Understanding reaction kinetics, deprotonation and solvation of bronsted acidic protons in heteropolyacid catalyzed synthesis of biorenewable alkyl levulinates

In search of a 'descriptor' for Bronsted acid-catalyzed biorenewable transformations in a complex reaction environment, two concepts related to the reactivity of Bronsted acid catalysts are explored. A simple reaction involving the esterification of levulinic acid in three different alcohol mediums (ethanol, 1-propanol, and 1-butanol) is experimented with two different Keggin heteropolyacid (HPA) catalysts to synthesize alkyl levulinates. On the same HPA catalyst, and different solvent medium, apparent activation energies of the esterification reaction are observed to increase by an average of similar to 5 kJ/mol on increasing the alkyl chain length of the alcohol medium by one carbon. Obtained apparent activation energies are corresponding with the solvation energies of the Bronsted proton in the respective alcohol medium. In contrast, on changing the HPA catalyst and keeping the same alcohol medium, the apparent activation energies are observed to differ by an average of similar to 19 kJ/mol. This directly correlates with the difference (similar to 20 kJ/mol) in the vapor phase deprotonation energies (DPE) of the two HPA catalysts. Thus, in the solvent environment, DPE values and the degree of solvation of the Bronsted acidic protons are describing the reactivity of the HPA catalysts.