Polybenzimidazole (PBI) with a strong size-sieving ability and excellent thermal stability is a leading membrane material for H2/CO2 separation at elevated temperatures. For example, commercial m-PBI exhibited H2 permeability of 9.0 Barrer and H2/CO2 selectivity of 14 at 100 C. Incorporating porous fillers in polymers to form mixed matrix materials (MMMs) is an effective way to improve their performance. However, most porous fillers have lower H2/CO2 selectivity than PBI, leading to higher H2 permeability but lower H2/CO2 selectivity in the MMMs. In this study, we demonstrate the in-situ synergistic growth of crystalline zeolite imidazole framework-8 (ZIF-8) and polymer-incorporated aZIF-8 MMMs (CPAMs). The formed porous crystalline ZIF-8 (cZIF-8) increases gas permeability. More importantly, PBI contains benzimidazole groups on the backbones (similar to the precursor for ZIF-8, i.e., 2-methylimidazole) and induces the formation of amorphous ZIF-8, enhancing interfacial compatibility and tightening the PBI nanostructures. The structures of CPAMs were thoroughly characterized using FTIR, WXRD, TGA, and SEM. The crystalline ZIF-8 NPs have sizes of 0.7 - 1 μm, and amorphous ZIF structures distribute evenly in the films. For example, the formation of 15 mass% ZIF-8 in PBI improves H2 permeability and H2/CO2 selectivity by ~100% at 35 C. The effects of feed gas composition, water vapor, temperature, and operation time on the separation properties were investigated. The CPAM shows stable H2/CO2 separation performance (H2 permeability of 42 Barrer and H2/CO2 selectivity of 28) at 200 C, breaking the permeability/selectivity tradeoff, surpassing the Robeson upper bound and demonstrating its potential for precombustion carbon capture. This work, for the first time, unveils a new platform of MMMs comprising functional polymer-incorporated amorphous ZIFs with hierarchical nanostructures for various applications.

In situ synergistic growth of crystalline and polymer-incorporated amorphous ZIF-8 in polybenzimidazole achieving hierarchical nanostructures for H2/CO2 separation

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