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.     

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     

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.     

Condensation Reactions of Softwood and Hardwood Lignin Model Compounds Under Organic Acid Cooking Conditions

In order to examine the condensation reactions of softwood and hardwood lignin during organic acid cooking, mixtures of benzyl alcohol type lignin model compounds with guaiacyl and syringyl units as the sources of benzyl cations and creosol and 5-methoxycreosol as the sources of electron-rich aromatic carbons were cooked under acidic pulping conditions.

From the yield of the condensation products in the initial reactions, it was shown that the carbonium cations of guaiacyl nuclei were more reactive than those of syringyl nuclei.

Syringyl type aromatic ring carbons had higher reactivities than guaiacyl type ones.

The cleavage of the benzyl ether bond in syringyl compounds was slower than that of guaiacyl compounds.

The diphenylmethane structures formed by the condensation reaction between veratryl alcohol and 5-methoxycreosol were found to be unstable under the strong acidic cooking conditions.

It is concluded that the condensation reactions between benzylic cations from the guaiacyl compounds and the electron rich aromatic ring carbons of syringyl ones are very fast, but the condensation products are unstable under the strong acidic pulping conditions.

When the guaiacyl nuclei react as an electron-rich aromatic carbon, the reaction is slower but the condensation product is much more stable.

These differences in reactivities, and the stabilities of the condensation products, may contribute to the resistance of softwood toward complete delignification on acid pulping.     

Exploring the Selective Demethylation of Aryl Methyl Ethers with a Pseudomonas Rieske Monooxygenase

Biocatalytic dealkylation of aryl methyl ethers is an attractive reaction for valorization of lignin components, as well as for deprotection of hydroxy functionalities in synthetic chemistry. We explored the demethylation of various aryl methyl ethers by using an oxidative demethylase from Pseudomonas sp. HR199. The Rieske monooxygenase VanA and its partner electron transfer protein VanB were recombinantly coexpressed in Escherichia coli and they constituted at least 25 % of the total protein content. Enzymatic transformations showed that VanB accepts NADH and NADPH as electron donors. The VanA–VanB system demethylates a number of aromatic substrates, the presence of a carboxylic acid moiety is essential, and the catalysis occurs selectively at the meta position to this carboxylic acid in the aromatic ring. The reaction is inhibited by the by-product formaldehyde. Therefore, we tested three different cascade/tandem reactions for cofactor regeneration and formaldehyde elimination; in particular, conversion was improved by addition of formaldehyde dehydrogenase and formate dehydrogenase. Finally, the biocatalyst was applied for the preparation of protocatechuic acid from vanillic acid, giving a 77 % yield of the desired product. The described reaction may find application in the conversion of lignin components into diverse hydroxyaromatic building blocks and generally offers potential for new, mild methods for efficient unmasking of phenols.     

Estimation of the Aromatic Units in Lignin by Nucleus Exchange - A Reassessment of the Method

A series of lignin model compounds was subjected to the nucleus exchange reaction (boron trifluoride-phenol at 180°C) to evaluate the application of the procedure to the study of wood and pulp lignin structure. The nucleus exchange reaction of the series of compounds gave rise to catechol in a range of yields, and in every case no starting material remained. In contrast to earlier reports, lignin model diphenyl ether and biphenyl compounds also gave rise to catechol. On the basis of these results and application of the nucleus exchange reaction to woodmeals and kraft pulps, we conclude that the nucleus exchange reaction does not allow accurate measurement of aromatic units in lignin.     

Phototransformation of metobromuron in the presence of TiO2

The photocatalytic degradation of the herbicide metobromuron was studied, using TiO₂ photocatalysts (Degussa P25 and Millennium PC50, PC100, PC105 and PC500). Intermediate products were identified by means of liquid chromatography–mass spectrometry (LC–MS–MS) using electrospray (ES) interfacing technique. In order of importance, the main reaction pathways were demethoxylation of the urea chain, ring oxidation leading to ortho hydroxylation with respect to the N atom, substitution of Br by OH, ring oxidation with formation of the meta hydroxylated compound. Demethylation of the demethoxylated product occurred as a secondary reaction. Formation of bromide ions was observed too. In the presence of 2-propanol used as a hydroxyl radical scavenger, demethoxylation was drastically inhibited while the other reactions were only partly inhibited. It suggests that demethoxylation was exclusively due to the oxidation by hydroxyl radicals, whereas the other reactions involved positive holes too. The degradation rate of metobromuron varied in the order: PC50 < PC100 < PC105 < PC500 < Degussa P25. TiO₂ Degussa P25 was therefore the most efficient photocatalyst although its surface area was lower than that of most of Millennium TiO₂.     

桉木光诱导氧化降解研究

本文研究了尾叶桉（Eucalyptusurophylla）木粉在光照射下其木素和多聚糖的变化行为．研究表明，在光照射下木素及木素结构单元中的甲氧基含量降低，多聚糖含量降低，铜价升高．UV分析可知，木素光降解产物中羰基或与苯环共轭的乙烯双键结构较多．经GC－MS分析鉴定出尾叶桉SCMP浆40种低分子量木素光氧化降解产物，其中丁香基型占58．10％，愈疮木基型占18．75％，其它占21．93％，主要降解产物包括丁香醛、2－羟基－3－丁香基丙酸、3，4－二羟基苯甲酸,2－丁香基乙醇、丁香酸、丁香酸和香兰素等.另外也研究了光降解过程中木素自由基变化行为。ESR分析测定表明，光照射产生的自由基在纯的氮气或二氧化碳中相对稳定，在空气或氧气中游离基迅速衰减，从而证实光诱导氧化作用是木材或富含木素化学机械浆光降解的重要作用。     

Ni-P/HZSM-5催化木质素降解制备酚类化学品

采用Ni-P复合改性HZSM-5催化剂催化木质素降解制备高附加值的单酚类化学品,探讨了催化剂种类、金属负载量、反应温度、反应时间以及溶剂种类对木质素催化降解制备酚类化合物的影响。同时采用X射线衍射仪（XRD）、比表面积和孔径分析仪（BET）、化学吸附仪（NH₃-TPD）、热重分析仪（TG）以及气相色谱质谱联用仪（GC/MS）对催化剂以及液相产物进行分析表征,同时探讨其催化失活以及再生机制。结果表明：Ni、P高度分散在HZSM-5催化剂的表面,Ni的添加有效地弱化了C－C键,致使β-O-4和α-O-4发生断裂,有效地提高了木质素加氢解聚的活性,减少了焦炭的生成,但催化剂的再生水热稳定性较差,重复使用性较低。当采用甲醇为供氢试剂,在反应温度为220℃,氢气压力为2MPa,反应时间为8h,催化剂负载量为10%,NaOH为共催化剂时,其木质素的转化率为98.6%,酚类化合物的含量达到74.97%。产物以苯酚、愈创木酚和紫丁香酚为主,低温促进了紫丁香酚的产生。     

Study of the nitroxyl radical catalyst in aerobic oxidative cleavage and functionalization of lignin model compounds

The selective aerobic oxidation of hydroxyl groups, α-oxyamination of benzylic ketones, and selective cleavage of Cα-Cβ linkages in lignin model compounds were studied using nitroxyl radical catalysts and sodium nitrite. The less hindered nitroxyl radical catalysts (ABNO, 1-Me-AZADO) were found to be more reactive in comparison to TEMPO. We identified a novel method to convert α,γ-dihydroxyl β-O-4 lignin model compounds directly into corresponding α-oxyaminated products and 1,2-keto esters. A new pathway for cleavage of Cα-Cβ linkage in β-O-4 link p-hydroxyphenyl (H-units) model compounds to produce useful aromatic monomers has also been demonstrated.     

Polyhydroxy fatty acids and their derivatives from plant oils

A novel process for the industrial production of hydroxylated fatty acids involves epoxidation of plant oils and their derivatives, followed by catalytic epoxy ring opening in the presence of water or other hydrogen donors, such as alcohols, diols, and amines. Depending on the starting material, epoxidation followed by opening of the oxirane ring leads to fatty acids that contain vicinal diol groups or to other substituted hydroxylated fatty acid derivatives. As an example for the preparation of a substituted hydroxylated fatty acid derivative, the reaction of epoxidized rapeseed oil with monobutylamine as hydrogen donor is described. Apart from the intended formation of hydroxyl groups with vicinal aminoalkyl groups, partial aminolysis of the ester compound was also observed. Another example describes the reaction of epoxidized rapeseed oil with different molar proportions of 1,4-butanediol as hydrogen donor. Depending on the molar proportion of the hydrogen donor, interesterification, or intermolecular ether formation were observed as side reactions. The properties of various technical hydroxylated fatty acids and their derivatives, prepared according to this novel process, are given, and potential applications of these products are suggested.     

Reactivities of Guaiacyl and Syringyl Lignin Model Phenols Towards Oxidation with Oxygen-Alkali

A range of guaiacyl and syringyl lignin model phenols was treated with oxygen in 1M potassium hydroxide solution at 70°C. The reactions were monitored by high performance liquid chromatography and gas chromatography-mass spectrometry. The reactions of the phenols, which followed pseudo-first-order kinetics, were faster for syringyl than for guaiacyl phenols. For the various 4-substituted syringols the reactivities were in decreasing order CH₂-syringyl > CHOH[sbnd]CH₃ π C₃H₇ ⁿ > CH₂OH > COOH > CHO > CO[sbnd]CH₃. Reaction of 1-guaiacylpropane in 1M potassium hydroxide with oxygen gave products of oxidative scission of the aromatic ring and no dehydrodimer, whereas at pH 11.5 some dehydrodimer was among the reaction products. Vanillyl alcohol and syringyl alcohol yielded vanillin and syringaldehyde, respectively, as minor oxidation products. However, the reaction sites for the series of phenols were generally the aromatic rings rather than the side-chains. Oxidation of alkaline solutions of the phenols with oxygen at 1.0 MPa pressure and 110 and 150°C gave similar mixtures of acids and hydroxyacids.     

Effects of Solvation and Ionization on 13C NMR Spectra of Lignin Model Compounds,Spruce Milled Wood Lignin and Kraft Lignin

The effects of alkali and polar aprotic solvent on the aromatic carbons signals in ¹³C NMR (Carbon-13 nuclear magnetic resonance) spectra of lignin model compounds and spruce milled wood lignin (MWL) were studied. It was found that in 1 M aqueous NaOH signal shifts of C-1 and C-4 carbon atoms in the aromatic ring were the most noticeable in lignin models with free phenolic hydroxyl groups, which are ionized under the conditions. A similar effect in the spectra of the studied model compounds was observed in 0.5 M aqueous NaOD-deuterated dimethyl sulfoxide (DMSO) mixture (DMSO: water ratio 3:7 v/v). The model data help explaining changes in the ¹³C NMR spectra of MWL and lignin in situ dissolved in spruce kraft black liquor caused by ionization. In the ¹³C NMR spectra of spruce black liquor the signals of phenolic and non-phenolic lignin units are clearly separated and do not overlap with the signals of the carbon atoms of carbohydrates and other aliphatic products of wood degradation. The data obtained are useful in understanding the important role of solvation and ionization processes leading to lignin solubilization.     

Hyperbranched polyols from hydroformylated methyl soyate

Hydroformylation of methyl soyate produces a mixture of fatty acid methyl esters with zero, one, two, and three hydroxyl groups, the major component being with two hydoxyls (around 50%). Polymerization of methyl esters of hydroxy fatty acids gives a hyperbranched product with a different content of hydroxyl groups depending on the degree of conversion. Molecular weights can be controlled by controlling the degree of conversion but also using monofunctional components. A range of hyperbranched polyols with acceptable viscosities and functionalities, suitable for flexible applications, was obtained by stopping the reaction at varying degrees of conversion. Monte-Carlo simulation of the polymerization of hydroxylated methyl soyate gave molecular weights and polydispersity which were compared with experimental values. Although hydroxylated methyl soyate contains considerable amounts of mono- and difunctional fatty acids, the system produces a physical gel at the highest conversions. This is due to very high molecular weights and was confirmed by experiments and the simulation. The simulation unexpectedly gave lower molecular weights but wider distribution than the experiments. This discrepancy was explained by the combination of experimental difficulties and possible side reactions leading to higher molecular weights. Functionality of polyols determined from gel points at critical NCO/OH ratios was reasonably close to predictions. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

针叶木硫酸盐浆中残留木素的GIF体系仿酶降解机理的研究

由于针叶木硫酸盐浆中的残留木素含有大量难降解的LCC结构及其它缩合型木素结构,所以硫酸盐浆的分离木素可以用作难降解型木素大分子的一种结构模型。文章采用Cu2+/吡啶/过氧化氢组成的GIF仿酶体系对马尾松硫酸盐未漂浆的纤维素酶酶解木素(CEL)进行了仿酶降解,并对降解产物进行了分离和化学组成分析。通过FT-IR以及GC-MS测定,解析了CEL在降解过程中的结构变化,并对CEL的仿酶降解机理进行了探讨。研究结果表明:这种仿酶降解方法对浆中残留木素有较强的降解能力,一部分木素大分子被氧化降解为香草醛和香草酸等低分子化合物,使得降解后的产物中羰基和羟基增多。研究发现木素在仿酶体系中存在苯环及其侧链上C-H键的破坏、木素结构中Ca-Cb连接键的断裂、苯环的开环等反应。因此,该项研究可以为开发纸浆的仿酶漂白技术提供理论依据。     

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。     

神府煤甲醇萃取物的GC/MS及FTIR分析

用GC/MS和FTIR技术结合气相色谱外标定量分析方法,对神府煤甲醇萃取物进行了定性和定量分析,讨论了神府煤甲醇萃取物的化学组成及其结构特征.结果显示,神府煤甲醇萃取物的GC/MS可测组分主要由多环芳烃和含氧化合物组成,其中芳烃以2～3环的烷基取代稠环芳烃为主,含氧化合物中以酮类和酯类化合物居多;结果还检测到种类较多的含氮化合物和少量的含硫、磷、氯的化合物.GC/MS不可测组分的IR分析显示,其中主要官能团有各类羟基、羰基、芳环和亚甲基、甲基等,表明其中可能存在分子量较大的稠环芳香族或其酚、酸、酮、酯类及烷基衍生物的化合物.     

31P NMR Spectroscopy in Wood Chemistry. I. Model Compounds

The quantitative reaction in an NMR tube of 1,3,2-dioxaphospholanyl chloride (I) with compounds bearing active hydrogens was explored as a simple method for derivatizing the labile centres known to occur in lignins. Derivatives of phenols, alcohols and carboxylic acids with (I) gave ³¹P chemical shifts which appeared in different ranges of the NMR spectra. Any ortho substitution onto the aromatic ring of phenols significantly affected the magnitude of the ³¹P NMR chemical shifts, while para and/or meta substituents had a much smaller effect. A clear distinction between guaiacyl, syringyl and unsubstituted phenolic hydroxyls can thus be made in mixtures of model compounds. Primary alcohols were clearly distinguished from secondary and tertiary alcohols in their derivatives with (I), while different ³¹P NMR signals for derivatives of erythro and threo forms of lignin-like model compounds were identified. While alcohols, phenols and simple carboxylic acids on reaction with (I) gave derivatives that were substitution products, aldehydes reacted via a distinctly different addition mechanism; ketones did not react at all.     

Mechanisms of Oxidative Degradation of Carbohydrates During Oxygen Delignktcation. I. Reaction of Methyl β-D-Glucopyranoside with Photochemically Generated Hydroxyl Radicals

Abstract

Hydroxyl radical is considered to be the major species causing degradation of carbohydrates during oxygen delignification. In this study, reactions involving a carbohydrate model compound and either photochemically generated hydroxyl radical or superoxide from potassium superoxide were carried out to investigate the cleavage of glycosidic linkages. Experiments show that hydroxyl radicals are responsible for the degradation of glycosidic linkages in methyl β-D-glucopyranoside by a substitution reaction displacing D-glucose. Once the glycosidic linkage is broken, reducing carbohydrates undergo a series of reactions forming aldonic acids and lower order aldoses. Control experiments established that no reaction occurs in the absence of UV light under otherwise identical conditions.     

木质素酚醛树脂的固化动力学及机理研究

通过差示扫描量热法（Dsc）和红外光谱（IR）分别研究了木质素酚醛树脂（LPF）的固化动力学及机理。DSC分析表明,LPF的固化反应起始温度比酚醛树脂高,终止温度及活化能比酚醛树脂低,因此,LPF凝胶点比酚醛树脂高,贮存稳定性比酚醛树脂好,固化速度比酚醛树脂快;IR分析表明,LPF的固化机理与酚醛树脂相似,都是羟基之间及羟基与芳环上的氢之间的缩合反应。不同的是LPF固化后仍有部分醚键结构。