Aldehyde-Assisted Lignocellulose Fractionation Provides Unique Lignin Oligomers for the Design of Tunable Polyurethane Bioresins

Abstract

Thanks to chemical stabilization, aldehyde-assisted fractionation (AAF) of lignocellulosic biomass has recently emerged as a powerful tool for the production of largely uncondensed lignin. Depolymerization of AAF lignin via ether cleavage provides aromatic monomers at near theoretical yields based on ether cleavage and an oligomeric fraction that remains largely unexploited despite its unique material properties. Here, we present an in-depth analytical characterization of AAF oligomers derived from hardwood and softwood in order to elucidate their molecular structures. These bioaromatic oligomers surpass technical Kraft lignin in terms of purity, solubility, and functionality and thus cannot even be compared to this common feedstock directly for material production. Instead, we performed comparative experiments with Kraft oligomers of similar molecular weight (Mn ∼ 1000) obtained through solvent extraction. These oligomers were then formulated into polyurethane materials. Substantial differences in material properties were observed depending on the amount of lignin, the botanical origin, and the biorefining process (AAF vs Kraft), suggesting new design principles for lignin-derived biopolymers with tailored properties. These results highlight the surprising versatility of AAF oligomers towards the design of new biomaterials and further demonstrate that AAF can enable the conversion of all biomass fractions into value-added products.

Dual Valorization of Lignin as a Versatile and Renewable Matrix for Enzyme Immobilization and (Flow) Bioprocess Engineering

“…Similar values were encountered for the l ‐alanine, 3.7 mg and 1.2 mg for the oligomer and the kraft lignin, respectively. These differences are likely linked to the very different features of lignin oligomers vs. kraft lignin present [43] . The better performance of lignin oligomers might notably come from their lower molecular weight ( M w =1940 g mol −1 ; M n =1050 g mol −1 ) compared to softwood kraft lignin ( M w =5370 g mol −1 ; M n =1483 g mol −1 ), which might make product release easier compared to a more entangled matrix.…”

“…Lignin oligomers where produced by stabilization with isobutyraldehyde according to previously reported methods [12,43] . Briefly, 1 kg of softwood, a mixture of spruce and pine previously size‐reduced in a cutting mill equipped with a 6‐mm grid, was pretreated with isobutyraldehyde (800 mL), dioxane (5.5 L) and 37 wt% HCl (250 mL) at 80 °C for 3 h. After neutralization with NaHCO 3 (250 g), cellulose was separated from the liquor by filtration and washed with MeTHF.…”

“…Lignin oligomers where produced by stabilization with isobutyraldehyde according to previously reported methods. [ 12 , 43 ] Briefly, 1 kg of softwood, a mixture of spruce and pine previously size‐reduced in a cutting mill equipped with a 6‐mm grid, was pretreated with isobutyraldehyde (800 mL), dioxane (5.5 L) and 37 wt% HCl (250 mL) at 80 °C for 3 h. After neutralization with NaHCO 3 (250 g), cellulose was separated from the liquor by filtration and washed with MeTHF. The liquor and the washing liquid were pooled and concentrated down under reduced pressure before being added dropwise into hexanes to precipitate lignin that was recovered as a powder after filtration and drying in a vacuum oven (45 °C).…”

Protected lignin biorefining through cyclic extraction: gaining fundamental insights into the tuneable properties of lignin by chemometrics

“…An example is aldehyde-assisted fractionation (AAF), where the aldehyde as an additive provides chemical protection to the lignin structure. [16][17][18] The protected lignins have a high content of aryl ether bonds and are therefore suitable for catalytic depolymerization such as hydrogenolysis and reductive catalytic fractionation (RCF), to yield monomers and smaller oligomers, which can further be used as biobased fuels or as platform chemicals. [19][20][21] For sustainability purposes, there is a need to create value from all the streams resulting from a biorefining process.…”

Strategies for the Removal of Polysaccharides from Biorefinery Lignins: Process Optimization and Techno Economic Evaluation

“…Despite the challenges, which are (i) the structural complexity and heterogeneity of lignin, (ii) the variability of biomass sources and treatment processes, and (iii) the presence of impurities [11], the number of publications about lignin isolation, purification, fractionation, chemical modification, and potential applications has grown exponentially in the recent years [4,12,13]. These applications include binders [14], surfactants [15], dispersants for cement [16], carbon fibers [17,18], epoxy, polyurethane and phenol formaldehyde resins [19][20][21][22], thermoplastic elastomers [23], and fire retardants [13] among others. Given the applicability in diverse segments, lignin valorization could play an essential role in the bio-based economy and will contribute to the mitigation of climate change [24].…”