Tröger's base-based copolymers with intrinsic microporosity for CO2 separation and effect of Tröger's base on separation performance

Abstract

“Polymers of intrinsic microporosity” (PIMs) have recently received considerable attention as powerful and promising membrane materials for gas separation, especially for CO₂ separation. In this work, a new type of PIM copolymer derived from 2,3,8,9-tetrahydroxy-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (THTB) and 5,5′,6,6′-tetrahydroxy-3,3′,3′,3-tetramethylspirobisindane (TTSBI) copolymerized with 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN) was synthesized and their properties for CO₂ separation were investigated. The molecular structures of the copolymers were characterized by ¹H NMR and the molecular weights were determined by GPC. Gas transportation measurements demonstrated that the TB-based copolymers exhibit an improved selective capacity for the separation of CO₂/N₂, CO₂/CH₄ and O₂/N₂, while showing a small decrease of permeability. A detailed investigation on the effect of TB on the performance of the copolymers was undertaken by wide-angle X-ray diffraction analysis, BET and fractional free volume measurements, and molecular modeling analysis. It was concluded that the introduction of TB units in the copolymers leads to more efficient chain stacking and gives the copolymers a smaller pore width distribution as compared to the homopolymer derived only from TTSBI and TFTPN (referred as PIM-1). As a consequence, the introduction of TB enhanced the CO₂ selectivity, owing to the increase of CO₂ solubility when passing through the copolymer membranes.