Highly selective synthesis of γ-valerolactone from levulinic acid at mild conditions catalyzed by boron oxide doped Cu/ZrO2 catalysts

Abstract

Cu/ZrO2 has been recently demonstrated as a promising non-precious metal catalyst for the efficient hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL), while suffering from insufficient catalytic activity and stability with relatively high reaction temperature (up to 200 °C). Herein, a series of Cu/ZrO2 catalysts modified with boron oxide was prepared and applied for hydrogenation of LA to GVL. The introducing boron oxide effectively improved the activity and lifespan of Cu/ZrO2 catalyst for hydrogenation of LA into GVL. The optimal 3BCu/ZrO2 catalyst afforded complete LA conversion along with 100% GVL yield at 150 °C low temperature. Besides, 3BCu/ZrO2 showed outstanding long-term stability and ran stably with GVL yield up to 90% for at least 100 h in continuous fix-bed tests, making it a promising catalyst of industrial applications for LA hydrogenation. Furthermore, the boron oxide doped and free Cu/ZrO2 catalysts were fully characterized by using ICP, BET, N2O titration, XRD, XPS, H2-TPR/TPD, and NH3-TPD. Boron oxide strongly interacted with surface copper species, which in turn restrained the aggregation of copper particles and improved the copper dispersion during thermal treatment and reaction. The strong interaction of boron oxide with surface copper species mainly accounted for the excellent catalytic activity and stability of boron oxide doped Cu/ZrO2 catalysts. Besides, doping boron oxide also increased specific surface area, modulated distribution of surface Cu0 and Cu+, facilitated adsorption of H2, and improved surface acidity of Cu/ZrO2 catalysts, which also played positive roles on the catalytic activity.