Preparation And Catalytic Performance Of Recoverable And High-activity Magnetic Composite Materials Based On Keggin-type POMs

Polyoxometalates?POMs?have been widely used in acid catalysis,redox catalysis,and photocatalysis because of their special spatial structure,high charge density and reversible redox properties.Since most of POMs are easily soluble in polar solvents,resulting in the difficulty of catalyst recovery.The construction of a recoverable,stable and high active composite catalytic materials based on POMs is an urgent and challenging issue in the research of POMs chemistry.In this thesis,selective POMs with good acid-base stability and catalytic activity are firmly loaded on the functionalized magnetic carrier,on which the noble metal nanoparticles are further deposited in situ.As-prepared composite catalysts display efficient photocatalytic activity,are very stable and durable,can be easily recovered.The several important factors influencing catalytic activity and stability of the composite materials,such as the component,structure and morphology of materials were investigated,and the photocatalytic mechanism was discussed.In addition,several organic-inorganic hybrid materials composed of isopolymolybdate,transition metal and organic ligands have been synthesized,and their catalytic and adsorption properties have been investigated.The main research achievements are attached below:1.Preparation and photocatalytic activity of Fe₃O₄@SiO₂@mSiO₂-PW₁₂-Ag(PW₁₂=PW₁₂O₄₀)composite nanocatalysts.In order to improve the active sites and catalytic properties of POMs in the composite catalyst,a mesoporous magnetic carrier?Fe₃O₄@SiO₂@mSiO₂?with high specific surface area and pore volume was selected as a support.The PW₁₂ was loaded on the mesoporous magnetic carrier by the wet-impregnation method.Treating at 300°C,PW₁₂ was stably supported on the surface of the support,and further deposited Ag NPs on the surface of Fe₃O₄@SiO₂@mSiO₂-PW₁₂ by in situ reduction of Ag⁺ions,a stable and easily recoverable multi-composite magnetic photocatalyst was obtained.Photocatalytic experiments reveal that Fe₃O₄@SiO₂@mSiO₂-PW_12x-Ag_y?x,y=18.6%,5.3%?exhibits the highest photocatalytic activity,which can completely degrade methyl orange?MO?in simulated sun light after 2 h.In the composite catalyst,the surface plasmon resonance effect?SPR?of Ag NPs retards the rapid recombination of photogenerated electrons?e^-?and holes?h⁺?in PW₁₂,improving light absorption and photocatalytic performance.The composite catalyst has the characteristics of durable activity and easy separation.The magnetic composite catalyst can be quickly recovered by using an external magnetic field,and the activity remains unchanged after used for 5 times.2.Preparation and catalytic performance of Fe₃O₄@SiO₂@PDA/SiW₁₁V-M(SiW₁₁V=SiW₁₁VO₄₀^6-,M=Ag,Au)composite nanocatalysts.In order to improve the dispersibility of noble metal nanoparticles and firmly load the POMs on the surface of the carrier,polydopamine?PDA?with excellent adhesion and good physicochemical properties and SiVW₁₁O₄₀^6-(SiW₁₁V)with good acid-base stability are encapsulated on the surface of the magnetic carrier,and further the noble metal particles in situ are deposited,a uniform distribution of noble metal nanoparticles,a stable and recoverable multi-component composite magnetic catalyst Fe₃O₄@SiO₂@PDA/SiW₁₁V?M=Ag,Au?was obtained.The Fe₃O₄@SiO₂@PDA/SiW₁₁V_x-Ag_y?x,y=14.1%,5.2%?sample can be completely degrade MO after 135 min under simulated sun light.In the composite catalyst,Ag NPs,SiW₁₁V and PDA can absorb ultraviolet and visible light in a wide wavelength range,so they can be excited in sunlight,and SPR of Ag NPs can delay the rapid recombination of photoelectrons?e^-?and holes?h⁺?,so the composite catalyst exhibits good catalytic activity under simulated sunlight.The composite catalyst also exhibits good stablity in wide pH range?pH=2-10?and most organic solvents.In addition,Fe₃O₄@SiO₂@PDA/SiW₁₁V_x-Au_y?x,y=14.1%,9.1%?catalyst exhibits good catalytic activity on the reduction of 4-nitrophenol?4-NP?with the reaction rate constant of8.48×10^-3 s^-1.Their catalytic activies remain unchanged after used for five times.3.Preparation and catalytic performance of Fe₃O₄@TiO₂@PDA/SiW₁₁V-Ag composite nanocatalysts.In order to further improve the photocatalytic activity of the composite catalyst,photoactive Fe₃O₄@TiO₂ was selected as the carrier.A stable,high photocatalytic activity andrecoverablemulti-componentcompositemagneticcatalyst Fe₃O₄@TiO₂@PDA/SiW₁₁V-Agwassuccessfullyprepared.Fe₃O₄@TiO₂@PDA/SiW₁₁V_x-Ag_y?x,y=12.4%,5.3%?showed the highest photocatalytic activity,and it can completely degrade MO under visible light??>420 nm?in 2 h.In the composite catalyst,except for Ag NPs,SiW₁₁V and PDA can absorb UV-visible light in a wide wavelength range,and the heterojunction among these components can effectively suppress the recombination of e^-and h⁺,therefore improve light absorption.Compared with the above Fe₃O₄@SiO₂@PDA/SiW₁₁V_14.1-Ag_5.2 which degraded 31.2%MO in 2hours under the same conditions,the catalytic efficiency of the catalyst was significantly improved.The composite catalyst exhibits high catalytic activity under simulated visible light,and the activity remains unchanged after used for five times.In addition,five new organic-inorganic hybrid materials composed of the isopolymolybdate,organic ligand and transition metal ions were synthesized under hydrothermal conditions.Compounds 1-3 were obtained under hydrothermal conditions by adjusting the pH of the reaction:[Cu₂^II?bbx?₂(?-Mo₈O₂₆)]·2H₂O?1?[Cu₂^II?bbx?₄(?-Mo₈O₂₆)]?2?H₂[Cu₂^I?bbx?₂?Mo₂O₈?]?3?Compounds 1 and 3 were used as heterogeneous catalysts to catalyze the reaction of persulfate?PMS?oxidized methylene blue?MB?.After 2 h,79%of MB can be degraded compared with 44%without catalyst.Compounds 4 and 5 are composed of[Mo_x O_y]^n-chain and transition metal complexes:[Cu?Hbib?₂(?-Mo₈O₂₆)]?4?[?Ni?H₂O??₂?bib?₂(?-Mo₈O₂₆)]·2H₂O?5?Compounds 4 and 5 exhibit good adsorption properties for dye MB,and can selectively adsorb MB molecules from aqueous solutions of MB,MO and rhodamine.The saturated adsorption amount?Qe?of compound 4 to MB is 551.3 mg·g^-1.