Isomer-Tailored Carbon Molecular Sieve Membranes with High Gas Separation Performance.

Partial substitution of the asymmetric 3,3',4,4'-biphenyl dianhydride monomer (aBPDA) into the backbone of a 6FDA-BPDA-DAM (6FDA=4,4'-hexafluoroisopropylidene, DAM=diaminomesitylene) diphthalic anhydride-based copolyimide based on symmetrical BPDA (sBPDA) was used to study membrane structure-processing-property relationships for gas separation. Properties of the polymer membrane as well as derived carbon molecular sieves (CMS) membranes were compared with copolyimides without the asymmetric monomer structure. CMS membranes derived from the copolyimides are very attractive for CO2 /CH4 separation. aBPDA provides the copolyimide with additional packing-inhibited structures compared with the symmetric ones and yields a corresponding CMS membrane with very high CO2 permeability and good CO2 /CH4 selectivity. This work, therefore, outlines a new strategy for tuning CMS membrane structure to meet separation performance needs.