Ind. Eng. Chem. Res. 1997, 36, 3619-3624
3619
Catalytic Hydrogenation of 4-(Hydroxyamino)-2-nitrotoluene and 2,4-Nitroaminotoluene Isomers: Kinetics and Reactivity Giovanni Neri,† Maria G. Musolino,‡ Lucio Bonaccorsi,‡ Andrea Donato,† Lucina Mercadante,‡ and Signorino Galvagno*,‡ Facolta` di Ingegneria, Universita` di Reggio Calabria, Via E. Cuzzocrea 48, I-89100 Reggio Calabria, Italy, and Dipartimento di Chimica Industriale, Universita` di Messina, Cas. Post. 29, I-98166 Sant’Agata di Messina, Italy
The kinetics of the liquid-phase hydrogenation of 4-(hydroxyamino)-2-nitrotoluene, 4-amino-2nitrotoluene, and 2-amino-4-nitrotoluene were studied in ethanol over a 5% Pd/C catalyst, in the temperature range between 283 and 323 K and at a pressure of 0.1 MPa. The reaction rates have been described by a Langmuir-Hinshelwood type model with adsorption of the organic species and hydrogen on a different type of active site. The rate constant, the adsorption constants, and the activation energy for each reaction have been determined by a nonlinear regression analysis. The effect of molecular structure on the reactivity of mono- and dinitro derivatives has been also investigated. Introduction Aromatic amines are important industrial chemicals used in the synthesis of a wide variety of products (Rylander, 1967; Stratz, 1984). For example, 2,4diaminotoluene (2,4-DAT) is the raw material for the manufacture of tolyl diisocyanate. The industrial process involved in the preparation of the toluenediamine is the catalytic hydrogenation of 2,4-dinitrotoluene (2,4DNT) which is carried out in the presence of a palladium catalyst. Notwithstanding the great industrial interest for this reaction, few papers, focusing their attention on the reaction mechanism, have however appeared in the literature (Kut et al., 1987; Janssen et al, 1990a,b; Neri et al., 1995a,b). In previous papers we have investigated the reaction mechanism of the catalytic hydrogenation of 2,4-DNT over a commercial 5% Pd/C (Neri et al. 1995a,b). We have shown that under the experimental conditions used (323 K, pressure of 0.1 MPa, and ethanol as solvent) 2,4-DNT is hydrogenated to 2,4-DAT through a complex reaction network, involving parallelconsecutive reaction steps. The 4-(hydroxyamino)-2nitrotoluene (4HA2NT) and the nitroaminotoluene isomers, 4-amino-2-nitrotoluene (4A2NT) and 2-amino-4nitrotoluene (2A4NT), are the main reaction intermediates detected. The overall reaction mechanism is however not yet fully understood, and different reaction pathways have been proposed (Janssen et al., 1990a,b; Neri et al., 1995a,b). In the present paper we report the kinetics of the hydrogenation of 4HA2NT and 2,4-nitroaminotoluene isomers over a 5% Pd/C in order to clarify the mechanism of the catalytic hydrogenation of 2,4-DNT. No references on these reactions have been found in the literature. The experimental data have been treated with mathematical models by using kinetic expressions derived on the basis of a simple Langmuir-Hinshelwood model with adsorption of organic species and hydrogen on different types of active sites. The kinetic parameters have been calculated by a nonlinear regression analysis. † ‡
Universita` di Reggio Calabria. Universita` di Messina. S0888-5885(95)00505-7 CCC: $14.00
The results obtained in this work are compared with those previously reported for the 2,4-DNT hydrogenation and the effect of molecular structure on the reactivity of mono- and dinitro derivatives is also discussed. Experimental Section Materials. The materials used were 4-amino-2nitrotoluene (Aldrich, purity 97%), 2-amino-4-nitrotoluene (Aldrich, purity 99%), ethanol (Fluka 95%, analytical grade), and ultrahigh purity hydrogen (Multigas >99.9%). Reagents and solvent were employed without further purification. 4HA2NT was prepared by hydrogenation of 2,4-DNT over 5% Pd/C catalyst (Montecatini Tecnologie). The reaction was stopped at total conversion of 2,4-DNT. In order to avoid the oxidation of arylhydroxylamine, the catalyst was separated immediately by filtration and the solvent was removed by using a rotating film evaporator. The obtained residue was dissolved in CHCl3 and purified by using a silica gel column and CHCl3 as eluent. The 4HA2NT was identified by IR, NMR, and mass spectroscopy. Kinetic Experiments. The hydrogenation of nitro compounds was carried out in a 100 mL five-necked flask, equipped with a reflux condenser and a thermocouple. Constant temperature ((0.5 °C) was maintained by circulation of silicone oil in an external jacket connected with a thermostat. The catalyst used was a 5% Pd/C supplied by Montecatini Tecnologie in the form of a powder with an average grain size, dp, of about 2025 µm. The catalyst (20-80 mg) was then added to the required amount of solvent (25 mL of 95% ethanol) and reduced at 323 K for 1 h under H2 flow. After the mixture cooled to reaction temperature, 25 mL of a solution (0.01-0.1 M) of reagent in ethanol was added through one arm of the flask. The reagent solutions contain hexadecane as internal standard. The reaction mixture was stirred with a stirrer head with permanent magnetic coupling at a stirring rate of 500 rpm, and the reaction was carried out at a pressure of 0.1 MPa under H2 flow. The progress of the reaction was followed by analyzing a sufficient number of samples withdrawn from the reaction mixture. Product analysis was performed with a gas chromatograph (Carlo Erba model 4200), equipped © 1997 American Chemical Society
3620 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
with a packed column (6 ft × 1/8 in., Tenax GC, 60-80 mesh) and a flame ionization detector. Quantitative analysis was carried out by calculating the area of the chromatographic peaks with an electronic integrator (HP 3396 Series II). Chemical analysis was also performed with a liquid chromatograph (Waters Model 510), equipped with a UV detector (λ ) 254 nm). The separation was carried out on a reversed-phase packed column (µ-Bondapak C18, 3 × 300 mm) using a mixture of solvents (acetonitrilewater). In order to exclude the presence of mass transfer effects, preliminary experiments have been performed with different amounts of catalyst, with different catalyst grain sizes, and at different stirring rates. Results and Discussion Kinetic Model. The kinetic model used to describe the catalytic hydrogenation of 4HA2NT and 2,4-nitroaminotoluene isomers follows that previously reported for the hydrogenation of 2,4-DNT (Neri et al., 1995b). Therefore, only the most important points are here recalled. The hydrogenation reactions have been described by a Langmuir-Hinshelwood type model, assuming noncompetitive adsorption of the organic species and hydrogen on the catalyst surface (Kut et al., 1987; Janssen et al., 1990b). The surface reaction between the organic substrate and H2 was assumed to be the rate limiting. The rate expression for the surface reaction, from component i to component j, is given by the equation
r ) WcatkijΘiΘH
(1)
where Wcat is the weight of the catalyst, kij represents the rate constant and Θi and ΘH are the fractional surface coverages of the organic substrate and hydrogen, respectively. Θi and ΘH are given by the equations
KiCi Θi ) 1 + KiCi + ΣjKjCj
(2)
ΘH ) KHC*H2
(3)
Wcatkij′KiCi 1 + KiCi
Figure 2. Hydrogenation of 4HA2NT over 5% Pd/C in ethanol solvent, T ) 283 K, [4HA2NT] ) 0.0175 mol/L: (2) 4HA2NT; (3) 4A2NT; ([) 2,4-DAT. Comparison between experimental and calculated concentrations with model II.
kinetic runs, minimizing the sum of squares of the objective function m
where Ki, Kj, KH and Ci, Cj, C*H2 represent the adsorption equilibrium constants and the concentrations of the ith and jth component and hydrogen dissolved in the liquid phase, respectively. Equation 3 was derived under the assumption that, at low H2 pressure, the catalytic hydrogenation of the nitro-substitued aromatic compounds is of first order in hydrogen (Janssen et al., 1990a; Kut et al., 1987; Pawlowski et al., 1981). Substituing eqs 2 and 3 into eq 1 and neglecting the adsorption term related to products, ΣjKjCj, in order to decrease the number of kinetic parameters to be determined, the rate equation for the hydrogenation of nitro compounds can be simply expressed as
r)
Figure 1. Hydrogenation of 4HA2NT over 5% Pd/C in ethanol solvent, T ) 283 K, [4HA2NT] ) 0.0175 mol/L: (2) 4HA2NT; (3) 4A2NT; ([) 2,4-DAT. Comparison between experimental and calculated concentrations with model I.
(4)
where kij′ ) kijKHC*H2. The term KHC*H2 was assumed to be constant in the range of temperatures investigated (Cargill, 1978; Kut et al., 1984) and has been incorporated into the rate constant. The kinetic parameters have been obtained by a multiresponse nonlinear regression of the experimental
SS )
fi2 ∑ i)1
(5)
were fi is the difference between the experimental and calculated data and m is the number of observations. Hydrogenation of 4HA2NT. Hydrogenation of 4HA2NT in the presence of 5% Pd/C has been carried out in the range of temperatures between 283 and 323 K and at a pressure of 0.1 MPa. Figures 1-3 show typical conversion-time plots for the reaction carried out at 283 and 313 K. The reaction proceeds through the intermediate 4A2NT, while no evidence of the presence of other intermediates has been found. At the lowest temperature, the formation of 2,4-DAT starts together with the formation of 4A2NT. In Figure 4 the plot of the selectivity to 2,4-DAT as a function of conversion at different reaction temperatures is reported. At the low temperature a high initial selectivity to 2,4-DAT of about 18% was observed. This indicates that a fraction of toluendiamine derives directly from 4HA2NT. With an increase of the reaction temperature, the initial selectivity decreases. In order to describe this type of behavior, different kinetic models have been tested. A simple consecutive scheme (model I) was first used to fit the experimental data
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3621 4HA2NT
r3
r1
2,4-DAT r2
(model II)
4A2NT
This reaction mechanism can be described by a total of three indipendent rate equations. The corresponding calculated concentrations were obtained by integration of the following material balance:
dC4HA2NT/dt ) -r1 - r3 ) -Wcat(k12′′C4HA2NT + k13′′C4HA2NT)/ (1 + K4HA2NTC4HA2NT) (9) Figure 3. Hydrogenation of 4HA2NT over 5% Pd/C in ethanol solvent, T ) 313 K, [4HA2NT] ) 0.0313 mol/L: (2) 4HA2NT; (3) 4A2NT; ([) 2,4-DAT. Comparison between experimental and calculated concentrations with model II.
dC4A2NT/dt ) r1 - r2 ) Wcat(k12′′C4HA2NT - k23′′C4A2NT)/(1 + K4HA2NTC4HA2NT) (10) dCDAT/dt ) r2 + r3 ) Wcat(k23′′C4A2NT + k13′′C4HA2NT)/(1 + K4HA2NTC4HA2NT) (11)
Figure 4. Selectivity to 2,4-DAT as a function of conversion during 4HA2NT hydrogenation at different reaction temperatures: (1) 283 K; (0) 303 K; (b) 323 K. r1
r2
4HA2NT 98 4A2NT 98 2,4-DAT
(model I)
The component concentrations were obtained by the following material balance
dC4HA2NT/dt ) - r1 ) -Wcatk12′′C4HA2NT/(1 + K4HA2NTC4HA2NT) (6) dC4A2NT/dt ) r1 - r2 ) Wcat(k12′′C4HA2NT - k23′′C4A2NT)/(1 + K4HA2NTC4HA2NT) (7) dCDAT/dt ) r2 ) Wcatk23′′C4A2NT /(1 + K4HA2NTC4HA2NT) (8) where kij′′ ) kij′K4HA2NT. This set of equations was integrated numerically with a standard Runge-Kutta-Merson algorithm with selfadjusting step size. The minimization of the weighted sum of squares of the residuals was performed by a specific optimization routine, based on several combined search procedures, which ensured quick convergence (Buzzi-Ferraris, 1970). Figure 1 shows the best fitting of the experimental results for the reaction carried out at a temperature of 283 K. The mean square error, MSE ) 17.2, is very high indicating that this model does not describe adequately the reaction pathway. Kinetic runs were then fitted by the following triangular reaction scheme (model II):
Figures 2 and 3 show the results of the mathematical modeling obtained for the runs carried out at 283 and 313 K, respectively. The agreement between the experimental points and the calculated curves is satisfactory in all ranges of temperatures investigated and the values of MSE are lower than those derived from model I (Table 1). Only at the higher temperature is the error calculated with the two models similar. Then, from a statistical view, model II fits well the experimental data in all ranges of temperature investigated. Table 2 shows the optimal estimates of rate constant kij′′ and the adsorption constant K4HA2NT calculated with model II. As the reaction temperature decreases, the values of rate constants for steps 1, 2, and 3 become comparable and the reaction proceeds through a consecutive-parallel scheme, leading to a significant amount of 2,4-DAT which derives directly from 4HA2NT. At higher temperature (323 K) the value of the rate constant k13′′ is small with respect to values of k12′′ and k23′′, and the hydrogenation of 4HA2NT proceeds preferentially through steps 1 and 2. The true activation energy, Ea, for paths 1 and 3 has been calculated by plotting the ln of kinetic constants k12′ and k13′ as a function of inverse temperature (Figure 5). The true activation energy for path 2 cannot be calculated because of the lack of an estimate of the adsorption constant relative to 4A2NT. The values of activation energy calculated (Ea ) 11.9 ( 0.3 kcal/mol for step 1 and 7.5 ( 2.6 kcal/mol for step 3) confirm the lower sensitivity to temperature of path 3. Hydrogenation of 2,4-Nitroaminotoluene Isomers. The hydrogenations of 4A2NT and 2A4NT were carried out in the same experimental conditions adopted for the reduction of 4HA2NT. Typical conversion-time plots are shown in Figures 6 and 7. The disappearance of 2A4NT follows a zero-order kinetic up to high conversions (>80%) whereas the rate of reduction of 4A2NT shows a most accentuate tailing due to a higher order of reaction. The 2,4-DAT is the only reaction product detected in both reactions. The formation of the 4-(hydroxyamino)2-aminotoluene (4HA2AT) reported by Janssen (1990a) and related to partial hydrogenation of the p-nitro group (p-NO2) to -NHOH group has been observed during the hydrogenation of 2A4NT at low temperature. However,
3622 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 1. Values of MSE for 4HA2NT Hydrogenation Carried Out at Different Temperatures with Models I and II MSE temp (K)
model I
model II
323 313 303 283
5.1 7.5 23.8 17.2
5.4 3.9 5.7 5.3
Table 2. Optimal Estimatesa of the Kinetic Parameters for the Hydrogenation of 4HA2NT over Pd/C with Model II temp (K)
k12′′, min-1 gcat-1
k23′′, min-1 gcat-1
k13′′, min-1 gcat-1
K4HA2NT, L/moles
323 313 303 283
3.09 ( 0.42 2.49 ( 0.29 1.58 ( 0.42 0.96 ( 0.28
1.89 ( 0.19 1.18 ( 0.06 0.41 ( 0.05 0.70 ( 0.11
0.48 ( 0.13 0.63 ( 0.22 0.46 ( 0.20
90.1 ( 19.7 120.1 ( 30.6 151.4 ( 58.4 372.9 ( 155.7
a
With 95% confidence limits.
Figure 7. Hydrogenation of 2A4NT over 5% Pd/C in ethanol solvent, T ) 313 K, and [2A4NT] ) 0.05 mol/L: (]) 2A4NT; ([) 2,4-DAT. Comparison between experimental and calculated concentrations. Table 3. Optimal Estimatesa of the Kinetic Parameters for the Hydrogenation of 2A4NT and 4A2NT over Pd/C 2A4NT
4A2NT
temp (K)
k12′′, min-1 gcat-1
K2A4NT, L/mol
k12′′, min-1 gcat-1
K2A4NT, L/mol
323 313 303 283
4.21 ( 0.46 5.47 ( 1.39 3.10 ( 1.00 3.75 ( 2.85
81.2 ( 16.5 102.6 ( 34.9 141.4 ( 56.9 415.3 ( 342.2
0.41 ( 0.06 0.36 ( 0.05 0.44 ( 0.05 0.18 ( 0.05
20.7 ( 8.0 32.6 ( 8.7 48.4 ( 7.5 84.2 ( 32.4
a
With 95% confidence limits.
Figure 5. Arrhenius plot for the kinetic constants of steps 1 and 3 in the hydrogenation of 4HA2NT: (4) k12′; (9) k13′
Figure 8. Arrhenius plot for the kinetic constants k12′ in the hydrogenation of 4A2NT and 2A4NT: (0) 4A2NT; (b) 2A4NT.
Figure 6. Hydrogenation of 4A2NT over 5% Pd/C in ethanol solvent, T ) 313 K, and [4A2NT] ) 0.05 mol/L: (3) 4A2NT; ([) 2,4-DAT. Comparison between experimental and calculated concentrations.
this compound was not taken into account in the kinetic analysis owing to its low concentration ( K4HA2NT, K2A4NT > K4A2NT. The adsorption constants decrease with the substitution of -NO2 group(s) with electron-releasing groups. The same trend is observed in the work of Janssen et al. (1990b), where the relative
Figure 9. Temperature dependence of the adsorption constants of mono- and dinitro derivatives: ([) KDNT; (boxes) K4HA2NT; (]) K4A2NT; (2) K2A4NT. Values of KDNT and K4HA2NT (hatched box) were taken from Neri et al. (1995b). Table 4. Rate Constants, k′, for the Hydrogenation of o-NO2 and p-NO2 Groups on Different Substrates (Tr ) 323 K) substrate
reducible group
k′, mol/(gcat min) 102
2,4-DNT
p-NO2 o-NO2 p-NO2 o-NO2
9.56 1.67 5.18 1.98
2A4NT 4A2NT
adsorption constants Qi ) Ki/KDNT show values always lower than 1, confirming the weaker adsorption of mononitro derivatives with respect to 2,4-DNT. Moreover, the lower adsorption constant of 4A2NT compared to 2A4NT shows that also steric effects play an important role in determining the interaction between the substrate and the catalyst surface. In order to correlate the reactivity to electronic and/ or steric factors, the hydrogenation rate constants of the -NO2 groups are reported in Table 4. The o-NO2 group is found less reactive than p-NO2, probably due to steric hindrance of the methyl group. Moreover, on the comparison of the reduction rate constants of the same nitro group in different substrates, two trends are observed. Rate constants for the p-NO2 group decrease with the substitution of the -NO2 group in ortho (with respect to the methyl group) with a more electronreleasing group. Instead, for the hydrogenation of the o-NO2 group, similar values for the rate constants were observed suggesting that the steric effect of the methyl group prevails over the electronic factor. Conclusions The catalytic hydrogenation of the intermediates 4HA2NT and the two 2,4-nitroamino derivatives, 2A4NT and 4A2NT, has given additional information on the complex reaction network of 2,4-DNT hydrogenation. The results of this work support well the reaction mechanism previously suggested (Neri et al., 1995a,b). It has been confirmed that the hydrogenation of 4HA2NT occurs through a triangular reaction pathway leading to 4A2NT and 2,4-DAT. Moreover, it has been proved that the reactivity of -NO2 groups is linked with two main factors. The presence of electron-releasing substituents as well as steric effects having a marked effect on rate of hydrogenation. The hydrogenation rate constants have been found to decrease (i) with increasing the electron-releasing effect of the second substituent and (ii) when the nitro group to be hydrogenated is close to the methyl group. Hydrogenation of the p-NO2 group has been found sensitive mainly to electronic
3624 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
factors, whereas for the reduction of the o-NO2 the steric effect prevails. Nomenclature Ci ) concentration of ith component (mol L-1) C*H2 ) liquid phase concentration of hydrogen (mol L-1) dp ) average catalyst grain size (µm) Ea ) activation energy (kcal mol-1) kij ) rate constant for reaction I f J (mol gcat-1 min-1) kij′ ) kij KH C*H2 (mol gcat-1 min-1) kij′′ ) kij′ Ki (L gcat-1 min-1) Ki ) adsorption constant of ith component (L mol-1) KH ) hydrogen adsorption constant (L mol-1) r ) reaction rate (mol L-1 min-1) t ) reaction time (min) T ) temperature (K) Θi ) fractional surface coverage of ith component ΘH ) fractional surface coverage of hydrogen Qi ) heat of chemisorption of ith component (kcal mol-1) Wcat ) mass of catalyst suspended in the liquid phase (g) Abbreviations 2,4-DNT ) 2,4-dinitrotoluene 2,4-DAT ) 2,4-diaminotoluene 4HA2NT ) 4-(hydroxyamino)-2-nitrotoluene 4A2NT ) 4-amino-2-nitrotoluene 2A4NT ) 2-amino-4-nitrotoluene
Literature Cited Buzzi-Ferraris, G. An Optimization Method for Multivariable Functions. Presented at the Working Party on Routine Computer Programs and the Use of Computers in Chemical Engineering; Florence, Italy, 1970. Cargill, R. W. Solubility of Helium and Hydrogen in Some Water + Alcohol Systems, J. Chem. Soc., Faraday Trans. 1 1978, 74, 1444. Janssen, H. J.; Kruithof, A. J.; Steghuis, G. J.; Westerterp, K. R. Kinetics of the Catalytic Hydrogenation of 2,4-Dinitrotoluene. 1. Experiments, Reaction Scheme, and Catalyst Activity. Ind. Eng. Chem. Res. 1990a, 29, 754-766.
Janssen, H. J.; Kruithof, A. J.; Steghuis, G. J.; Westerterp, K. R. Kinetics of the Catalytic Hydrogenation of 2,4-Dinitrotoluene. 2. Modeling of the Reaction Rates and Catalyst Activity. Ind. Eng. Chem. Res. 1990b, 29, 1822-1829. Jones, W. H.; Benning, W. F.; Davis, P.; Mulvey, D. M.; Pollak, P. I.; Schaeffer, J. C.; Tull, R.; Weinstock, L. M. Selective Hydrogenation of 2,4-Dinitro (alkyl) benzenes over Raney Copper Catalyst. Ann. N. Y. Acad. Sci. 1969, 158, 471-481. Karwa, S. L.; Rajadhyaksha R. A. Selective Catalytic Hydrogenation of Nitrobenzene to Phenylhydroxylamine. Ind. Eng. Chem. Res. 1987, 26, 1746-1750. Kut, O. M.; Yuculen, F.; Gut, G. Selective Liquid-phase Hydrogenation of 2,6-Dinitrotoluene with Platinum Catalysts. J. Chem. Technol. Biotechnol. 1987, 39, 107-114. Kut, O. M.; Buehlmann, T.; Mayer, F.; Gut, G. Kinetics of LiquidPhase Reduction of 2,4-Dimethylnitrobenzene to 2,4-Dimethylaniline by Hydrogren with Pd/C as Catalyst. Ind. Eng. Chem. Proc. Des. Dev. 1984, 23, 335-337. Neri, G.; Musolino, M. G.; Milone, C.; Visco, A. M.; Di Mario, A. Mechanism of 2,4-Dinitrotoluene Hydrogenation over Pd/C. J. Mol. Catal. A: Chemical, 1995a, 95, 235. Neri, G.; Musolino, M. G.; Milone, C.; Galvagno S.; Kinetic Modeling of 2,4-Dinitrotoluene Hydrogenation over Pd/C. Ind. Eng. Chem. Res. 1995b, 34, 2226. Pawlowski, J.; Kricsfalussy, Z. Reaktionskinetische Untersuchungen in Drei-Phasen Systemen, dargestellt am Beispiel der Dinitrotoluol-Hydrierung. Chem.-Ing.-Tech. 1981, 53, 652-654. Rylander, P. N. Catalytic Hydrogenation over Platinum Metals; Academic Press: New York, 1967. Stratz, A. M. The Hydrogenation of Aromatic Nitro Compounds to Aromatic Amines. In Catalysis of Organic Reactions; Kosak, J. R., Ed.; M. Dekker: New York, 1984; p 335. Turner, N. J. Biocatalytic Reductions. Chem. Ind. 1994, 15, 592595.
Received for review August 11, 1995 Revised manuscript received January 3, 1997 Accepted April 18, 1997X IE950505B
Abstract published in Advance ACS Abstracts, June 1, 1997. X
