Switchable Behavior of Ru–TiO₂ Catalysts in HMF Conversion

Abstract

5-Hydroxymethylfurfural (HMF) is a platform chemical that can be catalytically valorized into high-value-added chemicals. Despite the extensive use of titania-supported metal catalysts in HMF conversion, the specific influence of the TiO₂ support on the formation of metal active sites, their dispersion, and their role in HMF conversion is not addressed sufficiently. In this work, we investigated five TiO₂ supports, four anatase- and one rutile-dominant, varying in surface area and acidity, which were loaded with 1 wt % Ru using RuCl₃. Characterization results revealed that the Ru environment, charge distribution, and surface features (transition from Ru–Cl to Ru–O species) varied depending on the TiO₂ support used. Their catalytic performance was assessed in HMF hydrogenation. Despite identical Ru loadings, the Ru/TiO₂ catalysts exhibited remarkably different catalytic activities and selectivity. High-surface-area anatase TiO₂ led to the formation of smaller Ru particles and supported the conversion of HMF to 5-methylfurfural. In contrast, lower surface area anatase and rutile supports favored the formation of larger Ru particles and redirected the reaction course from 5-MF toward the hydrogenation route, yielding primarily 2,5-bis(hydroxymethyl)furan. This study revealed the switchable behavior of Ru/TiO₂ catalysts and exposed the critical role of TiO₂ structural and morphological features for reaction pathways in HMF valorization over Ru/TiO₂. These insights provide a refined framework for the rational design of oxide-supported catalysts tailored for the selective conversion of biomass.