Catalytic wet peroxide oxidation of vanillic acid as a lignin model compound towards the renewable production of dicarboxylic acids

Abstract

Lignin can be depolymerised and used as a feedstock to obtain renewable raw-materials, providing a green alternative to fossil counterparts. Among others, C4 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 (15mol%) and tartaric (5mol%) acids, as the main products. At pH=4.0, the production of succinic acid was improved (7.4mol%), with VA conversion achieving 78% after 2.0h of reaction. At alkaline pH, H2O2 reactivity is higher, leading to C4-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.