Sulfated niobia and zirconia catalysts for the synthesis of ethyl levulinate

Abstract

The esterification of levulinic acid into ethyl levulinate is a key reaction for biomass valorization, and the development of highly active and stable solid acid catalysts is essential to improve process efficiency. In this context, this work evaluates how sulfation strategies affect the acidic properties and catalytic behavior of niobium pentoxide- and zirconium dioxide-based catalysts in levulinic acid esterification with ethanol. Catalysts were synthesized by wet impregnation followed by filtration or evaporation, using sulfuric acid or ammonium sulfate as sulfur sources, and characterized by X-ray diffraction, N₂ physisorption, laser diffraction, potentiometric titration with n-butylamine, and FT-IR with pyridine adsorption. Sulfation modified not only the density and strength of acid sites, but also their Brønsted/Lewis nature, promoting the formation of strong and very strong acid sites, particularly in zirconia-based catalysts. Catalytic tests showed that sulfated zirconia prepared by wet impregnation–evaporation with ammonium sulfate achieved the highest performance, reaching up to 98% conversion at 140 °C. A multiparametric analysis revealed statistically significant correlations between levulinic acid conversion and BET surface area, the number of strong and very strong acid sites, and the relative Brønsted/Lewis acid site concentration, demonstrating that catalytic activity is governed by the generation of strong Brønsted acid sites and their stability under reaction conditions.