Influence of crystal expansion/contraction on zeolite membrane permeation

X-ray diffraction was used to measure the unit cell parameters of B-ZSM-5 and SAPO-34 zeolite powders as a function of adsorbate loading at 300K, and in one case, at elevated temperatures. Most adsorbates expanded the zeolite crystals below saturation loading at 300K: n-hexane and SF₆ in B-ZSM-5 and methanol and CO₂ in SAPO-34 zeolite. As the loadings increased, the crystals expanded more. Changes in the unit cell volumes of B-ZSM-5 and SAPO-34 zeolite powders correlated with changes in permeation through zeolite membranes defects. When the zeolite crystals expanded or contracted upon adsorption, the defect sizes decreased or increased. In B-ZSM-5 membranes, the fluxes through defects decreased dramatically when n-hexane or SF₆ adsorbed. In contrast, i-butane adsorption at 300K contracted B-ZSM-5 crystals at low loadings and expanded them at higher loadings. Correspondingly, the flux through B-ZSM-5 membrane defects increased at low i-butane loadings and decreased at high loadings because the defect size increased at low loadings and decreased at high loadings. At 398 and 473K, n-hexane expanded the B-ZSM-5 unit cell more than at 300K. The silicalite-1 and B-ZSM-5 unit cell volumes expanded similarly upon n-hexane adsorption at 300K, indicating boron substitution had little effect on volume expansion.