Synthesis and characterization of imidazolium-mediated Tröger's base containing poly(amide)-ionenes and composites with ionic liquids for CO2 separation membranes

Abstract

Considerable attention has been given to polymeric membranes either containing, or built from, ionic liquids (ILs) in gas separation processes due to their selective separation of CO₂ molecules. Achieving high-performance CO₂ separation membranes with enhanced permeability and selectivity relies mainly on rationally designing the molecular substructure and molecular composition of the polymer matrix. In this work, we have exclusively explored a facile synthetic route to incorporate aromatic amide linkages onto an ionene backbone derived from imidazolium-mediated Tröger's base moieties, yielding a novel rigid polyamide-ionene material (“Im-TB-PA ionene”). We have optimized two novel Im-TB-PA ionene polymers via Menshutkin reactions between N,N′-(1,4-phenylene)bis(4-(chloromethyl)benzamide) and equimolar amounts of two isomeric diimidazole-functionalized Tröger's base monomers with ‘ortho’ or ‘para’ regiochemistry. The two resulting Im-TB(o&p)-PA ionenes exhibited high molecular weights and excellent solubilities in polar organic solvents, serving homogeneous and mechanically stable blend membranes with “free” ILs. The structural and physical properties, as well as the gas separation behaviors of both the Im-TB(o&p)-PA ionenes and their IL composite counterparts ([Im-TB(o&p)-PA] + [IL]), were further extensively investigated. The membrane with an optimal composition and polymer architecture ([Im-TB(o)-PA] + [IL]) exhibited outstanding permselectivites for CO₂/CH₄ (46.73), CO₂/N₂ (51.74), and CO₂/H₂ (4.38) gas pairs together with the best CO₂ permeability of 47.2 barrer. Overall, this study provides a promising strategy to explore the benefits of Im-TB-PA ionenes to separate CO₂ from flue gas, natural gas, and syngas streams, while opening new possibilities in polymer design with strong candidate materials for other practical applications.