Stabilityand Reactivity Of Clathrate Methane Hydrate

Methane hydrate is considered to be an unexploited fuel source with a huge reserves in the world, and many coutries around the world treat the exploitation of methane hydrate as an important research project. The understanding of the properties of methane hydrate is the precondition for methane hydrate exploring. In present work, we mainly study the stability and reactivity of methane hydrate by quantum chemistry method, mainly in two aspects:one is to discuss the stability and reactivity of the single cages with or without guest molecules, and explore the stability of 435663CH4 cage. Another is to discuss the difference on stability and reactivity of the double cages.The research of stability and reactivity of the single cages mainly contains two aspects. Firstly, We consider five different clathrate hydrates, namely 435663,512,51262,51264, and 51268 cutting from the crystalline structures of SⅠ, SⅡ, and SH for the study of the stability and reactivity of clathrate hydrates with or without guest molecule (CH4, C3H8, C10H16). All the structures are optimized using the B3LYP method on 6-31++G (2d,2p) basis set conditions. Two Dynamic processes (face translation and face rotation) are preformed to obtain the energy of destroying the cages at B3LYP/6-31++G (2 d,2 p) level. A program is designed to achieve the average binding energy among water molecules at B97D/6-31++G (2d,2p) level. Chemical hardness(η) is also calculated at M052X/6-31++G (2d,2p) for the assessment of the reactivity of these cages. Evidences suggest that in the trem of stability changes:compared with 435663 cage,435663CH4 is more stable; the stability of 512CH4 cage is similar with that of 512; for 51262, 51264 and 51268 cages, the guest molecule make the stability of the cages decrease. In term of the reactivity changes:compared with 435663CH4, 435663 is more reactive; the reactivity of 512CH4 cage is similar with that of 512; 51262CH4,51264C3H8,51268C10H16 are more reactive when compared with their corresponding empty cages.Secondly,435663CH4 is regarded as the research object. In the present work, tetragonal face translation and face rotation were preformed to change the relative positions of the tetragonal face in 435663CH4 to explore the possibility of the CH4 molecule escaping from the cage. Meanwhile, the corresponding energy barriers of the CH4 molecule escape were calculated during the escaping course. The results show that, when the tetragonal face leaves 0.30 nm away from the rest of cage, CH4 molecule can overflow the cage smoothly. If 0.25 nm translation is carried out on the tetragonal face first, further rotation can lower the energy barrier of the CH4 molecule escape.We consider five different clathrate hydrates, namely 435663CH4/512CH4,512CH4/512CH4,5I262CH4/5I2CH4,51264CH4/512CH4 and 51268CH4/5I2CH4 for the study of the stability and reactivity of double cages. Three kinds of dates are obtained at at B3LYP/6-31++G(2d,2p), B97D/6-31++G(2d,2p) and M052X/6-31++G(2d,2p) levels of theory, they are the average binding energy among water molecules, the total energy of all considered cages, and the chemical hardness (η). Evidences suggest that in every double cage system, the pristine single cages are more stable than those in the double cages, and double cage is more reactive, two single cages which consist the double cage become more reactive because of the existence of each other, in addition, we treat the formation of double cages as a series of chemical reactions, the difference in energy of reactions, △Er are calculated. The results shown that the formation processes of 435663CH4/512CH4,512CH4/512CH4,51262CH4/512CH4, and 51262 CH4/512CH4 are endothermic, and the larger cage combining with 512CH4, the more heat is released. The formation process of 51268CH4/512CH4 is endothermic.