The Nature and Mechanism of Selectivity Decrease of the Acid-catalyzed Fructose Conversion with Increasing the Carbohydrate Concentration

Abstract

To understand the effect of decreasing the process selectivity with increasing the carbohydrate concentration, the influence of levulinic acid and glucose additions on the fructose acid-catalyzed conversion was studied. Levulinic acid yield significantly decreases by initial addition of levulinic acid and glucose to the reaction mixture. Humins were found to include structural fragments of levulinic acid, carbohydrates, and 5-hydroxymetylfurfural. According to the mechanism suggested, the carbocation formed from fructose interacts with water producing target products (5-HMF and levulinic acid) or with levulinic acid or another carbohydrate molecules giving humins. In these assumptions the levulinic acid formation in the process is a reaction of first order with respect to substrate concentration, and the humic substance formation is a reaction of second order with respect to the initial carbohydrate concentration. These two process characteristics, water to substrate ratio and the reaction orders distinction, are the reasons of decreasing the carbohydrate conversion selectivity with increasing the substrate concentration.