Pierre Boutry

The mechanism of vinyl acetate formation by gas-phase catalytic ethylene acetoxidation

Abstract The synthesis of vinyl acetate by vapor phase oxidation of ethylene-acetic acid mixtures has been studied using catalysts prepared by palladium deposition on silica or alumina supports. The kinetic study of this reaction served to determine the influence of palladium dispersion, the partial pressures of acetic acid, oxygen, ethylene, and water vapor, and the role of alkaline acetates usually recommended for catalyst activation. The results obtained, with or without water vapor in the gas mixture, enable a mechanism suggested which explains the influence of the various parameters studied.

Ethylene oxidation to acetic acid with Pd-V2O5 type catalysts: I. Gas-solid reactions

Palladium added to V2O5 enhances the reducibility of this oxide. The reduction can then be performed at low temperature so as to produce selectively a reduced phase with the formula V4O9. The influence of ethylene partial pressure on the initial reduction rate of Pd-V2O5 and the influence of oxygen partial pressure on the initial oxidation rate of the reduced phase have been established. Kinetic equations are proposed for these two gas-solid reactions. The selective formation of acetaldehyde and acetic acid occurs during the reduction. In the presence of both ethylene and oxygen reactants, i.e., for the condition of catalytic reaction, we observe a change of the initial composition of the solid toward a final state which depends on the partial pressure of the reactants. Qualitatively, from an initially oxidized form, there is a partial reduction, while from an initially reduced form, V4O9, there is a partial oxidation.

Ethylene oxidation to acetic acid with Pd-V2O5 catalysts: II. Kinetics of the catalytic reaction

Abstract The kinetics of the catalytic oxidation of ethylene into acetic acid over Pd-V 2 O 5 was investigated. We studied, in a first stage, the enhancement of catalytic performance by the addition of palladium to V 2 O 5 . Then, the influence of reactant partial pressures on the initial catalytic reaction rates and on the final composition of the catalyst was established. The parallelism observed between enhancement of V 2 O 5 reducibility and of catalytic performance, and the possibility of obtaining acetaldehyde and acetic acid from ethylene oxidation by lattice oxygen of V 2 O 5 , leads us to invoke a redox mechanism for the catalysis. Moreover, the simultaneous formation, at low conversions, of acetic acid and acetaldehyde is due to the occurrence of the consecutive oxidation process in the adsorbed phase, according to what has been called a rakelike scheme. A kinetic equation is proposed which fits the experimental results and permits the calculation of kinetic constants from these experimental results. An attempt has been made to determine, on the basis of the simplest redox mechanism, the theoretical steady state of bulk composition of the catalyst, as a function of reactant partial pressure. Comparison with experiment shows that there is a reasonable agreement if we consider the approximation which we have been obliged to make.

Influence of the degree of crystallinity and of the composition on the activity of MoO3-Bi2O3 catalysts

Abstract The influence of the degree of organization of the catalyst has been examined for the oxidation of butene into butadiene on bismuth molybdate. The results obtained show that the specific activity increases when the degree of crystallization decreases. The variation of this physical property of catalysts was easily obtained by bringing about the complete fusion of catalytic masses and by cooling at varying rates. Furthermore, the amorphous phases prepared in this way were processed at varying temperatures so as to cause their progressive crystallization. The structural stability of the solids obtained and the stability of the catalytic performances during the entire duration of the experiments proved to be quite good, thus enabling the above correlation to be determined. The variation of the Bi:Mo ratio also showed that, no matter what the degree of crystallization may be, catalytic activity appears at a value of about Bi:Mo = 1.