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

摘要

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 CeO_x-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.