Heterogeneous catalytic hydrogenation of 5-ethylidene-2-norbornene in the liquid phase

The liquid-phase hydrogenation of E/Z-5-ethylidene-2-norbornene (ENB) in the presence of the 0.25%Pd/γ-Al₂O₃ palladium catalyst (PC-25) under mild conditions proceeds sequentially and stereoselectively via the intermediate product E/Z-2-ethylidenenorbornane to form endo/exo-2-ethylnorbornane. All reaction products were identified, the main reaction routes were determined, and the material balance was studied. No destruction of the norbornane framework occurs in the reaction. The kinetic data indicate a distinct separation of the process steps and quantitative hydrogenation of the norbornene bond in the first step. The zero kinetic order with respect to substrate was determined for the first step. The activation energies for each step were determined (16 kJ mol⁻¹ for the hydrogenation of the norbornene double bond in E/Z-ENB and 25 kJ mol⁻¹ for the hydrogenation of the ethylidene bond). The decisive role of the additional strain energy concentrated on the intracyclic double bond of norbornenes in selective adsorption on the active site of the catalyst and, as a consequence, the full kinetic separation of the steps were shown.