Scaling of lignin monomer hydrogenation, hydrodeoxygenation and hydrocracking reaction micro-kinetics over solid metal/acid catalysts to aromatic oligomers

A bottom-up approach was formulated and used as a strategy in our work to systematically investigate the removal of lignin functionalities and the hydrogenolysis of the typical bonds relevant for lignin. In the previous studies, the role of metals and supports of 16 commercial catalysts (Pt, Pd, Rh, Ru, Ni, Cu supported on C, Al2O3, SiO2, SiO2-Al2O3, HZSM-5, TiO2) was demonstrated for the catalytic activity as well as the interaction, synergy and interplay of different active sites. The present study is focused on the hydrogenolysis and hydrodeoxygenation of lignin dimer model compounds, particularly the cleavage of C-C and ether bonds. A few selected catalysts (Pt/C, Pt/Al2O3, Ni/Al2O3 and Cu/Al2O3) from the previous segment were tested here at different temperatures. Parameters (rate constants and activation energies) that describe the kinetic rates of oxygen-containing groups removal, ring hydrogenation and cracking reactions were optimized for two types of active sites (metallic and acidic) by a micro-kinetic model. While the work with eugenol resulted in transferable rate constants of different active sites across catalysts, the rate constants are transferable also from monomers to dimers. Specifically, constants for the HDO of eugenol were successfully applied for dimers reactions (mostly for the reactions of formed monomers). In this study it was shown that the hydrogenation of partially hydrogenated dimer proceeds slower than the hydrogenation of unsaturated reactant itself. Furthermore, OH group promotes, while OCH3 hinders hydrogenation. Cracking of C beta-O was more promoted over the Ni, Cu and acidic active sites, while the cracking in other positions was more significant on the Pt active sites. 2-dimensional transferability of kinetic results for five model compounds demonstrates a fundamental description of mechanism and intrinsic kinetics for lignin defunctionalisation and depolymerisation.