Ultra-selective gas phase catalytic hydrogenation of aromatic nitro compounds over Au/Al2O3

We report, for the first time, the exclusive (and time invariant) gas phase hydrogenation of p-chloronitrobenzene to p-chloroaniline over Au/Al2O3 where 393 K <= T <= 523 K (Delta E-a = 49 kJ mol(-1)). Under the same conditions, Pd/Al2O3 promoted the formation of nitrobenzene and aniline, i.e. composite hydrodechlorination and hydrogenation. Reaction over Au-Pd/Al2O3 delivered equivalent activity/selectivity to that obtained with Pd/Al2O3. Temperature programmed reduction analysis and hydrogen uptake measurements suggest there is negligible Au-Pd interaction in the bimetallic. Exclusive formation of the corresponding haloaniline is also demonstrated over Au/Al2O3 for a series of mono- and di-substituted halonitroarenes; the activity sequence is consistent with electron withdrawing substituent activation. (C) 2007 Elsevier B.V. All rights reserved.