The role of hydrogen partial pressure in the gas phase hydrogenation of p-chloronitrobenzene over alumina supported Au and Pd: a consideration of reaction thermodynamics and kinetics

The gas phase hydrogenation of p-chloronitrobenzene (p-CNB) over alumina supported Au and Pd has been subjected to thermodynamic and kinetic analyses where the H-2 partial pressure was varied from excess (H-2/p-CNB up to 2300) to lean (stoichiometric) conditions. The catalysts have been characterised by temperature-programmed reduction (TPR). H-2 chemisorption/temperature-programmed desorption (TPD), BET surface area/porosity, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. Both catalysts exhibited nano-scale metal particles (mean diameter = 4.5 nm (Au) and 2.4 nm (Pd)) with the formation (from XPS analysis) of electron-rich Pd delta- but no significant electron transfer between Au and Al2O3. Under thermodynamic control, cyclohexane is the only product where H-2/p-CNB >= 10 with product (p-chloroaniline, aniline, chlorobenzene, benzene and cyclohexane) dependence on H-2/p-CNB in the range 1-8; at a given H-2/p-CNB, reaction temperature (373-473 K) has a negligible effect. Under conditions of catalytic control, Au/Al2O3 generated p-chloroaniline as the sole product while Pd/Al2O3 promoted hydrodechlorination to nitrobenzene and hydrogenation to aniline. A kinetic model is presented that accounts for the rate dependence on H-2 partial pressure, where the maximum turnover frequency delivered by Au/Al2O3 (with a lower H-2 adsorption coefficient) was an order magnitude lower than that recorded for Pd/Al2O3. Adsorption of p-CNB on Pd delta- via the aromatic ring is proposed that serves to activate both -NO2 and -Cl for attack whereas Au/Al2O3 selectively activates -NO2, leading to exclusive production of p-chloroaniline. (C) 2012 Elsevier B.V. All rights reserved.