Investigation of Kraft Lignin Solubility in Protic Ionic Liquids and Their Aqueous Solutions

Conversion of Lignin to Nitrogenous Chemicals and Functional Materials

Lignin has long been regarded as waste, readily separated and discarded from the pulp and paper industry. However, as the most abundant aromatic renewable biopolymer in nature, lignin can replace petroleum resources to prepare chemicals containing benzene rings. Therefore, the high-value transformation of lignin has attracted the interest of both academia and industry. Nitrogen-containing compounds and functionalized materials are a class of compounds that have wide applications in chemistry, materials science, energy storage, and other fields. Converting lignin into nitrogenous chemicals and materials is a high-value utilization pathway. Currently, there is a large amount of literature exploring the conversion of lignin. However, a comprehensive review of the transformation of lignin to nitrogenous compounds is lacking. The research progress of lignin conversion to nitrogenous chemicals and functional materials is reviewed in this article. This article provides an overview of the chemical structure and types of industrial lignin, methods of lignin modification, as well as nitrogen-containing chemicals and functional materials prepared from various types of lignin, including their applications in wastewater treatment, slow-release fertilizer, adhesive, coating, and biomedical fields. In addition, the challenges and limitations of nitrogenous lignin-based materials encountered during the development of applications are also discussed. It is believed that this review will act as a key reference and inspiration for researchers in the biomass and material field.

Tris(2-hydroxyethyl)ammonium-Based Protic "Ionic Liquids": Synthesis and Characterization

The proton transfer associated with the synthesis of protic ionic liquids (PILs) is often incomplete, meaning that the parent compounds may coexist with the ionic species. However, PILs are proposed for many applications as pure compounds without analysis of their ionicity. This work focuses on tris(2-hydroxyethyl)ammonium-based PILs with lactate, hydrogen succinate, hydrogen malate, and dihydrogen citrate anions. The interest of these anions lies in their low toxicity and capacity to disrupt the hydrogen-bonding network inherent to biopolymers. To improve current synthesis methods of this kind of PILs, which frequently lead to impurities derived from decomposition of reactants, working in the absence of solvents and at moderate temperatures is proposed. Through NMR studies, the ionicity of these systems was found to be low, from 20% to 86%, so the widely used term "ionic liquid" is not rigorous and must be used with caution. The un-ionized acid and base species coexist with the corresponding ionic forms, and this has to be considered in the studies involving these chemicals. The thermal characterization of the PILs was carried out. The influence of the anion on the thermal stability was found to be low. Isothermal thermogravimetric analysis showed that mass loss of these PILs starts at temperatures close to 350 K.

Thermal Stability of Protic Ionic Liquids

While protic ionic liquids (ILs) have found great success as solvents for a broad range of applications, little is known about their degradation when exposed to temperatures above ambient for extended periods of time. Here, we report the thermal stability of six protic ILs, namely, ethylammonium nitrate, ethylammonium formate, ethylammonium acetate, ethanolammonium nitrate, ethanolammonium formate, and ethanolammonium acetate. The effect of heating each ionic liquid to 60 °C for 1 h or 1 week (sealed or open to the atmosphere) was evaluated by considering the changes to water content, pH, mass, thermal phase transitions, and molecular structure after each treatment. Heating each of the six ILs when sealed led to measurable shifts in their water content and 10 wt % pH, but there was no significant change in their mass, thermal phase transitions according to differential scanning calorimetry (DSC), or molecular structure using proton nuclear magnetic resonance (1H NMR) spectra, indicating that the samples were largely unchanged. The samples that were heated open to the atmosphere also displayed no significant changes after 1 h but displayed significant changes after 1 week.

Constructing protic porous ionic liquids via one-step coupling neutralization reaction for extraction-adsorption coupled desulfurization

Porous ionic liquids (PILs) offer a distinctive combination of liquid-like fluidity and solid porosity, making them well-suited for various applications including separation, catalysis, and energy storage. Nevertheless, the design limitations and complex synthesis processes have hindered the development of PILs. Here, the one-step coupling neutralization reaction (OCNR) method has been first proposed for the controllable synthesis of functionalized protic porous ionic liquids (PPILs). Specifically, three types of PPILs have been synthesized based on tuning the position of the corona amino functional groups. The results indicate the crucial role of protic ion pairs in the formation of pure liquid PPILs with low viscosity. The extraction efficiency has obviously increased after introducing the porous materials from 38.5% to 51.9%. The results showed PPILs play good extraction-adsorption coupled desulfurization (EADS) performance. The density functional theory (DFT) results show that both the protic ion pairs and the porous structure have significant roles in EADS, with the former offering CH···π interactions, while the latter provides hydrogen bonding (CH···O) interactions. Ultimately, the strategy simplifies the synthesis process, providing a new idea for the directional design of low-viscosity PILs with specific functions.

Critical Techniques for Overcoming the Diffusion Limitations in Heterogeneously Catalytic Depolymerization of Lignin

Heterogeneously catalyzed depolymerization of lignin to value-added chemicals is increasingly attractive but highly challengeable. Particularly, the diffusion limitation of lignin macromolecule to the solid catalyst surface is a big barrier, which significantly decreases the yield of monomer while increasing char formation. Therefore, for the potential industrial utilization of lignin, new knowledge focused on the size of lignin particles is of great importance to offer guidance for promoting lignin depolymerization and suppressing condensation in the heterogeneously catalytic systems. In this Review, the size of lignin particles and macromolecules are summarized. Previous approaches for improving the mass diffusion including enhancing the solubility of lignin and exploitation of hierarchical and "solubilized" materials are also discussed. Based on these, a constructive perspective is proposed. Thus, this work provides a new insight on the rational design of heterogeneous catalytic techniques for efficient utilization of the aromatic polymer of lignin.

Ultrasound enhanced solubilization of forest biorefinery hydrolysis lignin in mild alkaline conditions

In the forest biorefinery, hydrolysis lignin (HL) is often dissolved with high concentration NaOH solution, followed by acid precipitation to obtain purified HL. For the first time, this study evaluates the effect of ultrasound (US) on the dissolution of industrially produced HL in aqueous NaOH solutions and the acid precipitation yield of HL. The solubility of HL in mild aqueous NaOH solutions was studied with and without US treatment at 20 kHz concerning the solid-to-liquid ratio, molecular weight of dissolved fractions and structural changes in dissolved HL. Results showed that the solubility of HL at 25 °C was strongly dependent on NaOH concentration. However, the US treatment significantly improved the solubility of HL, reaching a solubility plateau at 0.1 NaOH/HL ratio. US treatment enhanced the solubilization of HL molecules with higher MW compared to conventional mixing. The increase of HL solubility was up to 30 % and the recovery yield of purified lignin with acid precipitation was 37 % higher in dilute NaOH solution. A significant result was that the M_w of dissolved HL in homogeneous alkali solutions decreased with US treatment. SEC, HSQC and ³¹P NMR analyses of dissolved HL characteristics showed that both, the mechanoacoustic and sonochemical solubilization pathways contribute to the dissolution process. However, US does not cause major changes in the HL structure compared to the native lignin. Indeed, US technology has the potential to advance the dissolution and purification of HL in biorefineries by reducing the amount of chemicals required; thus, more controlled and environmentally friendly conditions can be used in HL valorization.

Lignin as a Renewable Building Block for Sustainable Polyurethanes

Currently, the pulp and paper industry generates around 50-70 million tons of lignin annually, which is mainly burned for energy recovery. Lignin, being a natural aromatic polymer rich in functional hydroxyl groups, has been drawing the interest of academia and industry for its valorization, especially for the development of polymeric materials. Among the different types of polymers that can be derived from lignin, polyurethanes (PUs) are amid the most important ones, especially due to their wide range of applications. This review encompasses available technologies to isolate lignin from pulping processes, the main approaches to convert solid lignin into a liquid polyol to produce bio-based polyurethanes, the challenges involving its characterization, and the current technology assessment. Despite the fact that PUs derived from bio-based polyols, such as lignin, are important in contributing to the circular economy, the use of isocyanate is a major environmental hot spot. Therefore, the main strategies that have been used to replace isocyanates to produce non-isocyanate polyurethanes (NIPUs) derived from lignin are also discussed.

Pretreatment of Jerusalem artichoke stalk using hydroxylammonium ionic liquids and their influences on 2,3-butanediol fermentation by Bacillus subtilis

Pretreatment of lignocellulose is a vital step for biological production of bio-chemicals and bio-fuels. In this work, the pretreatment of Jerusalem artichoke stalk (JAS) by hydroxylammonium ionic liquids was evaluated based on pretreatment efficiency including polysaccharide recovery and enzymatic digestibility, and influence of ionic liquids on 2,3-butanediol fermentation using Bacillus subtilis. The results showed ethanolammonium acetate (EOAA) was efficient in JAS pretreatment, and maximum cell density was increased 25% when EOAA concentration was not greater than 0.3 mol/L in medium, while the total concentration of acetoin and 2,3-butanediol was 15% greater than the control at 0.1 mol/L EOAA. After the pretreatment under optimized conditions of 170 °C for 5-h and liquid-solid ratio of 18, about 87% cellulose and 75% hemicellulose were recovered, and glucose yield of 64% and xylose of 66% were obtained after 24-h hydrolysis of JAS residue by cellulase (15 FPU/g) with solid loading of 10 wt%.

Extraction and isolation of lignin from ash tree (Fraxinus exselsior) with protic ionic liquids (PILs)

Protic ionic liquids (PILs) have been considered effective solvents for the selective separation and recovery of cellulose from lignocellulosic biomass. However, PILs can also be utilized for the extraction and conversion of lignin into fuels and value-added products. The objective of this work was to study the extraction of lignin from ash tree (Fraxinus exselsior) hardwood biomass using three different PILs-pyridinium acetate, pyridinium formate [Py][For], and pyrrolidinium acetate. Fiber analysis was used to determine the biochemical composition of the left-over biomass after lignin separation. FTIR and NMR were applied to determine the structure of dissolved lignin. Additionally, the regeneration potential and recyclability of PILs were assessed. Our results demonstrate that treatment with [Py][For] at 75 °C yields the highest percentage of lignin dissolution from biomass. This indicates that PILs could be used for Kraft lignin dissolution as well as separation of lignin from raw, milled biomass.

Utilization of Cellulose to Its Full Potential: A Review on Cellulose Dissolution, Regeneration, and Applications

As the most abundant natural polymer, cellulose is a prime candidate for the preparation of both sustainable and economically viable polymeric products hitherto predominantly produced from oil-based synthetic polymers. However, the utilization of cellulose to its full potential is constrained by its recalcitrance to chemical processing. Both fundamental and applied aspects of cellulose dissolution remain active areas of research and include mechanistic studies on solvent-cellulose interactions, the development of novel solvents and/or solvent systems, the optimization of dissolution conditions, and the preparation of various cellulose-based materials. In this review, we build on existing knowledge on cellulose dissolution, including the structural characteristics of the polymer that are important for dissolution (molecular weight, crystallinity, and effect of hydrophobic interactions), and evaluate widely used non-derivatizing solvents (sodium hydroxide (NaOH)-based systems, N,N-dimethylacetamide (DMAc)/lithium chloride (LiCl), N-methylmorpholine-N-oxide (NMMO), and ionic liquids). We also cover the subsequent regeneration of cellulose solutions from these solvents into various architectures (fibers, films, membranes, beads, aerogels, and hydrogels) and review uses of these materials in specific applications, such as biomedical, sorption, and energy uses.