Solution and solid state13C-NMR of barley straw lignins

Grass lignins are formed by the polymerization of phenoxy radicals and contain a variety of carbon-carbon and carbon-oxygen bonds. They are similar to the hardwood lignins, but differ by containing a substantial proportion of esterified cinnamic acids. Detailed nuclear magnetic resonance studies in conjunction with chemical analysis have given new information on the structure of grass lignins. Milled straw lignins (MSL) from barley were examined by both solution and solid-state (CP/MAS) NMR before and after acetylation. The assignment of the carbon-13 (100 MHz) solution spectra was achieved using model compound data, nuclear Overhauser enhancement (NOE) suppression, and insensitive nuclei enhanced by polarization transfer (INEPT) techniques. The NOE suppression permitted quantitative analysis of lignin, giving information on the ratio of specific carbon atoms. Use of the relaxation agent, chromium acetylacetonate, enabled accumulation of sufficient spectral data to give a spectrum suitable for integration after 90 h. The INEPT technique, which had not previously been used for lignin analysis, was successfully applied to acetylated MSL. This technique increased signal intensities 3–4-fold and simplified the spectrum by inverting methylene carbons and eliminating or inverting quaternary carbons. Comparison of this spectrum with the normal spectrum permitted accurate assignment of quaternary and methine carbons. The solid-state carbon-13 CP/MAS NMR was used to examinein situ lignin and the isolated MSL. The¹³C-CP/MAS spectrum ofin situ lignin shows that cellulose and hemicellulose resonances dominate with little evidence ot the aromatic structure of lignin. the¹³C-CP/MAS of MSL shows reduced carbohydrate resonances and increased aromatic resonances. The extent of modification to the barley straw was estimated and results indicate the presence oflignin-carbohydrate complexes. Detailed information on the nature of the linkages between lignin components and between lignin and carbohydrate components has been obtained from these spectra.     

Structure of Alkali Lignins Fractionated from Ricinus communis and Bagasse. 3. IR Spectra

Abstract

The lignin fractions isolated by one- and multistage soda and sulfate cookings showed almost identical IR spectra, indicating the similarity of the lignin skeletal structure throughout the plant. However, the absorbances reveal some differences. Similarity of the spectra includes: 1) chelation and bonding of the hydroxyl groups. 2) Stretching vibration of C-H bonds in methyl, methoxyl, and methylene groups. 3) Stretching vibration of C≡N. 4) Carbonyl unconjugated β-ketone, conjugated acids, or esters at 1725 cm^?1. There is no change in the intensity of absorption at this band from that at 1515 cm^?1 with the cooking stage. 5) Aromatic skeletal vibration at 1610 and 1515 cm^?1, affected by ring substituents at 1425 cm^?1. 6) The band at 1465 cm^?1 showed a higher intensity for soda and soluble kraft lignins than for insoluble kraft ones. 7) The band at 1370 cm^?1, assigned to phenolic OH bending, is affected by the methoxyl group. 8) The absence of condensed guaiacyl and the presence of syringyl and uncondensed guaiacyl. Assignments for hardwood lignin are shown for soda and soluble kraft lignins of bagasse, while those for softwood lignin are shown for soda, soluble, and insoluble kraft lignins of Ricinus communis and for insoluble kraft lignin of bagasse. A relation exists between the carbohydrate's lignin and the band at 920 cm^?1. Lignins from Ricinus communis are of higher guaiacyl to syringyl ratios than those from bagasse. The presence of C—S vibration and the absence of thiol groups for kraft lignins are indicated.     

Strategies for the Removal of Polysaccharides from Biorefinery Lignins: Process Optimization and Techno Economic Evaluation

The utilization of biorefinery lignins as a renewable resource for the production of bio-based chemicals and materials remain a challenge because of the high polysaccharide content of this variety of lignins. This study provides two simple methods; (i) the alkaline hydrolysis-acid precipitation method and (ii) the acid hydrolysis method for the removal of polysaccharides from polymeric biorefinery lignin samples. Both purification strategies are optimized for two different hardwood hydrolysis lignins, HL1 and HL2, containing 15.1% and 10.1% of polysaccharides, respectively. The treated lignins are characterized by polysaccharide content, molecular weight, hydroxyl content, and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). Preliminary techno-economic calculations are also carried out for both purification processes to assess the economic potential of these technologies. The results indicate that both protocols could be used for the purification of HL1 and HL2 hydrolysis lignins because of the minimal polysaccharide content obtained in the treated lignins. Nevertheless, from an industrial and economic perspective the acid hydrolysis technology using low acid concentrations and high temperatures is favored over the alkaline hydrolysis-acid precipitation strategy.     

Destruction of aspen wood by the fungusPhanerochaete sanguinea quantitative1H and13C NMR spectroscopies of biologically degraded lignin

The lignin of mechanically ground aspen wood and lignins isolated from aspen wood attacked by the fungusPhanerochaete sanguinea have been investigated by quantitative¹H and¹³C NMR spectroscopies. It has been shown that the biodestruction of the lignin takes place through the cleavage of alkyl-aryl and aryl-aryl bonds and is accompanied by demethylation (demethoxylation) reactions, and the oxidation of C_α and C_γ atoms. In addition to reactions in which the C—C bonds are cleaved, the formation of ether bonds has been observed. An interconnection has been shown between the variations in the amount of functional groups, fragments, and the bonds in biolignins and the loss in mass of the wood. A method is proposed for evaluating the carbohydrate content in lignin preparations using the NMR method.     

Quantitative1H and13C NMR spectroscopies of cottonplant dioxane lignin

A comparative analysis has been made of the dioxane lignins of the stems, seedling shoots, and seed coats of the cotton plant. The ratio of p-hydroxyaromatic, guaiacyl, and syringyl units in the macromolecules of the lignins has been determined, and so have the amounts of functional groups and bonds. It has been established that the main difference between the dioxane lignins from seedling shoots and seed coats of the cotton plant, on the one hand, and the lignin from the woody stems, on the other hand is a lower content of syringyl and guaiacyl units in the former.     

Quantitative1H and13C NMR spectroscopies of cottonplant dioxane lignin

A comparative analysis has been made of the dioxane lignins of the stems, seedling shoots, and seed coats of the cotton plant. The ratio of p-hydroxyaromatic, guaiacyl, and syringyl units in the macromolecules of the lignins has been determined, and so have the amounts of functional groups and bonds. It has been established that the main difference between the dioxane lignins from seedling shoots and seed coats of the cotton plant, on the one hand, and the lignin from the woody stems, on the other hand is a lower content of syringyl and guaiacyl units in the former.Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 798–808, November–December, 1994.     

Lignin: its functions and successive flow

Phenolic lignin-based polymers with structure-variable function have originally been designed, and a process has been developed for synthesizing them directly from native lignins. The key point of the process is to set up two different solvents, which are immiscible each other, for selective modification and separation of lignin and carbohydrates: hydrophobic solvent for hydrophobic lignin and hydrophilic solvent for hydrophilic carbohydrates. The native lignins, three dimensional network polymers, are subjected to selective modifications at the interface between both phases to give linear type polymers (lignophenols) composed mainly of 1,1-bis(aryl)propane type units, almost quantitatively. The process provides a new system for successively utilizing lignocellulosics in the molecular level.     

Structure of Alkali Lignins Fractionated from Ricinus communis and Bagasse. 1. Chemical Constituents

Fractionation of alkali lignins of the soda and sulfate pulping processes of Ricinus communis and bagasse was carried out by using successive equal concentrations of the alkaline reagent. Soda lignins were soluble in organic solvents, while the sulfate ones were sparingly soluble. Thus, two fractions of the sulfate lignins, soluble and insoluble, could be obtained from acetone. The different alkali lignin fractions were subjected to elemental and functional group analyses. For both Ricinus communis and bagasse, the carbon content of the fractions of the various types of lignin is in the order: soluble sulfate > soda > insoluble sulfate, while the methoxy is in the order: soda > soluble sulfate > insoluble sulfate. The phenolic OH content, as well as OH/C₉ of soda lignins of bagasse, are lower than those of soluble sulfate lignins. For Ricinus communis, the phenolic OH content and OH/C₉ is higher for some of the fractions (first three stages of cooking) of soda lignin than the corresponding fractions of kraft (sulfate) lignin, while the reverse takes place for the other fractions. For the same type of lignin, the fractions showed changes in their carbon, methoxyl, and phenolic OH contents. The change may be regular, i.e., increase or decrease with the order of stage of cooking, or irregular. Molecular weights of the different alkali lignins which are soluble in organic solvents ranged between 750 and 840.     

Analytical pyrolysis study of biodelignification of cloned Eucalyptus globulus (EG) clone and Pinus pinaster Aiton kraft pulp and residual lignins

This study centred on the analysis of lignin in situ of cloned eucalypt and pine kraft pulps. Trametes versicolor laccase-violuric acid system (LMS) delignifications were performed on a softwood (Pinus pinaster) and a hardwood (Eucalyptus globulus) conventional kraft pulp with an initial kappa number of 34.5 and 15.5, respectively. The LMS treated pulps were then subjected to alkaline extraction stages (E). The kappa number data show that LMS is effective at biodelignifying both softwood and hardwood kraft pulps. However, under the conditions employed in this study, a greater level of biodelignification was obtained with LMS E. globulus (hardwood) than with LMS P. pinaster (softwood), but the amount of lignin removed was higher for the softwood pulp. The original milled wood samples, kraft pulps, biodelignified kraft pulps, and isolated residual lignin and milled wood lignins from the two wood samples have been characterized by pyrolysis-gas chromatography/mass spectrometry. The analysis of the pyrograms indicates that the lignin compositions of the two wood species and corresponding pulps are very different, as expected; however, the knowledge of the chemical mechanisms of delignification is very limited and requires additional work. Analytical pyrolysis is one the few degradative methods for the analysis of biopolymers that has shown a sufficient degree of success.     

Studies on polymers and composites from lignin and fiber derived from sugar cane

Sugarcane fiber (i.e. bagasse) lignin has a larger fraction of aromatics unsubstitution in the ortho position than hardwood or softwood lignin and hence has the greater ability to be derivatized. Furthermore, organosolv lignin has a higher purity than sulfonated and kraft lignins. This work examines the purification of organosolv lignin derived from bagasse and the physico‐chemical properties of the lignin and lignin‐phenol formaldehyde (PF) resin coatings, and composites. The wetability tests have shown that lignin and lignin‐PF resin films are effective water barrier coatings, though the contact angles of lignin‐PF resin films were considerably less than the wax films. The overall mechanical properties (i.e. peak stress, peak strain and modulus) of the bagasse fiber composites were lower than the values obtained with the composites without the inclusion of bagasse fiber. Copyright © 2007 John Wiley & Sons, Ltd.     

Elucidation of lignin structure by quantitative 2D NMR

Quick quantitative HSQC (QQ‐HSQC) was applied to quantitative evaluation of different inter‐unit linkages in an array of milled softwood and hardwood and technical lignins by using the guaiacyl C2 and syringyl C2–C6 signals as internal standards. The results were found to be highly reproducible and comparable with earlier literature reports. The advantage of QQ‐HSQC NMR analysis of lignin is contemporary detection and quantification of lignin inter‐unit linkages with a direct, non‐destructive method requiring short acquisition times.     

Beech Lignin—Proposal of a Constitutional Scheme

The high molecular weight material lignin consists of phenylpropane units linked together by a variety of bond types. During the past eight years, two newly developed degradation procedures have permitted the first direct determinations of the nature of these bonds. The first procedure affords a very mild partial hydrolysis of benzyl ether bonds. Eleven dimeric, trimeric, and tetrameric degradation products were obtained in this way from spruce and beech lignin: they exhibited three different kinds of bonds between the C₉ structural units, and their structures have all been elucidated. In the second procedure, the most important kind of bond in lignin, i. e. the arylglycerol-β-aryl ether bond, can be subjected to directed cleavage under mild conditions after introduction of a suitable neighboring group. On application to beech lignin, 91 % of the material was degraded giving monomeric to tetrameric phenols. Complete structural elucidation of the twenty dimeric phenols isolated and a knowledge of their relative yields and the yields of the other fractions obtained by gel filtration permitted a structural scheme to be set up for beech lignin in which the C₉ structural units are linked together by no less then ten different kinds of bonds. The structural scheme, which can be readily explained biogenetically, has the same elemental composition as natural beech lignin. Further support for the structural scheme comes from a comparison of the ¹³C-NMR spectrum of natural beech lignin and a ¹³C-NMR spectrum calculated for the proposed structure on the basis of about fifty lignin model substances.     

Isolation of soil lignins by combination of ball-milling and cellulolysis: Evaluation of purity and isolation efficiency with pyrolysis/GC/MS

CuO oxidation is traditionally used for soil lignin study, although, like other degradation methods it might give access only to a part of the lignin polymers. For structural characterization, lignins are conventionally isolated from plant material by combining ball-milling, cellulolysis and solvent extraction to recover a milled wall enzymatic lignin (MWEL) fraction. This method might isolate condensed lignins, which are not accounted for CuO oxidation. MWELs are still associated with polysaccharides. This study aimed to evaluate if the MWEL method can be used complementary to CuO oxidation for soil lignin studies. We assessed the purity of isolated lignins by pyrolytic characterization of MWEL as well as the efficiency of the isolation method by characterization of the MWEL-free residues. MWELs isolated from maize stems and leaves and soil were primarily composed of lignin units, with few associated polysaccharides. MWELs isolated from soils were more degraded and contained less polysaccharides than MWEL from maize tissues. Fewer lignin-derived pyrolysis products were detected in the soil residue compared to the residues obtained from plant tissues. The MWEL method appeared very efficient in isolating relatively pure lignins from soil, most probably because the lignin–polysaccharide complex is less intact than in plants.     

Change in the structural elements of cotton-plant lignins in the various vegetation periods

Summary 1. The dioxane lignins of the cottonplant of various vegetation periods have been studied, and it has been shown that the lignin changes in the course of the development of the plant: the amount of methoxy groups increases and the amount of phenolic hydroxy groups decreases.2. Gel chromatography of the lignins has shown that they are all polydisperse with a predominance of the high-molecular-weight fraction.3. The 3,4-diethoxybenzaldehyde detected in the products of the alkaline nitrobenzene oxidation of dioxane lignin of the early vegetation period after its ethylation with diethyl sulfate is a direct proof of the presence of catechol structural units in cotton-plant lignin.4. A quantitative determination of the catechol structures in the lignins of the cottonplant has shown that their amount decreases as the plant develops.5. By decomposing the lignins with sodium and liquid ammonia, the presence in them of guaiacyl, syringyl, and p-coumaryl structural units has been confirmed and it has been shown that the lignin becomes less condensed as the plant grows, which is apparently connected with an increase in the degree of its methoxylation.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 100–107, January–February, 1977.     

Destruction of aspen wood by the fungusPhanerochaete sanguinea quantitative1H and13C NMR spectroscopies of biologically degraded lignin

The lignin of mechanically ground aspen wood and lignins isolated from aspen wood attacked by the fungusPhanerochaete sanguinea have been investigated by quantitative¹H and¹³C NMR spectroscopies. It has been shown that the biodestruction of the lignin takes place through the cleavage of alkyl-aryl and aryl-aryl bonds and is accompanied by demethylation (demethoxylation) reactions, and the oxidation of C and C atoms. In addition to reactions in which the C—C bonds are cleaved, the formation of ether bonds has been observed. An interconnection has been shown between the variations in the amount of functional groups, fragments, and the bonds in biolignins and the loss in mass of the wood. A method is proposed for evaluating the carbohydrate content in lignin preparations using the NMR method.Wood Chemistry Branch of the Institute of Organic Chemistry, Siberian Division of the Russian Academy of Sciences, Irkutsk. Irkutsk State University. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 547–557, July–August, 1994.     

Cleavage by sodium in liquid ammonia of kenaf lignins and the biological activity of the cleavage products

The natural lignin of kenaf stems and fractions of the dioxane lignin of kenaf stems isolated previously were cleaved by sodium in liquid ammonia in order to study their structure. It was established that the kenaf lignins consist of three types of structural units: p-coumaryl, guaiacyl, and syringyl, with a predominance of the guaiacyl types. An antioxidation activity of the monomeric products of the degradation of the kenaf lignins has been found.     

Novel tetrahydrofuran structures derived from beta-beta-coupling reactions involving sinapyl acetate in Kenaf lignins

Free radical coupling of sinapyl gamma-acetate or cross-coupling between sinapyl acetate and sinapyl alcohol yields novel tetrahydrofuran beta-beta-(cross-)coupled dehydrodimers. Such substructures are therefore anticipated in naturally acetylated lignins, e.g. in Kenaf, if sinapyl acetate is a component of the lignin monomer pool. The DFRC (derivatization followed by reductive cleavage) method, modified by replacing all acetyl reagents and solvents with their propionyl analogs (DFRC'), allows the analysis of naturally acetylated lignins. DFRC' treatment of the sinapyl acetate-derived dimers or crossed dimers gave diagnostic products that retain at least one acetate group on a sidechain gamma-position; the products have been authenticated by comparison of their mass spectra and GC retention times with those of synthesized compounds. DFRC' of Kenaf lignins produces the same diagnostic products as from the dimers, implicating the presence of the various tetrahydrofuran units in Kenaf lignins. With data from the model compounds in hand, NMR analysis of Kenaf lignins elegantly confirms the presence of such substructures in the polymer, establishing that acetates on Kenaf lignins arise through incorporation of sinapyl acetate, as a lignin precursor, via enzyme-mediated radical coupling mechanisms.     

Chemical transformations of lignin during the biodegradation and organosolv pulping of deciduous wood

The lignins from the liquors in the organosolv pulping of aspen wood treated with wood-destroying fungi have been investigated using quantitative¹H and¹³C NMR spectroscopy and exclusion liquid chromatography. It has been shown that the biodegradation of lignin takes place in different ways according to the complex of enzymes produced by the fungi. Phanerochaete sanguinea causes degradation through predominant cleavage at alkyl-phenyl bonds, and Trametes villosus at the C-C bonds of the aliphatic chain. In addition to degradation reactions, polymerization (condensation) reactions also take place with the appearance of new C_ar-O-C and C_ar-C bonds. It has been established that the biological pretreatment of aspen wood ensuring partial degradation of the lignin leads to its more ready extraction in the process of organosolv pulping.Wood Chemistry Division of the Irkutsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 603–610, July–August, 1995. Original article submitted May 23, 1994.     

Cleavage by sodium in liquid ammonia of kenaf lignins and the biological activity of the cleavage products

The natural lignin of kenaf stems and fractions of the dioxane lignin of kenaf stems isolated previously were cleaved by sodium in liquid ammonia in order to study their structure. It was established that the kenaf lignins consist of three types of structural units: p-coumaryl, guaiacyl, and syringyl, with a predominance of the guaiacyl types. An antioxidation activity of the monomeric products of the degradation of the kenaf lignins has been found.Institute of the Chemistry of Plant Substances of the Uzbek SSR Academy of Sciences, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 738–742, September–October, 1987.     

Cotton plant lignins

This paper generalizes the results of studies on lignins isolated from the stems, bolls, and seed hulls of cotton plants of a number of varieties as functions of the vegetation periods of the plants. The UV, IR and¹H and¹³C NMR spectra of the lignins have been studied. The dioxane lignin (DLA) and the natural lignins have been subjected to nitrobenzene oxidation and to cleavage with sodium in liquid nitrogen. A structure of the lignin macromolecules consisting of 18 phenylpropane structural units has been done up. Information is given on the use of lignins in agriculture, the food industry, and the paint and varnish industry. The anticorrosion and sorption properties of hydrolysis and modified hydrolysis lignins have been studied.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 40 64 75. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 465–496, July–August, 1997.