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.

Sustainable biomass hydrodeoxygenation in biphasic systems

“…8a. 120 The water/octanol biphasic system allows the in situ transfer of the product into the protective octanol phase, thus avoiding the formation of side reactions and slowing down the deposition of carbonaceous matter. Clearly, the use of the water–oil biphasic system hinders the deactivation of the active site, while optimizing the quality of the product and simplifying the separation step.…”

“…8a. 120 The water/octanol biphasic system allows the in situ transfer of the product into the protective octanol phase, thus avoiding the formation of side reactions and slowing down the deposition of carbonaceous matter.…”

“…8 (a) In situ catalytic conversion of lignin in the water/octanol biphasic system. 120 (b) The selective HDO of lignin to low molecular weight bio-oil over a Mo-Ru/AC catalyst. 127 Fig.…”

Metal-based ionic liquids: effective catalysts in aqueous media for the selective production of vanillin from alkali lignin at room temperature

“…This is followed by an oxygen assault that produces a peroxy radical. 17 The obtained quinone ring is no longer stable, so the water molecule will attack at the C α position. Finally, the electron density at C α -OH will resonate to generate the carbonyl product and water molecule via cleavage of C α -C β bond.…”

“…They reported a total of 64 monomers after depolymerization demonstrating the capability of the catalytic system to fragment the links but selectivity was poor. 17 Likewise, Zhang et al carried out lignin oxidation to vanillin (10.9 wt%) using CuSO 4 (10 wt%) as a catalyst, and H 2 O 2 as oxidant at 140 °C. The selectivity of vanillin was not covered in this study, and industrial use is challenging due to the high temperature.…”