The Dual Role of tert-Butanol toward Highly Selective Electrocatalytic Oxidative Cleavage of Lignin β-O-4 Cα-Cβ Bond to Aromatic Aldehydes

Abstract

The electrochemical selective cleavage of β-O-4 linkages, the most abundant interunit bonds in lignin, to produce aromatic aldehydes is frequently hindered by overoxidation and the formation of undesired co-products. Herein, we report a 45 vol.% tert-butanol (tBuOH)-containing microenvironment strategy to suppress aldehyde overoxidation during electrocatalytic cleavage of β-O-4 lignin linkages in a mild pH 9 aqueous electrolyte. This oxidative strategy is applicable to a wide range of electrode materials, including commercial and synthetic catalysts, achieving aldehyde selectivity of up to 96%. The high selectivity is attributed to two complementary roles of tBuOH co-solvent: (i) the formation of a hydrophobic boundary layer at the electrode interface that suppresses aldehyde hydration, thereby preventing its subsequent oxidative transformation to the corresponding carboxylic acid, and (ii) the efficient quenching of reactive oxygen species that would otherwise promote further oxidation of the aldehyde. Leveraging this mechanistic insight into overoxidation suppression, we successfully extended this mild electrocatalytic cleavage strategy to 18 diverse β-O-4, β-1, and α-O-4 lignin model dimers as well as real kraft lignin, yielding the corresponding substituted aromatic aldehydes with high selectivity.