Abstract
AbstractThe behavior of CH4 hydrate growth and CH4/H2O migration in confined interconnectivity space between the surface of ZIF-8 is studied by molecular dynamics simulation. To investigate the main growth position and direction of hydrate when CH4 is dissolved in aqueous solution and the effect of CH4/H2O migration to promote hydrate growth on the surface of hydrophobic ZIF-8 with pre-adsorbed water. The system of three-phase and four components containing CH4, H2O, hydrate and ZIF-8 is constructed before MD simulation. Microstructure properties are analyzed for regions containing hydrate near the surface of ZIF-8 (region1) and hydrate in the center of interconnectivity space (region2). The self-diffusion coefficients of H2O, CH4 are calculated, self-diffusion coefficients of CH4 and H2O are 0.47, 0.85 m2/s and 0.37, 0.54 m2/s in region1 and 2, respectively. The results demonstrate that the hydrate preferentially grows near the surface of ZIF-8 rather than the region of center. Because of the competitive adsorption for CH4 between ZIF-8 and hydrate surface, CH4 distribution is mainly concentrated in cavities and center of confined interconnectivity space, whereas H2O distribution is focused in the region near surface of ZIF-8 and center water shows the ordered structures by facilitating hydrogen bonds formation. Therefore, two-way migration of CH4/H2O further promote cooperativity between physical adsorption and hydrate growth in confined space, the nanopores of ZIF-8 could serve as sites of CH4 adsorption and preferentially enhance hydrate growth near the region of ZIF-8 and expand by a layer-by-layer mechanism for growth of the hydrate. Furthermore, hydrate growth in confined space is directional. The preferential growth directions of hydrate are boundary and center rather than the surface of ZIF-8. This study provides microscopic insight into CH4 storage by adsorption-hydration hybrid in ZIF-8, which can be favorable for the investigation of storage and utilization of hydrate in future.KeywordsMethane adsorption-hydration hybridCH4 hydrateMetal-organic frameworkMolecular dynamics simulation
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