Studies On The Catalysts For One-Step Oxidative Esterification Of Methacrolein To Methyl Methacrylate

Methyl Methacrylate(MMA)is a widely used industrial raw material with high added value.It is the monomer of plexiglass.The traditional production process of MMA is so called acetone cyanohydrin process,which requires highly toxic hydrocyanic acid.Methacrolein(MAL)one-step oxidation esterification process does not involved using HCN,therefore it is a green process in the production of MMA.Thus the commercialization of this process is to achieve high selectivity of MMA in production.While the key to this process is to develop a catalyst that can activate the substrate efficiently to result in high selectivity.The contributions of this work are as follows:through the treatment of protective agents and doping of second metals,the structure of catalysts is regulated,and high activity and selectivity of palladium-based catalysts are prepared.The structure-activity relationship and reaction mechanism of catalysts are to some extent explored by means of characterization of catalysts.In the first part of this paper,one-step oxidative esterification of MAL to MMA on monometallic palladium-based catalyst was studied.The results showed that the catalyst of Pd supported on MgO-Al2O3,support modified by alkaline earth metal,is more advantageous for MAL oxidative esterification.On this basis,the palladiumbased catalysts were treated with different protective agents.The catalytical performance evaluation results showed that the conversion of MAL and the selectivity of MMA for the catalysts treated with different protective agents were significantly different.Therefore,BET,XRD,TEM,HRTEM,TG,CO2-TPD,FT-IR,XPS,CO-TPR,H2-TPR were used to explore the effect of protective agent treatment on the oxidation performance of palladium-based catalysts.The results inllustrate that the protective agent can not only adjust the size of Pd particles,but also affect the electron transfer between Pd and the supports metal.The Pd species on the surface of palladium-based catalysts treated with PVA have electron-rich properties and have the ability of strong linear adsorption to C=O.This species can activate aldehyde carbonyl groups in MAL preferentially,thus reducing the occurrence of side reactions and improving the catalytic selectivity.In the second part of this paper,one-step oxidati ve esterification of MAL to MMA on bimetallic catalysts was studied.Pd-Pb bimetallic catalysts were loaded by step impregnation method.After optimizing the preparation conditions of catalysts and exploring different loading amounts,excellent Pd-Pb based catalysts were successfully obtained.The conversion of MAL and the selectivity of MMA were Pd-Pb based catalysts both above 90%,and the catalysts had excellent stability.The characterization results showed that,in the Pd-Pb bimetallic catalysts,the Pb species entered the lattice of Pd to form Pd3Pb alloy.In the alloy,electrons are transferred from Pb to Pd.Electronrich Pd can then enhance the activation ability of C=O bond in MAL.Moreover,the doping of Pb inhibits the formation of β-Pd[H]species,therefore reduces the adsorption of Pd to H species.In addition,the "poisoning" effect of Pb to Pd also weakens the adsorption of Pd to oxygen species,thus reducing the hydrogenation reaction and deep oxidation reaction,finally improving the activity of oxidative esterification reaction.Pd3Pb alloy structure can also be stably loaded on the support to reduce the loss of Pd-Pb species and thus improve the life of the catalyst.