Synthesis and gas separation performance of Tröger's base-containing polyimides derived from 9,9-Bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride

Two imide-containing diamine monomers were prepared using 9,9-bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride (6FcDA) and 2,6-dimethylaniline or 2,6-diisopropylaniline as the starting materials. Two polyimides of intrinsic microporosity (PIM-PIs), i.e., 6FcDA-Me2NH2-TB and 6FcDA-iPr2NH2-TB, were then synthesized via TB polymerization of imide-containing diamines and dimethoxymethane in trifluoroacetic acid. The surface areas, fractional free volumes, and interchain distances of the PIM-PIs were in the range of 555–569 m2 g−1, 0.16–0.20, and 0.60–0.67 nm, respectively. These values followed the order of 6FcDA-Me2NH2-TB < 6FcDA-iPr2NH2-TB owing to the larger size of isopropyl substituents relative to methyl groups. Furthermore, 6FcDA-Me2NH2-TB exhibited obvious π-π stacking interactions as evidenced by the diffraction peaks at ∼ 26 ° in wide angle X-ray diffraction patterns. Consequently, 6FcDA-Me2NH2-TB showed higher perm-selectivity but lower gas permeability as compared to 6FcDA-iPr2NH2-TB. Particularly, both polymers had a CO2 permeability of > 2900 Barrer and a CO2/CH4 selectivity of > 10, located between the 1991 and 2008 Robeson’s upper bound for CO2/CH4 separation.