Changes of Lignin-Solvent Systems Induced by Ultraviolet Light Irradiation

Doctoral thesis, 2024

Lignin is the most abundant natural source of aromatic compounds and the second most abundant biopolymer. Despite efforts over the last decades to develop methods of lignin valorisation, the vast majority of lignin separated from cellulose for pulp and paper production is burned to generate steam and electricity. The structural heterogeneity and chemical recalcitrance of lignin have complicated efforts to convert lignin into higher value chemicals or materials. Photochemical methods of lignin valorisation offer potential benefits for reaction selectivity and energy efficiency, and most of this work has been focused on heterogeneous or homogeneous photocatalytic methods.

In this work, we investigate the use of catalyst-free photochemical reactions to depolymerise lignin, produce small molecule products, and/or modify lignin’s functional groups. Our reactions were performed by irradiating lignin dissolved in acetonitrile, dimethyl sulfoxide, or aqueous sodium hydroxide solutions with ultraviolet light. NMR and other analysis methods were used to evaluate changes in the solutions throughout the reaction time. We also investigated the impact of the photosensitizer benzophenone and the impact that oxygen and water content in the solution had on the photoreactions.

Our results show that at least three small molecules (hydrogen peroxide, formic acid, and methanol) were formed in the photoreactions we conducted. Aromatic rings were lost throughout the reaction time. We observed aspects of lignin photoreactions which were solvent-dependent, including rate of decrease of aromatic rings and rate of increase of formic acid. We did not observe significant effects of benzophenone, water content, or oxygen content, although it seems likely that reactive oxygen species are involved in the reactions. Observation of solvent-dependent lignin fluorescence intensities suggests that the solvent-dependent reaction behaviour of lignin may be related to the influence of solvent on lignin’s photophysical behaviour.