Theoretical Studies Of The Intermolecular Interactions In Gas Hydrates And The Dissociation Behavior Of Gas Hydrates

Gas hydrates have definitely promising role in energy and environment field, especially for methane hydrate which called "fired ice" and hydrogen hydrate which has huge potential for storing hydrogen. It is estimated that the amount of organic carbon in fired ice deposits on earth is twice as much as which included in the fossil energy, so fired ice is considered as backup energy in future. Moreover, carbon dioxide can be sequestrated under the ocean floor in the form of carbon dioxide hydrates to reduce the greenhouse effect. Because of that, many governments are required to strengthen the studies on gas hydrates, so as to be in leading position in the technology for exploiting fired ice deposits, storing hydrogen in the form of hydrogen hydrate, and sequestrating carbon dioxide in the form of carbon dioxide hydrate in the world.The computational methods employed in previous papers on gas hydrates cannot give accurate description for the intermolecular non-covalent interaction in gas hydrates. As the importance of non-covalent interacting system improving, many DFT methods added long-range dispersion correction are developed in recent years. Compared with the benchmark results calculated at the high-level ab initio methods, we assessed the ability of twenty DFT methods to describe the intermolecular interaction in methane hydrate. Among the evaluated DFT methods, the performance of coB97X-D, X3LYP, B3LYP, M06-2X and B97-D to describe the hydrogen bonding energy between water molecules is excellent; at the same time, coB97X-D, M06-2X and B97-D is the best methods to describe the van der Waals interaction between methane molecule and water cage. Considering both computational precision and cost, we recommended both ωB97X-D/6-311++G(2d,2p) and M06-2X/6-311++G(2d,2p) with BSSE correction is the best calculating scheme to describe the intermolecular interaction in methane hydrate; in addition, B97-D/6-311++G(2d,2p) without BSSE correction is also a reasonable choice.Many important applications of carbon dioxide hydrate are closely related to the dissociation process of carbon dioxide hydrate, but there are only few studies on the dissociation behavior of carbon dioxide hydrate, and the microscopic dissociation mechanism of carbon dioxide hydrate has not been fixed until now. Through molecular dynamic simulation and first-principle calculation, we simulated the dissociation behavior of carbon dioxide hydrate, methane hydrate and hydrogen hydrate. Then based on the barrier of guest molecules diffusing from the inner of the water cage to outer, we revealed the physical origin under the dissociation behavior. We find the dissociation mechanism of carbon dioxide hydrate can be summarized as three steps:firstly, as the temperature increasing, the diffusion behavior of water molecules become severe, and the crystal structure of carbon dioxide hydrate distorted and even damaged as the water molecules diffusing; secondly, the carbon dioxide molecules trapped in the water cavities are released through the small opening and dispersed in the water solution in the form of small bubble; finally, the crystal structure of carbon dioxide hydrate is completely destroyed, and the small bubbles gather into a large bubble in the water solution. Moreover, the dissociation behavior of carbon dioxide hydrate not only depends on the overall occupancy of guest molecules, but also is related to the specific type of water cavity occupied by guest molecules; the more the small water cavities is occupied, the easier the dissociation is. In addition, as the higher diffusion barrier of guest molecules, the dissociation behavior of carbon dioxide is similar to that of methane hydrate; both the hydrogen bonding clathrate skeleton of water molecules firstly damaged, and then the trapped guest molecules are released. But for hydrogen hydrate, as the lower diffusion barrier of hydrogen molecules, the endohedral hydrogen molecules firstly diffuse freely in the structure framework of hydrate, and the local crystal structure is damaged as the unsteady empty water cavities generated by the hydrogen molecules diffusion, and then the trapped hydrogen molecules are released through the small openings, finally the regular crystal structure of hydrate is completely destroyed.