Pathways and their usage in the conversion of carbohydrates by aqueous barium hydroxide: insights from hyperpolarized and quantitative NMR

Abstract

The conversion of lignocellulosic biomass and its components (carbohydrates, lignin) to precursor chemicals is expected to both expand the range of industrial chemicals and to reduce the dependence on fossil resources. Challenges in the green transition arise from the multifunctionality of biosourced reactants and from the difficulty in identifying highly selective processes for their conversion to useful precursors. Among the promising precursor molecules that can be derived from carbohydrates through chemocatalysis are lactate and its ester variants. Chemocatalytic conversion of glucose by concentrated aqueous solutions of barium hydroxide at ambient temperatures in the absence of oxygen has been described as highly selective, but high-resolution NMR or MS characterizations of the product mixture and mechanistic insights are sparse. Here, we employ sensitivity enhanced (“hyperpolarized”) NMR to track the reaction cascade of hexose conversion by barium hydroxide and directly visualize transient enol and dihydroxyacetone species as the intermediates in the chemocatalytic pathway, similar to biochemical glycolysis. Quantitative NMR indicates that the conversion of glucose by barium hydroxide under anaerobic conditions indeed is highly selective for the formation 2-hydroxy-3-deoxy acids, which can include longer equivalents than lactate. C4–C6 analogues (metasaccharinic acids) can in total account for nearly the same carbon fraction as lactate in a reaction avoiding the formation of oxidation products.