Preparation And Photo-oxidation Performance Of Single-dispersed Wheel-shaped Polyoxometalate Composites

The oxidation of olefins is one of the most important reactions used in industry to produce high value chemical products.Conventional industrial oxidation processes usually use strong oxidants such as peroxy acids,HNO₃ and HIO₄.This results in the following problems:deep oxidation of reactants,poor selectivity,difficult product separation and low conversion,etc.Finally,a large amount of environmentally hazardous waste is generated.In addition,the lifetime and price of the catalyst are factors that cannot be ignored.Therefore,olefin oxidation catalysts that are inexpensive,green,non-toxic,recyclable and easy to separate have been a hot topic of research.One of them is polyoxometalates（POM）,which is metal oxygen clusters with unique structures,reversible redox properties and high photoelectric sensitivity.POM has good oxidation properties because of the large number of oxygen atoms on the surface and is less prone to over-oxidation of the reactants,So POM is considered as a promising organocatalyst.However,several problems can occur during the reaction of POM as catalysts,such as 1.severe molecular agglomeration,2.low reactant selectivity and 3.poor catalyst recovery.These deficiencies can cause both a significant reduction in the catalytic effect of the catalyst and an increase in the cost of using the catalyst.To this end,this thesis focuses on single-disperse functionalised POM composites to address the problem of POMs agglomeration during the reaction process and improve the catalytic performance of the catalyst.We carried out the following two works:Cu₁₀POM was obtained by introducing multiple copper ions into the P₈W₄₈（POM）,followed by physical mixing of the POM(Cu₁₀POM)and the thermally oxidized etched carbon nitride nanosheets（CNNS）under acidic（p H=3）conditions,and the Cu₁₀POM was dispersed into the carbon nitride nanosheets by electrostatic adsorption and the Cu₁₀POM was single-dispersed.The catalyst CNNS/Cu₁₀POM-200 with aⅡ-shaped heterojunction was used as an oxygen source to catalyse the oxidation of cyclohexene with high selectivity to produce cyclohexene oxide in 96.5%yield and 97.6%selectivity.In this work,the electron-rich whorled polyoxometalates were single-dispersed on CNNS by electrostatic adsorption,providing a new idea for the design of low-cost,green photocatalytic materials.In order to expose the surface of the P₈W₄₈ and the single-dispersed state of the POM can be more stable during the reaction,POM was assembled with Zinc oxides.In this work,POM@Zn O-X with different contents of POM were prepared and the composites were characterised morphologically and structurally.The oxidation performance of POM@Zn O-X was investigated by catalyzing the oxidation of indene under light conditions at room temperature.It was found that the catalysts assembled into two-dimensional nanosheet structures of POM@Zn O-3 catalyzed the ninhydrin oxidation with high selectivity to produce ninhydrin alcohol under room temperature light conditions,and the yield of ninhydrin alcohol could reach 98.8%within 24 h with selectivity greater than 99.5%compared with Zn O and POM materials not assembled into two-dimensional nanosheet structures,and their photocatalytic performance of ninhydrin oxidation was significantly enhanced.The assembly of POMs into single clustered ordered nanosuperstructures facilitates the catalytic reaction,and the assembly method provides a new idea for the preparation of functionalized clustered single-disperse catalysts.