(198f) Reaction Engineering Investigations of the Uncatalysed Cyclohexane Oxidation in A Microstructured Reactor

A flow apparatus with a microstructured reactor was developed for reaction engineering investigations of the uncatalysed cyclohexane oxidation. Uncatalysed cyclohexane oxidation is a highly exothermic radical partial oxidation reaction. It is known to be autocatalytic and hazardous. Due to an intensification of heat and mass transport and an enhanced safety the microstructured reactor allows elevated operation conditions (?new process window?), e.g. at higher pressures and temperatures than in conventional industrial bubble columns. Thus, with the microstructured reactor working at increased reaction rate a higher space time yield could be expected. (see Tab. 1).

Tab.  SEQ Tab. \* ARABIC 1: Comparison of the performance of a bubble column and a microstructured reactor [1, 2].

The reason for the decrease in selectivity observed may be with the temperature dependence of the intrinsic kinetics or by the increased specific surface of the microstructured reactor. This effect was investigated by varying the diameter of the microchannel.

A significant influence of the specific channel surface on the dependence of cyclohexyl hydroperoxide concentration upon residence time was observed (Fig. 1). With reduction of the channel diameter and increase of the specific channel surface the maximum of the cyclohexyl hydroperoxide concentration decreased and shifted towards shorter residence times. Simultaneously an earlier increase of the product concentrations of cyclohexanol and cyclohexanone as well as carboxylic acids was detected (not shown as diagram). It can be assumed that cyclohexyl hydroperoxide is decomposed on the channel walls, most likely into hydroxyl and cyclohexyloxy radicals. Hydroxyl radicals will start new radical chains (through reaction with surrounding cyclohexane) and, thus, the consumption rate of cyclohexane and the formation rate of products increases. Cyclohexyloxy radicals underlie ring opening and are further oxidized to carboxylic acids.

Fig.  SEQ Abb. \* ARABIC 1: Concentration of the peroxide versus residence time for different diameters of the channel, T = 200°C, p = 80 bar, 90 wt% cyclohexane (rest toluene).

Kinetic measurements are in progress in order to develop a kinetic model which properly describes cyclohexane oxidation in a microstructured reactor. This model must consider surface reactions of the cyclohexyl hydroperoxide and is based on the reaction mechanism of Jacobs et al.^{[3, 4, 5]}.

This project was financially supported by the German Federal Ministry of Education and Research (16SV1992) and the BASF.

[1]     W. B. Fisher, J. F. van Reppen, Cyclohexanone Cyclohexanol, Kirk-Othmer Encyclopedia of Chemical Technology, Wiley-VCH, 2000.

[2]     M. T. Musser, Cyclohexanol and Cyclohexanone, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2000.

[3]     I. Hermans, T. L. Nguyen, P. A. Jacobs, J. Peeters, Autoxidation of Cyclohexane: Conventional Views Challenged by Theory and Experiment, ChemPhysChem, 6, 637 ? 645, 2005.

[4]     I. Hermans, P. A. Jacobs, J. Peeters, To the Core of Autocatalysis in Cyclohexane Autoxidation, Chem. Eur. J., 12, 4229 ? 4240, 2006.

[5]     I. Hermans, P. A. Jacobs, J. Peeters, The Formation of Byproducts in the Autoxidation of Cyclohexane, Chem. Eur. J., 13, 754 ? 761, 2007.