Tetrahydrofuran and hydrogen form Structre II (sII) clathrate hydrates. They are formed at less severe conditions (7-10 MPa and 270-280 K)1-2 than pure H2 hydrates (200 MPa and 250 K)3. While they are formed at less severe conditions, 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.
To determine the stable H2 occupancy in small cavity, we have performed ab initio calculations using GAMESS package. 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 molecule is placed in the large cavity. We have then determined the binding energies of the singly occupied and the doubly occupied double cavities at MP2 level using 3-21(G) basis set. Our preliminary calculations indicate that the doubly occupied binary clathrate is as stable as the singly occupied binary clathrate.
References: 1. 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). 2. 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). 3. 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).