Acid-Catalysed Furfural Conversion: Enhanced Kinetic Insights into Formic Acid and Humin

Abstract

Furfural is a key platform molecule derived from lignocellulosic biomass and an essential precursor for biofuels, chemicals, and polymer materials. Its stability under acidic hydrothermal conditions critically affects biomass conversion efficiency. While numerous studies have explored furfural formation and xylose conversion, a comprehensive kinetic model that quantitatively captures furfural degradation pathways—particularly to formic acid and humins—and their interactions remains limited. This study addresses this gap by evaluating multiple kinetic models to identify the most accurate representation of furfural conversion behavior and by determining detailed kinetic parameters. Experiments were performed by reacting furfural with sulfuric acid at 140-180 C using furfural concentrations of 0.05–0.63 M and acid concentrations of 0.05-1.00 M. Among the tested models, Model 3 showed the highest predictive accuracy, with R² values of 98.04% for furfural conversion and 90.35% for formic acid formation. The estimated activation energies (Ea1 = 107.84 ± 0.53 kJ mol^-1, Ea2 = 81.75 ± 0.76 kJ mol^-1, and Ea4 = 117.07 ± 0.96 kJ mol^-1) indicate that humin formation has the lowest energetic barrier. In contrast, the autocatalytic formation of formic acid becomes significant only at elevated temperatures. Reaction orders of 0.81 for furfural, 0.62 for sulfuric acid, and 1.49 for formic acid reveal that both catalytic acidity and autocatalysis govern the overall reaction rate. Operational conditions strongly influence product selectivity: higher acid and furfural concentrations accelerate reactions but promote humin formation, whereas lower concentrations favor formic acid production. Overall, this study provides a holistic kinetic framework that accurately predicts the behavior of key species, offering valuable insights for reactor design and process optimization in furfural-based biorefineries.