Lignin as a renewable aromatic resource for the chemical industry

Valorization of lignin plays a key role in the further development of lignocellulosic biorefinery processes for biofuels and biobased materials production. Today’s increased demand for alternatives to fossil carbon-based products expands the interest and the need to create added value to the unconverted lignin fraction. The aim of the research was to study the potential of lignin to become a renewable aromatic resource for the chemical industry. Lignin can be considered as an abundantly available and cheap raw material for the manufacturing of an array of products. Development of applications needs to go hand in hand with the anticipated increased production of technical lignins derived from the pulp and paper industry and the emerging lignocellulosic biorefinery industry. Two promising lignin applications are studied in this thesis:

1)         the use of lignin in wood adhesives

2)         the use of lignin for the production of aromatic chemicals

PCA modeling was performed aimed at the prediction of the application potential of different technical lignins for wood adhesive production. The lignins and their fractions could be classified in different clusters based on their structure dependent properties. Lignins exhibiting sufficient reactive sites, medium molar mass and low level of impurities are most promising candidates for the development of lignin based wood adhesives. Both lignin reactivity and formaldehyde-free crosslinking agents are needed to develop emission-free adhesives. Periodate oxidation was studied as method to improve the lignin reactivity. Alternatives to formaldehyde- based glues are under investigation and a combination of lignin and furans might be an interesting concept to develop renewable adhesives. 

In this research a reliable SEC methodology was developed for the analysis of the molar mass distribution of a wide range of different lignins. The major drawback of this method is that the molar masses are calculated on a relative basis to sulfonated polystyrenes. Using MALDI-TOF-MS and prior fractionation of lignin did not solve all problems associated with the determination of the absolute molar mass of lignin.   

Supercritical depolymerisation of lignin using a carbon dioxide/acetone/water fluid resulted in a depolymerised lignin oil. In this oil some monomeric compounds are present in relatively high amounts up to 3.6% (based on dry lignin). These products maybe further isolated by downstream processing to obtain purified fine chemicals. For continuous operation of this supercritical process, the formation of char should be further limited. 

The results presented in this thesis are expected to contribute - together with the many on-going activities worldwide - to the increased commercial utilisation of lignin in the future. Moreover, the obtained results contribute to the increasing knowledge on lignin analysis, chemistry and reactivity.