(100e) Stability Analysis of H2 Clathrate Hydrates by Ab Initio Calculations

Gas hydrates are crystalline compounds, also known as clathrates. The guest (gas) molecules are entrapped in a cage-like structure of hydrogen bonded water molecules. Their general formula is Mn(H2O)p where M: gas, n: number of gas molecules, p: number of water molecules. Hydrogen clathrates are a potential storage medium for safe storage of hydrogen with a storage density of 5wt%. Hydrogen form Structre II (sII) clathrate hydrates and pure H2 hydrates form at 200 MPa and 250 K3. Recently, adding THF (tetrahydrofuran) dramatically reduces hydrogen hydrate formation pressure (5 MPa) but the H2 storage capacity of binary THF+H2 clathrate hydrates (1-4 wt %) is less than that of pure H2 hydrates (5.03 wt%). In binary THF+H2 clathrate hydrates, THF enters only the large cavities and H2 enters both the small and large cavities. The occupancy of H2 in large cavity is 4 and in small cavity it is either 1 or 2.

sII clathrate hydrates of pure hydrogen and binary hydrates of THF + H2 are studied using ab initio calculations to determine the stable occupancies of small cavities. We have considered one dodecahedron (small cavity) and one hexakaidecahedron (large cavity). These two cavities are attached to each other as in Str. II hydrate to form a double cavity. One or two H2 molecules are placed in the small cavity and one THF (or 4H2 molecules) molecule is placed in the large cavity. We have then determined the binding energies of the singly occupied and the double occupied double cavities at MP2 level using various basis sets (3-21G, 3-21G(2p), 3-21++G(2p), 6-31G, 6-31G(2p), and 6-31++G(2p)). Different basis sets yielded different stable occupancies. The results from the highest basis set (6-31++G(2p) with ZPE correction) indicate that single occupancy is as favorable as double occupancy in both the cases of pure H2 hydrates and THF+H2 double hydrates.

References:

1. W. L. Mao, H. K. Mao, A.F. Goncharov, V. V. Struzhkin, Q. Guo, J. Hu, J. Shu, R. J. Hemley, M. Somayazulu, and Y. Zhao, ?Hydrogen Clusters in Clathrate Hydrate?, Science, 297, 2247-2249 (2002).

2. L. J. Florusse, C. J. Peters, J. Schoonman, K. C. Hester, C. A. Koh, S. F. Dec, K. N. Marsh, and E. D. Sloan, ?Stable Low-Pressure Hydrogen Clusters Stored in a Binary Clathrate Hydrate?, Science, 306, 469-471 (2004).

3. H. Lee, J-W Lee, D. Y. Kim, J. Park, Y-T Seo, H. I. Zeng, L. Moudrakovski, C. I. Ratcliffe, and J. A. Ripmeester, ?Tuning Clathrate Hydrates for Hydrogen Storage?, Nature, 434, 743-746 (2005).