Quantitative structural characterization and thermal properties of birch lignins after auto‐catalyzed organosolv pretreatment and enzymatic hydrolysis

Abstract BACKGROUND To achieve the goals of economically feasible auto‐catalyzed organosolv pretreatments in bioethanol production, chemical conversion of the isolated lignin is needed. However, the structures and properties of lignin molecules produced after pretreatment have not been thoroughly investigated before its effective utilization. RESULTS The study focused on the auto‐catalyzed ethanol–water pretreatment of southwest birch, with the aim to clarify the structural transformations of birch lignin after pretreatment. Chemical structural elucidation of the isolated lignins was performed using multiple NMR methodologies (31P‐, 13C‐ and 2D‐HSQC NMR techniques). Results showed that the amount of β‐O‐4 linkages decreased in the order of AEOL (auto‐catalyzed ethanol organosolv lignin) < EHLP (enzymatic hydrolysis lignin, pretreated) < EHLU (unpretreated). The homolytic cleavage of β‐O‐4 linkages resulted in an increase of free phenolic hydroxyl groups and carboxylic acids in AEOL and EHLP compared with that of EHLU. In addition, α‐ethoxylation was the only modification in the auto‐catalyzed ethanol organosolv pretreatment (AEOP). Moreover, the thermal stability of the lignin samples is related to its inherent and condensed structures. CONCLUSIONS These findings would facilitate the further utilization of lignin as starting material for developing value‐added products in chemical and catalytic process. © 2013 Society of Chemical Industry