The Structures And Stabilities About Clathrate Methane Hydrate With Water Molecule Vacancy

Methane hydrate is a new green energy which will take place of theconventional energy, such as coal, oil et al. Because of its high energydensity, no burning pollution, reserves amazing, methane hydrate hasalready attracted global attention. However, till now, people have not foundan effective mining method to exploit and employ this green energy. In thepresent work, starting from the decomposition field about methane hydrate,we discussed the changes of structures, energies, and the possibility of CH₄escaping from the cage after the cage losing one or two water molecules, inorder to describe the relationship between structure vacancy of methanehydrate and its stability, hope to provide a new approach for miningmethod study. On basis of the two initial structures about CH₄@(H₂O)₂0and CH₄@(H₂O)₂4of the S cell with their symmetry, we analysed anddesigned a series of structures, which were removed one or two watermolecules in different locations, then formed two kinds of CH₄@(H₂O)₁9, 12kinds of CH₄@(H₂O)₁8with CH₄@(H₂O)₂0and4kinds of CH₄@(H₂O)₂3,44kinds of CH₄@(H₂O)₂2with CH₄@(H₂O)₂4. Then, used theB3LYP/6-31+G**method to optimize these vacancy structures, andcomputed their energies with the BSSE method on the B3LYP/6-311++G**level, so that the total energy of each structure and theinteraction between subject (the cage) and object (the methane molecule)would be clear. In addition, with the same covalent bond lengths of H-O,the same angles of H-O-H and even the same orientation of hydrogenatoms, when increased all of O-O side lengths by the same multiples tonearly none interaction among the molecules, calculated the energy of theloose cavity, and the difference of two energies (the total cavity energy andthe loose cavity energy) is the H₂O-H₂O_s binding energy. Finally, thestability of methane hydrate would be concluded with the changes of thestructure parameters and energy, caused by removing water molecules.The results show that, methane can escape from the hydrate cage onlywhen two water molecules are vacated in a same ring. In these hydratecages, their side length changes are mainly attributed to their hydrogenbonds, since the O-H covalent bond and H-O-H angle varies very little.Furthermore, for the isomer hydrates formed with equal-rings, theirstabilities may be contributed by the energy of hydrogen bond, but forthose formed with non equal-rings, their stabilities have a trend to be lowerwith the number of five-ring decreasing and four-ring or seven-ring et al. big rings increasing.