Synthesis And Catalytic Activities Of Polyoxometalates-based Magnetic Easy Recovery Composite Catalysts

Study on novel easy recovery and high efficient catalysts has been one of challenging and attractive research fields in chemistry and chemical engineering.Polyoxometalates(POMs)have been widely used as catalysts owing to their excellent acid-base property,reversible redox property and tunable structure and composition.Normally,POMs can be utilized as both acid catalyst and oxidation reduction catalyst,even as the multifunction catalysts.However,most of POMs are usually soluble in polar solvents,causing difficulties in the recovery,separation and recycling of the catalysts.On the other hand,insoluble POMs salts also suffer from some disadvantages,such as lower specific surface area and the lower catalytic activities.Therefore,it's still challenging and urgent to design and synthesize POMs-based heterogeneous with higher catalytic activities and easy recovery.In this dissertation,POMs-based magnetic easy recovery catalysts which combined POMs and metal nanoparticles have been studied.The main research achievements are attached below:(1)The synthesis and the catalytic activities of polyoxometalates-loaded spherical magnetic core-shell catalystsNovel magnetically recyclable catalysts Fe3O4@mSiO2-NH/POM were fabricated by immobilizing four POM(POM = H3PMo12O40,H3PW12O40,H5PV2M012O40 and H5PV2W10O40)on amino-functionalized mesoporous silica coated Fe3O4 core-shell nanoparticles,respectively.The obtained catalysts were characterized by using SEM,TEM,FTIR,XRD,N2 sorption-desorption,and magnetometry.The catalytic properties were evaluated for a series of alcohols that were selectively oxidized to the corresponding aldehydes or ketones using hydrogen peroxide as the oxidant.The results showed that POMs can be effectively loaded on mesoporous silica coated Fe3O4,and H5PV2W10O40-loaded magnetic catalyst presents the highest catalytic activity with 98%conversion of benzyl alcohol and 99%selectivity to benzaldehyde.After the reaction,the catalyst could be separated easily with an external magnetic field,and the recovered catalyst could be reused at least five times without any loss of catalytic activity.(2)The synthesis and the catalytic activities of polyoxometalates and metal nanoparticles composite spherical magnetic core-shell catalysts.Magnetically recyclable catalysts Fe3O4@mTiO2-NH/PMo12/M(M = Au,Ag and Pd)were successfully fabricated by combining Au,Ag or Pd nanoparticles and H3PMo12O40 immobilized amino-functionalized titanium dioxide coated Fe3O4 core-shell.The dispersed metal nanoparticles were assembled onto magnetic core-shell through the in situ reduction of metal ions by electrochemical reducted H3PMo12O40.The morphology,structure and component of catalysts were confirmed FT-IR,SEM,TEM,XRD,UV-Vis spectra and XPS.The catalytic properties were evaluated by using a model reaction based on the reduction process of 4-nitrophenol into 4-aminophenolin the presence of NaBH4 as the reductant and the mechanism for the enhancement of the catalytic activity of obtained catalysts were also investigated.The results show that Fe3O4@mTiO2-NH/PMo12/Pd(0.54%)catalyst exhibited excellent catalytic activities.The apparent kinetic rate constant(kapp)was calculated to be about 18.71×10-3 s-1.After the reaction,the catalyst could be separated easily with an external magnetic field,and the recovered catalyst could be reused at least nine times without any loss of catalytic activity.(3)The synthesis and the catalytic activities of polyoxometalates and palladium nanoparticles composite functional spindle-shaped magnetic core-shell catalysts.Ellipsoidal magnetically multifunction catalyst ?-Fe2O3@mTiO2-NH/PMo12/Pd was successfully fabricated by assembling metal nanoparticles onto titanium dioxide coated?-Fe2O3 core-shell through the in situ reduction of metal ions by electrochemical reducted H3PMo12O40.The as-prepared catalyst samples were characterized by FT-IR,SEM,XRD,XPS and TEM.The results revealed that the monodispersed metallic Pd nanoparticles with average size of 10?15 nm were deposited on the surface of magnetic core-shell.The as-prepared catalyst can serve as an efficient nanoscale magnetic stir bar in a microscopic droplet system.Because conventional stirring methods cannot be used in nanoscale reactors such as microdroplets or micelles,even may damage to the structure of catalyst in mixing by collision.The catalytic activities of the samples were evaluated by the reduction of methylene blue and reduction f 4-nitrophenol the presence of NaBH4.The results show that?-Fe2O3@mTiO2-NH/PMOi2/Pd(1.7%)exhibited excellent catalytic activities.It can completely catalytic reduct 4-nitrophenol in 3 min and catalytic reduction of methylene blue in 4 min.The magnetic stirring bars can effectively stir the reaction solution within microdrops to accelerate mass transfer in microscopic catalytic systems.After the reaction,the catalyst could be separated easily with an external magnetic field,and the recovered catalyst could be reused at least six times without any loss of catalytic activity.