Lignins as macromonomers for polyurethane synthesis: A comparative study on hydroxyl group determination

The hydroxyl group contents of four technical lignins [Indulin AT (Meadwestvaco), Alcell (Repap), Curan 27‐11P (Borregaard LignoTech), and Sarkanda (Granit SA)] were investigated in view of their valorization as polyols in polyurethane synthesis. The different hydroxyl group contents were determined by the following methods: titration and ¹H‐NMR, ¹³C‐NMR, and ³¹P‐NMR spectroscopy. The titration method chosen was on the basis of a standard method commonly used to characterize commercial polyols for polyurethanes synthesis. The values of the total and phenolic hydroxyl contents determined by the different techniques were found to be in good agreement. For the total hydroxyl contents, coefficients of variation of 5.6% (Alcell), 3.2% (Indulin AT), 2.3% (Sarkanda), and 6.2% (Curan 27‐11P) were established. For the phenolic hydroxyl contents, a good correlation was observed between data obtained from ³¹P‐NMR and ¹³C‐NMR for all lignin samples, except for the Sarkanda lignin, for which a relatively high coefficient of variation (12.6%) was found. For softwood lignins (Indulin AT and Curan 27‐11P), the phenolic hydroxyl content determined by ¹H‐NMR was always lower than that deduced from ³¹P‐NMR and ¹³C‐NMR spectroscopy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Quantitative Phosphorus-31 NMR Analysis of Six Soluble Lignins

By using a set of lignin samples, which have been subjected to thorough analyses by the international wood chemistry community, the recently developed quantitative method of ³¹P NMR spectroscopy was comprehensively examined. The values of total phenolic hydroxyl groups and those of total hydroxyl groups were found to favourably compare with those obtained by other laboratories, applying independent methods of analysis. Furthermore, the application of quantitative ³¹P NMR spectroscopy offered additional detailed structural information for the examined lignins which was in accord with literature accounts for similar wood species and lignin preparations. More specifically, the steam explosion lignins from aspen and yellow poplar woods and that produced by ball milling/enzyme hydrolysis of cottonwood were found to contain relatively high amounts of β-O-4 structures. In contrast, the kraft, organosolv, and the acid hydrolysis processes were found to induce significant chain scission on the resulting lignins. Ball milled cottonwood lignin contained the highest frequency of β-O-4 bonds and the lowest amount of free phenolic hydroxyls. The erythro form of β-O-4 structures were invariably predominant in the lignins from aspen, yellow poplar and cottonwood, in accord with the conclusions of previous reports on hardwood lignins. Thus, the application of quantitative ³¹P NMR spectroscopy offered the detailed chemical composition of the examined lignins.     

Quantitative Phosphorus-31 NMR Analysis of Lignins,a New Tool for the Lignin Chemist

A novel quantitative method has been developed for the determination of the various types of hydroxyl groups present in lignins. The syringyl, guaiacyl and p-hydroxyphenyl free phenolic groups, as well as the primary, and the secondary hydroxyl groups (belonging to individual erythro and threo forms of the aryl-glycerol β-O-4 ether strctures) can be quantitatively determined from a ³¹P NMR experiment. This is made possible by phosphitylating lignins with 1,3,2 dioxaphospholanyl chloride, followed by ³¹P NMR spectroscopy, in the presence of a relaxation reagent (chromium acetylacetonate) and an internal standard. The various aspects leading to the development of this technique are discussed together with relevant statistical information pertaining to the reproducibility and quantitative validity of the method. This simple and novel form of spectroscopy may become a valuable resource to the lignin chemist, because it can supply detailed quantitative information about the structure of a soluble lignin sample.     

Solid-State 13C CP/MAS NMR for Alkyl-O-Aryl Bond Determination in Lignin Preparations

Several lignin preparations (Freudenberg lignin, Björkman lignin, and Pepper lignin), technical lignins (soda, soda-AQ, Kraft, Kraft-AQ, and hydrolysis), dimeric lignin model compounds, and different polysaccharides (galactoglucomannan, arabinogalactan, xylan, and arabinan) were analyzed by means of solid-state ¹³C CP/MAS NMR. Signals assignment in solid-state NMR lignin spectra was performed on the basis of the conducted studies and earlier published data. It was established that there exists strong linear correlation (r = 0.985) between Alkyl-O-Aryl inter-unit bond content in lignin and integral signals intensity in NMR spectra in the range of chemical shifts of 96–68 ppm. The integral signals intensity was measured in correlation with the reference integral signals in the range of chemical shifts of 162–102 ppm, typical for aromatic carbon atoms. To eliminate the effect, caused by carbohydrates contained in lignin, the correction factor of 0.67% of the area of integration per 1% of carbohydrates was determined. It was shown that the solid-state ¹³C CP/MAS NMR method allowed to determine Alkyl-O-Aryl bond content in both soluble and insoluble lignin preparations, and also to determine methoxyl groups content in soluble preparations.     

The Lignin of the Diffuse Porous Angiosperm Tree Triplochyton scleroxylon K. Schum with Low Syringyl Content

“Milled wood lignin” was isolated from Triplochyton scleroxylon tree (wawa MWL) and subjected to elemental analysis before and after acid treatment, and to analytical pyrolysis. Both FT-IR and ¹H NMR spectra were recorded. For comparison, lignins from spruce and beech isolated by the MWL, dioxane/HCl (DIL), and Cuene (CUL) techniques were also characterized. The residual carbohydrate content of the lignins was estimated. Wawa MWL is a GS lignin with 88% G and 12% S content. The utility of analytical pyrolysis and FT-IR for lignin classification is discussed. FT-IR spectroscopy, calibrated with data of 50 MWLs (PLS approach), gives reliable results with a minimum of experimental work. The yields of S phenols obtained by pyrolysis has to be multiplied with a factor around 0.5 for correction if the S content of a lignin is low. The cross-linking indices and OH_phen/OH_aliph ratios of lignins increase in the order MWL < DIL < CUL. ¹H NMR spectroscopy is also helpful to recognize the S/G ratio of a lignin.     

The isolation and characterization of lignin of kenaf fiber

In this article, milled wood lignin (MWL) was isolated and purified from retted kenaf fiber, the lignin obtained was characterized by elemental analysis, FTIR, ¹H‐NMR, and ¹³C‐NMR spectroscopy. The C₉ formula is calculated for kenaf fiber MWL as C₉H_9.32O_3.69(OCH₃)_1.30. The spectra of FTIR, ¹H‐NMR, and ¹³C‐NMR show the kenaf fiber lignin to be of the G/S type with high proportion of syringyl (S) unit. The numbers of phenolic and aliphatic hydroxyl groups in the kenaf fiber MWL are estimated to be 0.14 and 1.31, respectively, per C₉ unit. The OH_aliph is 90.3% in total numbers of hydroxyl groups of kenaf fiber MWL, and the OH_ph is 9.7%. It is evident that the β‐O‐4 structures are mainly linkage in the MWL of kenaf fiber, which contain more erythro stereochemistry type in β‐O‐4 units than thero stereochemistry type. In general, the characteristics of lignin of kenaf fiber are similar to that of hardwood. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Lignin and Other Aromatic Substances Released from Spruce Wood During Pressurized Hot-Water Extraction,Part 1: Extraction,Fractionation and Physico-Chemical Characterization

Ground spruce wood was extracted with pressurized hot water in an accelerated solvent extractor (ASE) at 170°C during 20, 60, and 100 min. Released aromatic substances (from 2.2 to 2.5% on wood basis) were isolated on XAD-7 resin and fractionated into lignin, oligomeric aromatic substances (OAS), and compounds insoluble in methanol (IMC). The separated aromatic fractions, and the ground wood before and after extraction, were characterized by physico-chemical methods (GC, GC-MS, HPLC-SEC, TG, DSC). The major part of the aromatic substances was oligomeric aromatic substances (OAS). This fraction increased significantly with the extraction time, while the yield of dissolved lignin increased only slightly with extraction time. Isolated lignins had a lower molar mass and differed considerably from spruce milled wood lignin (MWL). The isolated lignins were more stable against oxidative thermodegradation than spruce MWL. The UV-extinction coefficients at 280 nm were lower for the isolated lignins than for MWL.     

Structural characteristics of different softwood lignins according to 1D and 2D NMR spectroscopy

Abstract

Along with cellulose, lignin is one of the most common polymers of plant origin. Numerous studies show that the complex and irregular structure of lignin is still unknown. First of all, the existing problems are connected with the peculiarities of lignins as a natural chemical compound. In this paper the general regularities of the chemical structure of dioxane lignins isolated from the softwood – larch (Lárix sibírica), pine (Pinus cembra) and juniper (Juníperus commúnis). The studies were carried out using a combination of ¹³C, ³¹P and 2D NMR spectroscopy. According to HSQC spectra it was established that the main structural fragments are the structures of β-aryl ether, followed by phenylcoumarans, with lower amounts of other substructures. Signals belonging to a methyl substituted phenylcoumaron were first identified on the NMR spectra of softwood dioxane lignins. Signals related to the structure of 3′-O-methylated dihydroquercetin were found in the composition of larch dioxane lignin.     

BEHAVIOR OF EUCALYPTUS GLOBULUS LIGNIN DURING KRAFT PULPING. II. ANALYSIS BY NMR,ESI/MS,AND GPC

ABSTRACT

Residual and dissolved lignin from different phases of kraft delignification of Eucalyptus globulus wood were isolated and characterized by 1D and 2D ¹H NMR, ¹³C NMR, Electrospray Ionization Mass Spectrometry (ESI/MS), and gel permeation chromatography (GPC). During the temperature rise period, below 70°C, about 20% of the lignin was dissolved without significant structural changes. Above 70°C, the lignin suffered significant degradation/fragmentation in the cell wall prior to dissolution. The lignin ether-linked syringyl units were the most susceptible to alkaline degradation. Through the course of pulping, the residual lignin (RL) revealed a gradual increase of aliphatic moieties of unknown structure, as well as a decrease of native structures such as phenylcoumaran and pino-/syringaresinol lignin units. A significant decrease of the β-O-4 structures content in RL was detected only at the final cooking temperature. The lignin dissolved in the black liquor (BL) consisted of highly branched oligomers with rather low molecular weight (average mass 800–1000 u). A part of BL (about 30%) was chemically linked to carbohydrates and possessed a large molecular weight distribution (500–4000 u). BL showed a progressive decrease in β-O-4 and pino-/syringaresinol structures and formation of enol ether and stilbene structures. The GPC analyses showed a continuous decrease of the molecular weight of both the residual and dissolved lignins during the pulping process, particularly in the residual stage.     

Chemicals Generated During Oxygen-Organosolv Pulping of Wood

In this paper, we have characterized the black liquors from oxygen-organosolv pulping of Eucalyptus globulus wood in ethanol/water solutions (50:50, V/V) and acetic acid/water solutions (80:20, V/V).

After pulp separation, the organic solvent has been distilled and recycled; the precipitated lignin has been recovered by filtration. The degradation products in solution have been easily separated by simple extraction techniques. Five main groups of compounds have been isolated: solid lignin (11–12% o.d.w.), sugars (10–17% o.d.w.), aliphatic acids (12,5%), aromatic acids (2% o.d.w.) and low molecular weight phenols (1% o.d.w.). Most of these compounds were identified by HPLC and GC. Solid lignins have been analyzed by FTIR and ¹³C NMR spectroscopy.

It has been çoncluded that oxygen-organosolv pulping process can be a new and interesting way for pulp preparation with little pollution and possibility of total biomass recovery; alternatively, it can be used for the fractionation and valorization of the components of lignocellulosics, including agriculture residues.     

Condensation Reactions of Lignin During Oxygen Delignification Under Acidic Conditions

Spruce wood (Picea abies) has been subjected to delignification by oxygen under acidic conditions using different solvent media. The residual and removed lignins were submitted to permanganate oxidation and the products analyzed by GC and GC-MS. Results from the analysis of the residual lignins indicate that lignin “condensation” depends on the nature of oxidation medium. It was found that the addition of acetone to the aqueous liquor decreases both acid catalyzed and radical side reactions. Lignin “condensation” in water containing media is dominated by radical reactions. Results from the analysis of the lignin removed during oxygen delignification in an acetone/water medium indicate that significant amounts of “condensed” structures are present, which are relatively stable towards oxidation. The presence of diphenylmethane, diarylether and biphenyl type structures in the removed lignin was confirmed by CP-MAS ¹³C NMR spectroscopy.     

Fractionation of Lignocellulosic Materials for the Biorefinery: Separation and Characterization of Lignin from Calamagrostis angustifolia Kom

Dewaxed Calamagrostis angustifolia Kom was pretreated with hot water at 60 and 90°C for 3 h, and then the residue obtained was successively treated with 70% ethanol, and 70% ethanol containing 0.2%, 1.0%, 2.0%, 4.0%, and 8.0% NaOH at 80°C for 3 h. The dissolved components were subjected to further separation to get eight lignin fractions, which were characterized by gel permeation chromatography, Fourier transform infrared, and sugar analysis. All the lignin fractions had small weight-average molecular weights between 810 and 2580 g/mol. Two typical lignins, L₃ (prepared with 70% ethanol) and L₅ (prepared with 70% ethanol containing 1.0% NaOH), were further analyzed using ¹H, ¹³C NMR and HSQC spectroscopy. Signals from guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units observed in aromatic/olefinic region of HSQC spectra indicated that the lignin from Calamagrostis angustifolia Kom could be classified as “GSH” lignin. In aliphatic-oxygenated region, β-O-4′ together with small amounts of β-5′, β-β′, and p-hydroxycinnamyl alcohol end group were the main interunit linkages observed. Aqueous ethanol, which could avoid the cleavage of ether bonds in lignin at neutral condition, was more effective than water on lignin extraction.     

Graft copolymers of lignin with electron poor alkenes

The synthesis of copolymers between lignin and electron poor alkenes is described. Lignin from steam‐exploded pine, from steam‐exploded straw, and organosolv were used as starting materials. Beforehand, lignins were fully characterized by using elemental analysis, ultraviolet spectroscopy, gel permeation chromatography (GPC), Fourier transform infrared (FTIR), and both¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. The synthesis of copolymers was performed using a previously described procedure utilizing calcium chloride and hydrogen peroxide as reagents. FTIR of copolymers showed absorptions due to the presence of both lignin and the electron withdrawing group on the alkene. GPC analysis showed the presence of fractions with high molecular weights: the M_z of lignin from pine was 3729 while the copolymer with methyl acrylate showed M_z = 383790. Differential calorimetry showed the presence of glass transitions in the range of ?9 to 4.5°C due to the presence of grafted polyalkene chains. When acrylonitrile was used as starting material DSC analysis of the product showed a glass transition at 119°C, which can be attributed to grafted polyacrylonitrile chain. Lignin from steam explosion could be a good raw material in the preparation of graft copolymers. Furthermore, lignin from pine gave better results than that from straw. Finally, lignin from steam explosion gave better results than organosolv lignin. These results can be explained on the basis of the structural properties of used lignins. Both UV and ¹³C NMR spectra showed that lignin from pine contained a consistent amount of double bonds. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1163–1171, 2003     

Effects of Solvation and Ionization on 13C NMR Spectra of Lignin Model Compounds,Spruce Milled Wood Lignin and Kraft Lignin

The effects of alkali and polar aprotic solvent on the aromatic carbons signals in ¹³C NMR (Carbon-13 nuclear magnetic resonance) spectra of lignin model compounds and spruce milled wood lignin (MWL) were studied. It was found that in 1 M aqueous NaOH signal shifts of C-1 and C-4 carbon atoms in the aromatic ring were the most noticeable in lignin models with free phenolic hydroxyl groups, which are ionized under the conditions. A similar effect in the spectra of the studied model compounds was observed in 0.5 M aqueous NaOD-deuterated dimethyl sulfoxide (DMSO) mixture (DMSO: water ratio 3:7 v/v). The model data help explaining changes in the ¹³C NMR spectra of MWL and lignin in situ dissolved in spruce kraft black liquor caused by ionization. In the ¹³C NMR spectra of spruce black liquor the signals of phenolic and non-phenolic lignin units are clearly separated and do not overlap with the signals of the carbon atoms of carbohydrates and other aliphatic products of wood degradation. The data obtained are useful in understanding the important role of solvation and ionization processes leading to lignin solubilization.     

Hydroxymethylation of technical lignins from South American sources with potential use in phenolic resins

This work investigates the valorization of sodium lignosulfonate, kraft, and organosolv lignins from South America. A detailed characterization of the lignins and their chemical modification by hydroxymethylation through its reaction with formaldehyde were performed. The characterization included measurements of moisture, ash, carbohydrate contents, elemental and thermogravimetric analysis, and functional groups, molar mass distributions by Fourier transform infrared spectroscopy, and size exclusion chromatography, respectively. Also, reactive aromatic hydrogens ( H_Ar) were quantified by the measurement of phenolic hydroxyl groups (P-OH) content by UV–Vis spectroscopy. The different initial formaldehyde/lignin weight ratios (0.07, 1.47), temperatures (40, 50, and 70 °C), and pHs (9, 11); and the following of hydroxymethylation reactions by UV–Vis spectroscopy were investigated. All lignins resulted attractive for the use as replacement of phenol in phenolic resins, but sodium lignosulfonate was the most appropriate due to its water solubility. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47712.     

Structural characterization of residual lignins isolated with cyanamide-activated hydrogen peroxide from various organosolvs pretreated wheat straw

The posttreatment of various organosolvs pretreated wheat straw with cyanamide-activated hydrogen peroxide was studied. About 44–80% of the total residual lignin and 38–85% of the total residual hemicelluloses were released or degraded during the posttreatment with 1.8% H₂O₂–0.18% cyanamide at 50°C under pH 10.0 for 4 h from different aqueous organic acids or alcohols pretreated straw. The seven degraded residual lignin preparations were subjected to a comprehensive physicochemical and structural characterization by UV, FTIR, and ¹H and ¹³NMR spectroscopy, and GPC. The nitrobenzene oxidation method was also applied to the in situ lignins. It was found that the seven residual lignin preparations contained large amounts of noncondensed syringyl and guaiacyl units, together with fewer noncondensed p-hydroxyphenyl units, esterified p-coumaric acid, and mainly etherified ferulic acid. All of the lignin fractions are free of associated polysaccharides and had molecular-average weights ranging between 2980 and 3820 g mol⁻¹. Analysis of these low molecular weight degradation products revealed an oxidation of residual lignin had occurred. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Structural Characterization of the Lignins from the Green and Yellow Bamboo of Bamboo Culm (Phyllostachys pubescens)

Milled wood lignins were isolated from green and yellow bamboo (Phyllostachys pubescens), termed MWLg and MWLy, respectively. The structural features of the lignin preparations and the remaining LCC were characterized by quantitative ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy and two-dimensional heteronuclear signal quantum coherence NMR (2D HSQC NMR) spectroscopy. The results indicated that the main substructures in MWLg and MWLy were β-O-4 alkyl-aryl ether, resinol, phenylcoumaran, spirodienone, and α,β-diaryl ether, and their abundances per 100 Ar units were 38.2 and 39.8, 6.9 and 6.3, 3.8 and 2.9, 1.7 and 2.1, 0.4 and 0.3, respectively. The S:G:H ratios of MWLg and MWLy were estimated to be 40:54:6 and 51:44:5, respectively. The γ-acylation of green bamboo lignin was 17.22%, lower than that of yellow bamboo lignin (21.12%). Moreover, a flavonoid compound (tricin) was also detected in the MWLg and MWLy. Some carbohydrates remained in the purified MWL preparations, which were considered as the lignin-carbohydrate complexes (LCC).     

Extraction and characterization of lignins from maize stem and sugarcane bagasse

Lignins were isolated from maize stem and sugarcane bagasse by using mild dioxane or acidic dioxane solution. The result of nitrobenzene oxidation of the isolated lignins shows that there is a high proportion of p‐hydroxyphenyl alcohol in the lignins of maize stem and sugarcane bagasse. The lignins isolated from maize stem and sugarcane bagasse have relatively same value of the weight‐average (M _w = 3405–3868 g mol⁻¹) and number‐average (M _n = 1411–1612 g mol⁻¹) molecular weights, and polydispersity (M _w/M _n = 2.24–2.51). Acidic dioxane treatment did attack the β‐aryl ether structures in lignins, in particular for β‐aryl syringyl ethers, and broke the ester bonds between arabinose and ferulic acid that etherified to lignins, and it also cleaved lots of bonds in hemicellulosic polymer. The proportion of β‐O‐4 (threo) guaiacyl units is higher than that of β‐O‐4 (erthreo) guaiacyl units. The phenyl glycoside and benzyl ether linkages between lignin and hemicelluloses are also demonstrated in NMR analysis. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Physicochemical characterization of lignins from rice straw by hydrogen peroxide treatment

Extraction of the dewaxed rice straw with 1% NaOH at 55°C for 2 h and following treatment without and with 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0% hydrogen peroxide (H₂O₂) at 45°C for 12 h at pH 11.5 resulted in a dissolution of 68.3, 85.4, 89.4, 92.3, 92.3, 94.3, and 95.1% of the original lignin, respectively. Meanwhile, the two‐stage treatment together solubilized 67.2, 77.2, 78.7, 83.7, 85.5, 87.3, and 88.5% of the original hemicelluloses and degraded 2.5, 9.8, 11.8, 12.1, 15.6, 16.4, and 17.8% of the original cellulose under the conditions given, respectively. Analyses of these lignins revealed that alkali‐soluble lignin fractions did not suffer sever oxidation, but nearly 60% of the original lignin was dissolved out during the first stage of alkali treatment. In the second stage of alkaline peroxide treatment, the residual lignins were substantially released and enriched in oxidized carbonyl and carboxyl groups. In comparison, the isolated eight pure lignin samples were further characterized by both destructive methods such as alkaline nitrobenzene oxidation and nondestructive techniques such as ultraviolet (UV), Fourier transform infrared (FTIR), and carbon‐13 magnetic resonance spectroscopy (¹³C‐NMR) as well as gel permeation chromatography (GPC), and the results are reported. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 719–732, 2001     

Comparative study of three lignin fractions isolated from mild ball‐milled Tamarix austromogoliac and Caragana sepium

Six lignin fractions from mild ball‐milled Tamarix austromogoliac (TA) and Caragana sepium (CS) were sequentially isolated with 80% dioxane containing 0.05M HCl at 75°C for 4 h, 50% aqueous ethanol containing 1M triethylamine at 70°C for 4 h, and 8% aqueous NaOH at 45°C for 3 h. The results showed that the successive treatments made it possible to isolate lignin from wood with a high yield and purity, in which 89.4 and 90.6% of the original lignin from TA and CS were released, respectively. The lignin fractions isolated with the three‐step method were analyzed with Fourier transform infrared, ¹H‐ and ¹³C‐NMR, alkaline nitrobenzene oxidation, and gel permeation chromatography. It was found that the three lignin fractions isolated from TA were rich in syringyl units, and the molar ratio of the relatively total moles of vanillin, vanillic acid, and acetovanillin to the relatively total moles of syringaldehyde, syringic acid, and acetosyringone decreased from 1: 2.6 to 1 : 3.2 to 1: 3.6 in the lignin preparations, whereas this ratio in the corresponding lignin fractions isolated from CS was found to be 1.4 : 1, 1.1 : 1, and 1 : 1.4, respectively. More importantly, the results revealed that the sequential extractions of the mild ball‐milled TA and CS with 80% acidic dioxane, 50% alkaline ethanol, and 8% aqueous NaOH under the conditions used did not significantly cleave the β–O‐4 and α–O‐4 linkages in lignin macromolecules. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008