Energy & Fuels, Vol.34, No.11, 15004-15010, 2020
Revealing the Growth of H-2 + THF Binary Hydrate through Molecular Simulations
The use of H-2 as an ideal fuel is restricted by storage and transportation. It is proposed that THF is a promising promoter for storing H-2 in the form of hydrates under mild conditions. However, the molecular-level details of the formation of H-2 + 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 H-2 + THF binary hydrate. We find that H-2 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 H-2 + 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 H-2 + 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 5(12) cages by H-2. The occupancy rate of 5(12)6(4) by H-2 at different temperatures is correlated with the growth rate of the binary hydrate. As a result, H-2 storage capacity is associated with the trend of the growth rate. Moreover, increasing pressure will increase the average number of H-2 molecules in each 5(12)6(4) cage, and triple H-2 clusters begin to fill 5(12)6(4) at 110 MPa and 250 K, which dramatically enhances the H-2 storage capacity in the binary hydrate.