Spirobisindane based polyimide as efficient precursor of thermally rearranged and carbon molecular sieve membranes for enhanced propylene/propane separation
Abstract High performance thermally rearranged (TR) and carbon molecular sieve (CMS) membranes made from an intrinsically microporous polymer precursor PIM 6FDA OH are reported for the separation of propylene from propane. Thermal rearrangement of PIM 6FDA OH to the corresponding polybenzoxazole (PBO) membrane resulted in a pure gas C 3 H 6 /C 3 H 8 selectivity of 15 and C 3 H 6 permeability of 14 Barrer, positioning it above the polymeric C 3 H 6 /C 3 H 8 upper bound. For the first time, the C 3 H 6 /C 3 H 8 mixed gas properties of a TR polymer were investigated and showed a C 3 H 6 permeability of 11 Barrer and C 3 H 6 /C 3 H 8 selectivity of ~11, essentially independent of feed pressure up to 5 bar. The CMS membrane made by treatment at 600 degC showed further improvement in performance as demonstrated with a pure gas C 3 H 6 /C 3 H 8 selectivity of 33 and a C 3 H 6 permeability of 45 Barrer. The mixed gas C 3 H 6 /C 3 H 8 selectivity dropped from 24 to 17 from 2 to 5 bar feed pressure due to a decrease in C 3 H 6 permeability most likely caused by competitive sorption without any evidence of plasticization.