Sustainable Solvent Systems for Use in Tandem Carbohydrate Dehydration Hydrogenation

Abstract

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 13C 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.