A formal kinetic model describing the main reaction pathways yielding demethoxylated monomeric phenols from wheat straw lignin in a hydrogen-enriched solvolysis reaction system using ethanol as cosolvent has been developed. The simplified lump model combines both a detailed focus on the deoxygenation reactions at molecular level as well as the reactions of global bulk lumps. The results show that depolymerization of lignin structures is quickly achieved and that deoxygenation reactions largely follow a main route pathway via intermediate methoxyphenols and catechols to give stable phenols in the liquid phase. Observed ethyl group substituents of phenolics are seen to originate from either alkylation reactions of the intermediates, or directly from the depolymerizing structures. A high correlation between primary 4-ethylphenol products and para-coumaryl units in the examined lignin suggests that main reaction pathways are strongly dependent on the type of plant lignin used.

Lignin forms an important part of lignocellulosic biomass and is an abundantly available residue. It is a potential renewable source of phenol. Liquefaction of enzymatic hydrolysis lignin as well as catalytical hydrodeoxygenation of the main intermediates in the degradation of lignin, that is, catechol and guaiacol, was studied. The cleavage of the ether bonds, which are abundant in the molecular structure of lignin, can be realised in near-critical water (573 to 673 K, 20 to 30 MPa). Hydrothermal treatment in this context provides high selectivity in respect to hydroxybenzenes, especially catechol. RANEY Nickel was found to be an adequate catalyst for hydrodeoxygenation. Although it does not influence the cleavage of ether bonds, RANEY Nickel favours the production of phenol from both lignin and catechol. The main product from hydrodeoxygenation of guaiacol with RANEY Nickel was cyclohexanol. Reaction mechanism and kinetics of the degradation of guaiacol were explored.

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This thesis, submitted for the degree of Philosophiae Doctor at the University of Bergen, consists of two parts. The first includes an introduction, a summary of the work and main results from the papers in the second part, an overall conclusion and future work suggestions. The second part consists of six research papers and manuscripts, and one conference proceedings paper. The main part of the work has been performed at the Department of Chemistry, University of Bergen in the period 2010-2012, including several guest research stays

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