SnCl4-catalyzed isomerization/dehydration of xylose and glucose to furanics in water

Abstract

A number of Lewis acid catalysts were screened for their effectiveness in converting both xylose and glucose in aqueous media to furfural and 5-HMF, respectively. While other catalysts were found to be more active, SnCl₄ was identified as the most selective Lewis acid. Hydrolysis of SnCl₄ was observed at various concentrations and temperatures resulting in the production of Brønsted acidic protons in a 3.5 : 1 ratio to Sn⁴⁺ at all SnCl₄ concentrations above 60 °C. As a consequence, there was no need to add a Brønsted acid in order to promote the dehydration of either xylose or glucose. SnCl₄-promoted isomerization/dehydration of xylose and glucose at 140 °C in water resulted in conversions of 55% and 33%, respectively, after 2 h of reaction, and furfural and 5-HMF selectivities of up to 58% and 27%, respectively. Significant conversion of sugars to humins was observed in both cases, and in the case of glucose, degradation of 5-HMF to levulinic and formic acids was also noted. The effects of secondary reactions could be greatly suppressed by extraction of the furanic product as it was produced. Using n-butanol as the extracting agent, xylose and glucose conversions of 90% and 75%, respectively, were observed after 5 h of reaction, and the selectivities to furfural and 5-HMF increased to 85% and 69%, respectively. Small additional increases in the furfural and 5-HMF selectivities were obtained by adding LiCl to the aqueous phase without much effect on the conversion of either sugar. In this case, the selectivities to furfural and 5-HMF were 88% and 72%, respectively, after 5 h of reaction at 140 °C.