Highly selective and efficient solvent-free transformation of bio-derived levulinic acid to γ-valerolactone by Ru(II) arene catalyst precursors

The selective and efficient solvent-free hydrogenation of bio-based levulinic acid (LA) to γ-valerolactone (GVL) was achieved with new pyridylimine ruthenium(II) complexes as catalyst precursors. The hydrogenation reactions were performed in the presence of formic acid as hydrogen source using a catalyst loading as low as 0.1 mol% with potassium hydroxide or triethylamine (Et₃N). 4-Hydroxyvaleric acid (HVA) was produced only when KOH was used, whereas reactions involving Et₃N were selective to GVL. At 150 °C, >96% LA conversions were achieved with 100% GVL selectivity. Recyclability of catalyst precursors was demonstrated by running three consecutive reactions where 100% conversion and selectivity was maintained. In-situ NMR studies show that hydrogen gas is formed by the decomposition of formic acid to carbon dioxide and hydrogen. Ru-hydride species have been detected, by ¹H NMR, and are believed to be the catalytically active species, and a mechanism of the reaction has been proposed.