Catalytic Oxidation of Lignins into the Aromatic Aldehydes: General Process Trends and Development Prospects

Peroxodicarbonate - a renaissance of an electrochemically generated green oxidizer

The direct anodic conversion of alkali carbonates in aqueous media provides access to peroxodicarbonate, which is a safe to use and green oxidizer. Although first reports date back around 150 years, its low concentrations and limited thermal stability have consigned this reagent to oblivion. Boron-doped diamond anodes, novel electrolyser concepts for heat dissipation, and the mixed cation trick allow record breaking peroxodicarbonate concentrations >900 mM. The electrochemical generation of peroxodicarbonate was already demonstrated on a pilot scale. The inherent safety is ensured by the limited stability of the peroxodicarbonate solution, which decomposes under ambient conditions to oxygen and facilitates subsequent downstream processing. This peroxide has, in particular at higher concentrations, an unusual reactivity and seems to be an ideal reagent when peroxo-equivalents in combination with alkaline base are required. The conversions with peroxodicarbonate include the Dakin reaction, epoxidation, oxidation of amines (aliphatic and aromatic) and sulfur compounds, deborolative hydroxylation reactions, and many more. Since the base equivalents also represent the makeup chemical for pulping plants, peroxodicarbonate is an ideal reagent for the selective degradation of lignin to vanillin. Moreover, peroxodicarbonate can be used as a halogen-free bleaching agent. The emerging electrogeneration and use of this green platform oxidizer are surveyed for the first time.

A comprehensive review of eclectic approaches to the biological synthesis of vanillin and their application towards the food sector

Vanillin, a highly regarded flavor compound, has earned widespread recognition for its natural and aromatic qualities, piquing substantial interest in the scientific community. This comprehensive review delves deeply into the intricate world of vanillin synthesis, encompassing a wide spectrum of methodologies, including enzymatic, microbial, and immobilized systems. This investigation provides a thorough analysis of the precursors of vanillin and also offers a comprehensive overview of its transformation through these diverse processes, making it an invaluable resource for researchers and enthusiasts alike. The elucidation of different substrates such as ferulic acid, eugenol, veratraldehyde, vanillic acid, glucovanillin, and C6-C3 phenylpropanoids adds a layer of depth and insight to the understanding of vanillin synthesis. Moreover, this comprehensive review explores the multifaceted applications of vanillin within the food industry. While commonly known as a flavoring agent, vanillin transcends this role by finding extensive use in food preservation and food packaging. The review meticulously examines the remarkable preservative properties of vanillin, providing a profound understanding of its crucial role in the culinary and food science sectors, thus making it an indispensable reference for professionals and researchers in these domains.

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Nature stays natural: two novel chemo-enzymatic one-pot cascades for the synthesis of fragrance and flavor aldehydes

Novel synthetic strategies for the production of high-value chemicals based on the 12 principles of green chemistry are highly desired. Herein, we present a proof of concept for two novel chemo-enzymatic one-pot cascades allowing for the production of valuable fragrance and flavor aldehydes. We utilized renewable phenylpropenes, such as eugenol from cloves or estragole from estragon, as starting materials. For the first strategy, Pd-catalyzed isomerization of the allylic double bond and subsequent enzyme-mediated (aromatic dioxygenase, ADO) alkene cleavage were performed to obtain the desired aldehydes. In the second route, the double bond was oxidized to the corresponding ketone via a copper-free Wacker oxidation protocol followed by enzymatic Baeyer-Villiger oxidation (phenylacetone monooxygenase from Thermobifida fusca), esterase-mediated (esterase from Pseudomonas fluorescens, PfeI) hydrolysis and subsequent oxidation of the primary alcohol (alcohol dehydrogenase from Pseudomonas putida, AlkJ) to the respective aldehyde products. Eight different phenylpropene derivatives were subjected to these reaction sequences, allowing for the synthesis of seven aldehydes in up to 55% yield after 4 reaction steps (86% for each step).

Capacity of Aqueous Solutions of the Ionic Liquid 1-Ethyl-3-methylimidazolium Acetate to Partially Depolymerize Lignin at Ambient Temperature and Pressure

Lignin is a very attractive and abundant biopolymer with the potential to be a biorenewable source of a large number of value-added organic chemicals. The current state-of-the-art methods fail to provide efficient valorization of lignin in this regard without the involvement of harsh conditions and auxiliary substances that compromise the overall sustainability of the proposed processes. Making an original approach from the set of mildest temperature and pressure conditions, this work identifies and explores the capacity of an aqueous solution of the nonvolatile ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) to partially depolymerize technical lignin (Indulin AT) by means of a treatment consisting in the simple contact at ambient temperature and pressure. Among a considerable number of valuable phenolic molecules that were identified in the resulting fluid, vanillin (yield of about 3 g/kg) and guaiacol (yield of about 1 g/kg) were the monophenolic compounds obtained in a higher concentration. The properties of the post-treatment solids recovered remain similar to those of the original lignin, although with a relatively lower abundance of guaiacyl units (in agreement with the generation of guaiacyl-derived phenolic molecules, such as vanillin and guaiacol). The assistance of the treatment with UV irradiation in the presence of nanoparticle catalysts does not lead to an improvement in the yields of phenolic compounds.

Ce-Zr-based mixed oxide catalyst for oxidative depolymerization of kenaf stalk (biomass) into vanillin

Since petroleum became depleted, rapid attention has been devoted to renewable energy sources such as lignocellulosic biomass to produce useful chemicals for industry (for instance vanillin). Three primary components of lignocellulose are lignin, cellulose, and hemicellulose. This paper uses microwave-assisted technology to oxidize the kenaf stalk (lignocellulosic biomass) and extract lignin to produce vanillin. Catalysts with variable acid-base and redox properties are essential for the mentioned effective conversion, for this reason, CeO2-CA, ZrO2-CA, and CeZrO2-CA catalysts were synthesized. The citrate complexation method was used for the catalyst synthesis and the physicochemical characteristics were analyzed by XRD, FTIR, FE-SEM, TEM, BET, and TPO. The characterization results demonstrated that CeZrO2-CA shows the smallest sized crystallites with a large specific surface area among the other chosen catalysts. For vanillin production, the effect of reaction temperature, reaction time, and catalyst loading was studied. It was observed that compared to other catalysts, CeZrO2-CA produced the highest vanillin yield of 9.90% for kenaf stalk for 5 wt% of CeZrO2-CA at 160 °C for 30 min. Furthermore, vanillin production using extracted lignin is studied keeping CeZrO2-CA as a catalyst and with the same operating parameters, which yielded 14.3% of vanillin. Afterward, the change in yield with respect to pH is also presented. Finally, the recyclability of catalyst is also studied, which showed that it has a strong metal support and greater stability which may give industrial applications a significant boost.

Direct Oxidation of Hibiscus cannabinus Stalks to Vanillin Using CeO2 Nanostructure Catalysts

Biomass lignin can be used to produce vanillin through an oxidation process. Although its purity is high, the processing time and separation efficiency are not ideal. This research aims to produce vanillin directly from Kenaf stalks without separating the lignin first from the lignocellulosic biomass. This method is greener because it does not require the separation of cellulose and hemicellulose from the biomass, thus minimizing the use of acid and alkaline solutions and saving time. A high oxygen storage capacity and release capacity of ceria as an oxidation catalyst contribute to the reversable redox properties between Ce⁴⁺ and Ce³⁺ in ceria lattice. Cerium oxide nanostructures were synthesized using a hydrothermal method treated under alkaline NaOH, followed by drying at 120 °C for 16 h and calcining at different temperatures between 400 and 600 °C for the direct oxidation of Kenaf stalks to vanillin under microwave irradiation. The catalysts were characterized for their physicochemical properties using XRD, N₂ adsorption-desorption isotherms and TEM. All synthesized CeO₂ nanostructures showed the presence of diffraction peaks assigned to the presence of cubic fluorite. The N₂ adsorption-desorption isotherms showed that all catalysts possess a Type IV isotherm, indicating a mesoporous structure. The TEM image shows the uniform shape of the CeO₂ nanostructures, while HRTEM images show that the CeO₂ nanostructures are single-crystalline in nature. All catalysts were tested for the direct oxidation of Kenaf stalks using H₂O₂ as the oxidizing agent in temperatures ranging from 160 to 180 °C for 10-30 min with 0.1-0.3 g catalyst loading under 100-500 W of microwave irradiation. The CeO₂-Nps-400 catalyst produced the highest vanillin yields of 3.84% and 4.32% for the direct oxidation of Kenaf stalks and extraction of lignin from Kenaf stalks, respectively. Compared to our earlier study, the highest vanillin yields of 2.90% and 3.70% for direct biomass and extracted lignin were achieved using a Ce/MgO catalyst.

Recovery of flavor compounds from vanilla bagasse by hydrolysis and their identification through UPLC-MSE

Vanilla is a globally treasured commodity, and the consequences of its unstable value affect social, environmental, economic, and academic ambits. The extensive range of aroma molecules found in cured vanilla beans is crucial to the complexity of this natural condiment and knowledge about their recovery is of the essence. Many strategies aim on reproducing the chemical intricacies of vanilla flavor, such as biotransformation and de novo biosynthesis. Few studies, however, aim at the exhaustion of the cured pods, of which the bagasse, after the traditional ethanolic extraction, might still bear a highly valued flavor composition. An untargeted liquid chromatography coupled with mass spectrometry (LC-MS^E) approach was applied to elucidate if sequential alkaline-acidic hydrolysis was effective in extracting flavor related molecules and chemical classes from the hydro-ethanolic fraction. Important vanilla flavor related compounds present in the hydro-ethanolic fraction were further extracted from the residue through alkaline hydrolysis, such as vanillin, vanillic acid, 3-methoxybenzaldehyde, 4-vinylphenol, heptanoic acid, and protocatechuic acid. Acid hydrolysis was effective on further extracting features from classes such as phenols, prenol lipids, and organooxygen compounds, though representative molecules remain unknown. Finally, sequential alkaline-acidic hydrolysis rendered natural vanilla's ethanolic extraction residues as an interesting supplier of its own products, which could be used as a food additive, and many other applications.

Peroxodicarbonate as a Green Oxidizer for the Selective Degradation of Kraft Lignin into Vanillin

Lignin, the world's largest resource of renewable aromatics, with annually roughly 50 million tons of accruing technical lignin, mainly Kraft lignin, is highly underdeveloped regarding the production of monoaromatics. We demonstrate the oxidative depolymerization of Kraft lignin at 180 °C to produce vanillin 1 in yields up to 6.2 wt % and 92 % referred to the maximum yield gained from the quantification reaction utilizing nitrobenzene. Using peroxodicarbonate (C₂ O₆ ^2- ) as "green" oxidizer for the degradation, toxic and/or harmful reagents are prevented. Also, the formed waste can serve as makeup chemical in the pulping process. Na₂ C₂ O₆ is synthesized in an ex-cell electrolysis of aqueous Na₂ CO₃ at BDD anodes, achieving a yield of Na₂ C₂ O₆ with 41 %. At least, the oxidation and degradation of Kraft lignin is analysis via UV/Vis and NMR spectroscopy.

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

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

Emission Characteristics and Formation Mechanism of Carbonyl Compounds from Residential Solid Fuel Combustion Based on Real-World Measurements and Tube-Furnace Experiments

This study updated carbonyl compound (CC) emission factors (EFs) and composition for residential solid fuel combustion based on real-world measurements of 124 fuel/stove combinations in China and explored the CC formation mechanism using tube-furnace experiments with 19 fuels and low/high temperatures to explain the impact of fuel and stove on CC emission characteristics. The average EF_CC values for straw, wood, and coal were 1.94 ± 1.57, 1.50 ± 0.88, and 0.40 ± 0.54 g/kg, respectively. Formaldehyde and acetaldehyde were the most abundant species, accounting for 40-60% of CCs, followed by acetone (∼20%), aromatic aldehydes (∼10%), and unsaturated aldehydes (∼5%). Different from formaldehyde and acetaldehyde, other species showed significant variation among fuel types. All these characteristics could be explained by the difference in the volatile content and chemical structure of fuel, such as aromatic in coal versus lignin in biomass. The improvement in stove technology reduced CC emissions by 30.4-69.7% (mainly formaldehyde and acetaldehyde) among fuels but increased the proportion of aromatic aldehydes by 24.3-89.4%. Various CC species showed different formation mechanisms related to fuel property and burning temperature. The volatile matter derived from thermal pyrolysis of fuel polymers determined CC composition, while higher temperature preferentially degraded formaldehyde and acetaldehyde but promoted the formation of acetone and aromatic aldehydes. This study not only revealed emission characteristic of CCs from RSFC but also contributed to the improvement of clean combustion technology.

On the Oxidative Valorization of Lignin to High-Value Chemicals: A Critical Review of Opportunities and Challenges

The efficient valorization of lignin is crucial if we are to replace current petroleum-based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second-generation biorefineries produce large quantities of low-value technical lignin as a by-product, which is often combusted on-site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

The Laccase-Lig Multienzymatic Multistep System in Lignin Valorization

A laccase-Lig multienzymatic multistep system for lignin depolymerization was designed and developed. Studies were performed on pristine and fractionated lignins (Kraft and Organosolv) using a specific cascade of enzymes, that is, laccases from Bacillus licheniformis and from Funalia trogii, respectively for Kraft and Organosolv lignin, followed by the Lig system from Sphingobium sp. SYK-6 (β-etherases Lig E and Lig F, glutathione lyase Lig G). Careful elucidation of the structural modifications occurring in the residual lignins associated with the identification and quantification of the generated low-molecular-weight compounds showed that (i) the laccase-Lig system cleaves non-phenolic aryl glycerol β-O-4 aryl ether bonds, and (ii) the overall reactivity is heavily dependent on the individual lignin structure. More specifically, samples with low phenolic/aliphatic OH groups ratio undergo net depolymerization, while an increased phenolic/aliphatic OH ratio results in the polymerization of the residual lignin irrespective of its botanical origin and isolation process.

Catalytic Oxidation of Flax Shives into Vanillin and Pulp

This research deals with a process of catalytic oxidation of flax shives to vanillin and pulp. Catalytic oxidation of flax shives with molecular oxygen allows two main products to be obtained—vanillin with a yield of up to 12 wt.% of lignin, and pulp. Final forms of the catalyst particles (Cu2O and CuO) are agglomerates or monocrystals 0.5–5 µm in size. Acid prehydrolysis of the shives does not affect the oxidation, in contrast to pine-wood oxidation. Lignin prehydrolysis and oxidation was suggested as illustrate this difference. The dependence of the vanillin formation rate on stirring speed was studied. Minimum alkali consumption in the process was attained with a mild stirring speed of the reaction mass.

Engineering a Feruloyl-Coenzyme A Synthase for Bioconversion of Phenylpropanoid Acids into High-Value Aromatic Aldehydes

Aromatic aldehydes find extensive applications in food, perfume, pharmaceutical, and chemical industries. However, a limited natural enzyme selectivity has become the bottleneck of bioconversion of aromatic aldehydes from natural phenylpropanoid acids. Here, based on the original structure of feruloyl-coenzyme A (CoA) synthetase (FCS) from Streptomyces sp. V-1, we engineered five substrate-binding domains to match specific phenylpropanoid acids. FcsCIA^E407A/K483L, FcsMA^{E407R/I481R/K483R}, FcsHA^{E407K/I481K/K483I}, FcsCA^{E407R/I481R/K483T}, and FcsFA^{E407R/I481K/K483R} showed 9.96-, 10.58-, 4.25-, 6.49-, and 8.71-fold enhanced catalytic efficiency for degrading CoA thioesters of cinnamic acid, 4-methoxycinnamic acid, 4-hydroxycinnamic acid, caffeic acid, and ferulic acid, respectively. Molecular dynamics simulation illustrated that novel substrate-binding domains formed strong interaction forces with substrates' methoxy/hydroxyl group and provided hydrophobic/alkaline catalytic surfaces. Five recombinant E. coli with FCS mutants were constructed with the maximum benzaldehyde, p-anisaldehyde, p-hydroxybenzaldehyde, protocatechualdehyde, and vanillin productivity of 6.2 ± 0.3, 5.1 ± 0.23, 4.1 ± 0.25, 7.1 ± 0.3, and 8.7 ± 0.2 mM/h, respectively. Hence, our study provided novel and efficient enzymes for the bioconversion of phenylpropanoid acids into aromatic aldehydes.

Bio-Based Degradable Poly(ether-ester)s from Melt-Polymerization of Aromatic Ester and Ether Diols

Vanillin, as a promising aromatic aldehyde, possesses worthy structural and bioactive properties useful in the design of novel sustainable polymeric materials. Its versatility and structural similarity to terephthalic acid (TPA) can lead to materials with properties similar to conventional poly(ethylene terephthalate) (PET). In this perspective, a symmetrical dimethylated dialkoxydivanillic diester monomer (DEMV) derived from vanillin was synthesized via a direct-coupling method. Then, a series of poly(ether-ester)s were synthesized via melt-polymerization incorporating mixtures of phenyl/phenyloxy diols (with hydroxyl side-chains in the 1,2-, 1,3- and 1,4-positions) and a cyclic diol, 1,4-cyclohexanedimethanol (CHDM). The polymers obtained had high molecular weights (M_w = 5.3–7.9 × 10⁴ g.mol⁻¹) and polydispersity index (Đ) values of 1.54–2.88. Thermal analysis showed the polymers are semi-crystalline materials with melting temperatures of 204–240 °C, and tunable glass transition temperatures (T_g) of 98–120 °C. Their 5% decomposition temperature (T_d,5%) varied from 430–315 °C, which endows the polymers with a broad processing window, owing to their rigid phenyl rings and trans-CHDM groups. These poly(ether-ester)s displayed remarkable impact strength and satisfactory gas barrier properties, due to the insertion of the cyclic alkyl chain moieties. Ultimately, the synergistic influence of the ester and ether bonds provided better control over the behavior and mechanism of in vitro degradation under passive and enzymatic incubation for 90 days. Regarding the morphology, scanning electron microscopy (SEM) imaging confirmed considerable surface degradation in the polymer matrices of both polymer series, with weight losses reaching up to 35% in enzymatic degradation, which demonstrates the significant influence of ether bonds for biodegradation.

Hardwood and softwood lignins from sulfite liquors: Structural characterization and valorization through depolymerization

This work aims to evaluate the structural characteristics and study the oxidative depolymerization of lignins obtained from hardwood and softwood sulfite liquors. Lignins were obtained after ultrafiltration and freeze-drying of the sulfite liquors and characterized based on inorganic content, nitrobenzene oxidation, ¹³C NMR, and molecular weight determination. The structural characteristics achieved allow evaluating the potential of each lignin through oxidative depolymerization to produce added-value phenolic monomers. Hardwood and softwood lignins were submitted to alkaline oxidation with oxygen and the reaction conditions optimized to obtain a final oxidation mixture with the maximum yield of phenolic monomers. Through oxidation with O₂, hardwood lignin generates mostly syringaldehyde while lignin from softwood biomass mainly produces vanillin; moreover, a lower reaction time and the interruption of O₂ admission avoid the degradation of the oxidation products in the final mixture for both lignins, more evidenced to hardwood lignin due to its higher reactivity. From the results, it is possible to conclude that a phenolic aldehyde-rich oxidation mixture could be obtained, confirming the viability of lignin as raw material to produce added-value products as vanillin and syringaldehyde.

Review on the preparation of fuels and chemicals based on lignin

Lignin is by far the most abundant natural renewable aromatic polymer in nature, and its reserves are second only to cellulose. In addition to the rich carbon content, the structure of lignin contains functional groups such as benzene rings, methoxyl groups, and phenolic hydroxyl groups. Lignin degradation has become one of the high value, high quality and high efficiency methods to convert lignin, which is of great significance to alleviating the current energy shortage and environmental crisis. This article introduces the hydrolysis methods of lignin in acidic, alkaline, ionic liquids and supercritical fluids, reviews the heating rate, the source of lignin species and the effects of heating rate on the pyrolysis of lignin, and briefly describes the metal catalysis, oxidation methods such as electrochemical degradation and photocatalytic oxidation, and degradation reduction methods using hydrogen and hydrogen supply reagents. The lignin degradation methods for the preparation of fuels and chemicals are systematically summarized. The advantages and disadvantages of different methods, the selectivity under different conditions and the degradation efficiency of different catalytic combination systems are compared. In this paper, a new approach to improve the degradation efficiency is envisioned in order to contribute to the efficient utilization and high value conversion of lignin.

Microbial peroxide producing cell mediated lignin valorization

Lignin is one of the most abundant naturally occurring polymers and can produce value-added products such as vanillin and p-coumaric acid. In the current work, in-situ depolymerization of lignin for its valorization in a microbial peroxide-producing cell (MPPC) system was performed. It is an electrochemical cell that requires no external energy to produce H₂O₂ for the advanced oxidation process. Lignin in the MPPC system undergoes oxidative depolymerization to generate value-added products. The maximum open-circuit voltage (OCV) was 1.143 V, the current density and power densities were 14 mA/cm² and 13 mW/cm², respectively, along with the production of 26 mM of H₂O₂. The degradation of signature linkages such as β-β bond and β-0-4 bond were analyzed and confirmed using FTIR spectroscopy. Vanillin, p-coumaric acid, ferulic acid, etc. were generated during depolymerization and were detected using LC-QTOF-MS.

Assessment of the Efficiency of Chemical and Thermochemical Depolymerization Methods for Lignin Valorization: Principal Component Analysis (PCA) Approach

Energy demand and the use of commodity consumer products, such as chemicals, plastics, and transportation fuels, are growing nowadays. These products, which are mainly derived from fossil resources and contribute to environmental pollution and CO2 emissions, will be used up eventually. Therefore, a renewable inexhaustible energy source is required. Plant biomass resources can be used as a suitable alternative source due to their green, clean attributes and low carbon emissions. Lignin is a class of complex aromatic polymers. It is highly abundant and a major constituent in the structural cell walls of all higher vascular land plants. Lignin can be used as an alternative source for fine chemicals and raw material for biofuel production. There are many chemical processes that can be potentially utilized to increase the degradation rate of lignin into biofuels or value-added chemicals. In this study, two lignin degradation methods, CuO-NaOH oxidation and tetramethyl ammonium hydroxide (TMAH) thermochemolysis, will be addressed. Both methods showed a high capacity to produce a large molecular dataset, resulting in tedious and time-consuming data analysis. To overcome this issue, an unsupervised machine learning technique called principal component analysis (PCA) is implemented.

Degradation mechanism of a lignin model compound during alkaline aerobic oxidation: formation of the vanillin precursor from the β-O-4 middle unit of softwood lignin

We have proposed plausible reaction pathways involved in the chemical conversion of softwood lignin to vanillin through alkaline aerobic oxidation.

Lignin-Derived Syringol and Acetosyringone from Palm Bunch Using Heterogeneous Oxidative Depolymerization over Mixed Metal Oxide Catalysts under Microwave Heating

Biomass valorization to building block chemicals in food and pharmaceutical industries has tremendously gained attention. To produce monophenolic compounds from palm empty fruit bunch (EFB), EFB was subjected to alkaline hydrothermal extraction using NaOH or K2CO3 as a promotor. Subsequently, EFB-derived lignin was subjected to an oxidative depolymerization using Cu(II) and Fe(III) mixed metal oxides catalyst supported on γ-Al2O3 or SiO2 as the catalyst in the presence of hydrogen peroxide. The highest percentage of total phenolic compounds of 63.87 wt% was obtained from microwave-induced oxidative degradation of K2CO3 extracted lignin catalyzed by Cu-Fe/SiO2 catalyst. Main products from the aforementioned condition included 27.29 wt% of 2,4-di-tert-butylphenol, 19.21 wt% of syringol, 9.36 wt% of acetosyringone, 3.69 wt% of acetovanillone, 2.16 wt% of syringaldehyde, and 2.16 wt% of vanillin. Although the total phenolic compound from Cu-Fe/Al2O3 catalyst was lower (49.52 wt%) compared with that from Cu-Fe/SiO2 catalyst (63.87 wt%), Cu-Fe/Al2O3 catalyst provided the greater selectivity of main two value-added products, syringol and acetosyrigone, at 54.64% and 23.65%, respectively (78.29% total selectivity of two products) from the NaOH extracted lignin. The findings suggested a promising method for syringol and acetosyringone production from the oxidative heterogeneous lignin depolymerization under low power intensity microwave heating within a short reaction time of 30 min.

Synthesis of Ce/MgO Catalysts for Direct Oxidation of Hibiscus cannabinus Stalks to Vanillin

One possible method of producing vanillin from biomass is through controlled oxidation of lignin. Direct oxidation of kenaf stalks was chosen without having to separate the cellulose and hemicellulose components from the lignocellulosic biomass. This makes the process greener, as well as saving time. In this paper, Ce/MgO catalysts were developed for oxidation of kenaf stalks and kenaf lignin under microwave irradiation. The catalysts were characterized for their physicochemical properties using XRD and N2 adsorption–desorption isotherms. The synthesized MgO showed the presence of diffraction peaks assigned to cubic MgO while the 30Ce/MgO catalysts showed the presence of cubic fluorite of CeO2. N2 adsorption–desorption isotherms showed that all catalysts possess Type III isotherm according to IUPAC classification, indicating a nonporous structure. All catalysts were tested for direct oxidation of kenaf stalks under 300 W of microwave irradiation using H2O2 as the oxidizing agent at pH 11.5 and temperatures between 160 and 180 °C for 10–30 min with 5–15% catalyst loading. The highest vanillin yields of 3.70% and 2.90% for extracted lignin and direct biomass oxidation were achieved using 30Ce/MgO-48. In comparison, 7.80% and 4.45% were obtained using 2N of NaOH homogeneous catalyst for extracted lignin and direct biomass, respectively, at 170 °C for 20 min. The reusability test shows that 30Ce/MgO can be used up to three cycles without significant loss in catalytic activity. Other compounds detected were 4-vinylguaiacol, syringol and syringaldehyde.

From Lignin to Valuable Aromatic Chemicals: Lignin Depolymerization and Monomer Separation via Centrifugal Partition Chromatography

Lignin has long been recognized as a potential feedstock for aromatic molecules; however, most lignin depolymerization methods create a complex mixture of products. The present study describes an alkaline aerobic oxidation method that converts lignin extracted from poplar into a collection of oxygenated aromatics, including valuable commercial compounds such as vanillin and p-hydroxybenzoic acid. Centrifugal partition chromatography (CPC) is shown to be an effective method to isolate the individual compounds from the complex product mixture. The liquid-liquid extraction method proceeds in two stages. The crude depolymerization mixture is first subjected to ascending-mode extraction with the Arizona solvent system L (pentane/ethyl acetate/methanol/water 2:3:2:3), enabling isolation of vanillin, syringic acid, and oligomers. The remaining components, syringaldehyde, vanillic acid, and p-hydroxybenzoic acid (pHBA), were resolved by using ascending-mode extraction with solvent mixture comprising dichloromethane/methanol/water (10:6:4) separation. These results showcase CPC as an effective technology that could provide scalable access to valuable chemicals from lignin and other biomass-derived feedstocks.

Lignosulfonate-assisted synthesis of platinum nanoparticles deposited on multi-walled carbon nanotubes for biosensing of glucose

It is presented in this work that lignosulfonate (LS) can be preferentially adsorbed on the surface of multi-walled carbon nanotubes (MWCNT) giving rise to the functional platform for platinum nanoparticles (NPt) deposition. The novel MWCNT/LS/NPt hybrid material has been characterized by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS). The morphology of the MWCNT/LS/NPt electrodes has been investigated by atomic force microscopy (AFM). The electrochemical studies of MWCNT/LS/NPt hybrid material revealed strong electrocatalytic properties towards hydrogen peroxide. In addition, the effects of lignosulfonate amount adsorbed at the MWCNT on the voltammetric response of the hydrogen peroxide were discussed and used to select the optimal and effective conditions for the synthesis of the electrode material. An amperometric biosensor for glucose was developed based on the covalent linkage of glucose oxidase (GOx) at the MWCNT/LS/NPt. The enzyme electrode exhibited excellent electrocatalytic activity and rapid response for glucose. The linear range of the glucose determination was 50-1400 µM and LOD was quantified as 15.67 µM.

Production of oxalic acid from sawdust using coal fly ash as a catalyst

The production of oxalic acid from sawdust using a mixture of strong nitric acid and concentrated sulphuric acid with coal fly ash as a catalyst has been explored. Operating parameters affecting the reaction were determined to be temperature, mesh size and amount of fly ash catalyst, time and the $${\text{HNO}}_{3} :{\text{H}}_{2} {\text{SO}}_{4}$$ HNO 3 : H 2 SO 4 ratio. A maximum oxalic acid yield of 84% was obtained using a mixture of 60% $${\text{HNO}}_{3} {\text{ and }}40 \%{\text{ H}}_{2} {\text{SO}}_{4}$$ HNO 3 and 40 % H 2 SO 4 at 70 °C and a reaction time of 150 minutes. Coal fly ash with particle size of 50–100 μm proved to be a suitable and efficient catalyst, and the optimum quantity of catalyst employed was 5g of fly ash for every 100g of sawdust.

Investigating (Pseudo)-Heterogeneous Pd-Catalysts for Kraft Lignin Depolymerization under Mild Aqueous Basic Conditions

Lignin is one of the main components of lignocellulosic biomass and corresponds to the first renewable source of aromatic compounds. It is obtained as a by-product in 100 million tons per year, mainly from the paper industry, from which only 2–3% is upgraded for chemistry purposes, with the rest being used as an energy source. The richness of the functional groups in lignin makes it an attractive precursor for a wide variety of aromatic compounds. With this aim, we investigated the Pd-catalyzed depolymerization of lignin under mild oxidizing conditions (air, 150 °C, and aqueous NaOH) producing oxygenated aromatic compounds, such as vanillin, vanillic acid, and acetovanillone. Palladium catalysts were implemented following different strategies, involving nanoparticles stabilized in water, and nanoparticles were supported on TiO2. Significant conversion of lignin was observed in all cases; however, depending on the catalyst nature and the synthetic methods, differences were observed in terms of selectivity in aromatic monomers, mainly vanillin. All these aspects are discussed in detail in this report, which also provides new insights into the role that Pd-catalysts can play for the lignin depolymerization mechanism.

Cell Wall Composition of Hemp Shiv Determined by Physical and Chemical Approaches

The use of agricultural by-products in the building engineering realm has led to an increase in insulation characteristics of biobased materials and a decrease in environmental impact. The understanding of cell wall structure is possible by the study of interactions of chemical compounds, themselves determined by common techniques like Van Soest (VS). In this study, a global method is investigated to characterise the cell wall of hemp shiv. The cell wall molecules were, at first, isolated by fractionation of biomass and then analysed by physical and chemical analysis (Thermal Gravimetric Analysis, Elementary Analysis, Dynamic Sorption Vapor and Infra-Red). This global method is an experimental way to characterise plant cell wall molecules of fractions by Thermal Gravimetric Analysis following by a mathematical method to have a detailed estimation of the cell wall composition and the interactions between plant macromolecules. The analyzed hemp shiv presents proportions of 2.5 ± 0.6% of water, 4.4 ± 0.2% of pectins, 42.6 ± 1.0% (Hemicellulose–Cellulose), 18.4 ± 1.6% (Cellulose–Hemicellulose), 29.0 ± 0.8% (Lignin–Cellulose) and 2.0 ± 0.4% of linked lignin.

Catalytic Conversion of Lignins for Valuable Chemicals

Modern civilization is moving from fossil sources of raw materials and, consequently, energy to renewable resources: plant raw materials and solar and wind energy [...]

Oxidative Catalytic Fractionation of Lignocellulosic Biomass under Non-alkaline Conditions

Biomass pretreatment methods are commonly used to isolate carbohydrates from biomass, but they often lead to modification, degradation, and/or low yields of lignin. Catalytic fractionation approaches provide a possible solution to these challenges by separating the polymeric sugar and lignin fractions in the presence of a catalyst that promotes cleavage of the lignin into aromatic monomers. Here, we demonstrate an oxidative fractionation method conducted in the presence of a heterogeneous non-precious-metal Co-N-C catalyst and O2 in acetone as the solvent. The process affords a 15 wt% yield of phenolic products bearing aldehydes (vanillin, syringaldehyde) and carboxylic acids (p-hydroxybenzoic acid, vanillic acid, syringic acid), complementing the alkylated phenols obtained from existing reductive catalytic fractionation methods. The oxygenated aromatics derived from this process have appealing features for use in polymer synthesis and/or biological funneling to value-added products, and the non-alkaline conditions associated with this process support preservation of the cellulose, which remains insoluble at reaction conditions and is recovered as a solid.

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

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

Review of life cycle assessments of lignin and derived products: Lessons learned

In the last decade, the use of lignin as a bio-based alternative for fossil-based products has attracted significant attention, and the first LCAs of lignin and derived products have been conducted. Assessing side-stream products like lignin and potential benefits compared to their fossil counterparts presents complex methodological issues. This article provides a critical review of forty-two peer-reviewed LCAs regarding lignin and derived products. Methodological issues and their influence on the LCA results include the choice of the modeling approach and system boundaries, functional unit definition, impact categories considered, type of data used, handling multifunctionality and biogenic carbon modeling. The review focused on climate change impacts, as this is also the main impact category considered in most studies. Other impact categories in the comparison between lignin-based products and counterparts were also discussed with examples from the studies. Based on ten lessons learned, recommendations were provided for LCA practitioners to increase future consistency of environmental claims made about lignin and lignin-based products. The finding suggest that the environmental performance of lignin-based products is significantly affected by both 1) LCA methodological problems such as allocation practices and biogenic carbon modeling and 2) technical aspects such as the percentage of lignin in the composition of products and the selection of the fuel to replace lignin in internal energy uses. Beyond this, the reviewed LCAs showed that often lignin-based products offer better environmental performances than fossil-based products, especially for climate change.

Biomass-derived chemical substitutes for bisphenol A: recent advancements in catalytic synthesis

Bisphenol A is an oil-derived, large market volume chemical with a wide spectrum of applications in plastics, adhesives and thermal papers. However, bisphenol A is not considered safe due to its endocrine disrupting properties and reproductive toxicity. Several functional substitutes of bisphenol A have been proposed in the literature, produced from plant biomass. Unless otherwise specified, the present review covers the most significant contributions that appeared in the time span January 2015-August 2019, describing the sustainable catalytic synthesis of rigid diols from biomass derivatives. The focus is thereupon on heterogeneous catalysis, use of green solvents and mild conditions, cascade processes in one-pot, and continuous flow setups. More than 500 up-to-date references describe the various substitutes proposed and the catalytic methods for their manufacture, broken down according to the main biomass types from which they originate.

Recent Advances in Renewable Polymer Production from Lignin-Derived Aldehydes

Lignin directly derived from lignocellulosic biomass has been named a promising source of platform chemicals for the production of bio-based polymers. This review discusses potentially relevant routes to produce renewable aromatic aldehydes (e.g., syringaldehyde and vanillin) from lignin feedstocks (pre-isolated lignin or lignocellulose) that are used to synthesize a range of bio-based polymers. To do this, the processes to make aromatic aldehydes from lignin with their highest available yields are first presented. After that, the routes from such aldehydes to different polymers are explored. Challenges and perspectives of the production the lignin-derived renewable chemicals and polymers are also highlighted.

Downstream processing of lignin derived feedstock into end products

This review provides critical analysis on various downstream processes to convert lignin derived feedstock into fuels, chemicals and materials.

Efficient hydrogenolysis of aryl ethers over Ce-MOF supported Pd NPs under mild conditions: mechanistic insight using density functional theoretical calculations

The significant Pd⁰ content and optimum bonding of the reactant & product (higher adsorption energy of benzyl phenyl ether and lower desorption energy for phenol) are responsible for the exceptional catalytic activity of Pd/Ce-MOF.

Heterogeneous Catalyzed Thermochemical Conversion of Lignin Model Compounds: An Overview

Thermochemical lignin conversion processes can be described as complex reaction networks involving not only de-polymerization and re-polymerization reactions, but also chemical transformations of the depolymerized mono-, di-, and oligomeric compounds. They typically result in a product mixture consisting of a gaseous, liquid (i.e., mono-, di-, and oligomeric products), and solid phase. Consequently, researchers have developed a common strategy to simplify this issue by replacing lignin with simpler, but still representative, lignin model compounds. This strategy is typically applied to the elucidation of reaction mechanisms and the exploration of novel lignin conversion approaches. In this review, we present a general overview of the latest advances in the principal thermochemical processes applied for the conversion of lignin model compounds using heterogeneous catalysts. This review focuses on the most representative lignin conversion methods, i.e., reductive, oxidative, pyrolytic, and hydrolytic processes. An additional subchapter on the reforming of pyrolysis oil model compounds has also been included. Special attention will be given to those research papers using "green" reactants (i.e., H₂ or renewable hydrogen donor molecules in reductive processes or air/O₂ in oxidative processes) and solvents, although less environmentally friendly chemicals will be also considered. Moreover, the scope of the review is limited to those most representative lignin model compounds and to those reaction products that are typically targeted in lignin valorization.

Determination of hydroxyl groups in biorefinery resources via quantitative 31P NMR spectroscopy

The analysis of chemical structural characteristics of biorefinery product streams (such as lignin and tannin) has advanced substantially over the past decade, with traditional wet-chemical techniques being replaced or supplemented by NMR methodologies. Quantitative ³¹P NMR spectroscopy is a promising technique for the analysis of hydroxyl groups because of its unique characterization capability and broad potential applicability across the biorefinery research community. This protocol describes procedures for (i) the preparation/solubilization of lignin and tannin, (ii) the phosphitylation of their hydroxyl groups, (iii) NMR acquisition details, and (iv) the ensuing data analyses and means to precisely calculate the content of the different types of hydroxyl groups. Compared with traditional wet-chemical techniques, the technique of quantitative ³¹P NMR spectroscopy offers unique advantages in measuring hydroxyl groups in a single spectrum with high signal resolution. The method provides complete quantitative information about the hydroxyl groups with small amounts of sample (~30 mg) within a relatively short experimental time (~30-120 min).