Hindering effects in diffusion of CO2/CH4 mixtures in ZIF-8 crystals

Cage-type micro-porous materials such as LTA, CHA, SAPO-34, DDR, ERI, ZIF-7, and ZIF-8 have significant potential for use in membrane technologies for CO2 capture. The permeation selectivities are governed by a combination of adsorption and diffusion selectivities, each of which can be separately manipulated. In this study we have investigated the characteristics of CO2/CH4 mixture diffusion in ZIF-8 by monitoring the uptake within crystals using infra-red microscopy (IRM). The uptake profiles were fitted to determine the “effective” Fick diffusivities of the individual components in the mixture. The diffusivity measurements, conducted for a range of loadings, clearly demonstrate that the preponderance of CO2 at the window regions of ZIF-8 hinders the inter-cage transport of partner CH4 molecules. Such hindering effects serve to enhance the CO2/CH4 permeation selectivities across ZIF-8 membranes for high mixture loadings within the membrane above values anticipated on the basis of unary permeances.

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► CO2 molecules lodge preferentially near the windows of ZIF-8.
► CO2 hinders the inter-cage hopping of CH4.
► Hindering effects enhance diffusion selectivities in favour of CO2.