International Conference on Gas Hydrates (ICGH) (6th : 2008)

INVESTIGATIONS ON THE INFLUENCE OF GUEST MOLECULE CHARACTERISTICS AND THE PRESENCE OF MULTICOMPONENT.. Luzi, Manja; Schicks, Judith M.; Naumann, Rudolf; Erzinger, Jörg; Udachin, Konstantin A.; Moudrakovski, Igor L.; Ripmeester, John A.; Ludwig, Ralf 2008-07-31

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INVESTIGATIONS ON THE INFLUENCE OF GUEST MOLECULE CHARACTERISTICS AND THE PRESENCE OF MULTICOMPO-NENT GAS MIXTURES ON GAS HYDRATE PROPERTIES  Manja Luzi, Judith M. Schicks*, Rudolf Naumann, J?rg Erzinger GeoForschungsZentrum Potsdam (GFZ)  Telegrafenberg, 14473 Potsdam, GERMANY  Konstantin Udachin, Igor Moudrakowski, John A. Ripmeester The Steacie Institute for Molecular Sciences National Research Council of Canada  Ottawa, Ontario, CANADA  Ralf Ludwig Physikalische Chemie / Theoretische Chemie Universit?t Rostock  18059 Rostock, GERMANY  ABSTRACT In this study, we investigated the molecular characteristics of hydrates which were synthesized from gas mixtures containing the two isomers of butane, or the pentane isomers neopentane and isopentane, in excess methane. Thereto various techniques, including Raman spectroscopy, pow-der and single crystal X-ray diffraction and 13C NMR spectroscopy were employed. It turned out that  shape  and  conformation  of  the  guest  molecule  and  hydrate  structure  both  influence  each other. In case of the mixed butane hydrate it could be confirmed that n-butane is enclathrated in its gauche conformation. This was verified by Raman spectroscopy, single crystal X-ray diffrac-tion and calculated data. While isopentane is known as a structure H former, our results from powder X-ray diffraction, 13C NMR and ab initio calculations show that it can be also incorpo-rated  into  structure  II  when  the  hydrate  is  formed  from  a  neopentane/isopentane/methane  gas mixture.     Keywords: gas hydrates, guest molecular properties, constitutional isomer, rotational isomer  INTRODUCTION Although  most  naturally  occurring  gas  hydrates form structure I hydrates with methane as the main component, structure II hydrates that contain light hydrocarbon guest molecules like propane, isobu-tane or n-butane could be identified as well [1, 2]. Recently Lu et al. succeeded in verifying the exis-tence  of  structure  H  hydrates  in  nature.  The  E (51268)  cavity  of  structure  H  is  the  largest  cavity which  is  known  for  naturally  occurring  gas  hy-drates.  It  incorporates  large  guest  molecules  such as isopentane, n-hexane and even methylcyclohex-ane [3].  It has been well established that the guest molecule size influences the structure obtained when a guest material reacts with water to form a hydrate.   But  as  it  was  shown  before  the  relationship  be-tween structure and molecular size is not straight-forward [4]. There are still open questions regard-ing  the  influence  of  other  molecular  properties such as the guest molecule shape (conformation) as well as the presence of further guest molecules on the properties of the formed gas hydrate and vice versa.  Therefore  hydrates  were  synthesized  from gas mixtures containing the two isomers of butane, or the pentane isomers neopentane and isopentane, in  excess  methane.  The  composition  of  these  gas mixtures  was  kept  close  to  natural  conditions  [5, 6].  Recently it was shown that depending on the for-mation conditions either the shape of a guest mole-cule  or  its  water  solubility  has  a  great  impact  on  Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.    * Corresponding author: Phone: +49 331 2881487 Fax: +49 331 2881474 E-mail: the gas hydrate composition [7]. For a non-stirred water  system  the  smaller  isomer  is  preferentially incorporated  into  the  hydrate  phase.  The  hydrate composition is dominated by the different diffusion rates  of  the  isomers,  which  are  related  to  their shape.  When  the  water  phase  is  stirred  diffusion becomes  unimportant,  and  the  isomer  with  the higher  water  solubility  is  enriched  in  the  hydrate phase.  In  order  to  study  these  multicomponent  hydrates on  a  molecular  level  several  analytical  tools  are employed to verify the crystal structure and require the hydrate composition. Thereby, the emphasis of this work was kept on the role of the guest molecu-lar shape during the hydrate formation process and the interaction between guest molecule and hydrate lattice.   EXPERIMENTAL METHODS Sample preparation To synthesize hydrate 3 g freshly ground ice was loaded into a high pressure cell (internal volume: approximately 130 mL) which was precooled in a freezer at 253 K. Afterwards the pressure cell was connected to a pressure transducer and placed in a cooling  bath  at  271  K.  Before  pressurization  the cell was tempered 30 min in the cooling bath. To prevent the ice powder from melting, the incoming gas  was  cooled  and  the  reactor  was  pressurized very  slowly.  The  pressure  cell  was  stored  in  the cooling  bath  until  no  further  pressure  drop  was observed.   In case of the isopentane hydrate sample the liquid hydrocarbon  was  cooled  in  the  freezer  as  well.  1.9  mL  isopentane  were  dispersed  with  a syringe on the ice powder. After filling a 50 mL pressure cell  with  5  g  ice  and  the  liquid  hydrocarbon,  the cell was tempered in the cooling bath at 268 K and afterwards pressurized with methane. By the time a further  pressure  drop  could  not  be  observed  the temperature was increased to 274 K to enhance the conversion of ice into hydrate.  A list of the gas mixtures used in this study is pre-sented in Tables 1 and 2.   Gas mixture  CH4 (mol %) n-C4H10 (mol %) iso-C4H10 (mol %) Ia  96.8  3.2  - Ib  94.4  -  5.6 Ic  96.0  2.0  2.0  Table 1. Butane gas mixtures. Gas mixture  CH4 (mol %) neo-C5H12 (mol %) iso-C5H12 (mol %) IIIa  97.2  2.8  - IIIb  79.8  -  20.2 IIIc  98.0  1.0  1.0  Table 2. Pentane gas mixtures.  Sample analysis The hydrate crystal structure and lattice constants were obtained with a Bruker D8 Advance Powder X-ray  diffractometer  (Cu  K?  radiation,  ?  =  1.5406 ?) equipped with an Anton Paar low tem-perature  controller.  The  powder  X-ray  diffraction patterns  were  acquired  at  153  K  in  a  continuous scan mode. The patterns obtained were calibrated with reference to several ice Ih reflections.   The cage  occupancies  of  the  mixed  hydrate  sam-ples were determined by 13C magic angle spinning (MAS)  NMR  measurements  which  were  carried out on a Bruker DSX-400 NMR spectrometer. For this,  hydrate  samples  were  finely  ground  and packed  in a  7  mm  zirconia  rotor that  was  loaded into  a  variable  temperature  probe.  The  13C  NMR spectra were recorded at 173 K with ~ 2 kHz spin-ning  rate.  In  order  to  distinguish  the  solid  phase signals  from  that  of  the  liquid  (adsorbed  higher hydrocarbons)  the  spectra  were  obtained  by  high power  proton  decoupling  (HPDEC)  as  well  as cross polarization (CP) programs. The spectra were referenced  to  adamantane  as  external  chemical shift  reference  with  assigned  chemical  shifts  of  delta = 38.56 ppm and delta = 29.50 ppm at 298 K.   The compositional analysis of the hydrate samples was  supported  by  Raman  spectroscopy.  The  Ra-man  spectra  were  recorded  in  the  C-C  stretching vibration regions at 77 K on an Acton SpectroPro 2500i spectrometer equipped with a Witec confo-cal  microscope.  An  argon  ion  laser  operating  at 514.53 nm was used as excitation source. Naphtha-lene at 1382 cm-1 was used as reference.  In case of the n-butane ? isobutane ? methane hy-drate it was possible to collect a single crystal from the bulk sample which turned out to be a multiple twin. This hydrate crystal was analyzed by single crystal X-ray diffraction. The data were collected with Mo K? radiation (? = 0.71073 ?, 2? = 50.0, ? scan mode) on a Bruker SMART CCD diffracto-meter at 100 K. The structure was solved by direct methods  using  the  SHELXTL  suite  of  programs. All atoms were refined anisotropically. Hydrogen  atoms  on  guest  molecules  were  placed in calculated positions and allowed to ride on the parent atoms.   In addition to the sample analysis, ab initio quan-tum  mechanical  calculations  were  performed  to investigate  the  interaction  of  the  guest  molecule and the hydrate cage and the role of the different guest molecular conformations during the hydrate formation  process  in  more  detail.  The  ab  initio calculations  were  performed  with  Hartree  Fock (HF)  and  density  functional  (DFT)  methods  that are  implemented  in  the  GAUSSIAN  98  software package.  The  following  basis  set  was  used:  6-31 + G*. The geometries of the hydrate cages and the  entrapped  guest  molecules  were  fully  opti-mized  using  HF  and  B3LYP  levels  of  theory.  A more  detailed  description  for  this  method  is  pro-vided  in  the  literature [8].  To  support the experi-mental  results  from  13C  NMR  measurements  the chemical shifts were calculated for the synthesized hydrates as well.   RESULTS AND DISCUSSION All hydrate samples were analyzed with powder X-ray diffraction, Raman spectroscopy and 13C NMR spectroscopy to determine the crystal structure and the composition of the hydrate. The measurements were performed in regard to the shape and confor-mation of the guest molecule and the gas hydrate structure. In the following, three examples for the interaction of guest molecular properties and mo-lecular  properties  of  the  formed  gas  hydrate  are presented.   Butane hydrates  Isobutane ? Methane SII Hydrate  This  hydrate  was  formed  at  2.1  MPa  for  several days  while  the  main  pressure  drop  was  observed during  the  first  hours  after  pressurization.  After verifying  the  crystal  structure  by  powder  X-ray diffraction the hydrate was analyzed by 13C MAS NMR methods (Figure 1). The measured chemical shifts coincide with literature data of mixed isobu-tane ? methane hydrates (Table 3) [3]. Please note, that the carbon signals for isobutane in the hydrate phase  are  reversed  relative  to  their  order  in  the pure compound isobutane [9].   -15Chemical Shift [ppm]Intensity  Figure 1. CP/MAS 13C NMR spectra of isobutane ? methane hydrate; measuring temperature 173 K.  System  Chemical shift [ppm]  Peak assignment -4.46  CH4 in small cage -8.16  CH4 in large cage 26.46  CH3-signal of iso-C4H10 isobutane ? methane hydrate  23.69  CH-signal of iso-C4H10 25.2  CH-signal (liq.) [9]  24.3  CH3-signal  Table 3. Chemical shifts and peak assignments for isobutane  and  methane  in  the  hydrate  phase  and isobutane in the liquid phase.  It  is  important  to  mention  that  the  peak  assign-ments for the CH- and CH3-signal of pure isobu-tane are not consistent in the literature [10, 11]. For this  reason  we  decided  to  compare  our  data  with calculated 13C NMR data.  In  this  case  our  peak  assignments  for  13C  NMR spectra were also confirmed by the calculated data (Figure  2).  In  comparison  to  the  pure  compound (dashed  lines)  the  CH-signal  and  CH3-signal  of isobutane are reversed in the hydrate phase (solid lines). The peak positions of the calculated spectra (calculation temperature 0 K) differ from those of the experimental spectrum as they were referenced to  the  methane  peak  that  was  also  calculated  for this structure. CH3-signal of iso-C4H10 CH-signal of iso-C4H10   Figure 2. Calculated 13C NMR spectra of isobutane in  the  hydrate  phase  (solid  lines)  and  in  the  gas phase (dashed lines); calculation temperature 0 K.  Apart from that it should be noted that peak posi-tions can be shifted due to changes in temperature [12-14]. So, one might assume that the change of the  peak  order  is  a  consequence  of  the  different measuring  and  calculation  temperatures,  respec-tively.  But  to  the  authors?  knowledge  it  was  not reported so far that a change of the temperature can cause  a  change  in  the  peak  order.  Therefore  we suppose that the change of the peak order results from the enclathration of isobutane in the hydrate lattice.  This  assumption  is  supported  by  single  crystal analyses  (Figure  3).  As  mentioned  before,  it  was possible to collect a single crystal from a hydrate sample  that  was  synthesized  from  a  gas  mixture containing  2%  isobutane  2%  n-butane  and  98% methane  (gas  mixture  Ic).  This  combination  of guest  molecules  results  in  a  structure  II  hydrate. The  unit  cell  parameter  was  determined  with a  = 17.213 ? (space group Fd-3m). The measurement has  shown  that  the  small  cavities  are  completely filled with methane. 65% of the large cavities are filled  with  isobutane  and  35%  are  filled  with  n-butane. As presented in Figure 3 it was found for isobutane that the molecule has moved out of the cavity centre closer to the edge of the hydrate cage. Therefore an interaction of the guest molecule and the  hydrate  cage  appears  to  be  possible  which might arise in a change of the peak order in the 13C NMR spectrum.                          Figure 3. Guest molecule isobutane in the H (51264) cavity of structure II.  n-Butane ? Methane SII Hydrate  A conformational analysis of n-butane encaged in the  hydrate  lattice  is  presented  as  the  second  ex-ample  for  an  interaction  between  guest  molecule and hydrate cage.  As a linear alkane, the CH3CH2 fragments are able to rotate around the central C-C bond which results in  different  rotational  isomers.  The  energetically most stable conformer of n-butane is the trans (also called anti) form that has a C2h symmetry. A rota-tion of 120? about the central C-C bond results in the gauche conformer (symmetry C2) [15]. Several studies dealing with the rotational isomerism of n-butane demonstrate that Raman spectroscopy is an appropriate tool to distinguish the conformers [15-17].  In  the  presence  of  a  help  gas  like  methane,  n-butane  forms  a  structure  II  hydrate.  Two  studies deal with the conformation of n-butane in the hy-drate cage. Davidson et al. have performed dielec-tric  relaxation  measurements  on  an  S  II  hydrate consisting of hydrogen sulphide and n-butane [18]. The condition for dielectric absorption is a perma-nent dipole moment of the guest molecule. In con-trast to trans n-butane gauche n-butane does have a permanent  dipole  moment.  Apart  from  that,  trans n-butane  (van-der-Waals  diameter:  7.9  ?) is  sup-posed to be too large to fit into the large cage of structure  II.  As  a  dielectric  absorption  was  ob-served  for  this  hydrate  Davidson  et  al.  assumed that the smaller conformer gauche n-butane (van-der-Waals diameter: 7.1 ?) is encaged in the hy-drate lattice.  Secondly,  Subramanian  and  Sloan  presented  Ra-man data for an n-butane ? methane hydrate syn-thesized at 0.22 MPa and 273.7 K [19]. Comparing the Raman spectra of liquid n-butane and the gas hydrate they showed that trans-gauche band pairs  10   8   6   4   2   0 -20         -10          0           10         20          30          40 Chemical Shift [ppm] 1  CH-signal of iso-C4H10 in the   hydrate phase 2  CH3-signal of iso-C4H10 in the   hydrate phase  3  CH-signal of iso-C4H10 in the gas   phase 4  CH3-signal of iso-C4H10 in the gas   phase 1    2  3 4 occurring  in  the  liquid  spectrum  reduce  to  single bands in the hydrate spectrum. They claimed that these single bands correspond to the gauche bands of  the  pure  liquid  n-butane.  As  they  found  only bands generated by the gauche form of n-butane in the hydrate spectrum they concluded that the trans form is excluded from the hydrate lattice.  For this work an n-butane ? methane hydrate was synthesized at 4.3 MPa and 271 K. In addition to the  hydrate  sample  liquid  n-butane  was  analyzed by  Raman  spectroscopy  as  well.  The  liquid  n-butane was measured at room temperature and the hydrate spectrum was recorded at 77 K.  While  the  Raman  spectra  of  Subramanian  and Sloan  cover  a  wave  number  region  of  700  cm-1? 1150 cm-1 this study shows a region of 750 cm-1 ? 1600 cm-1 (Figure 4) [19].   750 850 950 1050 1150 1250 1350 1450 1550wave number [cm-1]intensity [a. u.]  Figure 4. Comparison of the Raman spectra of pure n-butane (liquid) and n-butane ? methane hydrate. The  bands  of  the  liquid  n-butane  spectrum  were labelled with ?t? and ?g? corresponding to the trans and gauche rotational isomers of n-butane.  The liquid and hydrate spectra obtained correspond very  well  to  the  literature  data  [15-17,  19-22].  A complete overview of all peak positions and vibra-tional  assignments  for  the  liquid  and  the  hydrate phase spectra is presented in Table 4. Based on the literature  data  the  observed  bands  for  liquid  n-butane  were  assigned  to  the  corresponding  trans and gauche conformer, respectively [15-17, 20-22]. Accordingly the peaks were labelled as ?t? (trans) and  ?g?  (gauche). The  band  at 1453 cm-1  is  very broad  and  can  be  attributed  to  both  conformers. Due  to  its  width  it  interferes  with  several neighbouring  bands  which  are  assigned  to  both conformers as well. It is important to mention that the  literature  data  are  not  consistent  in  this  fre-quency area.  Vibrational mode nunununu liquid n-butane  [cm-1] nu n-butane ? methane hydrate [cm-1] trans  conformer gauche  conformer 792  808    C-C stretch, CH3/2 rock 832  840    C-C stretch, CH3 rock 840  ?  C-C stretch   959  968    C-C stretch 983  989    CH3 rock 1060  ?  C-C stretch   1079  1083    C-C stretch 1137* [21]  1139    CH3 rock 1150  ?  CH3 rock   1172  1177    CH3/2 rock 1263* [21]  1261   CH2 twist, CH3 rock 1281  1282    CH2 twist 1304  ?  CH2 twist   1343* [16]  1345    CH2 wag 1383* [21]  1383 CH3 sym deform CH3 sym deform 1453 (very broad) 1446** 1465** 1474** CH3 deg deform CH3 deg deform  Table 4. Observed Raman frequencies nu [cm-1] and assignments  to  the  main  vibrational  modes  for liquid  n-butane  and  n-butane  ?  methane  hydrate (stretch = stretching, rock = rocking, twist = twist-ing, scis = scissors, wag = wagging, deform = de-formation, deg = degenerate, sym = symmetric)  *  Literature  data  for  pure  n-butane  that  were  not observed in the liquid phase spectrum **  Due  to  overlapping  vibrational  modes  of  both conformers a reliable assignment is not possible.   The measured region in the Raman spectra of liq-uid n-butane contains four trans-gauche band pairs. These band pairs are 832 cm-1 (g) ? 840 (t) cm-1, 792   g  832     g 840   t         959     g     983      g       1060              t     1079          g     1150     t                  1172            g  1281      g   1304    t 1453 t + g  808  840       968            989   1083        1177      1139   1261         1282         1345             1383    1446+1465  +1474            liquid n-C4H10             n-C4H10 ? CH4             hydrate 1060  cm-1  (t)  ?  1079  cm-1  (g),  1150  cm-1  (t)  ?  1172 cm-1 (g) and 1281 cm-1 (g) ? 1304 cm-1 (t). On the basis of the Pimentel-Charles ?Loose Cage ? Tight Cage Model? Subramanian and Sloan de-termined  that  the  Raman  bands  for  n-butane trapped in the hydrate cage are slightly shifted to higher  frequencies  compared  to  the  liquid  phase spectrum [19]. Thus, we can assign the band occur-ring at 808 cm-1 in the hydrate spectrum as the C-C stretching vibration that was detected at 792 cm-1 in  the  liquid  spectrum.  Considering  the  trans-gauche  band  pairs  of  the  liquid  spectrum  we  can designate  the  single  bands  in  the  hydrate  phase spectrum  at  840  cm-1,  1083  cm-1,  1177  cm-1  and 1282 cm-1 as the remaining gauche bands. The four corresponding  trans  bands  are  nonexistent  in  the hydrate  phase  spectrum.  In  contrast  to  the  liquid spectrum the hydrate spectrum includes four bands at 1139 cm-1, 1261 cm-1, 1345 cm-1 and 1383 cm-1. As shown in Table 4 these four bands were com-pared with literature data of n-butane. Keeping in mind  that  the  vibrational  frequencies  of  n-butane trapped in the hydrate cage are slightly shifted, the bands  at  1139  cm-1,  1261  cm-1,  1345  cm-1  and 1383 cm-1 presumably correspond to bands of pure n-butane  which  are  located  at  1137  cm-1, 1263 cm-1, 1343 cm-1 and 1383 cm-1. The literature bands at 1137 cm-1, 1263 cm-1 and 1343 cm-1 are assigned  to  gauche  n-butane.  The  signal  at  1383  cm-1  which  is  caused  by  a  symmetric  CH3 deformation  vibration  can  result  from  both  con-formers [20]. Therefore we assume that compared to the liquid phase the gauche conformer was en-riched  in  the  hydrate  phase  which  results  in  the appearance  of  these  four  bands.  Regarding  the multiple  peak  at  1450  cm-1  it  was  already  men-tioned that several vibrational modes of both con-formers overlap at this position. For this reason an exact  assignment  of  the  three  bands  appearing  in the hydrate phase Raman spectra is only specula-tive. Thus, the presence of a vibrational mode re-sulting  from  the  trans  conformer  cannot  be  ex-cluded, but it is very unlikely.  To sum up, four distinctive trans bands present in the  liquid  spectrum  of  n-butane  are  absent  in  the hydrate phase spectrum. The signal at 1383 cm-1 as well as the multiple peak at 1450 cm-1 can include vibrational  modes  of  both  conformers  in  theory. Considering these results and due to fact that the hydrate  spectrum  shows  three  additional  bands resulting from the gauche conformer it can be con-cluded  that  the  gauche  conformer  of  n-butane  is incorporated into the hydrate phase.  These  results  are  supported  by  the  single  X-ray diffraction  data  which  were  obtained  for  the  n-butane ? isobutane ? methane hydrate. As shown in Figure 6 n-butane occupies the H (51264) cavity of  structure  II.  It  turned  out  that  n-butane  is  en-caged in its higher energy gauche form.                        Figure  6.  Guest  molecule  n-butane  in  a  gauche conformation inside the large cavity of structure II.   On the basis of ab initio calculations it was possi-ble  to  generate  a  picture  of  n-butane  in  the  large cavity of structure II. At the end of the optimiza-tion process it was found as well that n-butane can be  incorporated  in  its  gauche  conformation  as shown in Figure 7.   Figure 7. Optimized structure of n-butane ? meth-ane  hydrate.  n-Butane  is  encaged  in  its  gauche conformation.  Pentane hydrates  Isopentane ? Neopentane ? Methane Hydrate  As the existence of structure H hydrates in nature could be recently verified this study was extended to hydrates containing constitutional isomers with five  carbon  atoms  [3].  Isopentane  is  known  as structure H former [23]. Its smaller structural iso-mer is neopentane which occupies the large cavity of structure II [24]. Isopentane has two stable con-formations  ?  trans  and  gauche  with  C1  and  Cs symmetry, respectively [25].  To come close to natural conditions, a gas mixture was employed which included 1% isopentane 1% neopentane and 98% methane (gas mixture IIIc). In order to evaluate the results from this gas mixture hydrate samples were synthesized which contained only one pentane isomer and methane. According to the 13C NMR spectra and powder X-ray diffrac-tion  pattern  it  could  be  verified  for  the  samples containing only one isomer that neopentane is in-corporated into the H (51264) cavity of structure II and isopentane is encaged in the E (51268) cavity of structure H (Figure 8a and 9a and 8b and 9b, re-spectively).  Figure 8 shows the results of CP/MAS 13C NMR measurements  for  all  three  mixed  hydrates.  The peak positions and assignments, which were con-firmed by literature data, are presented in Table 5 [9, 26].   -15Chemical shift [ppm]Intensity  Figure 8. CP/MAS 13C NMR spectra of a) neopen-tane ?  methane  hydrate,  b)  isopentane  ?  methane hydrate,  c)  isopentane  ?  neopentane  ?  methane hydrate.  In  order  to  show  the  isopentane  signals (marked by an arrow) the CH3-signal for neopen-tane was cut.  The carbon skeleton of isopentane was numbered according to IUPAC rules. It should be noted that Shin  et  al.  have  shown  before  that  isopentane  is incorporated in its gauche conformation into 51268 cavity of structure H [26]. Although the trans con-former is the low energy form and would fit into the  large  cavity  of  structure  H,  Shin  et  al.  con-firmed  by  HPDEC  13C  NMR  techniques  that  the smaller gauche conformer is preferred.  System  Chemical shift [ppm]  Peak assignment -4.62  methane in small cage -8.19  methane in large cage 33.10  CH3-signal of neo-C5H12 a) neopen-tane ? methane hydrate   27.24  C-signal of neo-C5H12 -4.39  methane in small cage -4.77  methane in medium cage 22.31  CH3-signal of iso-C5H12 (C1) 30.79  CH-signal of iso-C5H12 (C2) 32.29  CH2-signal of iso-C5H12 (C3) b) isopen-tane ? methane hydrate 11.62  CH3-signal of iso-C5H12 (C4) -4.58  methane in small cage -8.13  methane in large cage 33.17  CH3-signal of neo-C5H12 27.33  C-signal of neo-C5H12 29.42  CH-signal of iso-C5H12 (C2) 21.83  CH3-signal of iso-C5H12 (C1) c) isopen-tane ? neo-pentane ? methane hydrate (2.3 MPa) 8.61  CH3-signal of iso-C5H12 (C4)  Table 5. Chemical shifts and peak assignments for the pentane hydrates.  The 13C NMR spectra of the isopentane ? neopen-tane ? methane hydrate sample (Figure 8c) shows only  traces  of  isopentane  in  the  hydrate  phase. Neopentane  is  the  dominant  pentane  isomer  that was trapped in the hydrate cage. In order to influ-ence  the  hydrate  composition  the  samples  were synthesized  at  four  different  pressures:  2.3  MPa, 3.0 MPa, 3.5 MPa and 4.0 MPa. But no distinctive change  in  the  hydrate  composition  could  be achieved.  Therefore  the  sample  that  was  synthe-sized  at  2.3  MPa  was  chosen  as  representative (Figure 8c and Figure 9c).  For isopentane in the mixed pentane hydrate only carbon  signals  for  C1,  C2  and  C4  could  be  ob-served which were marked by an arrow in Figure 8c. Please note that the peak positions for isopen- b)  a)  c) tane  in  the  mixed  pentane  hydrates  are  slightly shifted  upfield  compared  to  isopentane  in  the isopentane  ?  methane  hydrate  (Figure  8b).  Only the isopentane ? neopentane ? methane sample that was prepared at 3.0 MPa shows a double peak for the CH3-group of isopentane where one peak coin-cides  with  the  peak  position  of  the  C1-signal  for isopentane in the structure H hydrate.  To verify the crystal structure of the isopentane ? neopentane ? methane hydrates powder X-ray dif-fraction  measurements  were  performed  and  com-pared  with  hydrate  samples  containing  only  one pentane  isomer  (Figure  9).  As  both  pentane  iso-mers  are  known  to  form  different  hydrate  struc-tures a mixture of reflections from structure II and structure  H  was  expected.  In  fact,  for  the  mixed pentane samples only structure II reflections could be  recorded.  Once  again  only  the  isopentane  ? neopentane  ?  methane  sample  synthesized  at  3.0 MPa  shows  one  small  reflection  that  can  be  as-signed to structure H hydrate.  16 18 20 22 24 26 28 30 32 34 362-Theta-ScaleIntensity  Figure  9.  Powder  X-ray  diffraction  pattern  of  a) neopentane  ?  methane  hydrate,  b)  isopentane  ? methane  hydrate,  c)  isopentane  ?  neopentane  ? methane hydrate.  The 13C NMR measurements have shown that the amount  of  isopentane  is  very  low  in  the  hydrate phase. So one might assume that the concentra-tion of structure H hydrate was too low to be de-tected by powder X-ray diffraction.  But  due  to  the  fact  that  the  carbon  signals  for isopentane  of  the  mixed  pentane  hydrate  are shifted  upfield  these  results  can  also  lead  to  an-other  assumption.  We  calculated  the  van-der-Waals diameter of gauche isopentane at first from the molar volume with an assumed spherical struc-ture of the molecule: 6.23 ? and secondly from an Onsager cavity: 7.86 ?. As gauche isopentane has no  ideal  spherical  structure  the  ?real?  van-der-Waals  diameter  lies  between  those  two  values. Regarding  the  ratio  of  the  molecular  diameter  of gauche  isopentane  to  the  cavity  diameter  of  the 51264 cage of structure II this leads to the conclu-sion that  the  gauche isopentane does also  fit into the  51264  cavity.  Thereto,  ab  initio  calculations were  performed  which  should  show  if  the  large cavity  of  structure  II  could  be  occupied  by  the gauche  conformer  of  isopentane  as  well.  In  fact, the  optimizing  process  for  a  structure  II  hydrate with  isopentane  in  its  gauche  conformation  was successful  (Figure  10)  and  shows  that  this  con-former  can  also  occupy  the  large  cavity  structure II. Thus, the observed upfield shift for isopentane in the mixed pentane hydrate might result from its enclathration in the 51264 of structure II. The driv-ing force for this effect is supposedly the presence of  neopentane,  another  higher  hydrocarbon  guest molecule that forms structure II.   Figure  10.  Optimized  structure  of  isopentane  ? methane  hydrate.  Isopentane  is  encaged  in  its gauche  conformation  in  the  H  (51264)  cavity  of structure II.  For this reason it can be concluded that although isopentane is a typical structure H former it might be also incorporated into the large cavity of struc-ture  II  when  another  higher  hydrocarbon  guest molecule is present. There is only one mixed pen-tane sample that shows traces of a structure H hy-drate. But the diffraction pattern of all other sam-ples shows only structure II reflections. Therefore we conclude that depending on the formation con-ditions isopentane is able to form either a structure II or a structure H hydrate or probably even coex-isting structure II and structure H hydrates. But to  a)  b)  c) verify  this  assumption  further  investigations  are necessary.   CONCLUSION  The aim of this study was to investigate the inter-action  of  guest  molecular properties  e.g. the  con-formation and molecular properties of the gas hy-drate formed. Therefore gas hydrates were synthe-sized which contained the constitutional isomers of butane or two constitutional isomers of pentane in excess  methane,  respectively.  By  means  of  three examples this interaction was illustrated.  For  the  isobutane  ?  methane  hydrate  it  was  ob-served that the peak positions for isobutane in the 13C  NMR  spectrum  are  reversed  relative  to  their order in the pure compound. With the help of sin-gle crystal X-ray measurements it could be shown that  isobutane  is  moved  out  of  the  cavity  centre. The resulting interaction of the guest molecule and the cavity wall is assumed to be the reason for the change in the peak positions.  On the basis of Raman spectra, single crystal X-ray diffraction  data  and  quantum  mechanical  calcula-tions it could be verified that n-butane occupies the large cavity of structure II in its gauche conforma-tion.  The results for isopentane ? neopentane ? methane hydrates  led  to  the  assumption  that  similar  to  n-butane  the  conformation  of  isopentane  plays  an important role during the hydrate formation proc-ess although the conformation itself is not expected to  change  significantly.  Depending  on  the forma-tion conditions e.g. the presence of another higher hydrocarbon  like  neopentane  the  ?ambivalent? gauche conformer of isopentane does influence the crystal structure of the hydrate and vice versa. In order to examine this phenomenon in more detail single  crystal  X-ray  measurements  for  these  hy-drates and additional ab initio calculations are un-der way.   SUPPLEMENTARY INFORMATION Details  of  single  crystal  data  collections  and  re-finement:  Structure  II  isobutane  ?  n-butane  ?  methane  hy-drate. Crystal size 0.15 x 0.15 x 0.15, cubic, space group:  Fd-3m,  a=17.213(1)?,  V  =  5099.9(2)?3, T=100.0(1)?K, ?calc= 1.033 mg/m3, 2?Max=50.00?, 117 parameters, 33 restrains, residual electron den-sity  max.  0.53,  min.  -0.22  e?-3.  Final  R  indices (I>2?(I)):  R1=  0.064,  wR2=  0.172  (28603  reflec-tions total, 275 unique, 246 (I>2 ? (I)).  ACKNOWLEDGMENTS Manja Luzi gratefully acknowledges the financial support  for  her  studies  at  the  National  Research Council Canada provided by the DAAD (German Academic  Exchange  Service)  and  thanks  Dr  Stephen  Lang  from  the  National  Research Council  Canada  for  his  technical  and  scientific support.   LITERATURE [1] Sassen R, MacDonald IR. Evidence of structure H  hydrate,  Gulf  of  Mexico  continental  slope.  Or-ganic Geochemistry 1994, 22 (6): 1029-1032. [2]  Davidson  DW,  Garg  SK,  Gough  SR,  Handa YP, Ratcliffe CI, Ripmeester JA, Tse JS, Lawson WF. Laboratory analysis of a naturally occurring gas hydrate from sediment of the Gulf of Mexico. Geochimica  et  Cosmochimica  Acta  1986,  50  (4): 619-623. [3] Lu H, Seo Y-t, Lee J-w, Moudrakovski I, Rip-meester JA, Chapman NR, Coffin RB, Gardner G, Pohlman  J.  Complex  gas  hydrate  from  the  Cas-cadia margin. Nature 2007, 445 (7125): 303-306. [4]  Schicks  JM,  Naumann  R,  Erzinger  J,  Hester KC, Koh CA, Sloan Jr. ED. Phase Transitions in Mixed  Gas  Hydrates:  Experimental  Observations versus Calculated Data. Journal of Physical Chem-istry B 2006, 110 (23): 11468-11474. [5]  Milkov  AV.  Molecular  and  stable  isotope compositions  of  natural  gas  hydrates:  A  revised global  dataset  and  basic  interpretations  in  the context of geological settings. Organic Geochemis-try 2005, 36 (5): 681-702. [6] Sassen R, Roberts HH, Carney R, Milkov AV, DeFreitas DA, Lanoil B, Zhang C. 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Journal of Molecular Structure 2000, 550-551: 67-91. [26] Shin K, Park Y, Hong J, Lee H. Gauche con-formation of acyclic guest molecules appearing in the large cages of structure-H clathrate hydrates. Korean Journal of Chemical Engineering 2007, 24 (5): 843-846.   


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