Fabrication of Sub-Tg Cross-Linked and Thermally Rearranged Poly (Benzoxazole-Co-Imide) Hollow Fiber Membrane Derived from Phenolphthalein-Based Copolyimide for Co2/Ch4 Separation

Abstract

The high temperature (above 400 ℃ typically) of thermal rearrangement reaction often caused pore collapse and skin layer densification of asymmetric hollow fiber, and leading to low gas permeance. To solve this problem, a novel hollow fiber membrane based on phenolphthalein was prepared and undergone a cross-linked thermally rearranged (XTR) treatment. The thermal labile lactone rings from the phenolphthalein moities were employed to cross-link the hollow fiber membrane at sub-Tg. Subsequently, the cross-linked hollow fiber was heated over 400 ℃ and turned into thermally rearranged membrane. The CO2 permeance increased by 70% when the hollow fiber precursor was first cross-linked at sub-Tg for 2 h. This demonstrated the efficiency of sub-Tg cross-linking, which mitigated the sub-layer collapsing as shown in SEM images. Additionally, by optimizing the hollow fiber spinning condition and the thermal treatment procedure, the CO2 permeance was further increased to 436.22 GPU with a CO2/CH4 selectivity of 30.63, surpassing all TR hollow fiber membranes for CO2/CH4 separation. When used high pressure CO2, CO2/CH4, and CO2/CH4/N2/ethane/benzene mixed gases as feeds, no significant signs of plasticization were observed. Therefore, the anti-plasticized high performance XTR hollow fiber membrane had a great potential for natural gas purification.

Keywords: Sub-Tg cross-linking, Thermal rearrangement, Hollow fiber membrane, Natural gas purification, Anti-CO2 plasticization