Separation and characterization of pyrolytic lignins from the heavy fraction of bio-oil by molecular distillation

•Extraction of the bio-oil heavy fraction yielded 19.88 wt% pyrolytic lignins.•Two pyrolytic lignins with high molecular weights had similar structures.•Low-molecular-weight pyrolytic lignin contained abundant active functional groups.

Different fractions of bio-oil could be enriched with various chemical families by molecular distillation. The distilled fraction could be upgraded by catalytic esterification, cracking, or steam reforming, whereas the heavy fraction was difficult to dispose of. In this study, we adopted the methanol–water method for primary separation of pyrolytic lignins and sugars in the heavy fraction to improve the efficiency of utilization of the heavy fraction. Ultimate analysis, gel-permeation chromatography, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and pyrolysis–gas chromatography/mass spectrometry were employed to characterize the pyrolytic lignins obtained by methanol–water and water extraction methods. The pyrolytic lignins consisted of similar elements, and their basic structures included etherified and non-etherified syringyl and guaiacyl units. However, low-molecular-weight pyrolytic lignin from the heavy fraction differed from the high-molecular-weight pyrolytic lignin and water-extracted pyrolytic lignin in molecular weight distribution, side chains, and interunit linkages. As the low-molecular-weight pyrolytic lignin consisted of tri- to pentamers and 0.14/Ar carbonyls, it had high reactivity. Interunit linkages of the three pyrolytic lignins contained β–β′ resinol moieties, while the low-molecular-weight pyrolytic lignin had the most abundant alkyl ether linkages.

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