The influence of ZnO, CeO2 and ZrO2 on nanoparticle-oxide-supported palladium oxide catalysts for the oxidative coupling of 4-methylpyridine

4,4′-Dimethyl-2,2′-bipyridine is a useful but expensive chelating agent. Having more efficient routes to the synthesis of this compound would be advantageous to the wide-spread use of this fine chemical. In this work, the effects of adding strongly interacting oxides (ZnO, CeO2, and ZrO2) to PdO catalysts supported on high surface area n-Al2O3(+), n-MgO, and n-TiO2 prepared via co- and sequential precipitation were investigated. The product yields obtained from these catalysts in the oxidative coupling of 4-methylpyridine are dependent on the additive, the support, and preparation method. Evidently, these are complex catalytic systems in that the PdO–additive and PdO–support interactions must be right to promote product formation while preventing palladium leaching and support or additive migration over the active Pd/PdO sites. Although PdO/n-ZnO catalysts are reasonably active in the coupling reaction, ZnO addition to PdO catalysts supported on n-Al2O3(+), n-MgO, or n-TiO2 does not increase the yield in any case. CeO2 and ZrO2 can increase the product yields in the reaction depending on the support used. Due to strong PdO–CeO2 interactions, the addition of CeO2 in some cases results in CeOx-migration and coverage of active PdO species or disrupts favorable PdO–support interactions leading to Pd leaching. Therefore, ZrO2 is the better additive with co-precipitated PdO/ZrO2/n-Al2O3(+) consistently producing yields in excess of 3.4 ± 0.1 g/g catalyst which is 36% higher than the 2.5 ± 0.16 g/g catalyst obtained from the PdO/n-Al2O3(+) (5 wt% Pd), the best catalyst previously reported for this reaction.

The co-precipitated PdO/ZrO2/n-Al2O3(+) catalyst gives the best yield to date in the oxidative coupling of 4-methylpyridine. Other additive or support oxides investigated cannot outperform this catalyst.Figure optionsDownload high-quality image (24 K)Download as PowerPoint slideHighlights
► Complex interactions between PdO, added oxide and support oxide identified.
► Too strong PdO-additive oxide interactions → palladium leaching or coverage.
► ZnO not a promoter, CeO2 → unstable catalysts (too strong interactions).
► ZrO2 is a true promoter for Al2O3-supported catalysts.
► PdO/ZrO2/n-Al2O3(+) prepared via co-precipitation best catalyst to date.