Chemical Transformations of Buddleja davidii Lignin during Ethanol Organosolv Pretreatment

Abstract

Detailed chemical structural elucidation of ethanol organosolv lignin (EOL) of Buddleja davidii was performed to determine the fundamental structure released from an ethanol organosolv pretreatment (EOP). Several nuclear magnetic resonance (NMR) techniques were used to analyze the structure of EOL, including quantitative 13C, 31P NMR, and qualitative DEPT-135 13C, 2D 1H−13C correlation NMR. As revealed by NMR, the aryl-O-ether linkage (β-O-4′) of lignin was extensively cleaved via homolysis during pretreatment and led to the formation of stilbene structures. Other linkages, such as resinol (β-β′) and phenylcoumaran (β-5′), were resistant to degradation. The high degree of condensation of EOL indicated that condensation reactions occurred but did not impede the delignification efficiency of EOP. Both guaiacyl and syringyl lignin were found to be reactive toward condensation during pretreatment. The results from gel-permeation chromatography showed that the degree of polymerization (DP) of lignin significantly decreased by ∼85%, facilitating lignin solubilization in ethanol.