Hierarchical Zr-BEA zeolites as catalysts for the transformation of levulinic acid to γ-valerolactone under mild conditions

Abstract

An important stage of cascade transformation of lignocellulosic biomass to value-added chemicals is the conversion of levulinic acid and its esters in γ-valerolactone, which has a potential application as a bio-based solvent, an intermediate in the production of polymers, food additives and bio-fuels. Herein, hierarchical Zr-BEA zeolites have demonstrated high catalytic performance and reusability in this reaction under mild conditions (115 °C, atmospheric pressure). The novel approach for the preparation of these zeolites with various Zr content has been proposed. This two-step post-synthetic method includes dealumination of hierarchical Al-BEA obtained in the highly concentrated reaction mixture followed by zirconium incorporation via wet impregnation. The hierarchical Zr-BEA zeolite with the highest degree of zirconium incorporation, i.e. 42 %, calculated as the ratio of Lewis acid site concentration and Zr content, is obtained using wet impregnation in dry ethanol and ZrCl₄ as a zirconium source. This catalyst provides a higher initial rate of γ-valerolactone formation, which is 1.0 mmol_GVL/g_cath, as compared to commercially-based Zr-BEA, for which the rate is 0.2 mmol_GVL/g_cath. The higher reaction rate over hierarchical zeolite is associated with the improved accessibility of its strong Lewis acid sites, which are the most active in Meerwein-Ponndorf-Verley reduction. An even higher initial rate is achieved over hierarchical Zr-BEA in the conversion of butyl levulinate since this ester does not block the basic framework oxygen of active Zr–O sites, as in the case of levulinic acid. The plausible mechanism for the transformation of levulinic acid over Lewis and Brønsted acid sites has been proposed.