Recent developments in enzymatic and microbial biosynthesis of flavor and fragrance molecules

Flavors and fragrances are an important class of specialty chemicals for which interest in biomanufacturing has risen during recent years. These naturally occurring compounds are often amenable to biosynthesis using purified enzyme catalysts or metabolically engineered microbial cells in fermentation processes. In this review, we provide a brief overview of the categories of molecules that have received the greatest interest, both academically and industrially, by examining scholarly publications as well as patent literature. Overall, we seek to highlight innovations in the key reaction steps and microbial hosts used in flavor and fragrance manufacturing.

High-Performance One-Component Epoxy Adhesive Based on the Synergistic Effect of Lignin-Derived Triaryl-Imidazole and Phytic Acid

Latent curing agents are essential in the formulation of one-component epoxy resins, yet they are seldom derived from fully biobased chemicals. In the present work, a fully biobased latent curing agent for epoxy resins (BIMPA) was produced by synthesizing an ionic complex of lignin-derived triaryl-imidazole (BIM) and phytic acid (PA). Benefiting from the synergistic effect of BIM and PA, the one-component epoxy resin, composed of BIMPA and commercially available E51, exhibits a storage stability of over 90 days. Upon heating, the ionic complex undergoes decomposition, liberating the active imidazole to cure the precursor. The resulting epoxy resins exhibited a flexural modulus of 3.09 GPa, a flexural strength of 107.47 MPa, a notched izod impact strength of 2.47 kJ/m3, and a shear strength of 41.02 MPa. The outcome can provide an effective supplement for the development of biobased epoxy resins.

State-of-the-art of lignin-derived carbon nanodots: Preparation, properties, and applications

Lignin-derived carbon nanodots (LCNs) are nanometer-scale carbon spheres fabricated from naturally abundant lignin. Owing to rich and highly heritable graphene like π-π conjugated structure of lignin, to fabricate LCNs from it not only endows LCNs with on-demand tunable size and optical features, but also further broadens the green and chemical engineering of carbon nanodots. Recently, they have become increasingly popular in sensing, bioimaging, catalysis, anti-counterfeiting, energy storage/conversion, and others. Despite the enormous research efforts put into the ongoing development of lignin value-added utilization, few commercial LCNs are available. To have a deeper understanding of this issue, critical impacts on the preparation, properties, and applications of state-of-the-art LCNs are carefully reviewed and discussed. A concise analysis of their unique advantages, limitations for specific applications, and current challenges and outlook is conducted. We hope that this review will stimulate further advances in the functional material-oriented production of lignin.

WOx boosted hollow Ni nanoreactors for the hydrodeoxygenation of lignin derivatives

Reasonable design of non-noble metal catalysts with hollow open structure for hydrodeoxygenation (HDO) of lignin derivatives to value-added chemicals is of great significance but challenging. Herein, a novel MOF-derived multilayer hollow sphere coated nickel‑tungsten bimetallic catalyst (Ni2-WOx@CN-700) was fabricated via by confined pyrolysis strategy using bimetallic MOFs as a self-sacrificial template, which exhibits robust activity for the typical model HDO of vanillin to 2-methoxy-4-methylphenol (Yield of 100 % at 140 °C for no less than 10 cycles). The characterizations revealed that WOx facilitated the dispersion of Ni nanoparticles and adjusted the acidic capacity of the catalyst through the formed Ni-WOx heterojunction. Density functional theory (DFT) calculations confirms that WOx species enhanced the electron-rich nature of the active sites, while the adsorption energies of H2 and vanillin on Ni-WOx decreased from -0.572 eV and - 0.622 eV on Ni to -3.969 eV and - 4.922 eV, respectively. These results further indicated that the high activity of Ni2-WOx@CN-700 was attributed to the Ni-WOx heterojunction. Based on the characterizations and the thermodynamic calculations, the reaction mechanism was proposed. In addition, the catalyst shows good substrate universality, which enables its good commercial application prospect.

Carboxymethylated Lignin Reinforcement of SPI Adhesive: Enhancing Strength, Antimicrobial, and Flame-Retardant Properties without Excessive Alkali Introduction

To address the challenges associated with formaldehyde emissions in engineered wood adhesives and simultaneously enhance adhesive properties related to water resistance, fire resistance, and mold resistance, a novel environmentally sustainable biomass-based adhesive was formulated. In this work, kraft lignin was carboxymethylated and then blended with the soy protein isolate (SPI)-based adhesive, the dry and wet shear strength of the plywood bonded by the resultant adhesive was enhanced from 1.10 and 0.63 MPa to 1.73 and 1.23 MPa, respectively, resulting in improvements of 157% and 195%. Carboxymethylated lignin (CML) significantly improved the mold resistance and flame-resistance residual rate of the adhesive and decreased the water absorption rate from 190% to 108%. Furthermore, the adhesive exhibits outstanding flame-retardancy, with self-extinguishing capability rendering it suitable for industrial production. In addition, we also evaluated the performances of resulting adhesives cured with different diepoxides and triepoxides, and the comparisons of the adhesive in this work to commercial urea glue and soy protein-based adhesives were conducted. To our delight, the SPI-10CML adhesive presented comparable or even improved performances, showing its promising practical applications such as for fire doors.

A Metallomicellar Catalyst for Controlled Oxidation of Alcohols and Lignin Mimics in Water using Open Air as Oxidant

Alcohol groups and β-O-4 (C-C) linkages are widespread in biomass feedstock that are abundant renewable resource for value-added chemicals. The development of sustainable protocols for direct oxidation or oxidative cleavage of feedstock materials in a controlled fashion, using open air as an oxidant is an intellectually stimulating task to produce industrially important value-added carbonyls. Further, the oxidative depolymerization of lignin into fine chemicals has evoked interest in recent times. Herein, we report the first example of a catalyst system that could activate molecular oxygen from atmospheric air for controlled oxidation and oxidative cleavage/depolymerization of feedstock materials such as alcohols, β-O-4 (C-C) linkages and real lignin in water under open air conditions. The selectivity of carbonyl products is controlled by altering the pH between ~7.0 and ~12.0. The current strategy highlights the non-involvement of any external co-catalyst, oxidant, radical additives, and/or destructive organic solvents. The catalyst shows a wide substrate scope and eminent functional group tolerance. The upscaled multigram synthesis using an inexpensive catalyst and easily available oxidant evidences the practical utility of the developed protocol. A plausible mechanism has been proposed with the help of a few controlled experiments, and kinetic and computational studies.

Electrocatalytic functional group conversion-based carbon resource upgrading

The conversions of carbon resources, such as alcohols, aldehydes/ketones, and ethers, have been being one of the hottest topics most recently for the goal of carbon neutralization. The emerging electrocatalytic upgrading has been regarded as a promising strategy aiming to convert carbon resources into value-added chemicals. Although exciting progress has been made and reviewed recently in this area by mostly focusing on the explorations of valuable anodic oxidation or cathodic reduction reactions individually, however, the reaction rules of these reactions are still missing, and how to purposely find or rationally design novel but efficient reactions in batches is still challenging. The properties and transformations of key functional groups in substrate molecules play critically important roles in carbon resources conversion reactions, which have been paid more attention to and may offer hidden keys to achieve the above goal. In this review, the properties of functional groups are addressed and discussed in detail, and the reported electrocatalytic upgrading reactions are summarized in four categories based on the types of functional groups of carbon resources. Possible reaction pathways closely related to functional groups will be summarized from the aspects of activation, cleavage and formation of chemical bonds. The current challenges and future opportunities of electrocatalytic upgrading of carbon resources are discussed at the end of this review.

Synthesis of lignin-based resin and fabrication of sustainable transparent wood based on bio-recycling concept

Transparent wood (TW) has attracted much attention in the field of energy saving building structural materials because of its high light transmittance, good thermal insulation performance and good toughness. However, the polymeric resins used in the present study to impregnate lignin-based wood templates are usually derived from petroleum-based chemical resources, which pose a fatal threat to human beings both in terms of consuming large amounts of resources and causing environmental pollution problems. It is therefore important to develop alternatives to petroleum-derived chemicals in renewable natural resources. Here, we report a green and sustainable TW production process based on the bio-recycling concept. Lignin-based sustainable resin (LSR) was prepared from waste lignin produced during delignification by polymerization of guaiacol. At the same time, according to FT-IR and NMR data analysis combined with previous studies, the synthesis mechanism of LSR was proposed, and this result provided a reference for bio-based resins made from biomass materials. The prepared lignin-based sustainable transparent wood (LSTW) has good light transmittance and good dimensional stability. In addition, the LSTW also shows good thermal insulation and indoor temperature regulation capabilities compared with the common glass.

LiCoO2 impregnated nano-hierarchical ZSM-5 assisted catalytic upgrading of Kraft lignin-derived liquefaction bio-oil

In this study, Kraft lignin-derived bio-oil was upgraded with LiCoO2 or Co3O4-impregnated hierarchical nano-ZSM-5 catalysts. The synthesized catalysts were characterized by N2-Ads-Des, XRD, XPS, NH3-TPD, FTIR, FESEM and ICP-OES analyses. Upon incorporation of LiCoO2 and Co3O4 onto the HZSM-5 support, the MFI structure of HZSM-5 remained intact. All the catalysts displayed a combination of Type-I and -IV isotherms. The upgraded bio-oil showed a significant increase in the amounts of alkylated guaiacols owing to the reduction in unsubstituted guaiacols, alkenyl guaiacols, and homovanillic acid. Hydrogenation, alkylation, and deoxygenation were the plausible bio-oil upgrading pathways. With the increase in cobalt content, weak acidity decreased through all the catalysts, while LiCoO2 provided supplementary acid sites that increased the total acidity of LiCoO2/HZSM-5 compared to the Co3O4/HZSM-5 catalyst. LiCoO2/HZSM-5 with a low cobalt content (5% and 10% Co) displayed high selectivity for the production of alkylated guaiacols owing to their strong acidity. The upgraded bio-oils showed an increase in carbon and hydrogen followed by a decrease in oxygen content. The maximum higher heating value (∼29.83 MJ kg-1) was obtained for the 10% Co (LiCoO2)/HZSM-5 catalyst. In general, LiCoO2/HZSM-5 outperformed the Co3O4/HZSM-5 catalyst. XRD of the spent 10% Co (LiCoO2)/HZSM-5 suggested the complete loss of lithium from the catalyst with the retention of the MFI structure of the HZSM-5 support. In this study, it was successfully demonstrated that the main constituent of the cathode material of spent lithium-ion batteries i.e. LiCoO2 could be employed to synthesize a novel and cheap catalyst for bio-oil upgrading while addressing the e-waste management issue in a sustainable manner.

Advanced biomass-based Janus materials: Classification, preparation and application: A review

In contrast to conventional particles characterized by isotropic surfaces, Janus particles possess anisotropic surfaces, resulting in unique physicochemical properties and functional attributes. In recent times, there has been a surge in interest regarding the synthesis of Janus particles using biological macromolecules. Various synthesis techniques have been developed for the fabrication of Janus materials derived from biomass. These methods include electrospinning, freeze-drying, secondary casting film formation, self-assembly technology, and other approaches. In the realm of Janus composite materials, those derived from biomass have found extensive applications in diverse domains including oil-water separation, sensors, photocatalysis, and medical materials. This article provides a systematic introduction to the classification of Janus materials, with a specific focus on various types of biomass-based Janus materials (mainly cellulose-based Janus materials, lignin-based Janus materials and protein-based Janus materials) and the methods used for their preparation. This work will not only deepen the understanding of biomass-based Janus materials, but also contribute to the development of new methods for designing biomass-based Janus structures to optimize biomass utilization.

Sustainable production of triazoles from lignin major motifs

An efficiently catalyzed synthesis of pharmaceutically relevant 1,2,3-trazoles from renewable resources is highly desirable. However, due to incompatible catalysis conditions, this endeavor remained challenging so far. Herein, a practical access protocol to 1,2,3-triazoles, starting from lignin phenolic β-O-4 with γ-OH group utilizing a vanadium-based catalyst is presented. A broad substrate scope reaching up to 97 % yield of 1,2,3-triazoles are obtained. The reaction pathway includes selective cleavage of double C-O bonds, cycloaddition, and dehydrogenation. Mechanistic studies and density-functional theory (DFT) calculations suggest that the V-based complex acts as a bifunctional catalyst for both selective C-O bonds cleavage and dehydrogenation. This synthetic pathway has been applied for the synthesis of pharmacological and biological active carbohydrate derivatives starting from biomass components as feedstock, enabling a potential sustainable route to triazolyl carbohydrate derivatives, which paves the way for lignin-based heterocyclic aromatics in the pharmaceutical applications.

Lignin derivatives-based hydrogels for biomedical applications

Recently, numerous studies have been conducted on renewable polymers derived from different natural sources, exploring their suitability for diverse biomedical applications. Lignin as one of the main components of lignocellulosic has garnered significant attention as a promising alternative to petroleum-based polymers. This interest is primarily due to its cost-effectiveness, biocompatibility, eco-friendly nature, as well as its antioxidant and antimicrobial properties. These characteristics could be more beneficial when incorporating lignin into the formulation of value-added products. Although lignin has a chemical structure that is suitable for various applications, these characteristics require modifications to guarantee that the resultant materials display the desired biological, chemical, and physical properties when applied in the creation of biodegradable hydrogels, particularly for biomedical purposes. This study delineates the recent modification approaches that have been employed in the creation of lignin-based hydrogels. These strategies encompass both chemical and physical interactions with other polymers. Additionally, this review encompasses an examination of the current applications of lignin hydrogels, spanning their use as scaffolds for tissue engineering, carriers for pharmaceuticals, materials for wound dressings and biosensors, and elements in flexible and wearable electronics. Finally, we delve into the challenges and constraints associated with these materials, discuss the necessary steps required to attain the appropriate properties for the development of innovative lignin-based hydrogels, and derive conclusions based on the presented findings.

Photocatalytic Conversion of Lignin Models into Functionalized Aromatic Molecules Initiated by the Proton-Coupled Electron Transfer Process

A mild and efficient method for lignin β-O-4 cleavage and functionalization was achieved via photocatalysis. This protocol exhibits a broad scope of lignin models and excellent compatibility of functionalization reagents, constructing a series of functionalized lignin-based aromatic compounds. Highly selective formation of alkyl radical species through a proton-coupled electron transfer and β-scission process provides the opportunity to form new C-C and C-N bonds by reaction with electrophilic reagents.

Preparation of uniform lignin/titanium dioxide nanoparticles by confined assembly: A multifunctional nanofiller for a waterborne polyurethane wood coating

We report a facile synthesis for lignin/titanium dioxide (TiO2) nanoparticles (LT NPs) at room temperature by confining assembly of lignin macromolecules. The LT NPs had a uniform nanosize distribution (average diameter ∼ 68 nm) and were directly employed as multifunctional nanofillers to reinforce a waterborne polyurethane wood coating (WBC). X-ray diffraction, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy revealed the mechanism by which formed TiO2 confined lignin assembly. The LT NPs considerably increased the tensile strength of a WBC film from 16.3 MPa to 28.1 MPa. The WBC-LT NPs exhibited excellent ultraviolet (UV) A and UVB blocking performances of 87 % and 98 %, respectively, while maintaining 94 % transmittance in the visible region. Incorporating LT NPs into the WBC enhanced the coating performance (the hardness, adhesion, and abrasion resistance) on wood substrates. A quantitative color and texture analysis revealed that the LT NPs increased the decorativeness of actual wooden products. After nearly 1800 h of UV irradiation, wood coated with the WBC-LT NPs exhibited good color stability, where the original color remained unchanged or even became brighter. In this study, value-added valorization of lignin is enabled by using organic-inorganic nanofillers and insights are gained into developing multifunctional WBCs.

Catalytic carbon-carbon bond cleavage in lignin via manganese-zirconium-mediated autoxidation

Efforts to produce aromatic monomers through catalytic lignin depolymerization have historically focused on aryl-ether bond cleavage. A large fraction of aromatic monomers in lignin, however, are linked by various carbon-carbon (C-C) bonds that are more challenging to cleave and limit the yields of aromatic monomers from lignin depolymerization. Here, we report a catalytic autoxidation method to cleave C-C bonds in lignin-derived dimers and oligomers from pine and poplar. The method uses manganese and zirconium salts as catalysts in acetic acid and produces aromatic carboxylic acids as primary products. The mixtures of the oxygenated monomers are efficiently converted to cis,cis-muconic acid in an engineered strain of Pseudomonas putida KT2440 that conducts aromatic O-demethylation reactions at the 4-position. This work demonstrates that autoxidation of lignin with Mn and Zr offers a catalytic strategy to increase the yield of valuable aromatic monomers from lignin.

Advances in Heterogeneous Catalysts for Lignin Hydrogenolysis

Lignin is the main component of lignocellulose and the largest source of aromatic substances on the earth. Biofuel and bio-chemicals derived from lignin can reduce the use of petroleum products. Current advances in lignin catalysis conversion have facilitated many of progress, but understanding the principles of catalyst design is critical to moving the field forward. In this review, the factors affecting the catalysts (including the type of active metal, metal particle size, acidity, pore size, the nature of the oxide supports, and the synergistic effect of the metals) are systematically reviewed based on the three most commonly used supports (carbon, oxides, and zeolites) in lignin hydrogenolysis. The catalytic performance (selectivity and yield of products) is evaluated, and the emerging catalytic mechanisms are introduced to better understand the catalyst design guidelines. Finally, based on the progress of existing studies, future directions for catalyst design in the field of lignin depolymerization are proposed.

Autoxidation Catalysis for Carbon-Carbon Bond Cleavage in Lignin

Selective lignin depolymerization is a key step in lignin valorization to value-added products, and there are multiple catalytic methods to cleave labile aryl-ether bonds in lignin. However, the overall aromatic monomer yield is inherently limited by refractory carbon-carbon linkages, which are abundant in lignin and remain intact during most selective lignin deconstruction processes. In this work, we demonstrate that a Co/Mn/Br-based catalytic autoxidation method promotes carbon-carbon bond cleavage in acetylated lignin oligomers produced from reductive catalytic fractionation. The oxidation products include acetyl vanillic acid and acetyl vanillin, which are ideal substrates for bioconversion. Using an engineered strain of Pseudomonas putida, we demonstrate the conversion of these aromatic monomers to cis,cis-muconic acid. Overall, this study demonstrates that autoxidation enables higher yields of bioavailable aromatic monomers, exceeding the limits set by ether-bond cleavage alone.

Lignin Modification for Enhanced Performance of Polymer Composites

The biopolymer lignin, which is heterogeneous and abundant, is usually present in plant cell walls and gives them rigidity and strength. As a byproduct of the wood, paper, and pulp manufacturing industry, lignin ranks as the second most prevalent biopolymer worldwide, following cellulose. This review paper explores the extraction, modification, and prospective applications of lignin in various industries, including the enhancement of thermosetting and thermoplastic polymers, biomedical applications such as vanillin production, fuel development, carbon fiber composites, and the creation of nanomaterials for food packaging and drug delivery. The structural characteristics of lignin remain undefined due to its origin, separation, and fragmentation processes. This comprehensive overview encompasses state-of-the-art techniques, potential applications, diverse extraction methods, chemical modifications, carbon fiber utilization, and the extraction of vanillin. Moreover, the review focuses on the utilization of lignin-modified polymer blends across multiple manufacturing sectors, providing insights into the advantages and limitations of this innovative approach for the development of environmentally friendly materials.

Confining redox-active metal sites in acidic porous scaffolds for the catalytic transformation of lignin-derived phenols to naphthenes

The hydrodeoxygenation transformation of lignin-derived phenols provides an attractive pathway for the production of renewable biofuels; however, harsh process conditions strongly hinder its practical application. Herein, we report a porous metal silicate (PMS) material, PMS-36, which consists of metallic nickel and Lewis acid AlIII sites inside the pores, demonstrating high efficiency in catalyzing the hydrodeoxygenation transformation of guaiacol under mild conditions. PMS-36 also exhibits robust stability, which can be attributed to the strong interaction and charge transfer between metallic Ni and AlIII Lewis acid sites inside the confined pores. This study shows the importance of synergistic and confinement effects in developing high-performance and stable heterogeneous catalysts for the chemical transformation of biomass and its derivatives.

Recent Advances of Lignin Functionalization for High-Performance and Advanced Functional Rubber Composites

The biomass lignin is the only large-volume renewable feedstock that is composed of aromatics but has been largely underutilized and is sought for valorization as a value-added material. Recent research has highlighted lignin as a promising alternative to traditional petrol-based reinforcements and functional additives for rubber composites. This review summarized the recent advances in the functionalization of lignin for a variety of rubber composites, as well as the compounding techniques for effectively dispersing lignin within the rubber matrix. Significant progress has been achieved in the development of high-performance and advanced functional rubber/lignin composites through carefully designing the structure of lignin-based additives and the optimization of interfacial morphologies. This Review discussed the effect of lignin on composite properties, including mechanical reinforcement, dynamic properties, antiaging performance, and oil resistance, and also the advanced stimuli-responsive performance in detail. A critical analysis for the future development of rubber/lignin composites is presented as concluding remarks.

Thermal, photonic, and electrocatalysis in lignin depolymerization research

In order to realize a sustainable bio-based future, it is essential to fully harness the potential of biomass, including lignin - a readily available biopolymer that ranks second in abundance and serves as a renewable source of aromatics. While lignin has traditionally been used for lower-value applications like fuel and power generation, unlocking its higher-value potential through diverse conversion and upgrading techniques is of paramount importance. This review focuses on the catalytic conversion of lignin, with a specific emphasis on selective depolymerization, a process that not only supports economically and environmentally sustainable biorefineries but also aligns with Green Chemistry principles, mitigating adverse environmental impacts. Furthermore, we provide a comprehensive discussion of reaction pathways and mechanisms, including C-O and C-C bond cleavage, among different catalysts. Lastly, we analyze and briefly discuss the prospects of rational catalyst design in biomass valorization.

Photoinduced Biomimetic Room-Temperature C-O Bond Cleavage over Mn Doped CdS

Selective C-O bond cleavage is an efficient way for the biomass valorization to value-added chemicals, but is challenged to be operated at room temperature via conventional thermal catalysis. Herein, inspired from the DNA biosynthesis which involves a radical-mediated spin-center shift (SCS) C-O bond cleavage process, we report a biomimetic room-temperature C-O bond cleavage of vicinal diol (HOCHCH-OH). We construct a Mn doped CdS (Mn/CdS) as a photocatalyst to mimic the biologic SCS process. The Mn site plays pivotal role: (1) accelerates the photo-induced carrier separation, promoting the hole-mediated C-H bond cleavage to generate carbon-centered radicals, and (2) serves as the binding site for -OH groups, making it to be an easier leaving group. Mn/CdS achieves 0.28 mmol gcat -1  h-1 of hydroxyacetone (HA) from glycerol dehydration at room temperature under visible light irradiation, which is 3.5-fold that over pristine CdS and 40-fold that over bulk MnS/CdS. This study provides a new biomimetic room-temperature C-O bond cleavage process, which is promising for the biomass valorization.

Orange Peel Waste as Feedstock for the Production of Glycerol-Free Biodiesel by the Microalgae Nannochloropsis oculata

The bioconversion of agri-food waste into high-value products is gaining growing interest worldwide. Orange peel waste (OPW) is the main by-product of orange juice production and contains high levels of moisture and carbohydrates. In this study, the orange waste extract (OWE) obtained through acid hydrolysis of OPW was used as a substrate in the cultivation of the marine microalgae Nannochloropsis oculata. Photoheterotrophic (PH) and Photoautotrophic (PA) cultivations were performed in OWE medium and f/2 medium (obtained by supplementing OWE with macro- and micronutrients of f/2 medium), respectively, for 14 days. The biomass yields in PA and PH cultures were 390 mg L-1 and 450 mg L-1, while oil yields were 15% and 28%, respectively. The fatty acid (FA) profiles of PA cultures were mostly represented by saturated (43%) and monounsaturated (46%) FAs, whereas polyunsaturated FAs accounted for about 10% of the FAs. In PH cultures, FA profiles changed remarkably, with a strong increase in monounsaturated FAs (77.49%) and reduced levels of saturated (19.79%) and polyunsaturated (2.72%) FAs. Lipids obtained from PH cultures were simultaneously extracted and converted into glycerol-free biodiesel using an innovative microwave-assisted one-pot tandem protocol. FA methyl esters were then analyzed, and the absence of glycerol was confirmed. The FA profile was highly suitable for biodiesel production and the microwave-assisted one-pot tandem protocol was more effective than traditional extraction techniques. In conclusion, N. oculata used OWE photoheterotrophically, resulting in increased biomass and oil yield. Additionally, a more efficient procedure for simultaneous oil extraction and conversion into glycerol-free biodiesel is proposed.

Towards bioresource-based aggregation-induced emission luminogens from lignin β-O-4 motifs as renewable resources

One-pot synthesis of heterocyclic aromatics with good optical properties from phenolic β-O-4 lignin segments is of high importance to meet high value added biorefinery demands. However, executing this process remains a huge challenge due to the incompatible reaction conditions of the depolymerization of lignin β-O-4 segments containing γ-OH functionalities and bioresource-based aggregation-induced emission luminogens (BioAIEgens) formation with the desired properties. In this work, benzannulation reactions starting from lignin β-O-4 moieties with 3-alkenylated indoles catalyzed by vanadium-based complexes have been successfully developed, affording a wide range of functionalized carbazoles with up to 92% yield. Experiments and density functional theory calculations suggest that the reaction pathway involves the selective cleavage of double C-O bonds/Diels-Alder cycloaddition/dehydrogenative aromatization. Photophysical investigations show that these carbazole products represent a class of BioAIEgens with twisted intramolecular charge transfer. Distinctions of emission behavior were revealed based on unique acceptor-donor-acceptor-type molecular conformations as well as molecular packings. This work features lignin β-O-4 motifs with γ-OH functionalities as renewable substrates, without the need to apply external oxidant/reductant systems. Here, we show a concise and sustainable route to functional carbazoles with AIE properties, building a bridge between lignin and BioAIE materials.

Deriving high value products from depolymerized lignin oil, aided by (bio)catalytic funneling strategies

Lignin holds tremendous and versatile possibilities to produce value-added chemicals and high performing polymeric materials. Over the years, different cutting-edge lignin depolymerization methodologies have been developed, mainly focusing on achieving excellent yields of mono-phenolic products, some even approaching the theoretical maximum. However, due to lignin's inherent heterogeneity and recalcitrance, its depolymerization leads to relatively complex product streams, also containing dimers, and higher molecular weight fragments in substantial quantities. The subsequent chemo-catalytic valorization of these higher molecular weight streams, containing difficult-to-break, mainly C-C covalent bonds, is tremendously challenging, and has consequently received much less attention. In this minireview, we present an overview of recent advances on the development of sustainable biorefinery strategies aimed at the production of well-defined chemicals and polymeric materials, the prime focus being on depolymerized lignin oils, containing high molecular weight fractions. The key central unit operation to achieve this is (bio)catalytic funneling, which holds great potential to overcome separation and purification challenges.

Advances of Modified Lignin as Substitute to Develop Lignin-Based Phenol-Formaldehyde Resin Adhesives

Traditionally, phenols used to prepare phenol-formaldehyde (PF) resin adhesives are obtained from phenolic compounds and various chemicals, which are extracted from petroleum-based raw materials. Lignin, a sustainable phenolic macromolecule in the cell wall of biomass with an aromatic ring and a phenolic hydroxyl group similar to those of phenol, can be an ideal substitute for phenol in PF resin adhesives. However, only a few lignin-based adhesives are produced on a large scale in industry, mainly because of the low activity of lignin. Preparing lignin-based PF resin adhesives with exceptional achievements by modifying lignin instead of phenol is an efficient method to improve the economic benefits and protect the environment. In this review, the latest progress in the preparation of PF resin adhesives via lignin modification, including chemical, physical, and biological modifications, is discussed. In addition, the advantages and disadvantages of different lignin modification methods for adhesives are compared and discussed, and future research directions for the synthesis of lignin-based PF resin adhesives are proposed.

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.

Efficient Fractionation and Catalytic Valorization of Raw Biomass in ϵ-Caprolactone and Water

Efficient fractionation and utilization of the whole biomass is particularly attractive but remains a great challenge, owing to the recalcitrance of biomass. In this study, a simple and efficient approach is developed to obtain high-purity cellulose with a delignification degree of 97.5 % in ϵ-caprolactone and water. FTIR spectroscopy reveals that ϵ-caprolactone and water act in synergy to remove lignin from raw biomass and afford cellulose with clear macrofibrils. A linear positive correlation between the contents of hemicellulose and lignin is observed for the separated cellulose pulp. This mixed solvent exhibits good performance for the removal of lignin from various agricultural and forestry wastes. Moreover, nearly complete transformation of the whole biomass constituents is achieved with Ni-Al catalyst.

Exploring the Oxidative Effects of the Microbial Electro-Fenton Process on the Depolymerization of Lignin Extracted from Rice Straw in a Bio-Electrochemical System Coupled with Wastewater Treatment

Lignin is a potential renewable feedstock to produce value-added compounds, but the overwhelming bulk of it is either burned for energy or discarded as waste. This paper addressed two critical issues: waste-to-value generation and management by demonstrating the in situ depolymerization of lignin extracted from waste rice straw utilizing the microbial electro-Fenton process in a microbial peroxide-producing cell (MPPC), a type of bio-electrochemical cell, for value addition while synchronously treating wastewater. The MPPC electrochemical voltage yields of 0.171 ± 0.05-0.497 ± 0.2 V produced 9 ± 0.43-34 ± 0.11 mM of H₂O₂, which was utilized to depolymerize lignin at various concentrations. Interestingly, a direct correlation was observed between lignin depolymerization and H₂O₂ concentration, while Fourier-transform infrared spectroscopy data revealed a constant disruption of the lignin structure accurately in the wavenumber region of 1000-1750 cm^-1 irrespective of the H₂O₂ concentration. Carboxylic acid derivatives, benzopyran, hexanoic acid, and other valuable compounds were detected in the LC QTOF MS data from the depolymerized lignin mixture. Remarkably, SEM analysis demonstrated morphological changes in depolymerized lignin induced by the oxidative effects of hydroxyl radicals. Biochemical oxygen demand and chemical oxygen demand removal was 60 ± 3-85 ± 1% in anodic wastewater treatment. This research provides a sustainable and efficient technique for lignin valorization and wastewater treatment.

Catalyst-Free Synthesis of Covalent Adaptable Network (CAN) Polyurethanes from Lignin with Editable Shape Memory Properties

Here a new strategy of catalyst-free direct synthesis of covalent adaptable network polyurethanes (LPUs) from lignin with editable shape memory effect is reported. Using unmodified lignin, PEG, and isocyanate under the condition of the isocyanate index less than 1.0 (NCO/OH<1.0), a variety of LPUs are obtained. When NCO/OH=0.8, a stable cross-linked network can be formed (ex. the gel content of LPU50-0.8 was 98±0.3 %). The activation energy (E_a ) value of LPUs is similar to that of polyhydroxyurethanes (PHUs), at around 110 kJ mol^-1 . With an increase of lignin content, the LPUs show a transition from ductile fracture to brittle fracture mode. And the mechanical properties of LPUs are significantly enhanced after extrusion processing, with the maximum modulus reaching 649±26 MPa and the maximum toughness up to 9927±111 kJ m^-3 . The improvement in mechanical properties is due to the homogenization of complex cross-linked network under the powerful external force of the extruder and the lignin that originally was free in the system participated in the exchange reactions. Moreover, LPUs can also be prepared continuously in one step by using an extruder as the reactor. In addition, LPU50-0.8 has an editable shape memory effect. This study develops a novel method for the synthesis of LPU from lignin with NCO/OH<1.0, showcasing new possibilities for value-added utilization of lignin, and expands the bio-based products portfolio from biomass feedstock to help meet future green manufacturing demands.

Nanogreen is the new future: the conversion of lignin and lignocellulosic wastes into nanomaterials

The lignocellulose biorefinery industry has assumed an important role within the current scenario. Lignin is an abundant and available biopolymer and one of the compounds present in the lignocellulosic waste. Therefore, processing lignin into new materials and nanomaterials, such as nanolignin, has attracted the attention of the scientific community. Lignin nanoparticles are materials that have excellent properties, such as biodegradability and non-toxicity, and have great potential as chelating agents, antimicrobials agents, UV protectors, nanofillers, adsorbents, catalysts, supercapacitors, emulsion stabilizers, delivered systems, drugs, and gene carriers. This review article covers the emergent scenario of nanolignin and the main aspects of scientific interest, such as the conversion and functionalization of lignin, the valorization of lignocellulose waste, and nanoparticle synthesis. A techno-economic evaluation of the biorefinery model of the nanolignin synthesis is presented based on the simulation of the process on the experimental and commercial databases available and reported by some authors. Finally, the techno-economic assessment is complemented by the life cycle assessment of various nanolignin synthesis pathways reported to evaluate the environmental implications and support this emergent technology development.

High value valorization of lignin as environmental benign antimicrobial

Lignin is a natural aromatic polymer of p-hydroxyphenylpropanoids with various biological activities. Noticeably, plants have made use of lignin as biocides to defend themselves from pathogen microbial invasions. Thus, the use of isolated lignin as environmentally benign antimicrobial is believed to be a promising high value approach for lignin valorization. On the other hand, as green and sustainable product of plant photosynthesis, lignin should be beneficial to reduce the carbon footprint of antimicrobial industry. There have been many reports that make use of lignin to prepare antimicrobials for different applications. However, lignin is highly heterogeneous polymers different in their monomers, linkages, molecular weight, and functional groups. The structure and property relationship, and the mechanism of action of lignin as antimicrobial remains ambiguous. To show light on these issues, we reviewed the publications on lignin chemistry, antimicrobial activity of lignin models and isolated lignin and associated mechanism of actions, approaches in synthesis of lignin with improved antimicrobial activity, and the applications of lignin as antimicrobial in different fields. Hopefully, this review will help and inspire researchers in the preparation of lignin antimicrobial for their applications.

Recent advancements in lignocellulose biomass-based carbon fiber: Synthesis, properties, and applications

A growing need to reduce the global carbon footprint has prompted all sectors to make significant efforts in this direction. For example, there has been much focus on green carbon fiber sustainability. For example, it was found that the polyaromatic heteropolymer lignin might act as an intermediary in synthesising carbon fiber. Biomass is seen as a potential carbon accommodated solid natural sources that protects the nature and has a big overall supply and widespread distribution. With growing environmental concern in recent years, biomass has gained appeal as a raw material for production of carbon fibers. Especially, the positives of lignin material include its reasonable budget, sustainability, and higher carbon content, which makes it a dominating precursor. This review has examined a variety of bio precursors that help produce lignin and have higher lignin concentrations. In addition, there has been much research on plant sources, lignin types, factors affecting carbon fiber synthesis, spinning methods, stabilization, carbonization, and activation the characterisation techniques used for the lignin carbon fiber to comprehend the structure and features. In addition, an overview of the applications that use lignin carbon fiber has been provided.

High yield production of 1,4-cyclohexanediol and 1,4-cyclohexanediamine from high molecular-weight lignin oil

The complete utilization of all lignin depolymerization streams obtained from the reductive catalytic fractionation (RCF) of woody biomass into high-value-added compounds is a timely and challenging objective. Here, we present a catalytic methodology to transform beech lignin-derived dimers and oligomers (DO) into well-defined 1,4-cyclohexanediol and 1,4-cyclohexanediamine. The latter two compounds have vast industrial relevance as monomers for polymer synthesis as well as pharmaceutical building blocks. The proposed two-step catalytic sequence involves the use of the commercially available RANEY® Ni catalyst. Therefore, the first step involves the efficient defunctionalization of lignin-derived 2,6-dimethoxybenzoquinone (DMBQ) into 1,4-cyclohexanediol (14CHDO) in 86.5% molar yield, representing a 10.7 wt% yield calculated on a DO weight basis. The second step concerns the highly selective amination of 1,4-cyclohexanediol with ammonia to give 1,4-cyclohexanediamine (14CHDA) in near quantitative yield. The ability to use RANEY® Ni and ammonia in this process holds great potential for future industrial synthesis of 1,4-cyclohexanediamine from renewable resources.

Production of aromatic hydrocarbons from lignin derivatives by catalytic cracking over a SiO2–Al2O3 catalyst†

Catalytic cracking of phenolic compounds to aromatic hydrocarbons is vital to the utilization of lignin. In this work, pristine amorphous SiO₂–Al₂O₃ was used as a catalyst to produce aromatic hydrocarbons from lignin-derived phenolics by catalytic cracking using methanol as the solvent. These catalysts were characterized by various techniques (XRD, NH₃-TPD, Py-IR, etc.) and evaluated on a fixed bed reactor using guaiacol as a model compound. The effects of reaction temperature, the flow of carrier gas, the molar ratio of guaiacol to methanol, and WHSV were investigated. 33-SA (SiO₂–Al₂O₃ with the SiO₂ content of 33%) exhibited the best catalytic activity due to its high content of Lewis acid sites (168.47 μmol g⁻¹). Co-feeding with methanol promoted the removal of oxygen atoms and improved the reaction system H/C_eff. Under the optimal conditions of 400 °C, 25 mL min⁻¹ N₂, a molar ratio of methanol to guaiacol of 25, and WHSV of 8/3 h⁻¹, the yield of aromatic hydrocarbons reached 57.93%. The deactivating species in the transformation of guaiacol into aromatic hydrocarbons on catalysts were also studied.

Catalytic cracking of phenolic compounds to aromatic hydrocarbons is vital to the utilization of lignin.

Exploring the compatibility between hydrothermal depolymerization of alkaline lignin from sugarcane bagasse and metabolization of the aromatics by bacteria

Although hydrothermal treatments for biomass fractionation have been vastly studied, their effect on the depolymerization of isolated lignins in terms of yield, composition, and compatibility of the produced lignin bio-oils with bioconversion is still poorly investigated. In this study, we evaluated the hydrothermal depolymerization of an β-O-4'-rich lignin extracted from sugarcane bagasse by alkaline fractionation, investigating the influence of temperature (200-350 °C), time (30-90 min), and solid-liquid ratio (1:10-1:50 m.v^-1) on yield of bio-oils (up to 31 wt%) rich in monomers (light bio-oils). Principal Components Analysis showed that the defunctionalization of the aromatic monomers was more pronounced in the most severe reaction conditions and that the abundance of more hydrophobic monomers increased in more diluted reactions. While the high-molecular-weight (heavy) bio-oil generated at 350 °C, 90 min, and 1:50 m.v^-1 failed to support bacterial growth, the corresponding light bio-oil rich in aromatic monomers promoted the growth of bacteria from 9 distinct species. The isolates Pseudomonas sp. LIM05 and Burkholderia sp. LIM09 showed the best growth performance and tolerance to lignin-derived aromatics, being the most promising for the future development of biological upgrading strategies tailored for this lignin stream.

Zeolitic Imidazolate Framework Decorated Molybdenum Carbide Catalysts for Hydrodeoxygenation of Guaiacol to Phenol

Bimetallic zeolitic imidazolate framework (BMZIF)-decorated Mo carbide catalysts were designed for the catalytic hydrodeoxygenation of guaiacol to produce phenol with high selectivity. A uniform layer of BMZIF was systematically coated onto the surface of the MoO3 nanorods. During carbonization at 700 °C for 4 h, BMZIF generated active species (ZnO, CoO) on highly dispersed N-doped carbons, creating a porous shell structure. Simultaneously, the MoO3 nanorod was transformed into the Mo2C phase. The resulting core@shell type Mo2C@BMZIF-700 °C (4 h) catalyst promoted a 97% guaiacol conversion and 70% phenol selectivity under 4 MPa of H2 at 330 °C for 4 h, which was not achieved by other supported catalysts. The catalyst also showed excellent selective cleavage of the methoxy group of lignin derivatives (syringol and vanillin), which makes it suitable for selective demethoxylation in future biomass catalysis. Moreover, it exhibits excellent recyclability and stability without changing the structure or active species.

Creative biological lignin conversion routes toward lignin valorization

Lignin, the largest renewable aromatic resource, is a promising alternative feedstock for the sustainable production of various chemicals, fuels, and materials. Despite this potential, lignin is characterized by heterogeneous and macromolecular structures that must be addressed. In this review, we present biological lignin conversion routes (BLCRs) that offer opportunities for overcoming these challenges, making lignin valorization feasible. Funneling heterogeneous aromatics via a 'biological funnel' offers a high-specificity bioconversion route for aromatic platform chemicals. The inherent aromaticity of lignin drives atom-economic functionalization routes toward aromatic natural product generation. By harnessing the ligninolytic capacities of specific microbial systems, powerful aromatic ring-opening routes can be developed to generate various value-added products. Thus, BLCRs hold the promise to make lignin valorization feasible and enable a lignocellulose-based bioeconomy.

A Diamine-Oriented Biorefinery Concept Using Ammonia and Raney Ni as a Multifaceted Catalyst

Diamines are important industrial chemicals. In this paper we outline the feasibility of lignocellulose as a source of diol-containing molecules. We also illustrate the possibility of turning these diols into their diamines in good to excellent yields. Central to these transformations is the use of commercially available Raney Ni. For diol formation, the Raney Ni engages in hydrogenation and often also demethoxylation, that way funneling multiple components to one single molecule. For diamine formation, Raney Ni catalyzes hydrogen-borrowing mediated diamination in the presence of NH₃.

Production of xylo-oligosaccharides and ethanol from corncob by combined tartaric acid hydrolysis with simultaneous saccharification and fermentation

In organic acid hydrolysis for xylo-oligosaccharides (XOS) production, organic acids with low flash points and explosion limits can lead to explosion and fire risk. Herein, tartaric acid (TA) as an organic acid with high flash point and no explosion limit was used in the hydrolysis of corncob to produce XOS. Then, the TA-hydrolyzed corncob was used for ethanol production. In TA hydrolysis of corncob, a 56.4 % XOS yield was obtained from the hydrolysate with the conditions of 170 °C, 60 mM TA and 10 min. Meanwhile, 92.1 % TA was recovered from the hydrolysate by the addition of calcium hydroxide. After simultaneous saccharification and fermentation of TA-hydrolyzed corncob, an 82.4 % ethanol yield was obtained with a solid loading of 25 % (w/v, 250 g/L) by Saccharomyces cerevisiae H06. This research provided a relatively safe, simple, and efficient technology for producing XOS and ethanol from corncob.

Simultaneous Generation of Methyl Esters and CO in Lignin Transformation

Lignin is an abundant renewable carbon source. Due to its complex structure, utilization of lignin is very challenging. Herein, we describe an efficient strategy for the simultaneous utilization of lignin, in which the methoxy groups in lignin react with carboxylic acids to generate methyl carboxylates and the other alkyl and phenyl carbons react with oxygen to predominantly form CO that can be used directly in carbonylation reactions. The method was applied to the methylation of various functionalized aryl and alkyl carboxylic acids, including natural compounds, to produce valuable chemicals, including pharmaceuticals. No solid or liquid residues remain after the reaction. Mechanistic studies demonstrate that a well-ordered C-C and C-O bond activation sequence takes place to realize total transformation of lignin. This work opens a way for transformation of the entire lignin polymer into valuable products, exemplified by the synthesis of the pharmaceutical, Ramipril, on a gram scale.

One-Pot Catalytic Conversion of Lignin-Derivable Guaiacols and Syringols to Cyclohexylamines

Cyclic primary amines are elementary building blocks to many fine chemicals, pharmaceuticals, and polymers. Here, a powerful one-pot Raney Ni-based catalytic strategy was developed to transform guaiacol into cyclohexylamine using NH₃ (7 bar) and H₂ (10 bar) in up to 94 % yield. The methodology was extendable to the conversion of a wider range of guaiacols and syringols into their corresponding cyclohexylamines. Notably, a crude bio-oil originating from the reductive catalytic fractionation of birch lignocellulose was transformed into a product mixture rich in 4-propylcyclohexylamine, constituting an interesting case of catalytic funneling. The isolated yield of the desired 4-propylcyclohexylamine reached as high as 7 wt % (on lignin basis). Preliminary mechanistic studies pointed at the consecutive occurrence of three key catalytic transformations, namely, demethoxylation, hydrogenation, and amination.