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.