(411e) Mechanism for the Replacement of CH4 in Methane Hydrates with CO2 in the Solid Phase

Methane hydrates is a kind of nonstoichiometric crystalline compound composed of water and methane at low temperatures and high pressures, and is regarded as a potential energy resource for the future. In addition to energy gas production, reducing global warming gas is also a concerned issue. One intriguing idea for the simultaneous recovery of energy and sequestration of global warming gas is proposed by the transformation of methane hydrate into carbon dioxide (CO₂) hydrate without melting the network of hydrogen-bonded water molecules. Some experiments have shown that methane hydrate can be changed into CO₂ hydrate by injecting liquid CO₂ into methane hydrate powders, and some theories has been proposed that there should be some porous vacancies formed by water molecules (water vacancy) in the hydrate structure to facilitate the replacement reaction. However, the exact mechanism is still unclear.

Molecular dynamics (MD) simulation has been a useful tool to unveil the molecular level details of gas hydrate. In this work, we use MD simulation to exam the moving or exchanging behavior of methane and CO₂ molecular in the crystalline hydrate with and without the presence of water vacancies. Our simulation results show that water vacancies are high energy sites in the hydrate structure. For the system with low concentration of water vacancy, we found that the vacancy propagates within the hydrate structure; however, its propagation does not stimulate the movement of methane or CO₂ molecule between cages. For the system with high concentration of water vacancy, the initially separated vacancies are found to aggregate into a special defect structure, centering around small (5¹²) cages and resulting in broken surrounding large cages (5¹²6²). The movements of methane or CO₂ molecular are found to take place only in such aggregated defect structures. To confirm the validity of our simulation, we also check the diffusion coefficient of methane and carbon dioxide molecular, which are found to be in good agreement with experiment.