The conversion of lignocellulosic biomass into fuels and chemicals requires effective utilization of the C₅ and C₆ sugars present in hemicellulose and cellulose, respectively, by either processing these fractions together or separating and processing them separately. While simultaneous processing, such as in gasification or pyrolysis, offers the potential for simplicity of operation, the fractionation of hemicellulose and cellulose allows the processing of each fraction to be tailored to take advantage of the different chemical and physical properties of these fractions, and provides increased flexibility of operation.

For example, chemical processing methods can be employed to convert C₅ sugars into fuels/chemicals in hemicellulose, while employing recent advances in biological conversions allows to convert the C₆ sugars in cellulose into fuels and/or chemicals. One can also take advantage of the physical properties of cellulose for pulp and paper applications.

Herein, we show that the hemicellulose fraction of lignocellulosic biomass can be converted into furfural [FuAL] and levulinic acid [LA] by using biphasic reactors with alkylphenol solvents that selectively partition furanic compounds from acidic aqueous solutions. These furfural and levulinic acid products are valuable compounds for a variety of chemical applications, and they serve as precursors for the synthesis of liquid transportation fuels.

—Gürbüz et al.

Earlier work on the production of furfural from C₅ sugars (i.e., xylose) suffered from the low concentrations of xylose (1–2 wt %) from hemicellulose deconstruction, the team noted. Although other work using ion-exchange resin catalyst showed good yield, the regeneration of the catalysts is problematic, they added.

The Dumesic team’s strategy is to use a biphasic system consisting of an extractive organic layer and an aqueous layer that contains a mineral acid. These biphasic systems achieve high concentrations of FuAL and LA, enabling the recovery of both products at the top of distillation columns, and eliminating issues related to deactivation and regeneration of solid acid catalysts, the authors said.

The basic steps of the process include:

• Solid biomass is subjected to mild pretreatment in a dilute-acid, aqueous solution to solubilize the hemicellulose as xylose.

• After filtering the solution from the solid cellulose and lignin, an organic solvent is added to the aqueous solution, and these liquids are heated in a biphasic reactor to achieve dehydration of xylose to FuAL.

• FuAL can be distilled from the solvent and sold as a chemical or converted to LA by first hydrogenating FuAL to furfuryl alcohol (FuOH) over a metal-based catalyst (e.g. , copper) and then reacting the FuOH with water in a biphasic reactor to form LA.

• Similar to FuAL, the LA product can be distilled from the organic solvent and sold as a chemical.

In the paper, they demonstrated three organic solvents—2-sec-butylphenol (SBP), 4-n-hexylphenol (NHP) and 4-propyl guaiacol (PG)—to be effective extracting agents for the production of FuAL and LA in these biphasic systems. These solvents (i) have high partition coefficients for extraction of FuAL, FuOH, and LA; (ii) do not extract significant amounts of mineral acids from aqueous solutions; (iii) have higher boiling points than the final product; and (iv) could potentially be synthesized directly from biomass (i.e., lignin), such that these solvents would not have to be transported to the site of the biomass conversion steps.

The biorefining strategy outlined here offers the production of FuAL and LA from the hemicellulose portion of lignocellulosic biomass utilizing biphasic systems with new solvent systems. In the case of xylose dehydration, the presence of an extractive organic solvent enables the continuous removal of the highly reactive product (FuAL) from the acidic aqueous medium to prevent further degradation. In the case of FuOH hydrolysis, the biphasic system avoids high concentrations of the highly reactive reactant and/or intermediates in the acidic aqueous medium to prevent oligomerization reactions of FuOH that result in the formation of solid humins.

In both of these reaction systems, the organic solvent extracts the majority of the FuAl and LA products, enabling the separation of these valuable products from the mineral acid in the aqueous layer. Use of solvents such as SBP, NHP and PG allows for the production of FuAL and LA at higher concentrations compared to monophasic reactions in water, leading to more efficient separation of these products at the top of distillation columns.

—Gürbüz et al.

Resources

• Gürbüz, E. I., Wettstein, S. G. and Dumesic, J. A. (2012) Conversion of Hemicellulose to Furfural and Levulinic Acid using Biphasic Reactors with Alkylphenol Solvents. ChemSusChemdoi: 10.1002/cssc.201100608