Valorization of Humins by Cyclic Levulinic Acid Production Using Polyoxometalates and Formic Acid.

At a time when increasing attention is paid to sustainability in chemistry, levulinic acid (LA) is one of the most important platform chemicals for the goal of overcoming our dependence on fossil raw materials. However, a so far limiting obstacle on the way to efficient LA production from biomass is the formation of undesirable humin byproducts. In this work, a new catalytic route for the effective utilization of these humin byproducts, enabling a cyclic synthesis of LA using formic acid (FA) as organocatalyst is proposed. Selective catalytic oxidation (SCO) of humins using the HPVMoO (HPA-2) polyoxometalate (POM) catalyst produces FA that can be isolated from the aqueous reaction mixture by using nanofiltration membranes accompanied by a complete catalyst recycling (>99 %). After concentration of FA by distillation, the latter can be used as organocatalyst for LA production from sugars, whereby the formed humins can in turn be separated and used as substrates for further FA production via SCO to close the catalytic cycle. By using FA as a green and sustainable acidic organocatalyst, relatively high yields of LA (up to 42 mol %) could be achieved. In the future this can potentially lead to the creation of a closed cycle for an environmentally friendly and efficient production of green LA without undesired humin formation.