CO2 and CH4 diffusivities through synthesized ZIF-8 nanocrystals: An experimental and theoretical investigation

Abstract

CO 2 separation is of paramount importance owing to global warming abatement, as well as sour natural gas sweetening. Mixed matrix membranes (MMMs), containing porous zeolitic imidazolate frameworks (ZIFs), in particular ZIF-8, have drawn noticeable attention due to their favorable separation performance. However, there are evidences of discrepancies in the reported values of gas diffusion coefficients/diffusivities through ZIFs, even for common well-studied gases, such as CO 2 and CH 4 in the literature. These discrepancies might be attributed to the surface/barrier effects, which are intensified by ZIF-8 nanocrystals downsizing. In this paper, an effort has been made to determine the impact of these surface effects on diffusivities of CO 2 and CH 4 through ZIF-8 nanocrystals. To do so, ZIF-8 nanocrystals ( ~ 100 nm), pristine poly(ethylene glycol diacrylate) (PEGDA) membrane, and PEGDA/ZIF-8 mixed matrix membranes (MMMs) were prepared. Gas adsorption/sorption measurements for CO 2 and CH 4 over a temperature range of (308–348) K were conducted. The Maxwell model was applied to obtain gas permeabilities in pure ZIF-8 nanocrystals by employing the measured values of permeabilities through fabricated MMMs. The solubilities in ZIF-8 nanocrystals were found using adsorption isotherms, and the corrected/intracrystalline diffusivities were subsequently obtained using solution–diffusion theory. Moreover, the apparent/extracrystalline diffusivity values, affected by the surface effects, were attained from transient uptake rate measurements, and were compared to the corrected ones. The surface effects were eventually determined via a single-resistance model. • Surface effects of ZIF-8 nanocrystals against diffusion of CO 2 and CH 4 were investigated. • Surface permeabilities of CO 2 and CH 4 in ZIF-8 were obtained in the range of 10 −8 cm s −1 . • Corrected transport diffusivities were corresponding to intracrystalline diffusivities. • Extracrystalline diffusivities were found seven orders of magnitude lower than the intracrystalline diffusivities.