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

Abstract

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.