254888 A Novel Bimetallic Catalyst for the Oxidative Esterification of Aldehyde
The direct oxidative esterification of methacrolein (MAL) with methanol to methyl methactylate (MMA) process [1, 2] in the presence of molecular oxygen and a catalyst under mild conditions has been developed in our laboratory. In contract to the widely used acetone cyanohydrin (ACH) process , it is an environmentally benign, and therefore the catalyst preparation is the key technology. As for the esterification is reversible reaction, the generated water not only impedes the forward chemical reaction but also forms a water film to retard the diffusion of reactants to active sites , the hydrophobic material styrene-divinylbenzene copolymer (SDB), with spherical architecture and inner abundant pores, was used for the reaction. Early work demonstrated that the monometallic Pd supported on SDB show higher catalytic performance than the hydrophilic catalysts supported on SiO2 and ¦Ã-Al2O3 with MMA selectivity at 54 % and MAL conversion at 44 % . However, MMA yield was not sufficient because of the oxidation of the metal and occasionally this caused a severe leaching of the metal into the reaction medium.
In the present work, in order to enhance the catalytic performance and further studied the reaction mechanism, a new bimetallic hydrophobic catalyst was prepared. The catalysts were prepared by incipient wetness impregnation method and preparation conditions of the catalyst were studied.
The catalysts using hydrogen as reduction reagent showed higher catalytic performance in the MMA selectivity and MAL conversion for the reaction. Correspondingly, the catalyst reduced by hydrogen shows higher surface area and smaller Pd particle size to form a fine dispersion of Pd, which is favorable to the catalytic performance, compared with catalyst using the methanol-formaldehyde as reduction reagent. Baed on the experimental temperatures from 150 °æ to 250 °æ,the catalytic performance peaks at the reduction temperature of 200 °æ. The reduction time was also discussed from 2 h to 14 h and Pd(Cl)2 was reduced completely after 10 h according to XRD patterns.
The influence of lead was also investigated and the experimental results demonstrate that varying the amount of Pb (from 0 to 2 wt % at the interval of 0.5 wt %) change the catalytic activity and 0.6 wt % Pb loading is the optimum in the highest MMA selectivity (86%) and MAL conversion (79%) for the reaction. Compared to the monometallic catalyst (Pd/SDB), the bimetallic catalyst (PdPb/SDB) show higher catalytic activity because catalyst particles are better dispersed on the support with smaller particle size and Pd-Pb atoms formed intermetallic Pd3Pb1 crystals.
Moreover, the investigation on reaction kinetics provides a useful tool for large-scale direct production of MMA with higher performances over the hydrophobic catalysts and the estimated order of the oxidative esterification is 2.1. The reaction mechanism was also proposed. The selectivity of MMA of the reaction over the monometallic Pd catalyst is lower than that of the Pd-Pb bimetallic catalyst is due to the production of CO2 and we can conclude that the atom Pd has relatively strong adsorption with O2. The influence of the promoter, Pb, for the oxidation esterifiction, has weaken the interaction of Pd and oxygen atom and then decreased the formation of CO2. We concluded that the promoter of Pb has an optimal amount and it affects the oxide species concentration directly which plays the critical role on the selectivity of the catalyst. The experimental results demonstrate that the bimetallic hydrophobic catalysts make the esterification reaction more efficient and provide widely use in the reaction involving water as a product or reactant.
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