Highly Selective Oxidation and Depolymerization of α,γ‐Diol Protected Lignin

Knock on wood: 2,3‐Dichloro‐5,6‐dicyano‐1,4‐benzoquinone is able to selectively remove the acetal and oxidize the α‐OH of acetal‐protected lignin. The resultant oxidized lignin generated syringyl and guaiacyl propane diketons in high monomer yield (31–36 % on the base of original Klason lignin) and with high selectivity (90 %) in formic acid/sodium formate, thanks to nearly complete extraction of lignin and preservation of the β‐O‐4 linkages.

Abstract

Lignin oxidation offers a potential sustainable pathway to oxygenated aromatic molecules. However, current methods that use real lignin tend to have low selectivity and a yield that is limited by lignin degradation during its extraction. We developed stoichiometric and catalytic oxidation methods using 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) as oxidant/catalyst to selectively deprotect the acetal and oxidize the α‐OH into a ketone. The oxidized lignin was then depolymerized using a formic acid/sodium formate system to produce aromatic monomers with a 36 mol % (in the case of stoichiometric oxidation) and 31 mol % (in the case of catalytic oxidation) yield (based on the original Klason lignin). The selectivity to a single product reached 80 % (syringyl propane dione, and 10–13 % to guaiacyl propane dione). These high yields of monomers and unprecedented selectivity are attributed to the preservation of the lignin structure by the acetal.

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