Bifunctional metallic-acidic mechanisms of hydrodeoxygenation of eugenol as lignin model compound over supported Cu, Ni, Pd, Pt, Rh and Ru catalyst materials

The hydrodeoxygenation (HDO) reaction kinetics of lignin monomer model compound eugenol was systematically investigated over various commercially available catalysts typically used for lignin valorisation by hydrotreatment. The role of noble metals Pt, Pd, Rh, Ru, and non-noble Ni and Cu on C was investigated in the previous studies, while the present work is focused on the support (Al2O3, SiO2, SiO2-Al2O3, TiO2, HZSM-5) effect on the catalyst activity and selectivity, being estimated experimentally (at 225, 275, 325 °C and 5 MPa of initial hydrogen pressure) and in silico. The micro-kineticmodel took into consideration mass transfer resistances through convective films (gas bubbles and catalyst particles), reaction kinetics in bulk liquid, adsorption/desorption and surface reaction kinetics on metallic and acidic active sites, considering the occurrence of all reactions on both types of sites. The contribution of each support was quantitatively evaluated in terms of adsorption, desorption, and reaction rate constants and activation energies. SiO2-Al2O3 and HZSM-5 showed superior contribution to the activity of the methoxyl and hydroxyl group removal from oxygenated aromatics and particularly saturated species. Except for the above-mentioned materials Caromatic–O hydrogenolysis activity was not substantially affected, while the acidity of the support notably promoted Caliphatic–O cleavage. Contribution to overall hydrogenation rate was practically negligible in presence of acidic sites. It has been also noted that the contribution of supports was not very prominent when very active noble metal phases were used (such as Ru), while it came to the fore in the case of almost inactive Cu, being responsible for formation of 4-propylphenol at 325 °C. Based on microkinetic modelling results, Sankey diagrams were formulated for the first time to represent pathway flux and graphically demonstrate the contribution of each catalytic reaction participating in the overall eugenol HDO mechanism over the used catalysts (and corresponding intermediates) within a complex reaction network, as well as the engagement of metallic and acidic sites.