Structural Analysis of Lignins from DifferentSources (original) (raw)
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Corn stover and rice straw lignin samples received from ethanol pilot plants, along with softwood kraft lignin samples, were characterized using pyrolysis GC-MS, 13 C CP/MAS NMR spectroscopy, and permanganate oxidation degradation. The lignins were then esterified using 1-methylimidazole as a catalyst in a pyridine-free reaction, and the thermal properties of the products were evaluated. Solid state NMR showed the rice straw lignin contained 18% residual polysaccharides. Pyrolysis GC-MS showed the softwood kraft, corn stover, and rice straw lignins to be G-type, H/G/S-type, and G/S-type, respectively. However, some discrepancy was apparent between the pyrolysis and permanganate oxidation studies as to the ratios of the monomeric makeup of the lignins. The kraft and rice straw lignins were determined to have high degrees of condensation, while the corn stover lignin was uncondensed. Little to no increase in solubility was noticed for corn stover or rice straw lignin esters in organic solvents. Glass transition temperatures (T g) of the lignin derivatives were determined by a combination of differential scanning calorimetry, dynamic mechanical analysis, and parallel plate rheometry.
Physico-Chemical and Thermal Characterization of Alkali-Soluble Lignins from Wheat Straw
Polymer Journal, 1998
Four alkali-soluble lignin fractions were extracted from dewaxed wheat straw with 3% sodium hydroxide at 45°C for 2, 5, 12, and 15 h, respectively. The pure alkali-soluble lignin preparations LA were obtained by using a two step precipitation method instead of the traditional ether precipitation procedure. The physico-chemical properties and structural features of the isolated pure lignin fractions were characterized by UV, FT-IR, 13 C NMR spectroscopy, and thermal analysis. The lignin in fraction LA-I is mainly composed of /J-O-4 ether bonds. The less common /J-5 and /J-/J carbon-<:arbon linkages are also present in the lignin structural units. The results obtained showed that the lignin fractions appeared to be strongly associated to hydroxycinnamic acids such as p-coumaric and ferulic acids and glucuronic acid or 4-O-methylglucuronic acid.
Molecular composition of water-soluble lignins separated from different non-food biomasses
Fuel Processing Technology, 2015
Lignin Alkaline hydrogen peroxide Giant reed Miscanthus DFRC 1 H-13 C HSQC-NMR 31 P-NMR Separation of water-soluble lignins from lignocellulosic biomass provides a new and still poorly exploited feedstock to increase the sustainability of biorefineries. We applied derivatization followed by a reductive cleavage (DFRC) method, 2D-HSQC-NMR, and 31 PNMR after 31 P-labeling, to investigate molecular composition in water-soluble lignins obtained by alkaline oxidation from three biomass materials for energy (miscanthus, giant reed and an industrially pre-treated giant reed). Chromatographic identification of lignin products cleaved by DFRC showed a large predominance of guaiacyl (G) units in all biomasses and a lesser abundance of syringyl (S) and p-coumaryl (P) monomers. Our S/G ratios disagree with those reported in literature by other lignin separation methods. Carboxyl functions (ferulic and pcoumaric acids) were revealed by heterocorrelated 1 H-13 C HSQC-NMR, and confirmed by 31 P-NMR spectra of 31 P-labeled lignin molecules. An understanding of molecular composition of water-soluble lignins from biomass sources for energy is essential for lignin most efficient exploitation in either industrial or agricultural applications.
Lignin is the most abundant natural source of renewable aromatic units and therefore, detail characteri-sation to unveil its chemical properties is a critical step for its utilisation. Nine black liquor samples from different plant origins namely sugarcane bagasse, Eucalyptus grandis, and Pinus gregii extracted from the Kraft, soda, soda-anthraquinone and sulphite pulping processes were considered. After lignin purification, when applicable, the samples were characterised by several common methods (wet chemical methods, Fourier Transformed Infra-Red spectroscopy, Gel Permeation Chromatography). Lignin monomer composition (H:G:S) was determined by thioacidolysis as well as a new pyrolysis method based on the use of an analytical setup which couples Thermo Gravimetric Analysis (TGA) for lignin devolatilisation, the capture of released volatile compounds in thermal desorption (TD) tubes, and the quantification of the captured phenols by TD-GC–MS (gas chromatography–mass spectroscopy). The TGA-TD-GC–MS, with the use of internal calibration, allowed the quantification of 5.5–12.9 wt.% of monomeric products based on dry weight of purified lignin. Pyrolysis of sugarcane lignin resulted in significant yield of furfural, which was explained by the conversion of residual sugar. Pyrolysis of pine lignin gave the lowest yield of syringyl-type phenols, which was consistent with characterisation results (low methoxy content and absence of FT-IR band characteristic of syringyl unit). Pyrolysis method had the advantage to break different types of chemical bonds, which is likely to give a product distribution more representative of the lignin. With TGA-TD-GC–MS the monomer proportion for purified lignin from the same plant species were found to be very comparable (deviation lower than 10% for each unit). Compared to thioacidolysis (known to be selective towards bond cleavage and suspected to overestimate S content), TGA-TD-GC–MS gave lower S/G ratio. The TGA-TD-GC–MS method has demonstrated to be a good alternative technique to study the H:G:S proportions of lignins with low ash content (<5%).
Analytical methods for lignin characterization. II. Spectroscopic studies
Cellulose chemistry and technology, 2006
Lignin' characterisation is a very difficult task, if considering its diversity with respect to both origin and method of separation. The heterogeneity of lignin is caused by variations in polymer's composition, size, crosslinking, functional groups, and linkage type between the phenyl propane monomers (p -hydroxyl phenyl, guaiacyl and syringyl units). The elaboration of well-defined analytical methods for lignin characterization is very important for its industrial applications as a raw material. Two groups of lignin from woody species and annual fibre crops have been studied by FT-IR, UV, fluorescence and 13 C NMR spectroscopy. The relative content of the different functional groups (p -hydroxy phenyl, guaiacyl and syringyl units) was appreciated by normalised intensities and deconvolution of the spectral bands. Correlation of the results provided by these methods permits differentiation between the structural characteristics of two lignin groups from the viewpoint of their particularities.
ANALYTICAL METHODS FOR LIGNIN CHARACTERIZATION.
Lignin characterisation is a very difficult task, if considering its diversity with respect to both origin and method of separation. The heterogeneity of lignin is caused by variations in polymer composition, size, crosslinking, functional groups, and linkage type between the phenyl propane monomers (p–hydroxy phenyl, guaiacyl and syringyl units). The elaboration of well-defined analytical methods for lignin characterization is very important for its industrial applications as a raw material. Two groups of lignin from woody species and annual fiber crops have been studied by FT-IR, UV, fluorescence and 13C NMR spectroscopy. The relative content of the different functional groups (p–hydroxy phenyl, guaiacyl and syringyl units) was appreciated by normalised intensities and deconvolution of the spectral bands. Correlation of the results provided by these methods allows the differentiation between the structural characteristics of the two lignin groups.
Some physical properties of acetosolv lignins from bagasse
Journal of Applied Polymer Science, 2008
Acetosolv-solubilized lignins were characterized by their solubility in different organic solvents, Fourier transform infrared, 13 C-NMR, UV, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis. Solvents having a solubility parameter in the range of 10-12.7 and a hydrogenbonding parameter in the range of 3.6-5 were considered good solvents for acetosolv lignins. Fourier transform infrared spectra of the lignins were typical for lignins containing p-hydroxy phenylpropane (H), guaiacyl (G), and syringyl (S) units. The lignins contained more conjugated and fewer nonconjugated C¼ ¼O groups, and the guaiacyl groups were etherified and condensed. 13 C-NMR confirmed partial acetylation of the lignins and the presence of b-O-4 and b-5 linkages. Acetosolv lignins also showed the typical UV spectrum of annual plants. The effects of the acetic acid concentration and pulping time on the molecular weights of the lignins were explained with the presieving and condensation concepts. The thermal behavior of the acetosolv lignins was also studied.
A Comprehensive Approach for Quantitative Lignin Characterization by NMR Spectroscopy
Journal of Agricultural and Food Chemistry, 2004
The structure of Eucalyptus grandis milled wood lignin (MWL) was investigated by 2D 1 H-13 C HSQC, HMQC, and 1 H-1 H TOCSY correlation NMR techniques and by quantitative 13 C NMR as well as by the permanganate oxidation degradation technique. The combination of 2D NMR and quantitative 13 C NMR spectroscopy of nonacetylated and acetylated lignin preparations allowed reliable identification and calculation of the amount of different lignin structures. About 85% of side-chain moieties were estimated on the structural level. This information was substantiated by data on the quantity of various functional groups and interunit linkages as a whole. A modified method for calculation of the h:g:s ratio has been suggested and compared with previously suggested approaches. E. grandis MWL has been determined to have an h:g:s ratio of 2:36:62. The amounts of various phenolic/etherified noncondensed/condensed guaiacyl and syringyl moieties were approximately estimated. E. grandis MWL contained ∼0.60/Ar of -O-4 moieties along with small amounts of other structural units such as pino/syringyresinol (0.03/Ar), phenylcoumaran (0.03/Ar), and spirodienone (0.05/Ar). The degree of condensation was estimated at ∼21%; the main condensed structures are 4-O-5 moieties (∼0.09/Ar). The structure of E. grandis MWL was compared with those of other lignin preparations isolated from various hardwoods.