Preparation Of Metal Salen Covalent Organic Framework And Property Research Of Photocatalytic Reduction Of CO₂

Making use of photocatalysis technology to reduce CO₂ into energy or high value-added chemicals is one of the ways to solve the greenhouse effect and energy crisis.The key lies in the preparation of efficient photocatalyst.Covalent organic framework materials（COFs）are excellent photocatalysts because of their large surface area,strong absorption of visible light,adjustable energy band and structure.Making use of the structural advantages of COFs that can construct metal coordination sites flexibly and constructing CO₂ adsorption and reduction sites by anchoring metal ions,which is an effective means to improve the CO₂ reduction performance of COFs photocatalysts.However,the catalysts of COFs photocatalytic CO₂ reduction are faced with difficulties in improving catalytic activity and selectivity,which limits the development of COFs photocatalysts.In order to solve the above problems,metal Salen-COFs of electron donor-acceptors（D-A）type were constructed in this paper to promote the separation of photogenerated charge of COFs.Ionic liquids with strong adsorption capacity for CO₂ were further combined with COFs to improve the performance of photocatalytic CO₂ reduction through synergism between host and guest.The main contents are as follows:The metal Salen-COFs of D-A structure was designed and synthesized,which promoted the separation of photogenerated carriers and improved the photocatalytic performance of CO₂ reduction to CO.The synthesized 5,5’,5"-（1,3,5-triazine-2,4,6-triphenyl）tri（2-hydroxy-3-methoxy）benzaldehyde（HMBA）was reacted with（1R,2R）-（-）-1,2-diamino-cyclohexane/ethylenediamine by Schiff base,while Zn（Cl O₄）₂·6H₂O was added in situ to synthesize HMBA-COF-1-Zn and HMBA-COF-2-Zn.The structure of COFs was determined by characterizations,such as Powder X-ray diffraction（PXRD）,Fourier transform infrared spectroscopy（FT-IR）and Solid state nuclear magnetic resonance（SSNMR）,and the energy band structure of the two COFs was calculated by Mott-Schottky and UV–Vis absorption spectra.Both of them meet the energy bandgap requirements of photocatalytic CO₂.Through photocatalysis of soild-gas mode for 5 h,the CO yields of HMBA-COF-1-Zn and HMBA-COF-2-Zn were 151.5μmol·g^-1 and 65.5μmol·g^-1,respectively.By comparing the two COFs,it can be concluded that HMBA-COF-1-Zn has smaller AC impedance,lower fluorescence intensity and higher surface photocurrent.The experimental results show that HMBA-COF-1-Zn has efficient photogenerated electron-hole separation and transfer ability.More electrons reach the catalyst surface to reduce the adsorbed CO₂molecules and thus have better catalytic performance.Comparing the AC impedance,surface photocurrent and PXRD,it is found that the HMBA-COF-1-M coordination Zn2⁺with different metal active centers has the best reduction performance.Co-host/guest COFs photocatalyst was prepared by utilizing the combination of COFs and ionic liquid with the ability to enrich CO₂.The host-guest collaborative photocatalyst was successfully prepared by mixing COFs with ionic liquid.The results showed that 10μL of 1-butyl-3-methylimidazole bromide（[BMIM]Br）combined with COFs（ILs@HMBA-COF-Zn）had the best catalytic performance.The photocatalytic performance of CO₂ reduction to CO of ILs@HMBA-1-COF-Zn and ILs@HMBA-COF-2-Zn can reach 501.2μmol·g^-1 and 393.6μmol·g^-1.The properties of HMBA-1-COF-Zn and HMBA-COF-2-Zn were improved by 3 times and 6 times,respectively.The CO₂ adsorption test showed that the COFs of the composite ionic liquid could greatly improve the CO₂ adsorption capacity and the photocatalytic performance.