Impact of steric hindrance in levulinic acid esterification to levulinate esters: kinetic and thermodynamic studies

Biomass-derived levulinic acid (LA) is a promising platform chemical for the sustainable production of fuels and chemicals such as levulinate esters (LEs). The hydrochloric acid-catalyzed LA esterification reaction parameters were investigated in a batch reactor to study the impact of the co-reactant alcohol alkyl chain length and branching. The LA esterification rate laws were found to be approximately first order with respect to LA, alcohol, and HCl molar concentrations. Increasing the alcohol alkyl chain length and branching decreased the rate of LE formation with methyl > ethyl > n-propyl > isopropyl. Their activation energies were determined to be 21, 26, 34, and 31 kJ/mol, respectively. Thermodynamic analysis showed that the reactions are endothermic, endergonic, and non-spontaneous with a more ordered transition state. The decreased rate of LE formation and increased activation barrier for LA esterification with alcohols containing longer or branched alkyl chains are likely attributed to steric effects and larger structural rearrangements needed to attain the transition state. The transition states for the methyl and isopropyl LEs are relatively entropically favorable, possibly due to solvent effects.