Lignin is the most naturally abundant source of aromatics for phenolic chemicals, bioenergy sources, and functional materials. Recent works are devoted to the acid-catalyzed organosolv extraction for improving the lignin valorization. However, β-aryl ethers are mostly cleaved to form carbon-carbon bonds under the acidic condition. In this work, lignin isolated with methanolic hydrochloric acid (MHL) was compared with conventional milled wood lignin (MWL). The methanol incorporated into lignin as α-OCH3 to prevent the cleavage of β-aryl ethers. As a result, the yield of MHL was over 12 times that of MWL. The MHL appeared a regular granular structure with an average diameter of 100 nm. Analysis of 13C-1H HSQC NMR spectra showed that MHL contained 42.6 % β-O-4 linkages whereas 35.9 % for the MWL, as well as 6.5 % β-5 and 3.2 % β-β, which was less than MWL. Moreover, 13C NMR spectroscopy confirmed that MHL was mainly isolated by the cleavage of the associated carbohydrates rather than the degradation of β-aryl ether bonds. Therefore, MHL can be utilized more as a native lignin representative than MWL for studying the physicochemical properties and the interior structures of the protolignin.
Keywords: Lignin isolation; NMR spectroscopy; Structural characterization.
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