Synthesis And Catalytic Oxidation Properties Of Transition Elements Keggin Type Heteropolyanions With Graphene Based Composite Catalysts

Polyoxometalates（POMs）are a kind of metal-oxides cluster compounds and there exist rich Mo,W,V and other transition metal elements in the skeleton structures.Because of the unique structure and excellent physical and chemical properties,POMs is widely concerned in many fields.However,in the field of catalysis,the POMs as a homogeneous catalyst,because of its small specific surface area,low solubility,is difficult to separate and recover from the reaction system.Graphene,which combines multiple features,such as large specific surface and low solubility,can be used as building blocks with POMs.In this thesis,transition metal-substituted polyacids and graphene are used as building blocks.The new material possesses the excellent catalytic characteristic which is unique to the two original materials;We successful prepared the same structure of the POM/GS composite in a green technology of low temperature with high pressure.The important conclusions of this article are as follows:1.Synthesis and characterization of transition elements tri-substituted Keggin type heteropolyanions with graphene POM/GS complexes.Transition elements substitute the polyacids of Keggin typeα-SiW₉ andβ-SiW₉ respectively.Simultaneously,A series of inorganic composite functional materials ofα-SiW₉M₃（M=V,Ti）andβ-Si W₉M₃（M=V,Ti）were prepared.Graphite and the above-mentioned polyacid are used as structural elements,GO is synthesized by the Hummers method,and the POM/GS composite is continuously reduced by the hydrazine hydrate method.The structure and morphology of the target products were characterized by the FT-IR,XRD,Raman,SEM and TG,and the results show that We successful prepared the POM/GS composite.The Keggin framework structure was kept perfectly and the POMs were connected with GS by strong chemical bonding.2.Study on the catalytic performance of styrene epoxidation.The epoxidation of styrene was selected as the model reaction,and a variety of the POM/GS solid catalysts were used to catalyze the reaction.The experimental data show that this type of catalytic material has high catalytic activity and selectivity for the epoxidation of styrene.For example,the complex ofα-SiW₉V₃/GS,which the catalytic activity is higher than theα-SiW₉ andα-SiW₉V₃,makes the conversion of styrene reach to 83.45%and the selectivity of styrene oxide reach to 53.42%.The optimal amount of catalyst for this reaction is 0.06 g;the optimal temperature is 70℃;the optimal time of reaction is 6 h;The catalytic effect of the composite was not obviously attenuated after the catalyst has been used circularly for five times.3.Synthesis and characterization of transition metal-substituted polyacid-based graphene materials at low temperature with high pressure.In order to explore a green technology to prepare the POM/GS complex,we carried out the experimental work of the low temperature with high pressure synthesis method.At the temperature of 260℃,the pressure of 4 MPa nitrogen atmosphere,We successful prepared the same structure of the POM/GS composite.The composition and structure of the new material were characterized by FT-IR,XRD,Raman,SEM and TG.The results confirmed the GS and Keggin-type POM unit structures in the complex.There existed great chemical bond effect between them,and the graphene has a high degree of stripping,a low degree of disorder,and less accumulation.