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The kraft process is the main process used for the production of chemical pulps. In this process, an efficient delignification is achieved, yielding bleachable grade pulps. In recent years, there h...

Replacing fossil-based resources with renewable alternatives is generally acknowledged as a critical component to address several of today's environmental concerns. In this context, lignocellulosic biomass is an attractive, sustainable resource. However, the constitutional biopolymers of interest are locked in the structural complexity of the plant cell walls, which defines their properties and contributes to fractionation recalcitrance. One of the key suspects restricting fractionation of the biopolymers in high yield is the presence of lignin–carbohydrate bonds forming a matrix referred to as lignin–carbohydrate complex (LCC). Nevertheless, covalent bonds between lignin and carbohydrates, remain one of the most controversial topics in lignocellulose chemistry. This challenge can be attributed to the slow progress made in their research, which also forms the basis for this review. Herein, we will critically discuss the literature with a particular focus on the latest characterization and analytical techniques. Discussions on existing techniques and, importantly the drawbacks with them should be compelling to researchers in the area, especially at this time when crucial issues surrounding the realization of biorefineries need to be addressed.

Recent years have seen the development of technically feasible methods to retrieve kraft lignin from the black liquor as solids or liquids. This opens enormous opportunities to position kraft ligni ...

As part of the continuing efforts in lignin‐first biorefinery concepts, this study concerns a consolidated green processing approach to obtain high yields of hemicelluloses and lignin with a close to native molecular structure, leaving a fiber fraction enriched in crystalline cellulose. This is done by subcritical water extraction of hemicelluloses followed by organosolv lignin extraction. This initial report focuses on a detailed characterization of the lignin component, with the aim of unravelling processing strategies for the preservation of the native linkages while still obtaining good yields and high purity. To this effect, a static cycle process is developed as a physical protection strategy for lignin, and advanced NMR analysis is applied to study structural changes in lignin. Chemical protection mechanisms in the cyclic method are also reported and contrasted with the mechanisms in a reference batch extraction process where the role of homolytic cleavage in subsequent repolymerization reactions is elucidated., Powers of consolidation: A consolidated green processing approach has been developed to obtain high yields of hemicelluloses and lignin with a close to native molecular structure, by subcritical water extraction of hemicelluloses followed by organosolv lignin extraction. A static cycle process is developed as a physical protection strategy for lignin with advanced NMR analysis to monitor any structural changes in the lignin.

The presence of covalent bonds between lignin and polysaccharides was investigated in dissolving pulps made with one-stage and two-stage acidic sulfite pulping for 100% pine heartwood raw material. ...

In a recent study, it was suggested that there could be direct associations between cellulose and lignin in mild alkaline cooked pulps. The observation was based on studies showing that the molecul ...

Hydrothermal extraction of hemicelluloses from lignocellulosic biomass for conversion to renewable materials or fuels has captured attention. The extraction is however partial and some lignin is codissolved. Herein, we investigated the role of molecular structure in the recalcitrance. Wood meal of Spruce and Birch were subjected to pressurized hydrothermal extraction at 160 °C for 2 h, which extracted 68–75% of the hemicelluloses. 2D heteronuclear single quantum coherence (HSQC) NMR, HSQC-TOCSY, and 13C NMR were applied for structural studies of both extracts and residues. Subsequent to the known partial hydrolysis of native carbon-2 and carbon-3 acetates in hemicellulose, some acetylation of primary alcohols on hemicelluloses and lignin was observed. Lignin carbohydrate complexes (LCC) were detected in both the extracts and residues. In Spruce extracts, only the phenyl glycoside-type of LCC was detected. Birch extracts contained both the phenyl glycoside and benzyl ether-types. In the hydrothermal wood r...

Lignin extracted from trees is one of the most abundant biopolymers on Earth. Quinone, a substructure in lignin, can be used for energy storage via reversible redox reactions through absorbing and releasing electrons and protons. However, these efforts have encountered hindrances, such as short life cycle, low cycling efficiency, and a high self-discharge rate. All of these issues are related to electrode dissolution by electrolyte solvents and the insulating nature of lignin. Addressing these critical challenges, for the first time, we use a reconfigurable and hierarchical graphene cage to capture the lignin by mimicking the prey-trapping of venus flytraps. The reconfigurable graphene confines the lignin within the electrode to prevent dissolution, while acting as a three-dimensional current collector to provide efficient electron transport pathways during the electrochemical reaction. This bioinspired design enables the best cycling performance of lignin reported so far at 88% capacitance retention for ...

The covalent connectivity between lignin and polysaccharides forming the so-called lignin–carbohydrate complexes (LCCs) is important to obtain fundamental knowledge on wood formation and may shed light on molecular aspects of wood processing. Although widely studied, unequivocal proofs of their existence in native-state biomass are still lacking, mainly because of harsh preanalytical fractionation conditions that could cause artifacts. In the present study, we applied a mild protocol for quantitative fractionation of LCCs and performed detailed structural studies using 2D HSQC NMR spectroscopy, 31P NMR spectroscopy, and thioacidolysis in combination with GC–MS and GC with flame ionization detection. The detailed structural analysis of LCCs, including both lignin and the carbohydrate skeleton, unveiled insights into the role of molecular structure of xylan on the type of lignin–carbohydrate (LC) bonds formed. More specifically, it is shown that xylan LCCs differ in the degree of substitution of hydroxyl fu...

Lignin is the most abundant renewable source of phenolic compounds with great application potential in renewable materials, biofuels and platform chemicals. The current technology for producing cellulose-rich fibers co-produces heterogeneous lignin, which includes an untapped source of monomeric phenolics. One such monomer also happens to be the main monomer in soft wood lignin biosynthesis, namely coniferyl alcohol. Herein, we investigate the potential of coniferyl alcohol as a platform monomer for the biomimetic production of tailored functionalized oligolignols with desirable properties for material synthesis. Accordingly, a bifunctional molecule with at least one carboxyl-ended functionality is included with coniferyl alcohol in biomimetic lignin synthesis to, in one pot, produce a functionalized lignin. The functionalization mechanism is a nucleophilic addition reaction to the quinone methide intermediate of lignin polymerization. The solvent system applied was pure water or 50% aqueous acetone. Several bi-functional molecules differing in the second functionality were successfully inserted in the lignin demonstrating the platform component of this work. Detailed characterization was performed by a combination of NMR techniques which include 1H NMR, COSY-90, 31P NMR, 13C NMR, 13C APT, HSQC, HMBC and HSQC TOCSY. Excellent selectivity towards benzylic carbon and a high functionalization degree were noted. The structure of lignin was tailored through the solvent system choice, with 50% aqueous acetone producing a skeletal structure favorable for a high degree of functionalization. Finally, material concepts are demonstrated using classical thiol–ene- and Diels–Alder-chemistries to show the potential for the thermoset and thermoplastic concepts, respectively. The functionalization concept presents unprecedented opportunities for the green production of lignin-based recyclable biomaterials.

Hardwoods constitute an essential renewable resource for the production of platform chemicals and bio-based materials. A method for the sequential extraction of hemicelluloses and lignin from hardwoods is proposed using subcritical water in buffered conditions without prior delignification. This allows the cascade isolation of mannan, xylan and lignin-carbohydrate complexes based on their extractability and recalcitrance in birch lignocellulose. The time evolution of the extraction was monitored in terms of composition, oligomeric mass profiling and sequencing of the hemicelluloses, and molecular structure of the lignin and lignin-carbohydrate complexes (LCCs) by heteronuclear single quantum coherence nuclear magnetic resonance (2D HSQC NMR). The minor mannan and pectin populations are easily extractable at short times (QC 20180509

The Glucuronoyl esterases (GE) have been proposed to target lignin-carbohydrate (LC) ester bonds between lignin moieties and glucuronic acid side groups of xylan, but to date, no direct observations of enzymatic cleavage on native LC ester bonds have been demonstrated. In the present investigation, LCC fractions from spruce and birch were treated with a recombinantly produced GE originating from Acremonium alcalophilum (AaGE1). A combination of size exclusion chromatography and (31) P NMR analyses of phosphitylated LCC samples, before and after AaGE1 treatment provided the first evidence for cleavage of the LC ester linkages existing in wood.