Research Of Catalysts For Oxidative Esterification Of Methacrolein

Methyl methacrylate (MMA) is a kind of important chemical raw materials, which is widely used. Currently, among the latest MMA production routes, isobutene oxidation has attracted the most attention because of its simple process, high economic benefit and less environmental pollution. Isobutene oxidation technology contains two steps:isobutene or tert-butyl alcohol oxidation to methylacrolein (MAL); and MAL oxidative esterification to MMA with methanol and oxygen.The key of the oxidative esterification process is the preparation of new catalysts. There are many kinds of catalysts suitable to this reaction, among which palladium supported catalysts perform best. The mild reaction condition and simple catalyst recovery are also attractive for Pd supported catalysts. Researches of catalysts for this process are relatively scarce. Most researchers have focused on series of additives on the active sites to improve the catalysts performance, but the composition of these catalysts is usually complex. We studied the influence of simple additives and different supports on the performance of catalysts in the oxidative esterification process. The main experimental results and conclusions are summarized as follows:As the commonly used carrier, SiO2was modified with different contents of alkaline earth metal oxide MgO, then loaded Pd and Pb by impregnation and liquid phase reduction method to acquire5wt%Pd3wt%Pb/MgO-SiO2catalysts. Oxidative esterification of MAL over these catalysts was carried out, and catalysts were characterized by XRD, TEM, XPS, etc. After modified with MgO, the peak intensity of the active site Pd3Pb decreased, while the peak width increased. XPS results showed an increase of the Pd0species. These results indicate that the crystal size was much smaller, the dispersion of the active sites, active site number and catalytic performance were improved. When the content of MgO was10wt%in the support, the catalytic performance was the best. When the content was too high, the MgO peak appeared in XRD patterns and the activity decreased.The content of Pb had great influence on the selectivity of MMA and the activity of the catalyst. Results of XPS and H2-TPD indicated that with the increase of Pb content, the number of active sites increased. We also studied the effect of Pd metal content, and found an ideal conversion of MAL about93%and the selectivity of MMA about97%when the content of Pd was5wt%, the content of Pb reached3wt%, and the content of MgO was10wt%.The kinds of supports have great influence on the catalytic performance. SiO2, Y-Al2O3, SBA-15, ZrO2, CaCO3and ZnO modified with10wt%MgO were studied as the supports, and5wt%Pd and3wt%Pb were supported on them to form final catalysts. SiO2supported catalyst exhibited the best catalytic performance, the yield of MMA could reach80%on ZnO and CaCO3. These results are related to the isoelectric point, pore structure and acid-base property of supports. Different preparation methods were compared as well. The catalyst prepared by formaldehyde reduction was better than by hydrogen reduction.Water formed during oxidative esterification of MAL to MMA, decreases the conversion of MAL and the selectivity of MMA. In order to reduce negative impact, we attempted to add A-type zeolite membrane to the reaction system, to remove water from the reaction system. However, the desired effect was not obtained, which needs further study.