Determination of Arylglycerol-β-aryl Ethers and Other Linkages in Lignins Using DFRC/31P NMR

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Abstract
An analytical method for lignins has been developed that involves derivatization followed by reductive cleavage (DFRC), depolymerization, and quantitative 31P NMR spectroscopy. This technique detects and quantifies the various ether linkages present in softwood residual kraft lignins (RKL) and milled wood lignins (MWL). In addition, the technique supplies new quantitative information about β-aryl ethers linked to condensed and noncondensed aromatic moieties, including dibenzodioxocins. Within RKL, β-aryl ether bonds connected to condensed phenolic moieties predominated over those connected to noncondensed phenolic moieties. In addition, the amount of DFRC monomers determined by gas chromatography was minute in the RKL but large in the MWL. This indicates that almost all noncondensed β-aryl ether linkages were cleaved during kraft pulping. The method offers new avenues for the detailed investigation of the bonding patterns of native and technical lignins. Keywords: Aryl ether bonds; kraft; lignins; milled wood lignin (MWL); 31P nuclear magnetic resonance (NMR); phosphorus; pulping