as shown in Figure 55B, indicating that dipolar interactions from protons are the main source of the line broadening. A second species is clearly observed at loadings greater than 4 molecules per unit cell as indicated by the vertical arrows. This new "high-loaded" form is the only species present at loadings greater than 7 molecules per u.c. and clearly indicates that there has been a change of phase.  The delay times between pulses are  sufficiently long, so that these spectra can be considered to be quantitatively reliable. The spectra at loadings of 5 and 6 molecules per u. c. can be represented as a sum of the spectra at loadings of 4 and 8 molecules present i n different proportions, and the relative proportion of the "high-loaded" phase gradually increases with increasing p-xylene concentration as shown i n Figure 56. Both low-loaded and high-loaded forms are simultaneously present  and  the  resonances all remain sharp, indicating that both are highly ordered and crystalline. The N M R results suggest that from 2 to 4 molecules per unit cell, the 2 9  S i spectra are i n principle consistent with a orthorhombic symmetry of 12 T-  sites. W h e n the loading is increased, part of the sample is transformed to the high-loaded form, and the rest remains i n the low-loaded form, resulting in a lower total energy, i n agreement w i t h the results found i n sorption studies (121,133),  143  The need for proton decoupling indicates that there are significant dipolar interactions between the H nuclei i n the organic molecules and the S i nuclei i n 1  2 9  the lattice, which suggests that at least some of the p-xylene molecules are immobile i n the channels.  Thus the cross-polarization technique might be  reasonably efficient for this "high-loaded" structure. Figure 55C shows the  29  Si  C P M A S N M R spectra corresponding to the spectra i n Figure 55A and B. The results are as expected: For the '8 molecules'case, the total experimental time was -10 m i n , while the spectrum o n the same sample i n Figure 55B took approximately 14 hours to obtain acceptable S / N .  molecules p-xylene per unit cell  Figure 56  The effect of p-xylene loading on the proportion of high-loaded form in the sample.  144  In contrast, cross polarization is very inefficient for the low-loaded form. This may be due to the motion of the adsorbed molecules relative to the lattice and/or the longer distances between the H and S i nuclei. Figures 57A and B show the 1  2 9  * H M A S N M R spectra of absorped p-xylene i n ZSM-5 at loadings of 2 and 8 molecules per u.c. respectively and reflect the different motions i n the two cases. The broad featureless peak w i t h a 20 k H z linewidth in the '8 molecules' case reflects strong dipolar interactions between the H nuclei and suggests that at 1  least some of the p-xylene are immobile. In contrast, the spectrum of the '2 molecules' case shows much narrow central lines (~ 400 H z linewidth for the highest peak) and a series of spinning sidebands, indicating that the adsorbed pxylene molecules show some motional freedom. In the figure, the resonances associated w i t h aromatic protons are indicated by  and aliphatic ones, '•'.  These results are in agreement w i t h deuterium solid-state N M R studies ^ 6 , Because of the lack of geometric information on the ' l o w loaded' form, it is difficult to estimate the contribution of  1  H - S i distances to the dipolar 29  interaction. Figure 58A shows the S i M A S spectrum with proton decoupling of the 2 9  sample loaded with 8 molecules of p-xylene per u.c. with a long relaxation delay 5 times the longest S i T|). The Lorentzian peaks from the deconvolution are 2 9  shown i n Figure 58B, and indicate that the asymmetric unit contains 24 T-sites. This is i n agreement with the recent single crystal X-ray study by Koningsveld and co-workers(*24)  145  van  A  i 60  1  1  40  1  20  1  0  1  -20  1  -40  P P M  B  T  1  100  Figure 57  1  1  1  r  1  0  PPM  1  1  1  1  1  -100  (A) * H M A S N M R spectrum of ZSM-5 loaded with 2 molecules of p-xylene per u.c, with a spinning rate of 2.5 kHz. (B) * H M A S N M R spectrum of ZSM-5 loaded with 8 molecules of p-xylene per u.c, with a spinning rate of 2J5 kHz.  146  Figure 58  (A) S i M A S N M R spectrum of the 2SM-5 sample with 8 molecules per u.c. with proton decoupling during acquisition and a recycle time of 375 s. (B) The individual Lorentzian curves from a deconvolution of Figure 58A. The numbers above the curves indicate relative peak areas. 2 9  147  C.  NATURAL-ABUNDANCE  TWO-DIMENSIONAL  RESOLUTION SOLID STATE N M R  2 9  SI  HIGH-  INVESTIGATIONS OF  THE  K N O W N L A T T I C E S T R U C T U R E S O F Z E O L I T E ZSM-5  I.  INTRODUCTION  The ZSM-5 structure is a particularly difficult one to investigate by 2D N M R connectivity experiments and represents the most demanding test possible of these techniques to date.  First, zeolite ZSM-5 is the most complex k n o w n  zeolite i n the terms of the size of the asymmetric unit. There are 12 T-sites for the Pnma structure and 24 T-sites for the other structures. A s many as 48  29  Si-0- Si 29  connectivities w i l l occur within the 2D plots, which means that 96 peaks w i l l appear i n a contour plot if every one is well- resolved.  Secondly, there is no  readily available starting point, which can be used to begin working through the connectivity scheme.  A l l of the T-sites have equal occupancies and occupy  positions on the surface of the channels, precluding the assignment of any of the resonance on the basis of either their relative intensities or their spin-lattice relaxation times as was done previously for zeolites ZSM-12 and ZSM-22. However, i n all of the different phases of ZSM-5 investigated, the overall topology is unchanged. resonances i n the I D  2 9  It may thus be possible to trace the changes of the  S i M A S N M R spectra induced by raising temperature  and/or sorbed p-xylene, and use this to relate some resonances i n the different spectra to each other. In this way the assignments of the different 2D spectra may be checked for self-consistency.  148  In this section, four samples of highly siliceous zeolite ZSM-5 are investigated by 2D N M R correlation experiments. They are the low-temperature monoclinic phase or the orthorhombic phases to which it is converted by the action of temperature and/or absorption of p-xylene.  A summary of the  different phases investigated is given i n Table 15. The schematic representations of the asymmetric units of the phases are shown i n Figure 59, and the expected connectivities for each of them are presented i n Tables 16-18 after references 104, 122 and 124.  Table 15 Sample  ZSM-5  Description of the four samples investigated Conditions  ambient temperature  (300K)  high temperature  (403K)  ZSM-5 with sorbed p-xylene  Space group (ref) monoclinic  formP2i/n (104)  T-site (in a.u.*)  24  monoclinic phase  orthorhombic form Pnma (123)  low-loaded form 2 molecules /u.c.  P n m a (121)  high-loaded form 8 molecules / u.c.  orthorhombic form Y1 2 2 (124)  12  300K  293K  Name given i n the discussion  X  X  X  * 'a. u.' Stands for asymmetric unit  149  24  orthorhombic phase (12 T-sites)  orthorhombic phase (24 T-sites)  Figure 59  Schematic representations of the structures of ZSM-5 in: (A) the orthorhombic phase (12 T-sites); (B) the monodinic phase (24 T-sites); (C) the orthorhombic phase (24 T-sites). (refs. 122,104 and 124 respectively)  150  Table 16  T-sites, Their Occupancies, and Connectivities for the Asymmetric Unit i n the Orthorhombic Phase (12 T-sites) of Zeolite ZSM-5 (Ref. 121) T-site  T T T T T T T T  2  3  4  5  6  7  8  9  occupancy  connectivity  I  1T  1  1T  4  1T :  IT!  ITlO  1T  3  1T :  1T  1  1T  1T  4  1T :  1T  1  1T|  5=  1T  1  I T !  2  2  1T  5  6  6  3  1T  1T  4  1T  3  8  12  7  1T =  lTn  1T =  1T  IT*  ITll  1T :  1T  6  1  1T  2  1  1T  4  iI  1  1T  2  ITT:  1  1T  6  1T  8  : II9: IT10  IT!  1T  9  :  TlO  1  Til  1  1T  Tl2  1  1T •  5  3  151  5  Z  9  1T : 7  1T  8:  9  12  IT-IOJ 1T : 1 T 1Q  12  l T : H12 n  T-sites, Their Occupancies, and Connectivities for the Asymmetric  Table 17  U n i t i n the M o n o d i n i c Phase of Zeolite ZSM-5 (Ref. 104) T-site  connectivity  occupancy 1T :  1 T : 1T :  1T  lTj:  1 T : 1T =  1T  1T :  1 T : ITig: 1T  1T :  1 T : IT13  1T :  1 T : l T : IT 13  1T :  1 T : "15  1T :  1 T : 1T  1T :  1 T : 1T :  1T  12  lTg:  1 T : 1T :  1T  21  1T :  1T„:  2  T  2  T  3  T  4  T  5  T  6  T  7  2  3  4  2  8  T  8  T  9  2  17  16  3  6  4  1T  5  6  22  8  24  19  n  IT21 IT-23  5  16  7  19  9  10  18  1T :  13 1 T : 1T :  1T :  l T : 1T :  1T  24  1T :  1 T : 1T :  1T  14  Tl4  IT13:  1T :  Tl5  1T :  18 1 T : 1T :  Tl6  lTj:  1 T : 1T :  Tl7  1T  1T :  18  1T :  18 1 T : 1T :  Tl9  1T :  1T : l T :  1T  20  1T :  1 T : 1T :  1T  24  1T :  1 T : 1T :  1T^  ITj:  1 T : 1T :  1T  1T :  1T :  IT3:  1T : 1T :  TlO  9  Til  5  8  Tl2  4  Tl3  T  T  6  1 :  3  4  20  14  T1  6  1T  10  :  12  n  15  5  10  1T  :  15  14  12  15  7  16  1T  :  14  9  n  7  21  19  20  9  22 1T  1T  19  20 1T  1T  16  1T  17  23 1T  1T  17  2  T  T  22 23  7  T4 2  152  21  10  17  12  1T  22 20  :  1T  23  24  IT23  Table 18  T-sites, Their Occupancies, and Connectivities for the Asymmetric Unit in the Orthorhombic Phase (24 T-sites) of Zeolite ZSM-5 (Ref. 124) T-site  occupancy  connectivity  I  1T :  1T :  1T :  1T  1I  lTj:  1T :  1T :  ITg  I  1T : 2  1T :  1T :  1T  I  1T  1 :  1T :  1T :  1T  I  1T :  1T :  lT :  1T  I  1T :  1T :  1T :  1T  I  1T : ITg : 1 T :  1T  23  1  1T :  1T :  1T :  1T  12  1  1T :  ITg:  1T :  1T  21  TlO  1  lTj:  1T :  1T„:  1T  22  Til  1  1T :  1T :  1T :  1T  19  T  1  1T :  ITg:  lT :  1T  24  Tl3  1  IT5:  1T :  1T :  1T  22  Tl4  1  IT13:  l T i : 1T :  1T  20  Tl5  1  1T :  1T :  1T :  1T  24  1  1T :  1T :  1T :  1T  19  Tl7  1  IT! : 1 T :  1T :  1T  23  Tl8  1  1T :  1T :  1T :  1T  21  Tl9  1  1T :  1 T „ : 1T :  1T  2 0  20  1  1T :  1T :  1T :  1T  2 4  T1  1  1T :  1T :  1T :  1T  2 2  T  1  1T :  1T :  1T :  1T  23  1  1T :  1TJ : 1T22:  1T  2 4  l  1T :  1T :  1T  23  T  2  T  3  T  4  T  5  T  6  T  7  T  8  T  9  T  T  1 2  16  2  4  2  4  1Q  3  6  4  6  3  5  6  n  3  5  4  19  2  6  5  3  14  13  7  9  10  9  10  n  14  5  7  14  9  16  18  16  15  16  14  12  15  18  17  18  1?  16  19  18  21  20  1 7  12  7  13  9  2  T  2 2  23  T4 2  10  7  12  153  13  21  7  15  1T : 20  n.  RESULTS A N D DISCUSSION a) Orthorhombic phase (12 T-sites) Figure 60 shows the I D  2 9  S i M A S N M R spectra for the four samples  studied with the assignments of the individual resonances from the 2D experiments (see on). The linewidths of all the resonances are approximately 12 H z except those i n the high temperature spectra which are -15 H z .  The  broadening at high temperature may be caused by field inhomogeneity because the static magnetic field was shimmed at room temperature or perhaps by small lattice distortions.  The sharpness of all resonances indicates that all of the  materials being examined are both highly ordered and crystalline.  The  numbering of the different resonances i n each spectrum comes from the 2D N M R spectroscopy of the present work (see later). The substantial changes observed i n the peak positions reflect clearly the changes i n local T-site geometries induced i n the structure by these transformations. Since the orthorhombic phase (12 T-sites) is more symmetric and has a less complicated structure, it was investigated first.  The phase transition from  monoclinic to orthorhombic symmetry can be induced either by raising the temperature or by adsorbing p-xylene. The I D N M R spectrum of ZSM-5 loaded w i t h 2 molecules per u.c. and that of pure ZSM-5 at 403K are shown i n Figures 60B and C respectively, and they are quite distinct. Although it is possible to trace some lines though a series of spectra at 373 K from samples containing various amounts of p-xylene and variable-temperature experiments on a sample w i t h 2 molecules of p-xylene per u. c , an unambiguous correlation between all of the peaks i n the two spectra cannot be established from I D spectra alone due to  154  4,18,12,24,3  -i  111  Figure 60  >  1  -112  '  1  >  1  - l l < -116 PPM  1  r  -11B  1  i  -12B  (A) ^ i M A S N M R spectrum of ZSM-5 at 300 K. (B) S i M A S N M R spectrum of the low-loaded form of ZSM-5 (2 molecules of pxylene per u.c.) at 300 K. (C) S i M A S N M R spectrum of ZSM-5 at 403 K. GO) &Si M A S N M R spectrum of the high-loaded form of ZSM-5 (8 molecules of p-xylene per u.c.) at 293 K. 29  29  155  peak crossing and/or overlap, and both species were investigated i n detail by 2D experiments A series of  2 9  S i 2D C O S Y experiments were carried out on the p-xylene  loaded form (2 molecules per u.c.) and the results from a typical experiment are shown i n Figure 61 . There are clear indications that a number of connectivities between different silicons exist, 12 of the expected 22 being clearly observed in the  figure.  However, there is not enough information available from this  experiment to assign individual resonances. In part, this is due to there being no unique signals i n terms of either relative intensities or spin-lattice relaxation times with which to begin assignments, as discussed above. In addition, the very large intensities which occur along the diagonal obscure those cross peaks close to it, limiting the number which can be observed and subsequently used in the spectral assignment. In an attempt to solve this problem, S i 2D experiments 2 9  using an I N A D E Q U A T E sequence were carried out. Figure 62 shows the results of such an experiment carried out on the same low-loaded material using the parameters indicated i n the figure caption.  A s can be seen, many  more  connectivities are observed, 21 i n all, which from Table 16 means that almost every single possible S i - 0 - S i bonding interaction has been detected, although 29  29  some of them are not well defined in terms of the two source resonances because of limited spectral resolution. The problem i n assigning the resonances arises from the difficulty i n finding a starting point as noted above. Careful inspection of Table 16 reveals that only four silicon atoms of the total of 12 have selfconnectivities, which are underlined in the table. T w o of them are i n the fourmembered rings (To, and T^Q) and the others are T7 and T ^ -  Because self  connectivity is not detected, these resonances w i l l show only three connectivities  156  —i  -110  Figure 61  1  -111  1  -112  1  -113  1  1  -1U -115 PPM  1  -116  1  -117  1  -116  r  Contour plot of a COSY 45 experiment on ZSM-5 with 2 molecules of p-xylene per unit cell with the projection in the F2 dimension shown on top. The temperature was 300K and 64 experiments were carried out with 576 scans in each experiment. A sweepwidth of 1700 H z , a fixed delay of 10 ms and 220 real data points were used. Sine bell squared apodization and power calculation were used for the data processing.  157  i n the 2D plots. From this, these four signals can be identified as the resonances F, H , K and L, as indicated b y the arrows i n Figure 62. The two silicons i n the four-membered ring are directly bonded and thus show a connectivity between them, identifying them as resonances K and L. There are two possibilities for starting the assignment from this point, i. e., T9 -» K or -» L. Each of these leads to a complete set of assignments of resonances, both of which are consistent with the connectivitiy scheme shown i n Table 16. One complete assignment from T9 -> K is shown i n the figure. The other assignment can be obtained from this one by the following Equation: T <->T . , i  T  i  7  T  whenl£i£6  i  19-i'  w  h  e  n  7  ^  i s  t l  1 2  2 8  H a v i n g two possible assignments is the case for all phases of ZSM-5, which is similar to ZSM-22 as discussed i n Chapter Four, reflecting the symmetry of the structure and a discrimination between these assignments cannot be, i n general, made from the N M R data alone.  Additional information which w o u l d help  discriminate i n favour of one assignment could be gained by combining the N M R data with geometric information from diffraction studies, as i n the case of ZSM-22.  Unfortunately, the present powder  diffraction  dataU26) is not  considered accurate enough to use for this purpose and the choice of the correct assignment w i l l be postponed until the discussion of the orthorhombic phase (12 T-sites) induced b y high temperature.  158  —I  -112  Figure 62  1  1  -112  1  1  -114 PPM  .  1  1  -116  1 —  -118  Contour plot of an INADEQUATE experiment on ZSM-5 with 2 molecules pxylene per unit cell carried out at 300K with a ID M A S N M R spectrum above. 36 experiments with 512 scans in each experiment were performed with a recycle time of 14 s. and the total experimental time was approximately 72 h. A sweepwidth of 800 Hz, fixed delay of 15 ms. and 140 real data points were used. Sine- bell and trapezoidal apodizations in the F and Fj dimensions respectively and a power calculation were used for the data processing. 2  159  O n raising the temperature, a phase transition f r o m the m o n o d i n i c to the orthorhombic phase (12 T-sites) occurs for pure ZSM-5.  Figure 63 shows the  results of a 2 D I N A D E Q U A T E experiment o n ZSM-5 at 403K. The assignment can be initiated at the same point as the case of the low-loaded f o r m of p-xylene/ ZSM-5 described above, and the resonances J, K and L indicated by arrows i n the figure are associated w i t h three T-sites among the four w h i c h have selfconnections. The assignment of K and L to TJQ and T9 can be made due to the connection between them. In this spectrum, the overlap of some resonances is more severe and only 18 connectivities out of the 22 are observed, making it more difficult to obtain a complete assignment.  A n effort to determine the  relationship of some resonances between the high temperature spectrum of Z S M 5 and that of the low-loaded sample was made from the published l i t e r a t u r e ^ ) , because two possible assignments of the low-loaded form of p-xylene/ ZSM-5 were obtained. Careful inspection of the spectra of ZSM-5 i n the low-loaded form at various temperatures and the spectra with increasing concentration of pxylene at 373  <49)  K  reveals that the highest field peak in all cases is due to the same T-site. W i t h this information and the assignment of resonances K, L and J, it is n o w possible to obtain two complete sets of assignments (Table 19), which are related to each other by Equation 28. In this case, combining the N M R and the X-ray diffraction data allows a completely unambiguous choice between the two possible assignments. Thus, a highly accurate single crystal refinement of the high temperature form of ZSM-5 has recently been carried out by van Koningsveld and co-workers ^23)  The  very l o w errors i n the positional parameters obtained i n this study mean that it  160  5''6''I2  L K  —i  -111  Figure 63  J I  1  -112  F  HG  1  -113  '•—i  PPM  -114  ED C  1  -115  B  •—i  -116  A  r  Contour plot of an INADEQUATE experiment on ZSM-5 at 403K with a ID M A S N M R spectrum above. 32 experiments with 352 scans in each experiment were carried out with the recycle time of 50 s. and the total time for the experiment was approximately 157 h. A sweepwidth of 550 Hz, fixed delay of 15 ms and 108 real data points were used. Sine-bell and trapezoidal apodizations in the F and F| dimensions respectively and a power calculation were used for the data processing. 2  161  may reliably be used to distinguish between the two possible assignments. The chemical shifts are plotted as a function of average T-T distances for both assignments, as shown i n Figure 64. A s can be seen the linear correlation is much better for assignment I, and this one is considered to be unique and is that presented with the I D spectrum i n Figure 63. Consequenly, the unambiguous assignment for the low-loaded p-xylene/ZSM-5 can be deduced as shown i n figure 62.  Table 19  T-sites and T w o Possible Assignments of the Resonances i n the Orthorhombic Phase (12 T-sites) of Zeolite ZSM-5 at 403 K T-site  Assignment I  Assignment II  Tl  I  F-H  2  C-E  F-H  3  C-E  B  4  B  C-E  5  F-H  C-E  6  F-H  I  7  J  F-H  8  A  C-E  9  L  K  K  L  C-E  A  F-H  J  T T T T T T T T T  io  Til T  1 2  162  0  Assignment  o  I  • o  O OO  o 17 T 3.07  1  1 ' 3.08  1 ' 3.09  1 3.10  1 » 3.11  1  o  1 3.12  Mean S i - S i Distance(A)  T 3.08  '  1  •  3.09  1 3.10  «  r 3.11  3.12  Mean S i - S i Distance(A) Plots of the S i chemical shifts as functions of the average T-T distances calculated from the data of reference 123 for the two possible assignments of the high temperature form of ZSM-5. 2 9  163  B) Monoclinic Phase (24 T-sites) It is well k n o w n that the structure of pure ZSM-5 at room temperature is monoclinic P 2 | / n , with 24 crystallographically inequivelant silicons in an asymmetric unit. Inspection of its I D spectrum (Figure 60A) shows that most of the spectral intensity occurs i n the centre of the spectrum. It can be anticipated that i n C O S Y experiments o n this form, most of the cross peaks w o u l d be obscured by the large diagonal peaks. Thus I N A D E Q U A T E experiments were chosen to investigate this phase. The result of such an experiment is presented in Figure 65, and 38 of the total 48 expected connectivities are observed. In this phase, each silicon is bonded to four different silicons through oxygen bridges in addition to having the same occupancy and similar  relaxation times. Hence,  it is again difficult to find any starting point to the assignment. O n raising the temperature from ambient, there are gradual changes i n the frequencies of the different resonances and then a more abrupt change at the transition temperature (49). i n the l o w field region, four resonances tend towards the frequencies of the two lowest field signals of the orthorhombic form (12 T-sites), as shown i n Figure 66. M a k i n g specific connections between the two phases cannot be absolutely justified because of the discontinuity at the transition temperature, but it was felt that the resonances corresponding to the silicon atoms i n the four membered ring i n the monoclinic phase were probably amongst those at lower field and particular attention was paid to those signals i n this regard. A s seen in Figure 65 there are 6 resonances at lower field (to the left of the large central peaks) and these were examined first. The connectivities of the four silicons i n the fourmembered ring, i.e. T9, TJQ, T I and T22/ are presented i n Table 20A, the 2  diagonal pairs connecting simultaneously to the other pairs. The connectivities of the six resonances are listed i n Table 20B.  164  Figure 65  Contour plot of an INADEQUATE experiment on ZSM-5 at 300K with a ID M A S N M R spectrum above. 36 experiments with 448 scans in each experiment were carried out with a recycle time of 12 s. and the total time for the experiment was approximately 54 h. A sweep width of 700 Hz, fixed delay of 15 ms and 160 real data points were used. Sine-bell and trapezoidal apodizations in F and Fj dimensions respectively and a power calculation were used for the data processing. 2  165 A  4 , 1 8 , 1 2 , 2 4 , 3  T  1  1  1  1  -110  -111  -112  -113  PPM  1  1  1  -114  -115  -116  165 B  1 —  -117  A graphical representation of the variation of chemical shift with temperature for zeolite ZSM-5. (ref. 138)  Table 20 A . K n o w n Connectivities of the Four Membered Ring T-Sites i n the M o n o d i n i c Phase of ZSM-5 (From Table 17)  Connectivities  T-site  B.  9  8  10  18  21  10  9  11  13  22  21  6  9_  20  22  22  1  10  21  23  Observed Connectivities of the Six Lowest Field Resonances From N M R Experiments (From Figure 65)  Connectivities  Resonance  S  A  I/M  T  C  I/M  U V  E  N/R  V V V  D  S  T  w x u  W  D  H  I/M  T  X  H  N/R  S  U  167  X  F r o m those connectivities underlined, it can be deduced that silicons 9, 22 and 10, 21 are the (S, TJ) and (V, X) pairs. There are again two possible assignments which are related to each other by Equation [29].  T  i «•* i + 1 2 '  T  i ** i - 1 2 '  T  T  w  w  n  h e n i <; 12  e  n  1  >  I^  1 2  29  In order to facilitate discussion, the resonances V and X are assigned to be T^Q and T j at this stage. In Table 20B the resonances V and W show the same 2  connectivities to D and T. T | Q and T j are connected to TJJ, T13 and Tg, T g 2  2  respectively besides Tg and T ^ . Table 17 shows that T5 is connected to T-Q and T13 too, while only T j is connected to Tg and T Q . Thus the assignments, V -> 2  2  TJQ, W -> T5 and X —> T j can be made, so resonance H corresponds to Tg. By 2  trial and error, the assignments of D -» T ^ and T -»  follow. A t this point,  almost all of the lower field resonances are assigned. When the process is traced to the higher field part, it is more difficult to make progress because of the severe overlapping of resonances and the lower resolution of the 2D experiment than that shown i n the I D spectrum o n the top of the 2D plot. In order to get a reliable connectivity scheme of resonances, the experiments were repeated by carefully optimizing all of the N M R experimental variables and using different temperatures to improve the resolution of specific signals. The data obtained were processed using different numbers of data points and w i n d o w functions to help identify the connected signals and resonances from which they originate. Figure 65 shows the result of one of these experiments and both the resolution and sensitivity are good enough to continue tracing the connection pattern. The  168  remaining assignments were accomplished mainly by trial and error.  The  second possible assignment from the alternative starting point can be obtained by exchanging pairs of T-sites as described i n Equation 29. During the course of this study, a single crystal refinement of the room temperature structure of ZSM5 was reported by van Koningsveld and co-workers (104). A s in the case of high temperature form, the chemical shifts are plotted as a function of the average TT distance for both assignments, as shown in Figure 67. The linear correlation of Assignment I, which is the one presented i n Figure 65, is much superior and this assignment is considered to be the correct one. From the results of the 2D experiments at room temperature and high temperature (Figures 63 and 65 respectively), the relationships between the resonances of the two forms can be made, as shown schematically in Figure 68. It can be seen that the changes of chemical shifts for most resonances are small. Variable temperature I D e x p e r i m e n t s ^ ^ ,  a s  mentioned before, show that there  are gradual changes i n the frequencies of the different resonances and then a more abrupt change at the transition temperature (Figure 66).  However the  results of this 2D work (Figure 68) show that the interruption is small. Although it is impossible to make specific connections between the two forms from the I D variable temperature experiments, it may be acceptable to link the resonances in groups. In fact, the shift trend of the individual resonance in Figure 66 is in general correct except that the lines marked Thus  most  of the  connections  experiments are reliable.  could lead wrong connections.  obtained from I D  variable  temperature  This w i l l be useful i n future studies of u n k n o w n  systems w h i c h have this k i n d of linkage to some k n o w n structure.  169  -108  i—»—i—«—i—•—r 3.06  3.07  3.08  3.09  Mean S i - S i  3.10  3.11 3.12  3.13  Distance(A)  I  Si <M •H  CO rH  id  1  1 3.06  i—i—i—«—i—•—i—  3.07  3.08  3.09  3.10  1  —i—«—r  3.11  3.12 3.13  Mean S i - S i Distance(A)  Figure 67  Plots of the ^Si chemical shifts as functions of the average T-T distances for the two possible assignments of the room temperature form of ZSM-5.  170  High Temperature Form L, k I I  X  J, I l l  WVUTSN/R  H/F I J  I/MHGFE  C/E |_J  D  BA U  CB  A  Low Temperature Form  Figure 68  Relationship of the resonances between the room- and high- temperature forms of ZSM-5.  171  C) Orthorhombic Phase (24 T-sites) A s discussed earlier, w h e n the loading of p-xylene is increased from 2 to 7 molecules per unit cell, another phase transition occurs. The new phase has orthorhombic symmetry w i t h space group of P 2 i 2 j 2 i w i t h the connectivity pattern presented i n Table 18. 2 D experiments were performed o n the high loaded sample using the basic pulse sequence shown i n Figure 36B and the cross polarization sequence to excite  2 9  S i nuclei. The results of an I N A D E Q U A T E  experiment are shown in Figure 69. A l l but one of the 48 connectivities are clearly observed in this case, but not every connectivity is well defined i n terms of the two resonances from which it originates because of the limited resolution of the spectrum.  There is nothing k n o w n regarding the relationship of the  various chemical shifts between this phase and the other phases, and thus no help can be obtained from all of the assignments discussed above except that the resonances of four-silicons in the four membered ring might well again be expected to appear at lower field. Particular attention was paid to resonances X and W , w h i c h are the only two better resolved resonances at lower field.  The  connectivities of the other resonances associated w i t h W. are given in Table 21 A . The first r o w i n this table shows the resonances associated w i t h W and each column presents the association of each of these resonances to other resonances i n the spectrum. It can be seen from the table that B and L are simultanously connected w i t h C and the resonances L and E may or may be not linked to the same resonance within the overlapped peaks, G/J.  Then, the first 12 T-sites are  analysed according to Table 21A o n the basis of diffraction data (Table 18) and 6 T-sites, T  1#  T4, T , Tg, T 5  1 0  and T , present this k i n d of connectivity pattern. 1 2  172  Figure 69  Contour plot of a CP-INADEQUATE experiment on ZSM-5 with 8 molecules of p-xylene per unit cell with a ID CP M A S N M R spectrum on top. 64 experiments with 1088 scans in each experiment were carried out with a recycle time of 3 s and the total time for the experiment was approximately 58 h. A sweepwidth of 846 H z , contact time of 20 ms., fixed delay of 16 ms. and 100 real data points were used. Sine-bell apodization and a power calculation were used for the data processing.  173 A  173 B  Table 21  Connectivities Related to Resonance W and T-site 1 i n the H i g h Loaded p-Xylene Form of ZSM-5.  A.  Observed Connectivities related to resonance W  w  B.  L  B  X  E  w  w  w  w  C  C  K  A  G/J  O  S/T  D  G/J  R  U/V  G/J  Connectivities of T-i from Diffraction Data (Table 18) 1  2 1  4 1  10  17  1  1  3  3  9  16  6  5  11  18  8  7  22  23  174  In the present orthorhombic phase (24 T- sites) two possible assignments can be made as in the monoclinic phase. Thus only the first 12 T-sites are considered. Resonance W is at lower field and is connected with only one other lower field resonance X. Thus W could not be a silicon i n the four membered ring, while X might be. A m o n g the 6 possible T-sites only T^ is connected with one of the four membered ring silicons. The assignment can thus be started at this point, W-> T|. The connectivities related to T | are given i n Table 21B. By comparison w i t h Tables 21A and B, the following assignments can be made: X-» T^Q, E-» Tyj, C-> T , (L, B) -» (T , T^), (A, D) -» ( T , T ) and (O, P) -» (T , T ) . In addition, Tg, 3  2  16  1 8  5  Tg and T23 are among the resonances of G/J.  7  The process of assignment  continues without much difficulty yielding the complete assignment shown i n Figure 69. There are two possible assignments with the same exchange rules as i n the monoclinic form (Equation 29). The solution to this problem again lies i n combining  the N M R results w i t h single crystal diffraction data^ 4) 2  -p  ne  chemical shifts are plotted as a function of average T-T distances for both assignments, as shown i n Figure 70. The linear correlation of Assignment I, which is the one presented in Figure 69, is much better and this one is considered to be unique. The method based on listing the connectivity patterns both of resonances from N M R experiments and of T-sites on the X R D data, as shown i n Table 21, can be generally used in assignments.  For the cases where all or almost all  connectivities are resolved, the assignments can be made without any additional information, such as intensities of signals,  values of nuclei, etc. Zeolites Z S M -  39, ZSM-12 and ZSM-22 are such cases, though the methods of assignment discussed i n previous chapters are more easy and direct. However, i n situations  175  where the resonances are severely overlapped, such as the cases of pure ZSM-5 i n the room temperature and high temperature forms, it is hard to apply this method. resonances.  Firstly, there is considerable uncertainty in the connections of Secondly, the clues heeded to propagate the assignments quickly  disappear due to peak overlap, even if a starting point can be found. Nevertheless, with the help of some other information it may be possible to make an assignment to this k i n d of 2D spectrum, as described i n the high loaded form of ZSM-5.  176  -110  I  Assignment -112  o o  •P <H •r»  Xi  -114  o °o  CO  (0  •116  s  -118  V  o o  O •H  0 O  -120 3.07  oo o  —I—  3.09  3.07  3.09  -«  3.11  Mean S i - S i  r 3.13  3.15  Distance(A)  3.11  Mean S i - S i Figure 70  I  3.13  3.15  Distance(A)  Plots of the S i chemical shifts as functions of the average T-T distances for the high loaded p-xylene form of ZSM-5. 29  177  D.  TWO-DIMENSIONAL  2 9  S I H I G H - R E S O L U S I O N SOLID S T A T E N M R  I N V E S T I G A T I O N O F T H E L A T T I C E S T R U C T U R E S O F Z E O L I T E ZSM-5 L O A D E D WITH P-DICHLOROBENZENE  I.  INTRODUCTION  It has been reported by Fyfe and co-workers  that the compounds p-  xylene, p-chlorotoluene and p-cUchlorobenzene, at a loading of 2 molecules per unit cell, induce essentially identical changes i n the I D % i M A S N M R spectrum 2  of zeolite ZSM-5, as mentioned i n Chapter One. This result indicates that the major contribution to the phase transition from the monoclinic phase (24 T-sites) to the orthorhombic phase (12 T-sites) in these cases is the size and shape of the sorbed organic molecules. This conclusion is based on the appearance of I D N M R spectra and has been confirmed by the powder X-ray diffraction patterns of these systems. 2D correlation N M R techniques have provided more detail and reliable information about the zeolite structures, as can be seen in the previous discussion, and thus can also be used to confirm the conclusions drawn from the I D N M R results. A s discussed i n Section B of this chapter, the dynamic behavior of pxylene molecules adsorbed i n the channels of ZSM-5 is dependent on the loading.  In the low-loaded form, p-xylene molecules are mobile o n the N M R  time scale, while they are relatively 'fixed' in the high-loaded form. If this is true i n the case of p-dichlorobenzene, it is expected that the dipolar and electronic interactions between the adsorbed p-dichlorobenzene and the framework of ZSM-5 w i l l be more efficient at high loadings. Therefore, the changes both i n  178  N M R spectra as well as i n the structure of ZSM-5 induced by very high loadings p-dichlorobenzene are not easily predictable.  Thus  ID  and 2D  NMR  investigations of the structure of ZSM-5 with different loadings of pdichlorobenzene were carried out to further investigate the interactions between sorbates and the host zeolite ZSM-5. H.  RESULTS A N D DISCUSSION a) I D M A S N M R experiments Sirnilar results to those obtained for the 'p-xylene case' (see Figure 56) are  observed for ZSM-5 with various loadings of p-dichlorobenzene, as shown in Figure 71. The  2 9  S i spectra at loadings of both 2 and 4 molecules per unit cell  show that the asymmetric units contain 12 T-sites, except that the '4 molecule' one shows a trace of extra intensities. This second species is clearly present at loadings of 6 and 8 molecules which has more than 12 independent T-sites. The C P sequence is much more efficient i n the high loaded form, as i n the case of pxylene. Figure 72 shows the deconvolution of the spectrum of the '8 m o l . / u . c ' system indicating that the asymmetric unit contains 24 silicon atoms with at least 15 independent T-sites.  However, the spectrum at this loading of  p-  dichlorobenzene appears somewhat different from that observed i n the case of pxylene except for the highest field portion.  It is, therefore, not possible to draw  any conclusions at this stage about the dominant interactions in the adsorption of 8 molecules of p-dichlorobenzene contributing to the phase transition as mentioned in the '2 molecule' case. Another difference between absorption of pxylene and p-dichlorobenzene is the behavior when raising the temperature.  179  j  -110 Figure 71  T  1  1  T—  •  -115  PPM  1  1  1  1  1  -120  S i M A S N M R spectra of ZSM-5 with proton decoupling during acquisition with increasing concentrations of p-dichlorobenzene . The numbers indicate the numbers of p-dichlorobenzene molecules sorbed per u. c  2 9  180  I  Figure 72  1  1  -112  1  1  -114  1  1  -116 PPM  1  1  -118  1  I  -120  (A) S i M A S N M R spectrum of ZSM-5 loaded with 8 molecules pdichlorobenzene per u.c. with proton decoupling during acquisition. (8) Computer simulation of the experimental spectrum as the sum of fifteen Lorentzian curves. (C) The individual Lorentzian curves. The numbers above the curves indicate relative peak areas. 2 9  181  Figure 73 shows the results of variable temperature experiments at a loading of 8 molecules of p-dichlorobenzene per unit cell, indicating that this phase is stable at least u p to 370 K, while i n the case of p-xylene, the sorbate desorb at elevated temperatures, and the spectrum of the 4 molecules form is found at a temperature of 373K.  These results indicate the interactions involving p-  dichlorobenzene molecules with each other and the internal surface of ZSM-5 are stronger than i n the p-xylene case. b) 2D I N A D E Q U A T E experiments The result of an I N A D E Q U A T E experiment on ZSM-5 loaded with 2 molecules of p-dichlorobenzene per unit cell is shown in Figure 74. A similar connectivity pattern to that found for p-xylene (Figure 62) was observed as expected.  T w o equally valid assignments based on Pnma symmetry  are  obtained, as i n all other ZSM-5 cases. One of the two assignments is compatible w i t h the result obtained for the p-xylene case, and this is considered the unique one as shown i n the figure. These 2D results for the '2 molecule' case confirm the conclusion obtained from the I D studies that the two organic molecules induce the same phase transition on ZSM-5 and since they have similar geometries the nature of the interaction at least for this form is primarily based on the size and shape of the organic molecules. A n I N A D E Q U A T E experiment on ZSM-5 loaded w i t h 8 molecules of pdichlorobenzene per unit cell was carried out, and the results are presented in Figure 75. The structure of this form has not been reported yet, and thus the space group of the high loaded form of p-xylene was considered first i n order to  182  1  Figure 73  1  1  1  1  1 -115  1  1  PPM  1  1  1 1 -120  1  1  r  Variable temperature S i CP M A S N M R spectra of ZSM-5 loaded with 8 molecules p-dichlorobenzene per unit cell. The temperatures in K are indicated. 2 9  183  5,1  -112  Figure 74  -114 PPM  -116  -113  Contour plot of an INADEQUATE experiment on ZSM-5 with 2 molecules of pdichlorobenzene per unit cell at 300 K with a ID M A S N M R spectrum above. 32 experiments with 192 scans in each experiment were carried out and the total time for the experiment was approximately 20 h. A sweepwidth of 680 Hz, fixed delay of 16 ms. and 140 data points were used before zero filling. Shifted sinebell apodizations in the F and Fj dimensions respectively and a power calculation were used for the data processing. 2  184  Figure 75  Contour plot of a CP-INADEQUATE experiment on ZSM-5 loaded with 8 molecules of p-dichlorobenzene per unit cell at 300 K with a ID CP MAS N M R spectrum above. 56 experiments with 960 scans in each experiment were carried out. A sweepwidth of 737 Hz, fixed delay of 16 ms. and 100 real data points were used. Shifted sine-bell apodizations in the F and Fj dimensions respectively and a power calculation were used for the data processing. 2  185A  23,12,20  t  1  -1J2  1  -113  1  -114  1  -115  1  PPM  -116  1 8 5 B  1  -117  1  -118  i -119  r  interpret the 2D N M R data. Based on the fact that the highest field part of the 2D plot is similar to that of the p-xylene case, it is possible that the resonances A and B in both cases are due to the same T-sites 18 and 4. The complete assignment can be obtained from this starting point and is shown in the figure.  Other  possible space groups were examined and were not compatible with the N M R data.  Hence the structure of ZSM-5 with a loading of 8 molecules of p-  dichlorobenzene per u. c. is associated w i t h the orthorhombic space group P2|2|2|.  Comparing the results of the two assignments, it is found that the  differences i n chemical shift corresponding to the same T-site are generally small. Therefore, the major interactions between both p-xylene and p-dichlorobenzene and the lattice at various loadings again appear to be based on size and shape. The  fact  that  cross polarization from the  *H  nuclei i n  the p-  dichlorobenzene sorbates to the S i nuclei is efficient for the high loaded form 2 9  provides a way to cross-check the proposed orthorhombic structure, P 2 | 2 | 2 | . This is based o n the fact that the magnetization transfer originates from the dipolar interaction between * H and S i , and the efficiency of the C P process is 2 9  very dependent o n the internuclear distance. The X R D data from ZSM-5 with 8 molecules of p-xylene per unit c e l l ^ ) [ 24  s  u s e  d to calculate the positions of the H  atoms attached to the benzene rings. Then the distances between H and S i can 1  2 9  be calculated using the coordinates of silicons i n this data set assuming the molecules are reasonably fixed in the positions indicated by X R D study. Table 22 lists all of the Si-H distances less than 4.0 A., where H^- H 4 represent the H atoms of the molecules i n the channel intersections and H5- H g those of the molecules i n the sinusoidal channels (Figure 76).  186  Figure 76  Positions of two independent p-xylene molecules in the channels of ZSM-5 in the form with 8 molecules per unit cell: XYL1 is located at the intersection of the straight and the sinusoidal, XYL2 lies in the sinusoidal channel, (ref. 124)  187  Calculated Si- H distances (< 4 A ) for ZSM-5 loaded with 8 p-xylene  Table 22  per unit cell (from Ref.124)  Si- atom  H- atom  Ti 2 T  no H H H 8 no H H Hi no H H6 no H H H no H no no H H H5 H6 H3 H2 no H6 H5 H3 H6 HI HI H7  T  3  T T 6 T 4  5  T  7  T T  8 9  TlO Til Tl2 Tl3 Tl4 15 Tl6 Tl7 T  Tl8 19 20 T1  T  T  2  22 23 T4 T T  2  7  7  8  H  4  8  4  2  7  2  2  3  5  Distance  (A)  Si- H interaction estimated*  3.8 3.1  3.6 3.0 3.9 3.4 3.5 3.6 3.7 3.4 3.2 3.4 3.4 3.6 3.6 3.2 3.3 35  3.6 3.1 35 3.7 3.7 33 3.4 3.7  * dipolar interaction between Si and H, S: strong; M : median; W: weak.  188  W M S  s w M  s  w M  W S S W S W W M S M W S M S S  Variable contact time C P N M R experiments o n Z S M - 5 w i t h a loading of 8 molecules of p-dichlorobenzene were carried out to probe similarities i n the geometries  of  the  high loaded p-xylene  form to the  corresponding  p-  dichlorobenzene case and the correctness of the assignments. Figure 7 7 shows the results of the N M R experiments and Figure 78 displays the intensities of some of the T-sites from Figure 77 as functions of the contact time.  Resonances  A , B, C and S, whose intensities grow faster at beginning are indicated by '*',and are associated to T-sites 18, 4, 3, and 7.  They all have stronger dipolar  interactions w i t h H as indicated in Table 22, while resonances E, R and X marked by '•' grow much more slowly and correspond to T j , T j g and TJCJ, characterized by ' W i n the table.  The good agreement between the N M R results on the 8  molecules of p-dichlorobenzene / Z S M - 5 and the X R D data of the corresponding p-xylene/ZSM-5 system confirms that: i) The proposed orthorhombic structure of symmetry P 2 2 } 2 for the high loaded form of p-dichlorobenzene/ZSM-5 is 1  1  correct, ii) The positions of the organic molecules i n the channels of Z S M - 5 in both cases are very similar, i.e. one is in the channel intersections and the other in the sinusoidal channels.  189  190  Figure 78  The intensities of some T-sites in the high p-dichlorobenzene loaded form of ZSM-5 as a function of the contact time.  191 A  1000 + 7 u.  *  -  A 4,18 3  5  e  +  •  A  • • •  " 4001  +  H  • 08  •  X B  1  1  6  15  X  •  + •  +  • • X  X  +  •  WW  •  •  200 H  0  1  •  0  20  10 contact  time  (ms)  30  T1 T7 T15 T16 T3 (T4+T18)/2  E.  CORRELATIONS BETWEEN  2 9  S I M A S N M R C H E M I C A L SHIFTS A N D  X-RAY D I F F R A C T I O N D A T A F O R H I G H L Y SILICEOUS ZEOLITES  I.  INTRODUCTION  A s has been shown earlier, high resolution  2 9  S i solid state M A S  NMR  spectroscopy has developed as an important complementary technique to diffraction studies for structural investigations of zeolites since the  29  Si N M R  chemical shift is a very sensitive probe of the local structure surrounding the silicon nuclei. In order to interpret the S i chemical shifts observed in structural 2 9  studies more quantitatively, various linear correlations based on bond length (139,140)^ bridging bond angle (141/142), bond strength (  143  \  mean T O T distance  (144)^ group electronegativity (145), and s-orbital h y b r i d i z a t i o n ^ ^ have been previously presented. These relationships are of particular interest for highly siliceous zeolites, where all T-sites experience the same Si[0Al] chemical environment.  The numbers and relative intensities of the resonances i n these  spectra provide direct information on the number of crystallographically inequivalent silicons, and their chemical shifts are sensitive to subtle changes i n the local framework structures. The geometric parameters from X-ray diffraction data used for interpreting the ^ S i M A S N M R spectra of highly siliceous zeolites are: a/2  1) the mean Si-O-Si bond angle, a, i n different forms, e.g. a ^ ) ; sin 4 2  ( 1 4 4  > ; cos a /(cos a -i)(146,147)  /  2  ) the mean Si-Si d i s t a n c e  ( 1 1 7 )  , 3) the mean  Si-O bond l e n g t h ^ * ) . The mean Si-O b o n d length i n highly siliceous zeolites 4  varies very little, from 1.592A to 1.604A with estimated standard deviations (ESD) of 0.005A for the 24 T-sites i n ZSM-5 w i t h 8 p-xylene molecules per  192  u . c P ) . Thus it is not sensitive enough for correlation studies. The cos a /(cos 2 4  a -1) function is the most accepted parameter in the case of Si-O-Si angular dependence^46)  Correlations using mean Si-Si distances include the effects of  both bond length and bond angle. In the study of structures with multiple Tsites, the matching of a N M R resonance in a I D spectrum with a particular silicon atom i n the crystal structure is ambiguous for structures with more than one T-site unless the resonance peak intensities and corresponding population parameters can be related uniquely  because these correlations are basically  empirical. A s can be seen in previous discussions, 2D correlation N M R experiments of highly siliceous zeolites provide unambiguous assignments of the resonances of N M R spectra to the corresponding T-sites directly if the assignment is unique and by combination w i t h X R D data when two equivalent assignments are possible from the N M R data.  U s i n g the peak assignments which have been  obtained from the 2D experiments discussed earlier, the reliability of various geometric correlations w i l l be examined and discussed i n more detail i n this section. Four linear correlations w i l l be considered, as presented in Equations 29- 32. 8 = a [mean (Si- Si)] + b  [29]  8 = a (mean a) + b  [30]  8 = a [cos a/(cos a-1)] + b  [31]  8 = a {mean [cos a/ (cos a -1)]} + b  [32]  where 8 is the S i isotropic chemical shift measured i n p p m . with respect to T M S 2 9  taking Q g M g as a secondary reference (see page 68), the Si- Si distance is the  193  separation between the target  2 9  S i nucleus and its first neighbor Si atom,  measured i n A and a is Si-O-Si angle. The measurement error i n 5 i n the present study is estimated to be ±0.05 p p m for resolved peaks and ±0.15 p p m for overlapping peaks. n.  DISCUSSION  A synchrotron powder X R D refinement data set of Z S M -  is taken as  a first example for the correlation study as it represents the highest quality of powder data available, being derived from both a sample of the highest possible crystallinity and by using a synchrotron X-ray source. Figures 79A and B are the plots of chemical shift vs the mean T-T distance and the mean cos a /(cos a -1) respectively and the lines of regression are drawn as dashed lines for reasons which w i l l be discussed below. A s can be seen from the figures, the data points correlate but not very well. In general, this is because the estimated standard deviations (ESD)  in the powder diffraction data are quite substantial, as  indicated in the figure. The information which can be d r a w n from these figures is only semi-quantitative; that is, it is possible to divide the T-sites into two groups, one is T5 Tj, T  2  and T j whose corresponding resonances should be at  higher field and the second group T4, T3 and T5 which should occur at lower field. Single crystal X-ray diffraction studies of zeolites provide much more accurate and detailed structure information. A s mentioned earlier, only a very few synthetic zeolites have been studied to date by these techniques due to the difficulty of growing large enough single crystals.  A single crystal  XRD  refinement on a small crystal (45X 100X 225 urn) of ZSM-22 was reported^ ) and 16  taken for this study. Figures 80A and B present the N M R and X R D correlation  194  E  Q.  a  CO u E 3.04  3.05  3.06  3.07  3.08  3.09  3.10  3.11  Mean Distance (A)  To 2  -113 0.44 Figure 79  0.45  0.46  T  0.47  5  N 0.48  mean cosa/(coscc -1)  N M R and XRD correlation diagrams of zeolite ZSM-12: (A) plot of chemical shift vs. the mean T-T distance; (B) plot of chemical shift vs. the mean coso/(cosa-l). The estimated standard deviations in the geometric parameters are indicated by the horizontal lines.  195  ~>  1 3.08  >  >  1 3.09  r  3.10  3.12  0  Mean Distance (A)  B  -no  N  S  1,-111 a. £  x:  s  -112  \  s  N  N  N N  "3 -113  N  Tl  o  X  1  T  N  N  '4  ESD r—(  S _ S  \ N  -115 0.45  2  0.46  0.47  X_  0.48  Mean cos a /(cos a -1) Figure 80  N M R and XRD correlation diagrams of zeolite ZSM-22: (A) , plot of chemical shift vs. the mean T-T distance; (B) . plot of chemical shift vs. the mean cosa/(cosa-l). The estimated standard deviations in the geometric parameters are indicated by the horizontal lines.  196  diagrams between the S i chemical shifts and the mean Si- Si distances and the 2 9  mean cos a /(cos a -1) respectively for ZSM-22. Both show the correct trends, i.e. the T-sites with longer Si- Si distances or larger Si-O-Si angles correspond to the resonances at higher field. The linear relationships between the N M R and X R D data are much better than those from powder X R D data of ZSM-12, and can be used with some confidence to choose the correct assignment from two equally valid possibilities obtained from 2D N M R studies, as was done i n Chapter Four. Several highly accurate single crystal refinements of various forms of zeolite ZSM-5 are a v a i l a b l e /  1 0 4  '  1 2 3  '  1 2 4  ^ where the ESDs in the Si- O- Si bond  angles are approximately 0.3° and the ESDs i n the Si-O bond lengths are ~0.004 A.  This is the highest quality data available to date for zeolite structures.  Figures 81 and 82 show the correlation diagrams between the chemical shifts and the X R D derived parameters for the ZSM-5  room-temperature structure as  indicated in the figure captions, and the bold lines in the figures are the results of the linear regression analysis. The results for the high temperature form of ZSM5 and the high loaded form of p-xylene/ZSM-5 as well as the room temperature one are summarized i n Table 23. In general, the four functions describe the variation of 6 Si[4Si] for ZSM-5 in the RT and H T forms quite well, as reflected i n the high linear correlation coefficients. However, for the high loaded sample the mean distance function shows a better linear trend. W h e n the three data sets are presented together (Figure 83), only the mean distance function shows a good correlation. This function was used to discriminate between the two possible assignments valid for the N M R data of Z S M - 5 i n various cases, as described in Section C. It is also possible using mean Si-Si distances to assign some  197  A  -108  118 T — — I — i — I 3.06 3.07 3.08 1  Mean  B  -  1  — i — i — i — i — i — i — i — i — 3.09 3.10 3.11 3.12 3.13  Distance (A)  108  118  ~ r — • " — i — ' — i — • — i — • — i — i — i — • — i — 146 148 ISO 152 154 156 158  Mean TOT Figure 81  1  — 160  Angle (°)  N M R and XRD correlation diagrams of zeolite ZSM-5 at room temperature: (A) plot of chemical shift vs. the mean Si-Si distance; (B) plot of chemical shift vs. the mean Si-O-Si angle. The estimated standard deviations in the geometric parameters are indicated by the horizontal lines.  198  0.45  Figure 82  0.46  0.47  0.48  0.49  M e a n c o s a / ( c o s a -1)  N M R and XRD correlation diagrams of zeolite ZSM-5 at room temperature: (A) plot of chemical shift vs. cosa/(cos 5-1), (B) plot of chemical shift vs. the mean cosa/(cosa-l). The estimated standard deviations in the geometric parameters are indicated by the horizontal lines.  199  Table 23  Linear Regression Analysis of Chemical Shift Against Various Geometric Parameters for the Different forms of ZSM-5  X  Sample ZSM-S(RT)  Sample ZSM-5 (HT)  Sample ZSM-5 (p-xylene)  sTsi  8= 234.6-112.4X  8= 242.6-115.2X  8= 243.3-115.4X  r*= 0.918  r= 0.952  r= 0.920  8= -25.73-0.5739X  8= -52.68-0.3932X  8= -26.05-0.5755X  r= 0.917  r= 0.949  r= 0.872  8= 11.51-265.6X  8= -12.45-213.0X  8= 18.26-281.2X  r= 0.909  r= 0.961  r= 0.853  8= 21.44-287.6X  8= 6.583-254.4X  8= 19.19-285.0X  r= 0.969  r= 0.968  r= 0.876  a cosa /(cosa-1 cosa /(cosa-li  * r stants for correlation coefficient  resonances in the highest and/or lowest fields if they are well separated. However, it is still very difficult to predict the chemical shifts of the T-sites from X R D data in multiple T-sites cases, such as ZSM-5, and vice versa. The reasons may be: a) The conditions including temperature and loading between X R D and N M R experiments might not be exactly the same, b)  In the case of the high  loaded form, the local environments of Si nuclei may depend not only upon the geometry, but also be affected by the presence of p-xylene molecules in the channels. Therefore errors are introduced, which can affect the accuracy of the correlation. Nevertheless, these correlations between  2 9  S i N M R chemical shifts  and X R D data from 2D N M R and single crystal X R D studies are the most reliable ones to date for multiple T-sites zeolites and w i l l be of use in future studies.  200  The accuracy of X R D data can substantially affect the precision of the correlation as shown above i n three different kinds of data sets, i.e. ZSM-12, ZSM-22 and ZSM-5. Collecting all reliable X R D and N M R data sets as well as the data discussed above, general correlation maps can be obtained and are shown i n Figures 84A and B. Both the mean Si-Si distances and mean cos a / (cos a - 1) show a linear relationship and their correlation coefficients are approximataely 0.84. Better X R D and N M R data measured under the same conditions w i l l be required in order to get more reliable correlations for the investigation of zeolite structures.  201  s  a a as  o  ZSM-5 RT  "a  •  ZSM-5 HT  +  ZSM-5  C0  u  loaded  1  JS  U  3.06  B  3.08  3.10  3.12  3.14  3.16  M e a n S i - S i d i s t a n c e (A)  -108  8 = 11.90 - 267.9X, r = 0.793  s  a a.  <>• •  W3  6 — i 0.46  0.47  0.48  M e a n c o s a / ( c o s a-1)  Figure 83  o  ZSM-5 RT  •  ZSM-5 HT  +  ZSM-5  loaded  0.49  N M R and XRD correlation diagrams of zeolite ZSM-5 in the three cases: (A) plot of chemical shift vs. the mean distance, (B) plot of chemical shift vs. the mean cosa/(cosa-l). The estimated standard deviations in the geometric parameters are indicated by the horizontal lines.  202  Figure 84  N M R and XRD correlation diagrams for all avaiable data sets: A) plot of chemical shift vs. the mean distance, B) plot of chemical shift vs. the mean cosa/(coso>l).  203  A  3.04  3.06  3.08  3.10  3.12  3.14  Mean Si-Si Distance (A)  3.16  •  ZSM-5 RT  • o • + • x  ZSM-5 HT ZSM-5 loaded ZSM-22 ZSM-12 Quartz Cristobalite  0.44  0.45  0.46  0.47  0.48  Mean cosa /(cosa -1)  0.49  •  ZSM-5 RT  • o • + • x  ZSM-5 HT ZSM-5 loaded ZSM-22 ZSM-12 Quartz Cristobalite  CHAPTER  SIX  APPLICATION OF TWO-DIMENSIONAL S i MAS NMR TECHNIQUES TO THE STRUCTURAL INVESTIGATION OF LESS WELL CHARACTERIZED ZEOLITES 29  A.  NATURAL-ABUNDANCE TWO-DIMENSIONAL  2 9  SI MAS NMR  INVESTIGATION OF THE THREE-DIMENSIONAL BONDING CONNECTIVITIES OF THE HIGH AND LOW TEMPERATURE F O R M S O F Z E O L I T E ZSM-11 I.  INTRODUCTION ZSM-11 is one end member of a family of pentasil zeolites, of which ZSM-  5 is the other member as discussed i n Chapter Five. selective catalysts ( 1  4 9 _  1 D. 5  They are both shape-  i n the ZSM-11 structure, the pentasil layers are  joined such that neighboring layers are related by a reflection plane, as shown in Figure 85-A, while i n ZSM-5 they are related by an inversion centre. The ZSM-11 framework  contains two straight intersecting channel systems with ten-  membered ring openings with free diameters of 5.1 X 5.4 A (Figure 85-B). Intergrowths between ZSM-11 and ZSM-5 are common and ZSM-5 may easily be obtained in pure form, but it is very difficult to synthesize ZSM-11 samples which are free both of intergrowths of ZSM-5 and of amorphous materials. For this reason, i n comparison with ZSM-5, considerably less work has been done on ZSM-11, and the quality of the structural data available is limited.  204  Figure 85  (A) Stacking sequence of layers in zeolite ZSM-11 (layers shaded). (B) The channel systems in ZSM-11. (ref. 152)  205  The structure of zeolite ZSM-11 originally proposed by Kokotailo and coworkers^ ) 2  w  a  s  based on powder X R D studies and model building, and has a  tetrahedral space group I4m2. This tetrahedral symmetry implies that there are seven crystallographically inequivalent sites with relative occupancies of 1: 1: 2: 2: 2: 2: 2. A schematic representation of the structure is shown in Figure 86 with the T-sites i n the asymmetric unit indicated by filled circles. A n early  2 9  Si MAS  N M R spectrum of zeolite ZSM-11 was presented by N a g y and c o - w o r k e r s ^ ^ , but  the  resolution was  inequivalent silicons.  insufficient to  resolve  any  crystallographically  Detailed studies of the zeolite ZSM-11 structure have  recently been presented by Fyfe and co-workers (154,155)  u  s  m  g a combination of  solid state N M R and synchrotron X-ray diffraction techniques.  This work  demonstrated that the lattice structure of ZSM-11 is temperature dependent, changing from a tetrahedral form of space group I4m2 to a lower symmetry form below ambient temperature. The higher symmetry form can be also induced by adsorption of some organic molecules, for example, n-octane, as is the case for ZSM-5 which was previously discussed in Chapter Five. Rietveld refinement of synchrotron powder x-ray diffraction data collected at 373 K  proceeded  smoothly i n space group I4m2 with 7 crystallographically inequivalent T-sites. However, the room temperature X R D data could not be smoothly refined to match the I D N M R data which suggested 12 T-sites i n the asymmetric unit, and the ZSM-11 structure at lower temperatures remains ill-defined at present.  206  Schematic representation of zeolite ZSM-11 lattice framework with the T-sites the asymmetric unit indicated by the filled circles, (ref. 152)  207  /  As  demonstrated i n the previous  chapters,  2 9  Si  2D  MAS  NMR  connectivity experiments have been successfully applied to investigate the three dimensional bonding i n S i enriched and natural abundance zeolites, most of 2 9  whose structures are well defined. In this section, 2D S i M A S N M R techniques 2 9  are applied to the case of zeolite ZSM-11 in both its high and l o w temperature forms where the structures are less well-known.  H.  EXPERIMENTAL H i g h l y siliceous zeolite ZSM-11 was synthesized hydrothermally based  on a modification of the method reported by  The templates used were  benzyltrimethyl ammonium hydroxide and tetrabutylammonium bromide. The calcined material was ion-exchanged with ammonium fluoride and then dealuminated by passing water vapor over the sample at 750°C for three days. After the exchange and dealumination procedures were carried out twice, the sample was treated w i t h 0.6 N  sodium hydroxide solution at 80°C for  approximately five minutes to remove poorly crystalline materials, then filtered and activated at 500°C for 2 hours. The I D and 2D S i M A S N M R spectra were obtained at 79.5 M H Z using a 2 9  Bruker MSL-400 spectrometer as described previously.  The w-octane loaded  sample was prepared by adding 14 m g of n-octane to 250 m g of ZSM-11. The sample was sealed and then kept i n an oven at 100°C for 2 hours i n order to reach an equilibrium distribution of the sorbate within the host zeolite.  2 0 8  m.  RESULTS AND DISCUSSION a) I D experiments on zeolite ZSM-11 i) Effect of soditim hydroxide treatment on ZSM-11 The effect of sodium hydroxide treatment on the resolution of the  N M R spectra is shown i n Figure 87. Close examination of Figure 87A indicated that the sample consisted of two parts: One was highly crystalline ZSM-11, as represented by the sharp lines i n the spectrum, and the other consisted of some less crystalline or highly disordered materials, which formed the broad base in the lower part of the spectrum.  After sodium hydroxide treatment, the  resolution is substantially improved (Figure 87B). The effectiveness of the base treatment may result from differences i n the solubilities i n alkaline solution between the highly crystalline ZSM-11 and less crystalline or amorphous materials^57)  j^q latter may be easier to dissolve, resulting i n a more highly  crystalline ZSM-11 sample after filtration. Some Si-O-Si defects and hydroxyl groups could be formed during the treatment, which could affect the resolution i n the N M R spectra and the thermal treatment at 500°C "heals" these crystal lattice defects, leaving a highly siliceous and crystalline sample, whose spectrum is superior to those previously published. ii) Variable-temperature N M R experiments 2 9  S i variable-temperature  MAS  NMR  spectra have been previously  published by Fyfe and co-workers^S) -^,3 3 resonances could be resolved in the temperature range of 268-373 K, but the resolution was insufficient to indicate the exact temperature range of the phase transition or to determine the best temperature at which to investigate the l o w temperature structure.  209  A  1  i  Figure 87  1  1 1  -110  1  1  1  1  PPM  1 -115  1  1  1  1  1 r  -122  (A) S i M A S N M R spectrum of zeolite ZSM-11 before sodium hydroxide treatment. (B) S i M A S N M R spectrum of zeolite ZSM-11 after sodium hydroxide treatment. 2 9  2 9  210  T w o variable-temperature  2 9  Si MAS  NMR  experiments were carried out  separately i n the temperature ranges of 273-318 K and 298-342 K in order to determine the optimum temperatures for the investigation of the  three  dimensional bonding connectivities of the high and l o w temperature forms of ZSM-11. The results are shown in Figures 88 and 89 respectively, and clearly indicate the existence of two distinct phases. The very high resolution of the spectra confirms the highly siliceous and ordered nature of the sample and makes it possible to establish a relationship between the resonances of the two phases. W h e n the temperature is below 316 K, 11 or 12 resonances are cleerly resolved i n the spectra. There are only gradual shifts of some of the resonances, which probably reflect the general thermal expansion of the lattice and the corresponding changes induced i n the local geometries of some of the silicons. The temperature for the best resolved spectrum is approximately 302 K, so this temperature was selected for the lower temperature 2D experiments. Between 316-329 K the resonances are broadened, reflecting some distortion of the lattice i n the region of the phase change. The phase transition itself occurs between 320327 K.  It is clear from Figure 89 that another phase forms with seven well  resolved resonances when the temperature exceeds 334 K and 342 K was chosen as an appropriate temperature to investigate this high temperature structure. Figure 90 shows the S i M A S N M R spectrum obtained at 302 K together with 2 9  its deconvolution i n terms of Lorentzian curves.  The  linewidths  are  approximately 11 H z without any resolution enhancement. Figure 91 shows a 2 9  S i N M R spectrum of ZSM-11 at 342 K, together w i t h its deconvolution. The  numbers above the curves indicate the relative peak intensities.  211  Figure 88  Variable temperature S i M A S N M R experiments (273- 318 K) on ZSM-11. 2 9  212  213  A  I  -110  Figure 90  -112  -114  PPM  -116  -118  (A) S i MAS NMR spectrum of ZSM-11 at 302K with one of the two possible peak assignments (see text). (B) The individual Lorentzian curves used in the simulation of A. The numbers above the curves indicate the relative peak areas. 29  214  —I  -110  Figure 91  1  1  -112  1  1  PPM  -114  1  1 —  -116  (A) S i M A S N M R spectrum of ZSM-11 at 342K with the peak assignments (see text). (B) The individual Lorentzian curves used in the simulation of A. The numbers above the curves indicate the relative peak areas. 2 9  215  b) 2D experiments on the high temperature and n-octane loaded forms of ZSM-11 The lattice structure of ZSM-11 i n its high-temperature form has been refined from synchrotron X-ray data i n detail in the space group of I 4 m 2 ^ ^ . The schematic representation of the structure is shown i n Figure 92A and the expected Si-O-Si connectivities are presented in Table 24. Figure 93 shows the results of a S i 2D I N A D E Q U A T E experiment carried out at 340 K. A l l nine of 2 9  the expected connectivities are clearly observed, and the different occupancies of the T-sites provide a starting point for the assignment of the resonances and the interpretation of the 2D N M R data, restricting the assignment of resonances G and C which have unit intensities to the T-sites 1 and 6. Resonance G presents one connectivity with A , while C shows two connectivities in Figure 93. The assignment can be started at G -»1 and C -» 6, and continued by A -> 2, F -» 3. The complete assignment is shown above the I D spectrum in Figure 90, and is unique. The resonances of silicons 1, 3, and 7, which are in the four-membered rings, appear at low-field, which is i n agreement with the previous observations i n the case of zeolite ZSM-5. Table 24  T-sites, Their Occupancies, and Connectivities for the Asymmetric Unit i n Zeolite ZSM-11 i n the Space Group I4m2 T-site  T  T T  l  2Tj: 2 T  2  lTj: 1T :1T :1T  2  1T :1T : l T  2  1T :1T :1T :1T  2  1T :1T :1T :1T  6  1  2T : 2 T  7  2  1T :1T :2T  2  T  3  4  5  T T  connectivity  occupancy  2  3  2  3  2  2  3  3  216  4  t f  4  4  5  5  4  5  1T  7  5  7  6  7  A  m  3  \3  /  7  (J)  ^5  I  5  /  B  Figure 92  (A) Schematic representation of the high-temperature form of ZSM-11, space group I4m2. The T-sites are indicated. (B) Schematic representation of the low-temperature form of ZSM-11. The Tsites are indicated.  217  3  —i  -109  Figure 93  1  -118  1  -111  1  -112  PPM  1  -113  1  1—  -114  -115  Contour plot of an INADEQUATE experiment on ZSM-11 at 340 K with a ID M A S N M R spectrum on top. 32 experiments with 224 scans in each experiment were performed with a recycle time of 22 s. The total time for the experiment was approximately 44 h. A sweepwidth of 540 Hz, fixed delay of 16 ms and 160 data points were used. Sine-bell apodizations in both F2 and Fj dimensions and a power calculation were used in the data processing.  218  This high-symmetry phase observed at elevated temperatures can occur at ambient temperature when the sample is loaded w i t h ~3 molecules of M-octane per 96 T- atoms unit cell of ZSM-11.  The results of a 2D I N A D E Q U A T E  experiment together w i t h the I D spectrum are shown i n Figure 94. The I D spectrum has a very similar appearance to that of high-temperature form and Table 25 lists the chemical shifts of the resonances for both cases. This similarity does not mean that the T-sites corresponding to the resonances are necessarily the same in both cases, although they may well be. The assignment of the 2D experiment (Figure 94) is obtained i n a similar way to that discribed above for the high-temperature case, with the results shown i n the figure.  Identical  assignments in both cases indicate that the two factors, temperature and adsorption of n-octane, have a similar effect o n the lattice structure both in terms of symmetry and geometry.  Table 25  Chemical Shifts of the Resonances of ZSM-11 in the T w o Cases of Symmetry I4m2 Chemical Shift (ppm)  Form of A  B  C  Resonance D  E  F  G  -116.8  -116.5  -114.2  -113.7  -112.5  -112.5  -111.6  high temp. -116.2  -115.8  -113.9  -113.5  -112.0  -111.7  -111.4  ZSM-11 n-octane  219  —I -112  Figure 94  1 - 1 1 3  1 -114 PPM  1  1  -115  -116  1— - 1 1 ?  Contour plot of an INADEQUATE experiment on ZSM-11 loaded with 3 molecules n-octane per unit cell at 300 K with a ID M A S N M R spectrum on top. 32 experiments with 128 scans in each experiment were performed with a recycle time of 12 s. The total time for the experiment was approximately 13 h. A sweepwidth of 545 Hz, fixed delay of 16 ms. and 80 data points were used. Sinebell apodizations in both F2 and Fj dimensions and a power calculation were used in the data processing.  220  C) Investigation of the low-temperature lattice structure of ZSM-11 Step 1. Discussion of the 2D N M R spectra of the low-temperature form. A 2D I N A D E Q U A T E experiment was performed at 303 K and the results are shown in Figure 95. There are a number of well-defined signals, and the splitting due to scalar coupling is observed i n almost all of them. The range of 29  S i - 0 - S i J couplings i n ZSM-11 from Figure 95 is between 9 and 16 H z , which 29  is consistent w i t h those previously observed (see Chapter Four).  From the  discussion given in previous chapters, it is obvious that i n the contour plot of a 2D I N A D E Q U A T E experiment, the connected signals occur equally spaced on both sides of the diagonal of the plot and the m a x i m u m number of connectivities that can occur for a single S i resonance is four i n the case of zeolites. From 2 9  these restrictions, the connectivities of the resonances can be assigned and are presented i n Table 26.  Due to the small differences in chemical shifts, the  connectivities among H , I and J are slightly ambiguous, and are indicated b y i n Table 26. A n interpretation of the data may be made i n the following manner. Firstly, the topology of the whole framework is assumed to remain the same except that the number of crystallographically inequivalent sites i n  the  asymmetric unit is changed after the phase transition. The asymmetric unit i n the low-temperature form has 12 distinct T-sites according to the 12 resonance lines observed i n S i N M R spectrum, which are defined as T|, T , 2 9  T  3 ' ' 4 ' ' 1*5' T  2  Tj and T v 2  T 7* as shown i n Figure 92B. Further, the connectivities inside  the asymmetric unit are fixed no matter what the space group is.  The  connectivities are shown i n Table 27. A relationship of the resonances between the high and l o w temperature forms can be obtained through careful inspection  221  Figure 95  Contour plot of an INADEQUATE experiment of ZSM-11 at 303 K with a ID M A S N M R spectrum on top. 32 experiments with 832 scans in each experiment were performed with a recycle time of 12 s. and the total time for the experiment was approximately 90 h. A sweepwidth of 622 Hz, fixed delay of 16 ms. and 200 data points were used. Sine-bell apodizations in both dimensions and a power calculation were used for the data processing. The inset in the lower right corner is an expansion of the region indicated by the dashed lines.  222 A  assignment I 7 13Y3 5 assignment II 7' 1373 5'  6 5'4 I, 6 54' 1  2 2'  1  4'2' ' i 42  L KJIH 6 B-A  E-B  F-B E-CO  8-AG-A  D-F «0H-C  2:  K-C  LG-E  1  •112  1  -113  r  -114 PPM  222B  J-A rK-A  Table 26  Connectivity Scheme of the Resonances of ZSM-11 i n the L o w Temperature Form from the 2D N M R Data of Figure 95 Resonance  Connectivities  A  B  K  G  J  B  A  D  E  F  C  D  E  H  K  D  B  C  G  F  E  B  C  G  I  F  B  D  H  J  G  A  E  D  L  H  C  F  L  I  E  L  f  J  A  F  H*  K  A  C  L  G  H  I  223  J* .* 1  Table 27  Connectivities of the T-sites within the Asymmetric Unit of the L o w Temperature Form of ZSM-11 T-site  Connectivities  1  2  2  1  3  4  3  2  6  7  4  2  4'  5  5  4  6  3  4'  7  3  7'  2'  3'  4'  3'  2'  7'  4'  2'  4  5'  4'  7'  3'  5'  6  7  of the results of the variable-temperature experiments (Figure 88 and 89). Below the transition, increasing the temperature induces gradual shifts of individual resonances (Figure 96A). Although there are ill-defined changes in the 10-15 K temperature range during the phase transition, it is possible to trace the variation i n the chemical shifts of the various resonances and to correlate them in groups between the two phases, as shown i n Figure 96B. Finally, the assignment of the  224  individual resonance to T-sites of the low-temperature form can be made using the information presented in Tables 26, 27, and Figure 96B. The assignment is started at K - » 1 , then resonances A and C are assigned to be 2 and 2', since four T | silicons form a four-membered ring.  A t this stage either A or C can be  selected to be 2, leading to two possible assignments which are equally valid. To proceed, one is arbitrarily chosen: i.e., A -> 2 and C -> 2'. From A -» 2, the other two assignments of B -> 4 and J -> 3 can be made, In a similar manner, the assignments of H -»3' and D -> 4' can be obtained from the assignment of C -> 2'. In this way a complete assignment is obtained.  The alternate assignment  corresponding to the exchange of X <-» X' is equally valid.  The two possible  assignments are shown above the I D spectrum of Figure 95. Step 2. Connectivities of the structure of ZSM-11 in the low-temperature form. The bonding connection between T-sites 5 and 5' indicates that the mirror plane, which is perpendicular to the a axis (Figure 92-A) is missing. Then T-site 3' must be connected to 3 and 7' respectively. Thus the connections marked with "  K  i n Table 26 are confirmed. The connectivity table of the low-temperature  structure can n o w be completed as shown in Table 28. If all T-sites X ' and X are changed to X, exactly the same connectivity pattern as for the high-temperature form shown in Table 24 is obtained. This is true for both of the possible assignments and it is proof of self-consistency since it means that the lower symmetry induced by the phase transition only removes the twofold degeneracy of the doubly occupied sites i n the asymmetric unit.  225  340-J  G  FE  D C  B A  320 310 H  —  r-  300  EUD  C  B  290-  280-  114  - 1 1 2  PPM  B  T-sites I resonances G of low T form  3 7 F.E  resonances I—'—i i i of highT K L,J,I,H G.F.E form Figure 96  4 2 B,A  5 6 D,C  r  D,C,B,A  (A) Graphical representation of the variation of chemical shift with temperature for the individual resonances in the ^ S i M A S N M R spectra of ZSM-11. (B) Proposed correlations of individual resonances between the low and high temperature forms.  226  Table 28  Complete Connectivities of T-sites in the L o w Temperature Form of ZSM-11 Connectivities  T-site 1  1  1  2  2'  2  1  3  4  5'  3  2  3'  6  7  4  2  4'  5  6  5  2'  4  5'  7  6  3  3'  4  4'  7  3  5  7'  7'  2'  1  3'  4'  5  3'  2'  3  6  7'  4'  2'  4  5'  6  5'  2  4'  5  7'  7'  3'  5'  7  7  227  Step 3. Consideration of some possible space groups. Careful examination of the asymmetric unit and the symmetry elements of space group I4m2 suggests that the symmetric element of a twofold axis parallel to the diagonal of the ab plane is lost when the phase transition occurs from the high- to low-temperature forms. From a subgroup-supergroup relationship, the subgroups of I4m2 could be 14* (tetragonal) if the mirror plane is missing, and be Pmm2 (orthorhombic) when the 4 fold screw axes are lost. From the general coordinates of equivalent positions of I4m2, values of the coordinates of the additional five T-sites, X', are estimated, which are good enough to run the O R F F E computer program to obtain the theoretical connectivities resulting from lattice symmetries.  A l l space groups that are subgroups of I4"m2 were  considered, and of these, only 14 leads to connectivities that match those in Table 28.  This space group is thus considered to be the correct one for the low-  temperature form of ZSM-11.  In the 2D work described earlier i n this thesis, the structures of the zeolites investigated were wdl-determined by X-ray diffraction experiments, especially i n the case of single crystal diffraction. The interpretation of these N M R data is based o n the X R D data. The successful assignments of the 2D N M R data are i n good support of the proposed structures, and i n some cases give more detailed information about the structures, for example the case of D D 3 R (see Chapter Three). Diffraction experiments are primarily sensitive to long- range orderings and periodicities and give information o n the average crystal structure. However, the quantitative interpretation of powder diffraction data, which one is often forced to use due to the lack of single crystals, is always hampered by loss  228  of information through signal overlap.  N M R spectroscopy probes the local  environments of the T-sites i n the unit cell, and is more sensitive than X R D to moderate deviations from a regular structure such as those induced by changes i n temperature, or the presence of organic molecules. By applying 2D N M R techniques a more complete picture of the zeolite structure can be obtained, as illustrated in previous chapters.  N M R has helped to solve the space group  ambiguities in ZSM-12 (see Chapter One).  In the case of ZSM-11, the room  temperature synchrotron diffraction data set showed almost no extra peaks which w o u l d indicate the lowering of symmetry and the refinement d i d not smoothly proceed to match N M R results with 12 T-sites in the asymmetric ^^155)  jn  c o n  t r a s t , the N M R is very sensitive to the subtle changes in the local  geometry of the T-atoms due to the phase transition.  A s seen above, the  connectivities of the low-temperature form of ZSM-11 have been deduced from N M R experiments alone and the knowledge of the structure of the hightemperature form, and the structure of the l o w temperature form is proposed to be 14. A l o w temperature synchrotron data set is currently being refined by the research group of Dr. H . Gies, U. Bochum, based on the structure derived from 2D N M R results. Thus N M R can provide important complementary information for the refinement of diffraction data.  229  B.  NATURAL A B U N D A N C E TWO-DIMENSIONAL  2 9  SI MAS NMR  INVESTIGATION OP THE THREE-DIMENSIONAL BONDING C O N N E C T I V I T I E S A N D S T R U C T U R E O F Z E O L I T E ZSM-23  I.  INTRODUCTION ZSM-23 is a medium- pore size and high silica zeolite first synthesized by  Plank, Rosinsk and Rubin (*58) have  either  i  t e  framework topology has been proposed to  orthorhombic symmetry,  Pmmn  with  7 crystallographically  inequivalent T-sites and 24 T atoms per unit cell (159,160^ symmetry, P 2 | m n also with 7 independent T-sites U60)_  o r  orthorhombic  j h e pore structure  consists of a one-dimensional channel along the a axis with teardrop- shaped openings of ca. 4.5 X 5.6 A . The projection (be- face) of the framework structure is presented in Figure 97.  Figure 97  The projection (be- face) of zeolite ZSM-23 lattice framework.  230  IL  EXPERIMENTAL The as-synthesized ZSM-23 sample was kindly provided by Dr. S. Ernst,  and was synthesized according to the literature (161)  u s  i g hydrothermal n  techniques with N , N , N , N ' , N ' , N'- hexamethylheptamethylenediammonium dibromide as template.  Powder X R D data were i n excellent agreement with  those previously reported^l^l^O). A highly siliceous and crystalline sample was obtained by calcination and ammonium exchange followed by steaming twice at 750 C for three days. Further steamings gave no improvement i n the quality of this material. High-resolution S i M A S N M R spectra were obtained at 79.5 M H z on a 2 9  Bruker M S L 400 spectrometer using the techniques previously discussed.  m.  RESULTS A N D DISCUSSION a) I D experiments The S i M A S N M R spectrum (Figure 98A) of the highly siliceous sample 2 9  used i n these experiments shows a series of sharp resonances at room temperature. These lines can be deconvoluted i n terms of nine signals of relative intensities 1:1:2:1:1:2:1:2:1, as indicated i n Figure 98C, reflecting an asymmetric unit w i t h at least nine independent T-sites. This is in clear disagreement with both of the proposed structures, i n which the asymmetric unit has seven T-sites of relative occupancies 1: 1: 2: 2: 2: 2: 2. Clearly, if the proposed framework is correct, the syrnmetry must be lower than postulated. Raising the temperature to 100°C does not induce large changes i n the spectrum which w o u l d indicate a transition to a higher symmetry form although there are small and gradual changes consistent with lattice expansion. A s the temperature is lowered, small  231  1—  -186  Figure 98  -iee  -ne  -112  — I —  -11*  •116  PPM  S i M A S N M R spectra of zeolite ZSM-23 and its deconvolution: (A) Experimental spectrum; (B) A computer simulation of A ; (C) The individual Lorentzian curves used in the simulation.  2 9  232  shifts again occur and the double intensity line marked as E/F is split into two lines of equal intensity giving a total of ten lines i n the spectrum. This behavior suggests that the intensities of all three double intensity lines i n the room temperature spectrum (B/C, E/F, I/p might be due to the degeneracy of two resonances w i t h unit intensity and that the asymmetric unit contains twelve Tsites of equal occupancy. b) 2D I N A D E Q U A T E experiments The results of a 2D I N A D E Q U A T E experiment at ambient temperature are shown i n Figure 99. There are a considerable number of connectivities clearly observable, as indicated i n the figure.  Taking the relative intensities of the  signals into account and using the constraint that the m a x i m u m number of connectivities is four for a given silicon atom, there are dear indications that the signals J / K and B/C i n the I D spectrum are also degenerate as previously indicated. Therefore, it is assumed that there are 12 independent T-sites in the structure.  If two resonances are very close i n frequency, the connectivity  between them may be anticipated to be of m u c h lower intensity or perhaps not observable at all due to the coupling being second-order so it is possible that some connectivities might not have been detected. Those connectivities which are clear and reproducible are indicated i n Figure 99. Thus, the results of present 2 D N M R experiments can not be i n agreement w i t h X R D data, suggesting at least that the proposed space group, P m m n , based o n powder X-ray diffraction data for zeolite ZSM-23 is i n error. It is hoped that the combination of synchrotron powder diffraction and high resolution solid state N M R experiments can solve the structure of zeolite ZSM-23.  233  —I  -108  Figure 99  -I  -189  1  -1 10  1  -111  1  -112  PPM  1  -113  1  -114  1—  -115  Contour plot of an INADEQUATE experiment of ZSM-23 at 300 K with a ID M A S N M R spectrum on top. 54 experiments with 864 scans in each experiment were performed with a recycle time of 12 s. and the total time for the experiment was approximately 156 h. A sweepwidth of 740 Hz, fixed delay of 16 ms and 200 data points were used. Sine-bell apodizations in both dimensions and a power calculation were used for the data processing. The dashed lines indicate connections which are not as well defined but which appear reproducible over a series of experiments.  234  CHAPTER SEVEN  CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK A.  CONCLUSIONS  The present work has demonstrated for the first time that twodimensional homonuclear correlation  2 9  S i M A S N M R , mainly I N A D E Q U A T E  and C O S Y types of experiments, can be successfully used to investigate the three-dimensional  silicon bonding networks  i n zeolites.  The structural  information obtained from this study includes: 1) Confirming the structure determined b y X R D techniques, when the 2D data can be successfully interpreted in terms of the k n o w n crystal structures, e. g. Z S M - 39, Z S M - 12, Z S M - 22 etc. 2) Providing additional details o n lattice structures. For example, i n the case of zeolite DD3R, careful analysis of the 2D data indicates that the symmetry is lower than that proposed. 3) Indicating that some proposed structures may be i n error if the number of resonances and the connectivity pattern of the 2D map are not i n agreement w i t h the number of independent T-sites and the 'theoretical' connectivity scheme based on the X R D data. Zeolite ZSM-23 is an example of this case. 4) Investigating the structures for some poorly- defined zeolites i n terms of determining an appropriate space group. For example, the structure of Z S M 11 i n the room temperature form is suggested to be 14 and that of ZSM-5 loaded w i t h 8 molecules of p- dichlorobenzene is proposed to have space group P 2 2 2 . 1  235  1  1  Thus it is felt that the 2D experiments developed and described in this thesis can be used i n the future with confidence in the investigation of unknown zeolite structures and may be extended to other three- dimensional lattice structures.  236  B.  SUGGESTIONS F O R F U T U R E W O R K  I.  APPLICATION  OF  DYNAMIC-  D O U B L E - R O T A T I O N (DOR)  ANGLE  NMR  SPINNING  TO THE  (DAS)  STUDY OF  AND  ZEOLITE  STRUCTURES A s mentioned i n Chapter One, all of the atoms making up the zeolite lattice have N M R active isotopes and thus can be investigated by solid-state N M R , which are  2 9  S i (4.6 %),  27  A 1 (100 %) and  1 7  0 (0.04%).  thesis is concentrated on the investigation of their most direct information on the lattice itself.  2 9  2 9  The work in this  S i spectra which give the  S i is a spin 1/2 nucleus and M A S  averages the orientation-dependent interactions to zero or to their average 'isotropic' values, giving N M R spectra with narrow resonances.  2 7  A l and  1 7  0,  however, are quadrupolar nuclei with 1 = 5/2 and their solid- state spectra are more complex. The line shape due to the central transition (+ 1/2 <-» - 1/2) is distorted and shifted by the second-order quadrupolar interaction and other transitions are usually too broad to be observed directly. Magic-Angle Spinning technique can average only the first order quadrupolar interaction but dynamic- angle spinning (DAS) and double rotation (DOR) (161,162) techniques can average the second-order as well as first-order broadening.  In double rotation experiments, the axis of the rotor is moved  continuously i n a cone by spinning the sample i n a spinner within another spinner, each w i t h its o w n spinning axis, and i n the case of dynamic angle spinning, the sample is contained within a single spinner but the orientation axis of the spinner is switched between two discrete angles with respect to the external magnetic field. The success of the D O R and D A S techniques has been  237  recently demonstrated for  1 7  0 in a variety of s i l i c a t e s ^ ^ , where it is possible to  resolve crystallographically ^equivalent oxygens. However,  1 7  0 results of D O R  or D A S on zeolites have not been reported to date and w o u l d be very informative. It is predictable that they w i l l be successful if applied to the highly siliceous zeolite systems described i n this thesis.  Furthermore  1 7  0 - Si 2 9  heteronuclear correlation experiments w i l l provide additional useful information on zeolite structures. Since the assignments of S i spectra can be obtained from 2 9  2 9  S i 2 D experiments as described i n this thesis, the interpretation of the  spectra  w i l l be possible through  Consequently, the correlation of  1 7  heteronuclear  correlation  1 7  0  experiments.  0 chemical shifts and T- O- T angles could be  studied, and it is particularly important that the  1 7  0 shifts w i l l correlate with  discrete angles and not averages as i n the case of S i . Thus it w i l l give more 29  significance to the  1 7  0 chemical shifts and a closer linking the two major  techniques, X R D and N M R , in zeolite structure studies.  n.  R O T A T I O N A L - E C H O D O U B L E - R E S O N A N C E (REDOR) N M R STUDIES OF ZEOLITES Magic- angle spinning has been widely used to reduce the broadening  effects of chemical shift anisotropy and dipole- dipole coupling i n order to obtain high-resolution solid state spectra, as discussed i n Chapter One. Therefore the direct detection of weak dipolar couplings i n M A S N M R experiments is often difficult.  In  order to solve this problem, Schaefer^5)  n a s  proposed the  rotational-echo double-resonance (REDOR) pulse sequence, which was derived from spin-echo double r e s o n a n c e ^ ^ ' ^ T ) , information.  to obtain molecular  geometric  This k i n d of experiment has been extended to rotational-echo  238  triple-resonance N M R <  1 6 8  ) and DANTE-selected R E D O R N M R ^  application of these techniques has concentrated on C 1 3  1 5  1 6 9  \  The  N labeled spin pairs.  Fyfe and Grondey have recently applied these types of experiment to the interactions involving quadrupolar n u c l e i ^ ^ , and investigated the dipolar interactions between  3 1  P and  2 7  A l pairs in a sample of VPI- 5. 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