Photochemical reactions of lignin: Opportunities for valorisation

Licentiate thesis, 2022

Lignin is the second most abundant biopolymer after cellulose and is the largest bio-based source of aromatic compounds. However, its heterogeneous and recalcitrant structure makes it difficult to depolymerise for use in renewable chemicals production. Photochemical reactions can be performed at mild conditions and can achieve efficient reaction pathways without the use of additional reagents. The potential efficiency gains in terms of energy and use of materials have led to renewed interest in photochemistry research, as we seek to improve the environmental sustainability of industry.
Concepts for valorisation of lignin through heterogenous and homogenous photocatalysis are currently being investigated. Although this photocatalyst research has shown some encouraging results, the synthesis, separation, and recycling of catalysts would add significant costs to a process. If it were possible to induce direct photochemical reactions with ultraviolet light, the difficulties associated with catalysis could be avoided, and it would bring a photochemical lignin valorisation concept closer to feasibility.
This thesis deals with the topic of how ultraviolet light can induce changes in lignin. The contents in Paper I describe our early work, which demonstrated that UV light causes changes to functional groups in lignin and that UV light can be used to extract lignin from sawdust. In our more recent experiments, we use 280 nm light from UV-LEDs using a homemade 3D printed photoreactor, the details of which are covered in Paper II. The results presented here also include a comparison between acetonitrile and aqueous NaOH as solvents for the photoreactions. The evaluation of whether benzophenone can be used as a photosensitiser to increase the reaction rate or yield toward desired products in reactions of Kraft lignin and 2-(benzyloxy)phenol, a model compound used to represent an ether bond between two phenolic rings, is covered in Paper III. We also needed to access whether our conclusions are robust against changes in lignin concentration and light intensity, and an analysis of this is included in the results and discussion. Lignin has also been observed to act as a photocatalyst. The beginning of an investigation into this phenomenon is covered in this thesis using the photooxidation of methanol to formate as an example. 1H NMR is the primary analysis technique used in this work. Results based on GC-MS and diffusion NMR are included to demonstrate their intended use in future work. The analysis across all areas focuses on using the analysis methods to estimate reaction rates and selectivity toward certain products, and aims to understand the connections between these results and the details of the reactions.
The results gathered so far have laid the groundwork for understanding the complex relationships between the properties of lignin, reaction conditions, and changes which occur upon irradiation with UV light. The end of the thesis discusses future plans for increasing our understanding of these photoreactions.