Ind. Eng. Chem. Rd. Res. D8V. 1982,21, 279-281
279
Hydrogenation of Nitrobenzene over a Nickel Boride Catalyst Dermot J. Colllns' and Andrew D. Smlth Depertment of Chemical and Envkonmentai Engineering, Speed Scientific School, University of Louisviiie, Louisville, Kentucky 40292
Burtron H. Davls Institute Of Mining and Minerals Research, University of Kentucky, Lexington, Kentucky 40583
Nickel boride, unlike Raney nickel, does not produce appreciable concentrations of reaction intermediates during the liquid phase hydrogenation of nitrobenzene. The formation of aniline and the disappearance of nitrobenzene follow zeroader kinetics. The reaction appears to be second, or higher, order in hydrogen pressure. While nickel boride, for similar amounts of catalyst, is somewhat less active than Raney nickel, it does have the advantage
of producing aniline free of intermediates throughout the course of the reaction.
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 of aniline usually employs fluidized or fixed bed vapor phase catalytic reduction of nitrobenzene. Liquid phase reductions have been reported, but these are limited. Brown et al. (1953) introduced the use of boranes and group 8 metal boranes in synthetic chemistry. Paul et al. (1952) reported that the reduction of Ni(I1) salts with sodium borohydride produced an active hydrogenation catalyst. Minor changes in the conditions of the metal salt reduction may result in major changes in the physical and chemical properties, catalytic activity, and selectivity of the metal boride. A granular black material (P-1nickel boride (Brown and Brown, 1963)) is formed when nickel(I1) acetate and sodium borohydride are reacted in an aqueous media. An active catalyst (P-2nickel boride (Brown and Brown, 1963)) is also produced when these salts react in ethanol. Wade et al. (1976) reported that nickel salt and sodium borohydride reduced in other alcohols resemble P-2 nickel borides. A widely accepted reaction mechanism (Figure 1)was deduced by Haber (1898) from the results of his work on the electrochemical reduction of nitrobenzene. Burge et al. (1980) found that the liquid phase hydrogenation of nitrobenzene with Raney nickel followed the Haber mechanism; azobenzene and azoxybenzene were identified as intermediate products. Nitrobenzeneconversion, aniline formation, and the formation and decomposition of intermediates were a series of zero-order reactions. Yao and Emmett (1962) measured the kinetics of the reduction of p-nitrophenol and nitrobenzene over Raney nickel and nickel powder catalysts at 25 "C and hydrogen pressures of 1atm or less. Nitrophenol reduction was first order with respect to hydrogen and zero order with respect to nitrophenol. The reduction of nitrobenzene was similar except for a sharp increase in the zero-order rate at the point of high azoxybenzene concentration. Burge et al. (1980) noted a similar increase in aniline formation. Brown and Henke (1922, 1923) studied the gas phase hydrogenation of nitrobenzene with a number of catalysts. Of these, nickel and copper catalysts produced only aniline.
The rate of vapor phase hydrogenation of nitrobenzene with copper catalysts was one-half order in both nitrobenzene and hydrogen (Gharda and Sliepcevich, 1960; Rihani et al., 1965). They also studied the reduction of nitrobenzene to an intermediate of the Haber mechanism azobenzene; while thallium, lead, and bismuth showed some selectivity for this product, catalyst activity rapidly declined during use. The present study was undertaken to compare the reaction pathway for nickel boride catalyzed reduction of nitrobenzene to that obtained with Raney nickel catalysts. 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. Wet catalyst slurry was added to give 0.1 mol of nickel/L of solution. This slurry was prepared following the procedure of Brown and Brown (1963), using methanol as the solvent. The average nickel content was 39 f 2%. The catalyst was stored under nitrogen if it was not used immediately. The reactor system was flushed with nitrogen, then pressurized to 1 atm with hydrogen. The liquid was agitated at 600 rpm while heating to the reaction temperature (65,80, and 100 "C). Hydrogen was then added during the run to maintain a constant partial pressure. Samples were withdrawn at intervals during the course of the reaction. A more detailed description is given by Smith (1979). A flame ionization gas chromatograph, equipped with a 10 f t X l / * in. stainless steel column packed with 5% OV17 on Veriport 30 was used for sample analysis. The temperature was programmed from 95 to 290 "C at 6 "C/min and held at 290 "C until elution was complete. The order of elution was aniline, nitrobenzene, azobenzene, and azoxybenzene. Results and Discussion Catalyst Loading. A series of runs were carried out at 100 "C and hydrogen partial pressure of 4 atm. The catalyst/nitrobenzene molar ratios were varied from 0.014 to 0.265 to determine the effect of catalyst concentration on the hydrogenation rate. Reaction mixture samples were analyzed for nitrobenzene content, and rate constants were determined for a zero-order reaction. These rate constants increased linearly with catalyst loadings (Figure 2). In this study a maximum rate, due to catalyst saturation, was
Q196-4321/82/ 122l-Q279$Ql.25/Q 0 1982 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2 , 1982
" /H*
I
0-
I
IV CcH5 -NH2
Figure 1. Nitrobenzene reduction pathways proposed by Haber (1898).
Y 0
20
40 60 80 TIME (mln.)
100
120
Figure 3. Typical concentration-time curves for the formation of aniline (0)and disappearance of nitrobenzene (0).
CATALYST CONCENTRATION ('b NICKEL IN NITROBENZENE)
Figure 2. Dependence of the rate of nitrobenzene conversion on the catalyst concentration (100 O C , 1 atm hydrogen partial pressure). 041
not reached. Yao and Emmett (1962) obtained a maximum rate due to catalyst saturation with h e y nickel but only at nickel concentration approximately tenfold greater than the maximum used in this study. Reaction Behavior. Three experimental runs were performed at 65,80, and 100 "C using a constant hydrogen partial pressure of 4 atm and a catalyst concentration of 0.061 mol of nickel/mol of nitrobenzene. The concentration-time curves (Figure 3 is typical) show constant rates of nitrobenzene disappearance and aniline formation. A significant buildup of reaction intermediates was not observed. Reaction intermediates (predominantly azobenzene and azoxybenzene) were never found to be greater than 2% of nitrobenzene. In contrast, Burge et al. (1980) found that over Raney nickel, reaction intermediate concentrations greater than 25% of the total nitro compounds were present. Furthermore, a dramatic increase in aniline production near the end of the reaction did not occur using nickel boride; both Yao and Emmett (1960) and Burge et al. (1980) observed this increase using a Raney nickel catalyst. The concentration-time curves for nitrobenzene disappearance followed zero-order kinetics using nickel boride catalyst. Burge (1979) and Yao and Emmett (1962) also obtained a zero order for this reaction with a Raney catalyst. The formation of aniline also follows zero-order kinetics. This differs from the results of Burge (1979) and Yao and Emmett (1962). Burge (1979) found aniline formation, because of the presence of the reaction intermediates, to be a series of zero-order steps which caused rate changes during the reduction. In contrast, we find that the rate of formation of aniline and the rate of disappearance of
031
0 2:
-1 . 0
2
3
4
5 678910
HYDROGEN PRESSURE (Am)
Figure 4. Dependence of the relative rate of nitrobenzene hydrogenation on the hydrogen partial pressure (0,Qand A are for data from three separate runs normalized to an equal rate a t 5 atm hydrogen pressure; solid line is for hydrogen order of 2; 80 O C , 0.061 mol of nickel/mol of nitrogenzene).
nitrobenzene are the same and constant throughout the reaction. Yao and Emmett (1962) found that nitrobenzene hydrogenation using Raney nickel catalysts was first order in hydrogen. We made three runs, using different batches of nickel boride to determine the rate at several hydrogen partial pressures. The data from the three runs were normalized to give equal rates a t 5 atm hydrogen pressure (Figure 4). While there is considerable scatter in the data, it seems clear that the order for hydrogen pressure is greater than 1. The line in Figure 4 is for a hydrogen order of 2; a reasonable fit is also possible for an order greater than 2 (e.g., 3 or 4). According to the Haber scheme (Figure l ) , the most direct pathway to aniline is via nitrosobenzene and phenylhydroxylamine (reactions I, 11, and IV). The ratio of reaction III/IV gives a measure of the contribution of the less direct routes. For the nickel boride catalyst the ratio III/IV is essentially zero, but for Raney nickel this ratio about 2.511. Thus, even though h e y nickel may be more active, the production of high concentrations of interme-
Ind. Eng. Chem. Rod, Res. Dev. 1982, 21, 281-284
boride. This is higher than previously reported values for Raney nickel and Adam's platinum. However, because of the complex nature of this reaction, these values should be viewed with caution. In summary, the nickel boride is a less active catalyst for the hydrogenation of nitrobenzenethan is Raney nickel. However, it is more selective for aniline formation throughout the course of the reaction. Unlike Raney nickel, reaction intermediates are always present in less than 2% of the initial concentration of the reactant. Literature Cited
Table I. Aniline Rate Constants Raney nickelb nickel boridea T,"C lo6 x k C 65 80
100
lo6 X k c T, "C min max
1.8d (1.8)e 70 3.8d (3.8)e 10.0d ( 1 0 ) e
85
0.9 1.0
16 15
28 1
mean 4 . 7 d (5.5)e 4.6d (5.3)e
a This work. Burge et al. (1980). Units: g-mol/g Rate for aniline formation. e Rate of Ni, s, atm of H,. for nitrobenzene disappearance.
Brown, C. A.; Brown, H. C. J. Am. Chem. Soc. 1963, 85, 1003. Brown, H. C.; Schleslnger, H. I.; Flnholt, A. E.; Glibrent. J. R.; Hyde,E. K. J . Am. Chem. Soc. 1953, 75, 215. Brown, 0. W.; Henke, C. 0. J. Wys. Chem. 1922, 26, 161, 272, 324, 631, 715. Brown. 0. W.; Henke, C. 0. J. W y s . Chem. 1923, 27, 52. Burge, H. D.; Collins, D. J.; Davis, B. H. I d . Eng. Chem. Prod. Res. Dev. i980, 79, 389. Oharda, K. H.; Siiepcevlch, C. M. Ind. Eng. Chem. 1960, 52, 417. Haber, F. 2.Elektrochem. 1898, 22, 506. Paul, R.: Bulsson, P.; Joseph, N. Ind. Eng. Chem. 1952, 44, 1008. Rihani, D. N.; Narayanan, T. K.; Doralswamy, L. K. Ind. Eng. Chem. Process Des. Dev. 1965, 4 , 403. Ryan, R. C.; Wilemon, G. M.; Daisanto, M. P.; PHtmn, C. U., Jr. J. Mol. Catal. 1975, 5 , 319. Smlth, A. D. M.Eng. Thesis, University of LoulsvUle, 1979. Smlth, H. A.; Bedolt. W. C. I n "Catalysis", Emmett, P. H., Ed., Vol. 111, Reinhold: New York, 1955. Wade, R. C.; Holah, D. 0.; Hughes, A. N.; Hul, 8. C. Catel. Rev. Sci. Eng. 1976, 74. 211. Yao, H. C.; Emmett, P. H. J. Am. Chem. Soc. 1962, 84, 1086.
diates could present processing difficulties under some conditions. Rate Constants. The rate constants, evaluated from the concentration-time data, are given in Table I. Also presented are values for aniline formation using Raney nickel from Burge et al. (1980). The average value is computed by assuming a zero-order process throughout the reaction, reflecting the mean production rate of aniline. The data in Table I show that the overall rate constant for Raney nickel is greater than for nickel boride. In addition, the maximum rate for Raney nickel exceeds the nickel boride rate, even though the nickel boride rate constant is larger than the smallest value for Raney. For nitrobenzene disappearance the rate constants for Raney nickel are similar to the ones for nickel boride. Burge et al. (1980) obtained a very low temperature dependence in the limited range covered; an apparent activation energy of 12 kcal/mol was obtained for nickel
Received for review September 29,1980 Revised manuscript received November 20,1981 Accepted December 31, 1981
GENERAL ARTICLES Effect of Polymer-Bound Amine Accelerators on the Radical- I nitiated Curing of Unsaturated Polyesters with Styrene Charles U. Plttman, Jr.;
and Slvananda S. Jada
Department of Chemkby, The University of Alabama, University, Alabama 35486
Polymer-boundtertiary amine accelerators were compared to their freely added monomeric analogues as catalysts for the curing of poly(diethyiene glycol maleate) prepolymers with styrene. Benzoyl peroxide was used as the initiator. The polymer-anchored accelerators gave shorter gel and curing times and lower energies of initiation than their monomeric analogues. Use of equivalent amounts of the bound accelerators led to resins wlth greater tensile strengths and h w r softening points and Brinell hardness values. Thus, the bound accelerators were more efficient, suggesting that this concept is worthy of further investigation.
Introduction Reagents (Mathuretal, 1980), drugs (Donaruma, 1974), and biocides (Pittman, 1980), when bound to polymers, frequently continue to exhibit their desired properties. However, their physical properties and those of the polymer may change. While many types of species have been chemically bound to polymers (Okawara et al., 1976), no reports exist in the literature where curing accelerators 0196-432118211221-0281$01.25/0
have been attached to polymers and employed. In this paper, tertiary amine accelerators have been chemically bond into poly(diethy1ene glycol maleate) prepolymen. It is well known that tertiary amines react with benzoyl peroxide to accelerate radical-initiated processes (Bemdtson and Turnen, 1954; Maltha and Damen, 1956). An old classic reaction, the curing of polyester prepolymen with styrene, initiated with benzoyl peroxide, was studied 0
1982 American Chemical Society
