Exploring tuning phenomena of THF-H-2 hydrates via molecular dynamics simulations

초록

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-H-2 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 H-2 and oxygen of H2O were optimized from ab-initio calculations with various configurations of H-2 and H2O molecules. These optimized off-diagonal parameters provided reasonable phase equilibrium of THF-H-2 hydrates compared to the simple mixing rule case, which is derived from weakened H-2-H2O interaction and active drifting of H-2 in molecules' 5(12) cages. With the ab-initio modified LJ parameters, the growth of THF-H-2 hydrates was investigated by molecular dynamics simulations under various compositions and pressures of the system. Cage occupancy of H-2 in 5(12) cages of the binary hydrates increased when the THF concentration was lower than 5.56 mol%, but double occupation of H-2 in 5(12) cages was not observed. Cage occupancy of H-2 in 5(12)6(4) cages significantly increased by lowering the THF concentration, and the total storage capacity of H-2 increased by multiple occupations of H-2 in the 5(12)6(4) cages. The growth rate of the THF-H-2 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 H-2 could be enhanced by the tuning phenomena when mobile THF molecules in the liquid phase formed binary hydrates with H-2. (C) 2022 Elsevier B.V. All rights reserved.

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