2226 Ind. Eng. Chem. Res. 1995,34, 2226-2231 KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetic Modeling of 2,4=Dinitrotoluene Hydrogenation over Pd/C Giovanni Neri Universita di Reggio Calabria, Facolta di Ingegneria, Via Cuzzocrea 48, I-89100 Reggio Calabria, Italy Maria G. Musolino, Candida Milone, and Signorino Galvagno* Universita di Messina, Dipartimento di Chimica Industriale, Cas. Post. 29, I-98166 Sant'Agata di Messina, Italy The kinetics of liquid phase hydrogenation of 2,4-dinitrotoluene (2,4-DNT)to 2,4-diaminotoluene (2,4-DAT) were studied in ethanol with a 5% Pd/C catalyst using a slurry reactor, in the temperature range between 278 and 323 K and at a pressure of 0.1 MPa. The reaction pathway has been described by a Langmuir-Hinshelwood model with a noncompetitive adsorption of the organic species and hydrogen on the active sites. Different reaction mechanisms have been tested. The best fitting of the experimental data was obtained with a reaction mechanism involving three parallel routes leading from 2,4-DNT to 4-(hydroxyamino)-2-nitrotoluene (4HA2NT), 4-amino-2-nitrotoluene (MNT), and 2-amino-4-nitrotoluene (2A4NT) which are then hydrogenated to 2,4-DAT through a series of consecutive reactions. The rate constants and activation energy for each reaction step have been reported. Introduction The production of aromatic diamines by catalytic hydrogenation of aromatic dinitro compounds is an important industrial process (Rylander, 1967; Stratz, 1984). Notwithstanding the great industrial interest for the hydrogenation of 2,4-dinitrotoluene (2,4-DNT), up to now only a few systematic studies have been reported in the literature (Dove11 et al., 1970; Bird and Thomp- son, 1980; Kut et al., 1987; Janssen et al., 1990a,b; Benedetti et al., 1991; Suh et al., 1992;Neri et al., 1994). Moreover, due to the difficulties of analyzing the reac- tion mixture, very limited kinetic work concerning this reaction has been published. In a recent paper (Neri et al., 1995) we have presented the results on the catalytic liquid phase hydrogenation of 2,4-DNT over a 5% PdC. It has been observed that under the experi- mental conditions used (323 K, pressure of 0.1 MPa and ethanol as solvent), 2,4-DNT is converted into the final product, 2,4-diaminotoluene (2,4-DAT), through a com- plex reaction network. The 4-(hydroxyamino)-2-nitro- toluene (4HA2NT) is the main reaction intermediate. The amino-nitro compounds 4-amino-2-nitrotoluene (4A2NT), and 2-amino-4-nitrotoluene (2A4NT) are the other components formed during the hydrogenation of 2,4-DNT. The liquid phase concentration of each com- ponent was determined by a combination of gas (GC) and liquid chromatography (HPLC). In this paper we have extended our investigation presenting a mathematical modeling of the hydrogena- tion of 2,4-DNT in the temperature range between 278 and 323 K. The treatment of the experimental results by mathematical models is very valuable because it allows the evaluation of the kinetic parameters which represent the basis for a scale-up of laboratory data to industrial scale. Besides, it permits a comparison between different reaction schemes for a better under- standing of the reaction mechanism. The simple Lang- muir-Hinshelwood model was applied to describe the Q888-5885l95l2634-2226$09.0OlQ reaction rates by assuming a noncompetitive adsorption of the organic species and hydrogen on the active sites. The experimental data have been tested with different reaction mechanisms with the aim of finding the reac- tion pathway that best fits the results of 2,4-DNT hydrogenation over palladium catalysts. Experimental Section The materials used were 2,4-dinitrotoluene (Aldrich, purity 97%),ethanol (Fluka 95%, analytical grade), and ultra-high-purity hydrogen (Multigas '99.9%). The main impurity of 2,4-DNT was 2,5-DNT. Reagents and solvent were employed without further purification. 4HA2NT was prepared by hydrogenation of 2,4-DNT over 5% PdC catalyst (Engelhard). The reaction was stopped at about 90% conversion. In order to avoid the oxidation of arylhydroxylamine, the catalyst was sepa- rated immediately by filtration and the solvent was removed by using a rotating film evaporator. The obtained residue was dissolved in CHC13 and purified by using a silica gel column and CHCl3 as eluent. The 4HA2NT was identified by IR, NMR, and mass spec- troscopy. The catalyst sample used was a 5% PdC supplied by Montecatini Tecnologie in the form of a powder with an average grain size of about 20-25 pm. The hydrogenation of 2,4-DNT was carried out in a 100 mL five-necked flask, equipped with a reflux condenser and a thermocouple. Constant temperature (f0.5 "C) was maintained by circulation of silicone oil in an external jacket connected with a thermostat. The catalyst (20-80 mg) was added to the required amount of solvent (25 mL of 95% ethanol) and reduced at 323 K for 1 h under H2 flow. After cooling to reaction temperature, 25 mL of a 0.1 M solution of 2,4-DNT in ethanol, containing hexadecane as an internal standard, was added through one arm of the flask. The reaction mixture was stirred with a stirrer head with permanent 0 1995 American Chemical Society