Factors affecting alcohol–water pervaporation performance of hydrophobic zeolite–silicone rubber mixed matrix membranes

Mixed matrix membranes (MMMs) consisting of ZSM-5 zeolite particles dispersed in silicone rubber exhibited ethanol–water pervaporation permselectivities up to five times that of silicone rubber alone and three times higher than simple vapor–liquid equilibrium (VLE). A number of conditions resulted in the deterioration in pervaporation performance of these MMMs. Long-term exposure to simple binary ethanol–water mixtures resulted in slow declines in ethanol permeability, water permeability, and selectivity. Treating the particles by quenching them in linear short-chain alcohols immediately after calcination protected the MMM against this performance decline. In addition, rapid and dramatic reductions in both ethanol permeability and selectivity occurred when the MMMs were exposed to a centrifuged yeast fermentation broth. Exposure to a clarified acetone/n-butanol/ethanol (ABE) fermentation broth also resulted in marked reductions in performance, albeit less severe and not as rapid as with the yeast broth. The effect of the broths was not easily reversed. The most effective regeneration scheme was soaking the MMM in 100% n-butanol, which fully recovered the original performance. Zeolite pore blockage due to adsorption of minor constituents in the broths, including organic acids, esters, alcohols, and esterification reaction products was identified as the likely cause of performance changes.