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.