(669d) Oxidation and Depolymerization of Alpha,Gamma-Diol-Protected Lignin into Aromatic Monomers

Lignin oxidation offers a potential pathway to sustainable 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. Here, we show that α,γ-diol protected lignin, which can be isolated from birch at near quantitative yields using propionaldehyde under acidic conditions, can be upgraded at high selectivities (>90%) to just two products. 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 38 mol% (in the case of stoichiometric oxidation) and 32 mol% (in the case of catalytic oxidation) yield (based on the extracted lignin). The monomer products contained 90-93% of the syringyl or guaiacyl propane diones and 7-10% of phenyl acids and aldehydes. These unprecedented selectivities are attributed to the preservation of the lignin structure by the acetal. Taking the high lignin isolation yield into account, this method also produced higher overall aromatic monomer yields (36 mol% for stoichiometric oxidation and 31 mol% for catalytic oxidation based on the original Klason lignin) compared to other lignin oxidation and depolymerization strategies.