Polybenzimidazole (PBI)/zeolitic imidazolate frameworks (ZIF-8) mixed matrix membranes for pervaporation dehydration of alcohols

We have synthesized nano-sized zeolitic imidazolate frameworks (ZIF-8) and prepared polybenzimidazole (PBI)/ZIF-8 mixed matrix membranes (MMMs) with uniform morphology comprising ZIF as high as 58 wt% by directly mixing the as-synthesized ZIF-8 suspension into the polymer dope for pervaporation dehydration of ethanol, isopropanol (IPA) and butanol. Pervaporation test data show that the water permeability of PBI/ZIF-8 (1:1) MMMs is about one order higher than the original PBI membrane (14,000–22,000 vs. 1200–2300 Barrer). The apparent activation energy calculated from the Arrhenius equation indicates that there is a 29.2% reduction of energy barrier for penetrant transports for PBI membranes consisting of 33.7 wt% ZIF-8 nano-particles. As a result, the 33.7 wt% ZIF-8 in PBI membrane increases water permeability four times without much decrease in selectivity for water. The 58.7 wt% ZIF-8 in PBI membrane has a very high water permeability but a relatively low selectivity. Furthermore, the MMMs also show effectiveness in suppressing ethanol-induced swelling on the polymeric matrix owing to its inorganic properties. Vapor sorption studies confirm that the enhancement of pervaporation performance is primarily attributed to the significant improvement in diffusivity selectivity. With high thermal stability, chemical resistance and excellent compatibility with PBI, ZIF-8 nano-particles are promising fillers for enhancing the overall separation performance of low permeability but high performance materials like PBI.