Sub-Tg cross-linked thermally rearranged polybenzoxazole derived from phenolphthalein diamine for natural gas purification

Thermally rearranged (TR) polymers have shown a great potential for gas separation. However, the temperature of TR reaction often goes beyond the glass transition temperature (Tg) of polymer precursor and results in pore collapsing and low gas permeance to asymmetric membranes. To solve this problem, we prepared a series of crosslinked thermally rearranged (XTR) polybenzoxazoles (PBO) at sub-Tg. We synthesized crosslinkable phenolphthalein-based polyimides via chemical imidization (EPPI) and azeotropic distillation (HPPI). The two polyimides were crosslinked at 300–335 °C to increase their Tg so that the subsequent TR reaction were accomplished at sub-Tg. The HPPBO-425 exhibited the best separation performance with a CO2 permeability of 122 barrer and a CO2/CH4 selectivity of 40.4 for pure gases. Moreover, the XTR PBO was not plasticized at a CO2 pressure of 30 atm. After aged for 60 days, the CO2 permeability decreased to 100 barrer and the CO2/CH4 selectivity increased to 44. When separating CO2/CH4 (1:1) mixed gases, the CO2 permeability gradually decreased from 120 to 74 barrer with a decrement in the CO2/CH4 selectivity from 47 to 26 because of the progressively saturated Langmuir sorption sites and the competitive sorption by CH4 decreasing the solubility of CO2. The excellent and stable CO2/CH4 separation property makes the sub-Tg XTR PBO an ideal material for fabricating hollow fiber gas separation membranes.