Studies On Pd/MgO Catalysts For Oxidative Esterification Of Methacrolein To Methyl Methacrylate

This dissertation contributes to the studies on the catalyst for oxidative esterification of methacrolein(MAL) to methyl methacrylate(MMA) in the presence of O₂ and methanol.The catalytic performances and structural features of the catalysts effective for the conversion of MAL to MMA have been investigated systematically.Pd was selected as the active component in our catalysts.Among many supported Pd catalysts investigated,Pd/MgO was found to be efficient for oxidative esterification of MAL to MMA.It was found that the reductants and reduction conditions used for preparation of the catalyst also influenced the catalytic performances.By using HCHO as a reductant in basic aqueous solution,we obtained the Pd/MgO catalyst with the best performance for the conversion of MAL to MMA. The effect of additives such as Pb,Bi,Fe or Sn was investigated,and the results showed that the addition of Pb to Pd/MgO could increase the selectivity of MMA while keeping the conversion of MAL.By optimizing Pb/Pd molar ratios and reaction conditions,we obtained a MMA selectivity of 90.8%at a MAL conversion of 97.4% over a 3.2 wt%Pb-6.1 wt%Pd/MgO catalyst under the following conditions:T=50℃,CH₃OH/MAL=40/1,O₂ pressure=0.2 MPa,reaction time=2h.It was found that the catalytic performances had some relationships with the acid-base properties of the supports.The support with weaker acidity or basicity,led to lower catalytic activity,whereas the support with either strong basicity such as MgO or strong acidity such as Al₂O₃ favored the conversion of MAL to MMA. Further study indicateded that the surface area of the Pd/MgO catalysts prepared with different reductants or under different conditions may be different.The catalytic performances of these Pd/MgO catalysts increased with the surface area.XRD and XPS studies suggested that a large part MgO had been transformed into Mg(OH)₂ during the preparation.TEM observations clarified that the mean size of Pd in the Pd/MgO catalysts could be changed by preparation conditions,but there seemed no definite correlation between catalytic performances and the mean size of Pd. Characterizations by EDS suggested the uniform dispersion of Pb and Pd in the Pb-Pd/MgO catalysts.XRD measurements combined with HRTEM studies revealed the formation of PbPd₃ compound in the Pb-Pd/MgO catalyst.This compound may be beneficial to the selective formation of MMA.