Ind. Eng. Chem. Prod. Res. Dev. 1980, 79, 389-391
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Intermediates in the Raney Nickel Catalyzed Hydrogenation of Nitrobenzene to Aniline Hal D. Burge and Dermot J. Collins" Department of Chemical and Environmental Engineering, Speed Scientific School, University of Louisville, Louisville, Kentucky 40208
Burtron H. Davls Institute for Mining and Minerals Research, University of Kentucky, Lexington, Kentucky 40583
Hydrogenation of nitrobenzene was carried out with a Raney nickel catalyst at constant pressure and temperature. The disappearance of nitrobenzene, the formation and disappearance of intermediates, and the formation of aniline were all zero-order reactions. Aniline formation exhibited three distinct regions. The experimental results can be explained in terms of adsorption strengths which increase with the increasing oxidation state of the nitrogen atom.
Introduction Nitroaromatic compounds are starting materials for the production of large quantities of aromatic amines; aniline is the amine synthesized in greatest amounts by this method. Large-scale production methods of aniline employ mainly fluidized or fixed bed vapor phase catalytic reduction of nitrobenzene. Liquid phase reductions have been reported but these are very limited. The widely accepted reaction mechanism was deduced by Haber (1898) from his work on the electrochemical reduction of nitrobenzene (Figure 1). The reduction of the nitro group to the amino group involves reduction of the nitrogen from its highest to its lowest oxidation state. The present work was an effort to define the reaction pathway(s) for the reduction of nitrobenzene using Raney nickel in the liquid phase. Brown and Henke (1922, 1923) studied the gas-phase hydrogenation of nitrobenzene over a number of catalysts. Nickel and copper catalysts produced only aniline. They also studied the problem of stopping the reduction at the azobenzene stage; while thallium, lead, and bismuth showed some selectivity for this product, the catalyst activity rapidly declined during use. The vapor phase reaction using copper catalysts showed that the rate of hydrogenation of nitrobenzene was one-half order with respect to both nitrobenzene and hydrogen (Gharda and Sliepcevich, 1960; Rihani et al., 1965). Yao and Emmett (1962) measured the kinetics of the reduction of p-nitrophenol and nitrobenzene over Raney nickel and nickel powder catalysts. Their work was carried out at 25 "C and at hydrogen pressures of 1 atm and less. The reduction of nitrophenol was first order with respect to hydrogen and zero order with respect to nitrophenol. The reduction of nitrobenzene was similar except that a sharp increase in the zero-order rate occurred at the point of high azoxybenzene concentration. On the other hand, Ryan et al. (1979) found that Rh&20),,, as a homogeneous or resin polymer anchored catalyst, was highly selective in reducing nitrobenzene and that aniline was the only product. Experimental Section The 4-L reactor, a Model 50 STN Chemineer Batch Autoclave reactor, was charged with 3.3 L of a 1.0 M nitrobenzene in methanol solution. The wet catalyst slurry 0196-4321/80/1219-0389$01.00/0
(W. R. Grace Grade 28,91% nickel) was charged to give 0.1 mol of nickel per liter of solution. The system was flushed with nitrogen, then with hydrogen, and left with 1 atm of hydrogen pressure. The liquid was agitated at 600 rpm while the temperature was increased to the reaction temperature (70 or 85 "C). Hydrogen was then added to maintain the partial pressure of hydrogen at 4 atm during the run. Samples were withdrawn at intervals during the course of the reaction. A more detailed description is given by Burge (1979). A flame ionization gas chromatograph, equipped with a 10-ft length of 1/8-in.glass tubing packed with 5% OV17 on Veriport 10, was used for sample analysis. The temperature was programmed from 93 to 177 "C at 5.6 "C/ min; it was held at 177 "C until elution was complete. The order and time of elution was: aniline (372 s), nitrobenzene (500 s), an unidentified compound (730 s), azobenzene (1175 s), and azoxybenzene (1950 9). Results Concentrations of the reactant and products as the reduction progressed are presented in Figures 2 and 3. The intermediates azoxybenzene and azobenzene were present during the reaction along with the aniline. Since the reaction mixture was sampled on a time basis and not continuously, the data were sparse in some places. To remedy this, the nitrobenzene and aniline curves were fitted and concentration values of the intermediates were calculated wherever possible by means of a material balance on nitrogen. Since 1.0 M solutions of aniline were obtained at the conclusion of both runs, the nitrogen balance appears justified. Solid data points are calculated values; the others are experimental data. At the later reaction time an unidentified compound appeared on the chromatograms. It eluted after nitrobenzene and before azobenzene. For purposes of plotting the datum point, it was assumed to contain two atoms of nitrogen (probably hydrazobenzene according to the Haber scheme). The hydrogenation runs showed three distinct regions of kinetic activity with respect to aniline formation. For clarity of discussion, these regions are identified in Figure 4 by means of a generalized chart that applies to both temperatures. The observed rate constants for decomposition and formation of all species are given in Table I. These values 0 1980 American Chemical Society
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NITROBENZENE CONCENTRATION
ANILINE FORMS AT THE LOWEST CONSTbNT
~
AZOXYBENZENE CONCENTRATION
CONSTANT RATE
I
1
I
I
I b Z O X Y B E N Z E N E FORMS AT A CONS
k-
REGION I -REGION
2 *REGION3
Figure 4. Regions of kinetic activity during nitrobenzene hydrogenation over Raney nickel. Table I. Observed Reaction Rate Constants for the 70 and 85 C (343and 358 K) Runs
"3 I
rate constants in mol/min atm g of cat.a
0.4
2b
0
4'0 6'0 ELAPSED T I M E
8'0
-
IO0 MINUTES
I20
I40
Id0
Figure 2. Concentration-time curves for nitrobenzene hydrogenation at 70 "C.
a
UNIDENTIFIED INTERMEDIATE
b
2'0
40 60 ELAPSED TIME
IO0 MINUTES
80
-
I20
I40
I60
Figure 3. Concentration-time curves for nitrobenzene hydrogenation at 85 OC.
assume a first-order dependency on hydrogen pressure, based on the findings of Yao and Emmett (1962). Discussion The concentration-time curves show that the rate of disappearance of nitrobenzene followed zero-order kinetics. Referring to the two curves, it can be seen that the initial concentration of nitrobenzene dropped from 1.0 M during heat-up. However, normalizing the curves (by defining zero time as that time at which the nitrobenzene concentration was 0.80 M) makes both graphs essentially identical; it follows that the rate constants are nearly identical for the two runs. The reaction intermediate azoxybenzene appeared immediately in both runs. The rate of formation followed zero order kinetics until the nitrobenzene concentration dropped to about 0.18 M. A t this point (end of region 1, Figure 4) the azoxybenzene concentration began to decrease rapidly, again by zero-order kinetics. The Haber
species
70°C
nitrobenzene aniline region 1 region 2 region 3 azoxy benzene region 1 region 2 region 3 azobenzene
-3.0 x 10-4
85 " C -2.9 x 10-4
4.9 x 10-5 8.9 x 10-4 8.4 x 10-4
5.2 x 10-5 8.4 x 10-4 1.9 x 10-4
1.2 x -4.3 x -5.5 x 2.1 x -4.6 x
10-4 10-4 10-4 10-4 10-4
1.3 x -4.9 x -6.1 x 2.3 x -5.6 x
10-4 10-4 10-4 10-4 10-4
Negative sign (-) denotes decomposition.
reduction scheme suggests that azoxybenzene is formed in parallel to the aniline produced by hydrogenating N phenylhydroxylamine. Hence the azoxybenzene formation rate is not only governed by the rate-limiting step in its formation but also by the competing parallel reaction pathway involving aniline. The reduction of nitrosobenzene (11, Figure 1) and the reaction of the resulting N-phenylhydroxylamine with another molecule of nitrosobenzene (111) must be relatively fast since the experimental data in both runs show that the azoxybenzene concentration exceeded the aniline concentration in region one. Nitrogen conservation dictates that the azoxybenzene concentration cannot increase if its decomposition reaction is faster than its formation reaction. This is inconsistent with the experimental data in region 2 (Figure 4) where azoxybenzene decomposition is faster than its formation in region 1; however, this can be explained in terms of relative adsorption strengths of nitrobenzene and azoxybenzene. If the nitrobenzene is more strongly adsorbed than azoxybenzene, the azoxybenzene decomposition reaction will be limited by catalyst active site availability; i.e., the nitrobenzene acts as a strongly adsorbed inhibitor. Two observations support this: (i) azoxybenzene decomposition does not occur until the nitrobenzene concentration falls below a limiting value (approximately 0.18 M), and (ii) azobenzene, the decomposition product of azoxybenzene, does not appear until the end of the azoxybenzene concentration increase period (region 1). In the hydrogenation of acetylene, ethylene is the only product until the acetylene decreases to a low concentration; after this time ethylene is hydrogenated a t a faster rate than acetylene (Bond, 1962). This has been explained
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
by a competitive adsorption where acetylene is much more strongly adsorbed than is ethylene; hence, ethylene hydrogenation only begins after the acetylene has been hydrogenated. In our study we have an analogous situation when we consider reactions I, 11, and 111 (Figure 1). Initially, the 1O:l ratio of nitr0benzene:nickel is high enough for nitrobenzene to prevent the adsorption of the intermediate azoxybenzene. Only when the nitr0benzene:nickel ratio becomes less than about 2:l is azoxybenzene able to compete with nitrobenzene for catalytic sites. The azoxybenzene concentration then decreases so that it reaches zero at about the same time that the azobenzene concentration reaches its maximum concentration. The azobenzene maximum concentration is lower than that of the maximum for azoxybenzene. At the same time, aniline concentration increases rapidly during the formation of azobenzene. The increase in aniline, over and above that provided by the remaining nitrobenzene, must come from the decrease in azoxybenzene as shown in the Haber scheme. However, the rate of formation of azobenzene must be faster than the rate of decomposition in order for the concentration to increase during the previous time period. This would be true unless the decomposition reaction were hindered by the presence of a more strongly adsorbed species, such as nitrobenzene and/or azoxybenzene. If it were completely hindered, the decomposition reaction would not proceed, the aniline formation rate would not increase, and the azobenzene concentration would reach much higher levels. Since this is not the case, the azobenzene decomposition reaction must be at least partially hindered until the nitrobenzene and azoxybenzene concentrations are sufficiently low. In this fashion, the azobenzene decomposition kinetics differ from that of azoxybenzene. In the absence of azoxybenzene, the decomposition of azobenzene is zero order. Aniline was immediately produced by the hydrogenation (IV) of N-phenylhydroxylamine by a zero-order reaction. Throughout region 1,N-phenylhydroxylamine reacts with hydrogen to form aniline or with nitrosobenzene to form azoxybenzene. During this period the selectivity (azoxybenzenefaniline) is 2.5. Beginning at the point of abrupt aniline increase, and throughout the second region, two pathways are responsible for aniline formation. The two-reaction pathway region continues until the concen-
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tration of nitrobenzene becomes essentially zero; at this point, and throughout region 3, only the decomposition of reaction intermediates is responsible for the aniline formation. The formation kinetics in these last two regions both appeared to be zero order. In one reaction pathway in the Haber mechanism, aniline product reacts with nitrosobenzene to yield azobenzene (VI) and subsequently two moles of aniline. The experimental data provide two reasons for believing that this reaction does not take place to an appreciable extent. First, aniline must be present on the catalyst surface in order to react with the nitrosobenzene since the nitrosobenzene is not desorbed. Although aniline is formed on the catalyst surface, it would have to compete with the reactive N-phenylhydroxylamine and hydrogen for nitrosobenzene and with the strongly adsorbed nitrobenzene for surface sites; this would appear to be an unfavorable competition. Secondly, the reaction product azobenzene was not detected in the bulk liquid for quite some time. If azobenzene were formed, our earlier discussion shows that it would not compete favorably for catalyst sites so long as nitrobenzene and azoxybenzene was present in large quantities. Since it was not detected in the bulk liquid, reaction VI did not occur measurably. The present results suggest that with a Raney nickel catalyst, the Haber mechanism is valid. However, the direct formation of azobenzene from the aniline product is not a significant reaction pathway. Rather, nearly all of the conversion occurs through the N-phenylhydroxylamine intermediate.
Literature Cited Bond, G. C. “Catalysis by Metals”, Academic Press Inc.: London, 1962; pp 70, 105, 127. Brown, 0. W.; Henke, C. 0. J. Phys. Chem. 1022, 26, 161, 272, 324, 631, 715. Brown, 0. W.; Henke, C. 0. J. Phys. Chern. 1023, 27, 52. Burge, H. D., M.S. Thesis, University of Louisville, 1979. Gharda, K. H.; Sliepcevich, C. M. Ind. Eng. Chern., 1980, 52, 417. Haber, F. 2.Nektrochem. 1808, 22, 506. Rihani, D. N.; Narayanan, T. K.; Doraiswamy, L. K. Ind. Eng. Chem. Process Des. Dev. 1085, 4, 403. Ryan, R. C.; Wilemon, G. M.; Dalsanto, M. P.; Pittman, C. U., Jr. J. Mo.l Cats/. 1979, 5, 319. Yao, H. C.; Emmett, P. H. J. Am. Chem. SOC. W62, 8 4 , 1086.
Receiued for review October 18, 1979 Accepted April 9, 1980
