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We have provided two feasible methods to prepare stable organic solvent nanofiltration (OSN) membranes by chemically cross-linking the polybenzimidazole (PBI) membranes fabricated from environmentally benign ionic liquids for the first time. A glutaraldehyde (GA)–water solution and a 1,2,7,8-diepoxyoctane (DEO)–n-heptane solution were employed to cross-link PBI membranes. The effects of these cross-linking agents on membrane stability and separation performance toward various solvents, including polar aprotic solvents, have been carefully examined. The GA cross-linking method, with little consumption of traditional organic solvents and energy, yielded a PBI membrane with a high and steady ethanol flux [3.69 L (m2 bar h)−1] and ethyl acetate flux [5.21 L (m2 bar h)−1]. Although PBI membranes cross-linked by DEO showed lower ethanol and ethyl acetate fluxes, they displayed impressive stability in aggressive solvents such as dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidinone (NMP) and N,N-dimethylacetamide (DMAc). Overall, PBI membranes cross-linked by both agents exhibited reasonable rejections which varied slightly from solvent to solvent due to different solute–solvent–membrane interactions. This study not only offers a new range of membrane materials and fabrication approaches for OSN, but will also enhance membrane-based green technology for the recovery of organic solvents.

This study has investigated the molecular interactions between ionic liquids and PBI and discovered a potentially green solvent to fabricate polybenzimidazole (PBI) membranes for water reuse and protein separation. Ionic liquid, 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc), exhibits superior efficiency in dissolving PBI under much lower temperatures and pressures compared to the traditional toxic N,N-dimethylacetamide (DMAc). this is because the acetate anions of [EMIM]OAc could form hydrogen bonding with PBI chains and effectively break up the interchain hydrogen bonding in PBI molecules. Molecular simulations have verified this mechanism by showing the largest amount of hydrogen bonding and the lowest interaction energy in the PBI–[EMIM]OAc system among the studied PBI–ionic liquid systems. The PBI–[EMIM]OAc solution also displays unique rheological properties significantly deviated from the traditional Cox-merz rule, and the shear thinning rheology at low shear rates implies a strong charge-ordered structure resulting from the intense hydrogen bonding. In addition, PBI ultrafiltration membranes have been prepared from PBI–[EMIM]OAc solutions by the non-solvent induced phase separation method and displayed a relatively thick sponge-like structure with a few macrovoids. After thermal treatment in ethylene glycol at 140 °C and chemical cross-linking by dichloro p-xylene, the PBI membranes achieve a high separation factor of 94.55 for a binary protein mixture containing bovine serum albumin and hemoglobin with the aid of combined effects from size exclusion and charge repulsion. This study provides a viable alternative to the current fabrication technology of PBI membranes in a “green” process with the aid of ionic liquids.