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

• Published in: Journal of catalysis Vol. 262; no. 2; pp. 235 - 243
• Main Authors: Cárdenas-Lizana, Fernando, Gómez-Quero, Santiago, Hugon, Antoine, Delannoy, Laurent, Louis, Catherine, Keane, Mark A.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 10-03-2009; Elsevier; Elsevier BV
• Subjects: Au/Al 2O 3; Catalysis; Catalysts; Chemistry; DRIFTS analysis; Exact sciences and technology; Gases; General and physical chemistry; Gold; H 2 chemisorption; p-Chloroaniline; p-Chloronitrobenzene; Pd–Au/Al 2O 3; Selective hydrogenation; Temperature; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; TPR; p-Chloroaniline; p-Chloronitrobenzene; Selective hydrogenation; Au/Al 2O 3; H 2 chemisorption; TPR; DRIFTS analysis; Pd–Au/Al 2O 3; Binary compound; Support; Activation; Hydrogenation; Synergism; Supported catalyst; Au/Al2O3; Particle; Chemical reduction; Precipitation; H2 chemisorption; Aniline; Ureas; Platinoid; Deposition; Aluminium oxide; Nitrogen oxide; Chemisorption; Catalytic reaction; Pd-Au/Al2O3; Transition metal; Palladium; Continuous phase; Reaction rate; Conversion; Infrared spectrometry; Heterogeneous catalysis; Impregnation; Urea; Transmission electron microscopy; Gas phase; Alumina
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2008.12.019

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.
Nano-scale Au particles supported on Al
2O
3 promote 100% selective gas phase hydrogenation of
p-chloronitrobenzene to commercially important
p-chloroaniline; we have established that the incorporation of Pd (Au/Pd
⩾
20, see TEM image) serves to elevate rate while retaining reaction exclusivity, an effect that we associate with bimetallic particle formation.