Clean production of chloroanilines by selective gas phase hydrogenation over supported Ni catalysts

We have established for the first time 100% selectivity in the continuous gas phase hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline for reaction over a series of oxide and carbon supported Ni catalysts (6 ± 2%, w/w) under mild reaction conditions (T = 393 K, P = 1 atm). Catalyst activation by temperature programmed reduction (TPR) is addressed, BET area and H₂ uptake measurements provided and mean metal particle sizes evaluated by transmission electron micrographic (TEM) analysis. The following activity sequence has been determined: Ni/Al₂O₃ > Ni/SiO₂ > Ni/Activated Carbon > Ni/graphite. Pd/Al₂O₃, as an alternative catalyst, delivered an appreciably higher activity but with the production of nitrobenzene (principal product) and aniline (secondary product), i.e. hydrodechlorination with subsequent –NO₂ reduction prevailed. Exclusive formation of the corresponding haloaniline is also demonstrated for the hydrogenation of o-chloronitrobenzene, m-chloronitrobenzene and p-bromonitrobenzene over Ni/Al₂O₃. A lower hydrogenation rate is established for p-CNB relative to nitrobenzene, consistent with a halogen substituent deactivation effect. While the Ni catalysts suffered a loss of activity with time-on-stream, exclusive selectivity to the haloamine product was maintained. These preliminary results can serve as a basis for the development of a cleaner, high throughput production of commercially important haloamines.