TY - JOUR

T1 - The role of hydrogen partial pressure in the gas phase hydrogenation of p-chloronitrobenzene over alumina supported Au and Pd

T2 - A consideration of reaction thermodynamics and kinetics

AU - Wang, Xiaodong

AU - Perret, Noemie

AU - Keane, Mark A.

PY - 2012

Y1 - 2012

N2 - 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 H2 partial pressure was varied from excess (H2/p-CNB up to 2300) to lean (stoichiometric) conditions. The catalysts have been characterised by temperature-programmed reduction (TPR), H2 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 Pdδ− but no significant electron transfer between Au and Al2O3. Under thermodynamic control, cyclohexane is the only product where H2/p-CNB ⩾ 10 with product (p-chloroaniline, aniline, chlorobenzene, benzene and cyclohexane) dependence on H2/p-CNB in the range 1–8; at a given H2/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 H2 partial pressure, where the maximum turnover frequency delivered by Au/Al2O3 (with a lower H2 adsorption coefficient) was an order magnitude lower than that recorded for Pd/Al2O3. Adsorption of p-CNB on Pdδ− 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.

AB - 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 H2 partial pressure was varied from excess (H2/p-CNB up to 2300) to lean (stoichiometric) conditions. The catalysts have been characterised by temperature-programmed reduction (TPR), H2 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 Pdδ− but no significant electron transfer between Au and Al2O3. Under thermodynamic control, cyclohexane is the only product where H2/p-CNB ⩾ 10 with product (p-chloroaniline, aniline, chlorobenzene, benzene and cyclohexane) dependence on H2/p-CNB in the range 1–8; at a given H2/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 H2 partial pressure, where the maximum turnover frequency delivered by Au/Al2O3 (with a lower H2 adsorption coefficient) was an order magnitude lower than that recorded for Pd/Al2O3. Adsorption of p-CNB on Pdδ− 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.

U2 - 10.1016/j.cej.2012.08.061

DO - 10.1016/j.cej.2012.08.061

M3 - Journal article

VL - 210

SP - 103

EP - 113

JO - Chemical Engineering Journal

JF - Chemical Engineering Journal

SN - 1385-8947

ER -