Sustainable Microwave-Assisted Aerobic Oxidation of Tomato Plant Waste into Bioaromatics and Organic Acids

Lignin is the second most abundant polymer in lignocellulosic biomass. A biorefinery approach to lignin valorisation, as an alternative to the use of heating, can produce bioaromatics, and this idea is attracting ever more interest. However, the isolation of lignin for further processing is really quite challenging, making the valorisation of native lignin (i.e., lignin in raw biomass) crucially important. An analysis of vanillin yields in relation to total reaction feedstock weight shows that the economic viability of the process depends on the valorisation of all biomass components. This paper reports a microwave-assisted, catalyst-free aerobic oxidation process for biomass. The protocol yields bioaromatics (mainly vanillin and syringaldehyde, with traces of p-hydroxybenzaldehyde and acetovanillone) and organic acids (38.4 wt % maximum total yield) in only 30 min. Vanillin is the most valuable of the aromatic compounds to be produced by lignin oxidation, accounting for 8.7 wt % of the initial lignin. The use of biomass as the starting material means that no pretreatment is needed for component separation. Furthermore, the use of air as the oxidant and the catalyst-free nature of the protocol render the process environmentally sustainable and scalable, as the energy consumption is counterbalanced by the clean production of high market value products.