Cobalt Porphyrin-Cross-Linked Poly(Ionic Liquid)s as Efficient Heterogeneous Catalysts for Carbon Dioxide Conversion under Mild Conditions
Yuxia Liu, Shiting Wang, Zhifeng Dai, Yubing Xiong
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.