The Study Of One-step Synthesis Of Methylmethacrylate From Methacrolein Over Heteropolycompounds Catalysts

Methylmethacrylate (MMA) has been prepared from isobutene by two-stage oxidation and one-stage esterification, which was a complex process and easy to be jammed by high melting point by-products. The modified process that prepared MAL from isobutene, and then produced MMA from MAL by one-step oxidation-esterification reaction have been developed. However, the new processes that needed a rather long operation time used Pd as active material of catalysts. Furthermore, its operating cost was also kept relatively expensive for recycling the excessive methanol, which was fed into reactor to promote the conversion of MAL. Heteropolycompounds (HPCs) are known to be very effective catalysts for both acid-catalyzed and redox reactions. Moreover, the properties of oxidation and acid catalysis of HPCs can be modified by changing their preparation methods or compositions. In this dissertation, HPCs have been modified by choosing proper preparation methods or compositions, so that some HPCs can be used as catalysts in one-step synthesis of MMA from MAL. The project was performed by both-step reactions, MAL oxidation and MAA esterification. HPCs were evaluated and chosen by MAL oxidation, then some of them were used in one-step oxidation-esterification. And the reaction mechanism of one-step oxidation-esterification had been investigated.Firstly, Three kinds of pure heteropoly acid and four kinds of pure heteropoly salts containing P, Mo and V were prepared. Their structure and acid properties were studied by different characterizations methods. Catalytic performances of the HPCs were investigated in MAL oxidation reaction. Among the HPCs, H₄PMo₁₁VO₄₀ showed a higher decomposition temperature and better catalytic activity and it became a preferential substrate of HPCs catalysts. Effects of preparation method on the structural characters properties and catalytic performances of HPCs were investigated. Compared to coprecipitation method, HPCs catalysts with the same chemical composition prepare by quinline sedimentation method had more surface area and stronger catalytic activities of MAL oxidation. Moreover, counter-ions affected heavily catalytic performances: Cu²⁺ increased the redox of the HPCs; Cs⁺ improved the surface area; and synergetic effect of Cs⁺ and Cu²⁺ increases the structural stability. Acid acid properties and structural stabilities of HPCs with Cs⁺, Rb⁺ and K⁺ as counter-ions increased with the increasing of their ion radius. And HPCs with La³⁺ and Ce⁴⁺ as counter ions had lower structural stability and less surface area. Their properties were similar to pure heteropoly acid. Anyhow, H₄PMo₁₁VO₄₀ as the parent acid, HPCs with Cs⁺ as main counter-ions, containing less Cu²⁺ and As³⁺, prepared by the quinline sedimentation method, showed better catalysis.Under the similar reactive conditions of MAL oxidation, catalytic behaviors of HPCs with alkali metals (Cs⁺, Rb⁺ and K⁺) and lanthanide (La³⁺ and Ce⁴⁺) were investigated in esterification and MAL oxidation-esterification. HPCs with Cs⁺ showed good catalytic performances. Further, effects of cesium content on the properties and catalytic performances of HPCs were studied. With the increasing of Cs content, surface area and structural stabilities of the HPCs increased, while the acidity and reducibility decreased. On the other hand, MAL conversions increased in the beginning and then decreased, and the total selectivity of MMA and MAA showed a similar trend. Anyhow, effects on activity of MAL oxidation were surface area. The acidity of HPCs affected MAA selectivity in some degree; the acidity was main factor for esterification of MAA. Furthermore, effects of acid amount on selectivity of MMA were obvious. The more acid amount corresponded to the more MMA selectivity.Certain H₃PW₁₂O₄₀ was added into CsCuPMo₁₁VO₄₀ to improve its acidity by the ion-exchange method or mechanical blend method in order to study effects of acidity on reaction mechanism. Differences were not observed in structural properties and catalysis of the composite HPCs. The more MMA selectivity was observed on the composite HPCs catalysts; however, MAL conversion and the total selectivity of MMA and MAA are rather lower. Especially, life of the composite HPCs catalysts was rather brief. As a result, simplex improvement of acid properties can not promoted the yield of reaction. Reasons may exist that the intense acidity help to adsorb more methanol, so that promote the esterification; on the other hand, excessively intense MAL adsorption caused coking for a while. There may be both active sites corresponding MAL and methanol in the HPCs respectively. Moreover, the competitive adsorption can be concluded in some active sites of the HPCs that adsorbed MAL and methanol.The reaction conditions of MAL oxidation, MAA esterification and MAL oxidation-esterification were studied over CsCuPMo₁₁VO₄₀. And effects of O₂/MAL and steam on MAL oxidation-esterification were evaluated. The results showed MAL conversion increased with increasing of O₂ partial pressure, at the same time, increasing of O₂ partial pressure leaded to improvement of deep oxidation. On the contrary, increasing of MAL improved the total selectivity of MMA and MAA. Steam help to keep a higher MAA selectivity and restrained formation of CO and CO₂. However, the excess steam leaded to decreasing of MMA selectivity. Generally, CsCuPMo₁₁VO₄₀ had better stability than else the HPCs. However, its deactivation was attributed to structural inactivation. Surface area and pore volume of the deactivated HPCs decreased. Furthermore, it can be identified that there were two kinds of structures, i.e., V₂O₅ andα-MoO₃, in the decomposed products.Studies of immobilized H_2.25Cs_0.7Cu_0.25As_0.1PMo₁₁VO₄₀ (PMo₁₁VO₄₀)as active composition were carried out in MAL oxidation and one-step synthesis MMA from MAL. The results showed the critical load amount of was about 40 % on SiO₂ carrier. Results of MAL catalytic oxidation showed MAL conversion was 83.5% and selectivity of MAA was 92.7% on PMo₁₁VO₄₀/(C₆H₆N₄O₂)₆ in certain reactive conditions. On the same catalyst, 4.1% MMA selectivity and 79.2% MAA selectivity with 94.1 MAL conversion can be obtained in one-step synthesis MMA from MAL.