Kinetics of the hydrogenation of 2,4-dinitrotoluene over a palladium on alumina catalyst

Abstract

The chemistry and kinetics of the catalytic hydrogenation of 2,4-dinitrotoluene (2,4-DNT) over a palladium on alumina catalyst have been investigated. The catalyst consists of porous cylindrical pellets of 4.2 × 4.2 mm in which a shell of Pd has penetrated to a depth of the order of 100 μm. The experiments have been performed in a three-phase batch reactor under isothermal and isobaric conditions. Two reactor temperatures of 308 and 345 K and a hydrogen pressure range 0.5–4 MPa have been chosen. Employing a chromatographic method, analyses of the reaction mixtures have been carried out and the reaction pathway has been studied—three stable intermediates have been identified. We have concluded that for both types of catalyst, independent of the palladium carrier, the chemistry of the conversion of DNT into DAT is the same. Mass transfer experiments have shown that external mass transfer resistances can be neglected. However, internal diffusion limitations had to be taken into account. To study the intrinsic reaction kinetics, a series of measurements with a finely crushed catalyst of particle diameter lower than 40 μm have been carried out. To describe the kinetics, a Langmuir—Hinshelwood model based on adsorption of hydrogen and organic species on different active sites has been applied. The kinetics parameters have been determined and compared with data for a Pd on carbon catalyst. The influence of the internal mass transfer resistances on the reaction rates has been quantitatively described with an effectiveness factor. At higher temperatures an additional series of experiments with pellets has been performed at 371 K in order to obtain an impression of the possibilities to extrapolate our correlations outside the experimental region. A good agreement between experimental and calculated hydrogen consumption rates is observed.