Photocatalytic conversion of lignin into aromatic monomers
Creators: name: Shao, Shibo
Publisher: name: The University of Edinburgh
Year: 2023
Subjects: lignin
photocatalysis
metal sulphide photocatalysts
aromatic monomers
C-C bond fragmentation

description: Lignin is the only abundant biomass with natural aromatic structures and has been considered as a promising green renewable resource to substitute the traditional fossil fuel feedstock for the production of monomer aromatic products. However, the conventional lignin depolymerisation processes are still based on thermos-catalysis, which usually require the addition of chemical reagents and high energy input and struggles with low reaction selectivity and high cost. Recently, photocatalysis has attracted increasing attention in the field of organic synthesis and chemical production, not only because it can drive the reaction with sustainable solar energy but also can generate unique photoinduced charged carriers to overcome the thermodynamic difficulty and perform the reaction under mild conditions. Therefore, the reaction directions could be precisely controlled with high selectivity for the target products. As for photocatalytic lignin conversion, the oxidation and reduction of the selective C-O or C-C interunit linkage fragmentation could be simultaneously performed under mild conditions, and thus the valuable natural aromatic structures of lignin could be retained for downstream processing. Here, the main challenges in the practical implementation of the photocatalytic lignin conversion are the slow reaction rates and unsatisfying selectivity to the target monomeric products. The key to addressing these problems relies on the development of novel photocatalysts and a better understanding of the reaction mechanisms. This thesis focuses on building the correlation between the material properties of catalysts, especially the electronic structures, the reaction pathway, the optimisation of reaction factors such as the solvent environment, the reaction rate, and the selectivity for the target products. This thesis starts with the investigation of the classic semi-conductor photocatalyst TiO2, which is one of the most promising photocatalytic materials. However, its practical applications are always limited by its wide band gap and fast recombination rate of the photoinduced charges. Inspired by the conventional TiO2 production from ilmenite in the area of metallurgy, a simple synthesising process for producing iron doped TiO2 was proposed in this work (Chapter 3). The optimal amount of iron dopant can tailor the electronic structure and suppress the recombination of photogenerated charges, which could lead to significantly improved photocatalytic activity in dye degradation reactions under visible light irradiation. Moreover, the developed Fe-doped TiO2 also showed potential in the conversion of lignin model compounds, which can drive the cleavage of C-C bonds in lignin model compounds and generate the corresponding monomeric aromatics. Among various types of interunit linkages in lignin, the aryl ether C-O bond is the major form of interunit linkage in natural lignin structures. Therefore, it is usually the target bonds to be cleaved in the conversion of lignin into monomeric aromatics. Metal sulphide photocatalysts had shown some potential in Cβ-O bond fragmentation. In Chapter 4, Zn/S rich phase of zinc indium sulphide photocatalysts were developed and exhibited superior catalytic activity in the cleavage of Cβ-O bond with the presence of water in the reaction system. Notably, compared to the reaction condition without water, the reaction selectivity for aromatic monomers increased by 170% and the PP-ol conversion rate raised by 58%. For the first time, the isotopic labelling experiments and kinetic isotope effects (KIE) measurements revealed that the hydrogen transfer from water to the final products and the hydrogen generate from photocatalytic water splitting are superior in facilitating the hydrogenolysis process of Cβ-O bonds. Water has been proven as an effective hydrogen donor in Chapter 4, and the role of water needs further investigation. In Chapter 5, the applicability of the photocatalyst was also improved. The developed quaternary CdxZ1-xnIn2S4 photocatalyst exhibited superior catalytic activity in the cleavage of various forms of aryl ether C-O bonds. Besides the hydrogens, this work found that the water can also provide oxygen atoms to the Cα-O bond fragmentation. The formation of carbon-centred radical intermediates was proved by in-situ EPR analysis, and the water-assisted reaction mechanism was systematically investigated. It is plausible that the hydrogen atoms from water dissociation could participate in the hydrogenolysis of Cβ-O bonds via a PCET process, whereas the hydroxyl radicals absorbed on the surface of the catalyst may combine with the formed carbon-centred radical intermediates and participate in the cleavage of Cα-O bond. Chapter 6 also focuses on the cleavages of C-C bonds in lignin, and the catalyst design strategy is very purposeful. Based on the understanding of the reaction mechanism, multiple defects in the graphitic carbon nitride can modify the electronic structures for better photocatalytic activity and act as the active sites to drive the specific semi-reactions of C-C bond fragmentation. Ultrathin nanosheet morphology was aimed to accelerate the migration of photogenerated charges. As a result, the prepared g-C3N4 exhibited significantly improved photocatalytic activity in the cleavage of C-C bonds in different structures (β-O-4 and β-1 structures). This chapter also proved that the reactive radicals derived from the oxygen atmosphere and photogenerated holes and electrons play essential roles in the reaction process. Overall, this thesis presented a detailed investigation about the photocatalytic cleavage of C-O/C-C interunit linkages in lignin and developed several photocatalysts for driving the corresponding reactions. Hopefully, the design conception of the photocatalysts and the investigation of the reaction mechanisms in this thesis can inspire the future works on photocatalytic lignin conversion process to realise the lignin valorisation and sustainable supply of aromatic chemicals in the future.