Renewable platform chemicals: Thermochemical study of levulinic acid esters

Equilibrium of the levulinic acid esterification with methanol, ethanol, and 1-butanol was studied in a broad temperature range (323 K-473 K) for reactions catalyzed by the enzyme Novozym 435 as well as for uncatalysed reaction systems. Thermodynamic equilibrium constants were derived with help of activity coefficients of reaction participants, which were predicted with the equation of state PC-SAFT. The liquid phase standard molar enthalpies of formation of methyl-, ethyl-, and n-butyl-esters of levulinic acid were measured by using the high-precision combustion calorimetry. Vapor pressures of these esters were measured by using the transpiration method. The standard molar enthalpies of vaporization of alkyl levulinates at 298.15 K were derived from vapor pressure temperature dependencies. Thermochemical data of alkyl levulinates were collected, evaluated, and tested for internal consistency. The high-level G4 quantum-chemical method was used for mutual validation of the experimental and theoretical gas phase enthalpies of formation of studied esters. Thermodynamic analysis of the levulinic acid esterification has been performed. Using the levulinic acid esterification with alcohols as the model reaction for an industrial processing of biomass conversion to fuels and useful platform chemicals, we have shown that qualitatively correct agreement between results from equilibrium study, from thermochemical data (combustion and transpiration), and from G4 calculations was observed. Reasonable combination of the quantum-chemical methods and of PC-SAFT modelling with empirical methods could serve to reduce experimental efforts for assessment of feasibility of the chemical processes of utilization of renewable feedstocks.