Heteropoly Catalysts For Oxidation Of Methacrolein To Methacrylic Acid

As an important chemical,methacrylic acid can be used for production of methyl methacrylate,coating and rubber additives.The traditional production process of methacrylic acid is Acetone Cyanohydrin method,which is low atom efficiency,and also needs toxic HCN and corrosive H2SO4 and produces?NH₄?2SO4 as solid waste.Oxidation of methacrolein to methacrylic acid is a green way to produce methacrylic acid,and a key step to produce methyl methacrylate.Keggin-type heteropoly acids can catalyze this reaction,such as molybdophosphoric and molybdovanadylphosphoric compounds.Their structure and properties can be tuned by changing their coordination atoms and counter ions.The active sites,influence of secondary structure on catalytic properties,effects of counter ions were still unclear.The low surface area of heteropoly catalysts leads to the low catalytic efficiency.Vanadium atoms can partially replace the molybdenum atoms in Keggin structure and improve the catalytic performance for oxidation of methacrolein to mechacrylic acid.However,the form of active vanadyl species and its changes during calcinations and reaction were unclear.So it's meaningful for design of efficient heteropoly catalysts to study active vanadyl species.Catalysts with different vanadyl species were prepared and studied to find out the preferable vanadyl species for oxidation of methacrolein to methacrylic acid,vanadium atom in Keggin structure(H₄PMo₁₁VO₄₀),VO²⁺ in secondary structure(HVOPMo₁₂O₄₀)and V₂O₅ mixed with H₄PMo₁₁VO₄₀(V₂O₅/H₃PMo₁₂O₄₀).With the changing of calcination temperature,all the vanadyl species changed.The VO²⁺ species which interact with Keggin structure turned to be the main active vanadyl species in all catalysts.The generated VO²⁺ species in H₄PMo₁₁VO₄₀ and V₂O₅/H₃PMo₁₂O₄₀ showed better performance than the ones in HVOPMo₁₂O₄₀ to improve catalytic performance.The active vanadyl species changed from independent ion form to a squashed square-pyramidal on Keggin structure or defect Keggin structure during oxidation reaction.The addition of Cs+ can improve the catalytic performance,thermal stabllity and surface area of molybdophosphoric acid and molydbovanadylphosphoric acid.It is an indispensable counter ion for efficient heteropoly catalysts.The addition of Cs+ also can change the secondary structure of heteropoly catalysts.A series of novel heteropoly catalysts(H₄PMo₁₁VO₄₀/Cs4PMo₁₁VO₄₀)with core-shell structure were designed and synthesized to investigate the influence of secondary structure of catalysts on catalytic performance.The effects of H₄PMo₁₁VO₄₀ supporting amount and hydrothermal treatment temperature on catalytic performance were investigated.The crystalline form of H₄PMo₁₁VO₄₀ changed from triclinic to cubic form due to the guidance effect of Cs4PMo₁₁VO₄₀.The suitable thickness of H₄PMo₁₁VO₄₀ layer on Cs4PMo₁₁VO₄₀ was a key point for the catalytic performance.The heteropoly catalysts with core-shell structure showed much better catalytic performance than the one with solid solution structure and molydbovanadylphosphoric acid.Copper can improve the catalytic performance of heteropoly catalysts obviously,and its states in catalysts and the mechanism of improvement of catalytic performance were still unclear.So the forms of copper in heteropoly catalysts and effects on oxidation of methacrolein to methacrylic acid were investigated.Copper was in form of Cu²⁺ and Cu+and complexes with 4 water molecules in the secondary structure of the catalysts at room temperature.The addition of copper could increase the crystalline size of catalysts,prevent the accumulation of catalyst crystal and make catalysts to be a kind of mesoporous materials with pore width around 4 nm.Copper can obviously improve the catalytic performance of catalysts.Copper can get an electron from Keggin structure to improve the oxidation ability of active sites during the oxidation of MAL and give an electron to Keggin structure to make the reduced active sites to be re-oxidized easily.This research supplied theoretical support for the design of efficient heteropoly catalysts.The low surface area of heteropoly catalysts was the key factor which restrict the improvement of theirs catalytic efficiency.Loading it on porous support is a way to improve their surface area and catalytic efficiency.However,heteropoly catalysts can react with silica support,which leads to the decomposition of supported catalysts during calcinations and reaction.To solve this problem,molybdovanadylphosphoric acid was supported on carbon nitride modified SBA-15 molecule sieves to improve its catalytic performance.Compared with bulk H₄PMo₁₁VO₄₀,catalytic efficiency and the methacrylic acid selectivity of supported catalysts were improved more than 3 and 2 times,respectively.H₄PMo₁₁VO₄₀ was successfully separated from SBA-15 by carbon nitride to avoid the interaction between catalysts and support.H₄PMo₁₁VO₄₀ reacted with amino group and entered pores in supports.Amino groups can decrease the chemical states of catalysts to improve MAA selectivity obviously and enhance thermal stability of the supported heteropoly catalysts.