Molecular Insight into the Growth of Hydrogen and Methane Binary Hydrates

H-2 is considered as the ideal fuel; however, the storage and transportation of H-2 limit its usage. Clathrate hydrates are candidate materials for H-2 storage and transportation. Because of the extreme conditions necessary to stabilize the pure H-2 hydrate, additives are proposed to stabilize a mixed H-2 hydrate. Compared to the widely studied H-2 + tetrahydrofuran binary hydrates, H-2 + CH4 binary hydrates contain a higher energy density. In this study, we study the growth of H-2 + 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 H-2 + CH4 hydrate formation, while H-2 acts as a kinetic promoter for H-2 + CH4 hydrate growth at some of our working conditions. We find that there is a maximum growth rate of H-2 + CH4 binary hydrates at 250 K when the pressure is 50 MPa, and at fixed temperature, the growth rate of H-2 + CH4 binary hydrates shows a positive correlation with pressure. We also find that adding H-2 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 5(12) and 5(12)6(4) cages by H-2 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 5(12) cages occupied by H-2, where the ratio of H-2 and CH4 occupied cages in the grown hydrate can be 3:1 at 250 K and 80 MPa. However, the occupancy of 5(12)6(4) cages by H-2 and CH4 remains relatively constant with increasing pressure. In addition, at our working conditions, 5(12)6(4) cages can be double-occupied by H-2, and several 5(12)6(4) cages can be occupied by H-2 and CH4 or triple H-2. Our simulations show that the solubility and diffusivity of guest molecules, especially CH4, in solution dominate the growth process.