Structure and properties of soda-type lignin from a modern biorefinery process

Due to the increasing concerns regarding the use of crude oil for fuel and chemical production, considerable efforts have been made to find renewable and sustainable alternatives. Lignocellulosic biomass is the most abundant carbon containing natural resource on earth and mainly consists of its structural components cellulose, hemicellulose, and lignin. Compared to the carbohydrate constituents, lignin is structurally completely different. It is derived from aromatic monomers which makes it an attractive feedstock for the production of renewable chemicals. For over a century the pulp and paper industry has been fractionating lignocellulosic biomass to isolate cellulose and to use the remaining hemicellulose and lignin for heat and energy production. To develop new products from lignocellulosic biomass, additional fractionation technologies need to be explored. The polymer structure of lignin is inherently complex and will structurally not only vary between plant species, but technical isolation methods will further alter its structure. As such each fractionation technology will yield a different type of lignin with different properties compared to other lignin. Due to these reasons, it is crucial to understand the structural features of different lignin to determine the best application area.

In this thesis three different lignin, one hardwood and two softwood lignin, from a pressurized hot water extraction followed by mild soda cooking biomass fractionation technology were thoroughly investigated. A precipitation and purification protocol were developed to yield lignin samples free of contamination. The different lignin fractions were analyzed and compared to native-like lignin samples. A wide variety of analytical methods were used and the combined information from the analysis could be used to determine key structural features and properties of the lignin. Also model compounds with specific structural features were synthesized and the spectroscopic data was used to assist in the structural elucidation of the analyzed lignin. As a proof of concept, the purified lignin was fractionated into even more homogeneous fractions by simple solvent fractionation. Chemical modifications were used to investigate how simple modifications affected the thermal properties of lignin and were also used for to assist the structural determination.

During this thesis a 31P PULCON methodology was implemented to one of the most used lignin analysis methods, the quantitative determination of hydroxyl groups in lignin by 31P NMR spectroscopy, which showed to have benefits compared to the standard protocol.