Review on Catalytic Cleavage of C–C Inter-unit Linkages in Lignin Model Compounds: Towards Lignin Depolymerisation

Carbon-wise utilization of lignin-related compounds by synergistically employing anaerobic and aerobic bacteria

BACKGROUND

Lignin is a highly abundant but strongly underutilized natural resource that could serve as a sustainable feedstock for producing chemicals by microbial cell factories. Because of the heterogeneous nature of the lignin feedstocks, the biological upgrading of lignin relying on the metabolic routes of aerobic bacteria is currently considered as the most promising approach. However, the limited substrate range and the inefficient catabolism of the production hosts hinder the upgrading of lignin-related aromatics. Particularly, the aerobic O-demethylation of the methoxyl groups in aromatic substrates is energy-limited, inhibits growth, and results in carbon loss in the form of CO2.

RESULTS

In this study, we present a novel approach for carbon-wise utilization of lignin-related aromatics by the integration of anaerobic and aerobic metabolisms. In practice, we employed an acetogenic bacterium Acetobacterium woodii for anaerobic O-demethylation of aromatic compounds, which distinctively differs from the aerobic O-demethylation; in the process, the carbon from the methoxyl groups is fixed together with CO2 to form acetate, while the aromatic ring remains unchanged. These accessible end-metabolites were then utilized by an aerobic bacterium Acinetobacter baylyi ADP1. By utilizing this cocultivation approach, we demonstrated an upgrading of guaiacol, an abundant but inaccessible substrate to most microbes, into a plastic precursor muconate, with a nearly equimolar yields (0.9 mol/mol in a small-scale cultivation and 1.0 mol/mol in a one-pot bioreactor cultivation). The process required only a minor genetic engineering, namely a single gene knock-out. Noticeably, by employing a metabolic integration of the two bacteria, it was possible to produce biomass and muconate by utilizing only CO2 and guaiacol as carbon sources.

CONCLUSIONS

By the novel approach, we were able to overcome the issues related to aerobic O-demethylation of methoxylated aromatic substrates and demonstrated carbon-wise conversion of lignin-related aromatics to products with yields unattainable by aerobic processes. This study highlights the power of synergistic integration of distinctive metabolic features of bacteria, thus unlocking new opportunities for harnessing microbial cocultures in upgrading challenging feedstocks.

Photocatalytic, Oxidative Cleavage of C-C Bond in Lignin Models and Native Lignin

It is challenging to realize the selective C-C bond cleavage of lignin β-O-4 linkages for production of high-value aromatic chemicals due to its intrinsic inertness and complex structure. Here we report a light-driven, chlorine-radical-based protocol to realize the oxidative C-C bond cleavage in various lignin model compounds catalyzed by commercially available TPT and CaCl2, achieving high conversion and good to high product yields at room temperature. Mechanistic studies reveal that the preferential activation of Cβ-H bond facilitates the oxidation and C-C bond cleavage of lignin β-O-4 model via chlorine radical. Furthermore, this method is also applicable to the depolymerization of natural lignin extracts, furnishing the aromatic oxygenates from the cleavage of Cα-Cβ bonds. This study provides experimental foundations to the depolymerization and valorization of lignin into high value-added aromatic compounds.

Enhancing Hydrogen Production from the Photoreforming of Lignin

Photoreforming of lignocellulose biomass is widely recognised as a challenging but key technology for producing value-added chemicals and renewable hydrogen (H2 ). In this study, H2 production from photoreforming of organosolv lignin in a neutral aqueous solution was studied over a 0.1 wt % Pt/TiO2 (P25) catalyst with ultraviolet A (UVA) light. The H2 production from the system employing the lignin (~4.8 μmol gcat -1  h-1 ) was comparable to that using hydroxylated/methoxylated aromatic model compounds (i. e., guaiacol and phenol, 4.8-6.6 μmol gcat -1  h-1 ), being significantly lower than that from photoreforming of cellulose (~62.8 μmol gcat -1  h-1 ). Photoreforming of phenol and reaction intermediates catechol, hydroquinone and benzoquinone were studied to probe the mechanism of phenol oxidation under anaerobic photoreforming conditions with strong adsorption and electron transfer reactions lowering H2 production from the intermediates relative to that from phenol. The issues associated with catalyst poisoning and low photoreforming activity of lignins demonstrated in this paper have been mitigated by implementing a process by which the catalyst was cycled through anaerobic and aerobic conditions. This strategy enabled the periodic regeneration of the photocatalyst resulting in a threefold enhancement in H2 production from the photoreforming of lignin.

Ruthenium ion catalysed C-C bond activation in lignin model compounds - towards lignin depolymerisation

Lignin is the most abundant renewable feedstock to produce aromatic chemicals, however its depolymerisation involves the breaking of several C-O and C-C inter-unit linkages that connect smaller aromatic units that are present in lignin. Several strategies have been reported for the cleavage of the C-O inter-unit linkages in lignin. However, till today, only a few methodologies have been reported for the effective breaking or the conversion of the recalcitrant C-C inter unit linkages in lignin. Here we report the ruthenium ion catalysed oxidative methodology as an effective system to activate or convert the most recalcitrant inter unit linkages such as β-5 and 5-5' present in lignin. Initially, we used biphenyl as a model compound to study the effectiveness of the RICO methodology to activate the 5-5' C-C linkage. After 4 h reaction at 22 °C, we achieved a 30% conversion with 75% selectivity towards benzoic acid and phenyl glyoxal as the minor product. To the best of our knowledge this is the first ever oxidative activation of the C-C bond that connects the two phenyl rings in biphenyl. DFT calculation revealed that the RuO4 forms a [3 + 2] adduct with one of the aromatic C-C bonds resulting in the opening of the phenyl ring. Biphenyl conversion could be increased by increasing the amount of oxidant; however, this is accompanied by a reduction in the carbon balance because of the formation of CO2 and other unknown products. We extended this RICO methodology for the oxidative depolymerisation of lignin model hexamer containing β-5, 5-5' and β-O-4 linkages. Qualitative and quantitative analyses of the reaction mixture were done using 1H, 13C NMR spectroscopy methods along with GC-MS and Gel Permeation Chromatographic (GPC) methods. Advanced 2D NMR spectroscopic methods such as HSQC, HMBC and 31P NMR spectroscopy after phosphitylation of the mixture were employed to quantitatively analyse the conversion of the β-5, 5-5' and β-O-4 linkages and to identify the products. After 30 min, >90% of the 5-5' and linkages and >80% of the β-5' are converted with this methodology. This is the first report on the conversion of the 5-5' linkage in lignin model hexamer.

Aerobic Oxidative Cleavage of C(OH)-C Bonds to Produce Aromatic Aldehydes Catalyzed by CuI -1,10-phenanthroline Complex

Effective cleavage and functionalization of C(OH)-C bonds is of great importance for the production of value-added chemicals from renewable biomass resources such as carbohydrates, lignin and their derivatives. The efficiency and selectivity of oxidative cleavage of C(OH)-C bonds are hindered by their inert nature and various side reactions associated with the hydroxyl group. The oxidative conversion of secondary alcohols to produce aldehydes is particularly challenging because the generated aldehydes tend to be over-oxidized to acids or the other side products. Noble-metal based catalysts are necessary to get satisfactory aldehyde yields. Herein, for the first time, the efficient aerobic oxidative conversion of secondary aromatic alcohols into aromatic aldehydes is reported using non-noble metal catalysts and environmentally benign oxygen, without any additional base. It was found that CuI -1,10-phenanthroline (Cu-phen) complex showed outstanding performance for the reactions. The C(OH)-C bonds of a diverse array of aromatic secondary alcohols were effectively cleaved and functionalized, selectively affording aldehydes with excellent yields. Detailed mechanism study revealed a radical mediated pathway for the oxidative reaction. We believe that the findings in this work will lead to many explorations in non-noble metal catalyzed oxidative reactions.

Production of Diethyl Maleate via Oxidative Depolymerization of Organosolv Lignin from Wheat Stalk over the Cooperative Acidic Ionic Liquid Pair

Lignin, the second largest component of biomass, is considered as an important alternative source of fossil reserves for the production of fuels and chemicals. Here, we developed a novel method to oxidatively degrade organosolv lignin into value-added four-carbon esters, particularly diethyl maleate (DEM), with the cooperative catalyst consisting of 1-(3-sulfobutyl) triethylammonium hydrogen sulfate ([BSTEA]HSO₄) and 1-butyl-3-methylimidazolium ferric chloride ([BMIM]Fe₂Cl₇). Under optimized conditions (1.00 MPa initial O₂ pressure, 160 °C, 5 h), the lignin aromatic ring was effectively cleaved by oxidation to form DEM with a yield of 15.85% and a selectivity of 44.25% in the presence of the synergistic catalyst of [BMIM]Fe₂Cl₇-[BSMIM]HSO₄ (1/3, mol/mol). The structure and composition analysis of lignin residues and liquid products confirmed that the aromatic units in lignin were effectively and selectively oxidized. Furthermore, the catalytic oxidation of lignin model compounds was explored for obtaining a possible reaction pathway of oxidative cleavage of lignin aromatic units to DEM. This study provides a promising alternative method for the production of traditional petroleum-based chemicals.

Application of Plasticizer Glycerol in Lignosulfonate-Filled Rubber Compounds Based on SBR and NBR

The work deals with the application of biopolymer fillers in rubber formulations. Calcium lignosulfonate was incorporated into styrene-butadiene rubber and acrylonitrile-butadiene rubber in a constant amount of 30 phr. Glycerol in a concentration scale ranging from 5 to 20 phr was used as a plasticizer for rubber formulations. For the cross-linking of the compounds, a sulfur-based curing system was used. The study was focused on the investigation of glycerol in the curing process; the viscosity of rubber compounds; and the cross-link density, morphology, physical-mechanical, and dynamic mechanical properties of vulcanizates. The study revealed that the application of glycerol as a plasticizer resulted in a reduction in the rubber compounds' viscosity and contributed to the better dispersion and distribution of the filler within the rubber matrices. The mutual adhesion and compatibility between the filler and the rubber matrices were improved, which resulted in the significant enhancement of tensile characteristics. The main output of the work is the knowledge that the improvement of the physical-mechanical properties of biopolymer-filled vulcanizates can be easily obtained via the simple addition of a very cheap and environmentally friendly plasticizer into rubber compounds during their processing without additional treatments or procedures. The enhancement of the physical-mechanical properties of rubber compounds filled with biopolymers might contribute to the broadening of their potential applications. Moreover, the price of the final rubber articles could be reduced, and more pronounced ecological aspects could also be emphasized.

A reactive molecular dynamics study of thermal pyrolysis behavior and mechanisms of lignin during the hydrothermal process: The function of the water molecules

The lignin hydrothermal processing is an important option but a full understanding of the role played by the water molecules in the depolymerization of lignin is still lacking. In order to clarify the role of the water molecules in the depolymerization of lignin, the evolution of chemical bonds, microstructural changes, and possible mechanisms of product generation were compared during the pyrolysis process under vacuum and water conditions using Reactive Molecular Dynamics Simulation. Compared with vacuum conditions, the role of water changes with temperature, identifying three stages: promotion (1200-1800 K)-inhibition (2100-2400 K)-promotion (2700-3000 K). Also compared with vacuum conditions, hydrothermal processing can promote the cleavage of the ether bonds while inhibiting the destruction of carbocycles. Water molecules promote the depolymerization of lignin into more C₄-molecules, thereby generating more combustible gas resources. Based on the research results, the pyrolysis conditions of lignin can be flexibly controlled to obtain solids, liquids or gases.

Biorefineries: Achievements and challenges for a bio-based economy

Climate change, socioeconomical pressures, and new policy and legislation are driving a decarbonization process across industries, with a critical shift from a fossil-based economy toward a biomass-based one. This new paradigm implies not only a gradual phasing out of fossil fuels as a source of energy but also a move away from crude oil as a source of platform chemicals, polymers, drugs, solvents and many other critical materials, and consumer goods that are ubiquitous in our everyday life. If we are to achieve the United Nations' Sustainable Development Goals, crude oil must be substituted by renewable sources, and in this evolution, biorefineries arise as the critical alternative to traditional refineries for producing fuels, chemical building blocks, and materials out of non-edible biomass and biomass waste. State-of-the-art biorefineries already produce cost-competitive chemicals and materials, but other products remain challenging from the economic point of view, or their scaled-up production processes are still not sufficiently developed. In particular, lignin's depolymerization is a required milestone for the success of integrated biorefineries, and better catalysts and processes must be improved to prepare bio-based aromatic simple molecules. This review summarizes current challenges in biorefinery systems, while it suggests possible directions and goals for sustainable development in the years to come.

Toward the Photocatalytic Valorization of Lignin: Conversion of a Model Lignin Hexamer with Multiple Functionalities

The valorization of biomass via photocatalysis is an area of expanding research with advances in new technologies and materials with a view toward enhanced sustainability being reported. A significant challenge within this field, however, is understanding the impact photocatalysis has on more recalcitrant compounds present in biomass, such as lignin. Moreover, the current state of lignin model compound research is still largely focused on the breakdown of small models containing typically only one linkage. Described herein is the use of TiO₂-mediated photocatalysis for the degradation of a representative hexameric lignin model compound which contains multiple linkages (e.g., 5-5', β-5, and β-O-4). The results revealed that while cleavage of the β-5 and β-O-4 occurred, the 5-5' appeared to remain intact within the identified reaction intermediates. To understand some of the more fundamental questions, a dimeric compound with a biphenyl linkage was synthesized and studied under photocatalytic conditions. The proposal of intermediates and pathways of degradation based on the studies conducted is presented and discussed herein.

Lignin Biodegradation and Its Valorization

Lignin, a rigid polymer composed of phenolic subunits with high molecular weight and complex structure, ranks behind only cellulose in the contribution to the biomass of plants. Therefore, lignin can be used as a new environmentally friendly resource for the industrial production of a variety of polymers, dyes and adhesives. Since laccase was found to be able to degrade lignin, increasing attention had been paid to the valorization of lignin. Research has mainly focused on the identification of lignin-degrading enzymes, which play a key role in lignin biodegradation, and the potential application of lignin degradation products. In this review, we describe the source, catalytic specificity and enzyme reaction mechanism of the four classes of the lignin-degrading enzymes so far discovered. In addition, the major pathways of lignin biodegradation and the applications of the degradative products are also discussed. Lignin-degrading bacteria or enzymes can be used in combination with chemical pretreatment for the production of value-added chemicals from lignin, providing a promising strategy for lignin valorization.

LigninGraphs: lignin structure determination with multiscale graph modeling

Lignin is an aromatic biopolymer found in ubiquitous sources of woody biomass. Designing and optimizing lignin valorization processes requires a fundamental understanding of lignin structures. Experimental characterization techniques, such as 2D-heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectra, could elucidate the global properties of the polymer molecules. Computer models could extend the resolution of experiments by representing structures at the molecular and atomistic scales. We introduce a graph-based multiscale modeling framework for lignin structure generation and visualization. The framework employs accelerated rejection-free polymerization and hierarchical Metropolis Monte Carlo optimization algorithms. We obtain structure libraries for various lignin feedstocks based on literature and new experimental NMR data for poplar wood, pinewood, and herbaceous lignin. The framework could guide researchers towards feasible lignin structures, efficient space exploration, and future kinetics modeling. Its software implementation in Python, LigninGraphs, is open-source and available on GitHub.

Nickel‐Catalyzed C sp2 −OMe Functionalization for Chemoselective Aromatic Homologation En Route to Nanographenes

A Ni‐catalyzed C_sp2−OMe ortho‐functionalization methodology to form chemoselectively alkyne monoannulation or aromatic homologation products is reported as a novel protocol towards the valorisation of substrates containing C_sp2−OMe units. Double activation of C_sp2−OMe and C_sp2−F bonds is also demonstrated. Further use of aromatic homologation products towards the synthesis of nanographene‐like compounds is described.

Thermal fluctuation and conformational effects on NMR parameters in β-O-4 lignin dimers from QM/MM and machine-learning approaches

Advanced solid-state and liquid-state nuclear magnetic resonance (NMR) approaches have enabled high throughput information about functional groups and types of bonding in a variety of lignin fragments from degradation processes and laboratory synthesis. The use of quantum chemical (QM) methods may provide detailed insight into the relationships between NMR parameters and specific lignin conformations and their dynamics, whereas a rapid prediction of NMR properties could be achieved by combining QM with machine-learning (ML) approaches. In this study, we present the effect of conformations of β-O-4 linked lignin guaiacyl dimers on ¹³C and ¹H chemical shifts while considering the thermal fluctuations of the guaiacyl dimers in water, ethanol and acetonitrile, as well as their binary 75 wt% aqueous solutions. Molecular dynamics and QM/MM simulations were used to describe the dynamics of guaiacyl dimers. The isotropic shielding of the majority of the carbon nuclei was found to be less sensitive toward a specific conformation than that of the hydrogen nuclei. The largest ¹H downfield shifts of 4-6 ppm were established in the hydroxy groups and the rings in the presence of organic solvent components. The Gradient Boosting Regressor model has been trained on 60% of the chemical environments in the dynamics trajectories with the NMR isotropic shielding (σ_iso), computed with density-functional theory, for lignin atoms. The high efficiency of this machine-learning model in predicting the remaining 40% σ_iso(¹³C) and σ_iso(¹H) values was established.

Effect of Physicochemical Properties of Co and Mo Modified Natural Sourced Hierarchical ZSM-5 Zeolite Catalysts on Vanillin and Phenol Production from Diphenyl Ether

The conversion of lignocellulose biomass to value-added chemicals is challenging. In this paper, the conversion process of diphenyl ether (DPE) as a model lignin compound to phenol and vanillin compounds involved a bifunctional catalyst in reaching the simultaneous one-pot reaction in mild conditions with a high yield product. The catalysts used in this conversion are hierarchical ZSM-5 zeolites and their cobalt oxide and molybdenum oxide impregnated derivate. The ZSM-5 zeolites were synthesized using alternative precursors from natural resources, i.e., Indonesian natural zeolite and kaolin. The physicochemical properties of the catalysts were determined with various characterization methods, such as: X-ray Diffraction (XRD), Fourier Transform Infra Red (FT-IR), Scanning Electron Microscope - Energy Dispersive X-ray (SEM-EDX), X-ray Fluorescence (XRF), Surface Area Analyzer (SAA), and NH3-Temperature Programmed Desorption (NH3-TPD). The catalytic activity on conversion of DPE substrates showed that the MoOx/HZSM-5 produced the highest %yield for phenol and vanillin products; 31.96% at 250 °C and 7.63% at 200 °C, respectively. The correlation study between the physicochemical properties and the catalytic activity shows that the dominance of weak acid (>40%) and mesoporosity contribution (pore size of ~ 9 nm) play roles in giving the best catalytic activity at low temperatures. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Local Structure Analysis and Modelling of Lignin-Based Carbon Composites through the Hierarchical Decomposition of the Radial Distribution Function

Carbonized lignin has been proposed as a sustainable and domestic source of activated, amorphous, graphitic, and nanostructured carbon for many industrial applications as the structure can be tuned through processing conditions. However, the inherent variability of lignin and its complex physicochemical structure resulting from feedstock and pulping selection make the Process‐Structure‐Property‐Performance (PSPP) relationships hard to define. In this work, radial distribution functions (RDFs) from synchrotron X‐ray and neutron scattering of lignin‐based carbon composites (LBCCs) are investigated using the Hierarchical Decomposition of the Radial Distribution Function (HDRDF) modelling method to characterize the local atomic environment and develop quantitative PSPP relationships. PSPP relationships for LBCCs defined by this work include crystallite size dependence on lignin feedstock as well as increasing crystalline volume fraction, nanoscale composite density, and crystallite size with increasing reduction temperature.

Complexation of Lignin Dimers with β-Cyclodextrin and Binding Stability Analysis by ESI-MS, Isothermal Titration Calorimetry, and Molecular Dynamics Simulations

Lignin derived from lignocellulosic biomass is the largest source of renewable bioaromatics present on earth and requires environmentally sustainable separation strategies to selectively obtain high-value degradation products. Applications of supramolecular interactions have the potential to isolate lignin compounds from biomass degradation fractions by the formation of variable inclusion complexes with cyclodextrins (CDs). CDs are commonly used as selective adsorbents for many applications and can capture guest molecules in their internal hydrophobic cavity. The strength of supramolecular interactions between CDs and lignin model compounds that represent potential lignocellulosic biomass degradation products can be characterized by assessing the thermodynamics of binding stability. Consequently, the inclusion interactions of β-CD and lignin model compounds G-(β-O-4')-G, G-(β-O-4')-truncG (guaiacylglycerol-β-guaiacyl ether), and G-(β-β')-G (pinoresinol) were investigated empirically by electrospray ionization mass spectrometry and isothermal titration calorimetry, complemented by molecular dynamics (MD) simulations. Empirical results indicate that there are substantial differences in binding stability dependent on the linkage type. The lignin model β-β' dimer showed more potential bound states including 1:1, 2:1, and 1:2 (guest:host) complexation and, based on binding stability determinations, was consistently the most energetically favorable guest. Empirical results are supported by MD simulations that reveal that the capture of G-(β-β')-G by β-CD is promising with a 66% probability of being bound for G-(β-O-4')-truncG compared to 88% for G-(β-β')-G (unbiased distance trajectory and explicit counting of bound states). These outcomes indicate CDs as a promising material to assist in separations of lignin oligomers from heterogeneous mixtures for the development of environmentally sustainable isolations of lignin compounds from biomass fractions.

Machine learning based analysis of reaction phenomena in catalytic lignin depolymerization

Heterogeneously catalyzed lignin solvolysis opens the possibility of transforming low value biomass into high value, useful aromatic chemicals, however, its reaction behavior is poorly understood due to the many possible interactions between reaction parameters. In this study, a novel predictive model for bio-oil yield, char yield and reaction time is developed using Random Forest (RF) regression method using data available from the literature to study the impact of surface properties of the catalyst and the weight averaged molecular weight of the lignin (M_w) used in the reaction. The models achieved a coefficient of determination (R2) score of 0.9062, 0.9428 and 0.8327, respectively, and feature importance for each case was explained and tied to studies that provide a mechanistic explanation for the performance of the model. Surface properties and lignin M_w showed no importance to the prediction of bio-oil yield and average pore diameter contributed 3% of feature importance to reaction time.

Monitoring Molecular Weight Changes during Technical Lignin Depolymerization by Operando Attenuated Total Reflectance Infrared Spectroscopy and Chemometrics

Technical lignins are increasingly available at industrial scale, offering opportunities for valorization, such as by (partial) depolymerization. Any downstream lignin application requires careful tailoring of structural properties, such as molecular weight or functional group density, properties that are difficult to control or predict given the structure variability and recalcitrance of technical lignins. Online insight into changes in molecular weight (M_w ), to gauge the extent of lignin depolymerization and repolymerization, would be highly desired to improve such control, but cannot be readily provided by the standard ex-situ techniques, such as size exclusion chromatography (SEC). Herein, operando attenuated total reflectance infrared (ATR-IR) spectroscopy combined with chemometrics provided temporal changes in M_w during lignin depolymerization with high resolution. More specifically, ex-situ SEC-derived M_w and polydispersity data of kraft lignin subjected to aqueous phase reforming conditions could be well correlated with ATR-IR spectra of the reaction mixture as a function of time. The developed method showed excellent regression results and relative error, comparable to the standard SEC method. The method developed has the potential to be translated to other lignin depolymerization processes.

Metal triflate formation of C12-C22 phenolic compounds by the simultaneous C-O breaking and C-C coupling of benzyl phenyl ether

Catalytic pathways to produce high carbon number compounds from benzyl phenyl ether require multiple steps to break the aryl etheric carbon-oxygen bonds; these steps are followed by energy-intensive processes to remove oxygen atoms and/or carbon-carbon coupling. Here, we show an upgrading strategy to transform benzyl phenyl ether into large phenolic (C₁₂-C₂₂) compounds by a one-step C-O breaking and C-C coupling catalyzed by metal triflates under a mild condition (100 °C and 1 bar). Hafnium triflate was the most selective for the desired products. In addition, we measured the effect of solvent polarity on the catalytic performance. Solvents with a polarity index of less than 3.4 promoted the catalytic activity and selectivity to C₁₂-C₂₂ phenolic products. These C₁₂-C₂₂ phenolic compounds have potential applications for phenol-formaldehyde polymers, diesel/jet fuels, and liquid organic hydrogen carriers.

Oxidative cleavage of C-C bonds in lignin

Lignin is an aromatic polymer that constitutes up to 30 wt% of woody biomass and is considered the largest source of renewable aromatics. Valorization of the lignin stream is pivotal for making biorefining sustainable. Monomeric units in lignin are bound via C-O and C-C bonds. The majority of existing methods for the production of valuable compounds from lignin are based on the depolymerization of lignin via cleavage of relatively labile C-O bonds within lignin structure, which leads to yields of only 36-40 wt%. The remaining fraction (60 wt%) is a complex mixture of high-molecular-weight lignin, generally left unvalorized. Here we present a method to produce additional valuable monomers from the high-molecular-weight lignin fraction through oxidative C-C bond cleavage. This oxidation reaction proceeds with a high selectivity to give 2,6-dimethoxybenzoquinone (DMBQ) from high-molecular-weight lignin in 18 wt% yield, thus increasing the yield of monomers by 32%. This is an important step to make biorefining competitive with petroleum-based refineries.

Cleavage of Organosolv Lignin to Phenols Using Nitrogen Monoxide and Hydrazine

From the variety of methods known for the depolymerization of organosolv lignin, a broad range of diversely substituted aromatic compounds are available today. In the present work, a novel two-step reaction sequence is reported, which is focused on the formation of phenols. While the first step of the depolymerization strategy comprises the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-catalyzed oxidation of organosolv lignin with nitrogen monoxide so that two waste materials are combined, cleavage to the phenolic target compounds is achieved in the second step employing hydrazine and potassium hydroxide under Wolff-Kishner-type conditions. Besides the fact that the novel strategy proceeds via an untypical form of oxidized organosolv lignin, the two-step sequence is further able to provide phenols as cleavage products, which bear no substituent at the 4-position.

Bioremediation of lignin derivatives and phenolics in wastewater with lignin modifying enzymes: Status, opportunities and challenges

Lignin modifying enzymes from fungi and bacteria are potential biocatalysts for sustainable mitigation of different potentially toxic pollutants in wastewater. Notably, the paper and pulp industry generates enormous amounts of wastewater containing high amounts of complex lignin-derived chlorinated phenolics and sulfonated pollutants. The presence of these compounds in wastewater is a critical issue from environmental and toxicological perspectives. Some chloro-phenols are harmful to the environment and human health, as they exert carcinogenic, mutagenic, cytotoxic, and endocrine-disrupting effects. In order to address these most urgent concerns, the use of oxidative lignin modifying enzymes for bioremediation has come into focus. These enzymes catalyze modification of phenolic and non-phenolic lignin-derived substances, and include laccase and a range of peroxidases, specifically lignin peroxidase (LiP), manganese peroxidase (MnP), versatile peroxidase (VP), and dye-decolorizing peroxidase (DyP). In this review, we explore the key pollutant-generating steps in paper and pulp processing, summarize the most recently reported toxicological effects of industrial lignin-derived phenolic compounds, especially chlorinated phenolic pollutants, and outline bioremediation approaches for pollutant mitigation in wastewater from this industry, emphasizing the oxidative catalytic potential of oxidative lignin modifying enzymes in this regard. We highlight other emerging biotechnical approaches, including phytobioremediation, bioaugmentation, Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-based technology, protein engineering, and degradation pathways prediction, that are currently gathering momentum for the mitigation of wastewater pollutants. Finally, we address current research needs and options for maximizing sustainable biobased and biocatalytic degradation of toxic industrial wastewater pollutants.

Thermophilic microbial deconstruction and conversion of natural and transgenic lignocellulose

The potential to convert renewable plant biomasses into fuels and chemicals by microbial processes presents an attractive, less environmentally intense alternative to conventional routes based on fossil fuels. This would best be done with microbes that natively deconstruct lignocellulose and concomitantly form industrially relevant products, but these two physiological and metabolic features are rarely and simultaneously observed in nature. Genetic modification of both plant feedstocks and microbes can be used to increase lignocellulose deconstruction capability and generate industrially relevant products. Separate efforts on plants and microbes are ongoing, but these studies lack a focus on optimal, complementary combinations of these disparate biological systems to obtain a convergent technology. Improving genetic tools for plants have given rise to the generation of low-lignin lines that are more readily solubilized by microorganisms. Most focus on the microbiological front has involved thermophilic bacteria from the genera Caldicellulosiruptor and Clostridium, given their capacity to degrade lignocellulose and to form bio-products through metabolic engineering strategies enabled by ever-improving molecular genetics tools. Bioengineering plant properties to better fit the deconstruction capabilities of candidate consolidated bioprocessing microorganisms has potential to achieve the efficient lignocellulose deconstruction needed for industrial relevance.

Lignin peroxidase in focus for catalytic elimination of contaminants - A critical review on recent progress and perspectives

Lignin peroxidase (LiP) seems to be a catalyst for cleaving high-redox potential non-phenolic compounds with an oxidative cleavage of CC and COC bonds. LiP has been picked to seek a practical and cost-effective alternative to the sustainable mitigation of diverse environmental contaminants. LiP has been an outstanding tool for catalytic cleaning and efficient mitigation of environmental pollutants, including lignin, lignin derivatives, dyes, endocrine-disrupting compounds (EDCs), and persistent organic pollutants (POPs) for the past couple of decades. The extended deployment of LiP has proved to be a promising method for catalyzing these environmentally related hazardous pollutants of supreme interest. The advantageous potential and capabilities to act at different pH and thermostability offer its working tendencies in extended environmental engineering applications. Such advantages led to the emerging demand for LiP and increasing requirements in industrial and biotechnological sectors. The multitude of the ability attributed to LiP is triggered by its stability in xenobiotic and non-phenolic compound degradation. However, over the decades, the catalytic activity of LiP has been continuing in focus enormously towards catalytic functionalities over the available physiochemical, conventional, catalyst mediated technology for catalyzing such molecules. To cover this literature gap, this became much more evident to consider the catalytic attributes of LiP. In this review, the existing capabilities of LiP and other competencies have been described with recent updates. Furthermore, numerous recently emerged applications, such as textile effluent treatment, dye decolorization, catalytic elimination of pharmaceutical and EDCs compounds, have been discussed with suitable examples.

Oxidative depolymerization of Kraft lignin to high-value aromatics using a homogeneous vanadium–copper catalyst

Kraft lignin is efficiently depolymerized under benign conditions into value-added aromatics and high-quality bio-oil using a facile vanadium–copper catalyst system.

Inhibition of Phenolics Uptake by Ligninolytic Fungal Cells and Its Potential as a Tool for the Production of Lignin-Derived Aromatic Building Blocks

Lignin is the principal natural source of phenolics but its structural complexity and variability make it difficult to valorize through chemical depolymerization approaches. White rots are one of the rare groups of organisms that are able to degrade lignin in ecosystems. This biodegradation starts through extracellular enzymes producing oxidizing agents to depolymerize lignin and continue with the uptake of the generated oligomers by fungal cells for further degradation. Phanerochaete chrysosporium is one of the most studied species for the elucidation of these biodegradation mechanisms. Although the extracellular depolymerization step appears interesting for phenolics production from lignin, the uptake and intracellular degradation of oligomers occurring in the course of the depolymerization limits its potential. In this study, we aimed at inhibiting the phenolics uptake mechanism through metabolic inhibitors to favor extracellular oligomers accumulation without preventing the ligninases production that is necessary for extracellular depolymerization. The use of sodium azide confirmed that an active transportation phenomenon is involved in the phenolics uptake in P. chrysosporium. A protocol based on carbonyl cyanide m-chlorophenyl hydrazone enabled reaching 85% inhibition for vanillin uptake. This protocol was shown not to inhibit, but on the contrary, to stimulate the depolymerization of both dehydrogenation polymers (DHPs) and industrial purified lignins.

Biomimetic Vanadate and Molybdate Systems for Oxidative Upgrading of Iono- and Organosolv Hard- and Softwood Lignins

Recently reported acetosolv soft- and hardwood lignins as well as ionosolv soft- and hardwood lignins were transformed into monomeric aromatic compounds using either a vanadate or a molybdate-based catalyst system. Monomers were generated with remarkable, catalyst-dependent selectivity and high depolymerisation yields via oxidative exo- and endo-depolymerisation processes. Using the vanadate–hydrogen peroxide system on acetosolv pine lignin, vanillin and isovanillin were produced as main products with depolymerisation yields of 31%. Using the molybdate system on acetosolv and ionosolv lignin, vanillic acid was the practically exclusive product, with depolymerisation yields of up to 72%. Similar selectivities, albeit with lower depolymerisation yields of around 50% under standardised conditions, were obtained for eucalyptus acetosolv lignin, producing vanillin and syringaldehyde or vanillic acid as products, by using the vanadate- or the molybdate-based systems respectively.

Greener Routes to Biomass Waste Valorization: Lignin Transformation Through Electrocatalysis for Renewable Chemicals and Fuels Production

Lignin valorization is essential for biorefineries to produce fuels and chemicals for a sustainable future. Today's biorefineries pursue profitable value propositions for cellulose and hemicellulose; however, lignin is typically used mainly for its thermal energy value. To enhance the profit potential for biorefineries, lignin valorization would be a necessary practice. Lignin valorization is greatly advantaged when biomass carbon is retained in the fuel and chemical products and when energy quality is enhanced by electrochemical upgrading. Though lignin upgrading and valorization are very desirable in principle, many barriers involved in lignin pretreatment, extraction, and depolymerization must be overcome to unlock its full potential. This Review addresses the electrochemical transformation of various lignins with the aim of gaining a better understanding of many of the barriers that currently exist in such technologies. These studies give insight into electrochemical lignin depolymerization and upgrading to value-added commodities with the end goal of achieving a global low-carbon circular economy.

Catalytic Cleavage of the C-O Bond in 2,6-dimethoxyphenol Without External Hydrogen or Organic Solvent Using Catalytic Vanadium Metal

Hydrogenolysis of the C-O bonds in lignin, which promises to be able to generate fuels and chemical feedstocks from biomass, is a particularly challenging and important area of investigation. Herein, we demonstrate a vanadium-catalyzed cleavage of a lignin model compound (2,6-dimethoxyphenol). The impact of the catalyst in the context of the temperature, reaction time, and the solvent, was examined for the cleavage of the methyl ethers in 2,6-dimethoxyphenol. In contrast to traditional catalytic transfer hydrogenolysis, which requires high pressure hydrogen gas or reductive organic molecules, such as an alcohol and formic acid, the vanadium catalyst demonstrates superior catalytic activity on the cleavage of the C-O bonds using water as a solvent. For example, the conversion of 2,6-dimethoxyphenol is 89.5% at 280°C after 48 h using distilled water. Notably, the vanadium-catalyzed cleavage of the C-O bond linkage in 2,6-dimethoxyphenol affords 3-methoxycatechol, which undergoes further cleavage to afford pyrogallol. This work is expected to provide an alternative method for the hydrogenolysis of lignin and related compounds into valuable chemicals in the absence of external hydrogen and organic solvents.

Stabilization strategies in biomass depolymerization using chemical functionalization

A central feature of most lignocellulosic-biomass-valorization strategies is the depolymerization of all its three major constituents: cellulose and hemicellulose to simple sugars, and lignin to phenolic monomers. However, reactive intermediates, generally resulting from dehydration reactions, can participate in undesirable condensation pathways during biomass deconstruction, which have posed fundamental challenges to commercial biomass valorization. Thus, new strategies specifically aim to suppress condensations of reactive intermediates, either avoiding their formation by functionalizing the native structure or intermediates or selectively transforming these intermediates into stable derivatives. These strategies have provided unforeseen upgrading pathways, products and process solutions. In this Review, we outline the molecular driving forces that shape the deconstruction landscape and describe the strategies for chemical functionalization. We then offer an outlook on further developments and the potential of these strategies to sustainably produce renewable-platform chemicals.

Pd–Ce/ZIF-8 Nanocomposite for Catalytic Extraction of Sinomenine from Sinomenium acutum

Sinomenine is a naturally occurring alkaloid and commonly used as one of the bioactive drug components in rheumatoid arthritis (RA) treatment in the clinic. Varying supported palladium-based catalysts have been synthesized and examined as heterogeneous catalysts for catalytic extraction of sinomenine from Sinomenium acutum. Among various examined supported catalysts, Pd–Ce/ZIF-8 (zeolitic imidazolate framework-8) demonstrates promising catalytic activity in the extraction reaction with an improved yield of 2.15% under optimized conditions. The catalyst composite can be recovered by centrifuging, and reused. A total of three catalyst recycling processes were performed with constant activity. The catalyst Pd–Ce/ZIF-8 has a particle size range of 2–12 nm and a total Pd–Ce loading amount of 5.1 wt% (ZIF-8).

Heteroatom-participated lignin cleavage to functionalized aromatics

Heteroatom-containing reagents triggered the catalytic cleavage of lignin linkages and functionalization of products simultaneously.

Technoeconomic Assessment of Hybrid Organosolv–Steam Explosion Pretreatment of Woody Biomass

This study investigates technoeconomic performance of standalone biorefinery concepts that utilize hybrid organic solvent and steam explosion pretreatment technique. The assessments were made based on a mathematical process model developed in UniSim Design software using inhouse experimental data. The work was motivated by successful experimental applications of the hybrid pretreatment technique on lignocellulosic feedstocks that demonstrated high fractionation efficiency into a cellulose-rich, a hemicellulose-rich and lignin streams. For the biorefinery concepts studied here, the targeted final products were ethanol, organosolv lignin and hemicellulose syrup. Minimum ethanol selling price (MESP) and Internal rate of return (IRR) were evaluated as economic indicators of the investigated biorefinery concepts. Depending on the configuration, and allocating all costs to ethanol, MESP in the range 0.53–0.95 €/L were required for the biorefinery concepts to break even. Under the assumed ethanol reference price of 0.55 €/L, the corresponding IRR were found to be in the range −1.75–10.7%. Hemicellulose degradation and high steam demand identified as major sources of inefficiencies for the process and economic performance, respectively. Sensitivity of MESP and IRR towards the most influential technical, economic and market parameters performed.

Ru-Catalyzed Oxidative Cleavage of Guaiacyl Glycerol--Guaiacyl Ether-a Representative -O-4 Lignin Model Compound

The introduction of efficient and selective catalytic methods for aerobic oxidation of lignin and lignin model compounds to aromatics can extend the role of lignin applications in biorefineries. The current study focussed on the catalytic oxidative transformation of guaiacyl glycerol--guaiacyl ether (GGGE)–a -O-4 lignin model compound to produce basic aromatic compounds (guaiacol, vanillin and vanillic acid) using metal-supported catalysts. Ru/Al2O3, prepared with ruthenium(IV) oxide hydrate, showed the highest yields of the desired products (60%) in acetonitrile in a batch reactor at 160 C and 5-bar of 20% oxygen in argon. Alternative catalysts containing other transition metals (Ag, Fe, Mn, Co and Cu) supported on alumina, and ruthenium catalysts based on alternative supports (silica, spinel, HY zeolite and zirconia) gave significantly lower activities compared to Ru/Al2O3 at identical reaction conditions. Moreover, the Ru/Al2O3 catalyst was successfully reused in five consecutive reaction runs with only a minor decrease in catalytic performance.

Transformation of lamotrigine by white-rot fungus Pleurotus ostreatus

One of the most persistent pharmaceutical compounds commonly found in treated wastewater is lamotrigine (LTG). It has also been detected in soils and crops irrigated with treated wastewater. Here we focused on the ability of the white-rot edible mushroom Pleurotus ostreatus to remove and transform LTG in liquid cultures. At concentrations of environmental relevance (1 and 10 μg L^-1) LTG was almost completely removed from the culture medium within 20 days. To elucidate the mechanism of LTG removal and transformation, we applied a physiological-based approach using inhibitors and a competing agent. These experiments were conducted at a higher concentration for metabolites detection. Based on identification of sulfur-containing metabolites and LTG N2-oxide and the effect of specific inhibitors, cytochrome P450 oxidation is suggested as one of the reaction mechanisms leading to LTG transformation. The variety and number of transformation products (i.e., conjugates) found in the current study were larger than reported in mammals. Moreover, known conjugates with glucuronide, glutathione, or cysteine/glycine, were not found in our system. Since the majority of the identified transformation products were conjugates of LTG, this study highlights the persistence of LTG as an organic pollutant in ecosystems exposed to wastewater.