Preparation Of Highly Stable C=C Bond Bridged Covalent Organic Framework And Its Photocatalytic Performance

Recently,semiconductor photocatalysis to achieve energy conversion and new chemical bond construction has provided a blueprint for a sustainable and carbon-neutral world.Photocatalyst is the key to photochemical energy conversion,and its system structure optimization involves light absorption capacity,carrier migration efficiency,active site,etc.For a long period of time,researchers mainly focused on the development or modification of new inorganic semiconductor materials.Until most recently,co-polymers constructed through organic synthesis strategies have also shown excellent photocatalytic properties.Among them,two-dimensional covalent organic frameworks?2D-COFs?with crystallinity have attracted widespread attention.Its fully?-conjugated frame structure can extend light absorption to visible light and near-infrared light region.Compared with amorphous polymers,highly ordered stacked?electron channels provide a way to promote exciton migration and transmission.However,the fabrication of highly stable COFs for their useful photocatalytic applications has always been a problem.Here,the innovative Knoevenagel strategy is used to construct the COF with sp² carbon bridge.Inline bipyridine can be used to chelate metals as the active center of photocatalytic reactions.The experimental results show that sp²c-COF_dpy-Co and sp²c-COF_dpy-Ni can be used for efficient CO₂ reduction and C-O coupling,respectively.The specific research contents are as follows:?1?The bipyridine structure is introduced into the covalent organic framework through the Knoevenagel reaction to synthesize the bipyridyl based sp²c-COF_dpy.The bipyridine unit can be used to further chelate metal ions such as Fe,Co,Ni,Cu,etc.,thereby introducing active sites in COFs.Characterization by XRD,FTIR,XPS,solid¹³C-CP/MAS NMR proved the successful synthesis of the material,and XAFS and other characterizations indicated that the metal ion in the COF existed in a single atom state.Attempt to use sp²c-COF_dpy-metal for photocatalytic CO₂ reduction.The experimental results show that sp²c-COF_dpy-Co exhibits the best photocatalytic activity and selectivity?CO,H₂?under visible light,reaching 1.0 mmol g^-1 h^-1 and 81.4%,respectively.Through transient absorption spectroscopy and in-situ infrared spectroscopy,the process of photocatalytic CO₂ reduction was determined.Co was used as the active site to allow electrons to be smoothly transferred from the catalyst surface to CO₂ for reduction.Compared with sp²c-COF_bp without bipyridine structure,the important role of bipyridine structure was proved.Combining with theoretical calculations,we speculated the possible ways of photocatalytic CO₂reduction,and then the structure-activity relationship of photocatalytic CO₂reduction based on COFs was proposed.?2?Sp²c-COF_dpy-Ni was used in the photocatalytic C-O bond coupling reaction.Under the irradiation of 6 W blue LED,sp²c-COF_dpy-Ni successfully converted p-bromobenzonitrile to p-cyanoanisole.After replacing the cyano group?-CN?with electron-donating groups or electron-withdrawing groups,good reaction results were still obtained,confirming the universality of the reaction.At the same time,when the amount of the reaction was expanded to 1.81 g?10 mmol?,the yield could still reach98%,and the cycled experiment and characterization of the recovered samples confirmed the high stability of COFs.