Preparation and characterization of thin-film zeolite-PDMS composite membranes

Thin-film zeolite filled PDMS composite membranes, as thin as 3μm, have been prepared. Two criteria are essential for the successful preparation of these membranes: (a) application of ultrafine silicalite crystallites (0.2-0.5 μm) and (b) prepolymerization of the suspension comprising silicalite, two-component polydimethylsiloxane (PDMS) polymer, and isooctane. The separation properties of the composite membrane were characterized by pervaporation and gas permeation. High flux and high selectivity were observed in the pervaporation of aqueous ethanol solution. Composite membranes, having a silicalite to PDMS ratio (w_s/w_p) of 3.3, give a selectivity of 34 for separating ethanol from water, while the flux remains 150 g/m²-hr, at 22°C. The composite membranes also show improved performance in gas permeation, e.g. for separating O₂ from N₂. Optimization experiments provide suitable conditions and compositions for the preparation of the membranes. Employing PDMS concentrations in the 3.5-4.0 wt.% range and silicalite to PDMS ratios from 1.5 to 3.3, highly-selective composite membranes can be fabricated with high reproducibility.