Fructose Epimerization to l-Sorbose in Water over Molybdenum Oxide: Reaction Kinetics and Mechanism Insights

Abstract

Herein, we report the significant production of l-sorbose via a chemical catalysis method over molybdenum oxide (MoO3) using fructose in a water medium. The heterogeneous catalytic setup could produce 32% l-sorbose and 55% selectivity via C5-fructose epimerization. However, the synthesis was accompanied by a minor formation of tagatose (∼4%) and allulose (∼1%) side products via 1,3-hydride shift and 1,2-hydride shift mechanisms, respectively, using the single fructose source. Moreover, the interconversion reaction (fructose–sorbose) exhibited temperature-dependent characteristics, with an activation barrier of 68.2 kJ/mol. Based on the kinetics, fructose is shown to have a preference for reaching equilibrium with sorbose, which forms through a single 1,4-hydride shift. Due to this, a delayed formation of side products was observed, which follows a series of hydride shifts between the intermediates. From the results, the catalyst is versatile for various epimerization reactions. Selective extraction of fructose from the postreaction medium for product enrichment via naphthalene-2-boronic acid liquid–liquid, however, resulted in the recovery of both fructose and sorbose (ketoses) because of identical characteristics. Thus, the catalytic setup represents an environmentally friendly and sustainable method for upscaling based on the process’s green matrices.