Polymer/molecular sieve composite membranes for gas and vapor separations
Polymer/molecular sieve composites are gaining headway as new materials for separations. While this idea has been around for almost two decades, the fabrication of successful membranes has been hindered by incompatibility of the two materials. Two examples of polymer/molecular sieve membranes are described to illustrate the importance of proper materials selection and possible strategies to overcome these challenges.
A layered aluminophosphate was incorporated into a polysulfone matrix to assess possible improvement in gas separation selectivity. Exfoliation of the layered material can lead to large aspect ratios which introduce tortuous pathways for gas transport as well as contribute to proper orientation of the platelets. Several attempts were made to exfoliate the layered aluminophosphate into thin sheets by chemical and mechanical means. However, these treatments destroyed the integrity of the pore structure, and in some cases, created undesirable defects in the composite membranes, leading to poor gas separation performance for N2, O2, and CO2. Although the permeability of the composites was either higher or lower than polysulfone, selectivities were generally lower in all cases and for all gas pairs tested. Since the proper microstructure was never obtained, the poor results in permselectivity are not unreasonable.
The second example details the formation of membranes of silicalite-1 and poly(1-trimethylsilyl-1-propyne) as composite materials for the separation of hydrocarbons. The effect of different particle morphologies was examined, but the membrane performance was better correlated with film quality. The separation of butane isomers in the composites showed significant improvement, giving increased permeability and simultaneous increased selectivity over the pure polymer. This is the first successful demonstration of the incorporation of a molecular sieve into a polymer matrix for butane separations. The composite membranes were also tested for separations of n-hexane/2,2-dimethylbutane and p-xylene/o-xylene.