Liquid-phase hydrogenation of nitrobenzene and o-nitrochlorobenzene in the presence of phosphorus-containing palladium nanoparticles

The properties of phosphorus-modified palladium catalysts in the liquid-phase hydrogenation of nitrobenzene and o-nitrochlorobenzene have been investigated. A general regularity in the reduction of the nitro compounds in the presence of the phosphorus-containing palladium catalysts is an autoacceleration observed at large P: Pd ratios of 0.7-1.5. The autoacceleration is due to a redox process between the intermediate (azoxy derivative) and PdP2, a component of the catalyst nanoparticles. A promoting effect of elemental phosphorus on the chemoselectivity of the Pd-P catalysts in o-nitrochlorobenzene hydrogenation has been discovered. The main factors responsible for the changes in the quantitative characteristics of the phosphorus-modified palladium nanoparticles are considered. Among these factors there are the nature of the nanoparticles, their size, and the state of the surface layer. The properties of the Pd-P catalysts are determined by the proportions of palladium clusters and palladium phosphides (Pd(0) and Pd (x) P) in the nanoparticles. The formation of the clusters and phosphides has been confirmed by X-ray diffraction and X-ray photoelectron spectroscopy.