Molecular Insight into the Growth of Hydrogen and Methane Binary Hydrates

H2 is considered as the ideal fuel; however, the storage and transportation of H2 limit its usage. Clathrate hydrates are candidate materials for H2 storage and transportation. Because of the extreme conditions necessary to stabilize the pure H2 hydrate, additives are proposed to stabilize a mixed H2 hydrate. Compared to the widely studied H2 + tetrahydrofuran binary hydrates, H2 + CH4 binary hydrates contain a higher energy density. In this study, we study the growth of H2 + CH4 binary hydrates for two sets of temperature and pressure conditions by using molecular dynamics simulations with atomic models. Our results show that CH4 acts as a thermodynamic promoter for H2 + CH4 hydrate formation, while H2 acts as a kinetic promoter for H2 + CH4 hydrate growth at some of our working conditions. We find that there is a maximum growth rate of H2 + CH4 binary hydrates at 250 K when the pressure is 50 MPa, and at fixed temperature, the growth rate of H2 + CH4 binary hydrates shows a positive correlation with pressure. We also find that adding H2 in the gas phase, decreasing temperature (not smaller than 240 K), or increasing pressure can dramatically reduce the percentage of empty cages in the grown hydrate. Moreover, with increasing temperature, the occupancy of 512 and 51264 cages by H2 decreases, and inversely, the occupancy of cages by CH4 increases when the temperature is above 240 K. With increasing pressure, there is an increase in the percentage of 512 cages occupied by H2, where the ratio of H2 and CH4 occupied cages in the grown hydrate can be 3:1 at 250 K and 80 MPa. However, the occupancy of 51264 cages by H2 and CH4 remains relatively constant with increasing pressure. In addition, at our working conditions, 51264 cages can be double-occupied by H2, and several 51264 cages can be occupied by H2 and CH4 or triple H2. Our simulations show that the solubility and diffusivity of guest molecules, especially CH4, in solution dominate the growth process.