Thermal effect on polyethyleneoxide-containing copolyimide membranes for CO 2/N 2 separation

Abstract

Recovery of CO 2 from gas mixtures is an important industrial objective in view of several aspects such as natural gas reforming, biomedical applications and even global warming. Polyimides consisting of flexible polyethyleneoxide (PEO) segments and rigid polyimide segments have been reported in recent years for this application. They exhibit high CO 2 permeability coefficients and relatively high CO 2/N 2 selectivity due to CO 2 strong affinity to the polar PEO segments. In this work a set of copolyimides with different contents of PEO have been prepared from 3,3´,4,4´-biphenyltetracarboxylic dianhydride, oxydianiline (ODA) and a diamine containing the PEO fragment. ODA-PEO ratios (w:w) used to prepare the polymers were 1:1, 1:2, 1:4 and 1:6. Preliminary permeability measurements established that the membrane with the ODA-PEO ratio (w:w) 1:2, which exhibited permeabilities of 23.87 and 0.39 Barrers, for CO 2 and N 2 respectively, showed the highest ideal selectivity of CO 2 over N 2 (61.20) when compared to the other members of the series. The thermal treatment of this membrane was carried out in an oven heated at 200°C, 250°C and 300°C. The heating at 200°C caused a large increase in N 2 and CO 2 permeability coefficients (1.05 and 57.25 Barrers respectively) which resulted in a slight decrease of the selectivity. It should be mentioned that while the permeability of CO 2 increased considerably (~300%), the selectivity just suffered a small drop (~10%), and hence, the productivity of this membrane is much better after the thermal treatment. Thermal treatment of the membrane above 200°C produced successive increments on the permeability coefficients of both gases, while ideal selectivity was maintained.