Modulating the properties and structure of lignins produced by alkaline delignification of sugarcane bagasse pretreated with two different mineral acids at pilot-scale

Sugarcane bagasse was pretreated with dilute phosphoric acid or sulfuric acid to facilitate cellulose hydrolysis and lignin extraction. With phosphoric acid, only 8 % of the initial cellulose was lost after delignification, whereas pretreatment with sulfuric acid resulted in the solubilization of 38 % of the initial cellulose. After enzymatic hydrolysis, the process using phosphoric acid produced approximately 35 % more glucose than that using sulfuric acid. In general, the lignins showed 95-97 % purity (total lignin, w/w), an average molar mass of 9500-10,200 g mol-1, a glass transition temperature of 140-160 °C, and a calorific value of 25 MJ kg-1. Phosphoric acid lignin (PAL) was slightly more polar than sulfuric acid lignin (SAL). PAL had 13 % more oxidized units and 20 % more OH groups than SAL. Regardless of the acid used, the lignins shared similar properties, but differed slightly in the characteristics of their functional groups and chemical bonds. These findings show that pretreatment catalyzed with either of the two acids resulted in lignin with sufficiently good characteristics for use in industrial processes.

Structural elucidation and targeted valorization of untractable lignin from pre-hydrolysis liquor of xylose production via a simple and robust separation approach

Effective separation of lignin macromolecules from the xylose pre-hydrolysates (XPH) during the xylose production, thus optimizing the separation and purification process of xylose, is of great significance for reducing the production costs, achieving the high value-added utilization of lignin and increasing the industrial revenue. In this study, a simple and robust method (pH adjustment) for the separation of lignin from XPH was proposed and systematically compared with the conventional acid-promoted lignin precipitation method. The results showed that the lignin removal ratio (up to 60.34 %) of this simple method was higher than that of the conventional method, and the proposed method eliminated the necessity of heating and specialized equipment, which greatly reduced the separation cost. Meanwhile, this simple method does not destroy the components in XPH (especially xylose), ensuring the yield of the target product. On the other hand, the obtained lignin was nano-scale with less condensed structures, which also possessed small molecular weights with narrow distribution, excellent antioxidant activity (8-14 times higher than commercial antioxidants) and UV protection properties. In conclusion, the proposed simple separation method could effectively separate lignin from XPH at low cost, and the obtained lignin had potential commercial applications, which would further enhance the overall profitability of industrial production.

The role of acetone-fractionated Kraft lignin molecular structure on surface adhesion to formaldehyde-based resins

One of the key strategies for valorizing kraft lignin (KL) into value-added products such as bio-based adhesives is to perform solvent fractionation of KL to produce lignin with improved homogeneity. Understanding the structure and properties of fractionated KL will aid in the selection of the best samples for certain applications. In this study, acetone-fractionated KL from softwood and hardwood was characterized to understand its chemical structure, elemental composition, molecular weight, and thermal properties. The results revealed that acetone-insoluble KL (AIKL) fractions from softwood and hardwood have greater molecular weight, polydispersity, glass temperature, carbohydrate content, aliphatic hydroxyl groups, and a variety of native wood lignin side chains. In contrast, acetone-soluble KL (ASKL) fractions have a significantly lower molecular weight and polydispersity, a lower glass-transition temperature, a more condensed structure, more aromatic hydroxyl groups, and fewer native wood lignin side chains. In addition, the ASKL samples demonstrated stronger adhesive force and work of adhesion toward phenol-formaldehyde (PF) and urea-formaldehyde (UF) resins than the AIKL samples, regardless of the lignin source. These findings suggest that ASKL has great potential as a substitute for phenol in PF resins and as a green additive to reinforce UF resins.

Structural elucidation and targeted valorization of poplar lignin from the synergistic hydrothermal-deep eutectic solvent pretreatment

Elucidating the structural variations of lignin during the pretreatment is very important for lignin valorization. Herein, poplar wood was pretreated with an integrated process, which was composed of AlCl₃-catalyzed hydrothermal pretreatment (HTP, 130-150 °C, 1.0 h) and mild deep-eutectic solvents (DES, 100 °C, 10 min) delignification for recycling lignin fractions. Confocal Raman Microscopy (CRM) was developed to visually monitor the delignification process during the HTP-DES pretreatment. NMR characterizations (2D-HSQC and ³¹P NMR) and elemental analysis demonstrated that the lignin fractions had undergone the following structural changes, such as dehydration, depolymerization, condensation. Molecular weights (GPC), microstructure (SEM and TEM), and antioxidant activity (DPPH analysis) of the lignins revealed that the DES delignification resulted in homogeneous lignin fragments (1.32 < PDI < 1.58) and facilitated the rapid assemblage of lignin nanoparticles (LNPs) with controllable nanoscale sizes (30-210 nm) and excellent antioxidant activity. These findings will enhance the understanding of structural transformations of the lignin during the integrated process and maximize the lignin valorization in a current biorefinery process.

Efficient fractionation of bamboo residue by autohydrolysis and deep eutectic solvents pretreatment

Bamboo processing residue, which is rich in parenchyma cells, was treated as huge waste in bamboo processing industry, such as reassemble bamboo and bamboo flooring. Herein, autohydrolysis and rapid different deep eutectic solvents (DES) delignification strategy were consecutively performed to remove hemicelluloses and lignin from bamboo processing residue. The xylooligosaccharides (XOS) with high yield (34.35%) was achieved in the autohydrolysis process. Results showed that alkaline DES pretreatment resulted in the highest glucose yield (88.22%) and relatively high delignification rate (83.75%) as well as well-preserved lignin structures. However, the lignin fractions obtained under acidic DES conditions were tending to assemble into lignin nanoparticles (LNPs) and having excellent antioxidant activity as compared to those obtained from alkaline DES system. In brief, the combination of autohydrolysis and rapid DES delignification can achieve orientated fractionation of the components from the industrialized bamboo.

Ultrafast alkaline deep eutectic solvent pretreatment for enhancing enzymatic saccharification and lignin fractionation from industrial xylose residue

An aspirational pretreatment method for efficient fractionation and tailored valorization of large industrial biomass can ensure the realizability of sustainable biorefinery strategies. In this study, an ultrafast alkaline deep eutectic solvents (DES) pretreatment strategy was developed to efficiently extract the lignin nanoparticles and retain cellulose residues that could be readily enzymatic saccharified to obtain fermentative glucose for the bioenergy production from industrial xylose residue. Results showed that the DES pretreatment had excellent delignification performance and the regenerated DES lignin nanoparticles exhibited well-preserved structures and excellent antioxidant activity, as well as low molecular weights and relatively uniform size distribution, which could facilitate downstream catalytic degradation for production of chemicals and preparation of lignin-based materials. Under the optimal condition (DES pretreatment: 80 °C, 10 min; saccharification: 10 FPU/g, 5 wt%, 100 mg/g Tween 80), the glucose yield of 90.12% could be achieved, which was dramatically increased compared to raw materials.

Organosolv and ionosolv processes for autohydrolyzed poplar fractionation: Lignin recovery and characterization

Biomass fractionation plays a major role in the search for competitive biorefineries, where the isolation and recovery of the three woody fractions is key. In this sense, we have used autohydrolyzed hemicellulose-free poplar as feedstock to compare two fractionation processes, organosolv and ionosolv, oriented to lignin recovery. The recovered lignins were then characterize by different techniques (NMR, GPC, TGA). Both treatments were tested at different temperatures to analyze temperature influence on lignin recovery and properties. The highest lignin recovery was obtained with the ionosolv process at 135 °C, reaching a solid yield of ~70%. Lignin characterization showed differences between both treatments. Lignins enriched in C-O linkages and G units were recovered with the organosolv process, where increasing temperature led to highly depolymerized lignins. However, lignins with higher C-C linkages and S units contents were obtained with the ionosolv process, producing more thermically stable lignins. In addition, increasing temperature caused lignin repolymerization when employing ionic liquids as solvents. Therefore, this work outlines the most important differences between ionosolv and organosolv processes for biomass fractionation, focusing on lignin recovery and its properties, which is the first step in order to valorize all biomass fractions.

Short-time deep eutectic solvents pretreatment enhanced production of fermentable sugars and tailored lignin nanoparticles from abaca

Deep eutectic solvents (DES) pretreatment is a promising approach to decrease "biomass recalcitrance" and boost the cellulose bioconversion as well as lignin valorization. In this study, a short-time DES pretreatment strategy was performed to enhance the production of high-yield fermentable sugars and tailored lignin nanoparticles (LNPs) from abaca. The glucose yield reached 92.4% under the optimal pretreatment condition (110 °C, 30 min), which was dramatically increased in comparison with that (9.5%) of control abaca. Simultaneously, nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) techniques indicated that the removed and regenerated DES lignin fractions displayed depolymerized structures and have relatively low molecular weight with relatively homogeneous morphology and narrow size distribution. Transmission electron microscope (TEM) analysis indicated that these lignin fractions are LNPs and the size of the optimal LNPs fraction is ranged from 30 nm to 50 nm. Moreover, all the DES lignin exhibited excellent antioxidant activities as compared to the commercial antioxidant butylated hydroxytoluene (BHT), which can be used as a promising natural antioxidant in industry. In short, this study demonstrated that the short-time DES pretreatment will improve the enzymatic digestibility and facilitate the controllable production and valorization of LNPs from abaca biomass, which will further promote the economic and overall benefits of biorefinery.

Reduction of lignin heterogeneity using aqueous two-phase system: A facile and universal "one-step-three-fractions" approach

As the most abundant aromatic biopolymer, lignin presents great potential to produce valuable materials and chemicals. However, its large-scale value-added application is still facing many practical challenges and one of them is the unstable properties caused by lignin heterogeneity. Herein, we developed a novel "one-step-three-fractions" fractionation strategy to reduce lignin heterogeneity using aqueous two-phase system (ATPS) composed of (NH₄)₂SO₄ and ethanol. In contrast to conventional step-wise fractionation processes, the proposed process subdivided heterogeneous lignin into three homogeneous fractions in only one step: the first fraction (F1) dissolved in the ethanol-rich top layer; the second fraction (F2) dissolved in the salt-rich bottom layer and the last fraction (F3) insoluble in both two layers. F2 presented the lowest molecular weight followed by F1 while F3 showed the highest molecular weight. With the increase of molecular weight, the contents of guaiacyl unit and β-O-4 linkage increased while the content of hydrophilic groups (carboxyl and aromatic hydroxyl) decreased significantly. Moreover, the ATPS exhibited satisfactory recyclability and the fractionation approach could be applied to different types/sources of lignin. Consequently, the work indicates that ATPS is a novel and effective way to fractionate lignin and reduce its molecular weight polydispersity and structural heterogeneity in one step.

Enzymatic conversion of pretreated lignocellulosic biomass: A review on influence of structural changes of lignin

The demands of energy sustainability drive efforts to bio-chemical conversion of biomass into biofuels through pretreatment, enzymatic hydrolysis, and microbial fermentation. Pretreatment leads to significant structural changes of the complex lignin polymer that affect yield and productivity of the enzymatic conversion of lignocellulosic biomass. Structural changes of lignin after pretreatment include functional groups, inter unit linkages and compositions. These changes influence non-productive adsorption of enzyme on lignin through hydrophobic interaction and electrostatic interaction as well as hydrogen bonding. This paper reviews the relationships between structural changes of lignin and enzymatic hydrolysis of pretreated lignocellulosic biomass. The formation of pseudo-lignin during dilute acid pretreatment is revealed, and their negative effect on enzymatic hydrolysis is discussed.

Structural features and antioxidant activities of Chinese quince (Chaenomeles sinensis) fruits lignin during auto-catalyzed ethanol organosolv pretreatment

Chinese quince fruits (Chaenomeles sinensis) have an abundance of lignins with antioxidant activities. To facilitate the utilization of Chinese quince fruits, lignin was isolated from it by auto-catalyzed ethanol organosolv pretreatment. The effects of three processing conditions (temperature, time, and ethanol concentration) on yield, structural features and antioxidant activities of the auto-catalyzed ethanol organosolv lignin samples were assessed individually. Results showed the pretreatment temperature was the most significant factor; it affected the molecular weight, S/G ratio, number of β-O-4' linkages, thermal stability, and antioxidant activities of lignin samples. According to the GPC analyses, the molecular weight of lignin samples had a negative correlation with pretreatment temperature. 2D-HSQC NMR and Py-GC/MS results revealed that the S/G ratios of lignin samples increased with temperature, while total phenolic hydroxyl content of lignin samples decreased. The structural characterization clearly indicated that the various pretreatment conditions affected the structures of organosolv lignin, which further resulted in differences in the antioxidant activities of the lignin samples. These results can be helpful for controlling and optimizing delignification during auto-catalyzed ethanol organosolv pretreatment, and they provide theoretical support for the potential applications of Chinese quince fruits lignin as a natural antioxidant in the food industry.

From lignin to valuable products-strategies, challenges, and prospects

The exploration of effective approaches for the valorization of lignin to valuable products attracts broad interests of a growing scientific community. By fully unlocking the potential of the world's most abundant resource of bio-aromatics, it could improve the profitability and carbon efficiency of the entire biorefinery process, thus accelerate the replacement of fossil resources with bioresources in our society. The successful realization of this goal depends on the development of technologies to overcome the following challenges, including: 1) efficient biomass pretreatment and lignin separation technologies that overcomes its diverse structure and complex chemistry challenges to obtain high purity lignin; 2) advanced chemical analysis for precise quantitative characterization of the lignin in chemical transformation processes; 3) novel approaches for conversion of biomass-derived lignin to valuable products. This review summarizes the latest cutting-edge innovations of lignin chemical valorization with the focus on the aforementioned three key aspects.

Insight into structure-reactivity relationships for the iron-catalyzed hydrotreatment of technical lignins

The viability of several technical lignins as a source for biobased platform chemicals was investigated via hydrotreatment using a cheap Fe-based limonite catalyst and without using a solvent. In general, high-quality oils (up to 29 wt% total monomers) with an average relative composition of 55% alkylphenolics and 27% aromatics were obtained. Detailed structural investigations showed that the S-G aromatic unit content of the lignins was the most important factor positively affecting overall oil yields. A second parameter was the lignocellulose processing method. Even though alkaline lignin isolation provides more recalcitrant lignins, their lower aliphaticity and methoxy group content partially limit char and gas formation. Finally, enhanced monomer yields could be obtained irrespective of the ether linkage content, and a high amount of β-O-4 linkages actually showed a slightly negative effect on monomer yields. Overall, the results demonstrate that this route is particularly suitable for processing residual lignin streams.

Evaluation of lignins from side-streams generated in an olive tree pruning-based biorefinery: Bioethanol production and alkaline pulping

In modern lignocellulosic-based biorefineries, carbohydrates can be transformed into biofuels and pulp and paper, whereas lignin is burned to obtain energy. However, a part of lignin could be converted into value-added products including bio-based aromatic chemicals, as well as building blocks for materials. Then, a good knowledge of lignin is necessary to define its valorisation procedure. This study characterized different lignins from side-streams produced from olive tree pruning bioethanol production (lignins collected from steam explosion pretreatment with water or phosphoric acid as catalysts, followed by simultaneous saccharification and fermentation process) and alkaline pulping (lignins recovered from kraft and soda-AQ black liquors). Together with the chemical composition, the structure of lignins was investigated by FTIR, ¹³C NMR, and 2D NMR. Bioethanol lignins had clearly distinct characteristics compared to pulping lignins; a certain number of side-chain linkages (mostly alkyl-aryl ether and resinol) accompanied with lower phenolic hydroxyls content. Bioethanol lignins also showed a significant amount of carbohydrates, mainly glucose and protein impurities. By contrast, pulping lignins revealed xylose together with a dramatical reduction of side-chains (some resinol linkages survive) and thereby higher phenol content, indicating rather severe lignin degradation during alkaline pulping processes. All lignins showed a predominance of syringyl units.