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.
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