Physical adsorption analysis of intact supported MFI zeolite membranes

We compare the adsorption properties of intact supported silicalite membranes with those of silicalite powder and of alumina supports using nitrogen and argon as adsorbates at 77 K. We disentangle contributions from the membrane and support and find that the support contributes significantly to the total quantity adsorbed due to its relative thickness. The micropore-filling regions of the adsorption isotherms of the powder and the supported membrane are nearly identical for the membranes studied, but the isotherms differ at higher pressures--the supported membranes exhibit a much higher quantity adsorbed than the powders. Despite this difference, no hysteresis is observed in the membrane isotherms, indicating a lack of mesoporosity (pores in the 2-50 nm range) in either membrane or support for this preparation. We estimate argon transport fluxes at steady state by assuming surface diffusion with both a constant and concentration-dependent Maxwell-Stefan diffusion coefficient in the zeolite and the support. Further, we use the respective adsorption isotherms to determine the thermodynamic correction factors--that is, the ratios of the Fick and Maxwell-Stefan diffusion coefficients--required to solve the diffusion equation. The estimated argon flux is virtually the same using adsorption data from powders and membranes. For the relatively thick supports used in our study (approximately 2 mm), we find that the support exerts a much greater influence on the predicted fluxes for a wide range of values of the ratio of the support to zeolite diffusion coefficients. We emphasize that the results are specific to the architecture of the supported membranes studied, and thus, the results should be interpreted accordingly.