Exploring tuning phenomena of THF-H2 hydrates via molecular dynamics simulations

Abstract

The tuning phenomenon is one of the effective strategies to enhance the gas storage density of clathrate hydrates at moderate pressure: small gaseous guest molecules can occupy some of the large cages of binary hydrates that are not occupied by thermodynamic promoters. This work explored tuning phenomena in THF-H2 hydrates at the molecular level by all-atomic molecular dynamics simulations for the first time. Before conducting molecular dynamics simulations, off-diagonal Lennard-Jones (LJ) parameters between the center of mass of H2 and oxygen of H2O were optimized from ab-initio calculations with various configurations of H2 and H2O molecules. These optimized off-diagonal parameters provided reasonable phase equilibrium of THF-H2 hydrates compared to the simple mixing rule case, which is derived from weakened H2-H2O interaction and active drifting of H2 in molecules’ 512 cages. With the ab-initio modified LJ parameters, the growth of THF-H2 hydrates was investigated by molecular dynamics simulations under various compositions and pressures of the system. Cage occupancy of H2 in 512 cages of the binary hydrates increased when the THF concentration was lower than 5.56 mol%, but double occupation of H2 in 512 cages was not observed. Cage occupancy of H2 in 51264 cages significantly increased by lowering the THF concentration, and the total storage capacity of H2 increased by multiple occupations of H2 in the 51264 cages. The growth rate of the THF-H2 hydrates depended on the THF concentration, and that dependency of the growth rate was due to the wrong adsorption of THF molecules at open small cage sites of the solution-hydrate interface. These results demonstrated that uptake of H2 could be enhanced by the tuning phenomena when mobile THF molecules in the liquid phase formed binary hydrates with H2.