The Role of Metal Halides in Enhancing the Dehydration of Xylose to Furfural

Abstract

AbstractThe dehydration of xylose yields furfural, a product of considerable value as both a commodity chemical and a platform for producing a variety of fuels. When xylose is dehydrated in aqueous solution in the presence of a Brønsted acid catalyst, humins are formed via complex side processes that ultimately result in a loss in the yield of furfural. Such degradative processes can be minimized via the in situ extraction of furfural into an organic solvent. The partitioning of furfural from water into a given extracting solvent can be enhanced by the addition of salt to the aqueous phase, a process that increases the thermodynamic activity of furfural in water. Although the thermodynamics of using salts to improve liquid–liquid extraction are well studied, their impact on the kinetics of xylose dehydration catalyzed by a Brønsted acid are not. The aim of the present study was to understand how metal halide salts affect the mechanism and kinetics of xylose dehydration in aqueous solution. We found that the rate of xylose consumption is affected by both the nature of the salt cation and anion, increasing in the order no salt<K+<Na+<Li+ and no salt<Cl−<Br−<I−. Furfural selectivity increases similarly with respect to metal cations, but in the order no salt<I−<Br−<Cl− for halide anions. Multinuclear NMR was used to identify the interactions of cations and anions with xylose and to develop a model for explaining xylose‐metal halide and water‐metal halide interactions. The results of these experiments coupled with 18O‐labeling experiments indicate that xylose dehydration is initiated by protonation at the C1OH and C2OH sites, with halide anions acting to stabilize critical intermediates. The means by which metal halides affect the formation of humins was also investigated, and the role of cations and anions in affecting the selectivity to humins is discussed.