Non-stoichiometric formation of formic and levulinic acids from the hydrolysis of biomass derived hexose carbohydrates

This study challenges the assumption often postulated in the literature regarding the stoichiometric formation of formic and levulinic acids from the acid hydrolysis of hexose carbohydrates. Acid hydrolysis experiments are conducted with 2.5 wt% H₂SO₄ in aqueous media with a series of reactants relevant to the hydrolysis systems of hexoses; d-fructose, d-galactose, d-glucose, d-mannose, 5-hydroxymethylfurfural, d-erythrose, levulinic acid, furfuryl alcohol, furfural, dihydroxyacetone, glyceraldehyde, pyruvaldehyde and formic acid at 150 °C. We show that the hydrolysis of 5-hydroxymethylfurfural, which is the main intermediate between hexose carbohydrates and levulinic acid does result in the stoichiometric formation of formic and levulinic acids. However, in all cases with hexose carbohydrates as reactant, formic acid is observed in excess fractions to levulinic acid, implying the common assumption is inaccurate. At steady-state conversions of the reactant, the formic and levulinic acid ratio for d-fructose, d-glucose, d-mannose and d-galactose is shown to be 1.08 ± 0.05, 1.15 ± 0.08, 1.20 ± 0.10 and 1.19 ± 0.08 respectively. Combining this work and pertinent literature suggests there are at least four potential pathways depending on reaction condition responsible for the excess formic acid; through furfuryl alcohol and furfural formation and through the transformation of d-erythrose and pyruvaldehyde.