Nanoporous 6FDA-Polyimide Benzimidazole Membranes for High-Temperature Hydrogen Recovery

Abstract

Development of nanomaterials with enhanced functional properties by efficient methods is a current scientific and technological challenge in high-temperature gas separation, particularly in hydrogen recovery. In this study, nanoporous polyimide benzimidazole films of a thermostable nanoporous polymer (6FDA-DAPBI) are prepared by the polycondensation of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI) in the presence of benzoic acid as a catalyst in phenol under Ar flow, followed by casting the reaction solution onto a glass substrate. For the first time, gas transport characteristics of the 6FDA-DAPBI film for individual gases such as He, H2, O2, N2, CO2, and CH4 in the range 100−250 °C, as well as diffusion coefficients, solubility coefficients, diffusion and permeability activation energies, and heats of sorption, are thoroughly investigated. The presence of nanoporous morphology is confirmed by the CO2 sorption employing the Dubinin−Radushkevich and NLDFT methods. At about 150 °C, a slope change for the permeability vs temperature dependence is observed. It indicates a polymer transition to a metastable state with the increased free volume and reduced gas apparent activation energies, which is maintained during further heating and cooling cycles. Above 150 °C, the Robeson diagram data for individual H2 and CO2 and for the H2/CO2 mixture of various compositions lie above the 2008 upper bound for each heating cycle. Variation of the component ratio in the H2−CO2 mixture reveals that the higher hydrogen concentration leads to an increase of the separation factor. For a hydrogen concentration of 60 vol % at 250 °C and pressure drop of 1 atm, the separation factor reaches 300−350, which is almost 40 times higher compared with the ideal selectivity due to the presence of nanoscale channels within the matrix. The presented findings indicate a high potential for applying the developed nanoporous material in high-temperature hydrogen recovery technologies.