Polybenzimidazole (PBI) Membranes Cross-Linked with Various Cross-Linkers and Impregnated with 4-Sulfocalix[4]arene (SCA4) for Organic Solvent Nanofiltration (OSN)

The chemical, pharmaceutical, and edible oil industries have employed enormous amounts of organic solvents to produce medicines, chemicals, and other goods. In order to recycle these valuable organic solvents, organic solvent nanofiltration (OSN) is crucial. However, choosing the optimal cross-linker is vital for producing membranes with improved performance and chemical resistance. In this study, integrally skinned polybenzimidazole (PBI) flat membranes for OSN were studied through a covalent modification employing α, α′-dibromo-p-xylene (DBX), α, α′-dichloro-p-xylene (DCX) or trimesoyl chloride (TMC), followed by ionic impregnation with 4-sulfocalix [4]arene (SCA4). The nucleophilic-electrophilic reactions between PBI and cross-linkers and the ionic interactions among SCA4, PBI and cross-linkers were investigated and elucidated. Due to the two-fold modifications, the resultant PBI membranes show enhanced molecular sieving capabilities and chemical stability. Especially, the membranes modified with DBX and DCX, and then SCA4 infiltration demonstrate a wide range of stability in harsh organic solvents such as n-methyl-2-pyrrolidone (NMP) and dimethyl acetamide (DMAc). They also possess impressive rejections against low molecular weight (Mw) dyes, reconfirming their superiority in terms of structural and chemical stability to PBI for practical OSN uses. However, the dye rejection is still influenced by the physical characteristics of the solvents and dyes as well as the interactions among the solute, solvent, and membrane. This work may provide a workable plan to design membranes with precise molecular sieving capabilities for OSN.