Correlation between Free-Volume Properties and Pervaporative Flux of Polyurethane−Zeolite Composites on Organic Solvent Mixtures

Micron-sized zeolite particles were incorporated into a polyurethane (PU) matrix to prepare ethylbenzene-selective membranes. The resulting composite membranes were used in the pervaporation (PV) of ethylbenzene/styrene (EB/ST) mixtures. The sorption, diffusion, and PV permeation behaviors as a result of zeolite addition were elucidated. Zeolite is less chemically compatible with organic solvents than PU and the PU−zeolite composites, which exhibited suppressed solvent solubilities compared with pristine PU. However, these membranes favor EB transport by diffusion selectivity. The diffusivity and permeation flux increases in parallel with the enlarged radius of the free-volume hole size (R 4 increasing from 3.46 to 3.64 Å using positron annihilation lifetime spectroscopy analysis) by increasing the zeolite content from 0 to 23%. The enlarged free volume at a zeolite loading of 23% promoted pure solvent diffusivities by 10% higher than that of the unfilled film. During the PV operation on the EB/ST mixture, a significant diffusion-coupling was observed, and the permeant diffusion coefficients from the binary mixture exceeded the pure solvent diffusivity. The permeation flux was greatly improved (up to 0.72 kg/m2·h) by zeolite addition without any detrimental effect on the separation efficiency.