Selective Gas Phase Hydrogenation ofp-Chloronitrobenzene over Pd Catalysts: Role of the Support

The gas phase (1 atm, 453 K) hydrogenation of p- chloronitrobenzene ( p -CNB) over a series of laboratory-synthesized and commercial Pd (1–10% wt) supported on activated carbon (AC) and non-reducible (SiO 2 and Al 2 O 3 ) and reducible (ZnO) oxides has been examined. Reaction over these catalysts generated the target p -chloroaniline ( p -CAN) (via selective hydrogenation) and nitrobenzene (NB)/aniline (AN) as a result of a combined hydrodechlorination/hydrogenation. A range of Pd nanoparticles with mean sizes 2.4–12.6 nm (from HRTEM and H 2 /CO chemisorption) were generated. Both the p -CNB transformation rate and H 2 chemisorption increased with decreasing Pd size. Residual Mo (from the stabilizer used in the synthesis of Pd colloids) suppressed activity, but this was circumvented by the use of poly( N -vinyl-2-pyrrolidone) (PVP). Pd/AC generated p -CAN and AN as principal products, Pd on SiO 2 and Al 2 O 3 exhibited hydrodechlorination character generating AN and NB, and Pd/ZnO promoted the sole formation of p -CAN at all levels of conversion. Reaction selectivity is linked to Pd electron density with the formation of Pd δ+ on AC and the occurrence of Pd δ− on SiO 2 and Al 2 O 3 . Reaction exclusivity to p -CAN over Pd/ZnO is attributed to the formation of PdZn alloy (demonstrated by XPS), which selectively activates the −NO 2 group. This is the first report that demonstrates 100% selectivity for p -CNB → p -CAN over supported Pd.