Lignin is the second most abundant component, next to cellulose, in lignocellulosic biomass. Large amounts of this polymer are produced annually in the pulp and paper industries as a coproduct from the cooking process—most of it burned as fuel for energy. Strategies regarding lignin valorization have attracted significant attention over the recent decades due to lignin’s aromatic structure. Oxidative depolymerization allows converting lignin into added-value compounds, as phenolic monomers and/or dicarboxylic acids, which could be an excellent alternative to aromatic petrochemicals. However, the major challenge is to enhance the reactivity and selectivity of the lignin structure towards depolymerization and prevent condensation reactions. This review includes a comprehensive overview of the main contributions of lignin valorization through oxidative depolymerization to produce added-value compounds (vanillin and syringaldehyde) that have been developed over the recent decades in the LSRE group. An evaluation of the valuable products obtained from oxidation in an alkaline medium with oxygen of lignins and liquors from different sources and delignification processes is also provided. A review of C4 dicarboxylic acids obtained from lignin oxidation is also included, emphasizing catalytic conversion by O2 or H2O2 oxidation.

Lignin can be depolymerised and used as a feedstock to obtain renewable raw-materials, providing a green alternative to fossil counterparts. Among others, C 4 dicarboxylic acids (DCA), like succinic, malic, maleic and fumaric acids, which can find applications in pharmaceuticals, food industry, and act as solvents, can be obtained from lignin oxidation. To investigate their formation, the oxidation of vanillic acid (VA), a lignin model compound, was studied under catalytic wet peroxide oxidation (CWPO) conditions, using titanium silicalite-1 (TS-1) as the catalyst. The effect of temperature, pH, and reaction time were studied. In a second phase, catalyst modification with transition metal oxides (Fe, Co, Cu) was tested. Results showed that oxidation under pH = 10.5 gives rise to complete VA conversion with hydroxylated DCA, namely malic (15 mol%) and tartaric (5 mol%) acids, as the main products. At pH = 4.0, the production of succinic acid was improved (7.4 mol%), with VA conversion achieving 78% after 2.0 h of reaction. At alkaline pH, H 2 O 2 reactivity is higher, leading to C 4-DCA degradation to low-molecular weight compounds. Catalyst desilication was observed, pointed out for the convenience of using neutral and acidic pH. In acidic pH, Fe and Cu catalysts enhanced VA conversion, and Fe catalyst was more selective towards succinic acid production.

Lignin depolymerization through peroxide oxidation produces dicarboxylic acids (DCA), especially C 4 -DCA, like succinic acid. In this work, the effect of methoxy substituents on C 4 -DCA production using peroxide oxidation of lignin model compounds (p-hydroxybenzoic acid, vanillic acid, and syringic acid) and hardwood and softwood lignin samples was studied. It was concluded that methoxy substituents increased the reactivity toward peroxide oxidation. The succinic acid yield was higher for the model compounds with fewer methoxy groups, achieving 5.8 wt % of succinic acid for p-hydroxybenzoic acid. For Eucalyptus globulus kraft lignin (hardwood lignin with guaiacyl and syringyl units), an increased reactivity was verified, and more succinic acid (3.5 wt %) was produced in a shorter time, comparatively with Indulin AT lignin (softwood lignin, with only guaiacyl units), which produced 2.7 wt %. This evidence suggests that E. globulus kraft lignin might be a better raw material than Indulin AT for succinic acid production by peroxide oxidation.