La cinétique de la décarboxylation des acides gras sur la thorine
Authors: Kuriacose J. C. , Jungers J. C. 
Published: September 2010

Previous work on metal hydrogenation catalysts carried out in this laboratory having demonstrated the importance of adsorption coefficients in determining the nature and kinetics of reactions, it was considered useful to extend the study to metal oxide catalysts.

The conversion of fatty acids into ketones on basic metal oxides is known to be a reaction that goes to completion and hence lent itself to a convenient study, first to verify whether observations made on metal catalysts could be extended to oxides and secondly, to elucidate the mechanism of the reaction through a complete study of its kinetics.

The acid chosen for the most complete study is acetic acid, since it reacts with measurable velocities at sufficiently low temperatures (250‐350°C). With the higher acids, as the molecular weight increases, higher temperatures have to be resorted to, which could favour the decomposition of the ketones formed.

The reaction of acetic acid on 0,5 g of thorium oxide followed in the static system by observing the increase in pressure at constant volume, obeys an apparent first order as can be seen from table 1. From the same table it would be clear that propionic acid on the other hand follows a third order. The results presented in fig. 3 show that the initial velocity increases with the initial pressure of the acid to reach a limiting value, thus indicating that the catalyst gets gradually saturated by the reactants. The results in fig. 4 however show that it is difficult to distinguish between a first or second order reaction by considering the initial velocity in function of the initial pressure.

The influence of temperature on the reaction is represented in fig. 5 and 7. The results for butyric, valeric and isovaleric acids were obtained in a flow system. The results in fig. 5 demonstrate the influence of surface coverage on the activation energy. The low energy of activation obtained for the higher acids could be due to unsaturation of the surface at the higher temperatures employed. A comparison of the results for acetic acid and propionic acid in fig. 4 indicates further, that as the molecular weight increases, a higher pressure is needed to saturate the surface. However, the results in fig. 7 show clearly that the velocity of reaction decreases considerably with increase in molecular weight.

A few preliminary experiments made in the static system fig. 2 indicated that the products are strongly adsorbed. The influence of the different products as well as diethyl ketone and acetic anhydride on the decarboxylation of acetic acid is given in tables 2 to 5 and fig. 6. While carbon dioxide, acetone, diethyl ketone and acetic anhydride fall into one family, water shows a completely different influence by inhibiting the reaction more strongly at the beginning and less at the end, so much so, that it cuts across the other curves.

Since a study in the static system did not permit the determination of the exact order of the reaction, resort was made to the dynamic system. Since reactions on surfaces should show their exact order when the concentrations of reactants are extremely small, the concentration of acetic acid on the surface was decreased by mixing it with acetone, methanol, water or benzoic acid. The results presented in fig. 8 show that in the case of methanol and acetone the velocity of formation of ketone would correspond better to a second order than first.

For a mixture of methanol and propionic acid the velocity of esterification is plotted against the concentration in fig. 10. The results for the esterification given in fig. 9 correspond to theoretical curves calculated for bimolecular reactions on the surface of the catalyst. In the case of propionic acid, using the adsorption coefficients obtained from the esterification reaction the velocity of ketonization is calculated using a bimolecular expression. The calculated curve and experimental points are given in fig. 10 which proves that the ketonization of acids on thorium oxide is a bimolecular reaction on the surface.

Since the catalysts known to favour ketonization of fatty acids are all basic oxides, one of the first hypothesis one would consider for the mechanism of the reaction would be the formation of a salt with the catalyst. To verify this hypothesis, the decomposition of thorium acetate was studied and the results are given in fig. 11 and fig. 7. The salt starts decomposing only above 270°C and once started continues at lower temperatures. The form of the curves indicates that the reaction takes place probably at an interface. This is further confirmed by the fact that when already mixed with thorium oxide, the salt starts decomposing at 200°C. The energy of activation corresponds to that obtained for the ketonization of acetic acid.

Acetic acid could be easily displaced from the salt by means of water vapour and the salt treated thus starts decomposing at 200°C.

The fact that water behaves differently from the other products (fig. 6 and 12) is explained thus: acetic acid adsorbed on the surface gives rise to hydroxyl and acetate groups; water too, when adsorbed, gives hydroxyl groups. Since water and acetic acid give rise to a common group on the surface, water could reduce the amount of acetic acid adsorbed, both by occupying the surface and regressing the equilibrium (hydrolysis of the salt). An equation could be worked out (page 531) on the basis of this hypothesis, which accounts for the form of the curves obtained with carbondioxide and acetone on the one hand and water on the other. The facility with which water displaces acetic acid from the salt further confirms this hypothesis.

A few experiments performed with acetic anhydride proved definitely that it could not be the intermediate product in the ketonization of the acids.

The results presented in this paper are explained completely on the basis of the Langmuir‐Hinshelwood treatment and show clearly that the ketonization of fatty acids is a bimolecular reaction on the surface of the catalyst taking place through the intermediate formation of the salt.