(583fh) Insights Into the Conventional Pd-Pb Bimetallic Catalyst

                                                                                                   Insights into the conventional Pd-Pb bimetallic catalyst

 Junxing Han

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China

E-mail: jxhan@home.ipe.ac.cn

Carboxylic acid esters are significant chemicals used in food additives, fragrances, medicaments, pesticides and surfactants¹. The development of synthetic strategies towards esters has attracted the interest of chemists for more than 100 years². Up to now, several routes have been established, such as esterification of carboxylic acids or carboxylic derivatives with alcohols, one-step hydrogenation-esterification of carboxylic acids with aldehydes², oxidation of aldehydes into carboxylic acids followed by esterification process as well as direct oxidative esterification of aldehydes with alcohols. Among them, direct oxidative esterification of aldehydes using molecular oxygen represents an efficient, green and promising approach, especially in the production of methyl methacrylate (MMA), an important monomer for polymers³, from methylacrolein (MAL) and methanol. According to the reported work, direct oxidative esterification of aldehydes could be catalyzed by transition metals (Ag, Au, Ir, Pd, Rh and Ru) or conducted with stoichiometric amounts of KMnO₄, CrO₃, V₂O₅, peroxovanadium, iodine or KHSO₅, but only the Pd-based catalyst was commercialized in the production of methyl methacrylate due to its high performance and clean process. However, conventional Pd-based catalysts load large amount of Pd (approximately 5 wt%)⁴, which results in high cost. The development of cost-effective catalysts with high performance and low palladium loading is an urgent but challenging task.

Monometallic Pd catalysts can not meet the requirements for the production of methyl methacrylate due to the low selectivity. By adding Pb to monometallic Pd catalysts, Pd-Pb intermetallic compounds formed which exhibit high activity and selectivity for the direct oxidative esterification of methylacrolein with methanol. It is commonly accepted that Pd is the active component and Pb is the promoter, and in the conventional Pd-Pb bimetallic catalysts the loading amount of Pd is equal or greater than that of Pb. Investigation on catalysts with low Pd loadings and high Pb loadings has not been reported.

In the present work, the high-performance and low-Pd-loading PdPb bimetallic catalyst was prepared by increasing the Pb loadings and the novel Pd₂Pb₈/Al₂O₃ (Pd₂Pb₈ represents the Pd and Pb loadings of 2 wt% and 8 wt%, respectively.) exhibited better catalytic efficiency than the commercial Pd₅Pb₅/MgO-Al₂O₃. In recent years, catalysis by bimetallic nanoparticles is of particular interest due to their unique features of tunable compositions, diverse structures, confined interfaces and synergetic effects. The protocol proposed in this context will be of significance for bimetallic catalysts to improve their efficiency and reduce the usage of noble metals.

References

1. Zhang Y., Cui X.J., Shi F., Deng Y.Q., Nano-Gold Catalysis in Fine Chemical Synthesis, Chem. Rev. 2012, 112, 2467-2505.

2. Tang Y., Yu W.J., Mo L.Y., Lou H., Zheng X.M., One-step hydrogenation-esterification of aldehyde and acid to ester over bifunctional Pt catalysts: A model reaction as novel route for catalytic upgrading of fast pyrolysis bio-oil, Energy Fuels, 2008, 22, 3484-3488.

3. Mikami Y., Takeda A., Oh-Kita M., Catalyst Used in Production of Carboxylic Acid Esters and Process for Producing These Esters, US patent, 1999, 5892102.

4. Diao Y.Y., Yan R.Y., Zhang S.J., Yang P., Li Z.X., Wang L., Dong H.F., Effects of Pb and Mg Doping in Al₂O₃-supported Pd Catalyst on Direct Oxidative Esterification of Aldehydes with Alcohols to Esters, J. Mol. Catal. A-Chem. 2009, 303, 35-42.