Cobalt Porphyrin-Cross-Linked Poly(Ionic Liquid)s as Efficient Heterogeneous Catalysts for Carbon Dioxide Conversion under Mild Conditions

Abstract

The cycloaddition reaction of CO2 and epoxides to form cyclic carbonates is a promising strategy to alleviate the climate change caused by CO2 in view of its high atomic economic efficiency and solvent-free and mild conditions. Inspired by the mechanism of cycloaddition reactions catalyzed by Lewis acid/Lewis base binary systems, we explored a series of heterogeneous catalysts functionalized by cobalt porphyrin cross-linked phosphonium salt poly(ionic liquid)s. Specifically, poly(4-vinylbenzyl chloride) (PVBnCl) was used as the polymer backbone. Then, PVBnCl was cross-linked by 5,10,15,20-tetra(4-pyridyl) porphyrin (TPyP) and quaternized by triphenylphosphine (PPh3). After complexing with CoCl2 and anion exchange, the as-prepared polymers CoTPyP-c-PVBnPPh3X were characterized using FTIR, SEM, TEM, XPS, TGA, ICP, and solid-state 13C NMR measurements. The results demonstrated that CoTPyP-c-PVBnPPh3X were efficient catalysts for the cycloaddition of CO2 and epoxides under mild conditions. Almost quantitative conversion of epoxides could be achieved at 80 °C and 1 atm CO2, and CoTPyP-c-PVBnPPh3X could be easily separated and reused for four cycles only with a little decrease in the catalysis activity. The synergistic catalysis of the Lewis acid metal center and nucleophilic halogen ion addressed the excellent performance of CoTPyP-c-PVBnPPh3X. Therefore, our findings provided a new solution for the development of efficient heterogeneous catalysts with multi-centers for CO2 conversion.