A facile and novel lignin isolation procedure - Methanolic hydrochloric acid treatment at ambient temperature

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.