Kinetic Studies of Disproportionation of Propylene over Several Supported MoO₃ Catalysts

Abstract

Kinetics of disproportionation of propylene over several supported molybdenum oxide catalysts were investigated by means of the conventional differential reactor technique. The catalysts used were as follows: MoO3-A1203 and MoO3-SiO2 catalysts, prepared by wet mixing method containing various amount of MoO3, and MoO3-Y-A1203 and CoO-MoO3-r-A1203 catalysts, prepared by impregnation method.
The initial rate data obtained for all catalysts could be correlated with the rate equation based upon a dual site Langmuir-Hinshelwood mechanism. These results could be interpreted by introducing the following concepts to the adsorption equilibrium constants of olefine.
(1) The stepwise equilibrium constants of olefines K1 and K2 were not the same value with each other by considering the free energy of each interaction between a metal ion and two olefins.
(2) The active centeres were also retarded by the adsorbed species which were different from the intermediate of this reaction.
The compensation effect was observed between the activation energy E and the preexponential factor A in the rate expression of k=A exp(-E/RT).
As the degree of the dispersion of MoO3 on carriers increased, both the activation energy E and the heats of adsorption Q increased, and the relation, 2 Q E, was obtained for all catalysts used. It was concluded from all results that the inhibiting effect on the active center of the catalyst by non-intermediate adsorbates was very important, and also that the entropy change in forming the activated complex was more important than the enthalpy changes of this reaction.