Hydrogenation of Terpenols into Sustainable Biofuel Additives: “Virtual” vs “Real” Hydrogen

Terpenols, which are available from renewable resources, are regarded as promising additives for fuels. However, due to the double bonds in the molecules, they can undergo chemical transformations that thwart their technical application. Hydrogenation of terpenols serves to remove the double bonds. The hydrogenation can be carried out directly with pressurized "real" hydrogen. Alternatively, transfer hydrogenation using the perhydrogenated cyclic hydrocarbons offers a safer way to remove double bonds in terpenols, as in this case a "virtual" hydrogen is stored inside the hydrogen donor molecule. The comparative thermodynamic analysis of the two options was the focus of this study. The experimental data on the absolute vapor pressures and enthalpies of vaporization of terpenols and their hydrogenated products were collected and evaluated using the complementary thermochemical measurements and structure property correlations. The enthalpies and entropies of the hydrogenation reactions were derived from the combination of experimental and quantum chemical data. It was found that for both reaction types the entropy contribution is insignificant and the reactions are driven by the enthalpy factor. As a consequence, the complete hydrogenation of the terpenols can be achieved under mild reaction conditions.