Revealing the Growth of H₂ + THF Binary Hydrate through Molecular Simulations

Abstract

The use of H2 as an ideal fuel is restricted by storage and transportation. It is proposed that THF is a promising promoter for storing H2 in the form of hydrates under mild conditions. However, the molecular-level details of the formation of H2 + THF binary hydrate are still unclear. In the present work, molecular simulations were conducted for two sets of temperature and pressure conditions to investigate the growth of H2 + THF binary hydrate. We find that H2 as a kinetic promoter can significantly accelerate the growth of a pure THF hydrate at 250 K and 50 MPa. Our results indicate that the evolution of the growth rate of H2 + THF binary hydrate with temperature and pressure is similar to that of the pure THF hydrate, where the growth rate is the maximum at 260 K and a fixed pressure of 50 MPa; pressure has a negligible impact on the growth rate of the binary hydrate at pressures of 10–110 MPa and a fixed temperature of 250 K. This means that THF molecules control the growth process of H2 + THF binary hydrate at our conditions except that the rearrangement of water molecules dominates the growth process at 230 K and 50 MPa. Increasing temperature and pressure can lower the percentage of empty cages in the newly formed binary hydrate, thus improving the occupancy of 512 cages by H2. The occupancy rate of 51264 by H2 at different temperatures is correlated with the growth rate of the binary hydrate. As a result, H2 storage capacity is associated with the trend of the growth rate. Moreover, increasing pressure will increase the average number of H2 molecules in each 51264 cage, and triple H2 clusters begin to fill 51264 at 110 MPa and 250 K, which dramatically enhances the H2 storage capacity in the binary hydrate.

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