Rational engineering of multifunctional ionic covalent organic frameworks for metal-free and efficient chemical fixation of CO2 under mild conditions

The transformation of CO2 into high value-added chemicals such as cyclic carbonates provide a promising technology to solve the exceeding CO2 emission. In the absence of solvent and co-catalyst, the development of highly efficient and recyclable metal-free catalyst with excellent performance under mild conditions still remains a great challenge. Herein, a series of multifunctional ionic covalent organic frameworks (COFs) was synthesized via post-synthesis modification strategy of covalently grafting functionalized imidazole on triazine-based COF skeleton. The influence of bridged alkyl chain, imidazole functional group, nucleophile and grafting amount on catalytic activity was investigated in the cycloaddition of CO2 with epichlorohydrin. Remarkably, IM-COF-Br-Vinyl could afford 96.49% yield and > 99% selectivity with a turnover frequency (TOF) value of 5434 h(-1) at 0.5 MPa CO2 pressure and 100 degrees C for 6 h, which was superior to other reported COF catalysts. The excellent catalytic performance was mainly ascribed to its excellent CO2 adsorption capacity and the synergistic effect of CO2-philic basic sites, hydrogen bond donors and nucleophilic anions. Additionally, the reusability and versatility of the catalyst was considered and the proposal catalytic mechanism was speculated according to the above conclusions. This strategy not only avoided the environmental pollution caused by catalytic system itself but also ensured the safety and low-energy consumption by realizing highly efficient catalytic conversion of CO2 under mild conditions. This provides an effective avenue for the design and synthesis of ionic liquid functionalized COF catalysts and the development of CO2 activation and utilization.