Aqueous conversion of monosaccharides to furans: were we wrong all along to use catalysts?†
Dehydration of the most relevant biomass derived monosaccharides, xylose, glucose and fructose, was investigated to attain value-added platform chemicals: furfural, hydroxymethylfurfural (5-HMF) and levulinic acid (LA). Reaction kinetics were studied in an aqueous environment in the presence and absence of the H-BEA zeolite. H-BEA demonstrated high activity, especially for isomerization of monosaccharides and rehydration of 5-HMF, while this work demonstrated that dehydration of sugars was governed predominantly by homogeneous reactions. Saccharide reactivity followed the trend of fructose > xylose > glucose, while 5-HMF was more susceptible to further conversion compared to furfural. The addition of H-BEA compromised 5-HMF yields (being an intermediate), and facilitated its further conversion to levulinic acid. The absence of a solid acid catalyst resulted in significant amounts of furfural and 5-HMF, whereas the addition of H-BEA resulted in higher yields of levulinic acid (26 mol% and 30 mol%). Excellent agreement between experimental and modelled values allowed the use of kinetic parameters for predictive modelling and optimization of the process conditions for maximizing the yields of 5-HMF and LA. The optimization was experimentally validated, where homogeneously catalyzed dehydration of fructose resulted in the highest 5-HMF yield (52 mol%) attained in 34 min at 234 °C due to the high activation energy (139 kJ mol−1) and reaction rate constants. In contrast, the highest LA yield was achieved at moderate operational temperatures (165 °C) and longer reaction times (10 h) as a result of a significantly lower energy barrier (60 kJ mol−1) determined for rehydration reactions.