Formation of CH4 Hydrate in a Mesoporous Metal-Organic Framework MIL-101: Mechanistic Insights from Microsecond Molecular Dynamics Simulations.

The formation of CH4 hydrate in a mesoporous metal-organic framework MIL-101 is investigated by microsecond molecular dynamics simulations. CH4 hydrate is observed to form preferentially in the outer space of MIL-101 cavities rather than inside the cavities; only when the hydrate formation is nearly complete in the outer space can stable hydrate form in MIL-101 cavities. The underlying reason is revealed to be the easy dissociation of small hydrate clusters formed in the nanospace of the cavities, because of the diffusion of CH4 molecules out of the cavities into the outer space. Compared with dry MIL-101, the CH4 storage capacity of H2O-saturated MIL-101 is drastically reduced as the cavities are occupied by H2O. When oversaturated with H2O, however, extra H2O molecules in the outer space of the cavities can form considerable CH4 hydrate, significantly promoting CH4 storage capacity. This study provides important mechanistic insights into the formation mechanism and process of CH4 hydrate in MIL-101 and will facilitate the design of emerging materials for energy storage.