6th International Conference on Gas Hydrates

RAMAN STUDY OF THE METHANE + TBME MIXED HYDRATE IN A DIAMOND ANVIL Englezos, Peter; Desgreniers, Serge; Ripmeester, John A.; Klug, Dennis; Susilo, Robin 2008

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  RAMAN STUDY OF THE METHANE + TBME MIXED HYDRATE IN A DIAMOND ANVIL CELL      Robin Susilo, Dennis D Klug*, and John A Ripmeester Steacie Institute for Molecular Sciences  National Research Council Canada 100 Sussex Drive, Ottawa, ON, K1A 0R6  CANADA  Serge Desgreniers Department of Physics University of Ottawa 150 Louis Pasteur, Ottawa, ON, K1N 6N5  CANADA  Peter Englezos Department of Chemical & Biological Engineering  University of British Columbia 2260 East Mall, Vancouver, BC, V6T 1Z3 CANADA   ABSTRACT It is well known that methane hydrate undergoes several phase transformations at high pressures. At  room  temperature  and  low  to  moderate  pressure,  methane  and  water  form  a  stable  cubic structure I (sI) hydrate that is also known as MH-I. The structure is transformed to a hexagonal phase (sH/MH-II) above 1.0GPa. Another phase transformation occurs above 1.9GPa where the filled ice structure (MH-III) is stable up to 40 GPa before a new high pressure phase transition occurs. Experiments at such high pressures have to be performed in a diamond anvil cell (DAC). Our main interest, though, is to form sH methane hydrate at a lower pressure than reported in previous studies but with some methane in the large cages consequently increasing the methane content.  This  can be  accomplished  by  introducing  the molecules  of  the  large  hydrate forming substance (tert-butyl  methyl  ether/TBME) at  a  concentration  slightly  below  the  stoichiometric amount  as  suggested  by  molecular  dynamics  simulations.  In  this  study  we  have  synthesized mixed methane hydrate of sI and sH and loaded the clathrate with methane into several DACs. Raman spectra were collected at room temperature and pressures in the range of 0.1 to 11.3 GPa. The existence of sH methane hydrate was observed down to 0.2 GPa. However, the existence of methane in the large cages was visible only at pressure higher than 1.0 GPa. The excess methane in the system apparently destabilizes the sH clathrate at pressure below 1.0 GPa as it transforms to sI clathrate.  Keywords: Structure H, hydrate, high pressure, methane, TBME  INTRODUCTION Interest in methane clathrate studies exists mainly due  to  its  importance  and  challenges  related  to energy  and  environmental  applications  [1].  This inclusion  compound  is  found  naturally  under  the permafrost regions, in marine environments and it has  been  postulated  to  exist  in  the  outer  solar system  where  methane  and  water  are  in  contact under suitable pressure and temperature conditions. Synthetic  clathrate  also  can  be  prepared  in  the laboratory  although  the  studies  have  been  carried * Corresponding author: Phone: +1 613 991 1238 Fax: +1 613 947 2838 Email: Dennis.Klug@nrc-cnrc.gc.ca   out mostly at low to moderate pressure conditions due  to  its  practicality.  High  pressure  clathrate studies  are  thus  far  mainly  relevant  to  planetary science only. Hence, it is challenging to see if the high pressure clathrate phases can be stabilized at lower  pressure  so  that  these  can  be  utilized  for practical applications.  It  is  well-known  that  methane  and  water  form  a cubic structure I (sI / MH-I) clathrate under low to moderate  pressure  conditions.  The  existence  of cubic structure II (sII) hydrate been reported only as  a  kinetic  product  that  eventually  transformed back to sI hydrate [2,3]. The existence of methane in  sI  hydrate  has  been  reported  up  to  ~1.0  GPa before it transforms into the hexagonal structure H (sH  /  MH-II)  [4-6].  The  hexagonal  structure  is transformed to a filled ice structure (MH-III) above ~1.9  GPa.  Methane  in  the  filled  ice  structure  is stable  up  to  ~40  GPa  before  it  transforms  into  a denser filled ice structure [7].   It is well known that the low pressure sH clathrate generally  requires  two  different  sizes  of  guest molecule for stability. It is uncommon for such a small  molecule  like  methane  to  fill  a  large  cage. Methane  occupies  the  large  cage  only  at  high pressures  (above  ~1.0  GPa).  Consequently, multiple  methane  molecules  are  required  to provide  the  much  needed  repulsive  forces  and without these the host large cage collapses. It was believed that an occupancy of between two to three molecules  is  more  likely  and  there  is  also  a dependency on the pressure and temperature [8,9]. Since multiple numbers of methane molecules can be incorporated in the large cage of sH hydrate, the methane  storage  density  is  obviously  higher.  The gas content thus can be increased by 40% to 100% depending  on  how  many  methane  molecules  that are captured inside the large cages. The drawback however is that the pressure required is in the giga-Pascal range, which is impractical.   Recent  molecular  dynamics  simulations  have indicated  that the  inclusion  of  small  amount  of  a well  known  sH  hydrate  former  such  as  tert-butyl methyl  ether  (TBME)  in  some  large  cages  may enable the clathrate to be more stable [9] and thus the  pressure  required  for  synthesis  is  expected  to be less. This is exactly the same principle that has been  reported  for  hydrogen  hydrates  where  a significant  pressure  drop  can  be  achieved  by adding a small amount of THF [10,11]. Obviously however, there is a trade off between the hydrate stability and a higher gas content.  The  objective  of  this  study  is  to  verify experimentally the MD simulation results that both methane and the TBME molecule may co-exist in the large cages and the clathrate is more stable at lower pressure than the pure sH methane clathrate. This  is  achieved  by  adding  less  TBME  than  the stoichiometric requirement.    EXPERIMENTAL METHODOLOGY  The low pressure experiments were conducted in a 50  ml  pressure  vessel.  The  clathrate  was synthesized  from  ~5  g  of  finely  ground  ice  with ~0.75 g of TBME. The TBME amount was chosen in such a way that it corresponds to ~80% of the stoichiometry required if all water is converted into hydrate. TBME is also one of  sH hydrate formers that  has  considerable  solubility  in  water  [12]. Synthesis  experiments  were  conducted  at  253  K with the initial starting pressures of 10 MPa and 20 MPa.  Approximately  a  1  MPa  pressure  drop  was observed in both experiments. The solid phase was analyzed  by  X-Ray  diffraction  and  NMR spectroscopy  to  obtain  structural  information  and clathrate composition [13].   It was reported that the kinetics are slow for tuning sII  clathrate  [14].  Hence,  it  is  expected  that  the encapsulation  of  methane  into  the  large  cages  is also  expected  to  be  slow.  The  hydrate  was therefore grown for two to six months in this study to  ensure  that  the  clathrate  composition  was homogeneous  and  high  conversion  was  achieved. In  addition,  the  vessel  was  placed  on  a  ball mill, thus  rotating  the  cell  inside  a  freezer  for  mixing. Three sets of stainless steel bars were placed inside the  vessel  to  create  fresh  surfaces  for  promoting the  clathrate  formation.  The  vessel  was  shaken manually several times to ensure the steel bars did not stick to the wall. The temperature of the freezer was increased to ~ 274 K after two days for several hours  to  speed  up  the  clathrate  conversion  [15] before lowering it back down to 253K. At the end of  the  experiment,  where  no  gas  pressure  change was observed, the clathrate was collected and kept in  liquid  nitrogen  temperature.  The  solid  phase properties  were  also  characterized  by  X-Ray diffraction  and  solid-state  NMR  spectroscopy  to determine  the  crystal  structure  and  composition.   The  experimental  procedure  for  the  solid  state analysis is given elsewhere [13].  Gasketed  diamond  anvil  cells  (DACs)  were employed  for  high  pressure  experiments.  The clathrate  synthesized  at  low  pressure  was  ground into  very  fine  powder  and  then  loaded  into  the DAC kept cold at liquid nitrogen temperature. The loading was performed with extra care to minimize sample contamination with ice. A ruby sphere was inserted into the DAC as a pressure indicator. The pressure  was  obtained  by  measuring  the  ruby luminescence  shift  with  respect  to  room temperature  and  atmospheric  pressure  conditions [16]. Raman spectra were collected at various spots on the sample. Special attention was taken around the C-H (~2900 to 3000 cm-1), O-H (~3000 to 3600 cm-1), and C-C spectral range (~500 to 1000 cm-1).   In  some  experiments,  extra  methane  was  added into the DAC sample chamber. This is because the amount  of  methane  gas  available  in  the  original system is only from the loaded clathrate where all cages  are  singly  occupied.  Hence,  adding  more methane  may  be  needed  when  multiple  methane molecules  go  into  the  large  cages  or  else  some hydrate lattice may decompose into water or a high pressure  ice  phase,  depending  on  the  pressure condition and temperature conditions. The methane gas  was  liquefied  by  condensing  the  gas  inside copper  coil  tubing  that  was  immersed  in  liquid nitrogen. Before adding the methane, the clathrate and  the  ruby  were  loaded  first  and  the  cell  was closed up to ensure that the clathrate stayed in the DAC.  Then  the  DAC  was  opened  and  some clathrate  sample  was  removed  from  the  DAC  to allow  enough  space  for  the  liquid  methane. Subsequently, the DAC was immersed in the pool of liquid methane for about 10-20 seconds before it was finally closed to trap extra methane.    RESULTS AND DISCUSSION Methane-TBME Clathrate at low pressure up to 20 MPa The  solid  phase  analyses  of  the  low  pressure clathrate are summarized in Table 1. The samples contained ~60% sH and 40% sI clathrate and the clathrate content was higher than >90%. The lattice constants  were  slightly  bigger  than  our  previous measurements [13] because of higher temperature from the current XRD measurements. Cage occupancy  Struc-ture Lattice constant   Pressure  thetaS  thetaM  thetaL ~10 MPa  0.68  -  1.00 sI  a = 12.00 (2) ~ 20 MPa  0.69  -  1.00 ~10 MPa  0.83  0.88  1.00 sH  a = 12.38 (2)   c = 10.10 (4)  ~ 20 MPa  0.79  0.96  1.00 Table 1. Solid state analysis of synthesized mixed methane and TBME clathrate.   Methane  occupied  both  cages  in  sI  but  only  the small and medium cages were filled by methane in sH clathrate as shown in Fig. 1. The occupancies of methane in the small cages of sI clathrate were smaller  than  for  pure  methane  sI  clathrate.  The methane  occupancies  in  the  medium  cages  of  sH are  also  larger  and  increase  with  pressure. However,  none  of  those  samples  contained methane  in  the  large  cages  of  sH  clathrate.  The TBME molecule is likely to occupy the large cage under low pressure conditions although the amount of TBME was less than the stoichiometric required. Hence, the subsequent experiment was  conducted in a DAC at higher pressure.   80 60 40 20 0 -20Intensity / a.u.Chemical shift / ppm0 -2 -4 -6 -8 -10sI: 512sI: 51262sH: 512sH: 435663 Figure 1. 13C solid-state NMR spectra of clathrate synthesized at low pressure. The methane signals are magnified. Other strong signals correspond to TBME in clathrate phase.    Methane-TBME Clathrate at high pressure up to 11 GPa  The methane-TBME clathrate that was synthesized at  20  MPa  was  used  for  the  high  pressure experiments. Raman spectra from the sH and filled ice regions at various pressures are summarized in Figure 2 and 3.    2800 2900 3000 3100 3200 3300 3400 3500 360050000100000150000200000250000Counts / a.u.Wavenumber / cm-10.80 GPa0.20 GPa0.50 GPa0.60 GPa1.98 GPa1.05 GPa2.14 GPa1.36 GPa1.59 GPa2.25 GPa1.96 GPa1.68 GPa******* Figure 2. Raman spectra of mixed methane+TBME sH clathrate in the DAC at room temperature.    2800 2900 3000 3100 3200 3300 3400 3500 360050000100000150000200000250000Counts / a.u.W avenumber / cm-1**3.42 GPa2.69 GPa2.90 GPa3.15 GPa7.30 GPa4.16 GPa7.74 GPa4.41 GPa5.04 GPa8.37 GPa6.91 GPa5.84 GPa Figure 4. Raman spectra of filled ice methane and ice VII in the DAC at room temperature.   Some  spectra  were  obtained  while  increasing  or decreasing the pressure, however they generally do not  differ.  The  spectra  obtained  while  increasing the  pressure  are  indicated  by  the  green  star.  The methane  peak  is  always  the  most  intense  one among  other  peaks.  In  fact,  the  peaks  from  the water and TBME are almost invisible at pressures below  2  GPa  compared  to  the  methane  peak. Hence,  the  O-H  stretching  from  the  ice  VI  and clathrate  in  Fig.  2  are  hard  to  see.  Those  peaks become more intense and visible only at pressures above  2  GPa.  The  arrows  in  Figs.  2  and  3  are provided  to  help  identify  the  peaks  from  TBME and  water  with  pressure.  The  pink  arrow corresponds to the C-H bond of TBME. The dark and light blue arrows are from the ice VII phase. The  Raman  shift  usually  increases  with  pressure for the C-H bond but goes in the opposite direction for the O-H bond. A strong signal is also seen in all spectra  at  ~3569  cm-1  that  comes  out  of  the reflection  from  the  aluminum  stage  and  is  most likely  due  to  stray  laser  light.  It does  not  change with pressure and hence it is used as a reference for the Raman shift.   The  existence  of  mixed  sH  methane+TBME clathrate  was  observed  down  to  0.2  GPa  in  our high  pressure  experiments.  This  is  a  substantial reduction  from  the  pure  sH  methane  clathrate (MH-II). However, it is difficult to determine if the clathrate is still stable below 0.2 GPa because it is close  to  the  lower  pressure  limit  for  this  DAC experiment. Moreover, the methane peak from the large cage of sH clathrate is not clearly seen below 1.0  GPa.  Only  one  methane  peak  is  visible  that presumably  is  from  both  small  (512)  and  medium (435663)  cages  of  the  sH  hydrate,  which  are indistinguishable  [13].  Consequently,  the  large cages  are  presumably  occupied  by  TBME molecules only. The methane peak from the large cages is evident only at higher pressures above 1.0 GPa.  Fig.  2  indicates  the  appearance  of  broad shoulders  at  the  higher  frequencies  that  are  from the methane in the large cages, as also reported by Kumazaki et al. [6]. The intensity ratio between the methane  Raman  peak  from  the  large  to (small+medium)  cages  are  found  anywhere between ~0.4 to 0.7 as it is dependent on pressure. Hence,  this  suggests  that  there  are  two  to  four methane molecules occupying a large cage.  In most cases, the initial pressure after loading was above 2.0 GPa, which is in the stability region of filled  ice  methane  structure  (MH-III).  Hence,  the subsequent  experimental  step  would  be to  reduce the pressure and record the spectra in order to see the low pressure sH  clathrate. In one  experiment, we successfully arrived at 0.57 GPa after loading and for this experiment the presence of both mixed sI and sH clathrate was observed. This is because initially  the  sample  contained  both  clathrate structures. However, the sI methane clathrate (MH-  I) was transformed into sH clathrate (MH-II) upon increasing the pressure above 1.0 GPa. Once the sI clathrate  was  gone,  reducing  the  pressure  below 1.0 GPa did not reform the sI clathrate but the sH clathrate remained although it was in the stability region of pure sI clathrate.  This suggests that the sH clathrate with TBME is more stable or perhaps driven by faster kinetics than the pure sI methane clathrate  to  form.  That  is  likely  the  reason  that mixed  sH  clathrate  was  observed  below  1.0  GPa without any sI clathrate.   y = 6.6214x + 2909.5R2 = 0.9744y = 16.287x + 2902.8R2 = 0.9017y = 8.4803x + 2916.4R2 = 0.9909y = 11.702x + 2919.9R2 = 0.98772900292029402960298030003020304030600.0 2.0 4.0 6.0 8.0 10.0 12.0Pressure / GPaWavenumber / cm-1MH-II (small cage) MH-II (large cage)MH-IIIsolid methaneLinear (MH-II (small cage) )Linear (MH-II (large cage))Linear (MH-III)Linear (solid methane) Figure 4. Raman shift change with pressure.  Fig. 4 summarizes the Raman shifts of all methane Raman  signals  with  respect  to  pressure.  The highest  wavenumber  is  from  the  solid  methane. The methane in all clathrate phases is shifted to a lower  frequency  than  that  of  pure  methane (gas/liquid/solid).  The  slopes  differ  among different phases. Hence, the change of slope from the  Raman  shift  with  pressure  can  be  used  to identify  the  phase  stability  and  change.  Initially, the methane peak in the two smaller cages of sH clathrate were observed at low pressure before the methane in the large cages became visible at ~1.0 GPa.  At  around  ~2.0  to  ~2.2  GPa,  the  phase transformation  occurs  from  sH  to  filled  ice structure that is stable up to 11 GPa.   The Raman shift corresponding to TBME and O-H bonds  are  shown  in  Figure  5.  Unfortunately,  the TBME  peaks  below  ~2.0  GPa  (bottom)  are  so weak and hence, the identification of TBME in the clathrate phase is only visible by looking at the C-C stretching vibration peaks. There are many C-C peaks  from  the  TBME  but  they  are  mostly  weak peaks  except  one  at  around  ~700  cm-1  that  is plotted in Figure 5 (top). As seen, there is a change of  slope  from  this  strong  C-C  signal  of  TBME observed at around ~2.0 GPa that is consistent with the observable phase transformation from the sH to filled ice structures. Hence, the TBME participated in  the  sH  clathrate  as  well  as  the  methane.  This confirms  the  co-existence  of  both  methane  and TBME in the large cage of sH hydrate and explains why  the  sH  clathrate  is  stable  at  lower  pressure than  the  pure  sH  methane  clathrate.  The  Raman shift change from the O-H bond of ice VII (blue) has similar negative slope with pressure.  y = 10.27x + 721.98R2 = 0.99y = 3.52x + 734.27R2 = 0.94720725730735740745750755760765770Wavenumber / cm-1y = -30.71x + 3464.65R2 = 0.87y = -32.13x + 3379.55R2 = 0.99y = 19.17x + 2964.42R2 = 0.9430003050310031503200325033003350340034500 1 2 3 4 5 6 7 8 9 10Pressure / GPaWavenumber / cm-1 Figure 5. Raman shift change from C-C bonds of TBME (top/pink), O-H bonds of ice VII (blue) and C-H bonds of TBME (bottom/pink) with respect to pressure.  Two more experiments were conducted by adding extra  methane  along  with  the  methane-TBME clathrate into the DAC after loading the clathrate. The  Raman  spectra  from  the  sample  at  pressures above ~2.0 GPa do not change with the presence of excess methane. The excessive amount of methane may in fact dominate and bury the methane peak from the clathrate phase underneath because of its high  concentration  in  different  regions  of  the sample.  Four  representative  Raman  spectra  are shown in Fig. 6.    2800 2900 3000 3100 3200 3300 3400 3500 360035000400004500050000550006000065000Counts / a.u.Wavenumber / cm-12.08 GPa2933MH-III2800 2900 3000 3100 3200 3300 3400 3500 360035000400004500050000550006000065000Counts / a.u.Wavenumber / cm-12.08 GPa2933MH-III2800 2900 3000 3100 3200 3300 3400 3500 360030000350004000045000500005500060000Counts / a.u.Wavenumber / cm-11.57 GPa2927 2941MH-IISolid methane (I)2800 2900 3000 3100 3200 3300 3400 3500 360030000350004000045000500005500060000Counts / a.u.Wavenumber / cm-11.57 GPa2927 2941MH-IISolid methane (I)2800 2900 3000 3100 3200 3300 3400 3500 3600400004500050000550006000065000700007500080000Counts / a.u.Wavenumber / cm-10.68 GPa29052917MH-IFluid methane2800 2900 3000 3100 3200 3300 3400 3500 3600400004500050000550006000065000700007500080000Counts / a.u.Wavenumber / cm-10.68 GPa29052917MH-IFluid methane2800 2900 3000 3100 3200 3300 3400 3500 36004000050000600007000080000Counts / a.u.Wavenumber / cm-11.67 GPa2928MH-II2840 2880 2920 2960 30004000050000600007000080000Counts / a.u.Wavenumber / cm-12800 2900 3000 3100 3200 3300 3400 3500 36004000050000600007000080000Counts / a.u.Wavenumber / cm-11.67 GPa2928MH-II2840 2880 2920 2960 30004000050000600007000080000Counts / a.u.Wavenumber / cm-1 Figure 6. Raman spectra around methane spectral region with excess methane.  The  top  spectrum,  recorded  at  2.08  GPa corresponds to that of the filled ice structure (MH-III)  with  ice  VII  before  it  transformed  to  the expected sH clathrate (MH-II) at 1.67 GPa. Upon releasing the pressure, the ice VII was transformed to  ice  VI  however  the  methane  peak  from  the smaller cages of sH clathrate were not seen. Only one strong peak was observed at 2928 cm-1 which supposedly belong to methane present in the large cages of sH clathrate (see the inset).   Luminescence  arising from  the  diamond  anvils  is detected in the same spectral range as the Raman signal from the C-C vibrations that have impeded the  confirmation  of  the  presence  of  the  TBME molecules in the mixed clathrate phase. Because of the  extra  methane,  the  solid  methane  peak  may also be observed in some spots as shown in Fig. 6. Interestingly,  the  sH  clathrate  was  not  stable  at pressures  below  ~1.0  GPa  where  it  was transformed into sI methane clathrate (MH-I). This is  seen  by  the  presence  of  methane  peak  at  2905 cm-1 that corresponds to methane in the large cage of  sI  clathrate  and  2917  cm-1  that  corresponds  to fluid methane at 0.68 GPa. The methane peak from the  small  cage  is  buried  under  the  fluid  methane peak.  2860 2880 2900 2920 2940 2960 2980Counts / a.u.Wavenumber / cm-1Clathrate only Clathrate + excess methaneMethane in small cageat ~2920 cm-1Methane in large cageat ~2928 cm-1 Figure  7.  Raman  spectra  in  sH  clathrate  region with and without excess methane at 1.68 GPa.  Fig.  7  shows  the  comparison  between  Raman spectra  of  this  clathrate  with  and  without  the addition  of  extra  methane  at  the  same  pressure (1.68  GPa).  As  seen,  the  spectra  with  clathrate loading  only  has  a  stronger  methane  signal  at ~2920  cm-1  from  the  small  and  medium  cages where the signal from the large cages appear only   as a shoulder at ~2928 cm-1. On the other hand the spectrum  of  the  clathrate  loaded  with  excess methane  shows  a  strong  signal  exactly  at  the position where methane from the large cage of sH clathrate is located. No shoulder is seen at around ~2920  cm-1.  This  is  rather  surprising  because  the molecular  dynamics  simulation  performed  at  the same  conditions  indicates  that  the  sH  clathrate  is not stable when all the small and medium cages are empty.  Hence,  there  is  a  good  possibility  that another  type  of  clathrate  structure  may  exist.  To identify this structure will remain as future work.  CONCLUSIONS Experiments  were  conducted  to  explore  the possibility  of  increasing  the  methane  content  in structure H clathrate. In order to minimize the high pressure required to stabilize the structure H, tert-butyl  methyl  ether  (TBME)  as  the  large  guest molecule  was  employed.  The  amount  of  TBME used was ~80% from the stoichiometric required to allow  methane  occupying  the  remaining  large cages. It was found that the clathrate consisted of mixed  sI  and  sH  with  the  ratio  of  2:3  when synthesized  at  low  pressure  up  to  20  MPa.  The occupancies of the cages were measured by solid-state  NMR  spectroscopy  and  no  methane  in  the large  cages  was  found.  The  clathrate  was  then loaded  into  several  diamond  anvil  cells  (DACs) with and without the addition of extra methane.   The  high  pressure  experiments  carried  out  in DACs were conducted between 0.1 to 11 GPa and room  temperature.  The  experiments  performed without  adding  more  methane  indicate  that  the phase  transition  of  methane  clathrate  from structure I (MH-I) to filled ice structure (MH-III) is practically  the  same  as  for  the  pure  methane hydrate. However, the mixed structure H clathrate of methane and TBME is more stable than pure sH methane  clathrate  (MH-II)  with  decreasing  the pressure. The existence of the mixed sH clathrate was observed down to 0.2 GPa. Unfortunately, the presence  of  methane  in  the  large  cages  of  sH clathrate was only visible at pressures greater than 1.0  GPa.  Nevertheless,  the  occurrence  of  both methane  and  TBME  in  the  large  cages  of  sH clathrate  has  been  demonstrated.  The  addition  of methane  did  not  seem  to  increase  the  methane occupancy in the large cages and stabilize the sH clathrate at pressure below 1.0 GPa.   ACKNOWLEDGEMENTS The  financial  support  from  Natural  Sciences  and Engineering  Research  Council  of  Canada (NSERC)  is  greatly  appreciated.  Robin  Susilo gratefully  acknowledges  financial  support  from Canada Graduate Scholarship (CGS).   REFERENCES [1] Koh CA, Sloan ED. Natural gas hydrates: Recent advances and challenges in energy and environmental applications. AIChE J. 2007; 53:1636-1643. [2] Chou IM, Sharma A, Burrus RC, Shu J, Mao H, Hemley RJ, Goncharov AF, Stern LA, Kirby SH. Transformation in Methane Hydrate. PNAS 2000; 97: 13484-13487. [3] Schicks JM, Ripmeester JA. The coexistence of two different methane hydrate phases under moderate pressure and temperature conditions: kinetic versus thermodynamic Products. Angew. Chem.  2004; 43: 3310-3313. [4] Loveday JS, Nelmes RJ, Guthrie M, Belmote SA, Allan DR, Klug DD, Tse JS, Handa YP. Stable methane hydrate above 2 GPa and the source of Titan's atmospheric methane. Nature 2001; 410: 661-663. [5] Loveday JS, Nelmes RJ, Klug DD, Tse JS, Desgreniers S. Structural systematics in the clathrate hydrates under pressure. Can. J. Phys. 2003; 81: 539-544. [6] Kumazaki T, Kito Y, Sasaki S, Kume T, Shimizu H. Single-crystal growth of the high-pressure phase II of methane hydrate and its Raman scattering study. Chem. Phys. Lett. 2004; 388: 18-22.  [7] Machida S, Hirai H, Kawamura T, Yamamoto Y, Yagi T. A new high-pressure structure of methane hydrate surviving to 86 GPa and its implications for the interiors of giant icy planets. Physics of the Earth and Planetary Interiors 2006; 155: 170-176.  [8] Alavi S, Ripmeester JA, Klug DD. Molecular dynamics study of the stability of methane structure H clathrate hydrates. J. Chem. Phys. 2007; 126: 124708. [9] Susilo R, Alavi S, Ripmeester JA, Englezos P. Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation. Fluid Phase Equilibria 2008; 263: 6-17. [10] Florusse LJ, Peters CJ, Schoonman J, Hester KC, Koh CA, Dec SF, Marsh KN, Sloan ED.   Stable low-pressure hydrogen clusters stored in a binary clathrate hydrate. Science 2004; 306: 469-471. [11] Lee H, Lee J, Kim DY, Park J, Seo YT, Zeng H, Moudrakovski IL, Ratcliffe CI, Ripmeester JA. Tuning clathrate hydrates for hydrogen storage. Nature 2005; 434: 743-746. [12] Susilo R, Lee JD, Englezos P. Liquid-liquid equilibrium data of water with neohexane, methylcyclohexane, tert-butyl methyl ether, n-heptane and vapor?liquid?liquid equilibrium with methane. Fluid Phase Equilibria 2005; 231: 20-26. [13] Susilo R, Ripmeester JA, Englezos P. Characterization of gas hydrates with PXRD, DSC, NMR, and Raman spectroscopy. Chem. Eng. Sci. 2007; 62: 3930-3939. [14] Seo YT, Kim DY, Lee H, Lee J Moudrakovski IL, Ripmeester JA. Molecular behavior and guest distribution in the cages of CH4 + THF double hydrate. Proc. Fifth Int. Con. Gas Hydrates 2005; Vol. II: 568-572, Trondheim, Norway.  [15] Susilo R, Ripmeester JA, Englezos P. Methane conversion rate into structure H hydrate crystals from ice. AIChE J. 2007; 53: 2451-2460. [16] Piermarini GJ, Block S, Barnett JD, Forman RA. Calibration of the pressure dependence of the R1 ruby fluorescence line to 195 kbar. J. Appl. Phys. 1975; 46: 2774-2780. 

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