Study On Visible-light Catalytic Oxidation Of Decatungstate Salt Controlled By Vanadium Substitution,Substrate Sensitization And Cation

Photocatalytic oxidation technology is getting more and more attention from researchers.Among them,decatungstates quaternary ammonium(DTs)has become a research hotspot in the field of photocatalytic oxidation synthesis due to its unique photophysical and photochemical properties.However,its unsatisfactory synthetic quality,non-stable structure,and especially low visible light response performance limit its wide application in visible light catalysis synthesis.In order to improve the visible light catalytic oxidation performance of DTs,in this paper,the visible light catalytic oxidation property of DTs was regulated by metal vanadium substitution,substrate sensitization and quaternary ammonium cation.The main results are as follows:1.Tetramethyl,tetrapropyl and tetrabutylammonium decatungstates catalysts(DTs)were synthesized and the effect of their quaternary ammonium cations on the synthesis quality,structural stability,redox ability,visible light response and photoexcited state,as well as selective oxidation of inactive hydrocarbons using molecular oxygen under visible light excitation.The results showed that the photocatalytic oxidation performance of DTs is gradually increased with the decreasing cationic alkyl chain of DT salt,which is mainly due to its improving synthesis quality,structural stability and oxidizing ability,as supported by XPS,FT-IR,UV-vis and PL spectra and CVs characterization,In addition,the additive water and hydrochloric acid solution have obvious promoting effect on the DTs photocatalytic oxidation system.This promotion also increases with the decreasing cationic alkyl chain,which is mainly because: 1)Under the illumination,water can stabilize the DTs structure and improve its redox cycle ability;2)hydrochloric acid can improve the oxidation and redox cycle ability of DTs,and stabilize its photoexcited state;3)Their respective promoting effects can be well combined together in the co-existence of these two additives.Moreover,since the hydrophilicity of DTs gradually increases with the shortening of decreasing cationic alkyl chain,the above additive effect is enhanced,and thus tetramethylammonium decatungstates(TMADT)with the high quality exhibits the best visible light catalytic oxidation performance.Cyclohexane conversion(41%)and cyclohexanone selectivity(82%)could be obtained under optimized conditions(irradiation time,28 h)2.The oxidation of biomass-derived 5-hydroxymethylfurfural(HMF)to 2,5-diformylfuran(DFF)and 2,5-furandicarboxylic acid(FDCA)is one of the key transformation technologies for large-scale biomass utilization.In this chapter,the selective oxidation of HMF by molecular oxygen at room temperature and atmospheric pressure under visible light driven was studied using the above tetraalkylammonium decatungstate(DTs)as catalysts.The results showed that the three DTs salts can efficiently catalyze this photooxidation reaction in the absence of alkali,providing DFF and FDCA as the main oxidation products.TMADT with the high quality and stable structure shows the best photocatalytic activity.In addition,aqueous hydrochloric acid as an additive can further improve the photocatalytic activity of TMADT,and under optimized conditions,27.3% DFF and 17.4% FDCA yields can be obtained.It is noteworthy that the substrate HMF can accelerate this photocatalytic reaction through the donor-receptor(DA)reaction,which is mainly due to self-sensitization effect of the substrate,which can improve the structural stability,redox performance,visible light utilization and separation of photogenerated electron-hole pairs of DTs.3.Vanadium-substituted tetramethylammonium decatungstate(V-TMADT)was synthesized and applied to the research of selective oxidation of inactive hydrocarbons by molecular oxygen under visible light excitation at normal temperature and pressure.The results showed that in comparison with TMADT,V-TMADT shows better catalytic performance in the photocatalytic oxidation of inactive hydrocarbons and can achieve 33% cyclohexane conversion with 80% cyclohexanone selectivity at illumination 12 h under the promotion of 2 M hydrochloric acid.Through FT-IR,UV-vis and PL spectroscopy characterizations,it was found that: 1)Vanadium substitution can improve the visible light response of TMADT under almost no affecting its structure,thus improving the utilization of light;In addition,vanadium substitution likely enhances the light excitation efficiency of TMADT.This research work may open up a new voyage for the development of hybridization engineering(HE)strategy of DTs.