Investigation of the chemical modifications of beech wood lignin during heat treatment

Holocellulose, Klason lignin and milled wood lignin (MWL) of beech wood were extracted before and after heat treatment and analysed using CP MAS ¹³C NMR, ¹³C NMR, ³¹P NMR and size exclusion chromatography (SEC). Experimental results showed that the thermal treatment degrades hemicelluloses and affects lignin polymer through depolymerisation due mainly to cleavage of β-aryl-ether linkages and recondensation reactions. The spectroscopic analysis of MWL demonstrated that these recondensation reactions involved mainly guaiacyl units through formation of 5,5′-biphenolic and diarylmethane structures.Analysis of molecular weight distribution of MWL by SEC indicated that average molecular weights of heat treated milled wood lignin were lower than those of native milled wood lignin.     

Proton transfer and coordination properties of aromatic α-hydroxy hydrazones

The prototropic properties of two nitro derivatives of aromatic α-hydroxy hydrazones [2,4-dinitro-N-phenyl-N′(2-hydroxy-1-phenylmethylene hydrazine) and 2,4-dinitro-N-phenyl-N′(2-hydroxy-1-naphthylmethylene hydrazine] have been analysed through electronic absorption, ¹H and ¹³C NMR and semiempirical molecular orbital methods. The metal binding properties of both compounds and of the unsubstituted hydrazones were evaluated from potentiometric and spectrophotometric data in dioxane-water mixtures. On the basis of the spectral analysis and PM3 semiempirical calculations, the structures of the formed complexes were proposed.     

Isolation and comparative characterization of a Björkman lignin from the saponified cork of Douglas-fir bark

Cork from Douglas-fir bark (Pseudotsuga menziesii) was separated, extracted and submitted to suberin depolymerization by transesterification with sodium methoxide in methanol. As a result a saponified cork fraction (cork_sap) with a yield of 19% was obtained. From cork_sap a milled cork lignin (MCL_sap) was isolated using the Björkman procedure with a yield of 0.75% (based on cork_sap, corresponding to 0.14% based on original cork). The isolated Douglas-fir MCL_sap was characterized by elemental analysis and OMe determination, FT-IR spectroscopy and analytical pyrolysis (Py-GC/FID). Data are presented in comparison with those of MCL_sap isolated from cork-oak (Quercus suber) and milled wood lignins (MWL) of spruce. All data revealed that the Douglas-fir cork lignin has a guaiacyl-type structure (G-lignin). Results of Py-GC/FID demonstrate that this polymer consists of approximately 97% guaiacyl, 2% p-hydroxyphenyl and 1% syringyl units.     

Structure and Pyrolysis Characteristics of Lignin Derived from Wood Powder Hydrolysis Residues

Physicochemical characteristics of wood powder acid hydrolysis residue (WAHR) were studied firstly in this study, and WAHL (lignin derived from WAHR) was separated successfully from WAHR based on an improved isolating method. The content of functional group such as phenolic hydroxyl group of guaiacyl, syringyl, and hydroxyl?Cphenyl units in WAHL were identified by ³¹P-NMR and DFRC (derivatization followed by reductive cleavage) method. Thermal degradation experiments were carried out on a thermogravimetric (TG) analyzer to show pyrolysis characteristics of WAHL. The compositions of pyrolysis products of WAHL were also studied throughout a pyrolysis?Cgas chromatography?Cmass spectrometry (Py?CGC?CMS) analyzer. It was shown that the pyrolysis of WAHL took place in a wide temperature range and there were two obvious peaks in the differential thermogravimetric diagram. Results of Py?CGC?CMS analysis indicated that pyrolysis products were mainly formed through cleavage of the ??-O-4 connection and multiple pyrolysis.     

Stabilization effect of lignin in natural rubber

A series of carbon black filled natural rubbers containing lignin was tested from the view point of their thermo-oxidative aging. Lignin is biopolymer that belongs to the main components of wood. Mechanical properties and crosslink density of lignin stabilized vulcanisates were measured before and after thermo-oxidative aging for 24, 72, 168, 240 and 408 h at 80 °C. The results were compared with those from NR vulcanisates stabilized with the commercial rubber antioxidant N-phenyl-N-isopropyl-p-phenylene diamine (IPPD). The results obtained show that lignin exerts a stabilizing effect in carbon black filled natural rubber. Its effect is comparable with that of conventional synthetic antioxidant. Moreover, the addition of lignin increased the stabilizing effect of IPPD.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. II. Fixation of the Cu/Cr system on wood

The kinetic behavior of a Cr⁶⁺/Cu²⁺ system in its reaction with wood has been elucidated. The same reactions with carbohydrates and lignin of wood for Cr^VI alone are still present. Approximately 20% of the copper is fixed to the guaiacyl units of lignin in the form of copper chromate; the remaining copper forms complexes with cellulose and mainly with the guaiacyl units of lignin. The Cr^VI/lignin guaiacyl unit complexes already found for the treatment of wood with CrO₃ are still the main Cr^VI complexes formed. Copper chromate is present as [Cu(H₂O)₂(guaiacol)]CrO₄(guaiacol) complexes, and CuCrO₄ bridges between different guaiacyl units of the lignin network appear to be likely. Substantial interference of the Cu²⁺ ions in the second-zone reactions of Cr^VI with wood occurs. The amount of Cr^VI and Cr^III fixed onto lignin and cellulose compares well with experimental values. Rate constants, energies of activation, and Arrhenius equations of the various reactions have been obtained. The amount of Cu²⁺ interference in the reaction has also been calculated.     

Fractionation and physico-chemical analysis of degraded lignins from the black liquor of Eucalyptus pellita KP-AQ pulping

The degraded Eucalyptus pellita kraft lignin from the black liquor of KP-AQ pulping was precipitated directly at pH ∼2.0 without further purifying, since the lignin obtained is more representative with a whole distribution of molecular weight. The precipitated lignin was fractionated into six fractions by successive extraction with organic solvents. A comparison study of the lignin heterogeneity between the fractions was made in terms of fractional yield, content of associated polysaccharides, alkaline nitrobenzene oxidation, molecular weight distribution, ¹H NMR and ¹³C NMR spectroscopy and thermal stability. It was found that the lignin fractions contained higher associated hemicelluloses and ratios of non-condensed syringyl/guaiacyl units which were extracted by organic solvents with higher Hildebrand solubility parameters. The results from GPC and TGA showed that the polydispersity and the thermal stability of the lignin fractions increased with increasing molecular weight. In the low molecular weight fraction, small amounts of β-aryl ether bond (β-O-4) surviving the KP-AQ pulping were detected by both ¹H and ¹³C NMR spectra.     

Extraction products from <Emphasis Type="Italic">Miscanthus</Emphasis> var. ‘Soranovskii’

The chemical structures of Miscanthus var. ‘Soranovskii’ lignin fractions released via extraction of lignin from the lignocellulosic feedstock using moderately heated acetone under atmospheric pressure, without acidic and alkaline catalysts, were studied. A blend of Miscanthus stems and leaves was pretreated with water under thermobaric conditions. The acetone organosolv process subsequently afforded a substance related to a lignin-like matter-acetone organosolv Miscanthus lignin (AOML). Non-destructive analytical techniques such as FTIR spectroscopy, gas chromatography-mass spectrometry, size-exclusion chromatography, and 2D NMR were used. The IR and NMR spectroscopies revealed the AOML structure to comprise all the three major types of phenylpropane units: guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H). The resultant acetone-organosolv lignin exhibits good solubility in polar solvents, moderate solubility in aromatic chemicals, and is insoluble in non-polar solvents, exhibiting the physicochemical properties of a thermoplastic polymer with a softening point of 67.0°C (onset 33.0°C, endset 81.5°C).     

Association behaviour of lignins and lignin model compounds studied by multidetector size-exclusion chromatography

SEC elution curves of spruce milled wood lignin (MWL) and guaiacyl lignin polymer models (G-DHPs) in N,N-dimethylformamide (DMF) exhibited a bimodal elution profile. Light scattering measurements indicated that these elution profiles were due to association effects between the molecules. This became apparent from the determination of high molar masses in the range 10(5)-10(8) g/mol. To study this effect, MWL and DHP were fractionated by precipitation in tetrahydrofuran (THF). The THF-insoluble fractions were found to be the fractions corresponding to the apparent high molar mass part of the DMF elution profiles. The THF-soluble fractions proved to be the less-associated fractions, with lower apparent molecular mass. The individual fractions proved to be rather stable in DMF. Accordingly, the bimodal elution profiles of the starting materials were not the result of an equilibrium between associated and molecular dispersed molecules but of different structures exhibiting a specific and stable association pattern. The different fractions were further characterised by SEC in THF after acetylation to determine molar masses in molecular disperse solutions.     

COSMO-RS Based Predictions for the Extraction of Lignin from Lignocellulosic Biomass Using Ionic Liquids: Effect of Cation and Anion Combination

A total of 34 cations belonging to 6 classes and 34 anions resulting in 1156 possible combinations are screened using the quantum chemical based COSMO-RS (COnductor-like Screening MOdel for Real Solvents) model. The Hildebrand solubility parameter (?? _H) is calculated using the predicted infinite dilution activity coefficient (?? ^??) of lignin in ionic liquids at 303.15 K. Initial benchmarking is performed by predicting the Hildebrand solubility parameter of lignin in ionic liquids. Comparison with literature values involving 12 ILs gives the average root mean square deviation (RMS) as 10.15?%. Except for anions based on hexafluorophosphate [PF₆], bis(oxalato(2)borate) [BOB], tetracyanoborate [B(CN)₄] and bis(trifluoromethylsulfonyl)amide [BTA], all the cation?Canion combinations have calculated solubility parameters equal to that of lignin at 303.15 K, indicating high solubilities for lignin.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. IV. Fixation of CCA to wood

The kinetic behavior of copper–chromium–arsenic (CCA) wood preservatives in their reaction with wood, lignin, cellulose, and their respective simple model compounds, guaiacol and D(+)-glucose, has been elucidated. Copper chromates^VI/lignin guaiacyl units and chromium^III arsenates/lignin guaiacyl units, which are stable and insoluble complexes formed during the reaction, have been characterized. No copper arsenates were formed during the reactions (which were carried out using a CCA of type C). The sequence, nature, and combined effect of the reactions of CCA with wood, lignin, and cellulose are similar to that already described for the “CrO₃ alone,”¹ Cu/Cr², and Cr/As³ systems. Rate constants, energies of activation, and Arrhenius equations have been obtained and reported. The distribution of Cr^VI, Cr^III, As^v, and Cu²⁺ in CCA-C-treated wood has been calculated. The results obtained have allowed meaningful comparisons of several commercial CCA formulations.     

DFT study of the reaction sites of N,N′-substituted p-phenylenediamine antioxidants

The geometry of N,N′-diphenyl-p-phenylenediamine (DPPD), N-phenyl-N′-(1′-methylbenzyl)-p-phenylenediamine (SPPD), N-phenyl-N′-(1,3-dimethyl-butyl)-p-phenylenediamine (6PPD), N-phenyl-N′-isopropyl-p-phenylenediamine (IPPD), and N-(1-methyl-1-phenylethyl)-N′-phenyl-p-phenylenediamine (CPPD) as well as of their dehydrogenation products has been optimized at B3LYP/6-31G^∗ level of theory. Our results support the idea of formation of stable ketimine Ph-NC structures (instead of quinonediimine structures) during consecutive dehydrogenation of SPPD, 6PPD, and IPPD antioxidants despite the formation of tertiary carbon-centered radicals in the first dehydrogenation step is energetically preferred for SPPD only.     

Synthesis and Characterization of New 5‐Linked Pinoresinol Lignin Models

Pinoresinol structures, featuring a β‐β′‐linkage between lignin monomer units, are important in softwood lignins and in dicots and monocots, particularly those that are downregulated in syringyl‐specific genes. Although readily detected by NMR spectroscopy, pinoresinol structures largely escaped detection by β‐ether‐cleaving degradation analyses presumably due to the presence of the linkages at the 5 positions, in 5‐5′‐ or 5‐O‐4′‐structures. In this study, which is aimed at helping better understand 5‐linked pinoresinol structures by providing the required data for NMR characterization, new lignin model compounds were synthesized through biomimetic peroxidase‐mediated oxidative coupling reactions between pre‐formed (free‐phenolic) coniferyl alcohol 5‐5′‐ or 5‐O‐4′‐linked dimers and a coniferyl alcohol monomer. It was found that such dimers containing free‐phenolic coniferyl alcohol moieties can cross‐couple with the coniferyl alcohol producing pinoresinol‐containing trimers (and higher oligomers) in addition to other homo‐ and cross‐coupled products. Eight new lignin model compounds were obtained and characterized by NMR spectroscopy, and one tentatively identified cross‐coupled β‐O‐4′‐product was formed from a coniferyl alcohol 5‐O‐4′‐linked dimer. It was demonstrated that the 5‐5′‐ and 5‐O‐4′‐linked pinoresinol structures could be readily differentiated by using heteronuclear multiple‐bond correlation (HMBC) NMR spectroscopy. With appropriate modification (etherification or acetylation) to the newly obtained model compounds, it would be possible to identify the 5‐5′‐ or 5‐O‐4′‐linked pinoresinol structures in softwood lignins by 2D HMBC NMR spectroscopic methods. Identification of the cross‐coupled dibenzodioxocin from a coniferyl alcohol 5‐5′‐linked moiety suggested that thioacidolysis or derivatization followed by reductive cleavage (DFRC) could be used to detect and identify whether the coniferyl alcohol itself undergoes 5‐5′‐cross‐linking during lignification.     

Lignin-degrading enzyme fromPhanerochaete chrysosporium

The extracellular fluid of ligninolytic cultures of the white-rot wooddestroying fungus,Phanerochaete chrysosporium Burds., contains an enzyme that degrades lignin model compounds as well as lignin itself (1). Like ligninolytic activity, the enzyme appears during idiophasic metabolism, which is triggered by nitrogen starvation. The enzyme has been purified to homogeneity by DEAE-Biogel A chromatography, as assessed by SDS polyacrylamide gel electrophoresis, isoelectric focusing, and gel filtration chromatography. These techniques also revealed a molecular weight of 42,000 daltons, and an isoelectric point of 3.4. The purified enzyme exhibits low substrate specificity. It is an oxygenase, but requires hydrogen peroxide for activity. The activity is optimum at 0.15 mM H₂O₂; at concentrations above 0.5 mM, H₂O₂is inhibitory. Model compound studies have shown that the enzyme catalyzes cleavage between Cα and Cß in compounds of the type aryl-CαHOH—CßHR-(R = -aryl or -O-aryl), and in the Cα-hydroxyl-bearing propyl side chains of lignin. This cleavage produces an aromatic aldehyde moiety from the Cα-portion, and a Cß-hydroxylated moiety from the Cα-portion. Cleavage between Cα and Cß in arylglycerol-Β-aryl ether structures leads indirectly to cleavage of the Β-aryl ether linkage, which is the most abundant intermonomer linkage in lignin. The Cß-hydroxyl oxygen comes from molecular oxygen, and not from H₂O₂, as determined by¹⁸O isotope studies. The pH optimum for these reactions is between 2.5 and 3.0; no activity is observed above pH 5. Formation of the expected aldehydes from spruce and birch lignins, and partial depolymerization of the lignins results from the action of the purified enzyme. In addition to Cα—Cß cleavage, the enzyme catalyzes aromatic alcohol oxidation, aryl methylene oxidation, hydroxylation at Cα and Cß in models containing a Cα—Cß double bond, intradiol cleavage in phenylglycol structures, and phenol oxidations.     

Homo- and hetero-nuclear 2D NMR techniques: Umambiguous structural probes for non-cyclic benzyl aryl ethers in soluble lignin samples

A number of two-dimensional NMR experiments are evaluated with respect to their ability to provide unambiguous evidence for the presence or absence of non-cyclic benzyl aryl ether (α-O-4) structures in soluble lignin samples. The most suitable techniques, in terms of both sensitivity and structural information content were the homonuclear Hartmann-Hahn (HOHAHA) and heteronuclear multiple quantum coherence (HMQC) techniques. By spiking an acetylated Pinus radiata MWL sample with an oligomeric α-O-4 ether model compound, it was possible to determine a detection limit of <0.3 structures per 100 lignin C9 units for both the HOHAHA and HMQC techniques. From this, and other work, it can be shown that if α-O-4 structures are present in MWL samples, they are present at a level below the detection limit of these experiments. The implications of these results are discussed in terms of lignin biosynthesis and reactivity.     

Presence of 5-hydroxyguaiacyl units as native lignin constituents in plants as seen by Py-GC/MS

The presence of 5-hydroxyguaiacyl moieties in the lignin from several plants has been assessed by Py-GC/MS. Different woody (eucalypt) and nonwoody (flax, hemp, kenaf, jute, sisal and abaca) angiosperms were selected for this study. The pyrolysis of whole fibers released lignin-derived products with p-hydroxyphenyl, guaiacyl and syringyl structures. Indeed, a series of compounds having a 5-hydroxyguaiacyl nuclei, including 3-methoxycatechol, 5-vinyl-3-methoxycatechol and 5-propenyl-3-methoxycatechol, were detected and identified in all samples, although in lower amounts than the normal guaiacyl or syringyl compounds. The analysis of the lignins isolated from the same plants also showed the same 3-methoxycatechol derivatives found after whole fiber pyrolysis. These compounds are supposed to arise from the pyrolysis of 5-hydroxyguaiacyl moieties, which are supposed to be native constituents of lignin in plants forming benzodioxane substructures.     

2-Hydroxybenzaldehyde (2-phenylquinazolin-4-yl)hydrazones and their ZnII complexes: synthesis and photophysical properties

N-(2-Phenylquinazolin-4-yl)-N??-salicylidenehydrazines and their Zn^II complexes were obtained. The structures and luminescent properties of these new quinazoline derivatives were examined.     

Influence of the glucosylated β-cyclodextrin inclusion on sulfur trioxide spin-adduct stabilizations and spin-trapping rate processes for DMPO-type spin traps

The effect of CD-inclusion on spin-trapping rates and spin-adduct decay rates for sulfur trioxide radical anion (SO₃ ^??) was investigated. SO₃ ^?? radical was produced with UV photolysis of sodium sulfite in basic aqueous solution, and spin-trapped with various spin traps, i.e., PBN (α-phenyl-N-t-butylnitrone), DMPO (5,5-dimethyl pyrroline-1-oxide), and three other phosphoryl DMPO-type spin traps. A modified β-CD, 6-O-α-d-glucosyl-β-cyclodextrin (G-β-CD) having better inclusion properties than β-CD, was employed. Upon adding excess G-β-CD, decay rates of SO₃ ^?? radical adducts significantly decreased in most spin traps. Half-lives of SO₃ ^?? radical adducts of phosphoryl spin traps were one to two orders of magnitude longer than that of PBN or DMPO, and the G-β-CD addition further extended the half-life time. The spin traps containing phosphoryl-group all showed higher SO₃ ^?? trapping rates than those of PBN and DMPO, but two phosphoryl spin traps achieved slower trapping rates by G-β-CD addition. In addition, the structures of CD-inclusion complexes of spin traps were established by means of 1D and 2D NMR measurements. Based on the results, the influences of inclusion on the spin-trapping rate processes and spin-adduct stabilizations were discussed. We conclude that substituents in DMPO-type spin traps may be modified to provide best spin-trapping capabilities in the presence or absence of CD.     

Convenient synthesis of chiral lignin model compounds via optical resolution: four stereoisomers of guaiacylglycerol-β-guaiacyl ether and both enantiomers of 3-hydroxy-1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-propan-1-one (erone)

Guaiacylglycerol-β-guaiacyl ether (GGE) is one of the most important phenolic compound for studying the chemistry and biochemistry of lignin. GGE contains two asymmetric carbons at the alpha and beta positions of its side chain; therefore, theoretically it can exist as four different stereoisomers. It has been proposed that a Gram-negative bacterium, Sphingobium sp. SYK-6 (formerly referred as Sphingomonas paucimobilis SYK-6), degrades GGE enantiospecifically via cleavage of the ether bond. The cleavage was thought to proceed in two steps, each catalyzed by a different enantiospecific enzyme. In the first step, the alcohol residue at the alpha position of the side chain in GGE was thought to be oxidized enantiospecifically by four distinct Cα-dehydrogenases (LigD, LigN, LigL and LigO), to produce two enantiomers of 3-hydroxy-1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-propan-1-one (erone). To study this enantiospecific degradation step by the four dehydrogenases, we synthesized all four stereoisomers of GGE and both enantiomers of erone and separated them into enantiopure samples. The stereoisomers and our synthetic methods to prepare them are useful both for microbial and chemical investigations of lignin degradation.     

Reduction of chromano–piperidine-fused isoxazolidines: Tandem intramolecular rearrangements leading to 2-(methylamino)-4-oxo-N-phenyl-N-propyl-4H-chromene-3-carboxamide

Reductive ring opening of isoxazolidine moiety of chromano–piperidine-fused isoxazolidines (3a–c) with HCOONH₄ and 10% Pd/C in a mixture of solvents (THF/MeOH) at ambient temperature, affords novel 2-(methylamino)-4-oxo-N-phenyl-N-propyl-4H-chromene-3-carboxamide (4), which is apparently derived from reductive NO bond cleavage followed by tandem intramolecular rearrangements. Plausible mechanistic rationale for the formation of compound 4 is proffered.