Gas permeation and separation properties of sulfonated polyimide membranes

Permeability and selectivity of pure gas H2, CO2, O2, N2 and CH4 as well as a mixture of CO2/N2 for sulfonated homopolyimides prepared from 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and 2,2-bis[4-(4-aminophenoxy)phenyl] hexafluoro propane disulfonic acid (BAPHFDS) were measured and compared to those of the non-sulfonated homopolyimide having the same polymer backbone. The polyimide in a proton form (NTDA-BAPHFDS(H)) displayed higher selectivity of H2 over CH4 without loss of H2 permeability. Strong intermolecular interaction induced by sulfonic acid groups decreased diffusivity of the larger molecules. The CO2/N2 (19/81) mixed gas permeation was investigated as a function of humidity. With increasing relative humidity from 0% RH to 90% RH, the CO2 permeability for NTDA-BAPHFDS(H) polyimide increased by more than one order of magnitude, and the selectivity of CO2/N2 also increased twice or more. On the other hand, the gas permeability for the non-sulfonated polyimide slightly decreased with increasing humidity. NTDA-BAPHFDS(H) polyimide displayed a CO2 permeability of 290×10−10 cm3 (STP) cm/(cm2 s cmHg) and a separation factor of CO2/N2 of 51 at 96% RH, 50 °C and total pressure of 1 atm.