Synthesis of polyimides containing Tröger's base and triphenylmethane moieties with a tunable fractional free volume for CO2 separation

Abstract

CO₂ separation from natural gas (CO₂/CH₄) or flue gas (CO₂/N₂) has a great significance for the sustainable development of the environment and society as well as industrial production. In this work, a series of rigid, bulky triphenylmethane (TPM)-derived diamines with different substituted groups (H, F, CF₃, and COOH) were designed and synthesized, and subsequently introduced into the backbone of Tröger's base-based polyimide by a one-step copolymerization. Attributed to the rigid and contorted features of both triphenylmethane and Tröger's base moieties, these polyimides demonstrated simultaneously enhanced stiffness and fractional free volume. Interestingly, the viability of “tunable” fractional free volume for these polyimides could be realized by varying the substituents on the TPM-derived diamines, illustrated by their differentiated densities, fractional free volumes and d-spacings. The resulting membranes presented greatly improved separation performance for some important industrial gas pairs, such as CO₂ removal from CH₄ or N₂. Typically, the POLY-CF₃ membrane with a CF₃ pendant group exhibited a largest CO₂ permeability of 187.1 Barrer, almost 23 fold higher than the commercial Matrimid® 5218, accompanied by comparable CO₂/CH₄ ideal selectivity (30.7 vs. 34.9) while the strong interactions between CO₂ and the carboxylic acid groups in the POLY-COOH membrane increased the solubility selectivity, resulting in the highest CO₂/CH₄ and CO₂/N₂ ideal selectivities.