Mechanism for the Replacement of CH4 in Methane Hydrates with CO2 in the Solid Phase via Molecular Dynamics Simulation

Clathrate hydrates are a class of nonstoichiometric crystalline compounds forming from water and small gas molecules, such as methane and carbon dioxides, at low temperatures and high pressures. Due to its abundance in nature, methane hydrate (MH) is regarded as a potential energy resource for the future. 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 used MD simulation to study the mechanism for the replacement of methane in MH using CO2 both with and without a hydrate interface. When a hydrate-liquid CO2 interface is present, replacement only occurred in the first few surface layers of hydrate structure. If water vacancy is introduced to the hydrate phase, the vacancy quickly diffuses to the interface and vanishes, and thus does not promote the replacement process. In the case of bulk hydrate crystal (no interface), we investigate how the concentration of gas molecules (occupancy) and water vacancy affects the diffusivity of methane and CO2 in the crystalline phase. For the system with low concentration of water vacancy, we found that the vacancy propagated within the hydrate structure; however, its propagation did not stimulate the movement of methane or CO2 molecule between cages. For the system with high concentration of water vacancy, the initially separated vacancies were found to aggregate and form larger clusters of defected structures, each of which has 5 or more water vacancies centering around a small (512) cage, and resulting in broken surrounding large cages (51262). The diffusion of methane or CO2 molecular were found to take place only in such aggregated defect structures. The diffusion coefficient of methane and carbon dioxide molecules in such systems were found to be in good agreement with experiment. The result of simulations suggest that the replacement of methane with CO2 only occurs within structures of with aggregated water vacancies, such as grain boundary or interface.