Structural elucidation of industrial bioethanol residual lignin from corn stalk: A potential source of vinyl phenolics

•31P and 2D HSQC NMR were used for the comprehensive assessment of lignin structure.•Lignin molecules were modified by cleavage, oxidation, condensation and demethoxylation.•Formation of ferulates was attributed to β-O-4 cleavage combined with α,β-elimination.•Selectivities of vinyl phenolics via Py-GC/MS reached 27.74% (DL) and 43.87% (CL).

Structure of industrial bioethanol residual lignin is still unknown which restricts its further utilization. In this work, two bioethanol residual lignin fractionations (named as DL and CL) were selected to investigate the chemical structure evolution of corn stalk lignin during bioethanol production (BEP) process via GPC, FTIR and NMR with milled wood lignin as the control. Results showed that lignin structural framework and main functional groups maintained after BEP process. Lignin degradation mainly occurred on the ether linkages, especially the β-O-4 linkages. Meanwhile, lignin oxidation unmasked by 31P and 2D HSQC NMR contributed to the growth of COOH groups, G′- and S′-type units. Condensation and demethoxylation were also confirmed during BEP process. Notably, benefited from β-O-4 cleavage and α,β-elimination, the content of ferulate substructures increased. Furthermore, TGA was used to determine the thermal stabilities of these lignin fractionations, and Py-GC/MS was employed to evaluate their pyrolytic products. Vinyl phenolics (4-vinylphenol and 4-vinylguaiacol) were dominant, and the total selectivities at 500 °C were up to 27.74% (DL) and 43.87% (CL). Overall, corn stalk bioethanol residual lignin can be used as a potential resource to produce vinyl phenolics.

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