Deactivation kinetics in the hydrogenation of nitrobenzene to aniline on the basis of a coke formation kinetics -: investigations in an isothermal catalytic wall reactor

The heterogeneously catalysed gas-phase hydrogenation of nitrobenzene to aniline on an alpha -alumina supported palladium catalyst is a highly selective reaction which however is deactivated by coking. A catalytic wall reactor (CWR) was proved to be a powerful tool for the determination of a deactivation kinetics on the basis of a coke formation kinetics. Nitrobenzene was identified as coke precursor, whereas aniline had no influence neither on the kinetics of the nitrobenzene hydrogenation nor on the kinetics of coking. The coking of the catalyst was investigated by monitoring axial coke profiles along the coated wall of the CWR with progressing time-on-stream. Analysing the coke formation at the coated section close to the inlet allows a fast determination of the fundamental dependencies of coke formation on partial pressures of reactants and on temperature. For the first coated section partial pressures correspond to the well-defined inlet values and do not depend on time. Fitting coke profiles and the nitrobenzene outlet concentrations allowed the conclusion that it is indispensable to differentiate between coke on the support and coke on the active site. Furthermore, both coke on the support and coke on the active site were found to be responsible for deactivating the hydrogenation of nitrobenzene. Thus, it was concluded that also the support is involved in hydrogenation of nitrobenzene due to hydrogen spillover. (C) 2001 Elsevier Science Ltd. All rights reserved.