Nitrobenzenes, hydrogenation

Carbon monoxide cyanogen, hydrogen cyanide nitrites arsine aniline, dimethyl aniline, toluidine nitrobenzene hydrogen sulphide (causes respiratory paralysis by impairment of oxygen utilization in the central nervous system). 

Yeong, K. K., Gavriilidis, A., Zape, R., Hessel, V., Catalyst preparation and deactivation issues for nitrobenzene hydrogenation in a microstructured falling film reactor, Catal. Today 81, 4 (2003) 641-651. 

In the hydrogenation of nitrobenzene the presence of a nitrene is rarely postulated (7, 8). In a previous study of nitrobenzene hydrogenation (7) we postulated, as part of the overall mechaiusm, the following step ... 

Figure 57.9. The effect of wt.% Al on the catalysts nitrobenzene hydrogenation activity after being leached with 20 wt.% NaOH at 75°C (A) and 110°C (B).

The results from nitrobenzene hydrogenation over Cu/silica are shown in Table 1. It can be seen that N-hexylaniline is only formed significantly once all the nitrobenzene has been consumed. No other by-products were detected. 

Hence the 30 % that are lost could be on the support and slowly react with 1-hexanol to form HA. However when aniline is reacted there is no significant loss of material, which suggests that aniline cannot interact directly with the surface hydroxyls. This suggests that the interaction between aniline and the support hydroxyls is not as simple as shown above, rather it is more likely that the reaction operates via a spillover mechanism involving an intermediate in the nitrobenzene hydrogenation sequence rather than aniline. The alkylation reaction between aniline and 1-hexanol takes place on the metal function, therefore the reaction with the missing aniline associated with the support will be slow as it requires a reverse spillover and a diffusion across the support surface. 

Finally we must address the difference in rate between the in situ alkylation of aniline at the end of nitrobenzene hydrogenation and that found for the direct reaction between aniline and 1-hexanol. One significant difference between the systems is the presence of water. Two moles of water are produced for every mole of nitrobenzene converted so the final water concentration will be in excess of the aniline concentration. In the direct reaction no water was present. Water is a known poison for a range of catalytic reactions [8-11] and we have shown in our own laboratories that water will reduce the rate of nitrobenzene hydrogenation [12], Therefore it is likely that the deactivation of the aniline reaction after nitrobenzene hydrogenation is due to inhibition by water. 

Figure 6 Hydrogen uptake vs. reaction time under standard nitrobenzene hydrogenation test conditions 5 mg Pt, temperature = 29°C.

The hydrogenation of nitrobenzene, nitrosobenzene and azobenzene has been studied singly and competitively. A kinetic isotope effect was observed with nitrobenzene but not with nitrosobenzene. Nitrosobenzene inhibits nitrobenzene hydrogenation in a competitive reaction, whereas azobenzene and nitrobenzene co-react but at lower rates. Taken together a more detailed mechanistic understanding has been obtained. 

Although nitrobenzene, nitrosobenzene and azobenzene are often observed in nitrobenzene hydrogenation, we are aware of no studies of competitive reactions. In this paper we report on the competitive hydrogenations and their mechanistic implications. 

Figure 2. Hydrogen/deuterium up-take during nitrobenzene hydrogenation...

The competitive hydrogenation of azobenzene and nitrobenzene in a 0.5 1 molar mix was examined. A ratio of 0.5 1 was used as two nitrobenzene units are needed to produce a single azobenzene. The reaction profile is shown in Figure 4. In this reaction the concentrations of both nitrobenzene and azobenzene dropped simultaneously and coincided with an increase in aniline concentration. Aniline was produced at a rate of 3.5 mmol.mm g, which is five times slower than nitrobenzene hydrogenation in the absence of azobenzene and two-and-a-half times slower than azobenzene in the absence of nitrobenzene. No other by-products were observed with this reaction. 

From Figure 5 it can be clearly seen that nitrosobenzene totally inhibited nitrobenzene hydrogenation. The rapid adsorption and formation of azoxybenzene indicated that nitrosobenzene was more strongly adsorbed than nitrobenzene and that, given its high surface concentration, the principal surface reaction was coupling to form azoxybenzene with loss of water as shown in the reaction sequence ... 

Hence the proposed reaction scheme has Ph-NOH(a) as the key surface intermediate. It can be formed from both nitrobenzene and nitrosobenzene. In this scheme nitrosobenzene is not an intermediate in nitrobenzene hydrogenation but a by-product formed under low surface hydrogen conditions. Proposed mechanisms are shown below. 

---