Photocatalytic Conversion of β-O-4 Lignin Model Dimers: The Effect of Benzylic Ketones on Reaction Pathway

The conversion of biomass towards value-added and platform chemicals has become the focus of extensive research these past two decades. One of the methods that has been increasingly studied is the use of semiconductor-mediated photocatalysis for biomass conversion. Titanium dioxide has previously been demonstrated to be an effective commercial catalyst for the cleavage of bonds within lignin and also cellulose and hemicellulose. Described herein is the deployment of TiO₂ for the cleavage of bonds within two β-O-4 lignin model compounds, one bearing a ketone in the α-position and the other an alcohol. The presence of a ketone in the benzylic position in one of the models had a pronounced effect under photolytic conditions, e.g., in the absence of a photocatalyst but with irradiation present. The subsequent reduction of the benzylic ketone resulted in observed sensitivity towards the irradiation and solely photocatalytic conversion was achieved. In addition, reaction products are proposed, which demonstrate a feasible method for β-O-4 cleavage in native lignin extracts.