Hydrogenation of nitrobenzenes

The reduction of the nitro group to yield aniline is the most commercially important reaction of nitrobenzene. Usually the reaction is carried out by the catalytic hydrogenation of nitrobenzene, either in the gas phase or in solution, or by using iron borings and dilute hydrochloric acid (the Bechamp process). Depending on the conditions, the reduction of nitrobenzene can lead to a variety of products. The series of reduction products is shown in Figure 1 (see Amines byreduction). Nitrosobenzene, /V-pbenylbydroxylamine, and aniline are primary reduction products. Azoxybenzene is formed by the condensation of nitrosobenzene and /V-pbenylbydroxylamine in alkaline solutions, and azoxybenzene can be reduced to form azobenzene and hydrazobenzene. The reduction products of nitrobenzene under various conditions ate given in Table 2. 

Dehydrogenation processes in particular have been studied, with conversions in most cases well beyond thermodynamic equihbrium Ethane to ethylene, propane to propylene, water-gas shirt reaction CO -I- H9O CO9 + H9, ethylbenzene to styrene, cyclohexane to benzene, and others. Some hydrogenations and oxidations also show improvement in yields in the presence of catalytic membranes, although it is not obvious why the yields should be better since no separation is involved hydrogenation of nitrobenzene to aniline, of cyclopentadiene to cyclopentene, of furfural to furfuryl alcohol, and so on oxidation of ethylene to acetaldehyde, of methanol to formaldehyde, and so on. 

A bench-scale study of the hydrogenation of nitrobenzene was investigated by Wilson [2]. In this study, nitrobenzene and hydrogen were fed at a rate of 65.9 gmol/lir to a 30 cm internal diameter (ID) reactor containing the granular catalyst. A thermocouple sheath, 0.9... 

Equations 6-94 and 6-97 are first order differential equations, and it is possible to solve for both the eonversion and temperature of hydrogenation of nitrobenzene relative to the reaetor length of 25 em. A eomputer program PLUG61 has been developed employing the Runge-Kutta fourth order method to determine the temperature and eonversion using a eatalyst bed step size of 0.5 em. Table 6-6 shows... 

Longitudinal temperature profile and conversion in a reactor for the hydrogenation of nitrobenzene... 

Phenylhydroxylamine rearranges in sulfuric acid to give mainly p-aminophenol. Industrial routes to this compound have been developed in which phenylhydroxylamine, formed by hydrogenation of nitrobenzene in sulfuric acid over platinum-on-carbon, is rearranged as it is formed. Conditions are adjusted so that the rate of rearran ment is high relative to the rate of hydrogenation of hydroxylamine to aniline (15,17,86). An easy way to obtain a favorable rate ratio is to carry out the reduction with about 1% DMSO present in the sulfuric acid (79,81). 

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 ... 

Aniline is produced by the hydrogenation of nitrobenzene. A small amount of cyclo-hexylamine is produced as a by-product. The reactions are ... 

In the manufacture of aniline by the hydrogenation of nitrobenzene, the offgases from the reactor are cooled and the products and unreacted nitrobenzene condensed. The hydrogen and inerts, containing only traces of the condensed materials, are recycled. 

Figure 4.5. (a) Process flow diagram hydrogenation of nitrobenzene to aniline (b) Information flow diagram... 

Aniline is manufactured by the hydrogenation of nitrobenzene in a fluidised bed reactor. The reactor operates at 250 °C and 20 bar. The reactor vessel is approximately 3 m diameter and 9 m high. Suggest suitable materials of construction for this reactor. 

In the manufacture of aniline by the hydrogenation of nitrobenzene, the off-gases from the reactor are cooled and the products and unreacted nitrobenzene condensed in a shell and tube exchanger. A typical composition of the condensate is, kmol/h aniline 950, cyclo-hexylamine 10, water 1920, nitrobenzene 40. The gases enter the condenser at 230 °C and leave at 50 °C. The cooling water enters the tubes at 20 °C and leaves at 50 °C. Suggest suitable materials of construction for the shell and the tubes. 

A fluidised bed is used in the production of aniline by the hydrogenation of nitrobenzene. Single-stage cyclones, followed by candle filters, are used to remove fines from the gases leaving the fluidised bed. 

In the production of aniline by the hydrogenation of nitrobenzene, the reactor products are separated from unreacted hydrogen in a condenser. The condensate, which is mainly water and aniline, together with a small amount of unreacted nitrobenzene and cyclo-hexylamine, is fed to a decanter to separate the water and aniline. The separation will not be complete, as aniline is slightly soluble in water, and water in aniline. A typical material balance for the decanter is given below ... 

The reductive alkylation reaction occurring during the hydrogenation of nitrobenzene in a 1-hexanol solvent is not between the aniline formed and the 1-hexanol but is between 1-hexanol and a surface species retained by the catalyst. This surface species is not formed during the aniline 1-hexanol reaction. There is also inhibition of the reaction between aniline and 1-hexanol. 

Aniline is an aromatic amine used in the manufacture of dyes, dye intermediates, rubber accelerators, and antioxidants. It has also been used as a solvent, in printing inks, and as an intermediate in the manufacture of pharmaceuticals, photographic developers, plastics, isocyanates, hydroquinones, herbicides, fungicides, and ion-exchange resins. It is produced commercially by catalytic vapor phase hydrogenation of nitrobenzene (Benya and Cornish 1994 HSDB 1996). Production of aniline oil was listed at approximately 1 billion pounds in 1993 (U.S. ITC 1994). Chemical and physical properties are listed in Table 1-2. 

Lu, P. et al., Polymer-protected Ni/Pd bimetallic nano-clusters preparation, characterization and catalysis for Hydrogenation of Nitrobenzene, J. Phys. Chem. B, 103, 9673, 1999. 

The aryl rings of acetophenone and methyl benzoate are preferentially hydrogenated, with only minor reduction of the substituents. In contrast, hydrogenation of nitrobenzene, under essentially the same conditions, produces aniline and nitro-cyclohexane in ca. 9 1 ratio, with an overall conversion of >79%. This observation has additional significance when compared with the hydrogenation of the nitro derivative of vinylbenzene (Table 11.25). In all cases, it is the C=C bond which is hydrogenated and, only after a prolonged reduction time, is 1-nitro-2-phenylethene completely reduced to the aminoethane [4],... 

Atomic charges, effective charges at reacting centres, and HOMO and LUMO energies have been calculated for nitrobenzene, nitrosobenzene, A-phenylhydrazine, diphenyldiazine, A,A -diphenyldiazine-A-oxide, and A,A -diphenyUiydrazine, compared with kinetic data for the hydrogenation of these compounds, and used to propose a mechanism for the hydrogenation of nitrobenzene. 

The challenge of monitoring batch processes for deviations was addressed by Gabrielsson et aP" (also taken up by Wong et al. °) with the hydrogenation of nitrobenzene to aniline as a model reaction. The study followed a factorial design of experiments. Process deviations could only be detected when the UV spectroscopic data set and the process data set were used in a combined manner, not seperately. 

Direct preparation of azo compounds in good yields is accomplished by treatment of nitro compounds with lithium aluminum hydride [576], with magnesium aluminum hydride [577], with sodium bis(2-methoxy ethoxy)aluminum hydride [575], with silicon in alcoholic alkali [331] or with zinc in strongly alkaline medium [578], Hydrazobenzene was obtained by controlled hydrogenation of nitrobenzene in alkaline medium (yield 80%) [572] and by reduction with sodium bis 2-methoxyethoxy)alumium hydride (yield 37%) [544],... 

Reduction of aromatic nitro group takes preference to the reduction of the aromatic ring. Under certain conditions, however, even the benzene ring was reduced. Hydrogenation of nitrobenzene over platinum oxide or rhodium-platinum oxide in ethanol yielded aniline while in acetic acid cyclohexylamine was produced [55]. Heating of nitrobenzene with formic acid in the presence of copper at 200° gave a 100% yield of aniline, whereas similar treatment in the presence of nickel afforded 67% of cyclohexylamine [71]. 

Aniline is an important derivative of benzene that can be made in two steps by nitration to nitrobenzene and either catalytic hydrogenation or acidic metal reduction to aniline. Both steps occur in excellent yield. Almost all nitrobenzene manufactured (97%) is directly converted into aniline. The nitration of benzene with mixed acids is an example of an electrophilic aromatic substitution involving the nitronium ion as the attacking species. The hydrogenation of nitrobenzene has replaced the iron-... 

Pyrrolidone is a lactone used for the production of nylon-4. This reactant may be produced by the reduction ammoniation of maleic anhydride. s-Caprolactam, used in the production of nylon-6, may be produced by the Beckman rearrangement of cyclohexanone oxime (structure 17.11). The oxime may be produced by the catalytic hydrogenation of nitrobenzene, the photolytic nitrosylation of cyclohexane (structure 17.9), or the reaction of cyclohexanone and hydroxylamine (structure 17.10). Nearly one-half of the production of caprolactam is derived from phenol. 

Calculate the conversion in a fluidized bed reactor for the catalytic hydrogenation of nitrobenzene to analine. 

Acetaminophen (Paracetamol) N- (4 -Flydroxypheny 1) acetamide Partial hydrogenation of nitrobenzene, A-acetylation... 

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. 

The hydrogenation of nitrobenzene progressed to aniline without any significant by-product formation, only trace amounts of azobenzene were formed (< 1 %) as the reaction went to completion. NMR analysis showed no detectable phenyl hydroxylamine in solution. The hydrogen uptake displayed a smooth curve and the rate of hydrogen consumption coincided with the rate of aniline production. The rate of hydrogenation of nitrobenzene to aniline was 15.5 mmol.mm. g. ... 

Azobenzene was also present during the hydrogenation of nitrosobenzene and a stepwise hydrogenation process of nitrosobenzene to azoxybenzene to azobenzene to aniline is a credible route. No azoxybenzene was observed during the hydrogenation of nitrobenzene. 

---