Elucidating the Role of Addenda and Hydrogen Atoms on the Acidity of Heteropolyacid to Produce Butyl Levulinate as a Green Diesel Additive

Abstract

The present study explores the synthesis of butyl levulinate (BL), a green diesel additive, using vanadium-incorporated Keggin phosphomolybdic acid (HPVM) catalysts. This study highlights the effect of addenda and hydrogen atoms on HPVM catalysts for BL production, thereby elucidating a correlation between addenda atoms, HPVM acidity, and BL yield. The catalysts are characterized using Raman spectroscopy, FTIR, XRD, ammonia-TPD, SEM-EDX, and ICP-OES techniques to determine their structural properties, acidity, and elemental composition. Reaction parameters, including time, temperature, catalyst concentration, stirring rate, and solvent volume, are systematically evaluated to optimize the conversion of levulinic acid (LA) to BL. A significant BL yield (>96%) is obtained using 25 wt.% H₄PMo₁₁VO₄₀ (H₄PVM) catalyst at 100 °C in 40 min. Kinetic analysis unveiled a pseudo-first-order reaction mechanism for BL synthesis with activation energies of 66.73, 68.81, and 71.19 kJ.mol⁻¹ for H₄PMo₁₁VO₄₀ (H₄PVM), H₅PMo₁₀V₂O₄₀ (H₅PVM), and H₆PMo₉V₃O₄₀ (H₆PVM) catalysts, respectively. Additionally, thermodynamic parameters, such as enthalpy of activation (ΔH*), entropy of activation (ΔS*), and Gibbs free energy of activation (ΔG*), are determined. Overall, this study highlights the complex interplay between acidity, activation energy, and BL yield, emphasizing the significance of the number of vanadium addenda atoms in catalyst performance.