The Catalytic Properties of Pd Nanoparticles Modified by Phosphorus in Liquid-phase Hydrogenation of o-chloronitrobenzene

The new Pd-P catalysts for o-CNB hydrogenation obtained by interaction of Pd(acac)(2) with white phosphorus in hydrogen, in N, N-dimethylformamide (DMF) under mild conditions were developed and studied. They exhibit higher selectivity to o-chloroaniline (74-94%) compared to Pd-black (56%). The introduction of phosphorus increases selectivity of palladium catalysts, however the activity values and P/Pd ratio are inversely related. The nature and the properties of Pd-P catalysts depend on P/Pd ratio. With the increase of the P/Pd ratio the average size of the particles decreases from 26 nm (for P/Pd = 0) to 5.6 nm (P/Pd = 0.3) and 4.8 nm (P/Pd = 1.0). Nanoparticles formed at P/Pd = 0.3 consist of palladium phosphide Pd6P and clusters Pd(0). Core-shell structure PdxP@Pd-n was proposed for them. The Pd(0) clusters in the Pd-P catalyst (P/Pd = 0.3) possess higher electron density (Pd3d(5/2) 334.52 eV) compared to metallic palladium. With the increase of the P/Pd ratio the fraction of Pd(0) reduces and P-enriched palladium phosphides are formed. The Pd-P catalyst with P/Pd = 1.0 mainly consists of Pd5P2 and PdP2. The Pd(0) content for this catalyst is below 8%. Dimethylammonium dihydro-and hydrophosphates (phosphites) formed via the partial hydrolysis of the solvent - DMF contribute to the stabilization of Pd-P nanoparticles. We suggest that the nature of the catalyst nanoparticles consisting of Pd and PdxP as well as electronic state of Pd(0) clusters are the main factors determining catalytic properties of the Pd-P catalyst in o-CNB hydrogenation.