Recent progress in observations of radiation-induced radicals in gas hydrates was reviewed. Two different types of intermolecular hydrogen transfer reactions were observed. One is hydrogen-picking reaction where alkyl radicals withdraw hydrogen of alkane guest molecules in the adjacent cages of alkane hydrates. The other is hydrogen-hopping reaction where carboxyl (HOCO) radicals transfer excess hydrogen to CO2 molecules in the adjacent cages of CO2 hydrate. In the CO2+methane mixed gas hydrates, the yield of radiation-induced radicals increases by reducing the inverse reactions of radical formation. The variety of intermolecular reactions in gas hydrates will attract us not only in physical chemistry but also in application area using new reaction field.
Statistical mechanical and simulation studies have been reviewed for the filled ice Ic and ice II containing hydrogen molecules. The occupancy of hydrogen inside the voids of ice Ic and ice II is examined using a hybrid grand-canonical Monte Carlo simulation in wide ranges of pressure and temperature. A simple theoretical model provides a global phase diagram of two-component system in which the phase equilibria among various phases can be predicted.
The elastic properties of methane-propane mixed gas hydrate with cubic structure II (MPH-sII) have been determined as a function of pressure by high-pressure Brillouin spectroscopy up to 1.30 GPa at 296 K. The pressure dependences of the obtained ratios of elastic constants to density C11/ρ and C44/ρ of MPH-sII are obviously different from those of pure methane hydrate with cubic structure I (MH-sI), and indicate that the crystal of MPH-sII is elastically soft and weak against the shear stress in comparison with that of MH-sI under high pressure. Those results suggest that the elastic properties and structural stability of gas hydrates depend on not only the guest molecule but also the gas hydrate structure.
Low-temperature and high-pressure experiments were performed with filled ice structures of methane hydrate and hydrogen hydrate under pressure and temperature conditions of 2.0-77 GPa and 30-300 K and 5.0-50 GPa and 10-300 K, respectively, using diamond anvil cell and helium-refrigerator cryostat. In-situ X-ray diffractometry and Raman spectroscopy revealed that orientational ordering of guest molecules occurred from disordered-rotation state at low temperature and high pressure, and that the guest ordering induced structural changes. For hydrogen hydrate, cubic structure was deformed to a tetragonal structure, while for methane hydrate axial ratios of orthorhombic structure were remarkably changed within the same orthorhombic structure.
The role of methane hydrate is discussed in demand situation of natural gases in Japan. Then, the necessity of methane hydrate enhanced recovery is shown in Japan project of methane hydrate. As one of the enhanced recovery methods, we are experimentally considering using carbon dioxide (CO2). The latest results and its estimation show a possibility of using CO2.