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

Two sets of Pd/C catalysts with 1-10 wt.% Pd content supported on active carbon were prepared by a conventional formaldehyde method. One set was prepared using H-2[PdCl4] complex and other one with Na-2[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 5 nm. Catalysts were tested for nitrobenzene hydrogenation in a stirred autoclave with the presence of methanol as a solvent, at 5 MPa and 50 degrees C. Ratio of Pd to nitrobenzene substrate was in every reaction mixture the same: 15.4 mg of palladium per mol of nitrobenzene. Specific initial catalyst activity (initial reaction rate) for catalysts with 1-4 wt% 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 4 wt.% 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. (c) 2014 Elsevier B.V. All rights reserved.