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

Abstract

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.