Chemoselective Hydrogenation of Nitrostyrene to Aminostyrene over Pd- and Rh-Based Intermetallic Compounds

A noble catalytic system based on intermetallic compounds was developed for the chemoselective hydrogenation of p-nitrostyrene (NS) to p-aminostyrene (AS). The main concept of the catalyst design was to construct polar active sites consisting of two types of metals with different electronegativities. We prepared a series of Pd- and Rh-based intermetallics and investigated their catalytic properties in detail in the catalytic transfer hydrogenation of nitrobenzene and NS in the presence of 4-methyl-1-cyclohexene (MC) or methanol as a hydrogen donor. FT-IR studies of adsorbed CO confirmed that the number of Pd ensembles exposed on a surface was greatly decreased by the formation of an intermetallic phase; hence, the particle surface consisted of two metal elements being coadjacent at the atomic level. The product distribution achieved with Pd catalysts was dependent on the electronegativity of the second metal element: more electronegative metals gave higher AS selectivity and lower p-ethylnitrobenzene selectivity. Rh catalysts selectively gave AS, and their AS yields increased as the electronegativity of the second metal increased. The results revealed that an increase in the electronegativity of the second metal element provided polar sites and enhanced the activation of methanol as a hydrogen donor, which accelerated the hydrogenation of the nitro group of NS and, hence, improved the yield of AS. The high selectivity of Rh catalysts was due to the absence of MC activation ability, which caused the hydrogenation of the vinyl group of NS. Pd₁₃Pb₉ exhibited the highest chemoselectivity toward AS (92%) among the investigated Pd catalysts. Moreover, RhPb₂ exhibited not only high AS selectivity (93%) but also the highest NS conversion (94%) among the investigated catalysts. RhPb₂ also exhibited high selectivity toward AS (91%), even when H₂ was used as a hydrogen source. Thus, intermetallics that contain Pb, which was the most electronegative metal used in this study, afforded good catalytic performance and were observed to be good catalysts for the chemoselective hydrogenation of NS.