Research On Preparation Of Salen-based Covalent Organic Framework And Application In Photocatalytic CO₂ Reduction

Artificial photosynthetic CO₂ reduction is an effective way to solve greenhouse gas pollution and realize the recycling of carbon resources.It is of far-reaching significance to develop an artificial photosynthetic catalyst with efficient visible light response.Two-dimensional covalent organic frameworks（COFs）photocatalysts have the advantages of excellent visible light absorption,outstanding specific surface area,and adjustable structure and energy band.In addition,the organic nature of COFs can flexibly anchor metal active sites and build CO₂ adsorption and catalytic active centers.However,the current COFs-based artificial photosynthetic catalysts still face the problems of low catalytic activity and the need to add precious metal photosensitizers and sacrificial agents.In this paper,the molecular complex model is built into the COFs framework,and at the same time,the electron donor-acceptor strategy is constructed to regulate the separation of COFs photo-generated charge.Furthermore,an ionic liquid with CO₂enrichment ability is introduced into the COFs pores,and the catalytic activity is further improved through the synergistic effect of host and guest.The main research contents are as follows:A new structure of COFs based on the coexistence of Salen units and electron donor-acceptor groups was constructed,and the performance of CO₂ production by solid-phase CO₂ reduction of artificial photosynthetic gas was studied by regulating the metal center.Using（1R,2R）-（-）-1,2-diaminocyclohexane and ethylenediamine,respectively,with 1,3,5-triazine-2,4,6-tris（4’-hydroxy-5’-Formylphenyl）benzene（THFB）undergoes Schiff base reaction,and at the same time,Zn（Cl O₄）₂·6H₂O is introduced in situ to synthesize THFB-COF-1-Zn and THFB-COF-2-Zn.Through X-ray powder diffraction（PXRD）,infrared spectroscopy（FT-IR）,solid-state nuclear magnetic（SSNMR）and other characterizations,as well as structural simulation methods,the crystalline ordered structure of COFs was determined.The band structures of the two COFs were determined by ultraviolet-visible absorption spectroscopy（DRS）and Mott-Schottky based tests.The rates of solid-phase CO₂ reduction to produce CO for THFB-COF-1-Zn and THFB-COF-2-Zn were 20.8 and 18μmol·g-1·h-1,respectively.The comparison experiment of the activity of different metal centers confirmed that the COFs anchored by the Zn metal center have the best photocatalytic CO₂ reduction advantage,and THFB-COF-1-Zn has stronger visible light response ability and photo-generated charge than THFB-COF-2-Zn.By introducing ionic liquids into the pores of COFs,a COFs-based host-guest photocatalyst was constructed,and the important role of ionic liquids in the process of artificial photosynthesis was explored.The ILs@COFs composite film catalyst was prepared by loading the ILs into the nanopore channels of COFs by the liquid-phase concentration method.The N₂ adsorption test results prove that the ILs were successfully loaded in the COFs pores,and the CO₂ adsorption test shows that the ILs can greatly promote the CO₂ adsorption of the catalyst.Scanning electron microscopy tests show that the thickness of ILs（56.4 wt%）@THFB-COF-Zn film is about 4.3μm,and the surface is uniform.In a gas-solid reaction system,the visible light catalytic CO₂ reduction performance of composite films with different ILs and different loadings of ILs was studied.The results showed that the photocatalytic CO₂ reduction production rate of ILs（56.4 wt%）@THFB-COF-1-Zn and ILs（56.4 wt%）@THFB-COF-2-Zn to CO were 351.42 and 217.03μmol·g^-1·h^-1,respectively,which are the parent THFB-COF-1-Zn and THFB-COF-2-Zn 16and 12 times.Photoelectric performance characterization and analysis showed that ILs（56.4 wt%）@THFB-COF-Zn has better photoresponse and more effective photo-generated charge separation ability than THFB-COF-Zn,and ILs can enrich CO₂.The activation energy of the catalytic reduction reaction and the synergistic effect of the metal center is that the host and guest cooperate with the photocatalyst to show excellent catalytic activity.