Chemoselective hydrogenation of o-, p- and m-chloronitrobenzene at ambient temperature on Au/Fe₂O₃ catalysts

Gold nanoparticles deposited on an α-Fe₂O₃ substrate were prepared by different procedures including the deposition-precipitation in presence of urea or NaOH, and by the traditional wet impregnation followed by calcination. Metal loading and catalyst structures were determined by ICP-MS and XRD analyses while the size and electronic nature of Au nanoparticles were assessed using TEM and XPS. The effect of the catalyst preparation methodology was evaluated in the catalytic hydrogenation of nitrobenzene (NB) carried out in a batch reactor at 25 °C and 20 bar of H₂ pressure. Among the catalysts studied, 1.0%Au/α-Fe ₂O₃ prepared by deposition-precipitation with urea recorded the best performance (TOF = 4.6 s^-1) and a selectivity toward aniline of 92%. The XRD and TEM data pointed out that the active sites are small gold particles (3.8 nm) highly dispersed on the substrate surface. Moreover, the effect of Au loading (0.5, 1.0 and 2.0 wt.%) was investigated in the hydrogenation reaction of o-, m- and p-chloronitrobenzene (o, m- and p-CNB) to o-, m- and p-chloroaniline (o-, m- and p-CAN) (a batch reactor; T = 25 °C; P = 20 bar of H₂) over Au/α-Fe₂O₃ catalysts prepared by deposition-precipitation with urea. Regardless of the CNB isomer, maximum activity was recorded for the catalyst with 0.5 wt.% Au loading (m-CNB conversion of 99.7%). At 6 h reaction time, the yields of o-CAN, m-CAN and p-CAN were 97, 93.7 and 91.7 mol.%, respectively. Catalytic activity decreases in the order: m-CNB > o-CNB > p-CNB, but this sequence does not depend on the Au loading. Other factors such as the electronic properties of Au nanoparticles and steric factors of Cl substitute appear to play a major role in the reaction.