Distinctive Aldose Isomerization Characteristics and the Coordination Chemistry of Metal Chlorides in 1-Butyl-3-methylimidazolium Chloride

Abstract

The catalytic isomerization of aldoses to ketones is an important fundamental step for the transformation of cellulosic biomass to biobased chemicals and liquid fuels. The results of this work reveal for the first time the distinctive coordination chemistry features of four classes of metal chlorides, CrCl3, VCl3, FeCl3, and PtCl2 in 1-butyl-3-methylimidazolium chloride ([BMIM]­Cl), that are well correlated to the drastically different catalytic performances of the metal chlorides in the isomerization of glucose. The relative bond strengths and the number of ligands to which the metal ions are coordinated by oxygen atoms of different sources and by chloride were studied by probing model compounds with in situ far-infrared (FIR) and by reaction studies. The superior performance of CrCl3 for this reaction is now distinguished from that of other metal chlorides, on the basis of its selective Cr­(III) ene-diol coordination chemistry. We also offer new insights into the mechanism involved in the conversion of glucose to 5-hydroxymethylfurfural (5-HMF). In situ FIR is established as a powerful tool in the study of the coordination chemistry of metal complexes in ionic liquids.

Differentiation of the Coordination Chemistry of Metal Chlorides in Catalytic Conversion of Glucose in Ionic Liquids

“…The results show that VCl 3 condensed the glycolaldehyde much more rapidly than CrCl 3 . It is therefore evident that VCl 3 favors the condensation of glucose or fructose to humins especially at the first stage of the reaction [55].…”

“…Most recently, we have developed an in situ FIR spectroscopy tool and successfully applied it to the study of the coordination chemistry of metal chlorides in [BMIM]Cl [55]. The metal chlorides that showed strikingly different catalytic performances in the conversion of glucose to 5-HMF in [EMIM]Cl, as shown in Fig.…”

“…However it becomes of limited use for structural investigation of inorganic compounds, such as metal oxides or salts, which are inactive in the MIR region. FIR offers great advantages in the characterization of those inorganic compounds [51][52][53][54], as well as coordination complexes [55], lattice vibrations [56,57], hydrogen bonds [58,59], and the skeleton vibration of ring molecule [58], due to the lower energy in the FIR than the MIR spectral region. Just as with MIR, FIR analysis also suffers from the interference of water vapor in the air especially in low frequency [60,61].…”

“…In order to reduce the interference of water, several methods may be adopted [60], such as background subtraction, reduction in moisture level by purging with dry inert gases, resolution reduction, and isolation of the workplace from the air with a physical barrier. In addition, the spectral information of far infrared below 200 cm −1 may be neglected as there is little meaningful absorption by most compounds of interest, but the absorption of water is very intense in this region [55]. As far-infrared spectroscopy analyzes the weakest energy, it remains much less developed and the least applied in the infrared region.…”

An ionic liquid–organics–water ternary biphasic system enhances the 5-hydroxymethylfurfural yield in catalytic conversion of glucose at high concentrations

“…The mechanism of humins formation is still not fully understood, but it has been proposed that humins are formed due to oligomerization of glucose, fructose (an intermediate in the 10 conversion of glucose to 5-HMF), 5-HMF, and/or other reaction intermediates. 2,6,29,30 At high glucose concentration, bimolecular and sequential oligomerization are favored, so side-reactions to form oligomers and humins become serious. The detected glucose dimmers manifested that some of the glucose has 15 condensed into glycopolymers, thus further reaction can result in excessive humins formation.…”