Visible Light Photocatalytic Systems Based On Niobium Oxide For Selective Oxidation Reactions

Niobium oxide（Nb₂O₅）has attracted extensive attention in the field of photocatalysis,owing to its excellent chemical and thermodynamic stability,abundant Lewis acid sites,and superior redox properties.Nevertheless,bare Nb₂O₅ only responds to ultraviolet light on account of its large intrinsic bandgap.Therefore,we prepare several visible-light photocatalytic systems through the strategies of designing organic ligand-Nb₂O₅ assemblies and heterojunction construction,which could be applied to selective oxidation reactions.Firstly,we synthesize alizarin red S-Nb₂O₅（ARS-Nb₂O₅）and 2,3-dihydroxynaphthalene-6-sodium sulfonate-Nb₂O₅（DHNS-Nb₂O₅）based on the strategy of designing organic ligand-Nb₂O₅ assemblies,which realize visible-light photocatalytic selective oxidation of amines into imines in cooperation with（2,2,6,6-tetramethylpiperidin-1-yl）oxyl（TEMPO）as a redox mediator.Herein,the ARS and DHNS are anchored on the surface of Nb₂O₅ by ortho-dihydroxyl bonding,which gives rise to ligand-to-metal-charge transfer（LMCT）.Notably,TEMPO could synchronize the intricate electron and proton transfers among the organic ligand-Nb2O5photocatalytic systems,amines,and dioxygen（O₂）,and ensure a faster and more stable cycle of organic ligands,which enhances the activity of the visible-light photocatalytic system significantly.Next,we prepare benzylamines-Nb₂O₅ surface complexes based on the strategy of assembling substrates amines on Nb₂O₅ induced by visible light,which realizes visible-light photocatalytic selective oxidation of amines into imines in cooperation with2,2,6,6-tetramethylpiperidine-1-oxoammoniumhexafluorophosphate（TEMPO⁺PF₆^-）as a redox mediator.Herein,substrates amines are anchored on the surface of Nb₂O₅ by dissociative adsorption.As a result,with adding TEMPO⁺PF₆^-as a redox mediator,the conversion of benzylamine proceeded over Nb₂O₅ is approximately 2.5 times higher than that without TEMPO⁺PF₆^-,and is approximately2 times higher than that with TEMPO as a redox mediator.This work highlights it is feasible to ameliorate Nb₂O₅-based photocatalytic systems with a suited redox mediator.Further,we realize visible-light photocatalytic selective oxidation of benzyl alcohols into benzaldehydes proceeded over Nb₂O₅.Then,we reveal the participation process of O₂ in the mentioned reaction by oxygen isotope studies.After eliminating the influence of product H₂O,it can be noticed that the oxygen atom in the formed aldehyde almost originates from the substrate alcohol.That is say,oxygen atoms in O₂are not transferred to the formed aldehyde.Lastly,we straightforwardly synthesize polyimide-Nb₂O₅（PI-Nb₂O₅）heterojunction by thermal condensation polymerization.PI-Nb₂O₅ shows enhanced absorption for visible light and high charge transfer efficiency.Subsequently,PI-Nb₂O₅realizes blue light-induced activation of O₂ for selective oxidation of sulfides.In conclusion,we successfully prepare several visible-light photocatalytic systems based on Nb₂O₅ for the selective oxidation of amines,alcohols and sulfides.This thesis offers insights into constructing photocatalytic systems based on metal oxide for selective oxidation reactions.