Two-Stage Acid-Catalyzed Conversion of Carbohydrates into Levulinic Acid

In this paper, we report on a two-stage acid-catalytic conversion of carbohydrates from hybrid poplar wood chips into levulinic acid (LA), a renewable platform chemical which can be used for fuels and chemicals. It was hypothesized that under the harsh acid conditions utilized during LA production, the pentose fraction in biomass would first form furfurals which would then polymerize with saccharides forming humins. These reactions would both reduce any potential value from the pentose fraction as well as lower the levulinic acid yield. To test this idea, a two-stage conversion process was designed starting with a mild acid extraction to remove the majority of the pentoses while maintaining the hexose sugars in a solid form utilizing previously described optimized conditions. LA was then produced by subjecting the extracted solids to a second more severe step. The temperature, time, acid concentration, and a liquor-to-wood ratio were varied and modeled to find the optimal conditions. The best conditions were high acid concentration, high temperature, and low substrate consistency, which produced a maximum molar yield of 66% based on the hexose content or 17.5 wt % based of the initial biomass. A comparison of this two-stage process to a single-stage process without the pentose extraction was performed both with biomass and model compounds, which indicated no pentose or furfural in the product stream and a marked molar yield decrease in LA yield from the presence of pentose sugars, validating our initial hypothesis.