Gas Separation Properties of Unsupported Carbon Molecular Sieve Membranes Obtained from Blends of High Temperature Rigid Aromatic Polymers

Carbon molecular sieve membranes (CMSM) is an emerging technology in gas separations that could offer an outstanding relationship of permeability and selectivity for different gas pairs. In the present work, CMSM’s from polyimide PI DPPTD-MIMA, polybenzimidazole (PBI) and their blends are reported. CMSM’s preparation was performed at a pyrolysis temperature of 600ºC following a method reported by Ning and Koros (2014). Wide angle X-ray diffraction (WAXD) measurements were performed on CMSM using a SIEMENS 5000 X-ray diffractometer with CuKα radiation (wavelength 1.54 Å). Diffraction patterns show an amorphous halo with two maxima d-spacing obtained by Bragg’s law (nλ=2d sinθ): the first between 6.17 - 6.37 Å and the second was located on ~4 Å. In CMSM`s obtained from PI DPPTD-MIMA/PBI blends, the intensity of each maxima d-spacing show a variation related with the presence of PBI in membrane precursors. Gas transport properties were determined by a variable-pressure constant-volume method. Gas permeability coefficients were measured at 35ºC and 2 atm upstream pressure for He, O₂, N₂, CH₄ and CO₂. Permeability coefficients follow an order related to gas kinetic diameters (P_He > P_CO2 > P_O2 > P_N2 > P_CH4). Higher permeability coefficient than in the PI DPPTD-MIMA and PBI polymer precursor was obtained for CO₂ (P_CO2 = 435.79 Barrer) and O₂ (P_O2 = 118.10 Barrer).CMSM from PI DPPTD-MIMA/PBI surpass separation performance trade-off for various industrial interesting gas pairs, CO₂/CH₄ (selectivity = 56.04) and O₂/N₂ (selectivity = 8.47) separation. It was found that PBI concentration in membrane precursors affects gas separation properties in CMS membranes by diminishing permeability coefficients for all gases tested and selectivity for the gas pairs studied. Gas permeability coefficients temperature dependence in CMS membranes between 35 and 65ºC at 2 atm upstream pressure was examined. The results show that gas permeability coefficients for CMSM membranes increase with temperature following an Arrhenius type behavior.