One of the most notable advances toward biorefinery in recent years was the discovery of new catalytic systems that enabled the conversion of glucose and cellulose to a number of potential platform chemicals. The first catalytic conversion of glucose to 5-hydromethylfurfural (5-HMF) was reported with the discovery that chromium [II,III] chlorides were the most effective catalysts in 1-alkyl-3-methylimidazolium chloride ionic liquids.  Specifically, CrCl2 and CrCl3 were found capable of effectively isomerizing glucose to fructose in 1-ethyl-3-methylimidazolium chloride or [EMIM]Cl solvent, which facilitated the dehydration of fructose to 5-HMF in high yield. A considerably large number of researches followed over the last six years that studied a combination of a wide variety of ionic liquids, other solvents, other catalysts, and process conditions. Importantly, the chromium chlorides remain the top performing catalysts in ionic liquids and in other solvents, including aqueous phase. Despite the enormous research progresses made on this important subject, the unique chemical bonding nature of the chromium chloride catalysts responsible for their superior catalytic performance over many other metal chlorides for the isomerization of glucose to fructose remains to be clarified. In this work, we for the first time used far infrared (FIR) as an in-situ tool, in combination with a series of carefully selected probing model molecules, to study the fundamental differences in the chemical nature and in the coordination bonding of different metal chlorides in the reaction systems consisting of carbohydrates, water, and 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid. The results allowed us to reveal the most prominent features of different classes of metal chlorides during the catalysis in-situ, and most importantly allowed us to correlate the unique chemical natures of the chromium chlorides to their superior catalytic performance in the conversion of carbohydrates in the studied systems.

Correlation between metal chloride catalysis and the coordination chemistry during carbohydrate conversions in ionic liquids

248th National Meeting (2014)

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