Sustainable Solvent Systems for Use in Tandem Carbohydrate Dehydration Hydrogenation

Monophasic separation-friendly solvent systems were investigated for the sustainable acid-catalyzed dehydration of fructose to 5-hydroxymethylfurfural (HMF). The HMF selectivity depends on both fructose conversion, temperature, and the amount of cosolvent present in the aqueous solvent mixture. Use of HMF-derived 2,5-(dihydroxymethyl)-tetrahydrofuran (DHMTHF) or low-boiling tetrahydrofuran (THF) as co-solvents results in increased selectivity (>70%) to HMF at fructose conversions of ca. 80%. Analysis of the fructose tautomer distribution in each solvent system by C-13 NMR revealed higher furanose fractions in the presence of these and other protic (tetrahydrofurfuryl alcohol) and polar aprotic co-solvents (DMSO) relative to water alone. Formation of fructosides and/or difructose anhydrides in the presence of the co-solvents causes lower selectivity at early reaction times, but reversion to fructose and dehydration to HMF at longer reaction times results in increasing HMF selectivity:with fructose conversion. In 9:1 DHMTHF:water, a 7.5-fold increase in the initial rate of HMF production was observed relative to water alone. This mixed solvent system is proposed for use in a tandem catalytic approach to continuous DHMTHF production from fructose, namely, acid-catalyzed dehydration of fructose to HMF, followed by its catalytic hydrogenation to DHMTHF.