Adsorption Isotherms of Lignin-Derived Compounds on a Palladium Catalyst

We have studied the interaction of lignin fragments obtained from catalytic fractionation with a heterogeneous palladium catalyst. By studying the adsorption of verified substrate and product molecules on the palladium surface, understanding of what governs adsorption and desorption dynamics of both substrates and products has been obtained. In addition, we have studied the kinetic isotope effect of hydrogen-transfer reactions occurring on the surface of the catalyst. These studies give insights into the thermodynamics of the process in which species from lignin-derived species adsorb to the catalyst surface, are then transformed by hydrogenation-hydrogenolysis reactions in a slow reaction step, and finally desorbed. It was found that the adsorption dynamics depended on the degree of unsaturation as well as the presence of methoxy groups on the aryl. Thereby, the adsorption is stronger for substrate molecules derived from lignin than for reduced molecules obtained after the rate-determining transfer-hydrogenation and hydrogenolysis transformations.