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Clathrate hydrates of hydrogen and neon

 

作者: Yuri A. Dyadin,  

 

期刊: Mendeleev Communications  (RSC Available online 1999)
卷期: Volume 9, issue 5  

页码: 209-210

 

ISSN:0959-9436

 

年代: 1999

 

出版商: RSC

 

数据来源: RSC

 

摘要:

Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) Clathrate hydrates of hydrogen and neon Yuri A. Dyadin,* Eduard G. Larionov, Andrei Yu. Manakov, Fridrich V. Zhurko, Evgeny Ya. Aladko, Tamara V. Mikina and Vladislav Yu. Komarov Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation. Fax: +7 3832 34 4489; e-mail: clat@che.nsk.su At pressures up to 15 kbar, hydrogen and neon were found to form classical clathrate hydrate structures, whereas argon and krypton, already known as polyhedral clathrate formers, can also form hydrates based on the ice II framework at high pressures.Small hydrophobic molecules (including monoatomic molecules) are known to form clathrate hydrates; these molecules can be subdivided into two different groups.The smallest molecules, hydrogen, helium and neon, all of linear dimensions less than 3.5 Å, constitute a group that can be incorporated into the small cavities in the lattices of ices Ih, Ic and II, thus forming solid solutions or clathrates very similar to the original ice frameworks. 1–3 Thus, they are not known to form clathrates based on polyhedral structures.The second group includes substances with larger molecular dimensions of 3.8 to 9.2 Å. These do not dissolve in ice frameworks, but form polyhedral clathrate structures, the best known of which are classical cubic structures I and II (CS-I and CS-II), and a hexagonal structure H (HS-III).4,5 A study of phase diagrams of the (Ar, Kr, Xe)–H2O systems at pressures up to 15 kbar showed6–8 that these gases can form classical clathrate hydrates, which are stable in a wide temperature and pressure range.However, in argon and krypton systems at pressures above 9.6 and 13.4 kbar, respectively, we found new hydrate phases, which are quite dense judging from the fact that their decomposition temperatures rapidly increase with pressure.This high-density packing is impossible in the case of typical clathrate structures where a single guest molecule occupies one cavity in a hydrate framework.† Here, we report the results of studying phase equilibria in the neon–water and hydrogen–water systems. All of the experiments were carried out by differential thermal analysis in an excess of gas. The approximate gas–water molar ratio was 6:1, while the ratio between the components in classical clathrates and ice II structures when every cavity was occupied by one guest molecule was inverse.The temperature was measured with a Chromel– Alumel thermocouple, and the pressure, with a Manganin manometer. The experimental technique was described in detail elsewhere.6–8 The results are shown in Figure 1.In both cases, the initial portions of the curves (lines aIg) represent decomposition of solid solutions of the gases in ice Ih.1 In our case, this manifests itself in a slight increase in the decomposition temperature of the solid solution as compared to the ice melting curve. The final portion of the curve (line bIg) in the hydrogen-containing system corresponds to decomposition of the hydrogen hydrate H2·6H2O with the ice II framework.3,9 Note that in both portions of the P–T curves the equilibrium was established rapidly (in about 20 min), and the results were well reproducible.The most complicated and interesting behaviour is observed in the pressure ranges from 1.0 to 3.6 kbar for hydrogen and from 1.9 to 3.7 kbar for neon systems (lines hlg).In both cases, the equilibria were attained very slowly (in several hours or longer). It is reasonable to suppose that, really, equilibrium states were not achieved in some experiments. The results can be reproduced only with a high excess of gas. We assign these † It is reasonable to consider the above hydrates as belonging to the same group because, provided that all the cavities are occupied by the same guest, their packing coefficients k6,10 are similar.For instance, for the CS-I xenon hydrate, k = 0.584; for the hypothetical xenon hydrates of other structures, they vary from 0.55 to 0.58, i.e., they are appreciably lower than in the case with the densest packing (k = 0.74). Therefore, the effect of pressure on them should be very similar.curves to decomposition of the classical hydrogen and neon clathrates. Figure 2 shows the decomposition curves of hydrate phases in the (Ar, Kr, Xe)–H2O systems6–8 and those of the systems under discussion. It is evident that in the systems with hard noble gases the decomposition curves of classical structure hydrates (solid lines) are top-shaped, and their sizes regularly decrease from xenon to argon hydrate, i.e., with a decrease in the guest molecule radius. The hydrogen and neon systems are adequately described by this scheme. Kuhs et al.11 showed that every two nitrogen molecules can occupy a large cavity in CS-I and CS-II.This is even more true of the equilibrium in the case of hydrogen and neon molecules.12 The presence of more than one guest molecules in a cavity, and slow diffusion of hydrogen and neon molecules in the already formed solid compound can result in rather slow establishment of equilibrium in the case of classical hydrate formation.Figure 2 shows that in all of the systems (except for the xenon system), in addition to the group discussed above, there is a group characterised by similar curves, which go up steeply with increasing pressure (dashed lines).An X-ray study of the compounds formed in the water–hydrogen system showed that this curve branch corresponds to decomposition of a hydrate phase based on the ice II framework. Similar decomposition curves for the systems strongly suggest that, at pressures above 4 kbar in the neon system, 9.6 kbar in the argon system, and 13.4 kbar in the krypton system, hydrates based on the ice II structure are formed.For argon and krypton molecules to be arranged in ice II channel cavities, a slight elongation of hydrogen bonds is required, which is quite realistic. Let us recall that, when the self-clathrate ice VII is formed (two ice Ic frameworks penetrate into each other) at pressures above 20 kbar, the hydrogen bond length increases from 2.76 to 2.96 Å.13 Thus, classical clathrate hydrates are formed in aqueous neon, 80 60 40 20 0 –20 T/°C P/kbar 0 4 8 12 16 metastable continuations of the curves H2–H2O system, this work H2–H2O system, ref. 9 Ne–H2O system, this work alg hlg blg i1hI i3I i5I i6I Figure 1 Decomposition curves of hydrates formed in the water–hydrogen and water–neon systems. Melting curves of ices (inl) are given for comparison: (a) solid solutions of gases in ice Ih, (b) hydrates of the respective gases based on the ice II crystal framework, (h) classical clathrate hydrate phase, (l) liquid phase, (g) fluid phase.Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) hydrogen, argon, krypton and xenon systems. Their stability decreases in the order from xenon to neon. In the first four systems, hydrates are formed on the basis of ice II, and the larger the guest molecules, the higher the pressure required for their formation. A xenon molecule appears to be too bulky to be arranged in an ice II framework cavity, and the CS-I hydrate remains stable at pressures at least up to 15 kbar.We are grateful to Professor J. A. Ripmeester and Professor V.I. Kosjakov for helpful discussions and valuable comments. We also express our gratitude to N. V. Udachina for translating this article into English. This work was supported by the Presidium of the Siberian Branch of the Russian Academy of Sciences (grant no. 97-18) and by the Russian Foundation for Basic Research (grant no. 97-03-33521a). References 1 A.Yu. Namiot and E. B. Bukhalter, Zh. Strukt. Khim., 1965, 6, 911 [J. Struct. Chem. (Engl. Transl.), 1965, 6, 873] (Chem. Abstr., 1966, 64, 10497b). 2 G. P. Arnold, R. G. Wenzel, S. W. Rabideau, N. G. Nereson and A. L. Bowman, J. Chem. Phys., 1971, 55, 589. 3 D. Londono, W. F. Kuhs and J. L. Finney, Nature, 1988, 332, 141. 4 G. A. Jeffrey and R. K. McMillan, Progr. Inorg. Chem., 1967, 8, 43. 5 J. A. Ripmeester, J. S. Tse, C. I. Ratcliffe and B. M. Powell, Nature, 1987, 325, 135. 6 Yu. A. Dyadin, E. G. Larionov, D. S. Mirinski, T. V. Mikina, E. Ya. Aladko and L. I. Starostina, J. Incl. Phenom., 1997, 28, 271. 7 Yu. A. Dyadin, E. G. Larionov, D. S. Mirinski, T. V. Mikina and L. I. Starostina, Mendeleev Commun., 1997, 32. 8 Yu. A. Dyadin, E. G. Larionov, T. V. Mikina and L.I. Starostina, Mendeleev Commun., 1997, 74. 9 W. L. Vos, L. W. Finger, R. J. Hemley and Ho-Kwang Mao, Phys. Rev. Lett., 1993, 71, 3150. 10 A. I. Kitaigorodsky, Molecular Crystals and Molecules, Academic Press, NewYork, part 1. 11 W. F. Kuhs, B. Chazallon, P. Radaelli, F. Pauer and J. Kipfstuhl, Proceedings of the 2nd International Conference on Natural Gas Hydrates, Toulouse, 1996, p. 9. 12 K. A. Udachin, J. Lipkowski and M. Tkacz, Supramolecular Chem., 1994, 3, 181. 13 L. Pouling, General Chemistry, Freeman and Co, San Francisco, 1970. ........ 80 60 40 20 0 –20 T/°C P/kbar 0 4 8 12 16 Xe–H2O system Kr–H2O system Ar–H2O system H2–H2O system Ne–H2O system i1hI i3I i5I i6I Figure 2 Decomposition curves of hydrates formed in the (Xe, Kr, Ar, H2, Ne) water systems. Solid lines represent decomposition of polyhedral clathrate hydrates formed in these systems (first portions of decomposition lines in hydrogen and neon systems correspond to decomposition of solid solutions of the gases in ice Ih), dashed lines represent decomposition of hydrates of the gases based on the ice II crystal framework. Dotted lines represent the melting curves of the gases. Melting curves of ices (inl) are given for comparison. .......... Received: 22nd January 1999; Com. 99/1432

 



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