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.     

裂解-气质联用分析氧漂过程中木素结构的变化

从针叶木硫酸盐原浆、氧漂浆以及氧漂后的废液中分离出残余木素,在四甲基氢氧化铵（TMAH）存在下,采用裂解气相色谱-质谱联用(Py-GC-MS)的方法研究了氧漂过程中木素分子结构的变化.对裂解产物的分析表明,木素裂解时产生多种甲基化的芳香化合物,以愈创木基型化合物为主;氧漂能够氧化和降解木素分子,产生更多的羧基化合物,氧化程度高的木素分子容易从纸浆中溶出,氧漂优先脱除沉积在纤维表面上的短侧链木素.     

Characterization of Lignins from Organosolv Pulping According to the Organocell Process. Part 4. Molecular Weight Determination and Investigation of Fractions Isolated by GPC

Soluble and residual spruce lignins were subjected to permanganate oxidation and thioacidolysis to elucidate changes in lignin's structure during Organocell pulping.

As expected degradation reactions known from soda pulping are dominant especially in stage 2, including extensive enol ether formation. The chemical effect of methanol appears to be restricted to a minor methylation of the α-C-atom which prevents quinone methide formation and thus suppresses subsequent condensation reactions. Other beneficial effects of methanol such as liquor diffusion into the chips and a better lignin solubility obviously contribute to the successful delignification of softwoods in Organocell pulping.     

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.     

Characterization of Residual Lignins Isolated from Unbleached and Semibleached Softwood Kraft Pulps

Abstract

Residual lignins in an unbleached and a semibleached softwood (Pinus taeda L.) kraft pulp were isolated by enzymatic hydrolysis of polysaccharides in the pulps. After purification, the residual lignins were characterized. A dissolved lignin was also isolated from the alkaline extraction spent liquor by acidification and characterized.

Results of the characterization indicate that extensive degradation of residual lignin in kraft pulp occurred during the first two stages of bleaching. The results also strongly support the previous finding that stable covalent linkages between residual lignins and carbohydrates in pulp any be the most probable cause for the residual lignins to resist delignification during kraft pulping and prebleaching.     

Influence of Supercritical CO2 Pretreatment of Corn Stover with Ethanol‐Water as Co‐Solvent on Lignin Degradation

The process of delignification during the pretreatment of corn stover in supercritical CO₂ with ethanol‐water as co‐solvent was investigated. After pretreatment, many lignin droplets were found deposited on the fiber surface which hinder cellulose digestibility. These lignin droplets were removed by ethanol‐water and after washing the optimal glucose yield increased significantly. Lignin degradation reactions competed with condensation reactions during pretreatment. The cleavage of ether bonds and the high solubility of lignin fragments in ethanol‐water co‐solvent were the key factors for lignin removal and degradation behavior during pretreatment.     

The Influence of the Bleaching Medium on Caustic Extraction Efficiency (II): Oxidized Lignin Solubility

Abstract

Caustic extractions performed with >50% ethanol solutions severely hindered lignin removal from chlorine dioxide (D₀) treated pulps. In this study, we show that the alkali solubility of an isolated D₀ lignin decreased as the concentration of ethanol in the medium was increased from 50 to 75%. In addition, the amount of dissolved lignin in the alkaline extraction effluents approached its solubility limit when the medium contained more than 50% ethanol. These results indicate that alkaline extractions of D₀-stage pulps with ethanol-rich media were limited by the lower solubility of ionized lignin fragments in these solutions as compared to water. An empirical solubility parameter analysis predicted that non-ionized D₀ lignins should be more soluble in ethanol than in water. Indeed, delignification with an acid extraction stage was improved when the water was replaced with ethanol.     

THE INFLUENCE OF THE BLEACHING MEDIUM ON CHLORINE DIOXIDE DELIGNIFICATION

Chlorine dioxide delignification (D₀) stages performed in 90% ethanol resulted in post-D₀ kappa numbers that were 2 to 2.5 units lower than those performed in water. In spite of the reduction in kappa number, various gravimetric and UV spectroscopic techniques for measuring residual lignin indicated that the ethanol medium did not significantly enhance D₀ stage lignin removal. Differences in post-D₀ kappa numbers between the ethanol-based and aqueous systems were ascribed to how the bleaching medium affects ClO₂ oxidation of residual lignin. Ethanol-based D₀ stages resulted in an oxidized lignin that contains fewer muconic acid structures and more quinone structures than the aqueous-based systems. Indirect evidence from sodium hydrosulfite reduction implied that quinone moieties consume less KMnO₄ than aromatic structures in lignin during the kappa number test.     

Kinetics of Wheat Straw Delignification in Soda and Kraft Pulping

Kraft and soda delignification kinetics of wheat straw, Triticum aestivum, have been studied as a function of OH “concentration, HS” concentration, and temperature. It was found that about 90% of the lignin is dissolved in the rapid initial phase. The rates of delignification in the bulk and residual phases were found to be similar to those of birch wood, Betula pubescens. The amount of residual phase lignin was found to be considerably less than in birch or spruce, Picea abies, and it was affected by the same factors, A model that accurately describes the delignification of wheat straw was derived.     

Lignin and Other Aromatic Substances Released from Spruce Wood During Pressurized Hot-Water Extraction,Part 2: Structural Characterization

Extraction of ground spruce sapwood with pressurized hot water in an accelerated solvent extractor (ASE) at 170°C during 20, 60, and 100 min resulted in isolation of galactoglucomannans and aromatic substances, including lignin. The isolated lignin preparations were characterized by spectrometric (UV, FT-IR, ¹H NMR, liquid and solid-state ¹³C NMR), chromatographic (RP-HPLC, HP-SEC, GC-FID, and GC-MS), conventional pyrolysis, thermally assisted hydrolysis, and methylation techniques in tandem with GC-MS, and classical wet chemistry (methoxyl groups, total and phenolic hydroxyl groups, derivatization followed by reductive cleavage—DFRC). The content of β-O-4 bonds in isolated lignins was similar to that in MWL and their proportion decreased with extraction time. The oxidation of isolated lignins and content of total hydroxyl groups were significantly increased with extraction time. The lignin structure underwent condensation and demethylation reactions during hot-water extraction. The induction of new phenylcoumaran substructures was proposed in isolated lignins.     

Physicochemical characterization of lignin recovered from microwave‐assisted delignified lignocellulosic biomass for use in biobased materials

Lignocellulosic biomass (Moso Bamboo, Chinese tallow tree wood, switchgrass, and pine wood) was subjected to a novel delignification process using microwave energy in a binary glycerol/methanol solvent. The physicochemical properties of the recovered lignin were analyzed prior to its application in the fabrication of polylactic acid (PLA)–lignin composites. The results showed that the samples had a high Klason lignin content (>75%) and retained their natural structure. Thermogravimetric analysis revealed that the recovered lignin exhibited a different thermal decomposition pattern from that of commercial lignins. All the recovered lignins had good solubility in common organic solvents (acetone, 1,4‐dioxane, THF, DMSO, and ethanol/water) and 1 mol/L NaOH solution. The addition of lignin into the PLA matrix resulted in the improvement in tensile properties of PLA–lignin composites. PLA films with low lignin content had good UV light‐resistant properties, indicating that the recovered lignin has potential in packaging of light‐sensitive products. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42635.     

Polyoxometalate catalyzed ozonation of chemical pulps in organic solvent media

Polyoxometalate (POM) catalyzed ozonation of chemical pulps in organic solvent media was found to be particularly effective and selective environmentally benign bleaching approach providing a way for substantial increase in pulp brightness, viscosity and degree of delignification in comparison with other ozone-based bleaching techniques. A series of tested low-boiling polar aprotic and protic organic solvents showed a well-defined capacity for ozonation improvement in the presence of Keggin-type heteropolyanion [PMo₇V₅O₄₀]⁸⁻ (HPA-5). Even moderate solvent proportion of 6% (w/w) in the reaction solution caused additional gain in brightness up to 3.4% ISO with simultaneous increase in pulp viscosity up to 8.8% and lignin removal up to 18.9% after HPA-5 catalyzed ozonation (0.8% O₃; 0.5 mM HPA), as compared with the control solvent-free process. An aqueous acetone solution was found to be the preferred reaction medium in terms of pulp brightening and delignification. Under optimized conditions, the POM-catalyzed ozonation of eucalypt kraft pulp in acetone/water solution showed remarkable brightness improvement by 15.1% ISO with additional lignin removal by 39.4% and increase in intrinsic viscosity by 3% in comparison with pulp bleached in water media.     

稻草木素生物降解的研究

介绍了我们最近对稻草木素生物降解研究进展。通过诱变选育分离出高降解木素的优良菌株,10天内脱木素率高达59%,明显地缩短了生物作用周期;利用电子显微镜技术,详细揭示白腐菌生物降解草类植物的微观过程和稻草组织内部以及纤维细胞壁的微观结构的变化,同时,发现所选育的菌种具有:(1)优先降解纤维胞间层,致使纤维能保持完整地分离;(2)没有发生纤维细胞壁变薄破坏的现象;(3)生物作用初期就能分解除去不利于造纸的薄壁细胞;(4)能选择性降解纤维次生壁上的木素,微细纤维结构不被破坏而裸露出来等非常可贵的优良特性。这些特性十分有利于生物制浆造纸。系统分析和研究了生物脱木素过程及一些重要生理条件对脱木素的影响规律。发现生物脱木素过程可分为三个阶段。不同阶段生理条件对脱木素的影响不同。通氧培养、添加营养氮源或碳源及控制底物的湿含量能促进生物脱木素作用。为了解稻草木素生物降解规律,利用核磁共振技术定量及定性分析比较生物作用前后木素的结构特征。结果表明木素大分子的生物降解主要是氧化反应,在菌的作用下,木素结构中 C_γ被氧化、C_α—C_β键、C_β—C_γ,键以及芳香环被氧化断裂。其降解顺序为:以酯键连结的对香豆酸为主体的对羟苯基单元,愈疮木基单元和紫丁香基单元。菌优先作用于纤维细胞间层的结论与电子显微镜观察结果一致。上述特征和规律显示白腐菌 P.conchatus 对发展草类生物制浆及其他脱木素工艺均具有巨大潜在应用优势。     

Effect of Soda-Additive Pulping on Molecular Characteristics of Residual Pulp Lignin

The soda-additive pulps of pine wood were compared to corresponding soda and kraft pulps in respect to the macromolecular properties of the residual pulp lignins. Results of measurements of number and weight—average molecular weights as well as branching parameters show that the presence of additives such as anthraquinone, methanol and O₂—pretreatment significantly causes the breaking of cross-links in the pulp lignin. On the other hand, ethylenediamine and hydroeulfide ion contribute mainly to extensive lignin degradation. The influence of used additives on the condensation reactions of the residual lignin has also been studied.     

Molecular Mass Distribution of Lignin from the Alkaline Pulping of Hardwood,Softwood, and Wheat Straw

Abstract

The behavior of lignin during kraft (hardwood, softwood, and wheat straw) and soda-AQ (wheat straw) pulping was studied, mainly in terms of delignification degree and molecular mass distribution (MMD). In the initial delignification phase (at 140°C for 15–60 min), a prominent part of the dissolved softwood kraft lignin (18–25 g/L, MM mostly > 3,000 Da) was found in the liquid phase of chip cavities, rather than in the external bulk black liquor (5–7 g/L, MM mostly < 3,000 Da). The maximum weight average MM values ( _w) of the soluble lignin under conventional cooking conditions were detected for the kraft softwood (4,100 Da), and kraft birch (3,400 Da) when the degree of delignification was 65–75%, corresponding to a residual lignin content of 5–10%. The maximum _w of the dissolved wheat straw kraft (5,050 Da) and soda-AQ (5,900 Da) lignins was clearly higher than that of wood-based kraft lignins (2,950–4, 100 Da).     

Dissolution of Lignin in Kraft Pulp by Oxygen Bleaching

Gel permeation chromatography of spent liquors from oxygen bleaching showed that cleavage of covalent linkages in the fiber was a prerequisite for the dissolution of the lignin. The reactions continued in the liquor phase. The cleavage was related to the oxidation of the lignin. With pulps pretreated with nitrogen dioxide an appreciable number of lignin linkages were cleaved also by the direct influence of the hydroxide ions on the nitrated lignin. In the alkaline medium the acidic sites in the fiber and in dissolved lignin fragments are ionized. Donnan-exclusion and an increased fiber swelling promote the transfer of the fragments into the liquor. Hydrophilic groups introduced into the lignin during oxygen bleaching promote the delignification.     

Structure and Reactivity of Spruce Mechanical Pulp Lignins Part II. Organosolv Fractionation of Lignins in a Flow-Through Reactor

Various spruce mechanical pulps were subjected to delignification with ethanol-water (1 : 1, v / v ) containing 0.1 M acetic acid at 175°C in a flow- through reactor. A thermomechanical pulp (TMP) and the corresponding samples derived from its bleaching (BTMP) and yellowing ( YBTMP ) treatments were delignified to a similar extent, about 70% of delignification degree, as compared with spruce wood. A series of five successive lignin fractions was recovered from each pulp sample and then characterized by various analytical methods. Large structural variations were observed within these fractions. The number of phenolic structures appeared to be an important factor influencing the dissolution of lignins in the ethanol-water medium. These lignin fractions were found to be different in the amount of β-aryl ether structures and in the relative importance of main carbon-carbon bonding patterns. The results are discussed in relation to the lignin fractionation in the flow-through reactor.     

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.     

Effects of Green Liquor (GL) and Sodium Carbonate (SC) Pretreatment on Structural Characteristics of Wheat Stem Lignin

The content and structure of lignin have been considered as important factors that affect both pretreatment and enzymatic hydrolysis of lignocellulosic biomass. In this work, wheat stems (WS) were pretreated using mild alkali including green liquor (GL) and sodium carbonate (SC). The results indicate that GL pretreatment exhibits better delignification selectivity and higher enzymatic digestibility than SC pretreatment. Analysis of ¹H–¹³C HSQC NMR and FTIR on cellulolytic enzyme lignin (CEL) preparations isolated from untreated and pretreated WS also proves that a certain amount of lignin degrades which leads to a decrease of β-O-4′ linkages. Under mild alkaline conditions, more guaiacyl units in lignin are removed than syringyl units, which results in a higher condensation degree and S/G ratio of CELs isolated from GL- and SC-pretreated stems. Compared with p-coumarate structures, ferulates in lignin are more stable under mild alkaline conditions.     

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.