Defence of doctoral thesis: Petter Paulsen Thoresen – Structure and property oriented organosolv lignin extraction

In this Thesis, organosolv fractionation of softwood (spruce), hardwood (birch and beech), and
herbaceous crops (wheat straw) was performed by applying various organosolv process conditions.
Among these were a novel steam-explosion/organosolv hybrid mode, and two ternary solvent systems
utilizing water/ethanol/acetone, and water/acetone/acetic acid. In addition, the effect of using
inorganic acidic catalyst (H2SO4) was investigated for all raw material classes. Also, alkaline catalyst
(NaOH) was investigated for wheat straw due to its reported high content of inorganics which presence
adds additional structural complexity to the lignocellulosic recalcitrance. Following the organosolv
fractionation, structural characterization was performed (content of cellulose, hemicellulose, lignin) in
the isolated product fractions. Additionally, in-depth characterization of the isolated lignins was
performed by combining Pyrolysis-Gas Chromatography Mass Spectroscopy (Pyr-GC/MS), Gel
Permeation Chromatography (GPC), and different modes of Nuclear Magnetic Resonance (NMR; 13C,
1H-13C, 31P). Complementary analytics, such as content of monomeric/oligomeric sugars, dehydration
products, total phenolics, and Size-Exclusion Chromatography (SEC) of the isolated product liquor,
were performed to provide a comprehensive understanding of the process. Special attention was given
the lignin structural changes occurring throughout the organosolv process. This was done as the use
of lignin for higher-value application is considered crucial for the economic viability and development
of a modern biorefinery. For this reason, significant focus was given to study how the lignin
characteristics translate into physical properties such as solubility. The latter property was measured
through solubility trials in binary aqueous acetone solutions, as well as solvent-based fractionation
(acetone-water system). A coherent picture was aimed for, where structural motifs and lignin
characteristics were correlated to reductions/enhancements in solubility at various water contents. To
finally bridge this with a potential down-stream application, DPPH radical scavenging was performed
in DMSO with and without intercalating Lithium Bromide (LiBr) for a selection of organosolv lignins, as
well as their acetone/water fractionated lignins. This was complemented with a UV absorption study
of the lignin solutions.

The results provide a descriptive span of chemical characteristics related to organosolv lignins, where
they range between highly native, to non-native. Whereas the former lignins are largely comprised of
inter-unit motifs such as β-O-4’, β-β’ and β-5’, the non-native lignins are instead better characterized
by a high content of oxidized sidechains, biaryls and bifurans, as well as showing strong indications of
having quaternary cross-linkers originating from ketone functionalities. The ketones are also found to
undergo aldol condensation with aldehydes formed throughout the process. Interestingly, these two
extremes in lignin characteristics yield highly varying physical properties, where the predominantly
native and non-native lignins for example display low and high solubility in pure acetone, respectively.

Simultaneously, the latter is more prone towards water-induced precipitation, whereas the former
instead require water to display complete (~100%) solubility at specific concentrations. These
properties and findings are eventually consistent with recent literature where lignin aggregates also dominate their dissolved state, and the interunit motifs dictate the affinity on forming such aggregates
which are important for both their dissolution, precipitation, but also their display of functionality such
as antioxidant activity.