The relevant role of TiO2 for enhancing the Ru/TiO2-catalyzed conversion of levulinic acid to γ-valerolactone

Abstract

Levulinic acid (LA), that can be obtained from the acidic hydrothermal carbonization of biomass, is a platform molecule with several possibilities to be transformed into useful chemicals. The hydrogenation of LA to γ-valerolactone (GVL) requires a bifunctional catalyst, with both metallic and acidic active sites. In this study, TiO₂ was the selected support due to its acidic properties, and Ru was the metal phase due to its high activity in this reaction. 1 wt% Ru/TiO₂ catalysts were prepared using either commercial P25 or TiO₂ prepared by hydrothermal (HT) synthesis (with or without the addition of a low concentration HCl solution). The catalysts were used without previous reduction in catalytic tests carried out under mild reaction conditions, and the influence of the surface chemistry, porosity, crystalline structure, and oxygen vacancies of the TiO₂ supports, as well as the interaction between Ru and the TiO₂ supports were studied. All the catalysts demonstrated promising results under mild reaction conditions (24 – 65 % GVL yield), being the catalytic activity enhanced when using the Ru/HT-TiO₂ catalysts compared to the analogous Ru/P25. This indicates that a higher surface area of the supports improves the catalytic performance. High GVL yield (65 %) and excellent reusability over three cycles were achieved with the best catalyst, synthesized by HT without using HCl solution. Although isolating the effect of individual variables remains challenging, our results have shown that the surface chemistry composition and content of the synthesized TiO₂ supports have some influence on the formation of Ru⁰ on them.