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.     

Evaluation of Reaction Efficiency of Thioacidolysis for Cleavage of β-O-4 Interunitary Linkages by Using β-O-4 Type Artificial Lignin Polymer

Abstract

Dimeric lignin model compounds with non-phenolic and phenolic moieties and a β -O-4 type artificial lignin polymer were subjected to thioacidolysis to evaluate the reaction efficiency of thioacidolysis for cleavage of β -O-4 interunitary linkage. The obtained yields of thioethylated monomeric products from the dimeric lignin model compounds reached nearly 100% under the conventional condition, whereas that from the artificial lignin polymer was as low as 74%. Neither prolonged reaction time nor increased concentration of ethanethiol (the thioacidolysis reagent) enhanced the resulting monomer yields from the polymer (69–79%). Thioacidolysis of the lignin model compounds followed by HPSEC analysis also showed the dimeric model compounds were degraded almost quantitatively, but that the artificial lignin polymer was not. Thioacidolysis followed by desulfurization gave at least one dimeric product resulting from incomplete β -O-4 cleavage at significant yield. These results suggested the conventional thioacidolysis could not achieve quantitative cleavage of β -O-4 linkages in lignin macromolecules.     

Studies on the Reactions of Phenolic and Non-Phenolic Lignin Model Compounds with Chlorine Dioxide

The reaction between chlorine dioxide and four different lignin model compounds (phenolic and non-phenolic, with or without an α-hydroxyl group) are studied. The key difference with previous studies is the effective elimination of hypochlorous acid by sulfamic acid during chlorine dioxide treatment. The elimination of hypochlorous acid, an important intermediate formed during chlorine dioxide bleaching, makes it possible to study the consumption of the lignin model compounds solely as a result of the action of chlorine dioxide.

Based on almost closed mass balances for the substrate and atomic chlorine it is found that the reaction between ClO₂ and lignin model compounds is generally characterized by three independent parallel reactions:

1) demethylation, 2) formation of 2-methoxy-p-quinone and 3) formation of muconic acid monomethyl ester and/or its derivatives. Although, the importance of each reaction depends on the functional groups of the substrate, the demethylation reaction is the most important. It is shown that non-phenolic lignin model compounds do react with ClO₂ when ClO₂ is supplied in large excess. However at conditions representative of industrial ClO₂ application levels, only the phenolic substrates react with chlorine dioxide when both phenolic and non-phenolic lignin model compounds are present. Contrary to the previous model compound studies done without effective elimination of hypochlorous acid, no chloroaromatic material is detected in the present investigation. This supports the view that the reaction intermediate hypochlorous acid (or chlorine) is solely responsible for the formation of chloro organic material during chlorine dioxide bleaching. Reaction mechanisms of the degradation of phenolic and non-phenolic lignin model compounds by CI0₂ are also discussed.     

Conversion of biomass model compound under hydrothermal conditions using batch reactor

Supercritical water has been focused on as an environmentally attractive reaction media where organic materials can be decomposed into smaller molecules. The reaction behavior of lignin model compound was studied in near- and supercritical water with a batch type reactor. Catechol was used as a model compound for aromatic rings in lignin. The reaction was carried out at temperatures of 643-693 K at various pressures under an argon atmosphere. The chemical species in the aqueous products were identified by gas chromatography mass spectrometry (GCMS) and quantified using high performance liquid chromatography (HPLC). The effect of pressure and reaction time on the conversion process of catechol was presented. The main products from the conversion of catechol was phenol and the value of global rate constant for catechol conversion (k) is 3.0 × 10⁻⁴-11.0 × 10⁻⁴ min⁻¹.     

Decomposition of a Lignin Model Compound under Hydrothermal Conditions

Lignin, which is the second most abundant polymeric aromatic organic substance in wood biomass after cellulose, and contains many oxygen‐based functional groups, has been proposed as an alternative source of chemical compounds. Guaiacol, a model compound for lignin, was reacted in supercritical water using a batch‐type reactor at temperatures of 653–673 K and various pressures under an argon atmosphere. The effects of temperature and reaction time at the same pressure were combined into a single severity parameter that was used to monitor the decomposition of guaiacol to its derived compounds. The main products in aqueous solution were catechol, phenol, and o‐cresol. The amounts present approached 40.73 wt %, 14.18 wt %, and 4.45 wt %, respectively. With an increase in the reaction time at the same conditions, the amount of guaiacol decreased and the quantity of derived compounds of guaiacol increased. Based on the experimental results, a reaction mechanism for the decomposition of guaiacol was proposed. The process investigated in this study may form the basis for an efficient method of wood biomass decomposition.     

Reactivity of Lignin and Lignin Models Towards UV-Assisted Peroxide

The comparative reactivities of a series of guaiacyl and syringyl lignin model compounds and their methylated analogues towards alkaline peroxide and UV-alkaline peroxide were investigated. The overall reaction was followed by monitoring the reduction of the substrate as a function of time, and in every case, the reaction showed pseudo-first-order kinetics. The reaction rates of most lignin models having identical sidechains with alkaline peroxide and with UV-alkaline peroxide were in the order syringyl > guaiacyl > 3,4,5-trimethoxyphenyl > veratryl. Thus phenols react faster than their methyl ethers, and an extra ortho methoxyl group promotes the reaction. Lignin models possessing electron-donating side-chains had generally higher reaction rates than those with electron-withdrawing sidechains. The reaction rates of the series of benzoic acids were 2–4 times higher at pH 11 than at pH 5. UV-peroxide degradation of a eucalypt kraft lignin was faster than that of a pine kraft lignin, and degradation was 1.4–1.6 times faster at pH 11 than at pH 5. The data are consistent with the formation of higher amounts of reactive radicals under alkaline conditions, and aromatic rings with greater electronegativities promoting reactions with the radicals.     

Discoloration of α-Carbonyl-Free Lignin Model Compounds Under UV Light Exposure

A series of α-carbonyl-free lignin model compounds was prepared and adsorbed onto bleached chemical pulp. The rates of discoloration of these materials, under simulated solar light exposure, were established and compared to that of peroxide bleached methylated (alkali and dimethyl sulfate) and nonmethylated chemithermomechanical pulps. Phenolic phenylcoumarone and phenolic stilbenes were found to be the most sensitive chromophores under UV irradiation, both inducing a strong yellowing. Also, monophenolic biphenyl and biphenylmethane entities were shown to be prone to discolouration in contrast to the biphenolic and dimethylated ones. Moreover, the catechol structures which are known to be easily oxidizable into ortho quinones, display significant yellowing only when their absorption spectra are shifted above 300 nm where the light source emits. The behaviour of the phenolic stilbenes and the biphenyl catechol is reminiscent of the behaviour of CTMP. This indicates the possible involvement of such structures as being mainly responsible for the discoloration of lignocellulosic materials under UV irradiation. A mechanism is proposed to account for the photoyellowing of the models involving a cyclohexadienone hydroperoxide as intermediate, which is formed by UV irradiation from a complex between the phenol and ground state oxygen.     

Dissolution of Wood with Acetyl Bromide Solutions-Reactions of Lignin Model Compounds

A Pinus radiata refiner mechanical pulp (RMP) sample and a range of lignin model compounds were treated with a solution of 25% w/w acetyl bromide (AcBr) in acetic acid to assess the reactions which take place during the dissolution of wood with the AcBr solution. The lignin model reactions were also carried out in solutions to which 4% w/w perchloric acid was added. The RMP sample gave a product which contained 9.2% bromine, and was severely degraded to low molecular weight products, the polysaccharides more so than the lignin fraction. The reactions of the lignin model compounds with AcBr included O-acetylation of phenolic and aliphatic hydroxyl groups, cleavage of β-ether bonds, C-acetylation of aromatic rings, particularly in positions para to methoxyl groups, demethylation of aromatic methoxyl groups and probable replacement of benzyl alcohol groups by bromine functions. The reactions were accelerated by perchloric acid, and the rates appeared to decrease in the order O-acetylation π bromine substitution >> β-ether cleavage π C-acetylation > demethylation. It is concluded that dissolution of wood in the AcBr solution is a consequence of depolymerisation of the polysaccharides and lignin, and acetylation to give products which would be soluble in the reaction medium.     

Engineering plastics from lignin. I. Synthesis of hydroxypropyl lignin

Hydroxypropylation of lignin in a batch reactor under alkaline conditions at 180°C was studied using propylene oxide (PO) by itself, and PO in combination with several ligninlike model compounds and with kraft lignin. While the PO homopolymerization rate increased rapidly at temperatures above 85°C, and was too fast to be determined accurately at 180°C, the addition of model compounds and lignin was found to delay homopolymerization in relation to the presence of ionizable functional groups. The observations are consistent with a reaction mechanism involving first order kinetics with regard to each alkoxide and PO concentrations. Where the reaction rates toward PO increase with increasing pK_a values, the reaction sequence proceeds in the order of declining basicity. Thus lignins with high acidity were found to be subject to greater degrees of modification than those with more neutral character. This explains the earlier observed beneficial effect of lignin carboxylation on the properties of lignin–PO reaction mixtures.     

Formation rate of benzyl cation intermediate from p-Hydroxyphenyl,guaiacyl, or syringyl nucleus in acidolysis of lignin

The benzyl cation intermediate is believed to be primarily formed in the acid-catalyzed reaction of lignin, and the reaction route of the intermediate determines which reaction products are afforded and whether lignin undergoes depolymerization or condensation. This study aimed to examine the formation rate of the benzyl cation intermediate from phenolic (P) or non-phenolic (N) lignin model compounds with different types of aromatic nuclei, namely p-hydroxyphenyl (H), guaiacyl (G), or syringyl (S). The rate was in the order of H > G > S for both P- and N-type model compounds and of P > N for all H-, G-, and S- type model compounds. The orders were successfully explained by the electron-donating or electron-withdrawing properties of the hydroxy and methoxy groups at the para- and meta- of the benzyl position, which is the reaction center in the formation of the benzyl cation intermediate.     

木质素模型物羟甲基化反应的机理研究

以香草醇等六种化合物为木质素模型物,研究了侧链α-C上官能团和苯环C-3、C-5位甲氧基的取代情况对羟甲基化反应的影响。对产物的红外光谱和核磁共振氢谱分析表明,模型物经过改性后可以引入羟甲基。对模型物在羟甲基化反应过程中结构特性变化的研究表明,羟甲基化反应可在木质素苯环未取代的C-3和C-5上进行。在pH=11时,甲醛消耗量和羟甲基生成量均达到最大值,此时羟甲基化反应程度最大。木质素模型物的α-C基团对苯环的羟甲基化反应也有重要的影响,与α-C为羟基的模型物比较,α-C为醛基的模型物较易发生羟甲基化。由于羟甲基化反应是在碱性条件下进行的,且参加反应的醛均不含α-H,过程中会发生坎尼扎罗反应,产生酸性物质,从而使反应体系的pH值下降。     

Stereo-Preference in the degradation of the erythro and threo isomers of β-O-4-type lignin model compounds in oxidation processes. part 2: In the reaction with hydroxyl and oxyl anion radicals under hydrogen peroxide bleaching conditions

We examined which isomer, the erythro or threo, of non-phenolic β-O-4-type lignin model compounds is stereo-preferentially attacked by hydroxyl radical and its conjugate base, oxyl anion radical, generated by the decomposition of hydrogen peroxide in the presence of ferric ion and its precipitates under hydrogen peroxide bleaching conditions. The intrinsic stereo-preference of oxyl anion radical was slightly toward the erythro isomer, while hydroxyl radical had a further smaller stereo-preference. These stereo-preferences can be explained by our prior knowledge that oxyl anion radical preferentially attacks the side-chain of the lignin model compounds rather than the aromatic nucleus. The amount of the degraded lignin model compounds became less great with decreasing pH, but reversely and intensively greater in the pH range below 10. This phenomenon can be attributed to the change in the decomposition mechanism of hydrogen peroxide accompanying the pH variation.     

木素醌型发色基团与过氧化氢反应特性的研究进展

概述了木素醌型发色基团与过氧化氢反应特性的研究进展。指出木素中醌型发色基团,特别是邻醌结构是高得率浆颜色的主要来源,只要能有效提高过氧化氢破坏木素中醌型发色基团的效率,就能得到高白度的高得率浆,降低高得率浆过氧化氢漂白成本。建议利用木素模型物与过氧化氢反应,并研究其反应特性,为高得率纸浆过氧化氢漂白的应用研究积累一定的理论基础。     

Xylanase-Resistant Xylan In Unbleached Kraft Pulp

A water-soluble chromophoric xylanase-resitant xylan fraction (LF-D) was separated from a hardwood unbleached kraft pulp (UKP) after hydrolysis with a cellulase/ xylanase-membrane bioreactor. LF-D contained over 70% unremovable inorganic atoms including Si, Na, and S, together with a β-1,4- linked xylan. A nucleus exchange reaction and a nitrobenzene oxidation showed that LF-D contained a trace amount of a lignin component abundant in quinoid structures which had been partly demethylated during the course of kraft pulping. On the other hand, a higher molecular weight residual lignin fraction (HF-P), which was obtained from an impermeable part of the enzymatic digests, was found to have a diphenylmethane structure. LF-D was partially decolorized by Coriolus versicolor and bacterial microflora without action of extracellular lignin peroxidase, Mn-peroxidase, laccase or xylanase.     

The xanthate method of grafting. IV. Grafting of acrylonitrile onto high-yield pulp

A series of pulps with different lignin content was prepared from a softwood sodium bisulfite high-yield pulp. Sodium chlorite at pH 4 served as the delignification agent. The resulting pulps were subjected to grafting with acrylonitrile using the cellulose xanthate–hydrogen peroxide redox system to initiate the copolymerization reaction. The resulting products were isolated and analyzed for homopolymer content by dimethylformamide extraction. The results indicate that the presence of larger amounts of lignin in pulp may have a favorable effect on grafting. The plots of total conversion as well as of polymer loading show a minimum centered around approximately 15% of lignin. This minimum is observed invariably at five different reaction times and it coincides with the maximum concentration of reductive groups in the pulps as well as with the maximum cation exchange capacity. The parameter of grafting efficiency has a tendency to increase with rising lignin content, reflecting the lower relative yields of homopolymer obtained with high-lignin pulps. This observation is discussed in relation to possible chain transfer reactions by lignin.     

Catalytic efficiency of natural and synthetic compounds used as laccase-mediators in oxidising veratryl alcohol and a kraft lignin, estimated by electrochemical analysis

The electrochemical properties of eighteen natural and synthetic compounds commonly used to expand the oxidative capacity of laccases were evaluated in an aqueous buffered medium using cyclic voltammetry. This clarifies which compounds fulfil the requisites to be considered as redox mediators or enhancers. Cyclic voltammetry was also applied as a rapid way to assess the catalytic efficiency (CE) of those compounds which oxidise a non-phenolic lignin model (veratryl alcohol, VA) and a kraft lignin (KL). With the exception of gallic acid and catechol, all assayed compounds were capable of oxidising VA with varying CE. However, only some of them were able to oxidise KL. Although the oxidised forms of HBT and acetovanillone were not electrochemically stable, their reduced forms were quickly regenerated in the presence of VA. They thus act as chemical catalysts. Importantly, HBT and HPI did not attack the KL via the same mechanism as in VA oxidation. Electrochemical evidence suggests that violuric acid oxidises both substrates by an electron transfer mechanism, unlike the other N-OH compounds HBT and HPI. Acetovanillone was found to be efficient in oxidising VA and KL, even better than the synthetic mediators TEMPO, violuric acid or ABTS. Most of the compounds produced a generalised increase in the oxidative charge of KL, probably attributed to chain reactions arising between the phenolic and non-phenolic components of this complex molecule.     

Reactions of Metal Ion Complexes with Lignin Model Compounds,Part II. Fe(TSPC) Catalyzed Formation of Oxidized Products in the Absence of Oxygen

The water soluble phthalocyanine complex trisodium tetra-4-sulfonatophthalocyanineiron(III) (Fe(TSPc)) was found to be an effective catalyst for the cleavage of the β-ether bonds in the phenolic lignin model compounds guaiacylglycol β-guaiacyl ether (1) and guaiacylglycerol β-guaiacyl ether (11). The products of these reactions were very different from those formed in the corresponding reactions catalyzed by anthraquinone (AQ) or Co(SPP).^1–4 In particular, they gave large quantities of oxidized products, even though the reactions were performed in the absence of oxygen or other added oxidant. Mechanisms have been proposed for the oxidation reactions involving 1 and 11. In both cases the first step involves one electron oxidation of the lignin model compound by the catalyst. The radical derived from 1 then undergoes further one electron oxidation and deprotonation to give 4′-hydroxy-3′-methoxy-l-(2″-methoxyphenoxy)acetophenone (8) whereas that derived from 11 undergoes Cα-Cβ bond cleavage to give vanillin (4). Reactions of the reduced form of the catalyst with 8 and the quinone methides produced from the phenolic models are important routes for guaiacol formation and regeneration of the oxidized form of the catalyst. The feasibility of these proposed reaction pathways was investigated by studying the reactions of the intermediate compounds with the catalyst.     

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

Abstract

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.     

Chemical Factors Underlying the More Rapid β-O-4 Bond Cleavage of Syringyl than Guaiacyl Lignin under Alkaline Delignification Conditions

This study aimed to clarify why the β-O-4 bond cleavage of syringyl lignin is more rapid than that of guaiacyl lignin under alkaline pulping conditions. We examined whether or not three chemical factors, acidity of the α-hydroxy group, nucleophilicity of the generated α-alkoxide, and leaving ability of the leaving phenoxide, are different between syringyl and guaiacyl lignins and control the rate of the alkaline-induced β-O-4 bond cleavage, employing dimeric non-phenolic β-O-4-type lignin model compounds (LMCs) and novel methods for estimating these three factors. The results indicated that the α-hydroxy groups of syringyl-type LMCs are relatively more acidic than those of guaiacyl-type and that syringyl nucleus is a better leaving group than guaiacyl nucleus in the β-O-4 bond cleavage. These factors result in the β-O-4 bond of syringyl lignin being more prone to the alkaline-induced cleavage than that of guaiacyl lignin.     

Organosolv lignin depolymerization with different base catalysts

BACKGROUND: Revalorization of lignin is one of the key economical requirements for the development of cost‐effective biorefinery processes. The lignin polyphenolic structure is ideally suited to transformation catalytically into lower molecular weight compounds such as phenols, aromatic acids, esters, ethers, etc., replacing those obtained from petroleum. RESULTS: Lignin was subjected to base catalyzed depolymerization paying attention to the base effect on the oil yield and composition. The oil yields and compositions varied (5–20%) strongly depending on the base used, suggesting that the reactions took place via different mechanisms. As a result, the monomeric compounds obtained were different (catechol, cresols, syringol, guaiacol). Residual lignin contents were high for most of the experiments (up to 45%), and repolymerization reactions were proved to be one of the main reasons for this behavior. CONCLUSION: The results showed that lignin depolymerization produced phenolic compounds that can be introduced into existing petrochemical industries. The base selection will govern the nature of the products obtained. Repolymerization was proved to take place and to be the reason for the limitation on oil production. Copyright © 2012 Society of Chemical Industry