Catalysis of Organic..

174Nitrobenzene HydrogenationHence the proposed reaction scheme has Ph-NOH(a) as the key surfaceintermediate. It can be formed from both nitrobenzene and nitrosobenzene. Inthis scheme nitrosobenzene is not an intermediate in nitrobenzene hydrogenationbut a by-product formed under low surface hydrogen conditions. Proposedmechanisms are shown below.Nitrobenzene hydrogenation:1. Ph-NO 2 Ph-NO 2 (a)2. H 2 (g) 2H(a)3. Ph-NO 2 (a) + H(a) Ph-NO(OH)(a)4. Ph-NO(OH)(a) + H(a) Ph-N(OH) 2 (a)5. Ph-N(OH) 2 (a) + H(a) Ph-N(OH)(a) + H 2 O6. Ph-N(OH)(a) + H(a) Ph-N(OH)H(a) phenyl hydroxylamine7a. 2Ph-N(OH)H(a) Ph-NO(a) + Ph-NH 2 (a) + H 2 O7b. Ph-NO(a) + Ph-NH 2 (a) Ph-N=N-Ph(a) + H 2 O7. 2Ph-N(OH)H(a) Ph-N=N-Ph(a) + 2H 2 O azobenzene8. Ph-N(OH)H(a) + H(a) Ph-NH(a) + H 2 O9. Ph-NH(a) + H(a) Ph-NH 2 (a) anilineReaction 7 is the sum of reactions 7a and 7b.Nitrosobenzene hydrogenation:1. Ph-NO Ph-NO(a)2. Ph-NO(a) + H(a) Ph-N(OH)(a)3. 2Ph-N(OH)(a) Ph-N=N(O)-Ph(a) + H 2 O azoxybenzene4. Ph-N=N(O)-Ph(a) + H(a) Ph-N=N(OH)-Ph(a)5. Ph-N=N(OH)-Ph(a) + H(a) Ph-N=N-Ph(a) + H 2 O6. Ph-N=N-Ph(a) + H(a) Ph-N=NH-Ph(a)7. Ph-N=N-Ph(a) + H(a) Ph-NH-NH-Ph(a)8. Ph-NH-NH-Ph(a) + H(a) Ph-NH(a) + Ph-NH 2 (a)9. Ph-NH(a) + H(a) Ph-NH 2 (a) anilineThe hydrogenation of nitrobenzene and nitrosobenzene are complex and arange of factors can influence by-product reactions, e.g. hydrogen availability,support acid/base properties (13, 14). In this study we have examinecompetitive hydrogenation between nitrobenzene, nitrosobenzene andazobenzene. This methodology coupled with the use of deuterium has furtherelucidated the mechanism of these reactions.Experimental SectionThe catalyst was prepared by impregnation of the powdered activatedcarbon support, Norit SX Ultra, (surface area 1200 m 2 g -1 ) with sufficientpalladium nitrate to produce a metal loading of 3 %. The resulting suspensionwas dried and calcined at 423 K for 3 hours. The dispersion of the catalyst was