Effects of γ-valerolactone in hydrolysis of lignocellulosic biomass to monosaccharides†

The current transition to a sustainable source of chemicals and energy is being driven by political, economic, and environmental concerns associated with petroleum-derived feed stocks. In this context, the conversion of lignocellulosic biomass, a renewable resource, into platform molecules and fuels has received increasing interest. In order for biomass to effectively contribute to reducing our dependence on petroleum, its efficient utilization is vital. Thus, major importance is being placed on the conversion of the hemicellulose (C5 sugars) and cellulose (C6 sugars) fractions of biomass into platform molecules, which then can, in turn, be further upgraded into chemicals and fuels. Recently, the use of organic solvents has been shown to be beneficial in the chemical conversion of lignocellulosic biomass. One such solvent is γ-valerolactone (GVL), which can be obtained from biomass and displays significant improvements in reaction performance for biomass conversion reactions compared to conversion in aqueous media, such as increased catalytic activity and higher selectivity to desired reaction products. For example, high yields of levulinic acid (∼70%) from cellulose using GVL as solvent have been achieved using Amberlyst 70 as catalyst compared to yields as low as 20% obtained in water. In addition, significant selectivity increases have been shown using GVL as solvent compared to reactions in water for the conversion of C5 sugars 3 and C6 sugars to their corresponding furanic components furfural and 5-hydroxymethylfurfural (HMF), respectively, which are valuable platform molecules. Previously, the production of monosaccharides from biomass in aqueous media has proven to be difficult due to the subsequent conversion of these sugars to their corresponding furanic components or degradation products, and therefore, current methods of monosaccharide production from biomass are cost prohibitive. However, we have recently developed a processing strategy to produce concentrated streams of C5 and C6 sugars (e.g., 130 g L ) from the cellulose and hemicellulose fractions of lignocellulosic biomass in GVL–H2O solvent mixtures using dilute concentrations of mineral acids (e.g., 0.005 M) at mild temperatures (e.g., 430–490 K) without using enzymes or ionic liquids. After separation, the resulting aqueous stream of soluble sugars offer a versatile platform for subsequent upgrading by chemical or biological processes. These results indicate that GVL is a promising solvent for biomass processing reactions; however, the fundamental scientific basis for solvent effects in biomass conversion reactions is limited at present. In the present study, we quantify the effects of GVL as a solvent with respect to changes in rates for acid-catalyzed biomass hydrolysis reactions compared to reactions carried out in water. Furthermore, we compare apparent activation energies for acid-catalyzed biomass hydrolysis and monosaccharide conversion reactions using both water and GVL as solvent. In particular, the liquid phase hydrolysis of cellobiose (i.e., a disaccharide of glucose units connected via a β(1→4) glycosidic bond) is catalyzed by acid and serves as a probe reaction in the present study (Scheme 1). This reaction produces glucose, which is a valuable biomass-