POSS-based poly(ionic liquid) catalysts for solvent- and cocatalyst-free cycloaddition of CO2 at ambient pressure: experimental and mechanistic insights

Abstract

The chemical fixation of CO₂ into high-value cyclic carbonates is of great significance for achieving CO₂ utilization (CCU) technology. In this study, a series of imidazolium-based ionic linkers, designated as IL-2C, IL-3C, IL-4C, IL-5C, and IL-6C, were synthesized using 1-vinylimidazole and five different dibromoalkane linkers. The linker design incorporates flexible alkyl chains, where increasing chain length modulates the distance between imidazolium centers, influencing the resulting polymer network structure. Subsequently, their corresponding poly(ionic liquid) (PILs) porous polymers (PIL-2C, PIL-3C, PIL-4C, PIL-5C and PIL-6C) were synthesized through free radical copolymerization with octavinyl POSS using 2,2'-azobis(2-methylpropionitrile) (AIBN) as the initiator, resulting in crosslinked porous networks. The structure, morphology, and thermal stability of the PILs were characterized by NMR, FTIR, XRD, BET, SEM and TGA. The results indicate that the synthesized poly(ionic liquid) catalysts possess a mixed microporous/mesoporous structure, enhancing their performance in various applications. The PILs were evaluated as catalysts for the cycloaddition of CO₂ to epoxides, with PIL-5C exhibiting the best performance under ambient pressure, solvent-free, and co-catalyst-free conditions, achieving a cyclic carbonate yield as high as 99.9%. Reaction kinetic studies revealed that the POSS-based polymer significantly reduces the activation energy of the CO₂ cycloaddition reaction, and density functional theory (DFT) simulations provide a complete reaction pathway. Furthermore, PIL-5C demonstrated excellent catalytic activity for epoxides of varying sizes. As a heterogeneous catalyst, the POSS-based polymer showed no structural changes after recycling, as confirmed by FTIR and SEM analyses. The catalyst could be reused for 6 cycles while maintaining high catalytic performance.

Graphical abstract

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