A Kinetic Investigation of Solvent Effects on Fischer Esterification for Biobased Chemical Production

The production of biomass-sourced esters by joint biological-chemical synthesis is a current subject of interest, as these high-value chemicals have applications as fuel additives, polymers, pharmaceuticals, and more. A key aspect of the process is the acid-catalyzed esterification of platform intermediates, but solvent effects on the reaction kinetics remain unclear. Previous work on the esterification of butanol and butyric acid suggested that a change in solvent from THF to toluene decreases the reaction order, potentially caused by site-blocking and asymmetric changes in activities of the reactants, products, and kinetically-relevant transition states. This study improved on the experimental procedures used in previous work to better capture the initial rate regime and extend the range of targeted concentrations. Experiments were run in isothermal, well-mixed batch reactors using Amberlyst-46 as an acid catalyst. Concentrations were quantified using GC-FID. Rates were determined by linear fitting of catalyst-normalized butyl butyrate concentrations over the reaction timescale. Fractional orders of about 0.7 were found for both reactants in the 0.1 to 60 mM concentration range. Rate order plots displayed no visible curvature, suggesting minimal influence of site blocking. Nonlinear regression modeling showed that adsorbed butanol was the most plausible abundant surface intermediate, although differences between model fits were small in comparison to the uncertainty present, suggesting that the kinetics data alone cannot be used to make a definitive assignment of which species is abundant on the surface. Future work intends to further extend the concentration range and reduce catalyst loading to limit conversion for high-leverage experiments. Characterization of potential solvent effects on selective may also be an important avenue of study.