Comparison of Copper and Vanadium Homogeneous Catalysts for Aerobic Oxidation of Lignin Models

Sustainable production of catechol derivatives from waste tung nutshell C/G-type lignin via heterogeneous Cu-NC catalytic oxidation

The sustainable production of catechol derivatives is a challenging task. Catechyl (C) and guaiacyl (G) lignins coexisting in waste tung nutshells are promising feedstocks to form valuable catechol derivatives, but the depolymerization of C/G lignin typically involves a catalytic reductive process that cannot produce these oxidized aromatic chemicals. Herein, we demonstrated that the sustainable production of catechol derivative aldehydes and acids from C/G lignin could be achieved through a heterogeneous copper-catalyzed oxidative process. Under optimized conditions, the Cu-NC-800 catalyst affords a 43.5 mg g-1 yield (8.9 wt%, based on Klason lignin) of aromatic aldehydes (protocatechuic aldehyde, vanillin) and acids (protocatechuic acid, vanillic acid). XRD and XPS analyses showed that CuO and Cu2O may be the active species during the heterogeneous oxidation of the Cu-NC-800 catalyst. This study opens new opportunities for the sustainable production of catechol derivatives from C/G-type lignin.

Molecular mechanism of the transformation of oxidized lignin to N-substituted aromatics

The cleavage of C-C bonds in oxidized lignin model compounds is a highly effective methodology for achieving lignin depolymerization, as well the generation of N-substituted aromatics. Here, density functional theory calculations were performed to understand the mechanism of the transformation of an oxidized lignin model compound (ligninox) and hydroxylamine hydrochloride to N-substituted aromatics. The reaction was proposed to proceed via an energetically viable mechanism featuring the initial production of HOAc acting as proton bridge. According to our calculations, Z-type oxime is the major intermediate of the reaction, with an energy barrier of 22.9 kcal mol-1, owing to the weak interactions between methoxy and oximino groups being stronger than that of E-type oxime. Additionally, the hydroxy addition is the rate-determining step, with an energy barrier of 27.0 kcal mol-1. Moreover, the huge net energy change of Beckmann and abnormal Beckmann rearrangements is the main overall thermodynamic driving force for producing N-substituted aromatics from oximes. The theoretical results have provided a clear picture of how ligninox transforms into N-substituted aromatics and are expected to provide valuable theoretical guidance for lignin depolymerization.

Applications of Vanadium, Niobium, and Tantalum Complexes in Organic and Inorganic Synthesis

Organometallic catalysis is a powerful strategy in chemical synthesis, especially with the cheap and low toxic metals based on green chemistry principle. Thus, the selection of the metal is particularly important to plan relevant and applicable processes. The group VB metals have been the subject of exciting and significant advances in both organic and inorganic synthesis. In this Review, we have summarized some reports from recent decades, which are about the development of group VB metals utilized in various types of reactions, such as oxidation, reduction, alkylation, dealkylation, polymerization, aromatization, protein synthesis, and practical water splitting.

Promising and efficient lignin degradation versatile strategy based on DFT calculations

The extraction of higher-value products from lignin degradations under mild conditions is a challenge. Previous research reported efficient two-step oxidation and reduction strategies for lignin degradation, which has great significance to lignin degradation. In this paper, the mechanism about the C-O bond cleavage of lignin with and without Cα oxidations has been studied systematically. Our calculation results show that the degradation of anionized lignin with Cα oxidations is kinetically and thermodynamically feasible. In addition, the calculations predict that the anionized lignin compounds without Cα oxidation also could be degraded under mild conditions. Moreover, we propose special lignin catalytic degradation systems containing the characteristic structure of "double hydrogen bonds." The double hydrogen bonds structure could further decrease the energy barriers of the C-O bond cleavage reaction. This provides a versatile strategy to design novel lignin degradation.

Anion-Dependent Catalytic C-C Bond Cleavage of a Lignin Model within a Cationic Metal-Organic Framework

The development of heterogeneous catalysts capable of selectively converting lignin model compounds into products of added value offers an exciting avenue to explore in the production of renewable chemical feedstocks. The use of metal-organic frameworks (MOFs) in such chemical transformations relies largely on the presence of accessible open metal sites found within highly porous networks that simultaneously allow for fast transport and strong interactions with desired substrates. Here, we present the first systematic study on the modulation of catalytic performance of a cationic framework, [Cu₂(L)(H₂O)₂](NO₃)₂·5.5H₂O (L = 1,1'-bis(3,5-dicarboxylatophenyl)-4,4'-bipyridinium), achieved through the exchange of anionic guests. Remarkably, the catalytic activity proves to be highly anion-dependent, with a nearly 10-fold increase toward the aerobic C-C bond cleavage of a lignin model compound when different anionic species are incorporated within the MOF. Moreover, we demonstrate that the cationic nature of the MOF, imparted by the incorporation of viologen moieties within the linker, tunes the electrophilicity of the active copper(II) sites, resulting in stronger interactions with the substrate. As such, the copper-based framework exhibits enhanced catalytic performance when compared to its neutral counterpart, emphasizing the appeal of charged frameworks for use as green heterogeneous catalysts.

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

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

Green and sustainable route for oxidative depolymerization of lignin: New platform for fine chemicals and fuels

Depolymerization of lignin biomass to its value-added chemicals and fuels is pivotal for achieving the goals for sustainable society, and therefore has acquired key interest among the researchers worldwide. A number of distinct approaches have evolved in literature for the deconstruction of lignin framework to its mixture of complex constituents in recent decades. Among the existing practices, special attention has been devoted for robust site selective chemical transformation in the complex structural frameworks of lignin. Despite the initial challenges over a period of time, oxidation and oxidative cleavage process of aromatic building blocks of lignin biomass toward the fine chemical synthesis and fuel generation has improved substantially. The development has improved in terms of cost effectiveness, milder reaction conditions, and purity of compound individuals. These aforementioned oxidative protocols mainly involve the breaking of C-C and C-O bonds of complex lignin frameworks. More precisely in the line with environmentally friendly greener approach, the catalytic oxidation/oxidative cleavage reactions have received wide spread interest for their mild and selective nature toward the lignin depolymerization. This mini-review aims to provide an overview of recent developments in the field of oxidative depolymerization of lignin under greener and environmentally benign conditions. Also, these oxidation protocols have been discussed in terms of scalability and recyclability as catalysts for different fields of applications.

Copper-Catalyzed Oxidative Cleavage of the C-C Bonds of β-Alkoxy Alcohols and β-1 Compounds

Copper-catalyzed aerobic oxidation conditions were employed to promote the C-C bond cleavage of β-alkoxy alcohols and β-1 compounds (lignin model compounds). Besides these compounds, various 1,2 and 1,3-diols were successfully converted to aldehydes. We propose the Cu(I)-catalyzed mechanism explaining the C-C cleavage of these 1,2 and 1,3-dihydroxy compounds and β-alkoxy alcohols based on XPS data. Although our reaction conditions do not include large excess of bases and elaborated ligand-modified catalysts, copper salts with/without Me-TBD show good catalytic activities for C-C bond cleavage of various lignin model compounds.

Product-oriented Direct Cleavage of Chemical Linkages in Lignin

Lignin is one of the most important biomacromolecules in the plant biomass and the largest renewable source of aromatic building blocks in nature. Selectively producing value-added chemicals from the catalytic transformation of renewable lignin is of strategic significance and meet sustainability targets owing to the excessive consumption of non-renewable petroleum resource, but remains a long-term challenge owing to the complexity of lignin structure. This Minireview provides a summary and perspective of the extensive research that provides insight into selectively catalytic transformations of lignin and its derived monomers via directed scissor of chemical linkages (C-O and C-C bonds) with product-oriented targets. Furthermore, some challenges and opportunities of lignin catalytic transformation are provided based on existing problems in this field for readers to discuss future research directions.

Electrochemical oxidation mechanisms for selective products due to C-O and C-C cleavages of β-O-4 linkages in lignin model compounds

Electrochemical oxidation is a promising and effective method for lignin depolymerization owing to its selective oxidation capacity and environmental friendliness. Herein, the electrooxidation of non-phenolic alkyl aryl ether monomers and β-O-4 dimers was experimentally (by cyclic voltammetry, in situ spectroelectrochemistry, and gas chromatography-mass spectroscopy) and theoretically (by DFT calculations) explored in detail. Compared to the reported literature (T. Shiraishi, T. Takano, H. Kamitakahara and F. Nakatsubo, Holzforschung, 2012, 66(3), 303-309), 1-(4-ethoxyphenyl)ethanol showed a distinguishable oxidation pathway, where the resulting carbonyl product surprisingly underwent a bond cleavage on alkyl-aryl ether to ultimately produce a quinoid like compound. In contrast, β-O-4 dimers, like 2-phenoxy-1-phenethanol and 2-phenoxyacetophenone also demonstrated electrochemical oxidation induced by C_β-O and C_α-C_β bond cleavages. For the oxidation products, the presence of the C_α-hydroxyl group in dimers was the key to selectively generate aldehyde-containing species under mild electrochemical conditions, otherwise it produces alcohol-containing products following a different mechanism compared to the C_α[double bond, length as m-dash]O containing dimers.

Vanadium-Substituted Phosphomolybdic Acids for the Aerobic Cleavage of Lignin Models-Mechanistic Aspect and Extension to Lignin

This work deals with the aerobic oxidative cleavage of C-C and C-O bonds catalyzed by the Keggin-type phosphovanadomolybdic acid (H₆[PMo₉V₃O₄₀], noted H₆PV₃). The latter was synthesized by an adapted hydrothermal procedure classically used for lower vanadium content and was tested as a catalyst for the aerobic cleavage of 2-phenoxyacetophenone (noted K1_HH) and 1-phenyl-2-phenoxyethanol (A1_HH) used as two lignin models. The operative conditions (solvent, catalytic loading, etc.) were adjusted on K1_HH and extrapolated to A1_HH. The cleavage of the alcohol model required more drastic conditions and therefore further optimization. Preliminary attempts on an Organosolv wheat straw lignin were performed too. From the kinetic study, high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) data, a mechanism of the cleavage of both models was proposed.

Advances in lignin valorization towards bio-based chemicals and fuels: Lignin biorefinery

Lignin is one of the most promising renewable sources for aromatic hydrocarbons, while effective depolymerization towards its constituent monomers is a particular challenge because of the structural complexity and stability. Intensive research efforts have been directed towards exploiting effective valorization of lignin for the production of bio-based platform chemicals and fuels. The present contribution aims to provide a critical review of key advances in the identification of exact lignin structure subjected to various fractionation technologies and demonstrate the key roles of lignin structures in depolymerization for unique functionalized products. Various technologies (e.g., thermocatalytic approaches, photocatalytic conversion, and mechanochemical depolymerization) are reviewed and evaluated in terms of feasibility and potential for further upgrading. Overall, advances in pristine lignin structure analysis and conversion technologies can facilitate recovery and subsequent utilization of lignin towards tailored commodity chemicals and fungible fuels.

Activating molecular oxygen with Au/CeO2 for the conversion of lignin model compounds and organosolv lignin

Au/CeO2 was demonstrated to be a high efficiency catalyst for the conversion of 2-phenoxyacetophenol (PP-ol) employing O2 as an oxidant and methyl alcohol as the solvent without using an erosive strong base or acid. Mechanistic investigations, including emission quenching experiments, electron spin-resonance (ESR) and intermediate verification experiments, were carried out. The results verified that the superoxide anion activated by Au/CeO2 from molecular oxygen plays a vital role in the oxidation of lignin model compounds, and the cleavage of both the β-O-4 and Cα-Cβ linkages was involved. Au/CeO2 also performed well in the oxidative conversion of organosolv lignin under mild conditions (453 K), producing vanillin (10.5 wt%), methyl vanillate (6.8 wt%), methylene syringate (3.4 wt%) and a ring-opened product. Based on the detailed characterization data and mechanistic results, Au/CeO2 was confirmed to be a promising catalytic system.

Catalytic oxidation of biorefinery corncob lignin via zirconium(IV) chloride and sodium hydroxide in acetonitrile/water: A functionality study

In order to realize the efficient utilization of biorefinery corncob lignin, the promising catalytic oxidation strategy was carried out by using ZrCl₄ and NaOH as the co-catalyst and dioxygen as the oxidant in MeCN/H₂O. GC/MS, GC-FID, and MALDI-TOF/MS were employed to recognize the produced monomers and oligomers, and GPC was used to monitor the molecular weight changes of lignin fragments. In addition, specific structural evolution of corncob lignin during ZrCl₄/NaOH-catalyzed oxidation were revealed by quantitative ¹³C (Q¹³C) and 2D HSQC NMR techniques. Results showed that the total yields of produced oxidation monomers reached 6.8 wt%, and aromatic aldehydes were the major species, in which vanillin and 4-hydroxybenzaldehyde were the two dominant products. After ZrCl₄/NaOH-catalyzed oxidation, the weight-average molecular weight of corncob lignin and its products decreased from 2000 Da to 300 Da after oxidation with 16 h. Moreover, Q¹³C NMR analysis showed the decrease percentage of CO aliphatic carbons (including methoxyl carbons), the increase percentage of CC aliphatic and carbonyl carbons, and the relative stable percentage of aromatic carbons with reaction prolonged. These results combined with the further confirmation from HSQC indicated the oxidative cleavage of CO aliphatic linkages and removal of methoxy groups within corncob lignin, as well as the formation of CC aliphatic bonds and carbonyl groups. The work presented a comprehensive insight into the catalytic oxidative depolymerization of biorefinery corncob lignin.

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Cobalt-Catalyzed Oxidation of the β-O-4 Bond in Lignin and Lignin Model Compounds

In this work we demonstrate the use of Co(acac)3 in combination with N-hydroxyphthalimide as an oxidant for the selective α-oxidation of the representative β-O-4 linkages in lignin model compounds. The oxidation reaction proceeds under mild conditions at 80 °C using 1,4-dioxane as the solvent and an oxygen atmosphere. The prior α-oxidation in the β-O-4 linkage of the lignin polymer is known to result in an easier cleavage of the adjacent C-O and C-C bonds because of a decrease in bond stability. Finally, the conditions were successfully transferred to kraft- and organosolv-lignin samples as proven by 2D-NMR (HSQC) experiments and gel permeation chromatography measurements.

Selective Oxidation of Lignin Model Compounds

Lignin, the planet's most abundant renewable source of aromatic compounds, is difficult to degrade efficiently to welldefined aromatics. We developed a microwave-assisted catalytic Swern oxidation system using an easily prepared catalyst, MoO₂ Cl₂ (DMSO)₂ , and DMSO as the solvent and oxidant. It demonstrated high efficiency in transforming lignin model compounds containing the units and functional groups found in native lignins. The aromatic ring substituents strongly influenced the selectivity of β-ether phenolic dimer cleavage to generate sinapaldehyde and coniferaldehyde, monomers not usually produced by oxidative methods. Time-course studies on two key intermediates provided insight into the reaction pathway. Owing to the broad scope of this oxidation system and the insight gleaned with regard to its mechanism, this strategy could be adapted and applied in a general sense to the production of useful aromatic chemicals from phenolics and lignin.

Perspective on Lignin Oxidation: Advances, Challenges, and Future Directions

Lignin valorization has gained increasing attention over the past decade. Being the world's largest source of renewable aromatics, its valorization could pave the way towards more profitable and more sustainable lignocellulose biorefineries. Many lignin valorization strategies focus on the disassembly of lignin into aromatic monomers, which can serve as platform molecules for the chemical industry. Within this framework, the oxidative conversion of lignin is of great interest because it enables the formation of highly functionalized, valuable compounds. This work provides a brief overview and critical discussion of lignin oxidation research. In the first part, oxidative conversion of lignin models and isolated lignin streams is reviewed. The second part highlights a number of challenges with respect to the substrate, catalyst, and operating conditions, and proposes some future directions regarding the oxidative conversion of lignin.

Redox-active ligand controlled selectivity of vanadium oxidation on Au(100)

Metal-organic coordination networks at surfaces, formed by on-surface redox assembly, are of interest for designing specific and selective chemical function at surfaces for heterogeneous catalysts and other applications. The chemical reactivity of single-site transition metals in on-surface coordination networks, which is essential to these applications, has not previously been fully characterized. Here, we demonstrate with a surface-supported, single-site V system that not only are these sites active toward dioxygen activation, but the products of that reaction show much higher selectivity than traditional vanadium nanoparticles, leading to only one V-oxo product. We have studied the chemical reactivity of one-dimensional metal-organic vanadium - 3,6-di(2-pyridyl)-1,2,4,5-tetrazine (DPTZ) chains with O2. The electron-rich chains self-assemble through an on-surface redox process on the Au(100) surface and are characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, and density functional theory. Reaction of V-DPTZ chains with O2 causes an increase in V oxidation state from VII to VIV, resulting in a single strongly bonded (DPTZ2-)VIVO product and spillover of O to the Au surface. DFT calculations confirm these products and also suggest new candidate intermediate states, providing mechanistic insight into this on-surface reaction. In contrast, the oxidation of ligand-free V is less complete and results in multiple oxygen-bound products. This demonstrates the high chemical selectivity of single-site metal centers in metal-ligand complexes at surfaces compared to metal nanoislands.

Toward the oxidative deconstruction of lignin: oxidation of β-1 and β-5 linkages

Production of monomers and other products from the oxidation of β-1 and β-5 lignin models.

Vinylation of Aryl Ether (Lignin β-O-4 Linkage) and Epoxides with Calcium Carbide through C-O Bond Cleavage

Calcium carbide has been increasingly used as a sustainable, easy-to-handle, and low-cost feedstock in organic synthesis. Currently, methodologies of using calcium carbide as "solid acetylene" in synthesis are strictly limited to activation and reaction with X-H (X=C, N, O, S) bonds. Herein, a mild and transition-metal-free protocol was developed for the vinylation of epoxides and aryl ether linkage (β-O-4 lignin model compound) with calcium carbide through C-O bond cleavage, forming valuable vinyl ether products. Calcium carbide plays a vital role in the C-O bond activation and cleavage, and in providing acetylide source for the formation of vinylated products. These exciting results may provide new methodologies for organic synthesis and new insights toward lignin- or biomassrelated degradation to useful products.

Study on the cleavage of alkyl-O-aryl bonds by in situ generated hydroxyl radicals on an ORR cathode

˙OH selectively attacks the active sites opposite to phenolic hydroxyl groups and leads to bond-cleavage of ether bonds.

Transition-metal catalyzed valorization of lignin: the key to a sustainable carbon-neutral future

The development of a sustainable, carbon-neutral biorefinery has emerged as a prominent scientific and engineering goal of the 21st century. As petroleum has become less accessible, biomass-based carbon sources have been investigated for utility in fuel production and commodity chemical manufacturing. One underutilized biomaterial is lignin; however, its highly crosslinked and randomly polymerized composition have rendered this biopolymer recalcitrant to existing chemical processing. More recently, insight into lignin's molecular structure has reinvigorated chemists to develop catalytic methods for lignin depolymerization. This review examines the development of transition-metal catalyzed reactions and the insights shared between the homogeneous and heterogeneous catalytic systems towards the ultimate goal of valorizing lignin to produce value-added products.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.