Selective Oxidation Of Methacrolein To Methacrylic Acid Over Phosphomolybdic Acid And Its Salts

Methyl methacrylate?MMA? is one of important acrylic resin monomers. It is mostly used in the production of organic glass and has a promising market prospect. The traditional MMA production route is the acetone cyanohydrin?ACH? process. This process uses extremely toxic and hazardous hydrogen cyanide?HCN? as raw material and generates a large amount of ammonium sulfate with a low atom economy. The ethylene and syngas based route developed by BASF including four steps, ethylene hydroformylation, condensation of propionaldehyde with formaldehyde, methacrolein oxidation and methacylic acid esterification, to produce MMA is a cleaner technology and use clean and low-cost feedstock, which can be produced from coal. The selective oxidation of methacrolein to methylacrylic acid is a key step in determining the total yield of MMA. The phosphomolybdic acid based catalysts have been successfully industrially applied in the reaction, but the single-pass yield still has great space to improve.One of simple and direct ways to modulate the properties and catalytic performance of heteropolyacid compounds is to change the counter-ions.In this work, different content of Cs⁺ was added in phosphomolybdic acid. The results showed that the catalysts with Cs⁺ still reserved the Keggin structure, and the thermal stability was enhanced. Additionally,adding Cs⁺ to H₃PMo₁₂O₄₀, the secondary structure transitioned from the triclinic crystal phase to the cubic phase, the acidic amounts decreased and the catalytic performance was improved. And Cs₂HPMo₁₂O₄₀ showed the best catalytic performance.Another series of phosphomolybdate catalysts were prepared by adding different content of Cu²⁺ to Cs₂HPMo₁₂O₄₀ and then characterized. The results showed that the addition of Cu²⁺ did not modify the primary structure and reserved the Keggin structure. According to the H2-TPR results, the increase of the oxidizing ability was found and proposed due to the presence of CuOx in the secondary structure. Because of the reduction of protons, the acid amounts of the catalysts determined by NH3-TPD decreased with the increase of Cu content. Meanwhile, the catalytic activity could be improved obviously by the substitution of Cu for proton, the conversion of methacrolein was improved from 65 %(Cs₂HPMo₁₂O₄₀) to 96 %(Cs₂Cu_0.05H_0.9PMo₁₂O₄₀) at 300 ?, but the selectivity to acetic acid was enhanced maybe induced by the increasing oxidizing ability with the existence of CuOx. Whereas, with the increase of Cu content, the increase of the oxidizing ability was followed by the decrease of the acidity, which could weaken theadsorption of methacrolein and strengthen the adsorption of methacrylic acid and acetic acid. Therefore, the synergy between oxidizing ability and acidity determined the distribution of the products.The other series of catalysts were synthesized by adding Fe³⁺ in Cs₂HPMo₁₂O₄₀ to study the role of Fe³⁺ . Results showed that the Keggin structure was still maintained after adding Fe³⁺ . H2-TPR results and NH3-TPD results showed that the addition of Fe³⁺ had barely influence on the oxidizing ability and some new weaker acidic sites formed at high Fe content?Fe > 0.15? proposed due to the presence of iron aqua complexes, and the total acid amounts of catalysts were not decreased. The catalysts with Fe applied for the selective oxidation of methacrolein could improve the conversion of methacrolein from 65 % to 88 %.Whereas, the different changing trends of the catalytic performance with the change of Cu²⁺ content was found on Cs₂Fe_xH_1-3xPMo₁₂O₄₀. The conversion of methacrolein increased with Fe³⁺ content, and the products distribution was almost invariable. Therefore, Fe³⁺ was expected to accelerate the reoxidation step of the catalysts by assisting the water transfer.