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Large-Cage Occupancies of Hydrogen in Binary Clathrate Hydrates Dependent on Pressures and Guest Concentrations Takeshi Sugahara,†,‡ Joanna C. Haag,‡ Ashleigh A. Warntjes,‡ Pinnelli S. R. Prasad,§,‡ E. Dendy Sloan,‡ Carolyn A. Koh,*,‡ and Amadeu K. Sum*,‡ DiVision of Chemical Engineering, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, Center for Hydrate Research, Chemical Engineering Department, Colorado School of Mines, Golden, Colorado 80401, and National Geophysical Research Institute, Council for Scientific and Industrial Research, Hyderabad 500007, India ReceiVed: June 11, 2010; ReVised Manuscript ReceiVed: July 27, 2010
Balancing the formation and storage pressure with the storage capacity is one of the most significant steps toward developing H2 storage in hydrates. The large-cage occupancies of hydrogen molecules in tetrahydrofuran (THF), acetone, cyclohexanone (CHONE), and methylcyclohexane (MCH) hydrates were investigated by Raman spectroscopy, volumetric gas release measurement, and X-ray diffraction analysis in a pressure region below the equilibrium pressure of pure H2 hydrates at 255 ( 2 K. The results from the measurements show that H2 molecules occupy the large cage of the structure II THF+H2, acetone+H2, and CHONE+H2 hydrates at the suitable pressures and concentrations of promoter guest species, while H2 molecules do not occupy the largest cage of the structure H MCH+H2 hydrates, even around 70 MPa. The present work reveals that the large-cage occupancy of H2 strongly depends on the pressure and the concentration of promoter guest species. The maximum storage amount of H2 in the acetone+H2 hydrate was 3.6 ( 0.1 wt %, similar to that of THF+H2 hydrate, at 74 MPa and 255 ( 2 K. Acetone is superior to THF and CHONE as the promoter based on the relation between pressure and the large-cage occupancy of H2 molecules. Introduction Gas hydrates are stabilized by guest species in the cavity of cages composed of the hydrogen-bonded water molecules. There are five types of hydrate cages commonly found, in increasing size: pentagonal dodecahedron (512-cage), dodecahedron (435663cage), tetrakaidecahedron (51262-cage), hexakaidecahedron (51264cage), and icosahedron (51268-cage). Three common unit cells (sI, sII,and sH) of gas hydrates are known to form from a few types of hydrate cages depending largely on the size and physical properties of the guest species. For example, the sI unit lattice consists of two 512-cages and six 51262-cages, and the sII unit lattice has sixteen 512-cages and eight 51264-cages. The sH hydrate consists of three different cages: three 512-cages, two 435663-cages, and one 51268-cage. Dyadin et al.1 reported the phase boundaries of the pure H2 hydrate, ice Ih and ice II with H2. Mao et al.2 then elucidated the structure of pure H2 hydrate and the multiple occupancies of H2 in the 512- and 51264-cages. Since the finding of pure H2 hydrates was reported, many researchers have investigated pure H2 hydrates3,4 and other guest (promoter) + H2 hydrates5-14 as a possible H2 storage medium. In particular, tetrahydrofuran (THF) has been used as a promoter molecule, as it can drastically reduce the formation pressure of H2 hydrates.5-9 Other than THF, promoter molecules such as quaternary ammonium salts7,10,11 and amines12,14 have also been found to alleviate the severe formation conditions for hydrates containing H2. One disadvantage of using a promoter molecule is that the promoter * To whom correspondence should be addressed. A.K.S.: phone +1 (303) 273-3873, fax +1 (303) 273-3730, e-mail
[email protected]. C.A.K.: phone +1 (303) 273-3237, fax +1 (303) 273-3730, e-mail
[email protected]. † Osaka University. ‡ Colorado School of Mines. § National Geophysical Research Institute.
molecule itself occupies some of the hydrate cages (e.g., 51264cages in sII) thus restricting H2 to only certain cages (e.g., 512cages). As a result, by increasing the stability of the hydrate, the overall storage capacity of H2 is compromised at concentrations lower than the pure sII H2 hydrate that has an ideal H2 storage amount of 3.81 wt %, assuming the H2 molecules singly occupy the 512-cages with clusters of four H2 in the 51264-cages. Lee et al.6 suggested the possibility of “forcing” H2 and THF molecules to compete for the 51264-cages of sII. With decreasing the THF concentration in the THF hydrate as an initial material, more H2 molecules were able to occupy the 51264-cages, thus increasing the H2 storage to 4.0 wt % (assuming double occupancy of H2 in the 512-cages) at 12 MPa. More recently, an alternative experimental method to that of Lee et al. was suggested, where the initial material was “frozen” THF solutions instead of THF hydrates.15 However, in contrast numerous researchers have reported the maximum amount of H2 stored in THF+H2 hydrates at ∼1.0 wt %8,9,16-20 and confirmed that the H2 amount in the hydrate is independent of the THF concentration.8,9,16-18 Recently, we reported new evidence for the presence of H2 clusters in the 51264-cages of sII at 255 ( 2 K and ∼70 MPa by proposing a new method of hydrate preparation.21 With this method, the H2 storage capacity increases to 3.4 wt % at 72 MPa and depends on the THF concentration below the eutectic composition22 of the THF + water binary system, similar to the result of Lee et al. at 12 MPa.6 Because the tuning effect suggested by Lee et al. is such an attractive concept, investigating the pressure effect on the tuning behavior has been important. Balancing the formation and storage pressures with the storage capacity is one of the most significant steps toward developing H2 storage in hydrates. In the present study, we have investigated the effects of the pressure, promoter [THF, acetone,
10.1021/jp105379x 2010 American Chemical Society Published on Web 08/19/2010
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cyclohexanone (CHONE), or methylcyclohexane (MCH)], and its concentration on the storage capacity of H2 at 255 ( 2 K by Raman spectroscopic measurement and volumetric gas release measurement. Experimental Procedures The solid mixtures of powdered ice (∼180 µm, from deionized water) and powdered solid promoter with desirable concentrations were prepared gravimetrically at liquid nitrogen (LN2) temperature. We used THF (ChromAR 99.8% minimum purity, Mallinckrodt Chemicals), acetone (Technical grade 99.7%, Industrial Chemical Company), CHONE (Reagent grade 99.8%, Mallinckrodt Chemicals), and MCH (Reagent grade 99%, Sigma Aldrich) as the promoter. Approximately 1 g of sample was then loaded into a high-pressure cell quenched in LN2 and then pressurized with H2 (Ultra High Purity 99.999%, General Air) up to about 60 MPa. Once pressurized with H2, the sample was placed in a temperaturecontrolled bath at 255 ( 2 K. As a result of both hydrate formation and temperature change, the pressure of the system was stabilized around 70-74 MPa after approximately 12-24 h. Under this temperature and pressure condition, pure H2 hydrate was not formed without any promoter.20 This agrees with the phase diagram reported by Lokshin et al.3 The cell was kept at 255 ( 2 K for 3-4 days. Once the cell was removed from the bath, it was quenched and kept in LN2 for approximately 15-20 min. The pressure in the cell was then released and allowed to reach atmospheric pressure for analysis. Details of the experimental setup for the hydrate formation have been described elsewhere.8 Raman spectroscopic measurements were performed on a LabRam HR spectrometer (Horiba-Jobin Yvon Instruments) with a 532 nm diode laser as an excitation source providing 6 mW at the sample. The laser was focused onto several points on the sample through a 20× objective with a superlong working distance (N.A. 0.25). There was no significant difference in the Raman spectra among the points on the sample in the concentration range in which H2 molecules occupied the 51264-cages. Scattered light was collected in backscatter geometry to a multichannel charge coupled device detector. The spectral resolution ranged from 0.5 to 0.9 cm-1. All measurements were made at atmospheric pressure (∼0.084 MPa) and LN2 temperature (∼76 K) immediately after the quench period of hydrate samples prepared at 255 ( 2 K. The powder X-ray diffraction (PXRD) pattern was measured on a Siemens D500 diffractometer with cobalt radiation (wavelength 0.1788965 nm) in the θ/2θ scan mode. The measurements were performed in the stepscan mode with a dwell time of 2 s and step size of 0.02°. The PXRD patterns were collected in the range 2θ ) 5° to 65°. The measurements were carried out at atmospheric pressure and 90 K with a small amount of R-Al2O3 powder for calibration. The PXRD pattern indexing and cell refinement were obtained with use of the Checkcell and PowderX programs23 and the initial lattice parameters24 for the refinement. To measure the amount of H2 stored, the prepared hydrate was placed in a highpressure cell at LN2 temperature. After the cell was closed, the hydrate sample was then allowed to dissociate at ambient conditions. Then the cell was connected to a Ruska gasometer (cat. no. 2331-801 Houston, TX) to measure the volume of gas released. In the measurements, both the evaporation of promoter (THF or acetone) and the expansion of the gas volume due to the temperature change from LN2 temperature were taken into consideration. Results and Discussion The PXRD pattern of the sII acetone+H2 binary hydrate formed at 255 ( 2 K and 74 MPa is shown in Figure 1 along
Figure 1. Powder X-ray diffraction patterns of the sII THF+H2 and sII acetone+H2 hydrates with the promoter mole fraction of about 0.005 recorded at ambient pressure and 90 K. The patterns include the contributions from ice (I) and R-Al2O3 (S).
Figure 2. Raman spectra corresponding to the H2 vibrons in the sII THF+H2, sII acetone+H2, sII CHONE+H2, and sH MCH+H2 hydrates prepared at 255 ( 2 K with the promoter mole fraction of about 0.005. All spectra were recorded at 76 K and ambient pressure. Vertical lines indicate ortho and para pairs in the sII hydrates: 1H2/512-cage (1S), 2H2/51264-cage (2L), 3H2/51264-cage (3L), and 4H2/51264-cage (4L).
with that of the sII THF+H2 binary hydrate. The lattice parameter of the sII (cubic, Fd3m) hydrates is a ) 1.717 ( 0.001 nm for THF+H2 hydrate and a ) 1.715 ( 0.001 nm for acetone+H2 hydrate. These lattice parameters obtained in the present study are similar to literature values.24 The structures and PXRD patterns of the sII CHONE+H2 and sH MCH+H2 hydrates have been reported previously.25,26 Figure 2 shows the Raman spectra of H2 vibrons in the sII THF+H2, sII acetone+H2, sII CHONE+H2 hydrates, and sH MCH+H2 hydrate with the promoter mole fraction (x) of ∼0.005 at 255 ( 2 K and 73 ( 2 MPa. All the Raman spectra were recorded at the LN2 temperature and ambient pressure immediately after the quench period of hydrate samples prepared at 255 ( 2 K and 73 ( 2 MPa. The broad peaks detected from 4168 to 4182 cm-1 are attributed to H2 molecules in ice pores.21 In a recent study, Wang et al.27 suggested that the Raman peaks of double H2 occupancy in the 512-cage of sII hydrates are detected in the range of 4140 to 4180 cm-1. All the Raman spectra indicate the 512-cage occupancy of H2 (vibron modes at 4124 cm-1 (ortho) and 4130 cm-1 (para) at LN2 temperature) as reported by Strobel et al.28 The Raman spectra of H2 vibrons in the THF+H2, acetone+H2, and CHONE+H2 hydrates also reveal the 51264-cage occupancy of H2 (there are six peaks of
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Figure 3. Raman spectra corresponding to the H2 vibrons in the sH MCH+H2 hydrates prepared at 255 ( 2 K and 74 MPa with different mole fractions (xMCH) of MCH. All spectra were recorded at 76 K and ambient pressure.
Figure 4. Raman spectra corresponding to the H2 vibrons in the sII acetone+H2 hydrates prepared at 255 ( 2 K and 74 MPa with different mole fractions (xacetone) of acetone. All spectra were recorded at 76 K and ambient pressure.
the ortho and para bands for double, triple, and quadruple H2 clusters in a region from about 4130 to 4155 cm-1 at LN2 temperature28), in addition to the 512-cage occupancy of H2. That is, Raman spectra clearly show that the THF+H2, acetone+H2, and CHONE+H2 hydrates exhibit the multiple occupancy of H2 in 51264-cages, while H2 molecules did not occupy the 51268cage of the sH MCH+H2 hydrate under the temperature and pressure conditions tested in the present study. The absence of H2 in the 51268-cage of the sH MCH+H2 hydrate is independent of the mole fractions of MCH from 0.0029 to 0.0286, as shown in Figure 3. CHONE and MCH cannot form a hydrate without a small molecule, known as a “help-gas”, and it is well-known that H2 plays a role as the help-gas toward the sII CHONE25 and sH MCH26,29 hydrate formations. These results reveal that H2 can occupy the 51264-cage of the sII hydrate, regardless of whether H2 is a help-gas or not toward sII hydrate formation. On the other hand, H2 molecules cannot occupy the 51268-cage of sH hydrates. This behavior is consistent with experimental observations that help-gas molecules (even common help-gas molecules like methane and xenon) are not found to occupy the 51268-cage of sH double hydrates except for a hexagonal structure of simple hydrates at extreme pressures.30,31 Figure 4 shows the Raman spectra of H2 vibrons in the acetone+H2 hydrate prepared with the different acetone concentrations. In the whole composition range, the characteristic
Sugahara et al.
Figure 5. Relation between the storage amount of H2 and the promoter concentrations in the THF+H2 and acetone+H2 hydrates at 255 ( 2 K and 74 MPa.
Raman peaks derived from the acetone molecule trapped in the acetone+H2 hydrate were detected with those from H2 molecules. Without acetone, pure H2 hydrate was not formed under the same temperature and pressure condition.21 All Raman spectra in Figure 4 include peaks at around 4130 to 4155 cm-1, although the peak intensities of H2 vibrons depend on the mole fraction of acetone. The sII acetone+H2 hydrates exhibit the multiple occupancy of H2 in 51264-cage at xacetone ) 0.0058-0.0568. Comparing with concentration dependence of Raman spectra in THF+H2 hydartes,21 H2 molecules can occupy 51264-cages of the sII acetone+H2 hydrates, even at the mole fraction close to the stoichiometric concentration of sII acetone hydrate, unlike the THF+H2 hydrate. This behavior may result from the difference in the phase diagram between the THF + water and acetone + water binary systems. The THF + water binary system has a eutectic point at xTHF ) 0.0106 (272.06 K and 1.1 kPa),22 while the acetone + water binary system has no eutectic point, showing as an incongruent type of phase diagram. Pure sII acetone hydrate is thermodynamically stable below 252 K.32 In the region above 252 K with the xacetone ) 0-0.12227, both the ice phase and the aqueous solution of acetone can coexist similarly to those in the region below the eutectic concentration of the THF + water binary system. The existence of the (supercooled) aqueous phase on the surface of ice particles may be important toward developing the tuning effect. As such, it appears that a correlation exists between the composition ranges of a possible aqueous phase and the tuning effect, although the experimental temperature is below the eutectic temperature in the THF + water system. In addition, the existence of H2 produces the stabilization of sII acetone hydrate, because the sII acetone+H2 binary hydrate is formed at 255 ( 2 K, a temperature higher than 252 K. The results of volumetric gas release measurements in the THF+H2 and acetone+H2 hydrates around 74 MPa are shown in Figure 5. In the case of THF+H2 hydrate (open circles), the storage amount of H2 in THF+H2 hydrate is around 1.0 wt % for the mole fraction close to the stoichiometry of THF hydrate. The amount corresponds to the single occupancy of H2 in the 512-cages of sII THF+H2 hydrate. The H2 storage amount in the present study is about 1.0 wt %, smaller than those reported by Lee et al.,6 who suggested double occupancy of H2 molecules in the 512-cages. The results in the present study, accompanied with the Raman spectra of H2 vibrons, show that the 51264-cage occupancy of H2 depends on the THF concentration, while a single H2 molecule occupies the 512-cage. Decreasing the mole fraction of THF from the stoichiometric composition of THF
Large-Cage Occupancies of H2 in Binary Hydrates
Figure 6. Raman spectra corresponding to the H2 vibrons in the sII THF+H2 hydrates with the mole fractions (xTHF) of THF of about 0.005 prepared at 255 ( 2 K and different pressures. All spectra were recorded at 76 K and ambient pressure.
hydrate to the eutectic concentration of the THF + water binary system, the storage amount of H2 is gradually decreased. In the THF concentration range from 0.0106 to 0.0556, unreacted ice (including H2 in the pores) existed with the H2+THF hydrate in which H2 molecules occupy 512-cages only. Note that the results of volumetric gas release measurement shown in Figure 5 include the storage amount of H2 in ice pores. In this concentration range, three phases (ice, gas, and H2+THF hydrate;all 51264-cages are occupied by THF molecules) coexist. With further decrease in the THF mole fraction, the storage amount of H2 is very sensitive to the THF mole fractions (xTHF). It is significantly increased up to 2.2 wt % H2 with xTHF ) 0.0054 and 3.4 wt % H2 with xTHF ) 0.0050. In the THF concentration region lower than 0.005, the H2 amount in the hydrate begins to decrease again. In this concentration range where H2 molecules occupy the 51264-cages, H2+THF hydrate coexists with only the gas phase. As noted in the Experimental Procedures section, there was no significant difference in the Raman spectra at different sampling spots in this concentration range, except for the Raman peaks corresponding to H2 molecules in the ice phase that were only occasionally detected. The existence of an ice phase with the 51264-cage occupancy of H2 (dependent on THF concentrations) is contrary to the Gibbs phase rule under a constant pressure and temperature condition, because the degree of freedom is 2 if the ice phase coexists with hydrate and gas phases. That is, if the temperature and pressure were fixed, the 51264-cage occupancy of H2 should be independent of THF concentrations. Therefore, either the ice phase or H2+THF hydrates having 51264-cage occupancy of H2 may be under metastable condition. In the acetone+H2 hydrate system (open triangles in Figure 5), the trends between the storage amount of H2 and the mole fraction of acetone are qualitatively similar to that of THF+H2 hydrate. However, the storage amount of H2 in the acetone+H2 hydrate is slightly higher than that in the THF+H2 hydrate in the whole composition range. Note that the concentrations measured have considered the contribution of evaporated acetone, assuming all acetone molecules in the sample were evaporated. The maximum storage amount of H2 in the acetone+H2 hydrate is 3.6 ( 0.1 wt % with xacetone ) 0.0058, while it was around 1.4 ( 0.4 wt % with the mole fraction close to the stoichiometry of acetone hydrate. Figures 6 and 7 show the pressure dependence of the Raman peaks corresponding to H2 vibrons in the THF+H2 and
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Figure 7. Raman spectra corresponding to the H2 vibrons in the sII acetone+H2 hydrates with the mole fractions (xacetone) of acetone of about 0.005 prepared at 255 ( 2 K and different pressures. All spectra were recorded at 76 K and ambient pressure.
TABLE 1: Summary on the Required Pressure That Each H2 Cluster Begins to Occupy the 51264-Cage of sII Hydrates at 255 ( 2 K in the THF+H2 and Acetone+H2 Binary Hydrate Systems with the Mole Fraction of Promoter Molecules (THF or Acetone) of ∼0.005 H2 clusters
THF+H2
acetone+H2
2 H2 3 H2 4 H2
∼40 MPa ∼55 MPa ∼70 MPa
25-30 MPa 25-30 MPa ∼70 MPa
acetone+H2 hydrates at 255 ( 2 K with xTHF and xacetone of about 0.005, respectively. In the THF+H2 hydrate, the Raman peaks corresponding to the double H2 cluster in the 51264-cage begin to appear around 4140 cm-1 at a pressure of about 40 MPa, while the single H2 molecules can occupy only the 512-cage at 16 MPa, as shown in Figure 6. With further increase in the pressure, the Raman peaks of triple and quadruple H2 clusters begin to appear at about 55 and 70 MPa, respectively. Note that these results are in contrast to those reported previously by Lee et al.,6 which claimed that multiple H2 molecules can occupy the 51264-cages even at 12 MPa. In the acetone+H2 hydrate, the Raman peaks corresponding to the double and triple H2 clusters in the 51264-cage appear even at 31 MPa, as shown in Figure 7, while H2 molecules can occupy only 512-cages at 21 MPa. In the pressure region from 31 to 66 MPa, the peak intensities of the double and triple H2 clusters in the large cage gradually increased depending on the pressure. In the pressure region between 66 and 74 MPa, the double and triple H2 cluster peaks drastically increased and the peaks of the quadruple H2 molecules began to appear. The required pressures for each H2 cluster to occupy the 51264-cage of sII in the THF+H2 and acetone+H2 hydrates at 255 ( 2 K are summarized in Table 1. Comparing the pressure effect of the 51264-cage occupancy of H2 between the THF+H2 and acetone+H2 hydrates, the required pressures for the double and triple H2 clusters in the acetone+H2 hydrate are lower than those in the THF+H2 hydrate, while those for the quadruple H2 cluster are about the same. Conclusions The effects of promoter molecules, their concentrations, and the pressure on the large-cage occupancy of H2 in the sII and sH hydrates have been investigated by Raman spectroscopy. The preparation method we proposed previously results in the
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51264-cage occupancy of H2 in the sII THF+H2, acetone+H2, and CHONE+H2 hydrates. On the other hand, H2 was not found to occupy the 51268-cage in the sH MCH+H2 hydrate. The storage amount of H2 in sII hydrates can be tuned by the promoter concentrations and the pressure of H2 in the THF+H2 and acetone+H2 hydrate systems. On the basis of the occupancy of the 51264-cages alone, acetone is a better promoter than THF. Acknowledgment. We acknowledge financial support from the United States Department of Energy, Basic Energy Sciences (DOE-BES) under Contract DE-FG02-05ER46242 (for T.S.) and the funding from NSF-REMRSEC, DMR 0820518 (for P.S.R.P.). A.K.S. acknowledges DuPont for a DuPont Young Professor Award. References and Notes (1) Dyadin, Y. A.; Larionov, E. G.; Aladko, E. Y.; Manakov, A. Y.; Zhurko, F. V.; Mikina, T. V.; Komarov, V. Y.; Grachev, E. V. J. Struct. Chem. 1999, 40, 790. (2) Mao, W. L.; Mao, H. K.; Goncharov, A. F.; Struzhkin, V. V.; Guo, Q. Z.; Hu, J. Z.; Shu, J. F.; Hemley, R. J.; Somayazulu, M.; Zhao, Y. S. Science 2002, 297, 2247. (3) Lokshin, K. A.; He Duanwei, Z. Y.; Mao, W. L.; Mao, H. K.; Hemley, R. J.; Lobanov, M. V.; Greenblatt, M. Phys. ReV. Lett. 2004, 93, 125503. (4) Strobel, T. A.; Sloan, E. D.; Koh, C. A. J. Chem. Phys. 2009, 130, 014506. (5) Florusse, L. J.; Peters, C. J.; Schoonman, J.; Hester, K. C.; Koh, C. A.; Dec, S. F.; Marsh, K. N.; Sloan, E. D. Science 2004, 306, 469. (6) Lee, H.; Lee, J.-W.; Kim, D. Y.; Park, J.; Seo, Y.-T.; Zeng, H.; Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Nature 2005, 434, 743. (7) Hashimoto, S.; Murayama, S.; Sugahara, T.; Sato, H.; Ohgaki, K. Chem. Eng. Sci. 2006, 61, 7884. (8) Strobel, T. A.; Taylor, C. J.; Hester, K. C.; Dec, S. F.; Koh, C. A.; Miller, K. T.; Sloan, E. D., Jr. J. Phys. Chem. B 2006, 110, 17121. (9) Anderson, R.; Chapoy, A.; Tohidi, B. Langmuir 2007, 23, 3440. (10) Chapoy, A.; Anderson, R.; Tohidi, B. J. Am. Chem. Soc. 2007, 129, 746. (11) Shin, K.; Kim, Y.; Strobel, T. A.; Prasad, P. S. R.; Sugahara, T.; Lee, H.; Sloan, E. D.; Sum, A. K.; Koh, C. A. J. Phys. Chem. A 2009, 113, 6415.
Sugahara et al. (12) Prasad, P. S. R.; Sugahara, T.; Sum, A. K.; Sloan, E. D.; Koh, C. A. J. Phys. Chem. A 2009, 113, 6540. (13) Tsuda, T.; Ogata, K.; Hashimoto, S.; Sugahara, T.; Moritoki, M.; Ohgaki, K. Chem. Eng. Sci. 2009, 64, 4150. (14) Ogata, K.; Tsuda, T.; Amano, S.; Hashimoto, S.; Sugahara, T.; Ohgaki, K. Chem. Eng. Sci. 2010, 65, 1616. (15) Seo, Y.; Lee, J.-W.; Kumar, R.; Moudrakovski, I. L.; Lee, H.; Ripmeester, J. A. Chem. Asian J. 2009, 4, 1266. (16) Hashimoto, S.; Sugahara, T.; Sato, H.; Ohgaki, K. J. Chem. Eng. Data 2007, 52, 517. (17) Struzhkin, V. V.; Militzer, B.; Mao, W. L.; Mao, H.; Hemley, R. J. Chem. ReV. 2007, 107, 4133. (18) Mulder, F. M.; Wagemaker, M.; van Eijck, L.; Kearley, G. J. ChemPhysChem 2008, 9, 1331. (19) Ogata, K.; Hashimoto, S.; Sugahara, T.; Moritoki, M.; Sato, H.; Ohgaki, K. Chem. Eng. Sci. 2008, 63, 5714. (20) Nagai, Y.; Yoshioka, H.; Ota, M.; Sato, Y.; Inomata, H.; Smith, R. L., Jr.; Peters, C. J. AIChE J. 2008, 54, 3007. (21) Sugahara, T.; Haag, J. C.; Prasad, P. S. R.; Warntjes, A. A.; Sloan, E. D.; Sum, A. K.; Koh, C. A. J. Am. Chem. Soc. 2009, 131, 14616. (22) Makino, T.; Sugahara, T.; Ohgaki, K. J. Chem. Eng. Data 2005, 50, 2058. (23) The software programs Checkcell and PowderX are available at http://www.ccp14.ac.uk: Checkcell; developed by Laugier, L.; Bochu, B., Laboratoire des Materiaux et du Genie Physique, Ecole Superieure de Physique de Grenoble. PowderX; developed by Cheng, D., Institute of Physics, Beijing. (24) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; Taylor & Francis-CRC Press: Boca Raton, FL, 2008. (25) Strobel, T. A.; Hester, K. C.; Sloan, E. D.; Koh, C. A. J. Am. Chem. Soc. 2007, 129, 9544. (26) Strobel, T. A.; Koh, C. A.; Sloan, E. D. J. Phys. Chem. B 2008, 112, 1885. (27) Wang, J.; Lu, H.; Ripmeester, J. A. J. Am. Chem. Soc. 2009, 131, 14132. (28) Strobel, T. A.; Koh, C. A.; Sloan, E. D. Fluid Phase Equilib. 2007, 261, 382. (29) Duarte, A. R. C.; Shariati, A.; Rovetto, L. J.; Peters, C. J. J. Phys. Chem. B 2008, 112, 1888. (30) Chou, I.; Sharma, A.; Burruss, R. C.; Shu, J.; Mao, H.; Hemley, R. J.; Goncharov, A. F.; Stern, L. A.; Kirby, S. H. Proc. Natl. Acad. Sci. 2000, 97, 13484. (31) Hirai, H.; Uchihara, Y.; Fujihisa, H.; Sakashita, M.; Katoh, E.; Aoki, K.; Nagashima, K.; Yamaoto, Y.; Yagi, T. J. Chem. Phys. 2001, 115, 7066. (32) Wilson, G. J.; Davidson, D. W. Can. J. Chem. 1963, 41, 264.
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