Catalysis of Organic..

172Nitrobenzene HydrogenationThe competitive hydrogenation of nitrobenzene and nitrosobenzene in 1:1molar mix was studied. The reaction profile is shown in Figure 5. In the earlystages of the reaction (< 50 min) only nitrosobenzene reacted, the concentrationof nitrobenzene remained almost constant. During this period the reactionprofile was similar to that found for nitrosobenzene hydrogenation in theabsence of nitrobenzene, however the rate of aniline production wassignificantly greater at 5.0 mmol.min -1 .g -1 . After 75 min nitrobenzenehydrogenation initiated and the rate of aniline production increased to 14.9mmol.min -1 .g -1 , similar to that for nitrobenzene hydrogenation in the absence ofnitrosobenzene.Mol % in solution100Nitrosobenzene Nitrobenzene2090 hydrogenation hydrogenation8070NBA 1560AZOXY50104030 NSB20AZO 510000 50 100 150 200Time (min)Figure 5. Reaction profile for competitive hydrogenation of NB and NSB.From Figure 5 it can be clearly seen that nitrosobenzene totally inhibitednitrobenzene hydrogenation. The rapid adsorption and formation ofazoxybenzene indicated that nitrosobenzene was more strongly adsorbed thannitrobenzene and that, given its high surface concentration, the principal surfacereaction was coupling to form azoxybenzene with loss of water as shown in thereaction sequence;Ph-NO Ph-NO(a)Ph-NO(a) + H(a) Ph-N(OH)(a)2Ph-N(OH)(a) Ph-N=N(O)-Ph(a) + H 2 ONote that the rate of hydrogen up-take and the rate of formation ofazoxybenzene were identical over the first 20 min of reaction (0.02 mol.min -1 g -1 )Mol % Azo/Azoxy in solution