The isomerization of glucose to fructose represents a key intermediate step in the conversion of cellulosic biomass to fuels and renewable platform chemicals, namely, 5-hydroxymethyl furfural (HMF), 2,5-furandicarboxylic acid (FDCA), and levulinic acid (LA). Although both Lewis acids and Brønsted bases catalyze this reaction, the basecatalyzed pathway received significantly less attention due to its lower selectivity to fructose and the poor yields achieved (<10%). However, we recently demonstrated that homogeneous organic Brønsted bases present a similar performance (∼31% yield) as Sn-containing beta zeolite, a reference catalyst for this reaction. Herein, we report on the first extensive kinetic and mechanistic study on the organic Brønsted base-catalyzed isomerization of glucose to fructose. Specifically, we combine kinetic experiments performed over a broad range of conditions (temperature: 80−120 °C; pH 9.5−11.5; reactant: glucose, fructose) with isotopic studies and in situ 1 H NMR spectroscopy. Pathways leading to isomerization and degradation of the monosaccharides have been identified through careful experimentation and comparison with previously published data. Kinetic isotope effect experiments were carried out with labeled glucose to validate the rate-limiting step. The ex situ characterization of the reaction products was confirmed using in situ 1 H NMR studies. It is shown that unimolecular (thermal) and bimolecular (alkaline) degradation of fructose can be minimized independently by carefully controlling the reaction conditions. Fructose was produced with 32% yield and 64% selectivity within 7 min.

Fructose is a key intermediate in the conversion of cellulosic biomass to biofuels and renewable platform chemicals. Biomass-derived glucose can be isomerized to fructose using either Lewis acid or Brønsted base catalysts. Lewis acids are typically preferred as alkaline conditions promote a large number of side reactions. It is widely admitted that only low fructose yields, below 10%, are achievable with inorganic bases. Here, fructose was synthesized with 32% yield using commercially available organic amines. Glucose conversion and fructose selectivity were comparable to Lewis acids, which opens new perspectives for the base-catalyzed pathway.