Study On The Synthesis Of Methyl Methacrylate By One-step Oxidative Esterification Over Hydrophobic Catalyst

In this paper, the one-step oxidative esterification of methacrolein （MAL） withmethanol to methyl methacrylate （MMA） in the presence of hydrophobic catalyst andO₂was studied. The catalytic performances and hydrophobicity were investigatedsystematically and comprehensively. The reaction kinetics was also studied and thepotential mechanism was proposed.The catalytic performances of catalysts over various supports （styrene-divinylbenzene copolymer （SDB）, Al₂O₃, SiO₂） were explored firstly. The results indicatedthat the catalytic activities increased with the increasing hydrophobicity. In addition,the Al₂O₃and SiO₂supports were hydrophobicated by dichlorodimethylsilane andtheir catalytic performances were investigated, which further proved the conclusionthat the catalytic performance was enhanced with increasing hydrophobicity ofcatalyst.Focusing on Pd/SDB catalyst, the optimum Pd loading were determined firstly.The effects of reaction temperature, reaction time, catalyst additive amount,alcohol/aldehyde molar rate on catalytic performances were studied at fixed pH andO₂flow rate and the optimal reaction conditions were obtained. Catalyst life and andthe stability of Pd component were also investigated. The structures and performancesof the catalysts were characterized by TG-DTG, FTIR, ESEM, TEM, XRD and XPS.Further, the effects of promoters such as Pb, Bi on the performance of Pd₃/SDBwere studied, and the results showed that the addition of Pb and Bi to Pd₃/SDBeffectively improved catalytic performance. Under the condition of reactiontemperature=50℃, alcohol/aldehyde molar rate=40/1, additive amount of catalyst=3wt%, reaction time=5h, by optimizing Pb（Bi）/Pd molar ratios, the MAL conversionof97.8%and MMA selectivity of90.2%were obtained on Pd₃Pb₁/SDB and the MALconversion and MMA selectivity on Pd₃Bi₁/SDB were93.5%and89.8%, respectively.Catalysts containing both Pb and Bi were also inverstigated and on Pd₃Pb₁Bi₁/SDB,the MAL conversion of98.1%and MMA selectivity of90.6%were obtained.The results of XRD and XPS characterization indicated that there were newphases of Pd₃Pb, Pd₃Bi₂and Pd₂PbBi formed due to the interaction between Pd and the promoters. The intermetallic compounds formed may facilitate the production ofMMA.Last, the macrokinetics of the one-step oxidative esterification of MAL to MMAon Pd3/SDB was carried out and reaction order and activation energy wereobtained.Thus, the kinetic equation was built up. On the basis of in situ FITR spectracombined with Langmuir-Hinshelwood, the tentative reaction mechanism wassuggested.