Influence of preparation method and palladium content on Pd/C catalysts activity in the liquid phase hydrogenation of nitrobenzene to aniline

Abstract

Two sets of Pd/C catalysts with 1–10wt.% Pd content supported on active carbon were prepared by a conventional formaldehyde method. One set was prepared using H2[PdCl4] complex and other one with Na2[PdCl4]. In spite of different metal loadings, average crystallite size of palladium particles estimated by XRD and TEM analyses was virtually the same in all the cases, from 3 to 5nm. Catalysts were tested for nitrobenzene hydrogenation in a stirred autoclave with the presence of methanol as a solvent, at 5MPa and 50°C. Ratio of Pd to nitrobenzene substrate was in every reaction mixture the same: 15.4mg of palladium per mol of nitrobenzene. Specific initial catalyst activity (initial reaction rate) for catalysts with 1–4wt.% Pd content for both sets of catalysts was comparable. It means that palladium complex used for the preparation had no-effect on catalyst activity. With palladium content higher than 4wt.% a significant drop in the catalytic activity and increase in amount of the leached palladium were observed, which should be prescribed for the formation of agglomerates of palladium crystallites (revealed by TEM) and consequent decrease in concentration of catalytic sites. Because of a decrease in catalytic activity, the reaction times were longer, which caused a higher metal leaching.