(629f) Effect of Metal Chlorides on Glucose Mutarotation and Implications on Humin Molecular Structure

The conversion of biomass derived monosacharides through catalytic pathways is an attractive approach towards the production of fuels and/or value-added chemicals. Among the various proposed thermocatalytic pathways, the dehydration of hexoses to 5-Hydroxymethylfurfural (5-HMF), a chemical which is considered a key intermediate in biomass processing, has received considerable attention. In this reaction network, the interconversion of sugars plays an import role. The presence of Lewis acids has reported to promote glucose ring-opening followed by a kinetic relevant intramolecular hydride shift from C₂ to the C₁ position of glucose¹. In addition, the possible changes in the anomeric equilibrium especially via the stabilization of the α-anomer of glucose have been highlighted as being among the reasons for improved selectivity towards fructose².

In this work, we evaluate in-situ Raman spectroscopy to study the effect of commonly used homogeneous Lewis acid catalysts on the glucose mutarotation, a step that occurs simultaneously with the isomerization reaction. First, a thorough analysis on the skeletal vibrational modes of glucose is undertaken in order to find a characteristic set of vibrational bands that can be used to monitor not only the final equilibrium level (as has reported in the open literature) but also the dynamic interconversion of glucose anomers. Next, by utilizing this set of peaks we find that the glucose anomerization rates follow the order of AlCl₃<CrCl₃<SnCl₄. In addition, we find that the presence of CrCl₃, even at small quantities, appears to promote the formation of α-glucose anomer probably due to strong interaction of α-glucose and the dissolved metal species. The effect of the nature of metal chlorides used on the molecular structure of byproducts, i.e. humins, is also studied by means of a combined Raman and ATR-FTIR approach³.

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