Construction of mesoporous Cu/ZrO2-Al2O3 as a ternary catalyst for efficient synthesis of γ-valerolactone from levulinic acid at low temperature

A ternary base metal catalyst, which contained metallic Cu, oxygen deficiencies derived from ZrO2, and acidic Al2O3, was constructed for the efficient synthesis of γ-valerolactone by levulinic acid hydrogenation under mild conditions. Excellent catalytic performance was obtained by fine tuning the proportion of these three active components. The optimized Cu/Zr-Al-3 catalyst achieved superior catalytic activity with a high mass activity of 220.5 mmol GVL·h−1·g−1 surf. Cu at 130 °C, which was nearly eight times higher than Cu/Al and nine times higher than Cu/Zr. More importantly, Cu/Zr-Al-3 also possessed excellent recycling stability, making it as a potential catalyst for industrial applications. The presence of ZrO2 in the ternary catalysts weakened the Cu–Al2O3 interaction and formed Zr–O–Al interaction in the form of Al2O3–ZrO2 solid solution, which facilitated the reduction of CuO species and in turn enhanced the formation of more Cu0 active sites on the catalyst surface. The existence of a large number of oxygen vacancies and acid sites on the optimized ternary catalysts was also demonstrated by XPS and NH3 temperature-programmed desorption results. Levulinic acid can be adsorbed strongly on oxygen vacancies by its carboxylic or carbonyl groups and then readily attached by active H atoms dissociated on Cu0 sites to form active intermediates, while the acid sites presented on the ternary catalyst surface promote subsequent dehydration and intramolecular esterification of adsorbed active intermediates and consequently facilitate the formation of γ-valerolactone. The adsorption of levulinic acid onto oxygen vacancies also avoided the leaching of active Cu metal via the formation of soluble Cu metal carboxylate complexes, resulting in outstanding recycling stability for the optimized ternary catalysts. The intimate synergies of these ternary active sites mainly accounted for the superior catalytic activity and excellent recycling stability of the Cu/Zr-Al-3 catalyst.