Effect of the annealing temperature of polybenzimidazole membranes in high pressure and high temperature H2/CO2 gas separations

Abstract

The effect of the annealing temperature of polybenzimidazole (PBI) membranes on H2/CO2 gas separations was investigated. Membranes annealed from 250 °C to 400 °C were tested for gas permeation with pure H2, CO2, and N2 gases and a H2:CO2 (1:1) mixture at 35 °C, 100 °C, 200 °C, and 300 °C and at pressures up to 45 bar. Gas permeation data show that permeability and selectivity of the membranes is significantly impacted by the annealing temperature, the presence of adsorbed water, and remaining casting solvent (DMAc). At a testing temperature of 35 °C, ideal H2/CO2 selectivities of 50, 49, and 66 with pure H2 permeabilities of 1.5, 0.8, and 1.5 Barrer were obtained for membranes annealed at 250 °C, 300 °C, and 400 °C, respectively. At this temperature, high gas mixture H2/CO2 selectivities of 41, 73, and 47 with H2 permeabilities of 1.03, 0.26, and 0.50 Barrer were also obtained for these membranes. At testing temperatures of 300 °C, both the ideal and gas mixture H2/CO2 selectivities dropped to 44, 28, and 30 (ideal, H2 = 45, 45, 44 Barrer) and to 19, 22, and 23 (mixture, H2 = 41, 43, and 44 Barrer) for membranes annealed at 250 °C, 300 °C, and 400 °C, respectively. As water was removed from the membranes at temperatures greater than 100 °C during permeation cycles, where the testing temperature was increased from 35 °C to 300 °C, the permselectivity properties of the membranes annealed at 400 °C became more reproducible. Permeabilities at 35 °C from a second permeability cycle increased, but H2/CO2 selectivities decreased to 21 for gas mixtures (H2 = 1.4 Barrer) and to 34 for pure gases (H2 = 2.2 Barrer). The results suggest that high annealing temperatures may induce changes in the configuration and conformation of the polymer chains, imparting distinctive permselectivity properties to the membranes. Activation energies of permeability for H2, CO2, and N2 from pure gases and H2:CO2 mixtures correlated with these changes as well.