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 CO 2 permeability of 122 barrer and a CO 2 /CH 4 selectivity of 40.4 for pure gases. Moreover, the XTR PBO was not plasticized at a CO 2 pressure of 30 atm. After aged for 60 days, the CO 2 permeability decreased to 100 barrer and the CO 2 /CH 4 selectivity increased to 44. When separating CO 2 /CH 4 (1:1) mixed gases, the CO 2 permeability gradually decreased from 120 to 74 barrer with a decrement in the CO 2 /CH 4 selectivity from 47 to 26 because of the progressively saturated Langmuir sorption sites and the competitive sorption by CH 4 decreasing the solubility of CO 2. The excellent and stable CO 2 /CH 4 separation property makes the sub-Tg XTR PBO an ideal material for fabricating hollow fiber gas separation membranes. [Display omitted] • Crosslinked thermally rearranged PBOs were synthesized at sub-Tg. • Gas permeability increased by 20 times while maintained selectivity after TR. • XTR-PBO resists plasticization and good for nature gas separation. • Phenolphthalein based diamine is good platform to XTR polymer. [ABSTRACT FROM AUTHOR]