The main aim of this project was to investigate the photochemical degradation pathways of lignin models using singlet oxygen and other photo-induced reactive species, in order to understand the photochemical transformations of the lignin polymer and other lignin-like polymers. The sensitized photolytic oxidation of lignin models containing the 13-0-4' lignin substructures using visible light, Rose Bengal and oxygen was studied in an attempt to understand and develop photocatalytic oxidation as a method for the conversion of lignin into added-value chemicals, Initial studies using a simple model dimer and attempting to replicate a published method[1] resulted in significant differences in the products when compared with the previous literature studies. A time study was carried out and a possible mechanism of a photo catalysed dimerization has been proposed. The observed radical coupling reaction has been shown to precede the eventual cleavage of a key benzylic ether bond in the tetramer and the formation of a quinone product and guaiacol by-product. Similar radical couplings were also observed for 13-5' dimer and trimer lignin models, Identification of the coupling products was obtained by spectroscopic characterization and corroborated by independent synthesis. Batch and flow reactors were tested and compared for the photolysis reactions. The flow reactor was proved to be more efficient than the batch reactor in terms of time-space conversion and yield. It was also shown that the flow reaction is a good method for the study of mechanism and optimization of the conditions required for the process, The dimerization process was then used for the preparation of three novel unsymmetrical coupling products. Successful cross-coupling of lignin models in this way provides a flexible new synthetic strategy for the preparation of more complex lignin models which is competitive with previous synthetic routes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:679240 |
| Date | January 2015 |
| Creators | Ji, Xiaoyue |
| Publisher | Queen's University Belfast |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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