Understanding the characteristic of methane hydrate equilibrium in materials and its potential application

Understanding the phase characteristic of methane hydrate in materials is of recent interest in several fields. In this study, methane hydrate dissociation conditions in nine water wetted porous materials (ZIF-8, MIL-53(Al), HKUST-1, nanoporous activated carbon, zeolite 13X, and four silica gels) with pore diameters ranging from 0.35 to 100 nm as well as in two nonporous quartz sands were systematically investigated. The adsorption properties of methane in some selected adsorbents were also studied. For microporous materials, our results confirmed that methane hydrate cannot form due to the size limitation for the nucleation of a methane hydrate crystal. In contrast, for mesoporous materials with a pore diameter of 6.2 nm or bigger, hydrate formed in their pores and significantly shifted dissociation temperatures and pressures compared to the bulk hydrate were observed. The largest temperature shift was found to be ∼6 K in materials possessing 6.2-nm-diameter pores, and this shift decreased to less than 1 K in structures having 45-nm-diameter pores. For macroporous materials with a pore diameter of 100 nm or for nonporous quartz systems, the influence of pore sizes on the methane hydrate phase equilibrium was found to be marginal. Furthermore, from studying the adsorption characteristic of methane in 25 wt% wetted MIL-53(Al), 30 wt% wetted HKUST-1, and 10 wt% wetted nanoporous activated carbon, we demonstrated that the penetration of water molecules into the pores of microporous materials was not only unable to form methane hydrate but also substantially deteriorated the gas adsorption ability. To exploit simultaneously adsorption and hydrate formation in wetted microporous materials for gas storage, both the hydrophobicity and aperture pore size of materials are critical toward the selection of promising adsorbents.