Modified CARDO-Based Copolyimides with Improved Sour Mixed-Gas Permeation Properties

Abstract

Chemical modification is an interesting tool to introduce functional groups into polymers’ backbones to tailor their properties in a desired fashion. Polymers used in membrane-based gas separation applications need to possess high gas permeability and selectivity during mixed-gas separation under harsh operational conditions of pressure and temperature. Herein, we report the modification of three copolyimides containing the moiety 9,9-bis(4-aminophenyl)fluorine (CARDO) through Friedel–Crafts alkylation to introduce bulky tert-butyl groups into their backbones. Membranes prepared from the modified polymers were evaluated using pure- and sweet mixed-gas under aggressive feed pressures. Based on the promising results obtained, a selected copolyimide was subjected to sour mixed-gas separation under feed pressures up to 500 psi. Mixed-gas separation studies, especially those containing hydrogen sulfide, are of great interest to develop membranes for use in sour natural gas reserve treatment. For example, the H2S and CO2 permeability coefficients of 6FDA-Durene/6FDA-CARDO(t-Bu) at 500 psi were recorded as 456 and 238 Barrer, respectively, where the H2S/CH4 and CO2/CH4 are 20.4 and 10.6, respectively. The obtained results are very promising and even superior to many glassy polymers reported previously. This work represents an example of the benefit of chemical modification on polymers to tailor their structure–property relationship. Future works will be focused on the fabrication and testing of asymmetric or composite hollow fibers to imitate the membrane modules used industrially.