Decomposition of Lignin Model Compounds Under Catalysed and Non-catalysed Hydrothermal Liquefaction Conditions

Abstract

The decomposition of lignin under Hydrothermal Liquefaction (HTL) conditions yields an aqueous product containing valuable aromatic chemicals in addition to solids and gaseous products. This thesis shows the kinetics and reaction pathways of lignin decomposition under HTL conditions using diaryl model compounds representative of the α–O–4 and β–O–4 ether linkages found in lignin. Additionally, aromatic aldehydes with varying degrees of hydroxy and methoxy substitution have been investigated to determine their respective reaction pathways and possible contribution to CO2 formation. A laboratory scale batch reactor with rapid heating and cooling profiles was designed and used to determine accurate reaction kinetic parameters between 140 – 360 °C. The α–O–4 and β–O–4 linkages decomposed via hydrolysis forming products prone to secondary reactions including condensation, alkylation, demethylation and dehydration. The β–O–4 linkage was more stable than the α–O–4 linkage and required higher temperatures to decompose. The overall reactivity of the model compounds and product distribution depends on the number and type of functional groups on the aromatic ring under HTL conditions.