Simultaneous Improvement of Permeance and Permselectivity of 3,3′,4,4′-Biphenyltetracarboxylic Dianhydride-4,4′-Oxydianiline Polyimide Membrane by Carbonization

A BPDA-ODA polyimide film was formed on the outer surface of a porous alumina support tube (outside diameter, 2.4 mm; inside diameter, 1.8 mm; void fraction, 0.48; average pore size, 140 nm). It was imidized and then carbonized at 500–900°C. Defects usually generated during carbonization were completely avoided by optimizing temperature profiles. Carbonization above 600°C remarkably increased the permeance of He, CO₂, CH₄, N₂, and C₂H₆ without losing the high permselectivity of the polyimide. The ultimate micropore volume determined by CO₂ sorption was also increased to 0.36 × 10^‒3 from 0.16 × 10^‒3 m³, kg^‒1. However, the micropore volume was not a dominant factor. The increase in permeance, which was mainly controlled by effective diffusivity of permeating species in the membrane, was dependent on pore-size distribution. The CO₂ permeance and CO₂/CH₄ permselectivity of the carbonized membrane were 10^‒7 mol•m^‒2•s^‒1•Pa^‒1 and 100, respectively, at 30°C.