Structural Features of Alkaline Dioxane Lignin and Residual Lignin from Eucalyptus grandis × E. urophylla

Capacity of Aqueous Solutions of the Ionic Liquid 1-Ethyl-3-methylimidazolium Acetate to Partially Depolymerize Lignin at Ambient Temperature and Pressure

Lignin is a very attractive and abundant biopolymer with the potential to be a biorenewable source of a large number of value-added organic chemicals. The current state-of-the-art methods fail to provide efficient valorization of lignin in this regard without the involvement of harsh conditions and auxiliary substances that compromise the overall sustainability of the proposed processes. Making an original approach from the set of mildest temperature and pressure conditions, this work identifies and explores the capacity of an aqueous solution of the nonvolatile ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) to partially depolymerize technical lignin (Indulin AT) by means of a treatment consisting in the simple contact at ambient temperature and pressure. Among a considerable number of valuable phenolic molecules that were identified in the resulting fluid, vanillin (yield of about 3 g/kg) and guaiacol (yield of about 1 g/kg) were the monophenolic compounds obtained in a higher concentration. The properties of the post-treatment solids recovered remain similar to those of the original lignin, although with a relatively lower abundance of guaiacyl units (in agreement with the generation of guaiacyl-derived phenolic molecules, such as vanillin and guaiacol). The assistance of the treatment with UV irradiation in the presence of nanoparticle catalysts does not lead to an improvement in the yields of phenolic compounds.

The isolation of lignin with native-like structure

Searching for renewable alternatives for fossil carbon resources to produce chemicals, fuels and materials is essential for the development of a sustainable society. Lignin, a major component of lignocellulosic biomass, is an abundant renewable source of aromatics and is currently underutilized as it is often burned as an undesired side stream in the production of paper and bioethanol. This lignin harbors great potential as source of high value aromatic chemicals and materials. Biorefinery schemes focused on lignin are currently under development with aim of acquiring added value from lignin. However, the performance of these novel lignin-focused biorefineries is closely linked with the quality of extracted lignin in terms of the level of degradation and modification. Thus, the reactivity including the degradation pathways of the native lignin contained in the plant material needs to be understood in detail to potentially achieve higher value from lignin. Undegraded native-like lignin with an as close as possible structure to native lignin contained in the lignocellulosic plant material serves as a promising model lignin to support detailed studies on the structure and reactivity of native lignin, yielding key understanding for the development of lignin-focused biorefineries. The aim of this review is to highlight the different methods to attain "native-like" lignins that can be valuable for such studies. This is done by giving a basic introduction on what is known about the native lignin structure and the techniques and methods used to analyze it followed by an overview of the fractionation and isolation methods to isolate native-like lignin. Finally, a perspective on the isolation and use of native-like lignin is provided, showing the great potential that this type of lignin brings for understanding the effect of different biomass treatments on the native lignin structure.

A Value-Added Utilization Method of Sugar Production By-Products from Rice Straw: Extraction of Lignin and Evaluation of Its Antioxidant Activity

To value-added utilization of the rice straw, two types of lignin were extracted from the by-products of sugar production. The ether-extracted lignin with a purity of 98.7% was extracted from the pretreatment filtrate with two times the concentrated filtrate volume of ether, where the lignin yield was 6.62 mg/g of the rice straw. The ball-milled lignin with a purity of 99.6% was extracted from the milled enzymatic hydrolysis residue with a 1,4-dioxane solution, where the revolution speed and grinding time were 300 rpm and 12 h, respectively. The yield of ball-milled lignin was 34.52 mg/g of the rice straw, which was 421.5% higher than that extracted from extract-free rice straw. In the process of rice straw pretreatment and lignin extraction, 76.43% by mass of phosphotungstic acid catalyst and approximately 98% by volume of 1,4-dioxane solution could be recycled and reused. Compared with the soda lignin extracted from papermaking black liquor, the scavenging rates of DPPH radical and ABTS+ radical of ether-extracted lignin increased by 36.26% and 41.18%, respectively, while the above scavenging rates of ball-milled lignin increased by 30.22% and 37.75%, respectively. Moreover, the reducing power of the two extracted lignins was also stronger than that of soda lignin. The ether-extracted lignin and ball-milled lignin have the potential to be developed as natural macromolecular antioxidants.

Extraction of Noncondensed Lignin from Poplar Sawdusts with p-Toluenesulfonic Acid and Ethanol

The traditional pretreatment leads to the recalcitration of C-C bonds during lignin fractionation, thus hindering their depolymerization into aromatic monomers. It is essential to develop an applicable approach to extract noncondensed lignin for its high-value applications. In this work, noncondensed lignins were extracted from poplar sawdust using recyclable p-toluenesulfonic acid for cleaving lignin-carbohydrate complex bonds effectively and ethanol as a stabilization reagent to inhibit lignin condensation. Lignin yield of 83.74% was recovered by 3 mol/L acid in ethanol at 85 °C for 5 h, and carbohydrates were well preserved (retaining 98.97% cellulose and 50.01% hemicelluloses). During lignin fractionation, the acid concentration and extraction time were the major drivers of condensation. Ethanol reacted with lignin at the α-position to prevent the formation of the condensed structure. The extracted lignin depolymerized over the Pd/C catalysts gave a yield of 50.35% of aromatic monomers, suggesting that the novel extraction process provided a promising way for noncondensed lignin production.

The novel method to reduce the silica content in lignin recovered from black liquor originating from rice straw

Difficulties in the production of lignin from rice straw because of high silica content in the recovered lignin reduce its recovery yield and applications as bio-fuel and aromatic chemicals. Therefore, the objective of this study is to develop a novel method to reduce the silica content in lignin from rice straw more effectively and selectively. The method is established by monitoring the precipitation behavior as well as the chemical structure of precipitate by single-stage acidification at different pH values of black liquor collected from the alkaline treatment of rice straw. The result illustrates the significant influence of pH on the physical and chemical properties of the precipitate and the supernatant. The simple two-step acidification of the black liquor at pilot-scale by sulfuric acid 20w/v% is applied to recover lignin at pH 9 and pH 3 and gives a percentage of silica removal as high as 94.38%. Following the developed process, the high-quality lignin could be produced from abundant rice straw at the industrial-scale.

Discrepancy of lignin dissolution from eucalyptus during formic acid fractionation

Understanding the structure and properties of lignin has important practical significance for its further applications. In this case, eucalyptus was fractionated with 88% formic acid at 101 °C for different durations, and the removal efficiency as well as the chemical structure of lignin at various stages were comparatively analyzed. The obtained data indicated that with increasing reaction time, lignin was continuously removed and the process could be divided into three stages. The lignin dissolution rate was fast first and then slow, and the molecular weight of the dissolved lignin increased with time. The lignin structure was condensed and the molecular weight increased with prolonged of reaction time. Structural analysis indicated that the β-O-4' structure was largely destroyed, the G-type lignin dissolved early, and the degradation of the S-type lignin became more intensive with increasing reaction time. This is of great help for reaction control as well as the further processing of lignin byproducts.

Technologies for Eucalyptus wood processing in the scope of biorefineries: A comprehensive review

Eucalyptus is the most widely planted type of hardwoods, and represents an important biomass source for the production of fuels, chemicals, and materials. Its industrial benefit can be achieved by processes following the biorefinery concept, which is based on the selective separation ("fractionation") of the major components (hemicelluloses, cellulose and lignin), and on the generation of added-value from the resulting fractions. This article provides a in-depth assessment on the composition of Eucalyptus wood and a critical evaluation of selected technologies allowing its overall exploitation. These latter include treatments with organosolvents and with emerging fractionation agents (ionic liquids and deep eutectic solvents). The comparative evaluation of the diverse processing technologies is carried out in terms of degree of fractionation, yields and selectivities. The weak and strong points, challenges, and opportunities of the diverse fractionation methods are identified, focusing on the integral utilization of the feedstocks.