Oxidation Of Lignin Model Compounds Research Articles

Electrochemical oxidation of lignin model compounds over metal oxyhydroxides on nickel foam

The lignin model compounds were efficiently cleaved into veratric acid and aliphatic carboxylic acids by the electrocatalyst nickel oxyhydroxide.

Theoretical investigation on the reaction mechanisms of O2‐initiated gas‐phase oxidation of lignin model compounds

AbstractTransforming renewable lignin into high value‐added chemicals is a forward‐looking strategy to address the resource waste caused by insufficient utilization of biomass resources. On this basis, studying the efficient conversion of lignin to aldehydes/acids and their reaction mechanisms has become an attractive topic. A systematic investigation of the gas‐phase oxidation reaction mechanisms of the three model compounds initiated by O2 was carried out at the atomic and molecular levels by using density functional theory (DFT). Further revealing of oxidation behavior on two reaction sites of phenolic hydroxyl group and hydroxymethyl group were accomplished in detail. The potential energy surface information of 21 possible reaction channels of two pathways were obtained at B3LYP/6‐311+G(d,p) level. The influence of substituent effects on the reaction energy barrier was estimated. The calculation results showed that the reactivity of phenolic hydroxyl group is stronger than that of hydroxymethyl group, because the reaction Gibbs potential barriers are lower by about 4.9–8.7 kcal/mol. The reaction energy barriers on phenolic hydroxyl group site and hydroxymethyl group site decrease with the increase of the number of methoxy groups. Revealing the oxidation processes of lignin model compounds will provide a deeper understanding on the reaction mechanism and provide theoretical support for further experimental research on the conversion of lignin into high value‐added chemicals.

(Invited) Nitroxyl-Derivatives As Cocatalyst for Lignin Decomposition by Dye-Sensitized Photoelectrochemical Cell

For stable and chemoselective decomposition of lignin model compound by DSPEC, metal based sensitizer was replaced by organic dye, 5-[4-(diphenylamino)phenyl]thiophene-2-cyanoacrylic acid (L1). For effective regeneration of dyes and oxidation of lignin model compounds(LMCs), bicyclic nitroxyl derivates, such as 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and 9-azabicyclo[3,3,1]nonan-3-one-9-oxyl (Keto-ABNO) were used. Keto-ABNO exhibited higher photo-electrocatalytic activity than TEMPO for the oxidation and decomposition of LMC. The superior reactivity with high selectivity of L1 sensitizer with Keto-ABNO mediator is attributed to the effective Caryl-Cα bond cleavage with 100% conversion yield for 24hrs at room temperature while it was 86% with TEMPO. The D–π–A type organic dye with a bicyclic nitroxyl radical mediator provides a possible strategic pathway for heterogeneous photocatalytic C–C bond cleavage at ambient condition.

A copper decorated porphyrin-based porous organic polymer: Evaluation in the oxidation of lignin model compounds

A novel porous organic polymer containing porphyrin units was synthesized and decorated with copper to produce the metallated POP, Cu-TriPOP. The native POP, Nat-TriPOP and metal-decorated Cu-TriPOP exhibited BET surface areas of 595 m2/g and 373 m2/g, respectively. Excellent thermal stability up to 300 °C was observed by thermogravimetric analysis. Transmission electron microscopy (TEM) showed a sea urchin/bush-like morphology. The metal-decorated POP, Cu-TriPOP, was evaluated in the oxidation of veratryl alcohol and other related substrates. Excellent catalytic performance was achieved with conversions of around 80%. The addition of ascorbic acid, a well-known radical scavenger, significantly reduced the activity, suggesting that a radical mechanism is most likely operative. In addition, excellent catalyst recovery and recyclability were achieved for up to five cycles with minimal decrease in activity.

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

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

MPSC6 binuclear complexes immobilized on graphene oxide for oxidation of lignin model compounds and lignin

An efficient process for the oxidative degradation of lignin using M-GO (MPSC6 binuclear complexes immobilized on graphene oxide) biomimetic catalysts in acetonitrile was developed in this work. Different M-GO catalysts (M=Co, Cu, Zn and Ni) were used to catalyze the depolymerization of lignin model compounds and organosolv lignin. The results showed that the immobilization of MPSC6 complexes on GO was favorable for regulating the valuable depolymerization of the model compounds and lignin. The optimal solid catalyst Co-GO exhibited high catalytic activity, which showed a 83.04% conversion of phenolic β-O-4 lignin model compound, a 91.26% conversion of veratryl alcohol and a 96.24% conversion of vanillyl alcohol with a catalyst/model compound ratio of 50 mg:10 mmol and a dosage of 15 mmol H2O2 at 80 °C for 3 h. The stuctural changes of organosolv lignin occurred in catalytic oxidation were analyzed in terms of O/C ratio, molecular weight and OH content of lignin samples, and a plausible mechanism involving the formation of aromatic products and muconolactone from lignin depolymerization over M-GO was also proposed.

Origins of Catalyst Inhibition in the Manganese-Catalysed Oxidation of Lignin Model Compounds with H2 O2.

The upgrading of complex bio‐renewable feedstock, such as lignocellulose, through depolymerisation benefits from the selective reactions at key functional groups. Applying homogeneous catalysts developed for selective organic oxidative transformations to complex feedstock such as lignin is challenged by the presence of interfering components. The selection of appropriate model compounds is essential in applying new catalytic systems and identifying such interferences. Here, it was shown by using as an example the oxidation of a model substrate containing a β‐O‐4 linkage with H2O2 and an in situ‐prepared manganese‐based catalyst, capable of efficient oxidation of benzylic alcohols, that interference from compounds liberated during the reaction can prevent its application to lignocellulose depolymerisation.

Investigating homogeneous Co/Br−/H2O2 catalysed oxidation of lignin model compounds in acetic acid

The catalytic oxidation of lignin model compounds by Co/Br−/H2O2 is investigated; substituting Br− for N-hydroxyphthalimide improved substrate conversion and product yield.

Sphingobacterium sp. T2 Manganese Superoxide Dismutase Catalyzes the Oxidative Demethylation of Polymeric Lignin via Generation of Hydroxyl Radical.

Sphingobacterium sp. T2 contains two extracellular manganese superoxide dismutase enzymes which exhibit unprecedented activity for lignin oxidation but via an unknown mechanism. Enzymatic treatment of lignin model compounds gave products whose structures were indicative of aryl-Cα oxidative cleavage and demethylation, as well as alkene dihydroxylation and alcohol oxidation. 18O labeling studies on the SpMnSOD-catalyzed oxidation of lignin model compound guiaiacylglycerol-β-guaiacyl ether indicated that the an oxygen atom inserted by the enzyme is derived from superoxide or peroxide. Analysis of an alkali lignin treated by SpMnSOD1 by quantitative 31P NMR spectroscopy demonstrated 20-40% increases in phenolic and aliphatic OH content, consistent with lignin demethylation and some internal oxidative cleavage reactions. Assay for hydroxyl radical generation using a fluorometric hydroxyphenylfluorescein assay revealed the release of 4.1 molar equivalents of hydroxyl radical by SpMnSOD1. Four amino acid replacements in SpMnSOD1 were investigated, and A31H or Y27H site-directed mutant enzymes were found to show no lignin demethylation activity according to 31P NMR analysis. Structure determination of the A31H and Y27H mutant enzymes reveals the repositioning of an N-terminal protein loop, leading to widening of a solvent channel at the dimer interface, which would provide increased solvent access to the Mn center for hydroxyl radical generation.

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.

Three-dimensional composite of Co3O4 nanoparticles and nitrogen-doped reduced graphene oxide for lignin model compound oxidation

3D composite of Co3O4 nanoparticles and N-doped reduced graphene oxide can effectively catalyze the oxidation of lignin model compounds.

Study the Mechanism of the Electrochemical Oxidation of Lignin Model Compounds

Lignin has a high potential to be a renewable source of aromatic compounds due to its polyphenolic structure. Electrochemical oxidation of lignin on nickel–cobalt electrodes introduced an environmentally friendly, and economic procedure to convert lignin to other value-added aromatic compounds. However, lack of knowledge about the mechanism of the lignin electrochemical oxidation process, because of the complexity of the molecule, makes the conversion process more elaborate and complex. Understanding the mechanism of the lignin model compounds, which represent building blocks and linkages of lignin molecule, contributes controlling the lignin depolymerization process. Electrochemical study allows us to understand the reaction pathway, and demonstrate the mechanism by the aid of cyclic voltammetry. In this work we have presented the electrochemical oxidation mechanism of the lignin model compounds in the typical three electrode cell beaker using a Pt rotating disc electrode (RDE) as the working electrode. The effects of the concentration of the compounds, supporting electrolyte pH, and working electrode rotation rate have been also studied through cyclic voltammetry experiments.

Regioselective Baeyer–Villiger oxidation of lignin model compounds with tin beta zeolite catalyst and hydrogen peroxide

One approach to lignin deconstruction involves oxidation of benzylic alcohol groups, followed by Baeyer–Villiger oxidation (BVO) and ester hydrolysis. In this study β-O-4 and β-1 lignin models were oxidized using a heterogeneous BVO system.

A comparison of the oxidation of lignin model compounds in conventional and ionic liquid solvents and application to the oxidation of lignin

The oxidation of lignin model compounds in ionic liquid solvents was investigated as a prelude to the oxidation of lignin in these solvents where the polymer is appreciably soluble.

Performance of cobalt titanate towards H2O2 based catalytic oxidation of lignin model compound

Mixed metal cobalt titanium oxide (CoTiO3) prepared by solution phase method has been evaluated for the liquid phase catalytic oxidation of vanillyl alcohol to vanillin using H2O2 as an oxygen source.

Oxidation of model lignin compounds with peracetic acid under homogeneous catalysis with polyoxometalates

Oxidation of lignin model compounds with peracetic acid using polyoxometalate manganese-containing sodium vanadomolybdophosphate Na11[PMo6V5O39Mn(OH)] as a catalyst has been studied. The effect of pH, concentration and nature of the oxidized compound, concentration of the catalyst and peracetic acid, and temperature on kinetics of oxidation and the products composition has been investigated.

A Metal‐Free, Carbon‐Based Catalytic System for the Oxidation of Lignin Model Compounds and Lignin

AbstractNitrogen‐containing graphene material (LCN) has been identified as an effective catalyst for the oxidation of β‐O‐4 and α‐O‐4 types of lignin model compounds in the presence of tert‐butyl hydroperoxide, to provide aromatic aldehydes, acids and other organic chemicals in high yield. The transformations of five lignin model compounds over LCN were investigated systematically. Instrumentation analysis, kinetic study and radical trapping experiments highlight the mechanistic features of the reaction, including: 1) the reaction pathway starts by benzylic CH or COH bond activation, followed by CαCβ or CαO bond cleavage, and finally further oxidation of intermediate aromatics; and 2) the reaction follows a free‐radical mechanism with all the key steps involving radical species. In addition, the LCN proved to be a highly stable catalyst; no significant activity decrease was observed for four consecutive runs, and X‐ray photoelectron spectroscopy analysis indicates negligible decrease in the content of the active nitrogen species in the catalyst. Notably, this new catalytic system can be extended to the oxidative depolymerisation of real lignin, to produce a significant portion of liquefied, low‐molecular‐mass products.

ReaxFF Study of the Oxidation of Lignin Model Compounds for the Most Common Linkages in Softwood in View of Carbon Fiber Production

Lignin is an underused but major component of biomass. One possible area of utilization is the production of carbon fiber. A necessary processing step is the stabilization of lignin fiber (typically in an oxygen environment) before high temperature treatment. We investigate oxidative, thermal conversion of lignin using computational methods. Dilignol model compounds for the most common (seven) linkages in softwood are chosen to represent the diverse structure of lignin. We perform molecular dynamics simulation where the potential energy surface is described by a reactive force field (ReaxFF). We calculate overall activation energies for model conversion and reveal initial mechanisms of formaldehyde formation. We record fragmentation patterns and average carbon oxidation numbers at various temperatures. Most importantly, we identify mechanisms for stabilizing reactions that result in cyclic and rigid connections in softwood lignin fibers that are necessary for further processing into carbon fibers.

Recyclable ionic liquid tagged Co(salen) catalysts for the oxidation of lignin model compounds

Ionic liquid tagged salen ligands containing two proximal 1,3-disubstituted imidazolium ionic liquid cores form cobalt(III) complexes capable of selectively oxidizing veratryl alcohol, a lignin model compound, to veratraldehyde using air or pure oxygen as the source of oxygen. Entrainment of these catalysts in either 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], or 1-butyl-3-methylimidazolium bistriflimide, [bmim][NTf2], hydrophobic ionic liquid solvents, results in biphasic reactions when water is used as the second solvent allowing the catalyst/ionic liquid phase to be recycled.

Unprecedented oxidative properties of mesoporous silica materials: Towards microwave-assisted oxidation of lignin model compounds

Abstract The unusual oxidative ability of mesoporous silicas towards oxidation of an important lignin model molecule, 1,2-(4-hydroxy-3methoxy-phenoxy) ethanol, apocynol under microwave irradiation is presented in this work. Mesoporous MCM-41, HMS, SBA-15 and amorphous silica were employed as catalysts in the present study. Different reactivities were obtained for the various silica materials. It was assumed that the substrate conversion and product selectivity were highly influenced by the nature of mesoporous silica materials. Based on the nature of the catalysts and reaction product profile, a plausible mechanism has been proposed.