ABSTRACT
The development of strategies allowing either the production of high value phenolics, or the isolation of properties-enhanced materials from technical lignins represents a fundamental step in the industrial upcycling of technical lignins. Both aims are met by the strategy presented in the present work, relying on the coupling of solvent-based fractionation with the oxidative action of a new type of alkaline-stable genetically modified bacterial laccase. The described approach succeeded in the tandem, high-yield and selective isolation of valuable lignin-monomeric compounds (MCs) and high molecular weight and hydrophobicity-tailored polymerised materials (PMs) from two technical lignins, namely softwood kraft lignin (SKL), and wheat straw organosolv lignin (WSL). With respect to MCs, higher yields as compared to similar studies (up to 17.2â mg/g) were achieved. PMs from SKL samples where characterised by an almost quadrupled Mw, while in the case of WSL the Mw was approximately doubled. Noteworthy, the reaction conditions were optimized in terms of reaction temperature, time, enzymatic loading, and alkalinity for the selective production of single MCs. Most interestingly, technical lignins as well as their fractions and the PMs deriving from their laccase-catalysed oxidation showed increased hydrophobicity.
ABSTRACT
The valorization of lignin into value-added products by oxidative conversion is a widely studied strategy. However, in many cases, this approach has limited scope for integration into industrial processes. The objective of our work is to maximize overall lignin utilization to produce diverse value-added products with a focus on integration in the existing industrial pulp and paper processes. The utilization of the sequential oxidation strategy using oxygen and ozone resulted in kraft lignin with a marked improvement in carboxyl content and also allowed the formation of vanillin and vanillic acid in the oxygen stage. The sequentially oxidized lignin (OxL-COOH) was then cured with poly(ethylene glycol) diglycidyl ether (PEG-epoxy) to form high-lignin-content (>48 wt %) vitrimers with high thermal stability, fast relaxation, swelling, and self-healing due to the presence of bond-exchangeable cross-linked networks. Overall, this study provides a novel approach for the multidimensional valorization of lignin and demonstrates an integrated approach for kraft lignin valorization in the pulp and paper industry.
ABSTRACT
The development of sustainable biorefinery products is confronted, among others, with the challenge of lignin and tannin valorization. These abundant, renewable aromatic biopolymers have not been widely exploited due to their inherent structural complexity and high degrees of variability and species diversity. The lack of a defined primary structure for these polyphenols is further compounded with complex chemical alterations induced during processing, eventually imparting a large variety of structural features of extreme significance for any further utilization efforts. Consequently, a protocol for the rapid, simple, and unequivocal identification and quantification of the various functional groups present in natural polyphenols, is a fundamental prerequisite for understanding and accordingly tailor their reactivity and eventual utility. Quantitative 31P NMR offers the opportunity to rapidly and reliably identify unsubstituted, o-mono substituted, and o-disubstituted phenols, aliphatic OHs, and carboxylic acid moieties in lignins and tannins with broad application potential. The methodology consists of an in situ quantitative lignin or tannin labeling procedure using a suitable 31P containing probe, followed by the acquisition of a quantitative 31P NMR spectrum in the presence of an internal standard. The high natural abundance of the 31P nucleus allows for small amounts of the sample (~30 mg) and short NMR acquisition times (~30-120 min) with well-resolved 31P signals that are highly dependent on the surrounding chemical environment of the labeled OH groups.
