Simultaneous Dehydration of Glucose and Xylose Present in a Process-Relevant Biorefinery Hydrolysate to Furfurals Using Heterogeneous Solid Acid Catalysts

In this work, we report the simultaneous dehydration of glucose and xylose present in a process-relevant biorefinery hydrolysate to furfural and 5-hydroxymethylfurfural (HMF) using heterogeneous solid acid catalysts in a microwave reactor. Initially, several solid acid catalysts with varied Brønsted and Lewis acidity were screened to evaluate their activity and selectivity in dehydration of pure glucose to HMF. A noticeable improvement in HMF yield from dehydration of 8 wt % glucose was obtained by combining an acidic ion-exchange resin (Purolite CT-275DR) with an amorphous silica-alumina catalyst (Davicat-3115) resulting in HMF yields of 27–33% using a homogeneous solvent system of aqueous dioxane (dioxane/water, 2:1 v/v) at 195 °C in 5 min. Under the same reaction conditions, catalysts, and solvent system but with the addition of NaCl in catalytic amounts (33–100 mM), a more than 2-fold increase in HMF yields (66–70%) was achieved for the dehydration of 8 wt % glucose, whereas furfural yields approaching 95% were achieved for the dehydration of 6 wt % xylose, when conducted separately. Notably, using the same catalyst and solvent system while slightly modifying the reaction conditions to 197 °C and 5 min, simultaneous dehydration of 4 wt % xylose and 9 wt % glucose present in a process-relevant corn stover hydrolysate resulted in furfural and HMF yields of 96 and 74%, respectively, resulting in a combined furfural yield of 80%. The results further showed that the pH of the reaction solution played an important role in maximizing product yields. A pH < 2 resulted in low HMF yields due to the increased formation of HMF degradation products, whereas a pH 2–3 gave high HMF yields by possibly stabilizing the reaction intermediates and product, suppressing the occurrence of side reactions.