Lignin Structural Modifications Resulting from Ethanol Organosolv Treatment of Loblolly Pine

Abstract

Milled wood lignin samples from untreated Loblolly pine, post-ethanol organosolv pretreatment, and ethanol organosolv dissolved lignin were isolated and characterized by quantitative 13C and 31P nuclear magnetic resonance (NMR) spectroscopy. β-O-4 linkages are the predominant substructures present in lignin from the starting material, as well as after the pretreatment. During organosolv treatment of Loblolly pine, acid-catalyzed cleavage of β-O-4 linkages and ester bonds were the major mechanisms of lignin cleavage. This degradative pathway results in the formation of a dissolved ethanol organosolv lignin, which is more condensed and has a lower molecular weight than the starting lignin. The residual lignin isolated from the solid pretreated residue was also more condensed. Results from quantitative 13C and 31P NMR show that the residual and dissolved lignin fractions after pretreatment have a higher abundance of phenols and carboxylic acids and lower aliphatic carbon content and may be suitable as antioxidants or other value-added lignin products.