Lignin oxidation with an organic peroxide and subsequent aromatic ring opening

Novel and Integrated Process for the Valorization of Kraft Lignin to Produce Lignin-Containing Vitrimers

The valorization of lignin into value-added products by oxidative conversion is a widely studied strategy. However, in many cases, this approach has limited scope for integration into industrial processes. The objective of our work is to maximize overall lignin utilization to produce diverse value-added products with a focus on integration in the existing industrial pulp and paper processes. The utilization of the sequential oxidation strategy using oxygen and ozone resulted in kraft lignin with a marked improvement in carboxyl content and also allowed the formation of vanillin and vanillic acid in the oxygen stage. The sequentially oxidized lignin (OxL-COOH) was then cured with poly(ethylene glycol) diglycidyl ether (PEG-epoxy) to form high-lignin-content (>48 wt %) vitrimers with high thermal stability, fast relaxation, swelling, and self-healing due to the presence of bond-exchangeable cross-linked networks. Overall, this study provides a novel approach for the multidimensional valorization of lignin and demonstrates an integrated approach for kraft lignin valorization in the pulp and paper industry.

Three Adhesive Recipes Based on Magnesium Lignosulfonate, Used to Manufacture Particleboards with Low Formaldehyde Emissions and Good Mechanical Properties

Adhesives represent an important part in the wood-based composite production, and taking into account their impact on the environment and human health, it is a challenge to find suitable natural adhesives. Starting from the current concerns of finding bio-adhesives, this paper aims to use magnesium lignosulfonate in three adhesive recipes for particleboard manufacturing. First, the adhesive recipes were established, using oxygenated water to oxidize magnesium lignosulfonate (Recipe 1) and adding 3% polymeric diphenylmethane diisocyanate (pMDI) crosslinker (Recipe 2) and a mixture of 2% polymeric diphenylmethane diisocyanate with 15% glucose (Recipe 3). The particleboard manufacturing technology included operations for sorting particles and adhesive recipes, pressing the mats, and testing the mechanical strengths and formaldehyde emissions. The standardized testing methodology for formaldehyde emissions used in the research was the method of gas analysis. Tests to determine the resistance to static bending and internal cohesion for all types of boards and recipes were also conducted. The average values of static bending strengths of 0.1 N/mm2, 0.38 N/mm2, and 0.41 N/mm2 were obtained for the particleboard manufacturing with the three adhesive recipes and were compared with the minimal value of 0.35 N/mm2 required by the European standard in the field. Measuring the formaldehyde emissions, it was found that the three manufacturing recipes fell into emission classes E1 and E0. Recipes 2 and 3 were associated with good mechanical performances of particleboards, situated in the required limits of the European standards. As a main conclusion of the paper, it can be stated that the particleboards made with magnesium-lignosulphonate-based adhesive, with or without crosslinkers, can provide low formaldehyde emissions and also good mechanical strengths when crosslinkers such as pMDI and glucose are added. In this way magnesium lignosulfonate is really proving to be a good bio-adhesive.

Antisolvent versus ultrasonication: Bottom-up and top-down approaches to produce lignin nanoparticles (LNPs) with tailored properties

In spite of the increasing amount of literature on the production and application of lignin nanoparticles (LNPs), little or no attention has been paid so far to the influence of different production methods on the properties of these nanostructures. Herein, we propose a comprehensive study to assess the impact of several factors on the color, morphology, colloidal stability, antioxidant capacity, and UV-shielding performance of LNPs. LNPs were obtained by two different routes: a bottom-up approach based on the self-assembly in a solvent-antisolvent system with acetone/lignin/water; or a top-down approach based on the ultrasonication of never-dried lignin aqueous suspensions. The starting lignin was extracted from elephant grass leaves or stems, so that the influence of anatomical origin and molecular weight could also be investigated. Moreover, lignin was oxidized prior to being converted into LNPs, allowing for comparisons between different oxidation degrees. This study showed that interesting properties of LNPs can be easily tailored and combined focusing on the various applications of these versatile nanostructures. In a model application, different types of LNPs were incorporated into poly(vinyl alcohol)-based nanocomposites, modulating the UV-protection capability of the polymer matrix.

Mechanical Properties and Formaldehyde Release of Particleboard Made with Lignin-Based Adhesives

The aim of this research was to evaluate the potential of magnesium lignosulfonate as adhesive in particleboard manufacturing. Diphenylmethane diisocyanate (PMDI) between 1% and 3% and glucose (1% of the lignosulfonate content) were added as potential cross-linkers in the adhesive formulations. Mixed beech and spruce wood, 30% beech wood and 70% spruce wood, were employed for the configuration of the panel structure. The density, mechanical properties and formaldehyde emission of single-layer particleboard were investigated. Spectroscopic analysis (FTIR) revealed structural changes brought by oxidation that may indicate depolymerization by the splitting of C-O-C bonds and formation of carbonyl groups. Mechanical properties were improved, and the highest average values were recorded for panels having as adhesives oxidized lignin with cross-linkers as follow: 15 N/mm2 (MOR), 3320 N/mm2 (MOE) and 0.48 N/mm2 (IB). The density profile presented higher values for faces in case of oxidized lignin panels. Changes were observed for oxidized lignin with cross-linker panels wherein the core had higher values. The results showed that the panels manufactured with adhesives composed of oxidized lignosulfonate (20% of the dried wood particles weight) and the addition of PMDI and glucose in various percentages have a positive influence on their formaldehyde release and mechanical properties requested by EN 312 (2004) standard.

Towards a new understanding of the retro-aldol reaction for oxidative conversion of lignin to aromatic aldehydes and acids

The retro-aldol reaction is one of the key steps involved in the oxidative conversion of lignin to aromatic aldehydes and acids. In principle, the retro-aldol reaction can proceed in the absence of oxygen. In this work, a new approach based on the influence of oxygen on the oxidation of lignin was investigated. In this approach, the duration of oxygen charged during the reaction was optimized to, for the first time, improve the yield of aromatic aldehydes and acids. The effect of reaction chemistry, time, temperature, and lignin feedstock plays a key role on the yield of aromatic aldehydes and acids. At 140 °C, oxidation of softwood Lignoboost kraft lignin for 40 min results in combined maximum yield of 5.17% w/w of vanillin and vanillic acid. In comparison, using the new approach in which oxygen was charged for only 20 min during the 40 min reaction improved this yield considerably to 6.95%. Further, yield improvement was obtained when applying this approach to different lignin feedstocks. Oxidation also increased the carboxyl content in lignin from 0.49 mmol/g to 1.41 mmol/g which represents a marked improvement. The current study provides new evidence showing that the oxidation reaction is a crucial pathway for lignin valorization.

Effect of lignin-based monomer on controlling the molecular weight and physical properties of the polyacrylonitrile/lignin copolymer

In this work, lignin was grafted with acrylonitrile to control the molecular weights and molecular architecture of polyacrylonitrile (PAN)/lignin copolymer. Lignin-acrylonitrile monomer (LA-AN) and its copolymers with AN were synthesized successfully. First, lignin was aminated (LA) and then grafted with 2-chloroacrylonitrile to prepare LA-AN. The copolymerization of LA-AN and AN was carried out using 2,2-azobis(2-methylpropionitrile) as initiator. The modification, grafting, and copolymerization were confirmed with Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and X-ray photoelectron spectroscopy. Contrary to the previous studies, gel permeation chromatography showed that the molecular weight of the copolymers was increased significantly due to the presence of lignin (up to 203,944). Viscosity analysis revealed that the addition of lignin reduces the viscosity of the copolymer solution. While thermogravimetric analysis showed improvement in the degradation temperature, and lowering of the melt temperature, as revealed by differential scanning calorimetry. These findings indicated that the attaching acrylonitrile on lignin molecules result in control of the molecular weight and molecular structure of PAN/Lignin copolymers which results in enhanced solubility, spinnability, and other properties associated with molecular weight.

Singlet oxygen mediated the selective removal of oxytetracycline in C/Fe3C/Fe0 system as compared to chloramphenicol

Reactive oxygen species (ROS) production for Fe⁰ is limited because of the formed iron corrosion products. In this study, C/Fe₃C/Fe⁰ composite which produces enhanced ROS has been specifically designed and fabricated to remove typical antibiotics (i.e., oxytetracycline (OTC) and chloramphenicol (CAP)) as a heterogeneous Fenton-like catalyst. The C/Fe₃C/Fe⁰ composite demonstrated excellent performance for both OTC and CAP removal as compared with Fe⁰ and biochar. Furthermore, X-ray photoelectron spectrometry, Fourier transform infrared spectrometry, high performance liquid chromatography-mass spectra and electron spin resonance analyses were conducted to elucidate the adsorption and degradation mechanisms. The adsorption of OTC and CAP was mainly dominated by H bonds and the electron-acceptor-acceptor on the surface of the C/Fe₃C/Fe⁰ composite, respectively. In particular, OH simultaneously induced the degradation of OTC and CAP, while ¹O₂ presented the selective oxidation to OTC. More specifically, the degradation of OTC over C/Fe₃C/Fe⁰ was stronger and faster than that of CAP, leading to 65.84% and 16.84% of removal efficiency for OTC and CAP, respectively. Furthermore, C/Fe₃C/Fe⁰ exhibited superior reusability and stability after regeneration, but regenerated Fe⁰ almost lost its reactivity. Therefore, the efficiency in situ generation of ¹O₂ using C/Fe₃C/Fe⁰ would shed new light on the selective oxidation of aqueous organic compounds.

Activation Mechanism of Lead(II) to Ilmenite Flotation Using Salicylhydroxamic Acid as Collector

In this study, salicylhydroxamic acid (SHA), which exhibits superior flotation performance to conventional collector benzohydroxamic acid (BHA), was first introduced in ilmenite flotation. The addition of lead(II) can significantly increase the recovery of ilmenite using SHA as collector. Thus, the adsorption mechanism of SHA on lead(II)-activated ilmenite surface was systematically studied using micro-flotation tests, adsorption analysis, zeta potential measurements, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Micro-flotation results revealed that SHA had stronger collecting ability than BHA, and ilmenite floatability could be activated by lead ions with either SHA or BHA as collector. Ilmenite showed good floatability at pH 6–8 (over 90% recovery) in the presence of Pb2+ and SHA. In such conditions, the main lead species of Pb(OH)+ and Pb2+ acted as active sites and caused positive surface potential shifts, thereby increasing the adsorbed amounts of negatively charged SHA on the surface of the mineral. FTIR and XPS analyses suggested that the lead species was chemically adsorbed on the surface of ilmenite to form active sites chelated by SHA. Moreover, the free lead ions in solution might form the Pb–SHA complexes to adsorb on the mineral surface, thereby increasing the floatability of ilmenite.

Investigation on the production of formic and acetic acids from lignin by ethanol organosolv treatment at mild conditions

Cellulose and hemicellulose are usually considered the sources of formic and acetic acids that are obtained during ethanol pulping process, while our research reveals that lignin is another critical source of acids in the process. In this research, the sample lignin was purified and treated under ethanol pulping conditions and the factors that influence the yields of acids including: ethanol ratio (0-100%), residence time (30-210 min), reaction temperature (150-200 °C) and the effect of residual oxygen in the vessel, were tested separately. The yields of acids were identified using UPLC, the volatile products were characterized by GC-MS and the residual lignin was characterized by ¹³C NMR. The results indicated that the residual oxygen in the reaction vessel acted as an oxidant and the maximum yields of formic and acetic acid are 5.5% and 4.8% (g/g-lignin) from reed and aspen lignin, respectively. For understanding mechanism of the reaction, six lignin model compounds (LMCs) were treated and analyzed in the same reaction conditions; the subsequent results showed that both formic and acetic acid could be detected for all the LMCs tested. On the bases of the experimental results, the reaction pathways have been proposed and discussed.