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

Adsorption-hydration hybrid method has been widely concerned due to its great application potential in CH4 storage/transportation. Monte Carlo and molecular dynamics simulations were performed to investigate the hybrid CH4 physisorption-hydrate growth in ZIF-8. The two-way migration phenomenon between water molecules inside the cavities and CH4 molecules outside the cavities first occurs in the physisorption process, which increases the CH4 adsorption capacity to a level close to that of dry ZIF-8. Due to the competitive adsorption for CH4 between ZIF-8 and hydrate surface, it is not conducive to the stability of intergranular CH4 hydrate before the CH4 adsorption capacity of ZIF-8 attains the maximum. After the maximum, intergranular hydrate could exist stably and grow continuously with the dissolution of CH4 . Meanwhile, partially adsorbed CH4 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 CH4 and water molecules, hydrate cannot form in the ZIF-8 cavities. As a result, under the synergistic effect of physisorption and intergranular hydrates, CH4 storage capacity is significantly increased in H2O-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 CH4/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 CH4 storage by adsorption-hydration hybrid in ZIF-8.