Green Preparation of Thin Films of Polybenzimidazole on Flat and Hollow Fiber Supports: Application to Hydrogen Separation

This work shows the preparation of thin films, with thickness from 70 nm to 1 μm, of meta-polybenzimidazole (m-PBI) on polyimide P84 supports. Ethanolic solutions of m-PBI were used to coat flat and hollow fiber supports of asymmetric P84 with m-PBI in a process where the coating and drying was performed at room temperature. A solution of NaOH in EtOH allowed the dissolution of the m-PBI powder, providing the perfect coating solution to build thin films of m-PBI without damaging the polymeric support. It also meant a green alternative, avoiding the use of toxic solvents, such as dimethylacetamide. The resulting membranes have been tested for the separation of H₂ mixtures at high temperature at different setups to allow checking their reproducibility. With 100 nm thickness the membranes showed their best gas separation performance. For flat membranes at 180 °C and 3 bar feed pressure a H₂ permeance of 48.5 GPU was obtained, with respective H₂/CO₂ and H₂/N₂ selectivities of 33.3 and 55.8. Besides, the hollow fibers under a feed pressure of 6 bar and tested at the same temperature showed near 90 GPU of H₂ with a H₂/CO₂ selectivity of 13.5 in the one-fiber module and over 39 GPU of H₂ with a H₂/CO₂ selectivity of 20.2 in the five-fiber module. Finally, the stability of the membranes was proved for 22 days at 180 °C.