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.