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

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.