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On the Mechanism of Lewis Acid Catalyzed Glucose Transformations in Ionic Liquids
Author(s) -
Pidko Evgeny A.,
Degirmenci Volkan,
Hensen Emiel J. M.
Publication year - 2012
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201200111
Subject(s) - chemistry , catalysis , reactivity (psychology) , lewis acids and bases , fructose , ionic liquid , selectivity , chloride , inorganic chemistry , ligand (biochemistry) , dehydration reaction , deprotonation , organic chemistry , ion , medicine , biochemistry , alternative medicine , receptor , pathology
A complementary computational and experimental study of the reactivity of Lewis acidic CrCl 2 , CuCl 2 and FeCl 2 catalysts towards glucose activation in dialkylimidazolium chloride ionic liquids is performed. The selective dehydration of glucose to 5‐hydroxymethylfurfural (HMF) proceeds through the intermediate formation of fructose. Although chromium(II) and copper(II) chlorides are able to dehydrate fructose with high HMF selectivity, reasonable HMF yields from glucose are only obtained with CrCl 2 as the catalyst. Glucose conversion by CuCl 2 is not selective, while FeCl 2 catalyst does not activate sugar molecules. These differences in reactivity are rationalized on the basis of in situ X‐ray absorption spectroscopy measurements and the results of density functional theory calculations. The reactivity in glucose dehydration and HMF selectivity are determined by the behavior of the ionic liquid‐mediated Lewis acid catalysts towards the initial activation of the sugar molecules. The formation of a coordination complex between the Lewis acidic Cr 2+ center and glucose directs glucose transformation into fructose. For Cu 2+ the direct coordination of sugar to the copper(II) chloride complex is unfavorable. Glucose deprotonation by a mobile Cl − ligand in the CuCl 4 2− complex initiates the nonselective conversion. In the course of the reaction the Cu 2+ ions are reduced to Cu + . Both paths are prohibited for the FeCl 2 catalyst.