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

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.