Studies on the vapour-phases oxidation of somealcohols in a fixed, bed reactor

Abstract

Mechanism of the vapour?phase oxidation of alcohols over ferric molybdate catalyst was investigated in a fixed?bed isothermal differential flow reactor. Ring?substituted 1?phenylethanols were used as the organic substrates for this purpose. Ferric and thorium molybdates were chosen as catalysts for detailed kinetic studies on the oxidation of these alcohols because of their good efficiencies and selectivities towards the formation of the corresponding acetophenones, in the temperature range covered. The unreacted alcohol and the ketone formed in the product stream were absorbed in aqueous acetic acid and quantitatively estimated by spectrophotometric techniques after suitable derivatization. The effect of various operating variables, namely, (i) time factor, (ii) temperature, (iii) partial pressure of alcohol and (iv) partial pressure of oxygen, on conversion and reaction rate of alcohol, was studied. The experiments were planned such that the data obtained would represent the intrinsic kinetics of the reaction. Absence of significant concentration and temperature gradients in the reactor under the experimental conditions was also checked by theoretical calculations. For elucidating the kinetics of the reaction, twenty?four different models were tried. The rate constants were computed by linear least?squares regression technique. For this purpose FORTRAN 10 programmes were used in a DEC 1090 computer system. Model discrimination studies included classical criteria and non?intrinsic parameter method. Amongst the models examined, the two?stage redox model-wherein the substrate reduces the catalyst, which in turn is oxidised by oxygen from the feed—was found to fit the observed data adequately. This model was developed on the basis of first?order dependence on 1?phenylethanol and half?order dependence on oxygen. The active role of molybdenum species as also the participation of lattice oxygen of ferric molybdate in the catalytic reaction were established. Linear free?energy criteria was applied to probe the nature of the activated complex associated with the slow step leading to the formation of the ketone. This suggests a rate?determining abstraction of the hydrogen on the ??carbon atom of the alcohol as a hydride ion by the catalyst. A mechanistic pathway based on these findings for the vapour?phase partial oxidation of alcohols to ketones over ferric molybdate catalyst has been proposed. The activation parameters have been analysed in terms of the isokinetic relationship and the Exner refinement thereof.