Stepwise Enrichment of Sugars from the Heavy Fraction of Bio-oil

Sugars are among the main compounds in bio-oil produced by biomass pyrolysis. However, they can easily form coke, resulting in fast deactivation of the catalyst and severe blockage of the reactor during catalytic upgrading of bio-oil. Molecular distillation may be performed to retain sugars and pyrolytic lignin in the heavy fraction and thus to provide a primary stage for the subsequent sugar separation and utilization. A new method involving extraction, heat treatment, and column chromatography was introduced to separate further the aqueous phase obtained from the bio-oil heavy fraction by methanol water extraction. Thus, monophenols and sugars were coextracted through these combined separation technologies. A fraction rich in monophenols was obtained by solvent extraction. Other impurities were further removed by heat treatment and column chromatography. An alcohol sedimentation method further separated the combined sugar-rich fraction into an ethanol-soluble fraction (SF-1) and an ethanol-insoluble fraction (SF-2). Gas chromatography-mass spectrometry, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and high-performance liquid chromatography were performed to characterize sugars in the obtained fractions. SF-1 mainly contained monosaccharides such as levoglucosan, glucose, and xylose, whereas SF-2 still had a small amount of cellobiose besides monosaccharides. The ultimate recovery rates for several identified monosaccharides were in the range of 75-86 wt %.