Catalyst testing at a lab scale in mild oxidation: Can you control the reaction temperature?

Abstract

Recent trends in catalysis have emerged using new processes and new raw materials in order to reduce the number of steps leading from petroleum distillates to chemical intermediates. Selective activation of the strong C H bonds present in alkanes by catalytic action of oxygen is a stimulating challenge and new efficient catalysts have appeared in some cases. Nevertheless, the selective materials exhibit very low activities and low selectivities at high hydrocarbon conversion. Extreme conditions (partial pressures, residence time) are, thus, chosen to improve the production rate, which can have dramatic consequences on the real working conditions of the catalytic surface, when laboratory tests are performed. Catalytic oxidation of isobutane forming methacrylic acid was chosen as an example and compared to oldest and ‘classical’ mild oxidations. It is shown that in micro-reactors, in apparent isothermal and differential operation, extreme thermal gradients between the catalyst surface and fluid can develop. Moreover, whatever the kinetics and thermal exchange, the search for productivity announced in patents causes runaway in all cases without external evidence. These effects could lead to an erroneous interpretation of results in terms of kinetics and mechanisms, to drastic decreases in selectivities and therefore to the rejection of the most active catalysts due to a low apparent selectivity which could be a consequence of poor testing conditions. Simulation of different reactor geometry shows that industrial-like pilot plants could be more stable and more selective than micro-reactors. The increase of catalytic bed volume appears as a major positive factor in this objective. The screening of catalysts for these new reactions must thus be carried out with extreme care in the choice of experimental parameters.