Continuous flow (Sulfated) Zirconia Catalysed Cascade Conversion of Levulinic Acid to γ-Valerolactone

Abstract

γ-Valerolactone (GVL) is a renewable and versatile platform chemical derived from sustainable carbon feedstocks. The cascade conversion of levulinic acid into GVL requires Brønsted and Lewis acid catalysed reactions. Here, a dual-catalyst bed configuration is demonstrated that promotes synergy between Brønsted acid sites in sulfated zirconia (SZ) and Lewis acid sites in ZrO2/SBA-15 for the liquid phase, continuous flow esterification and subsequent catalytic transfer hydrogenation (CTH) of levulinic acid to GVL. A saturated surface sulfate monolayer, possessing a high density of strong Brønsted acid sites, was optimal for levulinic acid esterification to isopropyl levulinate over SZ (>80 % conversion). A conformal ZrO2 bilayer, deposited over a SBA-15 mesoporous silica and possessing mixed Brønsted:Lewis acidity, catalysed CTH of the levulinate ester and subsequent dealcoholisation/cyclisation to GVL (>60 % selectivity). Maximum stable productivity for the dual-bed was 2.2 mmolGVL.gcat.h−1 at 150 °C, significantly outperforming either catalyst alone or a physical mixture of both. Flow chemistry is a versatile approach to achieve spatial control over cascade transformations involving distinct catalytically active sites.