Influence of adsorbate-induced crystal changes on zeolite membranes

Previous studies found dramatic decreases in permeation fluxes and higher than expected mixture selectivities upon adsorption of n-hexane, SF6 , and several other molecules in MFI membranes. The hypothesis was that MFI zeolite crystals expanded upon molecular adsorption and decreased the defect size within the membranes. Indeed, previous studies have shown that molecules change the zeolite unit cell dimensions. This thesis research directly measured the changes in zeolite unit cell dimensions due to adsorption at saturation and less than saturation loadings using XRD and optical microscopy, and determined that the unit cell changes correlate with the changes in membrane defect size, as estimated from various permeation measurements. First, changes in MFI unit cell parameters were directly measured using optical microscopy and single-crystal XRD. Expansion was seen for n-alkanes and alcohols, which correlated with permeation measurements. The percent volume expansion for C5-C13 n-alkanes also correlated linearly with the number of carbon atoms per unit cell. Next, it was found that SF6 expanded the B-ZSM-5 crystals below saturation loading at 300 K and n-hexane expanded B-ZSM-5 crystals below saturation at 300, 398, and 473 K. As the loadings and temperature increased, the crystals expanded more. In contrast, i-butane adsorption at 300 K contracted B-ZSM-5 crystals at low loadings and expanded them at higher loadings. Finally, XRD measurements showed that zeolite NaA crystals contracted at low methanol and water loadings, and expanded at higher loadings. Methanol and CO2 expanded the SAPO-34 structure and as the loading increased, the percent expansion increased. Changes in the unit cell volumes for B-ZSM-5 and zeolite NaA correlated with changes in permeation through zeolite membranes defects. Collectively, understanding how defect sizes change with adsorption may potentially be used to improve membrane selectivity by adding small amounts of adsorbates to the feed, and will also improve zeolite membrane characterization and synthesis methods.