NITRO - Conversion of Nitrobenzene to Aniline

Nitrobenzene and hydrogen are fed at the rate of ntp to a tubular reactor operating at atmospheric pressure and 450 K and containing catalyst with a voidage e. 

5 Simulation Tools and Examples of Chemical Engineering Processes 

The heat transfer coefficient, U, to the reactor wall and the coolant temperature is Tj. The nitrobenzene feed rate is Nafo with hydrogen in very large excess. The heat of reaction is AH. The heat capacity of hydrogen is Cp. 

Consider the steady state balances around the segment AV, as shown in Fig. 2 

The changes in the molar flow rate of A with distance Z at steady-state conditions are thus 

The above equations are solved with the initial conditions at the reactor entrance 

NONISOTHERMAL HYDRATION OP NITROBENZENE IN A PACKED BED TUBULAR REACTOR 

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The analogy between the behaviour of these diketonate complexes and benzene derivatives is indeed remarkable, and reaction sequences very reminiscent of those observed in organic aromatic chemistry are common. In Fig. 5-9, the reduction of a nitro to an amino derivative is illustrated, a conversion reminiscent of the reduction of nitrobenzene to aniline. It is usually necessary to use kinetically inert cP or d6 metal complexes in sequences of this type. 

It was also reported that the Pd nanoparticles formed in the CO2 microemulsion could catalyze other hydrogenation reactions such as the conversion of the nitro group (NO2) to amine (NH2). One reported example is the hydrogenation of nitrobenzene to aniline catalyzed by the Pd nanoparticles in a CO2 microemulsion. The conversion was > 99 % within 30 minutes in supercritical CO2 at 50 °C and 200 atm. 

By-products of the reaction are the anilines corresponding to the nitro compounds. With a substrate/catalyst ratio of 50, only in the case of Ar = 4-MeOC Ht is a complete conversion not achieved (entry 1). Moreover, in this case the reaction shows a poor selectivity. In the other cases, a complete conversion is also accompanied by a much better selectivity. The selectivity increases with an increase in the electron-withdrawing power of the substituent on the nitroarene (Table 6, entries 1-5). On the other hand, the amount of aniline formed during the reaction is quite insensitive to the nature of the substituents. This observation may be explained by the high tendency of some intermediate complex formed during the reaction to react with adventitious moisture more rapidly than any further step of the catalytic reaction. Adventitious moisture should be present in nearly constant amounts in the reaction mixtures and we have already shown that the Ru3(CO)i2/DIAN-Me system is one of the best catalysts for the reduction of nitrobenzene to aniline by CO/H2O. 

Nitrous oxide is reduced by CO in the presence of [Rh(C0)2C1]2 and base to N2 and C02 21 The mechanism of the reduction of aromatic nitro compounds by CO to give isocyanates has been studied by IR the catalyst is trans-PdCl2(pyridine)2.212 PeiCOg, Ru3(C0)x2 Rh6(C0)xg convert ortho-nitrostyrenes and CO into indoles in up to 75 selectivity (eq.21).213 Nil2(PPh3)2 under CO pressure catalyses the conversion of nitrobenzene and aniline to diphenylurea and CO2 the amine is used as the solvent.214 Ru3(C0)i2 catalyses the carbonylation of amines to formamides and the hydroamidation of olefins to carboxylic amides.215... 

The reduction of aromatic nitro-compounds is of exceptionally great interest, not only scientifically, hut also technically. The conversion of the hydrocarbons of coal tar into useful products began with the discovery of the nitration process the conversion, on the technical scale, of the nitro-group of nitrobenzene into the amino-group gave aniline, the starting material for the preparation of innumerable dyes and pharmaceutical products to aniline were added the homologous toluidines, xylidines, naphthylamines, and so on. 

The reduction of the nitro group by H2S in the presence of crystalline aluminosilicates yields amines, sulfur, and HgO (173). Thus nitrobenzene provides high selectivity for aniline upon reaction with a 3-fold molar excess of H2S at 200-300° using NaX or NaY catalyst. About 16% conversion of C6H5NO2 to C6H5NH2 was observed at 143 minutes on stream in a continuous flow run (LHSV = 0.5) at 300° over NaX only small amounts of low molecular weight products (excluding sulfur and H2O) were formed. The stoichiometry of the reactants and the temperature of this reduction must be carefully controlled. At low ratios of... 

The metabolism of nitrobenzenes to quinone-imines arises from a six-electron reduction of the nitro group to the corresponding aniline metabolite via the intermediate nitroso and hydroxylamine analogs. Aromatic ring hydroxylation by CYP ortho or para to the aniline nitrogen then generates the aminophenol derivative. The conversion of a nitrobenzene derivative to a quinone-imine is illustrated with the catechol-O-methyltransferase inhibitor tolcapone, an... 

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],... 

The first experiments which were carried out in the author s laboratory on organometallic phase-transfer catalysis were concerned with the reduction of nitrobenzenes (4) to anilines (5) by triiron dodecacarbonyl. Such a conversion was reported to occur in benzene containing methanol at reflux for 10-17 h, with the hydridoundecacarbonyltriferrate anion as the likely key intermediate (16). It was our expectation that the trinuclear iron hydride should be generated by phase-transfer catalysis and if so, effect reduction of nitro compounds (4) under exceedingly mild conditions. Indeed this was the case, as illustrated by the results shown in Table I (17). Not only is the reaction complete in 2 h or less using sodium hydroxide as the aqueous phase, benzene as the organic phase, and benzyltrieth-ylammonium chloride as the phase-transfer catalyst, but it occurs at room temperature and requires less metal carbonyl than when the reaction was... 

In order to circumvent the problem of the use of selenium, analogous systems based on the use of sulphur compounds have been developed [85-88]. Aromatic nitro compounds can be reduced by CO in water/methanol media at 120-150 °C and 1-1.5 bar pressure [85, 86]. From nitrobenzene, aniline was obtained with selectivity over 97 % at 100 % PhN02 conversion. The reaction proceeds in the presence of a multicomponent catalyst consisting of a base (preferably a strong base such as sodium hydroxide or methoxide) and sulphur compounds. The ratio of catalytic effectiveness of sulphur compounds is as follows S CS2 H2S COS = 1 1.3 10 10. Vanadium(V) compounds can be added to improve selectivity in aniline formation. Aromatic dinitro derivatives undergo this reaction and selectivity to one of the two main products (phenylenediamine and nitroaniline) can be switched by the choice of reaction conditions. The main byproduct of the reaction of nitrobenzene is PhNHCOOMe [85, 86]. It has been shown that, under the catalytic conditions, methyl phenylcarbamate can be hydrolysed to afford aniline. More forcing conditions (up to 300 bar CO) have also been employed in order to increase the activity [87]. The same catalytic system has been used to reduce nitrophenols to the corresponding aminophenols [88]. 

Hydrogenations of nitrobenzene and p-nitrotoluene over supported noble metal catalysts are often investigated as model reactions, as they consist of various elemental reactions vfith different intermediates, which can react with each other, as depicted in the scheme below for p-nitro toluene [150]. At short reaction times, the intermediates are predominantly formed, while complete conversion to p-methyl aniline is achieved at long reaction times. Aniline itself can react further to form side products such as cyclohexanol, cyclohexyl amine, and other species. 

A mixture of nitrobenzene, cupric carbonate, and NaO/f in50%-ethylenediamine heated 4 hrs. at 90° under 15 p.s.i.g. CO in a shaking apparatus aniline. Conversion ca. 100%. - By this method, nitro groups can be reduced selectively, especially in compounds containing other reducible groups such as double bonds or carbonyl groups. If CO is replaced by the reaction is too slow to be of preparative value. H. R. Appell, J. Org. Chem. 32, 2021 (1967). 

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