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

CO2 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 CO2 and CH4 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 CO2 and CH4 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 CO2 and CH4 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. [Display omitted] •Surface effects of ZIF-8 nanocrystals against diffusion of CO2 and CH4 were investigated.•Surface permeabilities of CO2 and CH4 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.