Kinetics of Homogeneous Bronsted Acid Catalyzed Fructose Dehydration and 5-Hydroxymethyl Furfural Rehydration: A Combined Experimental and Computational Study

We perform the first extensive experimental kinetic studies of fructose dehydration and 5-hydroxymethyl furfural (HMF) rehydration at low temperatures over a wide range of conditions (T similar to 70-150 degrees C; pH values 0.7-1.6 and initial concentrations of fructose (5-20%w/v) and HMF (2.5-10%w/v)). Guided from insights from our first-principles calculations, we perform kinetic isotope effect (KIE) experiments of labeled fructose to validate the rate-limiting step. Subsequently, we develop the first skeleton model for fructose dehydration and HMF rehydration that integrates the fundamental kinetic experiments and accounts for the KIE, as well as the distribution of fructose tautomers, which changes significantly with temperature, and a direct path of fructose conversion to formic acid. It is shown that the skeleton mechanism of two steps consisting of fast protonation and dehydration followed by intramolecular hydride transfer as the rate-limiting step can capture the experimental kinetics and KIE experiments well. Fructose dehydration is found to result in stoichiometric excess of formic acid relative to levulinic acid, produced directly from fructose. All reactions are shown to be pseudo-first order in both catalyst and substrate. These insights are incorporated in a continuous flow reactor model; higher temperatures improve the optimum yield of HMF, while HMF selectivity at low conversions is less sensitive to temperature.