Synthesis of glycerol levulinates in the presence of Sn(IV) catalysts: Evaluation of the catalyst nature and reactional parameters

Abstract

This study investigated, for the first time, the use of a series of complexes based on Sn(IV) as catalysts in the esterification of levulinic acid (LA) with glycerol (GLY): dibutyltin dichloride (Bu₂SnCl₂), dimethyltin dichloride (Me₂SnCl₂), butyltin trichloride (BuSnCl₃), methyltin trichloride (MeSnCl₃), butylstanic acid (BuSnO(OH)), di-n-butyltin oxide (Bu₂SnO) and dibutyltin dilaurate (Bu₂SnLau₂). The influences of various reaction conditions, including catalyst nature and loading, reaction time, proportion of reactants, and temperature, were investigated. The most active Sn(IV) complex (BuSnCl₃; k_ap = 52,8 × 10⁻³ min⁻¹) led to a GLY conversion of ∼ 74 % in the first 15 min and ∼ 97 % in 360 min, with a total formation of 26.5 % GML, 56.0 % GDL and 14.0 % GTL (360 min, 100 °C; molar ratio LA:GLI:CAT of 4:1:0.01). From the apparent rate constants (k_ap) determined at various temperatures, it was possible to estimate the activation energy (AE) for the GLY conversion, which was 12.5 kJ mol⁻¹ in the presence of BuSnCl₃ and 24 kJ mol⁻¹ for the reaction performed without a catalyst, representing a decrease of approximately 50 % in the AE. This auspicious result associated with the comparison with reactions performed and using conventional Bronsted acid catalysts (hydrochloric acid (HCl) and p-toluenesulfonic acid (p-TsOH)) shows the potential of systems based on Sn(IV), without the drawbacks associated with the use of the aforementioned acids, such as reactor corrosion, among others.