Nanoparticle oxides as catalyst supports for the oxidative coupling of 4-methylpyridine over palladium.

摘要

The use of nanoparticles as supports for palladium catalysts was studied for application to the oxidative coupling of 4-methylpyridine via C-H activation and C-C coupling. The product of this oxidative coupling is 4,4'-dimethyl-2,2'-bipyridine which is a useful but expensive chelating agent. Although palladium on alumina are traditionally poor catalysts in this reaction, an excellent palladium on nanoparticle alumina catalyst was developed which exceeds the activities of the traditional palladium on carbon catalyst. This nanoparticle catalyst only gives high activities when prepared by precipitation. Other nanoparticle oxide supports were found that give highly active palladium catalyst for the oxidative coupling reaction, including nanoparticle magnesia and zirconia. Additionally, some traditional porous supports were found to result in active catalysts, including gamma-alumina, titania, and silica, but only the porous titania had activities comparable to the best performing nanoparticle-supported catalysts. It was also determined that zirconia and ceria are effective additives in several of the palladium catalysts.;Optimization studies of the catalysts were also performed. It was determined that 2.5, 5, and 10% loadings of palladium on nanoparticle alumina give the same product yield per unit weight of catalyst, which indicates a structure sensitive reaction. Lowering the Pd loading to 1% decreases the yield palladium loadings of ∼ 2.5% were, thus, determined to be optimal on the nanoparticle alumina support. Additionally it was shown that titration to a pH of 11 gives more consistent results compared with titration to a simple stoichiometric excess (based on the amount of palladium(II) nitrate used).;In general, oxides with strong palladium-support interactions, and/or very high surface areas were the most active. This is attributed to electrophilic palladium oxide particles that can form on supports with strong metal-support interactions or to the numerous low-coordination edge or corner sites of the high surface area supports. The electrophilicity of these particles was determined by x-ray photoelectron spectroscopy (XPS). It was also shown that basic sites on the support are not necessary for forming an active catalyst, but that a high concentration of acidic support sites tends to give more active catalysts.