Porous organic polymers with diverse quaternary phosphonium units for chemical fixation of CO2 with low concentration

Low concentration (normal less than 15%) of CO2 is mainly resulted from emission of industrial processes, and its transformation into value-added chemicals over heterogeneous catalysts is of great significance for sustainable reduction of CO2 accumulation. Herein, we reported a series of porous organic polymers (POPs) with diverse quaternary phosphonium units constructed from phosphonium reaction of halogen-functionalized porous divinylbenzene polymers with phosphines. These POPs have large surface areas and hierarchical porosity, exhibiting excellent catalytic properties in cycloaddition reaction of CO2 and epoxides even under the co-catalyst free and mild conditions. Structural comparison of the catalysts reveals that the catalytic activities increase with the steric hindrance of the quaternary phosphonium units in the catalysts. For example, catalytic activity of the porous organic polymer with tricyclohexyl phosphonium chloride unit (POP-BnCl-CP) was higher than that of porous organic polymers with tributyl phosphonium chloride (POP-BnCl-BP) and triphenyl phosphonium chlo-ride (POP-BnCl-PP) units. Particularly, this difference was more obvious when low concentration of CO2 was employed in the reaction. In this case, the POP-BnCl-CP has the CO2 conversion at 94.3% with product selectivity at 99.0%, while the POP-BnCl-BP and POP-BnCl-PP have relatively low conversion and selectivity (67.6% and 99.0% for POP-BnCl-BP and 55.2% and 99.0% for POP-BnCl-PP). The high activity and excellent selectivity of the POP-BnCl-BP catalyst in the absence of any co-catalyst under mild conditions should be important for chemical fixation of CO2 with low concentration into value-added chemicals in the future.

Automated summary

In this study, porous organic polymers (POPs) with diverse quaternary phosphonium units were synthesized and used as catalysts for cycloaddition reaction of CO2 and epoxides. The POPs exhibited excellent catalytic properties even under mild conditions and without the need for a co-catalyst. The catalytic activities of the POPs were found to increase with the steric hindrance of the quaternary phosphonium units.