Catalytic Conversion of Biomass-Derived Levulinic Acid to γ-Valerolactone over Amphoteric Zirconium Hydroxide

A series of acid–base amphoteric amorphous Zr(OH)4 catalysts, synthesized with different amounts of leached SiO2, were investigated for catalytic transfer hydrogenation of liquid-phase levulinic acid (LA) to produce γ-valerolactone (GVL) with isopropanol as a hydrogen-donor solvent. The prepared Zr(OH)4 at pH of ∼10 under a reflux condition of water caused an inevitable dissolution of silicon species from glassware, which eventually altered the surface acid–basic properties of the amorphous Zr(OH)4. The number of Lewis and total acidic as well as medium-strength basic sites, prepared by simply changing the aging days of Zr(OH)4, was mainly responsible for an enhanced LA conversion of 91.5% and GVL selectivity of 77.6% with an apparent activation energy of ∼33.9 kJ/mol over Zr(OH)4 aged for 7 days [ZrOH (7)]. In addition, catalytic and thermal stabilities of ZrOH (7) were found with a stable LA conversion with smaller coke depositions, which were attributed to the stable preservation of the active sites caused by a proper amount of thermally stable Zr–O–Si phase formation.