Tuning clathrate hydrates: Application to hydrogen storage

Gas hydrates represent an attractive way of storing large quantities of gases such as methane, although to date there has been little effort to optimize the storage capacity and to understand the trade-offs between storage conditions and storage capacity. In this work, we proposed the peculiar cage occupancy dynamics observed at the mixed hydrates simultaneously containing both one gas and one liquid guest. Liquid guests participating in forming hydrates must be water-soluble and possess the sufficient interaction with surroundings in order to make strong host-guest networks. However, it seems to be almost impossible for small molecules such as hydrogen and methane to replace the relatively large liquid guest molecules and thus an appropriate condition needs to be made for small guests to be enclathrated in large cages. By lowering the concentrations of liquid guests the small gas guests are found to enter the large cages feeling like their home. We also tried to explain this natural phenomenon with solid solution theory for clathrate hydrates. Special types of water-soluble liquid materials, such as THF, 1,4-dioxane, and t-BuNH2, form sII double hydrates upon reacting with gas guest molecules. We expect that the storage capacities of gas guests largely depend on both the chemical nature of liquid guests and their relative concentration in host water. This tuning mechanism occurring in the mixed clathrate mixtures appears to be quite general and can be applied to energy and environmental systems including gas storage and transportation fields. (c) 2006 Elsevier B.V. All rights reserved.