ZrO₂ Is Preferred over TiO₂ as Support for the Ru-Catalyzed Hydrogenation of Levulinic Acid to γ-Valerolactone

Abstract

Catalyst stability in the liquid phase under polar conditions, typically required for the catalytic conversion of renewable platform molecules, is a major concern but has been only sparsely studied. Here, the activity, selectivity, and stability of Ru-based catalysts supported on TiO2, ZrO2, and C in the conversion of levulinic acid (LA) to γ-valerolactone (GVL) has been studied at 30 bar of H2 and 423 K in dioxane as solvent. All catalysts showed excellent yields of GVL when used fresh, but only the Ru/ZrO2 catalyst could maintain these high yields upon multiple recycling. Surprisingly, the widely used Ru/TiO2 catalyst showed quick signs of deactivation already after the first catalytic test. XPS, CO/FT-IR, TGA, AC-STEM, and physisorption data showed that the partial deactivation is not due to Ru sintering or coking but rather due to reduction of the titania support in combination with partial coverage of the Ru nanoparticles, i.e. due to a detrimental strong metal–support interaction. In contrast, the zirconia support showed no signs of reduction and displayed high morphological and structural stability even after five recycling tests. Remarkably, in the fresh Ru/ZrO2 catalyst, Ru was found to be fully atomically dispersed on the fresh catalyst even at 1 wt % Ru loading, with some genesis of Ru nanoparticles being observed upon recycling. Further studies with the Ru/ZrO2 catalyst showed that dioxane can be readily replaced by more benign solvents, including GVL itself. The addition of water to the reaction mixture was furthermore shown to promote the selective hydrogenation reaction.

Hydrogenation of levulinic acid to γ-valerolactone over anatase-supported Ru catalysts: Effect of catalyst synthesis protocols on activity

“…LA hydrogenation reactions were performed according to previously published standard conditions for water [38], and dioxane [39]. The hydrogenation reactions using water as the solvent were performed in a stainless steel batch autoclave (100 mL, Parr Instrument Company).…”

“…Standard reaction conditions were selected based our previous results [19,38,39], and are given in Table 2. The lower reaction temperature selected for the reaction in water (90°C) compared to dioxane (150°C) reflects the surprisingly high activity of Ru-based hydrogenation catalysts under aqueous conditions, for which the underlying reasons have been recently discussed [15,17,19,38,41,42].…”

Nature of Sintering‐Resistant, Single‐Atom Ru Species Dispersed on Zirconia‐Based Catalysts: A DFT and FTIR Study of CO Adsorption

“…Recently, Ftouni et al . compared the performances of Ru/TiO 2 and Ru/ZrO 2 catalysts in the liquid‐phase hydrogenation of levulinic acid, a key step in the upgrade of cellulosic biomasses to value‐added products.…”

The Promoting Effect of Ce on the Performance of Au/CexZr1−xO2 for γ-Valerolactone Production from Biomass-Based Levulinic Acid and Formic Acid

“…Ftouni et al indicate that ZrO 2 support shows better efficiency for the dispersion and reduction of metal Ru than TiO 2 , due to the detrimental strong interaction between metal and support, and all catalysts exhibit high efficiency for GVL production. However, only Ru/ZrO 2 catalyst shows good recyclability [33]. Du et al also report that Au/ZrO 2 shows much higher activity than Au/TiO 2 for LA hydrogenation to GVL, whereas Au/SiO 2 and Au/C are less active [1].…”