Molecular insights into hybrid CH4 physisorption-hydrate growth in hydrophobic metal–organic framework ZIF-8: Implications for CH4 storage

• CH 4 storage by adsorption-hydration hybrid was first studied at the molecular level. • Two-way migration phenomenon between CH 4 and H 2 O occurs in the physisorption process. • Competitive adsorption between ZIF-8 and hydrate goes against CH 4 hydrate formation. • Hydrophobic ZIF-8 surface facilitates hydrogen bond formation in interfacial water. • Synergistic effect of physisorption and intergranular hydrates promotes CH 4 storage. Adsorption-hydration hybrid method has been widely concerned due to its great application potential in CH 4 storage/transportation. Monte Carlo and molecular dynamics simulations were performed to investigate the hybrid CH 4 physisorption-hydrate growth in ZIF-8. The two-way migration phenomenon between water molecules inside the cavities and CH 4 molecules outside the cavities first occurs in the physisorption process, which increases the CH 4 adsorption capacity to a level close to that of dry ZIF-8. Due to the competitive adsorption for CH 4 between ZIF-8 and hydrate surface, it is not conducive to the stability of intergranular CH 4 hydrate before the CH 4 adsorption capacity of ZIF-8 attains the maximum. After the maximum, intergranular hydrate could exist stably and grow continuously with the dissolution of CH 4 . Meanwhile, partially adsorbed CH 4 molecules in the cavities migrate outwards and promote hydrate growth. These constitute the link between physisorption and hydrate growth. Because of the phase separation of CH 4 and water molecules, hydrate cannot form in the ZIF-8 cavities. As a result, under the synergistic effect of physisorption and intergranular hydrates, CH 4 storage capacity is significantly increased in H 2 O-oversaturated ZIF-8 compared with that in dry ZIF-8. Further, hydrophobic ZIF-8 surface facilitates hydrogen bonds formation in interfacial water by inducing ordered structures, and interfacial water as a medium promotes the contact between hydrate cage and ZIF-8 surface. An analysis of the diffusion properties of CH 4 /water molecules reveals that ZIF-8 cavities, hydrate cages, and intergranular liquid water exert inhibition on their self-diffusion coefficients in different degree. This study provides important microscopic insight into CH 4 storage by adsorption-hydration hybrid in ZIF-8.