Sulfated Niobia and Zirconia Catalysts for the Synthesis of Ethyl Levulinate

Abstract

The conversion of levulinic acid (LAc) into ethyl levulinate represents a relevant strategy to add value to compounds derived from lignocellulosic biomass-based biorefineries. This work reports the synthesis, characterization, and catalytic evaluation of catalysts based on sulfated niobium pentoxide and zirconium dioxide. For sulfation, wet impregnation followed by filtration (WI-F) or evaporation (WI-E) was studied, using sulfuric acid and ammonium sulfate as sulfur sources, leading to the development of new catalysts.The synthesized catalysts were characterized by X-ray diffraction, N₂ physisorption, laser diffraction, potentiometric titration with n-butylamine, and FT-IR with pyridine adsorption. Catalytic activity was evaluated in the esterification of LAc with ethanol, carried out at 70 and 140 °C with an alcohol:acid molar ratio of 12:1, 5% catalyst (based on LAc mass), a stirring rate of 700 rpm, and a reaction time of 5 h in a batch reactor.The catalysts were active in the esterification reaction, with those synthesized by WI-E using ammonium sulfate showing the best performance. At 70 °C, LAc conversion was low (8.6-19.7%), but increased significantly with temperature, reaching 43.9% and 98.4% at 140 °C for sulfated niobia and zirconia (via WI-E), respectively. After 15 h of reaction at 140 °C with these catalysts, conversion stabilized around 37% and 75%. A multiparametric analysis revealed significant positive correlations (p < 0.05) between LAc conversion and catalyst properties such as BET surface area, the number of strong and very strong acid sites (E₀ > 0), and the relative concentration of Brønsted/Lewis acid sites.