Accelerated formation of hydrate in size-varied ZIF-8 for CH4 storage by adsorption-hydration hybrid technology

CH4 storage by the adsorption-hydration hybrid is considered as the most potential method in CH4 storage/transportation. Metal-organic frameworks (MOFs) as porous materials have significant potential in CH4 storage. However, little is known with respect to the rapid formation and spatial growth of hydrate in MOFs. In this work, microstructures of hydrophobic ZIF-8 demonstrated inner cavities could act as CH4 physisorption sites. The experiment of CH4 adsorption-hydration hybrid in three particle sizes of ZIF-8 was carried out in visualization system, suggesting hydrate nucleation and growth preferentially occurred in micro bulges and surface gaps. Three morphologies of fog-like hydrate, hydrate fibers and hydrate chunks were observed in ZIF-8, depending on material surface and particle sizes. On this basis, a novel approach to facilitate hydrate formation was proposed by using different particle sizes ZIF-8 enhanced CH4 physisorption. The CH4 storage capacity and kinetics of adsorption-hydration hybrid under different water content were further investigated, providing suitable water content for exploring the effects of ZIF-8 particle size on hydrate formation kinetics. Compared with the three particle sizes range of ZIF-8, an extremely short hydrate induction time is 1.08 min, corresponding to the particle size of 0.5 ∼ 1.5 mm. In the particle sizes range studied, the rate of gas consumption demonstrated that CH4 physisorption is higher with ZIF-8 particle size of 0.5 ∼ 1.5 mm, and the rate of physisorption is negatively correlated with hydrate induction time. Finally, the similar kinetics effect of CH4 adsorption-hydration hybrid could be attained with three particle sizes of recycled ZIF-8. The property of the ZIF-8 is stable confirmed from the scanning electron microscope measurements after it undergoes two cycles. This study would shed light on the understanding of CH4 rapid storage by adsorption-hydration hybrid in MOFs, which provided new perspective to improve the efficiency of the natural gas storage.