Increasing the glucose loading in the 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid containing a dissolved CrCl3 catalyst system led to excessive formation of humins and a serious decrease in the 5-hydroxymethylfurfural (5-HMF) yield. A biphasic system containing glycol dimethyl ether (GDE) as the extraction phase, and [BMIM]Cl/CrCl3/glucose in combination with a partitioned amount of GDE and an appropriate amount of water as the reaction phase was found to be highly efficient for the reaction; CrCl3 catalyzed the formation of 5-HMF in 64.5 mol% yield from a very high glucose concentration (80 wt% with respect to the ionic liquid) at 108 °C. This 5-HMF yield in the [BMIM]Cl–GDE–H2O ternary biphasic system nearly doubled that obtained in the single [BMIM]Cl/CrCl3/glucose reaction phase. Importantly, the GDE phase contained about 56% of the generated 5-HMF without detectable contamination by the ionic liquid or carbohydrates. GDE served multiple functions: as a hydrogen-bond acceptor, it exhibited excellent extraction performance for 5-HMF; due to its low boiling point and suitable solubility saturation point in the ionic liquid, a sustained GDE bubbling phenomenon in the ionic liquid phase was observed that promoted the rate of inter-phase mass-transfer of 5-HMF in reactions; and GDE mediated the [BMIM]Cl phase to a reduced viscosity. In addition, an appropriate amount of water in the ternary system promoted the extraction efficiency of 5-HMF and also lowered the viscosity of [BMIM]Cl/glucose. The ionic liquid–organics–water ternary biphasic system has been demonstrated for high 5-HMF productivity and separation efficiency.
As green and effective depolymerization processes, CrCl 3 and CrCl 3 -catalyzed ethanol pretreatment were comprehensively investigated to improve cellulose conversion from pretreated residues in poplar in the present study. The influence of pretreatment temperature on delignification , structural characteristics of lignin, and the yield of glucose and ethanol was comparatively analyzed to emphasize the synergy effect of CrCl 3 and ethanol solvent. Particularly, CrCl 3 -catalyzed ethanol pretreatment at 200 °C resulted in the pretreated substrates with >94% removal of hemicellulose and lignin, consequently reaching 88.88% and 56.11% yields of glucose and ethanol, respectively. Lignin's structural change was observed by Gel permeation chromatography and 2D nuclear magnetic resonance spectroscopy and its relationship with the efficiency of enzymatic hydrolysis and fermentation was determined. Results indicated that the increased pretreatment temperature enhanced the lignin depolymerization with smaller molecular weight , fewer β-O-4 linkages, and higher S/G ratio, especially in CrCl 3 -catalyzed ethanol pretreatment, which was positively associated with poplar cellulose conversion. In addition to offering a comprehensive illustration of the sugar/ethanol generation during CrCl 3 -catalyzed organosolv pretreatment, this work also provided theoretical support for biofuel and biochemical production from poplar.
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