Experimental and Comparative Analysis on Properties and Performance of SO₃H-Functionalized Carbon Catalyst in Ethyl Levulinate Synthesis from Levulinic Acid

Hydrothermal sulfonation is a method that introduces sulfonic acid groups as active sites onto suitable support materials under autogenous pressure conditions. In this work, the effectiveness of hydrothermal sulfonation was studied on various lignin-carbons prepared at different thermal conditions of 400–600 °C for 1 and 2 h. The sulfonated lignin-carbon catalysts produced were characterized and evaluated for their catalytic performance in levulinic acid (LA) esterification under selected reaction conditions to synthesize ethyl levulinate (EL). The catalyst with the highest EL yield and LA conversion was selected for extended reaction time (1–6 h) to study the time-dependent performance in LA esterification. The influence of catalyst acidity and the surface area of the prepared catalysts on the reaction behavior was assessed and further analyzed through an extensive comparative study with related literature. The results indicated that all lignin-carbon catalysts exhibited enhanced porous structures and surface areas of 193–368 m²/g, along with amorphous characteristics. Additionally, a reduction in catalyst acidity was observed, decreasing from 1.3 to 0.5 mmol/g as the thermal conditions of lignin-carbon preparation increased. The catalytic activity was found to decrease for lignin-carbon prepared at higher thermal conditions. The catalyst performance achieved 75.9 mol% EL yield (TOF of 0.02 s⁻¹) with 76.5% LA conversion, and the EL yield increased to 81.1 mol% when the reaction time was extended to 5 h. The carbon catalyst can be reused over five reaction cycles with a decrease in EL yield from 81.1 mol% to 62.2 mol%. In comparative study, catalyst acidity plays an important role in catalyzing the esterification reaction of levulinic acid to ethyl levulinate, serving as a key design indicator for carbon-based catalysts. Catalyst acidity showed a positive correlation with reaction performance, and further increases in surface area (> 200 m²/ g) did not significantly enhance reaction performance. Besides, hydrothermal sulfonation demonstrated potential as a catalyst modification method and could be recommended for carbon-based catalyst preparation.