Competition between CH₄ hydrate formation and phase separation in a wetted metal-organic framework MIL-101 at moderate subcooling: molecular insights into CH₄ storage

Adsorption-hydration hybrid technology has emerged as a promising technology to store CH4 in porous materials, as it synergistically improves CH4 storage capacity by combining CH4 adsorption and hydrate formation. However, the fundamental mechanism involved in this technology remains elusive. Herein, we perform systematic molecular dynamics simulations to explore CH4 hydrate formation in a metal-organic framework MIL-101 at moderate subcooling. Simulation results reveal that at moderate subcooling, CH4 hydrate formation and phase separation of CH4 to form nanobubbles occur simultaneously, and these two processes compete with each other for CH4 molecules in the solution. The outcome of the competition is primarily governed by the relative stabilities of CH4 hydrate solids and CH4 nanobubbles, which are closely related to their sizes. It is revealed that CH4 hydrate formation occurs exclusively in the outer space of MIL-101 cavities, whereas phase separation of CH4 to form nanobubbles takes place in the MIL-101 cavities and their outer space simultaneously. The small nanobubbles in the MIL-101 cavities gradually shrink and finally disappear, as CH4 molecules therein diffuse out and grow into large nanobubbles and large hydrate solids in the outer space. Moderate subcooling appears to facilitate the formation of large ordered CH4 hydrate solids containing sI and sII domains. Additionally, it is found that lower subcooling and presence of MIL-101 both promote phase separation of CH4. In the evolution of large and small nanobubbles during phase separation, coalescence of CH4 nanobubbles and an interesting phenomenon similar to Ostwald ripening are observed. The molecular insights into the effects of the degree of subcooling on CH4 hydrate formation in MIL-101 provide bottom-up guidance on optimizing pressure-temperature conditions for CH4 storage in porous materials with adsorption-hydrate hybrid technology.