Preparation And Catalytic Properties Of Monolacunary And Vanadium-substituted Keggin-type Polyoxometalate Mesoporous Composite Materials

As environmental friendly catalysts,polyoxometalates?POMs?have been widely used in redox reactions,acid catalytic and photocatalytic reactions due to its unique structure and excellent performance.However,their high solubility in aqueous solutions and polar solvents limits their application with respect to recovery aspect.Anchoring POMs on active carriers with high specific surface area is an effective method to overcome this shortcoming.Meanwhile,the catalytic activity of the catalyst would be effectively improved due to the synergistic effect between POMs and carriers.In this thesis,the monolacunary Keggin-type anions(PW₁₁O₃₉^7-and SiW₁₁O₃₉^8-,abbr.as PW₁₁and SiW₁₁)with high reactivity were covalently anchored on TiO₂to achieve the mesoporous composites(PW₁₁/m TiO₂and SiW₁₁/m TiO₂).Then,ternary catalysts(Ag/PW₁₁/m TiO₂and Ag/SiW₁₁/m TiO₂)were obtaind after depositing silver nanoparticles,and further applied in photocatalytic and electrocatalytic reactions.Moreover,PWV₂/TiO₂-SiO₂composites were obtained via anchoring vanadium-substituted phosphotungstic acid(H₅PW₁₀V₂O₄₀,abbr.as PWV₂)on TiO₂-SiO₂carrier and employed as acid catalyst.The main works are as follows:1.PW₁₁/m TiO₂ and SiW₁₁/m TiO₂binary composites were prepared by combining PW₁₁and SiW₁₁with TiO₂via hydrothermal method.Afterwards,the ternary catalysts(Ag/PW₁₁/m TiO₂and Ag/SiW₁₁/m TiO₂)were prepared by depositing silver nanoparticles?Ag NPs?via photo-reduction method.It was found that Ag/SiW₁₁/m TiO₂dispalyed excellent performance in photodegration of methyl organe?MO?and tetracycline?TC?.Under simulated sunlight irradiation?320 nm???780 nm?for 30 min,MO can be completely degraded and the degradation rate of TC is 92%.The photocatalytic mechanism was proposed as well.Experiment results show that the introduction of Ag NPs can enhance light absorption and POMs can effectively inhibit the recombination of photo-generated electrons and holes.2.The electrocatalytic nitrogen fixation performance of the ternary catalysts(Ag/PW₁₁/m TiO₂and Ag/SiW₁₁/m TiO₂)was investigated as well.Under working potential of-0.7 V?vs.RHE?,Ag/SiW₁₁/m TiO₂exhibited excellent ammonia yield rate(148.9?g?mg^-1_cat?h^-1)with the faraday efficiency?FE?of 4.17%.The ammonia yield rate for Ag/PW₁₁/m TiO₂is 114.9?g?mg^-1_cat?h^-1together with the FE of 4.17%.The ternary catalyst as well displayed superior durability and reproducibility in the NRR process.3.Acid catalyst PWV₂/TiO₂-SiO₂composites were prepared by loading vanadium-substituted phosphotungstic acid(H₅PW₁₀V₂O₄₀,abbr.as PWV₂)on mesoporous binary oxide TiO₂-SiO₂carrier via sol-gel method.Its acid catalytic performance for the acetal reaction of benzaldehyde with ethylene glycol and the esterification of lauric acid with ethanol was explored.PWV₂/TiO₂-SiO₂displayed the best catalytic performance with benzaldehyde conversion rate of 96.2%,selectivity of acetal of 99%under optimal conditions?ethylene glycol to benzaldehyde=1.6:1?mol/mol?,the amount of catalyst=5wt%?relative to benzaldehyde?,T=90oC and t=2 h?,used cyclohexane as a water-carrying agent and an internal standard.Moreover,PWV₂/TiO₂-SiO₂exhibited the excellent catalytic performance on the esterification of lauric acid,with 92.0%yield of ethyl laurate together with selectivity of 99%,respectively,under optimal conditions?lauric acid to ethanol=1:9?mol/mol?,the amount of catalyst=5 wt%?relative to lauric acid?,T=78oC and t=3 h?,when employing dodecane as an internal standard.Synergy effect between Br?nsted and Lewis acid sites was attributed to the excellent activity of the catalysts,superior durability and reproducibility were achieved as well.