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.     

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.     

2D Nmr Study of Residual Lignin in Beech Kraft Pulp Combined with Selective Cleavage with Pivaloyl Iodide

Structures of acetylated residual lignin obtained from unbleached beech kraft pulp were investigated by 2D NMR in combination with selective cleavage of non-phenolic α-alkyl ether bonds with pivaloyl iodide generated in situ from pivaloyl chloride / sodium iodide. β-O-4 and resinol structures still remained in this polymer lignin. Furthermore, carbohydrates were proved to be linked glycosidically to benzyl carbons of lignin. Possibly this inhibits complete delignification during kraft pulping.     

乙酰溴法对小兴安岭碱法杨木黑液提取物成分的研究

衍生化后的还原裂解(DFRC)法作为木质素结构解析的新方法,可以对碱法杨木黑液提取物中大分子结构的β-O-4醚键和α芳基醚键实施选择性的高效断裂.实验采用DFRC法对碱法萃取的杨木木质素进行降解,通过GC及GC-MS,分析研究了小兴安岭碱法杨木黑液提取物的成分和结构.结果表明:碱法杨木黑液提取物中主要包括4类化合物:B...     

木质素及其模型化合物热解研究进展

木质素是世界第二大可再生生物质资源,实现木质素的资源化、高值化利用对国民经济发展具有重要意义。木质素热解因具有成本低廉、操作简单、可获得高附加值产品等优点受到了学者的广泛关注。本文综述了β-O-4、α-O-4、β-5等木质素模型化合物的热解机理,比较了模型化合物中不同种类连接键断裂的难易程度及生成产物的区别;概述了木质素热解过程的特性、影响因素及产物分布规律,阐述了催化剂对热解过程的影响,尤其是金属盐类和分子筛对热解产物组成的影响规律和催化作用机制,详细介绍了木质素热解制备酚类化学品和生物油的关键控制因素及产品特点;针对现有木质素热解技术存在的机理不明确、目标产物选择性低、产物提纯困难等问题,对探索热解机理、开发新型催化剂及生产工艺、提高经济效益等发展方向进行了展望,为木质素资源化利用提供了理论基础。     

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.     

Absolute Configuration of β- and α-Asymmetric Carbons within β-O-4-Structures in Hardwood Lignin

In this study, we investigated the proportion of erythro- and threo-forms of β-O-4-ether structures and their enantiomeric compositions in hardwood lignin by applying the ozonation method to birch wood meal. Optical activity was not substantially observed in either the erythronic or threonic acids obtained as the ozonation products of β-O-4-structures in birch wood meal. The proportions of the four stereoisomeric forms {(αS,βR)-erythro, (αR,βR)-threo, (αS,βS)-threo, and (αR,βS)-erythro forms} were estimated to be 37-38%, 13-14%, 12-13%, and 36-37% based on the yields of erythronic and threonic acids, and on their optical activities. The proportions suggest that the entire components of β-O-4-ether structures in birch wood lignin have R- and S-configurations at the β-carbon in approximately the same quantities {(βR)-β-O-4-structure: (βS)-β-O-4-structure = 50–52:48–50}; i.e., that the β-ether structures are essentially racemic. This estimation implies that, during lignin biosynthesis, an equal number of enantiomeric forms of β-O-4-bonded quinone methides were formed by radical coupling reactions.     

Copper Catalysts for Selective C?C Bond Cleavage of β‐O‐4 Lignin Model Compounds

The reactivity of homogeneous copper catalysts towards the selective C C bond cleavage of both phenolic and non‐phenolic arylglycerol β‐aryl ether lignin model compounds has been explored. Several copper precursors, nitrogen ligands, and solvents were evaluated in order to optimize the catalyst system. Using the optimized catalyst system, copper(I) trifluoromethanesulfonate [Cu(OTf)]/L/TEMPO (L=2,6‐lutidine, TEMPO=2,2,6,6‐tetramethyl‐piperidin‐1‐yl‐oxyl), aerobic oxidation of the non‐phenolic β‐O‐4 lignin model compound proceeded with good selectivity for C_α C_β bond cleavage, affording 3,5‐dimethoxybenzaldehyde as the major product. Aerobic oxidation of the corresponding phenolic β‐O‐4 lignin model proceeded with different selectivity, affording 2,6‐dimethoxybenzoquinone and α,β‐unsaturated aldehyde products resulting from cleavage of the C_α C_aryl bond. At low catalyst concentrations, however, a change in selectivity was observed as oxidation of the benzylic secondary alcohol predominated with both substrates.

     

Studies of the Photodegradation of Spruce Lignin by Nmr Spectroscopy

The light-induced degradation of lignin was studied using spruce MWL impregnated on handsheets prepared from cotton linters. The sheets were irradiated with simulated sunlight and the lignin was extracted. Solutions of untreated and irradiated lignin were studied by ¹H and ¹³C NMR spectroscopy. The results showed that α- and β- ether and α-β bond cleavage were the main reactions. The main degradation products were vanillin and vanillic acid. The amount of terminal groups containing aromatic aldehyde and carboxyl groups also incre- ased on irradiation. The existing α- C=O groups were transformed mainly to carboxyl groups. However, the results indicate that new α- C=O groups were generated on irradiation. The methoxyl content decreased and some degradation of aromatic rings took place. However, no formation of quinoid structures was observed. Degradation of phenylcoumaran units was evident, α- Ether cleavage in phenylcoumarans could possibly lead to formation of stilbenes and stilbene derived chromophores.     

Pulping Catalysts From Lignin (7). Nitrogen Dioxide Oxidation Of A Lignin Model Dimer

Oxidation of a lignin model disyringyl dimer with nitrogen dioxide (NO₂) in the presence of air and N-hydroxysuccinimide led to C₁-C_αcleavage with the formation of approximately equal amounts of 2,6-dimethoxy-p-benzoquinone (DMBQ) and glyceraldehyde-2-syringyl ether type structures. The result indicates that only the phenolic end syringyl units of a lignin polymer will be converted to DMBQ upon treatment with the current NO₂ reaction conditions. Internal (non-phenolic) lignin units, bonded by β-O-4 linkages, will resist oxidation.     

Selective Depolymerization of Lignin: Assessment of Yields of Monomeric Products

A method is developed to assess the limits of selective depolymerization, that is, splitting of aryl ether bonds to yield monomeric aromatic products, of native and technical lignins. The method and respective formula is based on measuring the difference between a sum of aryl ether bonds (α-О-4 and β-О-4) and sum of 5-5 and 4-O-5 bonds. The values calculated based on the proposed formula are correlated with published experimental data on catalytic degradation of lignin. In all cases, wood species and experimental conditions notwithstanding, the yields of monomeric aromatic products are found to be close to theoretically calculated for both softwood (23%) and hardwood (51%). Notice that in the actual experiments the yield in hardwood is twice as much as in softwood. In technical lignins (kraft, organosolv) the yields of monomeric products are lowed due to splitting of α-О-4 and β-О-4 bonds during pulping. The developed method is helpful in evaluation of technologies aiming to bring value-added products from lignin through selective degradation.     

Delignification Mechanism During High-Boiling Solvent Pulping. IV. Effect of a Reducing Sugar on the Degradation of Guaiacylglycerol-β-guaiacyl Ether

Abstract

A phenolic β-O-4 type lignin model compound, guaiacylglycerol-β-guaiacyl ether was treated with 70 wt% aqueous 1,3-butanediol solution in the presence of glucose at 160–200°C to investigate the effect of reducing sugars on the degradation of lignin during high-boiling solvent (HBS) pulping. Addition of glucose increased the formation of guaiacol, coniferyl alcohol, and its γ-ethers, and decreased the formation of radical coupling compounds dramatically. These results suggest that reducing sugars may stabilize phenoxy radicals formed by homolysis of phenolic β-ethers. The kinetic studies also revealed that the disappearance of the β-ether model compound was enhanced substantially by the presence of glucose, which suggests that in addition to homolysis of the β-ether, a reducing sugar-assisted β-ether cleavage may be involved under the conditions used.     

THERMAL INDUCED HOMOLYTIC SCISSIONS OF LIGNIN INTERUNITARY BONDS IN THE HARDWOOD LIGNIN SOLUTION. AN ESR STUDY COMBINED WITH A SPIN TRAPPING METHOD

An electron spin resonance (ESR) method combined with a spin trapping reagent was successfully applied to trap and characterize unstable free radicals which were generated by heat-treatment of the dimethylsulfoxide (DMSO) solution of a hardwood, Japanese beech (Fagus crenata) lignin. It was found, consequently, that two unstable secondary carbon radicals, ～ CH? in the solution were created and the resulting radicals were trapped as the stable nitroxide spin adducts when the DMSO solution was heat-treated in the presence of a spin trapping reagent: 2,4,6-tri-tert-butylnitrosobenzene (BNB) at ca. 91°C. This means that so-called alkyl phenyl ether bonds, ～ CH-O- phenyl, known as important lignin interunitary bonds were homolytically scissoned by the heat-treatment of the lignin solution. Further the detailed analysis of the observed ESR spectrum revealed that two positions of alkyl phenyl ether bonds, i.e., β-O-4 and/or α-O-4 bonds as the interunitary linkages in the lignin are homolytically scissioned, although the phenoxy radical, Ph-O ? as the counter radical of the secondary carbon radicals was not trapped by the BNB spin trap. This suggests that fairly large steric hindrances operate between the syringyl with two methoxy moieties at the ortho positions and/or guaiacyl moieties with a methoxy moiety at the ortho position, and the BNB molecule bearing two bulky ortho tert-butyl groups in the phenyl ring.     

Zeolite beta catalysts for n-C7 hydroisomerization

Zeolite β with Si/2Al ratios of 60, 100, and 200 were synthesized using tetraethlammonium hydroxide (TEAOH) as the structure-directing agent (SDA) in the absence of alkali metal cations. Pt, Pd and Pt-Pd catalysts supported on the zeolite β samples were studied in n-heptane (n-C7) hydroisomerization. The Pt/β catalysts showed a higher catalytic activity than the Pd/β catalysts. For the Pt/β with a Si/2Al ratio of 100, its n-C7 conversion and selectivity of C7 isomers were observed to be 87.06% and 75.48% respectively at 250^°C. The activity of n-C7 conversion was stable for at least 82 h. However, the selectivity of C7 isomers was gradually decreased with the reaction time. Experimental data also showed that the addition of Pd to catalyst Pt/β enhanced the n-C7 conversion, but lowered the selectivity of C7 isomers. Pd catalyst was also observed to minimize the formation of aromatics in comparison with Pt catalyst.     

Analysis of phenolic compounds obtained from bamboo microwave liquefaction for fast-curing phenol-formaldehyde resin preparation

Three wastes originated from outer surface, shallow surface, and inner surface of bamboo during bamboo-mat preparation were liquefied using microwave energy in a binary solvent (glycerol/methanol). The properties (yield, molecular structure, and molecular weight distribution) of the lignin fractions in the obtained phenolic compounds were contrastively analyzed with proto-lignin in the materials. Results showed that >58% of the lignin in the bamboo cell walls could be degraded and moved to the phenolic compounds via the cleavage of β-O-4′ aryl ether bonds during the microwave liquefaction process. Some polysaccharides still connected with the lignin fraction in the phenolic compounds through covalent bonds. The phenolic compounds displayed well solubility in common organic solvents (ethanol/water cosolvent, acetone, tetrahydrofuran, 1,4-dioxane, and dimethyl sulfoxide). A fast-curing phenol-formaldehyde resin was successfully synthesized with the phenolic compounds under the presence of MgO, and a 23.23% curing time reduction was achieved. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46952.     

Effects of Cα -Oxidation in the Fungal Metabolism of Lignin

C_α-Oxidation (benzyl alcohol oxidation) is a prominent reaction in the degradation of lignin by white-rot fungi. This study showed that such oxidation markedly retards metabolism of a nonphenolic β-O-4 model compound, 1-(3-methoxy-4-ethoxyphenyl)-2-(o-methoxyphenoxy)propane-1,3-diol, by cultures of Phanerochaete chrysosporium Burds. Surprisingly, however, selective chemical C_α -oxidation of spruce lignins enhanced their depolymerization by the cultures. Thus the decrease in intrinsic degradability of substructures is more than compensated by another effect of C_α-oxidation in lignin. One possibility is that the oxidation increases the accessibility of the lignin to enzymes by decreasing its steric complexity. This study also revealed that the β-O-4 model, like lignin in wood, is degraded in part via C_α-oxidation by P. chrysosporium. Reduction of the α-carbonyl groups thus formed does not occur. Addition of L-glutamate to ligninolytic cultures completely suppresses their competence to degrade the model compound, as it does their ability to oxidize lignin to CO₂. This result strengthens past evidence indicating that substructure models are metabolized by the same enzyme system as lignin.     

Thermomechanical Pulp and Light -Photoactivity of α-Carbonyl Group in Solid Lignin

Interaction of sunlight and UV light with thermomechanical (TMP) pulps, milled wood lignin (MWL) and various lignin model compounds in solid state was studied. It was found that TMP fibers were sensitive to these lights. During exposure, TMP lost its brightness as a function of irradiation time and increased its yellowness. The α-carbonyl group functioned as a light absorption center to promote photocleavage of C_α—C_β and β–0–4 linkages. Electron spin resonance (ESR) studies demonstrated that free radicals were generated in the interim. Phenoxy radicals appeared to be important radical intermediates that ultimately transformed into o—quinonoid structures.     

The Influence of Pulping Conditions on the Structure of Acetosolv Eucalyptus Lignins

Acetosolv lignins obtained under a variety of pulping conditions as regards acetic acid concentration, catalyst concentration, temperature and pulping time were characterized structurally by elemental analysis, FTIR and ¹H and ¹³C NMR spectroscopy, and determination of methoxyl group content. Increasing catalyst concentration and pulping time favours both the dissolution of lignin (by cleavage of α and β-O-4 bonds) and the occurrence of condensation reactions. Increasing temperature favours the degradation of syringyl units more than of guaiacyl units.     

Oxidative cleavage of C—O and benzene ring in lignin catalyzed by polyoxometalate ionic liquids

Lignin is the most abundant renewable aromatic polymer in nature and has the potential to replace or partially replace petroleum for the production of bulk chemicals. However, due to the complex molecular structure and chemical bond connection of lignin, the current comprehensive utilization of lignin generally has problems such as low target product selectivity. Therefore, the development of novel catalysts plays a significance role in the utilization of lignin. Herein, a novel and efficient polyoxometalate ionic liquid (POM-IL) catalytic system was provided for the conversion of lignin to diethyl maleate (DEM), a universally used biochemical, through the selective cleavage of the C—O bond and benzene ring of lignin in the ethanol solvent. 4-Methoxy-α-[(2-methoxyphenoxy)methyl]-benzenemethanol, a representative lignin model compound containing b-O-4 linkage, is used as the feedstock. Over [BSmim]CuH₂PMo₁₀V₂O₄₀, 98.5% of the lignin model compound can be converted at 180℃ for 5 h, affording 84.3%(mass) yield and 62.2% selectivity of DEM. Furthermore, this POM-IL could be handily recovered through self-separation with temperature control. Moreover, the catalyst of [BSmim]CuH₂PMo₁₀V₂O₄ shows good recyclability, where a satisfactory DEM yield is demonstrated even after six consecutive runs. Moreover, a plausible pathway for this process was proposed. Therefore, this work provides a new route for the selective conversion of renewable lignin to bulk petrochemicals.     

基于氢化断裂木质素及其模型化合物C-O键的研究进展

基于不同氢化断键模式,从催化剂类型综述了目前木质素及其模型化合物的氢化断裂C-O键研究的进展。为木质素及其模型化合物的研究提供相关的理论和实践基础。