Identification and structural characterization of oligomers formed from the pyrolysis of biomass

The bio-oil composition and oligomer structure requires detailed studies for an effective design of an upgrading process. An in-depth identification and structural characterization of oligomers formed from the pyrolysis of several typical biomass were performed. Oligomers (200-500 g/mol) accounted for about 55-75 % in the pyrolytic bio-oil. Characteristic oligomers analyzed by ESI-MS and MSn were classified to 5 groups, that is, hexose-derived oligomers (HO), pentose-derived oligomers (PO), hexose-pentose oligomers (HPO), lignin-derived oligomers (LO) and lignin-carbohydrate-complex oligomers (LCC), the stability of which in the solution were verified by computational studies. The hexose-derived oligomers, pentose-derived oligomers and HPO oligomers were formed by dehydration, while the LCC oligomers were formed by dehydration and decarboxylation, whereas the lignin-derived oligomers were formed by dehydration, decarboxylation and demethylation. The aldol-condensation reactions of carbohydrates in pyrolysis was proposed. Lignin-derived oligomers kept higher high heat value (> 23 MJ.kg(-1)) than other carbohydrate derived oligomers (16-24 MJ.kg(-1)). The pyrolysis of wheat straw and corncob residue preferred to form hexatetrose, while enteromorpha prolifera (E. prolifera) to form pentobiose. The pyrolysis of biomass with cellulose-lignin ratio of about 2.0 and cellulose-hemicellulose ratio of about 2.8 (pubescen and mulberry wood) resulted in mainly tetrameric-phenol oligomers.