Selective Liquid-Phase Hydrogenation of a Nitro Group in Substituted Nitrobenzenes over Au/Al₂ O₃ Catalyst in a Packed-Bed Flow Reactor

A series of substituted nitrobenzenes with the general formula XC₆ H₄ NO₂ (X=Cl, CH=CH₂ , or C(O)CH₃ ) dissolved in toluene were reduced with hydrogen over the 1.9 % Au/Al₂ O₃ catalyst at 60-110 °C and 10-20 bar in a three-phase packed-bed reactor operating in up-flow mode. Under these conditions, hydrogenation of isomeric ClC₆ H₄ NO₂ gives exclusively chloroanilines. Hydrogenation of 3-CH₂ CHC₆ H₄ NO₂ and 4-CH₃ C(O)C₆ H₄ NO₂ leads to the formation of 3-CH₂ CHC₆ H₄ NH₂ and 4-CH₃ C(O)C₆ H₄ NH₂ with selectivities of up to 93 and 97 % at substrate conversions of 98 and 100 %, respectively. Smooth catalyst deactivation was observed regardless of which substituted nitrobenzene was taken for hydrogenation. According to the results obtained by temperature-programmed oxidation of the spent catalyst, a carbonaceous deposit formed that might block the catalyst surface. Almost complete regeneration of the supported gold catalyst with retention of its high selectivity to hydrogenation of a nitro group was achieved in a flow of air at temperatures up to 400 °C to eliminate carbonaceous deposits.