Lignin dissolution model in formic acid-acetic acid-water systems based on lignin chemical structure

A review on the calculation and application of lignin Hansen solubility parameters

Hansen solubility parameters (HSPs) play a critical role in the majority of processes involving lignin depolymerization, separation, fractionation, and polymer blending, which are directly related to dissolution properties. However, the calculation of lignin HSPs is highly complicated due to the diversity of sources and the complexity of lignin structures. Despite their important role, lignin HSPs have been undervalued, attracting insufficient attention. This review summarizes the calculation methods for lignin HSPs and proposes a straightforward method based on lignin subunits. Furthermore, it highlights the crucial applications of lignin HSPs, such as identifying ideal solvents for lignin dissolution, selecting suitable solvents for lignin depolymerization and extraction, designing green solvents for lignin fractionation, and guiding the preparation of lignin-based composites. For instance, leveraging HSPs to design a series of solvents could potentially achieve sequential controllable lignin fractionation, addressing issues of low value-added applications of lignin resulting from poor homogeneity. Notably, HSPs serve as valuable tools for understanding the dissolution behavior of lignin. Consequently, we expect this review to be of great interest to researchers specializing in lignin and other macromolecules.

A universal approach for producing lignin-based monocomponent fiber by one-step ethanol fractionation

To achieve sustainable whole process of carbon-fiber production and high-value utilization of lignin, one-step ethanol fractionation followed by coaxial electrospinning was applied to produce lignin-based monocomponent carbon-fiber. To elucidate the mechanism, hydrothermal extracted poplar lignin (HPL) were obtained to be divide into two parts through ethanol fractionation, in which the ethanol-soluble lignin (ESL) was eletrcospun into fiber precursors. Then, to verify the universality of this method, four more lignin were extracted to produce fiber precursors, after which five kinds of carbon fibers were prepared by carbonization of the corresponding precursors. Structural analysis showed that ESL of HPL is a small and highly branched three-dimensional stereomolecules. Combined with the SEM results of fiber precursors, the mechanism which hydrogen bonding promotes fiber formation was elucidated. Among all five samples, carbon-fiber prepared from HPL possesses the minimum fiber diameter of 557 nm, the smallest interplanar spacing of 0.3909 nm, ID/IG value of 0.6345 and the largest specific surface area of 408.15 m²/g. This work proposes a universal method to prepare lignin-based monocomponent carbon-fiber, in which carbon-fibers prepared from HPL exhibits the best comprehensive performance and can be applied to capture radioactive iodine.

Highly Efficient Fractionation of Cornstalk into Noncondensed Lignin, Xylose, and Cellulose in Formic Acid

Traditional pretreatment of lignocellulose is usually conducted under higher acidic and high temperature conditions, which leads to both the degradation of sugar and the condensation of lignin, hindering the subsequent conversion. An effective approach to fractionate lignocellulose into 93.9% of noncondensed lignin, 99.4% of cellulose, 17.8% of xylose, and 66.7% of xylooligosaccharides under mild conditions was developed using the formic acid solution at 80 °C for 100 min. The β-O-4 bond content of lignin fractionated with formic acid (54.6 per 100 C9 units) was higher than dioxasolv lignin (48.4 per 100 C9 units), indicating that formic acid pretreatment well protected the ether bonds in lignin. Therefore, the hydrogenolysis of fractionated lignin contributed to 28.0% of aromatic monomer yield, which was comparable to dioxasolv lignin. As cellulose possesses a large amount of porosity because lignin was separated from lignocellulose, the hydrolysis of fractionated cellulose by molten salt hydrates gave a 96.4% of glucose yield.

An integral method for determining the molecular composition of lignin and its application

Lignin is a natural and renewable aromatic polymer, but only about 2% of lignin is utilized with high added value. Polydispersity and heterogeneity are the key reasons for the difficulty in separation, fractionation, characterization, purification and utilization of lignin. However, the molecular weight of lignin is still described from the overall perspective of number-/weight-average molecular weight (Mn and Mw), which if far from enough to understand the heterogeneous and dispersed lignin. To provide a tool for understanding the molecular weight of lignin from a molecular perspective, an integral method for quantifying the molecular characteristics of lignin molecules at arbitrary molecular intervals on the molecular weight distribution curve of lignin was established. The molecular contents of wheat straw lignin as well as its soluble and insoluble fractions at different intervals were calculated. The ease of fractionation of small molecules with weights lower than 8000 g/mol into soluble fractions, and that of large molecules with weights higher than 10,000 g/mol into insoluble fractions were quantitatively analyzed. The established integral method will significantly help in the understanding the properties of lignin at the molecular-level, as well as the fractionation and utilization of lignin.

Structural properties and antioxidation activities of lignins isolated from sequential two-step formosolv fractionation

In order to investigate the solubility behavior of lignin in formic acid (FA) solution Phragmites australis biomass was subjected to a sequential two-step formosolv fractionation using 88% FA followed by 70% FA to obtain four specific lignin fractions, designated as IFL-88%, IFSL-70%, IFIL-70% and IFL-EtAc. The structures of the four isolated lignin fractions were successfully characterized by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy (2D-HSQC NMR), thermogravimetric analysis (TGA), and gas chromatography-mass spectroscopy (GC/MS). Furthermore, the total phenolic content of the four isolated lignin samples was assessed by Folin-Ciocalteu analysis. The data from structural properties revealed that depolymerization of the isolated lignin fractions occurred via β-O-4 cleavage, accompanied by competitive condensation reaction. Interestingly, 70% aqueous FA could separate specific lignin fractions with different antioxidant capacities of ABTS˙⁺ and DPPH radical scavenging activity. Due to the high total phenolic hydroxyl content (25%) and low molecular weight (Mw = 2760 Da) and polydispersity index (PDI = 1.5), IFL-EtAc lignin showed excellent antioxidant activity at the same concentration of 2.0 mg mL⁻¹ in comparison with three other isolated lignin fractions, and it was even equal to that of commercial antioxidant butylated hydroxytoluene (BHT). These findings are helpful to separate specific lignins with higher value as potential antioxidants by sequential two-step formosolv fractionation in lignin chemistry.

Phragmites australis was subjected to a sequential two-step formosolv fractionation using 88% formic acid (FA) followed by 70% FA to obtain lignin fractions. The relationship between structure and antioxidation of the isolated lignin was elucidated.

Fabrication and Characterization of Composite Biofilm of Konjac Glucomannan/Sodium Lignosulfonate/ε-Polylysine with Reinforced Mechanical Strength and Antibacterial Ability

In order to enforce the mechanical strength and antibacterial ability of biofilm and explore the underlying mechanism, sodium lignosulfonate (SL) and ε-polylysine (ε-PL) were introduced to fabricate the composite film of konjac glucomannan (KGM)/SL/ε-PL in the present study. According to our previous method, 1% (w/v) of KGM was the optimal concentration for the film preparation method, on the basis of which the amount of SL and ε-PL were screened by mechanical properties enforcement of film. The structure, mechanical performance and thermal stability of the film were characterized by SEM, FTIR, TGA and tensile strength tests. The optimized composite film was comprised of KGM 1% (w/v), SL 0.2% (w/v), and ε-PL 0.375% (w/v). The tensile strength (105.97 ± 4.58 MPa, p < 0.05) and elongation at break (95.71 ± 5.02%, p < 0.05) of the KGM/SL/ε-PL composite film was greatly improved compared with that of KGM. Meanwhile, the thermal stability and antibacterial property of film were also enhanced by the presence of SL and ε-PL. In co-culturation mode, the KGM/SL/ε-PL composite film showed good inhibitory effect on Escherichia coli (22.50 ± 0.31 mm, p < 0.05) and Staphylococcus aureus (19.69 ± 0.36 mm, p < 0.05) by determining the inhibition zone diameter. It was revealed that KGM/SL/ε-PL composite film shows enhanced mechanical strength and reliable antibacterial activities and it could be a potential candidate in the field of food packaging.