Hydrothermal lignin liquefaction for the production of bifunctional components

Lignin is a heterogeneous polymeric structure containing different types of phenols and a variety of functional groups. Therefore, it is an interesting resource for producing aromatic building blocks for the chemical industry. Lignin derived monocyclic aromatic compounds like catechol could be used as compounds for production of polymers. However, no commercial process for aromatics production from lignin, except vanillin, has been established by now.

The focus of this project was to get a better knowledge of the behavior of lignocellulosic biomass and specifically lignin during hydrothermal liquefaction (HTL), a process with large potential for lignin depolymerization. In contrast to thermal degradation processes which are mainly radical chain reactions which end up in an enormous number of reaction products, solvolytic processes like HTL have the advantage to split very selectively ether bonds with the help of an alkaline or acid catalyst and lead to products with interesting functional groups.

In order to optimize the product spectrum of lignin depolymerization different reaction conditions were tested. Among the investigated reactor types little influence on the liquefaction was detected. However, it was found that hydrothermal conditions increase the yield and selectivity towards bifunctional molecules like catechol. With higher temperatures and longer retention times the catechol yields got lower. At temperatures above 350 °C nearly no catechol could be gained hydrothermically. This trend was shown to be true for different lignin types from different sources.

Several limitations of common analytical methods for the analysis of lignin and derivatives have been identified and partly elucidated in this work. For extensive discussion see link.

Based on the experimental data the basic reaction pathways of lignin to bifunctional compounds have been elucidated and modeled. A model was developed to predict the product mixture resulting from hydrothermal liquefaction of lignin under different reaction conditions (temperature, reaction times). This model will be used in the design of a biorefinery concept to follow up the project. Moreover the project significantly enhanced the understanding of lignin as a molecule, the analysis of lignin and its derivatives, and the reactions of lignin during HTL.