Elucidation of Lignin Structure through Degradative Methods: Comparison of Modified DFRC and Thioacidolysis

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Abstract
Milled wood and milled wood lignin (MWL) samples were subjected to DFRC and thioacidolysis. Despite the fact that both methods selectively cleave aryl ether bonds, substantial differences in results were obtained. Lignin thioacidolysis gave total molar yields of degradation monomer products in the range of 3.5−7 mol % higher than DFRC. GPC analysis showed that the thioacidolysis-treated lignin was degraded to a lower average molecular weight than that treated by DFRC. Contrary to results reported for lignin model compounds, these results indicate that the DFRC method does not completely or efficiently degrade the lignin polymer. In fact, the DFRC-degraded lignin retained much of the characteristics of the original MWL. Elemental analysis revealed the presence of bromine in the DFRC-treated lignin, and two-dimensional 1H−13C HMQC NMR spectroscopy showed the presence of β-O-4 linkages in the DFRC-treated lignin. No β-O-4 interunit linkages were detected in the thioacidolysis-treated lignin. These results are consistent with the lower monomer yields and the higher average molecular weight of the DFRC-treated lignin and indicate inefficiency in the chemistry of the method, probably due to steric constraints of the polymeric nature of lignin. Keywords: Lignin thioacidolysis; derivatization followed by reductive cleavage (DFRC); HMQC NMR spectroscopy; milled wood lignin (MWL); GPC; β-O-4 linkages