Synthesis of hydrogen-bond-donor-containing ionic covalent organic frameworks for carbon dioxide upgrading

Traditional catalysts those containing hydrogen bond donor (HBD) groups usually require harsh reaction conditions or the use of a co-catalyst to achieve high efficiency of cycloaddition of carbon dioxide (CO₂) with epoxides. In this study, an HBD-containing covalent organic framework (HBD-COF) is synthesized via a simple solvothermal method. Post-synthetic modification on HBD-COF affords an HBD-containing ionic covalent organic framework (HBD-iCOF). These two covalent organic frameworks (COFs) serve as efficient catalysts for the cycloaddition reaction of CO₂ with epoxides under mild conditions in the absence of solvent and co-catalysts. Notably, HBD-iCOF exhibits higher catalytic activity (up to 95.8 ± 0.5 % yield) than HBD-COF under similar conditions, which could be attributed to the synergistic epoxide-activation by imidazolium cation and Br^- anion, along with higher CO₂ affinity (isosteric adsorption heat (Q_st) of 44.5 kJ·mol^-1). Density functional theory (DFT) computations suggest that the higher CO₂ affinity of HBD-iCOF may stem from the joint π-π interactions and hydrogen bonding interactions between CO₂ and adsorption sites. In addition, kinetic studies reveal that HBD-iCOF also significantly reduces the overall reaction energy barrier.