Hydrothermal Depolymerisation of Kraft Lignin: The Influence of Capping Agents and Residence Time

Sammanfattning: Lignin, the aromatic macromolecule found in wood and other lignocellulosic biomass, is envisioned as a future renewable source of aromatic compounds that can be used as, for example, chemicals, fuel additives and resins. A way in which this can be realised is by depolymerising the lignin once it has been isolated from the lignocellulosic biomass. Hydrothermal methods that employ water as a reaction medium are suggested as ways of accomplishing this. However, reactive fragments prone to repolymerisation, indeed char formation, are formed during depolymerisation, causing operational difficulties in the process as well as yield losses of the desired components. The suggestion has been made to add chemicals, i.e. capping agents, to scavenge the reactive components in order to mitigate these issues. Careful selection of reaction parameters, such as residence time and temperature, are also crucial for the process. In this work, the depolymerisation of softwood kraft lignin has been investigated under hydrothermal conditions, with additions isopropanol, glycerol and guaiacol, to investigate their potential of functioning as capping agents. A customised batch reactor (99 ml) enabling swift heating of the reaction mixture was employed at 290-335 °C and 250 bar with low residence times (1-12 min). The result of the hydrothermal depolymerisation of lignin was an aqueous suspension with a strong smoky odour. No apparent separate liquid organic phase was formed, although a char fraction precipitated. Characterisation of the products indicated a rapid depolymerisation of the lignin, since inter-unit ether linkages were cleaved, which was later followed by a slower repolymerisation. Additions of isopropanol, glycerol and guaiacol, reduced the weight average molecular weights (Mw) of the product fractions. Only isopropanol appeared to be able to reduce the amount of char formed from the lignin. Subsequent aftertreatment of the product would be required to obtain a usable product.