Synthesizing Hypercrosslinked Polymers with Deep Eutectic Solvents to Enhance CO₂/N₂ Selectivity

Hypercrosslinked polymers (HCPs) are widely used in ion exchange, water purification, and gas separation. However, HCP synthesis typically requires hazardous halogenated solvents e. g., dichloroethane, dichloromethane and chloroform which are toxic to human health and environment. Herein we hypothesize that the use of halogenated solvents in HCP synthesis can be overcome with deep eutectic solvents (DES) comprising metal halides-FeCl₃, ZnCl₂ that can act as both the solvent hydrogen bond donor and catalyst for polymer crosslinking via Friedel Crafts alkylation. We validated our hypothesis by synthesizing HCPs in DESs via internal and external crosslinking strategies. [ChCl][ZnCl₂]₂ and [ChCl][FeCl₃]₂ was more suitable for internal and external hypercrosslinking, respectively. The specific surface areas of HCPs synthesized in DES were 20-60 % lower than those from halogenated solvents, but their CO₂/N₂ selectivities were up to 453 % higher (CO₂/N₂ selectivity of poly-α,α'-dichloro-p-xylene synthesized in [ChCl][ZnCl₂]₂ via internal crosslinking reached a value of 105). This was attributed to the narrower pore size distributions of HCPs synthesized in DESs.