(378l) 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. Due to the extreme conditions necessary to stabilize pure H₂ hydrate, additives are proposed to stabilize a mixed H₂ hydrate. Compared to the widely studied H₂ + THF 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 all 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 CH₄increases when the temperature is above 240 K. With increasing pressure, there is an increase in the percentage of 5¹² cages occupied by H_2, where the ratio of H₂ and CH₄ occupied cages in 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.