Kinetic study on the impact of acidity and acid concentration on the formation of 5-hydroxymethylfurfural (HMF), humins, and levulinic acid in the hydrothermal conversion of fructose

Abstract

In order to optimize the production of biobased platform chemicals, such as 5-hydroxymethylfurfural (HMF) and levulinic acid (LA), from biomass and sugars, reaction conditions have to be optimized. This article assesses the impact of acidity on the kinetic parameters of the hydrothermal conversion of fructose at 150 °C. The reaction parameters were varied in time (5–180 min), acid (NaHSO4, H3PO4, citric acid, and formic acid), and acid concentration (0.016 to 1.6 mol/l). The experimental data were evaluated with a novel kinetic model that includes the formation of HMF, LA, and humins. While it was expected that the dehydration rate of fructose to HMF and rehydration rate of HMF to levulinic acid depend linearly on the proton concentration, the results show that there is actually an exponential relationship. This can be explained by the participation of the acid rest ion in the dehydration and rehydration reactions. In contrast, the humin formation rate depends linearly on the proton concentration, and also, the sensitivity of this relationship is rather low. This clarifies why high acid concentrations or proton concentrations, respectively, increase the formation of LA to an expense of the humin formation.