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.     

Substituent Effects on 13C Chemical Shifts of Aromatic Carbons in β-O-4 and β-5 type Lignin Model Compounds

Abstract

Substituent effects on the chemical shifts of aromatic carbons in lignin model compounds have been elucidated from ¹³C NMR spectra of guaiacyl and syringyl type monomerio and β-O-4 model compounds and guaiacyl type β-5 model compounds. Evaluation of the observed values of substituent chemical shift (SCS) for the aromatic carbons leads to elucidation of a generalized SCS additivity rule, for estimation of the chemical shifts of aromatic carbons in ring A of β-O-4 and β-5 type substructures in model compounds and in ring B of β-O-4 substructures in lignin preparations, with errors of less than 1 ppm. The rule is applicable to substructures of both guaiacyl and syringyl types, using an appropriate parent compound as reference instead of benzene. Signals in the aromatic region of the ¹³C NMR spectra of β-O-4 and β-5 type model compounds are reassigned on the basis of the observed SCS's as well as APT spectra of the compounds.     

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.     

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.     

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.     

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.     

The Characteristics and Reactivity of Acid-Labile Aryl Ether Units in Wood Lignin

Wood lignin contains significant amounts of acid-labile aryl ether units, which play a significant role in lignin modification or delignification processes. We have evaluated the rate and reaction kinetics on the acid-catalyzed cleavages of aryl ether structures for wood lignin in situ based on the formation of phenolic hydroxyl groups. The content of acid-labile aryl ether units was quite uniform for a variety of softwood wood lignins (～4% per C₉ unit) and it varied appreciably among hardwood species, ranging from 4% for aspen to 9% for beech wood lignin. These variations, however, appear to be related to the content of syringyl units in wood lignin. The reactivity of these reactive aryl ether structures was noticeably higher for the spruce than for the aspen wood lignin. This difference in reactivity, based on the behavior of lignin model compound reactions, can be attributed to the influence of syringyl moieties in aspen wood lignin. It appears that most of the acid-labile aryl ether units in hardwood were associated with the syringy moiety being present as a benzyl unit, which is much less reactive than the corresponding guaiacyl moiety.     

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.     

Characteristics and Reactivity of Lignin in Acacia and Eucalyptus Woods

The chemical characteristics of lignin, including the Klason lignin content, acid-soluble lignin content, proportion of the aromatic ring types [syringyl ratio = syringyl/(syringyl + guaiacyl)], and proportion of the diastereomeric forms for the β-O-4 structure [erythro ratio = erythro/(erythro + threo)], and pulpability were investigated for 28 wood samples belonging to the genera Acacia and Eucalyptus. Although the lignin characteristics of these 28 woods varied widely, the chemical characteristics of the two genera could be categorized in two clearly distinguished groups on the basis of the syringyl ratio. Clear negative correlations were observed between both the syringyl and erythro ratios and the total lignin content (sum of the Klason and acid-soluble lignin contents) within each genus. In addition, the syringyl ratio correlated positively with the erythro ratio and acid-soluble lignin content, regardless of the genus. The existence of a clear, high correlation between the syringyl and erythro ratios supports the hypothesis that the aromatic ring type (syringyl ratio) is a decisive factor for controlling the diastereomeric forms of the β-O-4 structure (erythro ratio). Each of the wood samples was also subjected to Kraft pulping, and it was demonstrated that the woods with higher syringyl ratios were easier to delignify. This tendency is reasonably attributed to the high reactivity of the erythro-rich and syringyl-rich β-O-4 structures, and the low lignin content of these syringyl-rich woods.     

Determination of the Conformation and Isomeric Composition of Lignin Model Quinone Methides By NMR

Abstract

Proton and ¹³C NMR of representative guaiacyl and syringyl β-aryl ether quinone methides have shown that guaiacyl quinone methides, generated from the corresponding benzyl bromides, exist as isomeric pairs in approximately a 70:30 ratio, the major isomer having the 3-methoxyl group syn with respect to the side chain. The ring protons at the 2- and 6-positions in the syn-and the anti-isomera respectively are markedly deshielded by 3teric compression effects; concomitant shielding of these ring carbons is observed in the C-13 NMR. Nuclear Overhauser enhancement experiments define the major solution conformation of these species, the conformation being consistent with the observed predominance of threo-products resulting from nucleophilic addition reactions.     

Relationships between Hemicellulose Composition and Lignin Structure in Woods

The composition and absolute amount of neutral sugars were determined for 48 hardwood species (including 17 hardwoods of genera Acacia, 14 hardwoods of genera Eucalyptus, and 17 hardwoods of other genera) and 14 softwood species by alditol-acetate method, and their relationships to the syringyl ratio (syringyl/(syringyl+guaiacyl)) of lignin, which was determined by nitrobenzene oxidation, was investigated. In the hardwood species, an increase in the syringy ratio of lignin correlated with a tendency toward increased xylose/glucose, rhamnose/glucose, and arabinose/glucose ratios. However, the absolute amount of glucose in hardwood was maintained in a small range (0.4–0.5 g in 1 g sample), independent of changes in the syringyl ratio. In the softwood species, with increasing lignin content, the mannose/glucose ratio decreased, but the absolute amount of glucose remained almost constant. In both hardwood and softwood species, a strong correlation was suggested between lignin, indicated by higher syringy ratio, and hemicellulose, indicated by higher xylan/mannan ratio.     

Molecular Characteristics of Kraft-AQ Pulping Lignin Fractionated by Sequential Organic Solvent Extraction

Kraft-AQ pulping lignin was sequentially fractionated by organic solvent extractions and the molecular properties of each fraction were characterized by chemical degradation, GPC, UV, FT-IR, (13)C-NMR and thermal analysis. The average molecular weight and polydispersity of each lignin fraction increased with its hydrogen-bonding capacity (Hildebrand solubility parameter). In addition, the ratio of the non-condensed guaiacyl/syringyl units and the content of β-O-4 linkages increased with the increment of the lignin fractions extracted successively with hexane, diethylether, methylene chloride, methanol, and dioxane. Furthermore, the presence of the condensation reaction products was contributed to the higher thermal stability of the larger molecules.     

pKa-Values of Guaiacyl and Syringyl Phenols Related to Lignin

Abstract

The literature relating to the pK_a -values of guaiacyl- and syringyl-derived phenols has been thoroughly surveyed and summarized. In addition, the pK_a -values of a number of guaiacyl, syringyl and other phenols related to lignin have been determined using a spectrophotometric method combined with multivariate evaluation. Differences and similarities between the acidities of a number of substances are extensively discussed. The pK_a -value strongly affects the delignification during pulping, bleaching and leaching of lignin during pulp washing.     

Pulping with organic acids: 3-acetic acid pulping of Bagasse

Bagasse was subjected to acetic acid pulping; the effects of the addition of 0.5% H₂SO₄ to the pulping solutions (50, 70 and 90% acetic acid) and varying the liquor ratio, on pulp yield, delignification %, pentosan content and strength properties of the pulps obtained were evaluated. Addition of H₂SO₄ improved the delignification but at the expense of yield and pentosan content. The breaking length and burst deteriorated, whereas the tear was slightly improved. Raising the liquor ratio from 5:1 to 10:1 also reduced the strength properties of the pulps. Delignification of Casuarina was easier than bagasse. IR studies of the acetic acid lignins obtained from both raw materials showed that Casuarina lignins contained more syringyl nuclei than bagasse lignin; the latter resembled softwood lignins. Addition of H₂SO₄ to bagasse changed the syringyl-deficient to guaiacyl-deficient lignin. A relationship was found between the ease of delignification for both raw materials and the relative intensity of the bands corresponding to syringyl groups.     

Brightness Reversion of Mechanical Pulps Part XIII: Photoinduced Degradation of Lignin on Cellulose Matrix

Abstract

An acidic dioxane extraction procedure was employed to isolate lignin from softwood bleached chemithermomechanical pulp (BCTMP). The isolated lignin was characterized spectroscopically and applied on cellulose test sheets. The treated cellulose test sheets were shown to exhibit photoyellowing properties comparable to those of BCTMP. The photolyzed lignin was re-isolated and characterized using spectroscopic techniques. UV-VIS and FT-IR studies implied elimination of the guaiacyl structure of lignin and the formation of carboxyl and/or unconjugated carbonyl groups during the photoyellowing process. The steady-state emission intensity of the excited states of guaiacyl structures decreased with increasing photolysis time, suggesting the formation of quinones during photolysis. NMR spectroscopic analysis of the photolyzed lignin indicated that the formyl, methoxyl, and uncondensed phenolics decreased in concentration as the irradiation proceeds. In contrast, condensed lignin and carboxylic acids increased as photolysis time was increased. These results are explained within the mechanism of lignin photodegradation.     

Rapid Hydrothermolysis of Poplar Wood: Comparison of Sapwood,Heartwood, Bark,and Isolated Lignin

Abstract

Rapid hydrothermolysis at 350°C of poplar sapwood, heartwood and bark, as well as the lignin extracted from the sapwood and heartwood, gives oil and water soluble fractions whose chemical analysis is correlated with the nature of the feed stock. Results from cellulose and Douglas fir are included, and the poplar bark and heartwood lignin are shown to give an ether soluble oil that consists mainly of guaiacyl units while the sapwood product is mainly syringyl. However, the chloroform soluble oil from both sapwood and heartwood was mainly syringyl with the additional presence of carbohydrate derived material in the heartwood oil. The acetone-only soluble residue from sapwood was shown by pyrolysis mass spectrometry to be mainly cellulose derived while that from heartwood was mainly from lignin. Thus there is a fundamental difference in the behaviour of the poplar sapwood and heartwood that may arise from different distributions of the guaiacyl and syringyl units, and of the binding to the cellulose. The origin of the rapid hydrothermolysis products with respect to the major wood components is discussed. An empirical approach to the calculation of the lignin content based on the rapid hydrothermolysis fractions is presented.     

BEHAVIOR OF EUCALYPTUS GLOBULUS LIGNIN DURING KRAFT PULPING. I. ANALYSIS BY CHEMICAL DEGRADATION METHODS

ABSTRACT

Eucalyptus globulus wood was subjected to kraft pulping, reaching different extents of delignification. The residual and dissolved lignins were isolated by soft acidolysis and acidic precipitation, respectively, and submitted to analysis of residual sugars and methoxyl groups, as well as to analysis by nitrobenzene and permanganate oxidation and thioacidolysis. Results from both residual and dissolved lignins analyses indicated that in the initial phase of pulping there is a preferential removal of lignin enriched in guaiacylpropane (G) and p-hydroxyphenylpropane (H) units, which is highly condensed and bonded with polysaccharides, predominantly with xylan. During the bulk and residual phases of pulping, lignin enriched in syringylpropane (S) units is progressively removed, leading to the increase of S/G ratio of black liquor lignin. After a purification step of dissolved lignins, two fractions with distinct levels of carbohydrates and with different structural features were obtained. The overall results were interpreted in terms of the topochemistry of the kraft pulping process.     

Analysis of Lignin Aromatic Structure in Wood Based on the IR Spectrum

Abstract

A total of 17 softwoods and 48 hardwoods were analyzed by IR spectroscopy to examine if syringyl ratio (syringyl/(syringyl+guaiacyl)) calculated from nitrobenzene oxidation products can be precisely expressed by area ratios of characteristic peaks of lignin in IR spectrum. Area ratio of two peaks is referred to as that of two wavenumber domains, represented by “wavenumber 1/ wavenumber 2.” Examined peak area ratios were 1595/1509, 1509/1460, 1275/1220, 1130/1032, and 835/(855+815). Among these ratios, log(1595/1509) and log(1275/1220) showed significant linear relationship with the syringyl ratios with a correlation coefficient of 0.98 for all 65 woods. These two ratios could also be used to distinguish all the hardwoods from the softwoods.     

Investigation of the Chemistry of Oxygen Delignification of Low Kappa Softwood Kraft Pulp using an Organic/Inorganic Chemical Selectivity System

Abstract

Oxygen delignification (OD) of low kappa softwood kraft pulp was examined in two steps without inter‐stage washing as part of an overall program to evaluate the efficiency of a selectivity enhancement system consisting of phenol and magnesium sulfate. Black liquor carryover in the reaction system did not substantially affect delignification and the selectivity of these OD reactions. The residual lignins from both the original pulp and oxygen‐delignified pulp with and without the phenol/MgSO₄ selectivity enhancement system were prepared and characterized using NMR spectroscopy. The effluent lignins after oxygen delignification were also prepared and characterized. The lignin characterizations provided the basis for the rationalization of the selectivity observed. A significant finding of this study was that the phenol/MgSO₄ system in the oxygen delignification reaction appeared to hinder phenolic guaiacyl unit condensation. It also appeared to enrich the levels of p‐hydroxyphenyls in the residual lignin.     

Insoluble Lignin Models (4): Condensation Reactions of a Polymer-Bound Guaiacylpropanol Model

The reactions of syringyl alcohol with itself and with either a soluble phenol, 3-methoxy-4-hydroxyphenylpropane (guaiacylpropane, 13) or an insoluble polymer-bound phenol, guaiacylpropanol (14)¹, led to dimeric products. The low (11%), but significant yield of C5-Cα dimer 16 in the reaction of polymer-bound phenol with syringyl alcohol suggests that soluble lignin units are able to condense with insoluble lignin gels during the course of soda pulping. The yield of the C5-Cα dimer 15 in the soluble system was four times greater than that of the analogous heterogeneous dimer 16. Reactions of the polymer model 14 were not mass transfer limited.