Exploring lignin valorisation: the application of photocatalysis for the degradation of the β-5 linkage

Abstract: The valorisation of lignin has gained significant interest in bioenergy, which is driven by the abundance of the material coupled with the potential to form value-added compounds. As a result, the range of technologies deployed for this application has increased and more recently includes advanced oxidation processes such as photocatalysis. The complexity of lignin is challenging however, and therefore model compounds, which represent key linkages in the native structure, have become crucial as both a tool for evaluating novel technologies and for providing an insight into the mechanism of conversion. Previously, the β-O-4 dimer, the most abundant linkage found in native lignin, has been extensively used as a model compound. Described herein, however, is the first report of photocatalytic TiO2 technology for the degradation of a β-5 model dimer. Under low power UV-light emitting diode irradiation, complete degradation of the β-5 compound (6.3 × 10−3 mg ml−1 min−1) was achieved along with formation and subsequent removal of reaction intermediates. Investigation into the mechanism revealed within the first 2 min of irradiation there was the formation of a diol species due to consumption of the alkene sidechain. Although the data presented highlights the complexity of the system, which is underpinned by multiple oxidative reaction pathways, an overview of the key photocatalytic processes are discussed including the impact of acetonitrile and role of reactive oxygen species.