Efficient cycloaddition of CO2 and epoxides to cyclic carbonates using salen-based covalent organic framework as a heterogeneous catalyst

Salen-based covalent organic framework (COF), constructed by salen skeleton, has great potential applications as the heterogeneous catalyst due to its unique salen-functionalized structure and uniform porous microenvironment. In this study, the mesoporous crystalline salen-COF was facile synthesized via heating salicylaldehyde and ethylenediamine in the air and characterized by fourier transform infrared, powder X-ray diffractometer, Brunauer–Emmett–Teller, and inductively coupled plasma atomic emission spectrometer. The deliberately selected macromonomer endowed salen-COF with a large pore size (2.06 nm). After metalation with cobalt, the obtained salen-COF-Co exhibited high Co(salen) crystallinity as well as the well-retained COF structure. Under the optimized reaction conditions (0.25 mol% salen-COF-Co, 120 °C, and 2.0 MPa CO 2 ), the salen-COF-Co exhibited high reactivity and > 99% selectivity for the CO 2 cycloaddition with varieties of epoxides (12 examples) to afford multifunctional cyclic carbonates. Noting that the cyclic bis- and tris-carbonates were successfully obtained with high conversions and > 99% selectivities. Moreover, the unsubstituted trimethylene oxide and hydroxyl-functionalized substituted 3-ethyl-3-methyloloxetane were successfully converted to the corresponding six-membered cyclic carbonates with moderate conversions and > 99% selectivities. In addition, during at least five recycle tests, the robust heterogeneous salen-COF-Co exhibited excellent recyclability and reusability, maintaining high crystallinity and ordered porous structure with stable reactivity. This study provides a potential heterogeneous porous catalyst for selective CO 2 cycloaddition with epoxides and oxetanes.