Reactivity studies in water on the acid-catalysed dehydration of psicose compared to other ketohexoses into 5-hydroxymethylfurfural

The conversion of the four possible ketohexoses (fructose, tagatose, sorbose and psicose) into 5-hydroxymethylfurfural (HMF) was explored in water using sulphuric acid as the catalyst (33 mM H2SO4, 120 degrees C). Significant differences in reactivity were observed and tagatose (48% conversion after 75 mM) and psicose (35% conversion after 75 mM) were clearly more reactive than fructose and sorbose (around 20% conversion after 75 mM). The selectivity to HMF was found to be higher for fructose and psicose than for tagatose and sorbose. 2-Hydroxyacetylfuran (HAF) was shown to be a by-product for mainly sorbose and tagatose (as high as 2% yield). The results indicate that the relative orientation of the hydroxyl groups on C3 and C4 has a major effect on the reactivity and selectivity. This suggests that the dehydration towards HMF takes place via a mechanism with cyclic intermediates in which the C3-C4 bond is fixed in a ring structure. A reaction mechanism involving a bicyclic structure is proposed to explain the formation of HAF. The reactivity of the sugars was significantly lower in water than previously observed in methanol. (C) 2017 Published by Elsevier Ltd