Successful carbohydrate recovery from lignocellulosic biomass requires breaking intermolecular bonds in glucan and xylan chains while avoiding further reaction of the resulting glucose and xylose9. However, in neutral or dilute aqueous acid solutions (<10 wt % mineral acid systems), the resulting glucose further reacts to yield furans or other degradation products. The glucose degradation reactions significantly outpace cellulose depolymerization at temperatures below 250-350° C. depending on the acid content. This leads to the need for reaction systems with short residence times (10 ms to 1 min) at high temperatures (250-400° C.) in order to obtain a high selectivity to glucose at high conversion9,10, while minimizing degradation of the desired glucose product. These types of short residence time reactions are especially impractical when using heterogeneous starting products such as biomass. Higher yields are obtainable at lower temperatures and longer residence times using increased homogeneous catalyst concentrations such as high mineral acid concentrations and/or ionic liquids5,8. However, in both cases, the homogeneous catalyst is a very significant expense. Thus, recovering the catalyst is critical for the commercial viability of these processes. Ultimately, recovering and recycling the catalysts ends up being a significant component of the processing costs5,8,11. Cellulase enzymes operating at only 50° C. can achieve near complete conversion of cellulose. However, in these processes, the cellulose must be rendered accessible by a thermochemical pretreatment of the raw cellulosic feed stock. Both enzyme and pretreatment costs are significant obstacles toward the successful commercialization of these processes. For example, enzyme costs are consistently shown to account for between US $0.50 and $2.00 per gallon of ethanol (2009 dollars), a significant portion of the overall cost of production. See, for example, Heather L MacLean and Sabrina Spatari 2009 Environ. Res. Lett. 4 014001 and David B Wilson 2009 Curr Opin Biotechnol Volume 20, Issue 3, Pages 295-299.
Strategies have been developed to successfully produce glucose and xylose from biomass while avoiding further degradation despite using low catalyst concentration and low temperature. One such strategy involves flowing a solvent through a heated packed bed of biomass in a flow-through reaction system. This approach decouples the residence times of the solid carbohydrate polymer from its soluble counterpart12,13. These systems are typically limited by their ability to produce reasonably concentrated product solutions. Indeed, using an aqueous solution of 1 wt % H2SO4 as the extraction solvent, glucose yields of only 45-55% are achieved when using a 2-4 wt % sugar solution as the feedstock12.
In recent work, GVL-water solutions coupled with very dilute acid concentrations (>0.1 M H2SO4) or solid acid catalysts have shown the ability to solubilize lignocellulosic biomass and promote dehydration of glucose to levulinic acid and of xylose to furfural14-16.
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