Insights into Photosensitized Reactions for Upgrading Lignin

The conversion of lignin into valuable chemical products is important for the shift away from the petrochemical industry toward a more sustainable system of biorefineries. However, the recalcitrance and heterogeneity of lignin have made its selective depolymerization a difficult task. Photochemical methods of lignin conversion are being investigated because of the potential to operate photoreactors at milder temperatures and pressures than thermal methods and to achieve efficient reaction pathways. Furthermore, light-driven reactions facilitate reaction pathways that cannot be accessed by conventional/thermal methods. Most of the current research focuses on photocatalytic methods, which are interesting due to their potentially high selectivity, but come with the disadvantage of catalyst costs and separation requirements. In this work, we continue our investigation into the use of ultraviolet light-emitting diodes, which aims to utilize the advantages of photochemistry, while avoiding the use of expensive catalysts. Photosensitizers can participate in energy transfer, electron transfer, and hydrogen abstraction in photochemical reactions. Here, we investigated the effects of a common photosensitizer, benzophenone, on the photochemical conversion of lignin, and 2-(benzyloxy)phenol (2BP), a compound with an ether bond between two aromatic units. We monitored the conversion reactions using complementary techniques of 1H nuclear magnetic resonance (NMR), diffusion NMR, and in situ Fourier transform infrared (FTIR) spectroscopy. For 2BP, the reactions with benzophenone progressed slower and without a difference in the final product formation. However, several differences were observed in photoreactions utilizing Kraft lignin and benzophenone compared to those without benzophenone. For example, a faster decay of the 1H NMR peak corresponding to aromatic/phenolic protons and different changes in the shape of methoxy peaks were observed, indicating the formation of different products. This work demonstrates that benzophenone participates in the photoreactions of Kraft lignin and that the photoreactions of Kraft lignin and 2BP are different. Depolymerization of lignin into smaller fragments was confirmed with diffusion NMR, both with and without the photosensitizer.