Solvent Effects in Acid-Catalyzed Biomass Conversion Reactions

Reaction kinetics were studied to quantify the effects of polar aprotic organic solvents on the acid-catalyzed conversion of xylose into furfural. A solvent of particular importance is g-valerolactone (GVL), which leads to signifi- cant increases in reaction rates compared to water in addition to increased product selectivity. GVL has similar effects on the kinetics for the dehydration of 1,2-propanediol to propanal and for the hydrolysis of cellobiose to glucose. Based on results obtained for homogeneous Bronsted acid catalysts that span a range of pKa values, we suggest that an aprotic organic solvent affects the reaction kinetics by changing the stabiliza- tion of the acidic proton relative to the protonated transition state. This same behavior is displayed by strong solid Bronsted acid catalysts, such as H-mordenite and H-beta. The use of organic solvents is pervasive in the chemical industry, and recently it has been shown that organic solvents are beneficial in the chemical conversion of lignocellulosic biomass. (1-3) One such solvent is g-valerolactone (GVL), which can be produced from biomass and displays significant improvements in reaction performance for biomass conver- sion reactions compared to conversion in aqueous media, such as increased catalytic activity and higher selectivity to desired reaction products. (2, 3) Furthermore, we have reported that the simultaneous conversion of hemicellulose and cellulose can be achieved using GVL as a solvent in a single reactor, eliminating the need for pretreatment and/or sepa- ration steps. (4) Recently, we have taken advantage of accel- erated rates of cellulose and hemicellulose deconstruction in GVL-H2O solvent mixtures to develop a processing strategy to produce streams of C5 and C6 sugars (e.g., 130 g l � 1 ) from biomass. (5) Other polar aprotic solvents, such as g-lactones and tetrahydrofurans, have also shown comparable benefits to GVL in biomass conversion processes. (3) Herein, we report the effects of GVL and other polar aprotic solvents on acid-catalyzed biomass conversion reac- tions using acid catalysts that span a range of pKa values. The liquid-phase dehydration of xylose to furfural is catalyzed by Bronsted acids and serves as a probe reaction in the present study. We compare the reactivity trends displayed by these homogeneous acid catalysts in the liquid phase with the performance of solid acid catalysts, the latter of which have been shown to span a range of catalytic activities for the gas