Preparation And Photocatalytic Properties Of Zirconium-based Metal-Organic Framework(MOFs) And Composites

Metal-organic framework materials?MOFs?is a kind of three-dimensional reticulated crystal materials synthesized by self-assembly of metal ion and organic ligand,which has large specific surface area and rich topological structure.In addition,its structure has the characteristic of cutting,designability and adjustment,thus it was widely used in many fields.Compared with other materials,zirconium-based metal-organic framework materials,which is consist with high valence cation Zr⁴⁺as the center coordination atom,has higher thermal stability and chemical stability because of the strong affinity between zirconium ion and oxygen atoms in organic ligand,it can be used in complex reactions as catalyst.In this paper,the synthesis method of zirconium-based metal-organic framework material UiO-66-NH₂ was discussed,and its adsorption and photodegradation properties were also discussed.On the basis,the composite materials PMoV₂/UiO-66-NH₂ and PMoV₂/UiO-66-NH₂ were synthesized and their properties for direct catalytic oxidation of benzene to phenol under visible light were investigated.The details of the study were as follows:?1?A series of Zr-based MOFs were successfully prepared by conventional solvothermal method and microwave irradiation method.The products synthesized by microwave irradiation possessed a better dispersibility and a lager specific surface area than those synthesized by solvothermal method,and the crystal morphology can be regulated by adding monoacid.The adsorption kinetics of methylene blue by a series of UiO-66-NH₂ samples showed that the adsorption of methylene blue was in accordance with the pseudo-second-order kinetics,indicating that the adsorption of methylene blue by the adsorbents was chemical adsorption.It was found that the adsorption of methylene blue by synthesized Zr-MOFs was more in accord with freundlich isotherm adsorption model,indicating that the adsorption of methylene blue by synthetic samples was multi-layer adsorption,both linear and nonlinear.The adsorption experiment showd the synthesized UiO-66-NH₂ materials had high adsorption capacity for methylene blue.After five adsorption-desorption experiments,the samples with decreased adsorption capacity can degrade the dyes adsorbed on the surface of the adsorbents by photocatalytic reaction,so that the adsorption properties of mofs materials can be restored.Therefore,Zr-MOFs have potential to be applied to the treatment of dye.?2?Molybdovanaphosphoric acid(H₅PMo₁₀V₂O₄₀)with Keggin structure were prepared and loaded it on the surface of UiO-66-NH₂ to get the composite PMoV₂/UiO-66-NH₂ without the change of topological structure of UiO-66-NH₂.During the experiments of benzene hydroxylation under visible light,UiO-66-NH₂and PMoV₂/UiO-66-NH₂ were used as the photocatalyst and its photocatalytic performance under different reaction conditions was also studied.When the mole ratio of benzene to hydrogen peroxide was 1 to 2,the photocatalytic performance was highest and the yield of phenol was highest,too.The yield of phenol could be reached to biggest under the optimum conditions to reaction for 4 h,and it was not changed even prolong the reaction time,and the selectivity of phenol was 98.8%.The control experimental results show that the catalytic activity of the composite catalyst is higher than that of the catalyst with any single component,which proves that the composite catalyst plays a synergistic role.?3?UiO-66-NH₂ was used as substrate and reacted with salicylaldehyde to form Schiff base UiO-66-NH₂-SA,and then reacted with vanadium oxyacetylacetonate to produce the novel catalyst UiO-66-NH₂-SA-V.During the experiment of benzene hydroxylation under visible light,UiO-66-NH₂-SA-V was used as the photocatalyst and its performance was also studied under different reaction conditions.When the mole ratio of benzene to hydrogen peroxide was 2 to 3,the photocatalytic performance was highest and the yield of phenol was also highest.The yield of phenol could be reached to biggest?17.75%?under the optimum conditions to reaction for 4 h,and the selectivity of phenol was 100%,it was not changed even prolong the reaction time,it was noted that no peroxide product was detected.The repeated experiments showed that the efficiency of the catalyst remained unchanged after five cycles,which indicated that the bimetallic photocatalyst UiO-66-NH₂-SA-V synthesized by chemical bonding had good stability.Studies on the reaction mechanism show that the pores of the synthesized catalyst mixed with benzene are occupied by benzene,which prevents the contact between the formed phenol and the catalyst and ensures the high selectivity of the reaction.