High-performance gas separation membranes derived from thermal-oxidative block poly(benzoxazole-co-imide)

Abstract

Thermally rearranged(TR) polymers are usually prepared at an insert atmosphere and display high gas permeability, but their low gas selectivity results in high energy consumption. Thermal oxidation is an effective method to enhance the gas selectivity of microporous polymers. In this work, the effect of oxygen in the purge environment on imide-to-benzoxazole conversion and membrane structures was investigated. 1% oxygen content of the purge environment could restrain imide-to-benzoxazole conversion of 6FDA-DETDA/bisAPAF hydroxyl block polyimide(bHPI) membranes effectively, and oxygen induced the oxygen bridge formation, improving the gas selectivities of the resulting thermal-oxidative membranes with the increased oxygen content. Thermal-oxidative PBOCI membranes (TO-PBOCI) based on block poly(benzoxazole-co-imide)(PBOCI) precursor membranes were prepared to enhance gas separation performance of microporous polymers. The stable polybenzoxazole structures and microphase separation of polybenzoxazole blocks maintained membrane structures and the gas transport channels in the thermal-oxidative process at high temperature. Significantly, the remaining hydroxyl groups in PBOCI precursor membranes were beneficial for oxygen bridge formations of TO-PBOCI membranes, enhancing H2/CH4 selectivity. As a result, the TO-PBOCI membranes treated at a high temperature kept high H2 permeability and H2/CH4 selectivity, surpassing the 2008 Robeson upper bound. The study about TO-PBOCI membranes promotes a better understanding of the polymeric materials and process development of microporous polymer membranes for hydrogen separation applications.