Molecular aspects of glucose dehydration by chromium chlorides in ionic liquids

A combined experimental and computational study of the ionic-liquid-mediated dehydration of glucose and fructose by CrII and CrIII chlorides has been performed. The ability of chromium to selectively dehydrate glucose to 5-hydroxymethylfurfural (HMF) in the ionic liquid 1-ethyl-3-methyl imidazolium chloride does not depend on the oxidation state of chromium. Nevertheless, CrIII exhibits higher activity and selectivity to HMF than CrII. Anhydrous CrCl2 and CrCl36¿H2O readily catalyze glucose dehydration with HMF yields of 60 and 72¿%, respectively, after 3 h. Anhydrous CrCl3 has a lower activity, because it only slowly dissolves in the reaction mixture. The transformation of glucose to HMF involves the formation of fructose as an intermediate. The exceptional catalytic performance of the chromium catalysts is explained by their unique ability to catalyze glucose to fructose isomerization and fructose to HMF dehydration with high selectivity. Side reactions leading to humins by means of condensation reactions take predominantly place during fructose dehydration. The higher HMF selectivity for CrIII is tentatively explained by the higher activity in fructose dehydration compared to CrII. This limits the concentration of intermediates that are involved in bimolecular condensation reactions. Model DFT calculations indicate a substantially lower activation barrier for glucose isomerization by CrIII compared to CrII. Qualitatively, glucose isomerization follows a similar mechanism for CrII and CrIII. The mechanism involves ring opening of D-glucopyranose coordinated to a single Cr ion, followed by a transient self-organization of catalytic chromium complexes that promotes the rate-determining hydrogen-shift step.