Reactions Of Lignin Model Compounds Research Articles

Effect of traditional solvent on thermal decomposition mechanism of lignin: A density functional theory study

In order to understand the effect of traditional solvents on lignin pyrolysis, the decarbonylation and decarboxylation reactions of various phenylic lignin model compounds were theoretically investigated using DFT methods at M06-2×/6–31++G(d,p) level. The calculation results show that activation energy of the decarbonylation and decarboxylation reactions of lignin model compounds can be reduced when H2O/CH3OH existed. There are two types of reaction for the H2O/CH3OH during the pyrolysis. For first type, the synergistic reaction of lignin with H2O/CH3OH as hydrogen transfer carrier. The energy barriers of the main elemental reaction steps during this type of pyrolysis are about 285.0–300.0 kJ/mol (H2O) and 275.0–290.0 kJ/mol (CH3OH) (decarbonylation), 170.0–210.0 kJ/mol and 155.0–200.0 kJ/mol (decarboxylation). For another type, the synergistic reaction of lignin with H2O/CH3OH as hydrogen source. The energy barriers of the main elemental reaction steps during this type of pyrolysis are about 260.0–278.0 kJ/mol and 240.0–260.0 kJ/mol, 303.0–312.0 kJ/mol and 291.0–297.0 kJ/mol. Furthermore, the reaction temperature has the most significant impact on decomposition reaction of lignin in a methanol medium, suggesting that the reaction in the methanol medium is better than that in the water environment.

Visible-Light-Induced Selective C–C Bond Cleavage Reactions of Dimeric β-O-4 and β-1 Lignin Model Substrates Utilizing Amine-Functionalized Fullerene

Finding a selective and efficient fragmentation process under ambient conditions is pivotal for the generation of fuels and chemical feedstocks from lignoceullosic biomass. In the present study, visible-light and amine-functionalized fullerene-based photocatalyst-promoted photodegradation reactions of dimeric β-O-4 and β-1 lignin model compounds, containing varying numbers of methoxy substituents on the arene ring, were explored to find and develop mild, eco-friendly photochemical techniques for efficient delignification. The results showed that, in contrast to well-known organic photoredox catalysts, amine-functionalized fullerene photocatalyst promoted photochemical reactions of lignin model compounds could lead to more efficient lignin fragmentation reactions through a pathway involving a selective Cα-Cβ bond cleavage process, and in addition, Cα-hydroxyl moiety in lignin model compounds played a significant role in the success of the Cα-Cβ bond cleavage reaction of lignin model substrates.

Photophysics of Perylene Diimide Dianions and Their Application in Photoredox Catalysis.

The two‐electron reduced forms of perylene diimides (PDIs) are luminescent closed‐shell species whose photochemical properties seem underexplored. Our proof‐of‐concept study demonstrates that straightforward (single) excitation of PDI dianions with green photons provides an excited state that is similarly or more reducing than the much shorter‐lived excited states of PDI radical monoanions, which are typically accessible after biphotonic excitation with blue photons. Thermodynamically demanding photocatalytic reductive dehalogenations and reductive C−O bond cleavage reactions of lignin model compounds have been performed using sodium dithionite acts as a reductant, either in aqueous solution or in biphasic water–acetonitrile mixtures in the presence of a phase transfer reagent. Our work illustrates the concept of multi‐electron reduction of a photocatalyst by a sacrificial reagent prior to irradiation with low‐energy photons as a means of generating very reactive excited states.

Photophysics of Perylene Diimide Dianions and Their Application in Photoredox Catalysis

AbstractThe two‐electron reduced forms of perylene diimides (PDIs) are luminescent closed‐shell species whose photochemical properties seem underexplored. Our proof‐of‐concept study demonstrates that straightforward (single) excitation of PDI dianions with green photons provides an excited state that is similarly or more reducing than the much shorter‐lived excited states of PDI radical monoanions, which are typically accessible after biphotonic excitation with blue photons. Thermodynamically demanding photocatalytic reductive dehalogenations and reductive C−O bond cleavage reactions of lignin model compounds have been performed using sodium dithionite acts as a reductant, either in aqueous solution or in biphasic water–acetonitrile mixtures in the presence of a phase transfer reagent. Our work illustrates the concept of multi‐electron reduction of a photocatalyst by a sacrificial reagent prior to irradiation with low‐energy photons as a means of generating very reactive excited states.

The reaction of lignin model compounds during enzymatic bleaching with a Curvularia verruculosa haloperoxidase: impact on chlorination

AbstractRecent developments in enzymatic bleaching processes have led to replacement strategies of harsh chemicals by haloperoxidases. For this purpose, it is important to control the haloperoxidase-mediated formation of adsorbable organic halides (AOX). In this study, we studied the chlorination of monomeric and dimeric lignin model substrates. Guaiacol, acetovanillone, veratryl alcohol, pinoresinol and adlerol were treated with Curvularia verruculosa haloperoxidase and compared to a sodium hypochlorite treatment. High-performance liquid chromatography-diode array detection-mass spectrometry (HPLC-DAD-MS) analysis was employed for the characterization of the reaction products. Our results show that while treatment with haloperoxidases in the presence of sodium chloride and hydrogen peroxide leads to no improvement in AOX formation compared to chemical treatment with NaOCl, addition of ammonium chloride substantially lessens chlorination and promotes β-O-4 ether bond cleavage. The use of ammonium chloride in conjunction with enzymatic haloperoxidase-based bleaching could be a route to minimize lignin chlorination.

Effect of Metal Catalysts on Bond Cleavage Reactions of Lignin Model Compounds in Supercritical Water

Lignin depolymerization into its aromatic monomers is necessary for the valorization of lignocellulosic biomass, but cleavage of the C–O ether bonds and C–C bonds between the monomers is challenging. In this study, we investigated bond cleavage reactions of benzyl phenyl ether, diphenyl ether, and biphenyl, which served as models for lignin compounds with α–O–4, 4–O–5, and 5–5 linkages, respectively, by using various metal catalysts in supercritical water at 673 K and a water density of 0.5 g cm−3 without added hydrogen gas. Benzyl phenyl ether was decomposed without catalyst. We found that palladium, platinum, and rhodium catalysts supported on carbon showed activity for cleavage of the C–O ether bonds in benzyl phenyl ether and diphenyl ether but not for the bonds between the aromatic units of biphenyl. Using anisole as a model compound, we also found that methoxy groups, which are present in lignin containing coniferyl and sinapyl alcohol monomers, were converted into phenol groups or were decomposed. The reaction conditions described herein have great potential utility for the conversion of lignin, which can be decomposed and dissolved in supercritical water, into valuable aromatic compounds.

Ni/HZSM-5 catalyst preparation by deposition-precipitation. Part 2. Catalytic hydrodeoxygenation reactions of lignin model compounds in organic and aqueous systems

Nickel metal supported on HZSM-5 (zeolite) is a promising catalyst for lignin depolymerization. In this work, the ability of catalysts prepared via deposition-precipitation (DP) to perform hydrodeoxygenation (HDO) on two lignin model compounds in organic and aqueous solvents was evaluated; guaiacol in dodecane and 2-phenoxy-1-phenylethanol (PPE) in aqueous solutions. All Ni/HZSM-5 catalysts were capable of guaiacol HDO into cyclohexane at 523 K. The role of the HZSM-5 acid sites was confirmed by comparison with Ni/SiO2 (inert support) which exhibited incomplete deoxygenation of guaiacol due to the inability to perform the cyclohexanol dehydration step. The catalyst prepared with 15 wt% Ni, a DP time of 16 h, and a calcination temperature of 673 K (Ni(15)/HZSM-5 DP16_Cal673), performed the guaiacol conversion with the greatest selectivity towards HDO products, with an intrinsic rate ratio (HDO rate to conversion rate) of 0.31, and 90% selectivity to cyclohexane. Catalytic activity and selectivity of Ni/HZSM-5 (15 wt%) in aqueous environments (water and 0.1 M NaOH solution) was confirmed using PPE reactions at 523 K. After 30 min reaction time in water, Ni/HZSM-5 exhibited ∼100% conversion of PPE, and good yield of the desired products; ethylbenzene and phenol (∼35% and 23% of initial carbon, respectively). Ni/HZSM-5 in NaOH solution resulted in significantly higher ring saturation compared to the Ni/HZSM-5 in water or the NaOH solution control.

Probing the Lignin Disassembly Pathways with Modified Catalysts Based on Cu-Doped Porous Metal Oxides

Described are the selectivities observed for reactions of lignin model compounds with modifications of the copper-doped porous metal oxide (CuPMO) system previously shown to be a catalyst for lignin disassembly in supercritical methanol (Matson et al., J. Amer. Chem. Soc. 2011, 133, 14090–14097). The models studied are benzyl phenyl ether, 2-phenylethyl phenyl ether, diphenyl ether, biphenyl, and 2,3-dihydrobenzofuran, which are respective mimetics of the α-O-4, β-O-4, 4-O-5, 5-5, and β-5 linkages characteristic of lignin. Also, briefly investigated as a substrate is poplar organosolv lignin. The catalyst modifications included added samarium(III) (both homogeneous and heterogeneous) or formic acid. The highest activity for the hydrogenolysis of aryl ether linkages was noted for catalysts with Sm(III) incorporated into the solid matrix of the PMO structure. In contrast, simply adding Sm3+ salts to the solution suppressed the hydrogenolysis activity. Added formic acid suppressed aryl ether hydrogenolysis, ...

A review on thermal chemical reactions of lignin model compounds

Biomass-based energy fulfills the future need of sustainable humanity. The utilization of lignin which is a primary constituent of biomass still remains the world most difficult problem due to its highly aromatic polymeric structure. Investigations on lignin model compounds renew the knowledge on CC and CO bond cleavage chemistry and indicate directions for lignin deconstruction. This review is to present the state of the art of the chemical transformation of lignin model compounds including monomers and dimers based on catalyst classifications, with emphasis on the fundamental catalytic chemistry in different processes. Moreover, how ideas derived from concepts transform to strategies for controlling the reaction pathways is also discussed.

Inside Back Cover: Trimethylenemethane‐Ruthenium(II)‐Triphos Complexes as Highly Active Catalysts for Catalytic CO Bond Cleavage Reactions of Lignin Model Compounds (ChemCatChem 2/2013)

Inside Back Cover: Trimethylenemethane‐Ruthenium(II)‐Triphos Complexes as Highly Active Catalysts for Catalytic CO Bond Cleavage Reactions of Lignin Model Compounds (ChemCatChem 2/2013)

Trimethylenemethane‐Ruthenium(II)‐Triphos Complexes as Highly Active Catalysts for Catalytic CO Bond Cleavage Reactions of Lignin Model Compounds

Trimethylenemethane‐Ruthenium(II)‐Triphos Complexes as Highly Active Catalysts for Catalytic CO Bond Cleavage Reactions of Lignin Model Compounds

A computational study of pyrolysis reactions of lignin model compounds

Abstract Enthalpies of reaction for the initial steps in the pyrolysis of lignin have been evaluated at the CBS-4m level of theory using fully substituted β-O-4 dilignols. Values for competing unimolecular decomposition reactions are consistent with results previously published for phenethyl phenyl ether models, but with lowered selectivity. Chain propagating reactions of free radicals with a closed-shell dilignol are dominated by structures in which extensive electron delocalization occurs.

Reactions of lignin model compounds in ionic liquids

Abstract Lignin, a readily available form of biomass, is a potential source of renewable aromatic chemicals through catalytic conversion. Recent work has demonstrated that ionic liquids are excellent solvents for processing woody biomass and lignin. Seeking to exploit ionic liquids as media for depolymerization of lignin, we investigated reactions of lignin model compounds in these solvents. Using Bronsted acid catalysts in 1-ethyl-3-methylimidazolium triflate at moderate temperatures below 200 °C, we obtained up to 11.6% molar yield of the dealkylation product 2-methoxyphenol from the model compound 2-methoxy-4-(2-propenyl)phenol and cleaved 2-phenylethyl phenyl ether, a model for lignin ethers. Despite these successes, acid catalysis failed in dealkylation of the saturated-chain model compound 4-ethyl-2-methoxyphenol and did not produce monomeric products from organosolv lignin, demonstrating that further work is required to understand the complex chemistry of lignin depolymerization.

Study of kinetics of reaction of lignin model compounds with propylene oxide

Abstract The etherification of phenolic groups has been found to inhibit photodegradation in wood and lignin rich pulps. The precise understanding of kinetics of chemical reaction between lignins or their model compounds and the etherifying agent is the first step for developing a viable modification procedure. In this study, we have investigated the reaction of lignin model compounds (namely, phenol and guaiacol) with propylene oxide in aqueous media. The kinetics of etherification reaction was studied under varying pH conditions in the temperature range 30–60°C. The etherified reaction products were characterized by gas chromatogram-mass spectrum (GC-MS). The extent of etherification of phenols and the rate of chemical reaction was followed by UV-Visible absorption spectroscopy. The reaction between lignin model compounds and propylene oxide was indicated by a rapid reduction in the absorbance accompanied by the development of a new band corresponding to etherified products. The reaction kinetics was investigated at pH ∼12 under the condition of excess concentration of propylene oxide. The reaction followed first order kinetics and rate constants increased linearly with an increase in the temperature and concentration of propylene oxide. The MS fragment data of reaction product support the proposed reaction scheme. The activation energy of the reaction of propylene oxide with phenol and guaiacol, calculated with the Arrhenius equation, was 56.2 kJ mol-1 and 45.4 kJ mol-1, respectively.

Comparative Studies of Chlorine Dioxide Reactions with Muconic Acid Derivatives and Lignin Model Compounds

The reaction of chlorine dioxide with different types of lignin model compounds was investigated in order to compare the kinetics and to evaluate the amount of oxidant consumed by the different substrates. Complete reaction of lignin model compounds was observed at ClO2‐to‐substrate molar ratios of 0.9–1.2, which corresponds to an electron transfer varying between 5–6 equivalents per mole of substrate. Muconic acid derivatives also fully reacted, at a ClO2‐to‐substrate molar ratio of 1.2, with the oxidant consumption being about 4 equivalents per mole of substrate. The reaction of mixtures of phenolic, non‐phenolic, and muconic acid type substrates showed that the reaction rates of non‐phenolic and muconic acid type substrates were rather similar. This study suggests that further reaction between ClO2 and the primary lignin oxidation products, such as muconic acid type structures could be the cause of overconsumption of oxidant in a D stage.

The Reactions of Lignin Model Compounds with Hydrogen Peroxide at Low pH

Summary In peroxymonosulfuric acid bleaching, the presence of hydrogen peroxide is dependent on the reaction conditions and the conversion ratios used to generate the peroxy acid. Substantial amounts of hydrogen peroxide may be present in the reaction system under certain conditions. An understanding of the reactions of hydrogen peroxide under these conditions would be beneficial. Therefore, several simple lignin model compounds were reacted with acidic hydrogen peroxide, pH 1-3, at 70°C. In all cases the phenolic lignin model compounds reacted much faster than their non-phenolic counterparts. In fact, the extent of reaction was very much dependent on the structure of the lignin model compound. The α-hydroxyl compounds, 4-(1-Hydroxy-ethyl)-2-methoxy-phenol and 1-(3,4-Dimethoxy-phenyl)-ethanol, reacted faster than the corresponding α-carbonyl compounds with both reacting much faster than the aromatic compounds, with simple alkyl substituents. A new reaction mechanism for α-hydroxyl compounds is proposed, in which benzyl carbocation formation is followed by nucleophilic addition of hydrogen peroxide. Unlike the mechanisms proposed in the past, no evidence of aromatic hydroxylation via perhydronium ion was observed. The reactivities were very pH dependent, in that higher reactivity was associated with lower pH. Decreasing pH further increased the amount of condensation products identified, such that condensation was competitive with degradation. These condensation reactions were also present under the Caro's acid bleaching conditions at pH below 2. However, under all conditions the reactivity of acidic peroxide was found to be much less than that of peroxymonosulfuric acid.

Ring-Opened Products from Reaction of Lignin Model Compounds with UV-Assisted Peroxide

Reaction of a range of guaiacyl, syringyl, veratryl and 3,4,5-trimethoxyphenyl lignin model compounds with UV-alkaline peroxide gave rise to 26 aliphatic acids which were identified by gas chromatography-mass spectrometry. The acids derived from scission of the aromatic rings, and their total yields were less than 10% from most model compounds. The acids were both monocarboxylic and dicarboxylic. with either saturated or unsaturated carbon chains, and were variously substituted with hydroxyl, methoxyl and alkyl groups. There were more aliphatic acid products identified in the reaction mixtures of guaiacyl and syringyl lignin models than in those from veratryl and 3,4.5-trimethoxyphenyl models, although high numbers of products were not always reflected in the total yields. The identification of more products from phenols than from their methyl ethers reflects the degree of their degradation, and is consistent with the involvement of the superoxide radical anion as a reaction intermediate.

Reactions of Lignin Model Compounds with Chlorine Dioxide. Molecular Orbital Calculations

Based on previously proposed chemical mechanisms accounting for the formation of various oxidation products from both phenolic and non-phenolic lignin model compounds, semi-empirical molecular orbital calculations have been completed. The computational results indicate that the heats of reaction for the phenolic model compound are consistently lower than those for the etherified model. These data are in agreement with the experimental results which indicate that when both types of model compounds are present, the phenolic compounds are preferentially oxidized.

Aromatic Products from Reaction of Lignin Model Compounds with UV-Alkaline Peroxide

A series of guaiacyl and syringyl lignin model compounds and their methylated analogues were reacted with alkaline hydrogen peroxide while irradiating with UV light at 254 nm. The aromatic products obtained were investigated by gas chromatography-mass spectrometry (GC-MS). Guaiacol, syringol and veratrol gave no detectable aromatic products. However, syringol methyl ether gave small amounts of aromatic products, resulting from ring substitution and methoxyl displacement by hydroxyl radicals. Reaction of vanillin and syringaldehyde gave the Dakin reaction products, methoxy-1,4-hydroquinones, while reaction of their methyl ethers yielded benzoic acids. Acetoguaiacone, acetosyringone and their methyl ethers afforded several hydroxylated aromatic products, but no aromatic products were identified in the reaction mixtures from guaiacylpropane and syringylpropane. In contrast, veratrylpropane gave a mixture from which 17 aromatic hydroxylated compounds were identified. It is concluded that for phenolic lignin model compounds, particularly those possessing electron-donating aromatic ring substituents, ring-cleavage reactions involving superoxide radical anions are dominant, whereas for non-phenolic lignin models, hydroxylation reactions through attack of hydroxyl radicals prevail.

Reaction of lignin model compounds with pulp bleaching chemicals

Reaction of lignin model compounds with pulp bleaching chemicals