A Kinetic Model and Mechanism for Liquid-Phase Heterogeneous Hydrogenation of Dicyclopentadiene

The study investigates the main routes of liquid-phase hydrogenation of endo-tricyclo[5.2.1.02,6]deca-3,8-diene (dicyclopentadiene, (1)) in the presence of a PK-25 palladium catalyst (Pd/γ-Al2O3, 0.25% Pd). All the reaction products were identified, and the material balance was examined. Mild conditions were chosen for the hydrogenation of (1) to ensure that the norbornane framework was retained. For (1), like for other norbornene derivatives, the effect of prevalent adsorption of a norbornene double bond on an active site (AS) of palladium was confirmed, in contrast to other types of double bonds. Based on a combination of experimental and theoretical data, a consistent mechanism was proposed for the process, in which endo-tricyclo[5.2.1.02,6]decane (3) is obtained as the only final product. The kinetic order with respect to (1) was found to be zero within a wide range of its initial concentrations; the hydrogenation of the intermediate cycloalkene—endo-tricyclo[5.2.1.02,6]deca-3-ene (2)—was shown to have the first kinetic order. The activation parameters of the liquid-phase hydrogenation of both (1) and (2) were further determined. Based on the Langmuir–Hinshelwood approach and the concept of multiple adsorption of substrates on a single AS, an adequate kinetic model of the process was developed. It was shown that three process steps occurring by two routes significantly contributed to the reaction rate. The rate constants of these reaction steps and the adsorption constants of AS complexes with unsaturated compounds were estimated.