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ARTICLES Methane Hydrate Behavior under High Pressure Hisako Hirai,*,† Tadashi Kondo,‡ Masashi Hasegawa,‡ Takehiko Yagi,‡ Yoshitaka Yamamoto,# Takeshi Komai,# Kazushige Nagashima,# Mami Sakashita,§ Hiroyuki Fujihisa,§ and Katsutoshi Aoki§ Institute of Geoscience, UniVersity of Tsukuba, Tsukuba, Ibaraki 305, Japan, Institute of Solid State Physics, Tokyo UniVersity, Roppongi, Minato-ku, Tokyo 106, Japan, National Institute for Resources and EnVironment, Tsukuba, Ibaraki 305, Japan, and National Institute of Material Chemistry, Tsukuba, Ibaraki 305, Japan ReceiVed: July 28, 1999; In Final Form: NoVember 10, 1999
Phases changes in a water-methane system were investigated in a pressure range from 0.2 to 5.5 GPa using a diamond anvil cell. In-situ X-ray diffractometry and optical microscopy revealed methane hydrate behavior from growth to decomposition into high-pressure ice and solid methane at room temperature. Methane hydrate crystallized at 0.2-0.3 GPa from liquid, and it was compressed continuously until 2.3 GPa, maintaining structure I. Below 0.7 GPa the cage occupancy was unchanged. At 1.5 GPa methane hydrate partly decomposed to ice IV and fluid methane. The remaining methane hydrate kept structure I, but the cage occupancy was changed; i.e., small cages became vacant. At 2.1 GPa, coexisting ice VI transformed to ice VII and fluid methane solidified to phase I, while methane hydrate remained. At this pressure, structure I of methane hydrate was still maintained, and an additional change of cage occupancy occurred. The change in the cage occupancy is consistent with the change in compressibility observed on the compression curve. At 2.3 GPa, all of the methane hydrate decomposed into ice VII and phase I of solid methane.
Introduction Methane hydrate, a clathrate compound consisting of host water molecules and included guest molecules is found globally in oceanic deposits.1 Growth and decomposition of methane hydrate could cause serious environmental problems because methane is a more effective greenhouse gas than carbon dioxide.2 At the same time, methane hydrate is expected to be an abundant natural resource. Investigation of methane hydrate under high pressure, therefore, is of great importance in understanding the basic science of the water-methane system, as well as in finding a possible solution for urgent applied problems. The existence of gas hydrates has long been known, and a vast number of clathrate hydrates containing inert gases and organic molecules,2,3 even hydrogen,4,5 have been synthesized. The typical crystal structures for these clathrate hydrates are known as structures I6 and II,7 and several other structures have also been reported on the basis of experiments and theoretical calculations3,8,9,10. These previous studies focused mainly on such subjects as phase relations between water and guest,2,11 crystal structures or cage occupancy,12-15 physical properties,2,3,16 and the nature of hydrogen bonding.4 Most of these studies were made in the low-temperature region, with only a few studies made at room temperature and pressures above 1 GPa.4,17 * To whom correspondence should be addressed. Fax: +81-298-533990. E-mail:
[email protected]. † Institute of Geoscience, University of Tsukuba. ‡ Institute of Solid State Physics, Tokyo University. # National Institute for Resources and Environment. § National Institute of Material Chemistry.
It is known that the size of the guest molecule is the controlling factor in selecting between structures I and II in these hydrates.2,3 However, in some of the nitrogen, air and inert gas hydratessthose in which a peculiar cage occupancy has been observedsthe size of the guest molecule does not seem to determine the structure.12,15 In addition, some of these hydrates were formed at higher pressure.15 Therefore, to establish a general rule for determining hydrate structures, it is indispensable to consider the effects of both pressure and the resulting changes in cage occupancy. In the present study, a single phase of methane hydrate was synthesized in a diamond anvil cell (DAC) under very high pressure and room temperature. Methane hydrate behavior, from growth to decomposition, as well as the compression process associated with changes in cage occupancy was observed at up to 5.5 GPa by optical microscopy and by in-situ X-ray diffractometry using synchrotron radiation. Experimental Procedure High-pressure experiments were performed using a lever- and spring-type DAC. To control the pressure lower than 2 GPa, very soft springs were used. Pressures were measured by the ruby florescence method. The starting material was methane hydrate powder prepared by a conventional ice-gas interface method at 15 MPa and -10 °C. This material consisted of almost pure methane hydrate and contained a maximum of a few vol % of ice Ih. A single phase of methane hydrate was then prepared in the following manner. Prior to sampling, the diamond anvil and gasket were cooled by liquid nitrogen. The
10.1021/jp9926490 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/28/2000
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Figure 1. Optical micrographs and XRD patterns at 0.4 (a), 1.5 (b), 2.1 GPa (c), and 2.3 GPa (d). (a) All diffraction lines are assigned to those of structure I. (b) Coexistence of ice VI, structure I, and methane phase. Spotty diffraction lines correspond to those of ice VI, and the other lines correspond to structure I. The relative intensities are changed from (a). Especially, the 110 diffraction line observed at the lowest angle in (a) has disappeared. (c) Coexistence of ice VII, structure I, and phase I of solid methane. Strong spotty lines are from ice VII, and the diffraction of solid methane appeared. Others are from structure I, and the relative intensities are further changed. (d) Only ice VII and phase I of methane are observed. Bar scales are 100 µm.
sampling was carried out in a vessel cooled by liquid nitrogen to eliminate frost deposition. The gasket hole (sample chamber) was filled with starting material and sealed with the anvils. While the DAC was being warmed to room temperature, the sample melted and became a homogeneous liquid without removal of gaseous methane. By increasing the pressure slightly at room temperature, a single phase of methane hydrate was formed. A rapid increase of pressure results in formation of finegrained polycrystalline aggregates, while a slow increase results in the growth of larger crystals. By cycling the pressure slightly up and down while the liquid and methane hydrate coexisted, a single crystal of methane hydrate was fabricated. To obtain a uniform powder X-ray diffraction pattern, a fine-grained polycrystalline sample was prepared. Changes in microtexture and
phase were observed by optical microscopy and X-ray diffractometry (XRD). Results At pressures between 0.2 and 0.3 GPa, methane hydrate and liquid coexisted, and at 0.3 GPa the entire sample crystallized into methane hydrate. The XRD pattern obtained exhibited complete agreement with that of structure I.6 Methane hydrate was compressed continuously to approximately 0.7 GPa. The cell parameters of the cubic unit cell measured at 0.26, 0.30, 0.38, and 0.70 GPa were 1.188, 1.184, 1.172, and 1.141 nm, respectively. The relative intensities of the XRD, as well as the observed microtextures, remained almost unchanged within this pressure range. Figure 1a shows a typical optical micrograph
Methane Hydrate Behavior under High Pressure
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TABLE 1: Comparison of the Observed d Values and Relative Intensities at 0.4 GPa with Those Calculated for Structure Ia h k
l
1 1 2 0 2 1 2 1 3 1 2 2 3 2 3 2 4 0 4 1 3 3 4 2 4 2 3 3 4 2 4 3 5 1 5 2 5 3 5 3 6 0 6 1 5 3 6 2 5 4
0 0 0 1 0 2 0 1 0 0 0 0 1 2 2 0 0 0 0 1 0 0 2 0 0
d-cal int-cal d-obs a0 ) 1.17 nm X,Y ) 1.0 a0 ) 1.17 nm int-obs
4 1 1
4 3 2 4 3 3 4 4 2 6 1 1
0.8273 0.5850 0.5232 0.4777 0.3700 0.3377 0.3245 0.3127 0.2925 0.2838 0.2758 0.2616 0.2553 0.2494 0.2388 0.2340 0.2295 0.2173 0.2007 0.1978 0.1950 0.1923 0.1898 0.1850 0.1827
5 5 24 15 2 33 35 100 8 25 13 1 11 4 1 2 1 9 55 18 19 4 40 7 6
0.8215 0.5832 0.5217 0.4766 0.3693 0.3372 0.3243 0.3124 0.2927 0.2838 0.2755 0.2577 0.2552 0.2497 0.2367 0.2341 0.2295 0.2173 0.2008 0.1978 0.1950 0.1926 0.1900 0.1852 0.1826
5 4 24 13 2 3 34 100 10 24 13 1 10 4 2 3 2 9 52 12 12 3 18 4 4
a The values of d-cal and int-cal are those for a unit cell parameter of a0 ) 1.17 nm and for cage occupancies of X ) 1.0 and Y ) 1.0, respectively.
and XRD pattern of a sample at 0.4 GPa. Table 1 shows a comparison of the observed d values and relative intensities with those calculated on the basis of the crystal data,6 indicating good agreement between the two. At the pressure of 1.5 GPa, some of the methane hydrate decomposed to ice VI and a methane phase (Figure 1 b). The spotty diffraction pattern observed in Figure 1b-2 can be attributed to ice VI. All the diffraction lines of the remaining methane hydrate were attributed to structure I, but the relative intensities changed from those observed at lower pressures, indicating a change in cage occupancy discussed below. The unit cell parameter was 1.125 nm (V/V0 ∼ 0.86, V0 is for 0.26 GPa). No diffraction pattern corresponding to solid methane was observed, suggesting that methane was in either an amorphous or a fluid state.18,19 Many small patches appeared simultaneously at decomposition (Figure 1b-1). These patches were observed to be mobile, suggesting a fluid state, and probably corresponded to the methane phase. At the pressure of 2.1-2.2 GPa, ice VI transformed to ice VII, and a diffraction pattern corresponding to solid methane, phase I, appeared. At this time, some structure I of methane hydrate continued to survive (Figure 1c-1,2). The unit cell parameter was 1.123 nm. The cell parameter was almost the same as that at 1.5 GPa, but the relative intensities differed from both those at 1.5 GPa and those at 0.3-0.7 GPa, suggesting an additional change in cage occupancy. The small patches seemed to increase slightly with increasing pressure (Figure 1c-1). At the pressure of 2.3 GPa, the XRD pattern showed the presence of only ice VII and phase I solid methane, indicating complete decomposition of methane hydrate (Figure 1d-1,2). Until up to the pressure of 5.5 GPa no change was observed in XRD or microtexture. The lattice parameters of ice VI at 1.5 and 2.0 GPa were a0 ) 0.8102, b0 ) 0.6046, c0 ) 0.8790 nm and a0 ) 0.8039, b0 ) 0.5960, c0 ) 0.8734 nm, respectively. Those of ice VII at 2.1 and 2.3
Figure 2. Variation of volume ratio with pressure. The methane hydrate was continuously compressed until 2.3 GPa. The change in compressibility was observed between 0.7 and 1.5 GPa.
GPa were a0 ) 0.3363 and a0 ) 0.3350 nm, respectively. Comparing those lattice parameters with compression curves of pure ice VI and ice VII, any deviations suggesting trapping methane molecules in both ice structures could not be found, although methane molecules are likely to be trapped. The phase changes observed with increasing pressure occurred reversibly with decreasing pressure. The methane hydrate reverted to a homogeneous liquid at approximately 0.3 GPa, and large bubbles appeared in the liquid when the pressure was decreased further. Discussion With increasing pressure, the following phenomena were observed: coexistence of methane gas and liquid, crystallization of methane hydrate from liquid, continuous compression (Figure 1a), partial decomposition to ice VI and fluid methane (Figure 1b), further transition to ice VII and solid methane (Figure 1c), and complete decomposition to ice VII and solid methane (Figure 1d). Figure 2 shows the variation in volume ratios (V/V0) of methane hydrate with pressure. The volume ratios decreased continuously from 0.26 to 2.3 GPa. The compressibility is relatively larger below 0.7 GPa, while it is smaller above 1.5 GPa. Fitting by Birch-Murnaghan’s equation of state as K0′ ) 4, the bulk moduli, K0, were calculated to be 2.8 and 31 GPa for the former pressure region and for the latter region, respectively. This suggests that a certain structural change occurred between 0.7 and 1.5 GPa, although the fundamental crystal structure (structure I) of the methane hydrate was maintained up to 2.3 GPa. Relative intensities of the diffraction, however, varied considerably from 0.7 to 1.5 GPa. Since the relative intensity of the diffraction pattern is sensitive to the cage occupancy in the clathrate structure, the relative intensities were calculated on the basis of the previously established crystal data6 as a function of site occupancy parameters of both large (X-site) and small (Y-site) cages from 0 to 1 with 0.1 intervals. The representative relative intensities calculated for X ) 1, Y ) 1; X ) 1, Y ) 0; X ) 0, Y ) 1; and X ) 0, Y ) 0 are shown in Figure 3. The figure shows clear differences in the relative intensities with the changes in site occupancy. Among them, it is remarkable that the 110 diffraction, which appears at the lowest angle, disappears when the Y-site is vacant and the X-site is between 1 and 0.8. When both the X-site and Y-site are vacant, which may not occur in reality, the 110 diffraction appears due to a cavity-cavity scattering.20 Comparing the observed and calculated intensities, the site occupancies were estimated to be comparable to X ) 1-0.9
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Figure 3. Relative intensities calculated for X ) 1, Y ) 1; X ) 1, Y ) 0; X ) 0, Y ) 1; and X ) 0, Y ) 0. The structural models having these site occupancies are also drawn.
and Y ) 1-0.9 for the samples from 0.3 to 0.7 GPa. Table 1 shows good agreement between the observed intensities at 0.4 GPa and the calculated ones for X ) 1 and Y ) 1. The relative intensities observed at 1.5 and 2.1 GPa corresponded to X ) ∼0.9, Y ) 0 and X ) 0.9-0.8, Y ) 0, respectively. It is noteworthy that the Y-site (small cage) is vacant in both cases,although it had been fully occupied at lower pressures. This might be explained by the fact that above 1.5 GPa the framework of structure I was reduced to V/V0 ∼ 0.85, and thus
the small cage can no longer accommodate a methane molecule. This drastic change of occupancy might be related to the change in compressibility observed in the compression curve shown in Figure 2. Below 0.7 GPa both large and small cages are large enough to accommodate methane molecules, while above 1.5 GPa even the larger cage is too small to fully accommodate methane molecules, and the occupancy parameter decreases continuously. Above 1.5 GPa three phases, i.e., methane hydrate, ice VI
Methane Hydrate Behavior under High Pressure (or ice VII), and fluid methane (or solid methane), coexisted. According to the phase rule, the coexistence of these three phases may be a metastable behavior when the present system is regarded as a two-component system consisting of H2O and CH4. However, the phase transitions generating these three phases occur reversibly at 1.5 and 2.1 GPa. Such an experimental result is in general interpreted to be a stable behavior. Therefore, it is difficult, at present, to determine strictly that the observed result is stable or metastable behavior. In any case, the methane hydrate can survive by changing cage occupancies to those favorable under higher pressure without structure change. This characteristic behavior might derive from the flexibility or “versatility” of the clathrate structure, since this structure is built up from cages of different sizes. This flexibility is considerably high, so that the methane molecules that have been removed from the Y-site can potentially return to them. As for methane, high-pressure studies of its phase changes indicate that methane solidifies to phase I at 1.68 GPa and that at lower pressure methane is fluid.18,19 In the present study, small patches but no diffraction pattern corresponding to the methane phase were observed at 1.5 GPa, whereas at 2.1 GPa the diffraction pattern of phase I appeared. These results were consistent with the phase changes previously reported. A previous report on methane hydrate suggests that it has a denser structure before decomposition to ice VI.17 In that study, some surfactant was added to the water-methane system examined.17 In the present study, although a change in compressibility was observed, this was not due to changes in basic structure, but rather to changes in site occupancy, as discussed above. The present work, consisting of in-situ observation by XRD and optical microscopy using DAC, has allowed a more detailed
J. Phys. Chem. B, Vol. 104, No. 7, 2000 1433 understanding of the methane hydrate behavior under pressure at room temperature. The results indicate that high-pressure study is also indispensable for other clathrate hydrates. References and Notes (1) Kvenvolden, K. A. Chem. Geol. 1988, 71, 41. (2) Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases; Marcel Dekker: New York, 1990. (3) Jeffrey, G. A. In Inclusion Compounds; Atwood, J. L., Davis, J. E. D., McaNicol, D. D., Eds.; Academic: London, 1984; Vol. 1. (4) Vos, W. L.; Finger, L. W.; Hemley, R. J.; Mao, H. K. Chem. Phys. Lett. 1996, 257, 524. (5) Hutz, U.; Englezos, P. Fluid Phases Equilib. 1996, 117, 178. (6) McMullan, R. K.; Jeffrey, G. A. J. Chem. Phys. 1965, 42, 2725. (7) Mak, T. C. W.; McMullan, R. K. J. Chem. Phys. 1965, 42, 2732. (8) Ripmeester, J. A.; Tse, J. S.; Ratcliffe, C. I.; Powell, B. M. Nature 1987, 325, 135. (9) Calvert, L. D.; Srivastava, P. Acta Crystallogr. A 1969, 25, S131. (10) Brownstein, S.; Davidson, D. W.; Fait, D. J. Chem. Phys. 1967, 46, 1454. (11) Davidson, D. W. In Water: A ComprehensiVe Treatise; Franks, F., Ed.; Plenum: New York, 1973; Vol. 2. (12) Davidson, D. W.; Handa, Y. P.; Ratcliff, C. I.; Tse, J. S.; Powell, B. M. Nature 1984, 311, 142. (13) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1988, 92, 337. (14) Tse, J. S.; Ratcliff, C. I.; Powell, B. M.; Sears, V. F.; Handa, Y. P. J. Phys. Chem. A 1997, 101, 4491. (15) Kuhs, W. F.; Chazallon, B.; Radaelli, P. G.; Pauer, F. J. Inclusion Phenom. Mol. Recognit. Chem. 1997, 19, 65. (16) Davidson, D. W. In Natural Gas Hydrates: Properties, Occurrence and RecoVery; Cox, J. L., Ed.; Butterworth: Boston, 1983. (17) Dyadin, Y. A.; Aladko, E. Y.; Larionov, E. G. MenndeleeV Commun. 1997, 1, 34. (18) Hazen, R. M.; Mao, H. K.; Finger, L. W.; Bell, P. M. Apple. Phys. Lett. 1980, 37, 288. (19) Hebert, P.; Polian, A.; Loubeyre, P.; Toullec, R. L. Phys. ReV. B 1987, 36, 9196. (20) Tse, J. S. Private communication, 1999.
