The Inclusion of Hydrogen in Clathrate Hydrates of the Various Guests and Tuning of the tert-Butylamine Hydrate
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0927-EE06-01
The Inclusion of Hydrogen in Clathrate Hydrates of the Various Guests and Tuning of the tert-Butylamine Hydrate Huen Lee, Jong-won Lee, Do Youn Kim, and Jeasung Park Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea, Republic of
ABSTRACT Clathrate hydrates of structure II with organic cyclic oxides were identified to have the ability to include hydrogen molecules into their empty small cages at near-ambient temperature and moderate pressure. Solid-state NMR and Raman spectroscopy of the included hydrogen molecules indicate that the small cavities are both singly and doubly occupied by hydrogen molecules in the mixed hydrates. The inclusion mechanism can be derived from the critical progression of the cage occupancies as determined from in-situ kinetic experiments on formation and decomposition. The inclusion ability of hydrogen molecules was observed for several organic clathrate hydrate formers such as THF (tetrahydrofuran) and 1,4-dioxane, suggesting this to be a general property. A structural transition of the simple amine hydrate was suggested to produce a novel double CH4 + t-BuNH2 hydrate as confirmed by using microscopic analytical methods. The corresponding hydrate compositions were determined by using NMR spectroscopy to describe a tuning mechanism to increase the storage capacity. INTRODUCTION Gas hydrates, or more generally clathrate hydrates, are a type of inclusion compound and take the form of non-stoichiometic solid crystalline materials formed from water and relatively small guest molecules [1]. Under sufficient pressure of the guest material at specific temperatures, three-dimensional hydrate structures are formed by the stacking of polyhedral cages of hydrogen-bonded water molecules. Guest molecules with suitable sizes are enclathrated in the cages and their presence is necessary to give stability to the compounds formed. Although the guest molecules are physically enclosed within the cages, there are no direct chemical bonds between host water and guest molecules. In general, clathrate hydrates, commonly called gas hydrates, form three distinct structural families, known as structure I (sI), II (sII) and H (sH), which differ in the combination of cage types required to give a three dimensional structure [2]. Structure I and II are composed of small and large cages (small and large cages are pentagonal dodecahedra(512) and tetradecahedra(51262) respectively for sI, while small and large cages are pentagonal dodecahedra and hexadecahedra respectively for sII). However, structure H comprises small(512), medium(435663) and large(51268) cages. The trapping of molecular hydrogen is a topic of considerable interest today. Recent work by Mao et al. [3] has shown that hydrogen hydrate can be prepared at very high pressure and that it may play a role in the capture and storage of hydrogen in extraterrestrial environments. An intriguing feature of the hydrate known as structure II is
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