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

COMPLEX COEXISTENCE BEHAVIOR OF STRUCTURE I AND H HYDRATES Seo, Yutaek; Kang, Seong-Pil; Seo, Yongwon; Lee, Jongwon; Lee, Huen 2008-07-31

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   COMPLEX COEXISTENCE BEHAVIOR OF STRUCTURE I AND H HYDRATES   Yutaek Seo, Seong-Pil Kang Climate Change Technology Research Division, Korea Institute of Energy Research, Daejeon, Korea  Yongwon Seo Dept. of Chemical Engineering, Changwon National University, Korea   Jongwon Lee Dept. of Environmental Engineering, Kongju National University, Korea  Huen Lee Dept of Chemical and Biomolecular Engieering, KAIST, Korea   ABSTRACT 13C NMR spectroscopic analysis was carried out to clarify the formed hydrate structure in specific conditions on hydrate phase diagram of ternary methane, neohexane, and water system. The obtained NMR spectra at three different conditions suggested that both structure I and H were formed simultaneously and coexisted at 273.6 K and 50 bar. But, for both conditions of 273.6 K, 25 bar and 283.1 K, 50 bar the formed hydrate was identified as structure H only. These results showed that the pure CH4 hydrate of structure I was formed and coexisted with mixed CH4+neohexane hydrate of structure H in low temperature and high pressure region after passing through the phase boundary of pure CH4 hydrate. We have examined the structure coexistence at 273.6 K and 50 bar with other structure H formers of isopentane, methylcyclopentane, and methylcyclohexane. In case of isopentane, the obtained NMR spectrum showed that structure I and H coexisted and the amount of methane molecules in structure I was two times as many as in cages of structure H. However, there were no resonance lines of structure I when methylcyclohexane formed structure H with methane molecules.  Keywords: Structure coexistence, phase equilibrium, structure H, methane, neohexane                                                         Corresponding author: Phone: +82 42 860 3607 Fax +82 42 860 3134 E-mail: ytseo@kier.re.kr INTRODUCTION Gas hydrates are crystalline materials formed by a physical interaction between host water molecules and relatively light guest molecules [1]. At high pressure and low temperature conditions water molecules are connected by hydrogen bonds and form various cavities capable of entrapping guest molecules. Those non-stoichiometric crystalline compounds are divided into three distinct structures I, II and H, which differ in cavity size and shape [2]. Although the hydrate structures were usually known to be determined by the largest guest molecule in hydrate forming system, the complex structural characteristics indicating the occurrence of metastable structur II Xe hydrate have been reported [3] when a THF hydrate was exposed to hyperpolarized Xe. Recently, we have reported the heterogeneous state of mixed CH4+THF hydrate and liquid water from microimaging observation to follow water Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  consumption [4]. The heterogeneous mixture was transformed to solid mixture of pure CH4 and mixed CH4+THF hydrate when the temperature was decreased to low region of the phase boundary of pure CH4 hydrate. Such issues of complex processes that led to unexpected hydrate structure are of considerable importance, suggesting the requirement of cross exploration through phase equilibrium determination and spectroscopic analysis of hydrate structures. This study had been considered to explore the structural characteristics depending on the hydrate equilibrium conditions of ternary methane, neohexane, and water system. Although normally a single structure is expected to be stable in such systems, here we report the coexistence of structure I and H at different conditions of hydrate formation. Furthermore the similar experiment with other structure H formers showed the effect of molecular shape on determining the distribution of hydrate structures.  EXPERIMENTS To identify hydrate structure, a Bruker 400 MHz solid-state NMR spectrometer was used. Spectra were recorded at 243 K by placing samples within a 4 mm o.d. Zirconia rotor loaded into variable temperature probe. All 13C NMR spectra were recorded at a frequency of 100.6 MHz with magic angle spinning (MAS) at about 2 kHz. The radio frequency field strengths of 70 kHz corresponding to 5 s  90o pulses were used. The adamantane that assigned a chemical shift of 38.3 ppm at 300 K was used as an external chemical shift reference. A detailed method capable of resolving hydrate structures using NMR spectroscopic analysis can be found in literature [5, 6]. The hydrate samples were prepared in a mechanically stirred reactor. The reactor maintained at a constant temperature was filled with the liquid mixtures of water and neohexane, and then subsequently pressurized with pure CH4 gas to higher pressure than the corresponding hydrate equilibrium pressure at the same temperature. By stirring aqueous mixture solid hydrate formation was proceeded through induction, nucleation, and growth periods, successively. When hydrate formation process was completed, the formed hydrates were sampled by keeping both the cell and Zr-rotor immersed in liquid nitrogen to prevent any hydrate dissociation or distortion. This Zr-rotor having hydrate samples was then inserted to the solid-state NMR. RESULTS AND DISCUSSION In general, the phase equilibrium of pure CH4 hydrate was represented as three-phase equilibrium, H-Lw-V, curve and it meant the limit condition for formation and stable existence of hydrate phase. The presence of organic hydrocarbons clearly makes a drastic difference in phase behavior. Figure 1 showed the comparison of hydrate equilibrium conditions for pure CH4 and mixed CH4+neohexane hydrates. The pure CH4 hydrate could be formed at lower temperature and higher pressure conditions than the H-Lw-V curve (region I) and the formed hydrate was known to be structure I composed of 512 and 51262 cages. The addition of neohexane induced a hydrate forming condition to be more favorable by lowering pressure and raising temperature for forming the mixed hydrate when compared with the corresponding condition of pure CH4 hydrate. The equilibrium condition of mixed CH4+neohexane hydrate was represented as four-phase equilibrium, H-Lw-Lhc-V, curve because neohexane was immiscible to liquid water phase. At lower temperature and higher pressure conditions than the H-Lw-Lhc-V curve (region H), the formed mixed CH4+neohexane hydrate was known to be structure H composed of 512, 435663, and 51268 cages [7].  It is noted that the formation condition of pure CH4 hydrate assigned as region I was included into the formation condition of mixed CH4+neohexane hydrate assigned as region H. Accordingly, it is possible to think that the formation of structure I and H should compete each other for better occupation of guest molecules and stabilization of hydrate structure if we consider the hydrate formation in region I of Figure 1. To investigate the relationship between hydrate structure and phase equilibrium, we have formed the hydrate at three different conditions of point A, B, and C as seen in Figure 1. First the hydrate was formed at 273.6 K and 25 bar (point A, region H) to observe the spontaneous formation of structure H hydrate. Secondly, the hydrate was formed at 273.6 K and 50 bar (point B, region I), which was considered as possible region for simultaneous formation of structure I and H. Finally, to investigate the effect of changing the formation region the hydrate was formed again at condition of point B and then the temperature was increased to 283.1 K (point C, region H) after the hydrate formation reaction reached steady-state.  T/ K270 275 280 285 290P/bar020406080P h a s e  b o u n d a r y  f o r  m i x e d  C H4+ n e o h e x a n e  h y d r a t eP h a s e  b o u n d a r y  f o r  p u r e  C H4 h y d r a t eA  ( 2 7 3 . 6 K ,  2 5 b a r )B  ( 2 7 3 . 6 K ,  5 0 b a r ) C  ( 2 8 3 . 1 K ,  2 5 b a r )R e g i o n  HR e g i o n  I Figure 1. Hydrate phase boundary of pure CH4 (structure I, assigned as dotted line) and mixed CH4+neohexane (structure H, assigned as solid line) hydrates. Point A, B, and C represents sample obtaining conditions for 13C NMR spectroscopy.  Figure 2 represented a stacked plot of three 13C NMR spectra obtained at each point, which were used to identify the formed hydrate structures. Neohexane was showed as four distinct resonance lines because it was composed of six carbons and each resonance line was attributed to the individual functional group. However, in this study, 13C NMR spectra was showed from 0 to 10 ppm to focus on the methane molecules occupying the hydrate cages, which provide the structural information of hydrate samples such as structure type and guest distribution in hydrate cages. Figure 2 (a) representing 13C NMR spectrum of hydrate sample formed at point A indicate that the methane molecules are enclathrated into both 512 and 435663 cages of structure H at – 4.5 and – 4.9 ppm, respectively. The stoichiometric ratio of 512 and 435663 cages appears to be 3 : 2 in the unit cell of structure H [5]. The relative integrated intensities of NMR spectrum directly gave the value of occupancy ratio of methane molecules in 435663 cages to those in 512 cages, 22.144 ,, CHSCHM  , confirming the fact that methane molecules in 512 cages was more abundant than those in 435663 cages. The NMR spectrum provides no evidence of methane molecules occupying the 51268 cages, which was known to only accommodate the structure H formers, i.e. neohexane. This result suggested that the spontaneous formation of structure H was possible without an initiating medium to create hydrogen bonding in aqueous phase. However, the hydrate sample formed at point B showed different structural characteristics.   0 -2 -4 -6 -8 - 1 0-4.9-4.5C h e m i c a l  s h i f t  ( p p m )0 -2 -4 -6 -8 -10-6.7-4.9-4.5-4.3C h e m i c a l  s h i f t  ( p p m )0 -2 -4 -6 -8 -1 0-4.9-4.5C h e m i c a l  s h i f t  ( p p m )( a )( b )( c )-4.9-4.5-6.7-4.9-4.5-4.3-4.9-4.5 Figure 2. Stacked plot of 13C NMR spectra for ternary methane, noehexane, and water system. (a) structure H at point A (b) coexistence of structure I and H at point B (c) structure H at point C.  The 13C NMR spectrum in Figure 2 (b) showed two resonance lines at – 4.5 and – 4.9 ppm, which were identified as methane molecules in 512 and 435663 cages of structure H, respectively. The occupancy ratio of 22.144 ,, CHSCHM  was obtained from relative integrated intensities of both resonance lines. In addition, two more resonance lines were also detected at – 4.3 and – 6.7 ppm. Our previous work[8] revealed that both resonance lines came from pure CH4 hydrate because the methane molecules captured into 512 and 51262 cages of structure I was showed peaks at – 4.3 and – 6.7 ppm, respectively. The occupancy ratio appears to be 16.144 ,, CHSCHL  and the corresponding intensity ratio of 49.344 ,, CHSCHL II  is analogues to the stoichiometric ratio of 512 and 51262 cages in structure I. This result strongly suggested that mixed CH4+neohexane hydrate was formed at point B and simultaneously pure CH4 hydrate was also formed and coexisted with mixed hydrate. The 13C NMR spectrum obtained in point C as shown in Figure 2 (c) represents a change of structural characteristics in hydrate phase. There are only two resonance lines identified as methane molecules in 512 and 435663 cages of structure H at – 4.5 and – 4.9 ppm, respectively. The occupancy ratio was 36.144 ,, CHSCHM , which meant that the occupancy of methane in 512 cages decreased slightly than corresponding occupation in point A and B. It is noted that from the NMR spectrum both structure I and H coexisted at point B, but upon warming to point C the structure I was totally dissociated while structure H remained largely intact. Accordingly, it can be concluded from the NMR analysis based on equilibrium conditions that the pure CH4 hydrate formed and coexisted with mixed CH4+neohexane hydrate in low temperature and high-pressure region after passing through the phase boundary of pure CH4 hydrate. At this moment, it must be resolved whether the distribution of hydrate structure should be observed for other hydrocarbons forming structure H with methane molecules. We chose three specific structure H formers of isopentane, methylcyclopentane (MCP), methylcyclohexane (MCH) in order to observe the effect of molecular size and shape. For ternary system of selected hydrocarbon, methane, and water the hydrate sample was formed at 273.6 K and 50 bar, same condition of point B. The obtained 13C NMR spectra for three different structure H formers were shown in Figure 3. Isopentane was known to be one of alkane hydrocarbon to form structure H and as seen in Figure 3 (a) two resonance lines at – 4.5 and – 4.9 ppm revealed the formation of structure H hydrate, which indicate the methane molecules in 512 and 435663 cages. In addition, as discussed in Figure 2 (b), two resonance lines of methane molecules in 512 and 51262 cages of structure I also came from pure CH4 hydrate coexisting with mixed CH4+isopentane hydrate. The amount of methane molecules occupying the cages of structure I was two times as many as methane molecules in cages of structure H, while in case of neohexane the ratio of methane molecules of structure I to H was 0.9. Based on these spectra, there must be preferential distribution of structure I and H in ternary system containing methane and alkane hydrocarbons that could selectively participate in forming both hydrate structures.  0 -2 -4 -6 -8 -1 0-6.7-4.9-4.5-4.3C h e m i ca l  sh i f t  (p p m )0 -2 -4 -6 -8 -1 0-6.5-4.8-4.5C h e m i ca l  sh i f t  (p p m )0 -2 -4 -6 -8 -1 0-4.9-4.5C h e m i ca l  sh i f t  (p p m )( b)( c)( a)-6.7-4.9-4.5-4.3-6.5-4.8-4.5-4.9-4.5 Figure 3. (a) 13C NMR spectra of coexisting pure CH4 and mixed CH4+isopentane hydrates formed at 273.6 K and 50 bar for ternary methane, isopentane, and water system. (b) 13C NMR spectra of mixed CH4+MCP hydrate containing small amount of pure CH4 hydrate formed at 273.6 K and 50 bar for ternary methane, MCP, and water system. (c) 13C NMR spectra of mixed CH4+MCH hydrate formed at 273.6 K and 50 bar for ternary methane, MCH, and water system representing the overall hydrate phase was structure H.  However, following spectroscopic results of cycloalkane represent the different structural characteristics, suggesting the importance of molecular shape of hydrocarbons.  Figure 3 (b) was 13C NMR spectrum of hydrate sample for ternary methane, MCP, and water system. The methane molecules occupying the 512 and 435663 cages of structure H were represented as two resonance lines at – 4.5 and – 4.8 ppm, respectively, while there was only one resonance line indicating methane molecules in 51262 cages of structure I at – 6.5 ppm. The ratio of methane molecules of structure I to H was 0.3, suggesting the hydrate phase was largely structure H and there exist only small amount of structure I even though the resonance line of methane molecules in 512 cages of structure I might superimposed to resonance line of methane molecules in 512 cages of structure H. When the hydrate was formed in ternary system of methane, MCH and water, 13C NMR spectrum, Figure 3 (c), showed only two resonance lines of methane molecules occupying 512 and 435663 cages of structure H at – 4.5 and – 4.9 ppm respectively. There were no resonance lines of methane in cages of structure I. Accordingly, the overall hydrate phase could be identified as structure H. Figure 3 (b) and (c) suggested that the structure H was abundant during the hydrate formation process if cycloalkane hydrocarbons participated in the formation of structure H. The methane molecules played a role as help gas to form mixed hydrate with hydrocarbons and didn’t initiate the formation of structure I. Comparing with the results of linear alkane hydrocarbon, it is possible to suggest that cycloalkane is more preferable compounds to form structure H than linear alkane because the shape of cycloalkane is more available to fill and stabilize the 51268 cages of structure H.   CONCLUSIONS In this contribution, we observed the particular phenomena of simultaneous formation of two different hydrate structures and effect of molecular shape on formation process. Stable coexistence of structure I and H must be considered to develop the exploitation process of natural gas from marine sediments of Gulf of Mexico because the natural gas hydrate deposit was known to be structure H by powder X-ray diffraction studies. Moreover, in the petrochemical processes containing structure H formers, the developing kinetic inhibitors will have to prevent the both hydrate structures of I and H. Although the structural characterization has been carried out for important systems with natural gas components, many of the physical attributes of hydrate structure still remain unknown and need to be identified in detail. More precise and considerable amount of work needs to be done in order to resolve the formation rate of hydrate structures and interaction between two coexisting hydrate structures.   REFERENCES [1] Sloan, ED, Clathrate Hydrates of Natural Gases, 2nd edition; Marcel Dekker : New York, 1998.  [2] Ripmeester, JA, Ratcliffe, CI. The Diverse Nature of Dodecahedral Cages in Clathrate Hydrates As Revealed by 129Xe and 13C NMR Spectroscopy: CO2 as a Small-Cage Guest. Energy & Fuel 1998; 12 (2): 197-200. [3] Moudrakovski, IL, Ratcliffe, CI, Ripmeester, JA. Probing Transient Hydrate Structures with Hyperpolarized 129Xe NMR Spectroscopy: A Metastable Structure II Hydrate of Xe. Angew. Chem. Int. Ed. 2001; 40 (20): 3890-3892.  [4] Seo, YT, Lee, H, Moudrakovski, IL, Ripmeester, JA, Phase Behavior and Structural Characterization of Coexisting Pure and Mixed Clathrate Hydrates. ChemPhysChem. 2002; 4(4): 379-382. [5] Ripmeester, JA, Ratcliffe, CI, Xenon-129 NMR studies of clathrate hydrates: new guests for structure II and structure H. J. Phys. Chem. 1990; 94(25): 8773-8776. [6] Subramanian. S, Kini, RA, Dec, RA, Sloan, ED Jr. Evidence of structure II hydrate formation from methane+ethane mixtures. Chem. Eng. Science. 2000; 55(11): 1981-1999. [7] Ripmeester, JA, Tse, JS, Ratcliffe, CI, Poewll, BM, A new clathrate hydrate structure. Nature. 1987; 325: 135-136. [8] Seo, Y, Lee, H, Phase Behavior and Structure Identification of the Mixed Chlorinated Hydrocarbon Clathrate Hydrates. J. Phys. Chem. 2002; 106(37): 9668-9673.    

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