Poly-/metal-benzimidazole nano-composite membranes for hydrogen purification

Hydrogen energy offers many unique advantages and is becoming a promising alternative and renewable energy source. In the large scale hydrogen productive water-gas shift reaction, carbon dioxide is the main byproduct of this unit operation in the leaving stream and should be captured for effective H ₂ usage and environmental concerns. Therefore, the removal of CO ₂ is the primary step in the hydrogen purification. Comparing with traditional separation methods, membrane based separation technologies show the advantages of less energy deduction, more environmental friendly and smaller footprint of the operation unit. Most of available polymeric membrane materials employed currently can only be used below 150 °C and are not stable in much harsh high-temperature environments required for many industrial gas separations. As a unique polymer, polybenzimidazole (PBI) has remarkable resistance to high temperatures (up to 500 °C) with superior compression strength. However, this material shows relatively low gas permeability for directly gas separation usages. In this study, a novel scheme to fabricate nano-composite membrane materials containing fully dispersed nano-size zeolitic imidazolate frameworks (ZIF) has been proposed for the first time. By mixing the ZIF-7 nano-particles with polybenzimidazole (PBI), the resultant membranes not only achieve an unprecedented ZIF-7 loading as high as 50 wt %, but also overcome the low permeability nature of PBI. The membranes exhibit characteristics of high transparency and mechanical flexibility, together with enhanced H ₂ permeability and ideal H ₂/CO ₂ permselectivity surpassing both neat PBI and ZIF-7 membranes. Advanced instrument analyses have confirmed the unique ZIF-polymer interface and elucidate mixed matrix structure that contributes to the high ZIF loading and enhanced gas separation performance superior to the prediction from the Maxwell model. The high thermal stability, good dispersion of ZIF nano particles with minimal agglomeration and the attractive gas separation performance at elevated temperatures up to 180 °C indicate the practicability of this nano-composite material for hydrogen production and CO ₂ capture in realistic industrial applications under harsh and extreme environments.