Hydrogenation versus hydrogenolysis in the reaction of cis-2-butene-1,4-diol over supported platinum catalysts: Kinetic aspects

The liquid phase hydrogenation of cis-2-butene-1,4-diol has been studied in ethanol, using different platinum-supported catalysts, at a temperature ranging from 273 to 303 K and at a partial H2 pressure between 0.01 and 0.1 MPa. Formation of only hydrogenation and hydrogenolysis products was detected and their selectivity was found to depend on the acid-base characteristics of the support. A yield of 96% to the fully hydrogenated product, butane-1,4-diol, at 303 K and 0.1 MPa H2 pressure was obtained on Pt/MgO. The experimental data, in presence of platinum supported on TiO2, was explained on the basis of a two paths mechanism: one leading to butane-1,4-diol, as usually, through a σ-alkyl species and the other, affording cis-crotyl alcohol and 3-buten-1-ol, involving the same unique intermediate, η3-allyl. Reaction rates were described on the basis of a Langmuir-Hinshelwood type model, assuming a non-competitive adsorption of organic species and hydrogen on the platinum active sites. Kinetic parameters for individual reaction steps were determined by a non-linear regression analysis. Furthermore, an adsorption of the olefinic substrate in a tilted mode was discussed in order to explain the hydrogenolysis reaction.