High Yields of Aromatic Monomers from Acidolytic Oxidation of Kraft Lignin in a Biphasic System

Abstract

In this work, acidic oxidation in a biphasic water/octanol system was applied to mitigate condensation and increase the yields of high-value aromatic monomers from Kraft lignin, a waste product, and an industrially relevant feedstock. Biphasic depolymerization (BPD) is a one-pot multistep technique in which target molecules are protected from degradation in the reactive aqueous milieu (necessary for the cleavage of β-O-4 bonds as well as the recalcitrant high dissociation energy carbon–carbon moieties) by an in situ transfer of the products into the protective octanol phase. The results have shown overall monomeric yields up to 13 wt % and yields of vanillin up to 7 wt % (FeCl3 as a catalyst and O2 as an oxidant), significantly outperforming the previously reported monophasic and biphasic outcomes for Kraft lignin. In addition to increased yields, BPD also resulted in an expansion of the product pool from few molecules typically reported in the literature to 64 aromatic monomers detected at various concentration levels in the current work. The identified aromatic alcohols, carbonyl compounds, and carboxylic acids incorporating varying side chain lengths show potential as green and sustainable replacements to BTX (benzene, toluene, and xylene) petrochemicals. BPD as a process coupled mild conversion conditions, cheap oxidants, and common homogeneous catalysts with an in situ separation of the products from the reaction mixture to yield excellent results in converting industrial waste and recalcitrant feedstocks into value.