Added-Value Chemicals from Lignin Oxidation

Impact of accumulation of organic acids on the degradation of cellulose in historic paper

The acidity of historic paper, a property crucial for its preservation, is thought to mainly depend on the type of sizing. However, this research shows that during its degradation, paper acidity increases mainly due to the formation of non-volatile carboxylic acids, which accelerate acid-catalysed hydrolysis. Whether and how this accumulation depends on paper composition has not been studied systematically so far. A collection of 89 European paper samples, dated between 1844 and 1990 was analysed for organic acids content and other conservation-relevant properties. Oxalic acid was the acid determined in the highest concentrations and with the strongest influence on paper pH. Multivariate data analysis revealed correlations between lignin content and oxalic and formic acid contents, highlighting the dominant influence of lignin on acid production. The results also indicate that the content of oxalic acid in paper increases by approximately 0.01 mmol/g per year, resulting in a decrease of pH at 0.008 per year (assuming other variables remain constant). To assist in paper conservation, a portable, non-invasive Raman spectroscopic method was developed to determine the lignin content of historic paper, using partial least squares regression. The research results are significant as they demonstrate the dominant impact of oxalic acid on acidic paper conservation.

Impact of Thermal Treatment and Aging on Lignin Properties in Spruce Wood: Pathways to Value-Added Applications

Thermal modification is an environmentally friendly process that does not utilize chemical agents to enhance the stability and durability of wood. The use of thermally modified wood results in a significantly extended lifespan compared with untreated wood, with minimal maintenance requirements, thereby reducing the carbon footprint. This study examines the impact of varying modification temperatures (160, 180, and 210 °C) on the lignin of spruce wood using the ThermoWood process and following the accelerated aging of thermally modified wood. Wet chemistry methods, including nitrobenzene oxidation (NBO), size exclusion chromatography (SEC), thermogravimetry (TG), differential thermogravimetry (DTG), and Fourier transform infrared spectroscopy (FTIR), were employed to investigate the alterations in lignin. At lower modification temperatures, the predominant reaction is the degradation of lignin, which results in a reduction in the molecular weight and an enhanced yield of NBO (vanillin and vanillic acid) products. At elevated temperatures, condensation and repolymerization reactions become the dominant processes, increasing these traits. The lignin content of aged wood is higher than that of thermally modified wood, which has a lower molecular weight and a lower decomposition temperature. The results demonstrate that lignin isolated from thermally modified wood at the end of its life cycle is a promising feedstock for carbon-based materials and the production of a variety of aromatic monomers, including phenols, aromatic aldehydes and acids, and benzene derivatives.

Mild and efficient approach to aromatic backbone cleavage using copper-lignosulfonate/hydrogen peroxide system

This study investigates the dual role of copper ions in catalysis and complexation during the oxidation of lignosulfonates with hydrogen peroxide (H2O2) under alkaline conditions. The presence of copper ions reduces partial oxidation by 86 % compared to H2O2 treatment alone, enhancing overall conversion efficiency to 63 % under increased oxidative conditions. Analyses reveal that copper-lignosulfonate complexes facilitate redox cycling and hydroxyl radical generation through interactions with H2O2, confirming copper's dual functions. This mechanism mitigates the hindrance of sulfonic groups on hydroperoxide anions, leading to lignosulfonate degradation into dicarboxylic acids. These findings provide novel insights into the copper-lignosulfonate/H2O2 system, expanding the understanding of oxidative degradation mechanisms beyond traditional Fenton-like reactions. Furthermore, this system offers a simplified and efficient alternative for industrial applications, particularly in integration with the sulfite pretreatment process of woody biomass for producing valuable co-products.

Bioconversion of Food and Green Waste into Valuable Compounds Using Solid-State Fermentation in Nonsterile Conditions

Agro-industrial residues have transitions from being an environmental problem to being a cost-effective source of biopolymers and value-added chemicals. However, the efficient extraction of the desired products from these residues requires pretreatments. Fungal biorefinery is a fascinating approach for the biotransformation of raw materials into multiple products in a single batch. In this study, the ability of Trichoderma asperellum R to convert fruit scrap and green waste into value-added chemicals was tested in solid-state and in nonsterile conditions. A solid-state fermentation protocol for a tray bioreactor was developed using spawn as the inoculum for nonsterile substrates. T. asperellum R drove the fermentation of both substrates, shaping the metabolites that were enriched in the secondary plant metabolites. Strain R showed cellulase activity only when inoculated on fruit scraps, resulting in increased amounts of polysaccharides in the crude extract. This extract was also enriched in vanillic acid and limonoid, which are intriguing compounds due to the increasing interest in their potential as biological nitrification inhibitors or food additives. Finally, trimethoxybenzaldehyde, an interesting chemical building block, was identified in the extracts of the Trichoderma-guided fermentation. The overall results showed that the application of T. asperellum R has potential as a driver to facilitate the extraction of bioactive substances from nonsterile recalcitrant substrates.

Controlled chemical transformation of lignin by nitric acid treatment and carbonization

This study focuses on understanding the chemical reactions and results of Kraft lignin transformation through nitric acid treatment and subsequent carbonization. With its rich carbon content, lignin stands out as a promising candidate for the manufacturing of high-value carbon materials. The lignin underwent effective nitration, depolymerization, and oxidation under ambient conditions and at 40 °C, while a slight increase in reaction temperature significantly reduced the reaction time. The molecular weight Mw was effectively reduced from 4371 g/mol to 767 g/mol. The acid-treated lignin samples with incorporated nitro groups were further carbonized to create nitrogen-doped carbon structures. The resulting materials show stable nitrogen content (about at 5 wt%) even after carbonization due to the transformation of nitro groups into thermally stable pyridinic moieties, thereby exhibiting enhanced electrocatalytic properties compared to nitrogen-free carbon materials derived from Kraft lignin. The nitric acid-assisted treatment of lignin obviates the need for catalysts, and additional extraction or purification steps for preparing bio-derived carbon precursors, rendering it facile, fast, and cost-efficient.

Functional Materials from Biomass-Derived Terpyridines: State of the Art and Few Possible Perspectives

This review focuses on functional materials that contain terpyridine (terpy) units, which can be synthesized from biomass-derived platform chemicals. The latter are obtained by the chemical conversion of raw biopolymers such as cellulose (e.g., 2-furaldehyde) or lignin (e.g., syringaldehyde). These biomass-derived platform chemicals serve as starting reagents for the preparation of many different terpyridine derivatives using various synthetic strategies (e.g., Kröhnke reaction, cross-coupling reactions). Chemical transformations of these terpyridines provide a broad range of different ligands with various functionalities to be used for the modification or construction of various materials. Either inorganic materials (such as oxides) or organic ones (such as polymers) can be combined with terpyridines to provide functional materials. Different strategies are presented for grafting terpy to materials, such as covalent grafting through a carboxylic acid or silanization. Furthermore, terpy can be used directly for the elaboration of functional materials via complexation with metals. The so-obtained functional materials find various applications, such as photovoltaic devices, heterogeneous catalysts, metal-organic frameworks (MOF), and metallopolymers. Finally, some possible developments are presented.

Oxidative Carboxylation of Lignin: Exploring Reactivity of Different Lignin Types

The increased interest in the utilization of lignin in biobased applications is evident from the rise in lignin valorization studies. The present study explores the responsiveness of lignin toward oxidative valorization using acetic acid and hydrogen peroxide. The pristine lignins and their oxidized equivalents were analyzed comprehensively using NMR and SEC. The study revealed ring opening of phenolic rings yielding muconic acid- and ester-end groups and side-chain oxidations of the benzylic hydroxyls. Syringyl units were more responsive to these reactions than guaiacyl units. The high selectivity of the reaction yielded oligomeric oxidation products with a narrower dispersity than pristine lignins. Mild alkaline hydrolysis of methyl esters enhanced the carboxylic acid content of oxidized lignin, presenting the potential to adjust the carboxylic acid content of lignin. While oxidation reactions in lignin valorization are well documented, this study showed the feasibility of employing optimized oxidation conditions to engineer tailored lignin-based material precursors.

Production of vanillin via oxidation depolymerization of lignin over Fe- and Mn-modified TS-1 zeolites

Converting lignin into specific aromatic chemicals for utilization through depolymerization of lignin is an effective way to achieve high-value applications. There are many depolymerization methods that can do this, but there are problems such as harsh reaction conditions, low depolymerization efficiency and uncontrollable target products that need to be solved. This study reports a novel system for the oxidative depolymerization of alkali lignin using Fe- and Mn- modified TS-1 as a catalyst to assist in the highly selective production of vanillin. We also proposed a possible reaction pathway for the oxidative depolymerization of alkali lignin to produce vanillin catalyzed by Fe-Mn/TS-1 catalyst. The catalytic effects of TS-1, Fe/TS-1, and Fe-Mn/TS-1 catalysts on the oxidative depolymerization of lignin to produce phenolic monomers and vanillin were investigated. The results show that the modified catalysts can effectively improve the efficiency of linkage bond breaking in lignin, especially the β-O-4 bond, in which the inter-band transitions of Fe and Mn play an important role. The synergistic effect of the bimetallic-loaded catalyst (Fe-Mn/TS-1) could catalyze the oxidative depolymerization of lignin more efficiently than the monometallic-loaded catalyst (Fe/TS-1). This lignin oxidative depolymerization system produced 40.59 wt% bio-oil including 12.24 wt% phenolic monomers and 16.17 wt% re-lignin after the addition of Fe-Mn/TS-1 catalyst, owning the highest phenolic monomer yield. Surprisingly, this lignin oxidative depolymerization system exhibited high yield for vanillin (8.36 wt%) production. These results demonstrated that the Fe-Mn/TS-1 catalytic system has potential to produce vanillin from lignin under mild conditions.

From Waste to Value: Recent Insights into Producing Vanillin from Lignin

Vanillin, one of the most widely used and appreciated flavoring agents worldwide, is the main constituent of vanilla bean extract, obtained from the seed pods of various members belonging to the Orchidaceae family. Due to the great demand in the food confectionery industry, as well as in the perfume industry, medicine, and more, the majority of vanillin used today is produced synthetically, and only less than one percent of the world's vanilla flavoring market comes directly from the traditional natural sources. The increasing global demand for vanillin requires alternative and overall sustainable new production methods, and the recovery from biobased polymers, like lignin, is an environmentally friendly alternative to chemical synthesis. The present review provides firstly an overview of the different types of vanillin, followed by a description of the main differences between natural and synthetic vanillin, their preparation, the market of interest, and the authentication issues and the related analytical techniques. Then, the review explores the real potentialities of lignin for vanillin production, presenting firstly the well-assessed classical methods and moving towards the most recent promising approaches through chemical, biotechnological and photocatalytic methodologies, together with the challenges and the principal issues associated with each technique.

Catalytic Selective Oxidation of β-O-4 Bond in Phenethoxybenzene as a Lignin Model Using (TBA)5[PMo10V2O40] Nanocatalyst: Optimization of Operational Conditions

The catalytic oxidation of phenethoxybenzene as a lignin model compound with a β-O-4 bond was conducted using the Keggin-type polyoxometalate nanocatalyst (TBA)5[PMo10V2O40]. The optimization of the process's operational conditions was carried out using response surface methodology. The statistically significant variables in the process were determined using a fractional factorial design. Based on this selection, a central circumscribed composite experimental design was used to maximize the phenethoxybenzene conversion, varying temperature, reaction time, and catalyst load. The optimal conditions that maximized the phenethoxybenzene conversion were 137 °C, 3.5 h, and 200 mg of catalyst. In addition, under the optimized conditions, the Kraft lignin catalytic depolymerization was carried out to validate the effectiveness of the process. The depolymerization degree was assessed by gel permeation chromatography from which a significant decrease in the molar mass distribution Mw from 7.34 kDa to 1.97 kDa and a reduction in the polydispersity index PDI from 6 to 3 were observed. Furthermore, the successful cleavage of the β-O-4 bond in the Kraft lignin was verified by gas chromatography-mass spectrometry analysis of the reaction products. These results offer a sustainable alternative to efficiently converting lignin into valuable products.

The Biomodified Lignin Platform: A Review

A reliance on fossil fuel has led to the increased emission of greenhouse gases (GHGs). The excessive consumption of raw materials today makes the search for sustainable resources more pressing than ever. Technical lignins are mainly used in low-value applications such as heat and electricity generation. Green enzyme-based modifications of technical lignin have generated a number of functional lignin-based polymers, fillers, coatings, and many other applications and materials. These bio-modified technical lignins often display similar properties in terms of their durability and elasticity as fossil-based materials while also being biodegradable. Therefore, it is possible to replace a wide range of environmentally damaging materials with lignin-based ones. By researching publications from the last 20 years focusing on the latest findings utilizing databases, a comprehensive collection on this topic was crafted. This review summarizes the recent progress made in enzymatically modifying technical lignins utilizing laccases, peroxidases, and lipases. The underlying enzymatic reaction mechanisms and processes are being elucidated and the application possibilities discussed. In addition, the environmental assessment of novel technical lignin-based products as well as the developments, opportunities, and challenges are highlighted.

Scavengome of an antioxidant

The term "scavengome" refers to the chemical space of all the metabolites that may be formed from an antioxidant upon scavenging reactive oxygen or nitrogen species (ROS/RNS). This chemical space covers a wide variety of free radical metabolites with drug discovery potential. It is very rich in structures representing an increased chemical complexity as compared to the parent antioxidant: a wide range of unusual heterocyclic structures, new CC bonds, etc. may be formed. Further, in a biological environment, this increased chemical complexity is directly translated from the localized conditions of oxidative stress that determines the amounts and types of ROS/RNS present. Biomimetic oxidative chemistry provides an excellent tool to model chemical reactions between antioxidants and ROS/RNS. In this chapter, we provide an overview on the known metabolites obtained by biomimetic oxidation of a few selected natural antioxidants, i.e., a stilbene (resveratrol), a pair of hydroxycinnamates (caffeic acid and methyl caffeate), and a flavonol (quercetin), and discuss the drug discovery perspectives of the related chemical space.

Efficient cell factories for the production of N-methylated amino acids and for methanol-based amino acid production

The growing world needs commodity amino acids such as L-glutamate and L-lysine for use as food and feed, and specialty amino acids for dedicated applications. To meet the supply a paradigm shift regarding their production is required. On the one hand, the use of sustainable and cheap raw materials is necessary to sustain low production cost and decrease detrimental effects of sugar-based feedstock on soil health and food security caused by competing uses of crops in the feed and food industries. On the other hand, the biotechnological methods to produce functionalized amino acids need to be developed further, and titres enhanced to become competitive with chemical synthesis methods. In the current review, we present successful strain mutagenesis and rational metabolic engineering examples leading to the construction of recombinant bacterial strains for the production of amino acids such as L-glutamate, L-lysine, L-threonine and their derivatives from methanol as sole carbon source. In addition, the fermentative routes for bioproduction of N-methylated amino acids are highlighted, with focus on three strategies: partial transfer of methylamine catabolism, S-adenosyl-L-methionine dependent alkylation and reductive methylamination of 2-oxoacids.

New Insights into Green Protocols for Oxidative Depolymerization of Lignin and Lignin Model Compounds

Oxidative depolymerization of lignin is a hot topic in the field of biomass valorization. The most recent and green procedures have been herein detailed. Photochemical and electrochemical approaches are reviewed highlighting the pros and cons of each method. Mechanochemistry activated strategies are able to combine oxidation and depolymerization in the deconstruction of lignin. Homogenous and heterogeneous catalytic systems are exemplified stressing the green aspects associated with both the procedures. Solvent-free approaches as well as those carried out in alternative media are listed. Finally, the few examples of selenium catalyzed lignin valorization reported so far are cited.

Microwave-Assisted Lignin Wet Peroxide Oxidation to C4 Dicarboxylic Acids

Innovative methodologies, such as microwave-assisted reaction, can help to valorize lignin with higher productivity and better energy efficiency. In this work, microwave heating was tested in the wet peroxide oxidation of three lignins (Indulin AT, Lignol, and Eucalyptus globulus lignins) as a novel methodology to obtain C₄ dicarboxylic acids. The effect of temperature, time, and catalyst type (TS-1 or Fe-TS1) was evaluated in the production of these acids. The TS-1 catalyst improved succinic acid yield, achieving up to 9.4 wt % for Lignol lignin. Moreover, the microwave heating specifically enhanced Lignol conversion to malic acid (34 wt %), even without catalyst, showing to be an attractive path for the future valorization of organosolv lignins. Overall, compared to conventional heating, microwave heating originated a rapid lignin conversion. Nevertheless, for prolonged times, conventional heating led to better results for some target products, e.g., malic and succinic acids.

Simulation and optimization of organosolv based lignocellulosic biomass refinery: A review

Organosolv pretreatment can be considered as the core of the lignocellulosic biomass fractionation within the biorefinery concept. Organosolv facilitates the separation of the major fractions (cellulose, hemicelluloses, lignin), and their use as renewable feedstocks to produce bioenergy, biofuels, and added-value biomass derived chemicals. The efficient separation of these fractions affects the economic feasibility of the biorefinery complex. This review focuses on the simulation of the organosolv pretreatment and the optimization of (i) feedstock delignification, (ii) sugars production (mainly from hemicelluloses), (iii) enzymatic digestibility of the cellulose fraction and (iv) quality of lignin. Simulation is used for the technoeconomic optimization of the biorefinery complex. Simulation and optimization implement a holistic approach considering the efficient technological, economic, and environmental performance of the biorefinery operational units. Consequently, an optimized organosolv stage is the first step for a sustainable, economically viable biorefinery complex in the concept of industrial ecology and zero waste circular economy.