Base-Catalyzed Cascades of Monosaccharide Conversion to Formic Acid: Isotope Tracking Reveals Pathways and Their Optimal Usage under Mild Conditions in Water

Abstract

Carbohydrates are promising substrates for future precursors and fuels if operationally simple and high-yielding processes are developed. Attractive products include formic acid as a liquid-phase hydrogen-storage material that is safe to handle. Formate can be formed through the conversion of abundant monosaccharides such as glucose in aqueous alkaline solution of hydrogen peroxide. Details of this conversion and strategies for its optimal usage have remained elusive. Here, we show that real-time isotope tracking and quantitative NMR provide insights into the complex pathways of glucose conversion to formate and its byproducts. Contrary to previous belief, fructose is not a byproduct of these reactions. Instead, monosaccharides degradation and product composition is governed by competing (i) C–C bond cleavage adjacent to carbonyl groups and (ii) isomerization via the Lobry de Bruyn–Van Ekenstein transformation. The stronger acidity of hydrogen peroxide compared to protons in glucose is found to support glucose conversion to formate under moderately basic conditions. Alternative media with moderate basicity, such as silicate and phosphate salts instead of hydroxides, can provide efficient glucose conversion. By contrast, highly alkaline environments and insufficient oxidant concentrations increase glucose isomerization to fructose, which rapidly converts to C2–C5 aldonic acids as byproducts. High oxidant concentration and substrate configurations that ensure low aldose-to-ketose isomerization rates favor the formation of formate over C2–C5 aldonic acids by more than 20:1. Overall, insight into the competing pathways in the degradation cascade suggests strategies for optimizing the conversion of common aldoses to formate under kinetic control.