Superacid catalyzed triptycene-based polymer to enhance membrane permeability for molecular sieving of nitrogen over VOC

Abstract

Superacid catalysis, a suitable method for the synthesis of membrane materials owing to its facile polymerization procedure, has been extensively studied. However, superacid-catalyzed binary coplanar polymer membranes generally exhibit low permeabilities. In this study, a rigid 3D triptycene-based polymer was synthesized by the superacid catalysis of triptycene with trifluoroacetophenone and diphenyl ether to enhance membrane permeability for the molecular sieving of nitrogen over volatile organic compound (VOC). The synthesis of polymers with (CF₃PhET) or without triptycene (CF₃PhE) was investigated using different characterizations. The triptycene content of the synthesized polymers was optimized based on an analysis of the molecular weight, membrane-forming properties, and separation performance. The separation performances of membranes fabricated using CF₃PhE, CF₃PhET, and a mixture of CF₃PhE and triptycene were compared. Results showed that the introduction of non-coplanar triptycene in the membrane can increase permeability by nearly 60 times due to the enhanced free volume, from 30 Barrer for the CF₃PhE membrane to 1755 Barrer for the membrane with 5 ​mol% triptycene content for the separation of a 3 ​mol% nitrogen/cyclohexane mixture at 1 ​L/(m²·min). Furthermore, the rejection remains constant, which provides an effective idea for the synthesis of membrane materials with high performance using superacid catalysis.