Tailoring mixed matrix composite membranes for gas separations

Abstract

Polymeric gas separation membrane materials have improved significantly over the past two decades due to systematic optimization in backbone structure. Recent evidence suggests, however, that advantages of a purely polymer-based approach are reaching diminishing returns for important separations such as O2 and N2. Zeolites, carbon molecular sieves (CMS), and rigid rod polymers offer attractive transport properties but are difficult and expensive to process. Mixed matrix composite (MMC) membranes, incorporating molecular sieving materials within polymeric substrates, may provide economical, high performance gas separation membranes if defects at the molecular sieve/polymer interface can be eliminated. In addition, careful matching of the intrinsic permeability and selectivity of the support matrix and the molecular sieve domains is necessary. Theoretical O2 permeability and O2/N2 selectivity predictions are presented for such optimized membranes of zeolite 4A and CMS in Ultem® and Udel® matrices using two idealized expressions based on classical and more recent results. Positive and desirable deviations from these idealized results can be anticipated if a continuous molecular sieving phase is formed, but this special case is not required if a proper selection of the polymer matrix is made.