Pd-promoted selective gas phase hydrogenation of p-chloronitrobenzene over alumina supported Au

摘要

The gas phase hydrogenation of p-chloronitrobenzene has been investigated over alumina supported Au (ca. 1%, w/w) prepared by deposition-precipitation with urea (DP) and impregnation in excess solvent (IMP). Both catalysts were 100% selective in terms of -NO_2 group reduction, resulting in the sole formation of p-chloroaniline. Au-DP exhibited a smaller mean Au size (2.9 nm) compared with Au-IMP (4.5 nm) and delivered a higher (by a factor of 14) specific hydrogenation rate. Bimetallic Pd-Au/Al_2O_3 catalysts have also been prepared by DP and IMP with Au/Pd mol/mol = 8, 20 and 88. Catalyst activation by temperature programmed reduction has been monitored and the activated catalysts characterized in terms of H_2 chemisorption, TEM analysis and DRIFTS. DRIFTS measurements using CO as a probe molecule suggest the presence of bimetallic particles and surface Au-Pd interaction. A significant increase (by up to a factor of 3) in activity was observed for Pd-Au/Al_2O_3 (Au/Pd ≥ 20) compared with Au/Al_2O_3 where the exclusive conversion of p-chloronitrobenzene to p-chloroaniline was maintained. At a lower ratio (Au/Pd = 8), nitrobenzene was produced as a result of a Pd catalyzed hydrodechlorination step. Under the same reaction conditions, Au/Al_2O_3 + Pd/Al_2O_3 physical mixtures (Au/Pd = 20) delivered higher reaction rates but with the formation of nitrobenzene and aniline, i.e. products of hydrodechlorination and hydrogenation. We attribute the enhanced and exclusive production of p-chloroaniline over the supported bimetallics to a surface Pd-Au synergism. Our results establish the viability of Pd-promotion in the selective continuous gas phase catalytic hydrogenation of p-chloronitrobenzene over supported Au.