Synthesis of covalent organic frameworks via in situ salen skeleton formation for catalytic applications
Covalent organic frameworks (COFs) with highly ordered crystalline structures and uniform microenvironments have potential applications in the field of catalysis; however, their application is restricted by their harsh synthesis conditions and multi-step synthetic procedures. Herein, we report the facile synthesis of a new COF-salen via in situ salen skeleton formation by heating aldehyde and amine precursors in an air atmosphere. This COF-salen possesses an AA stacking eclipsed layered crystalline structure, micro/macro hierarchical pores, and high stability in acid or base medium and can be efficiently transformed to COF-salen–M (M = Co, Mn, Cu, Zn) with a well-retained ordered crystalline structure. The successful application of COF-salen–Co(III) and COF-salen–Mn, respectively, in epoxide hydration, which requires the cooperation of two salen–Co(III), and olefin epoxidation, in which the isolated salen–Mn functions as the active site, can be attributed to their unique layered crystalline structures; these structures can efficiently isolate the active sites by restricting their mobility and generate cooperation between nearby active sites in adjacent layers. COF-salen–Co exhibited much higher activity and stability than the corresponding amorphous polymers in cycloaddition reactions of epoxides with CO2, demonstrating the advantage of the crystalline structure in catalysis.