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.     

Characterization of the Residual Kraft Pulp Lignin In Situ by Sulfite Treatments

Abstract

The O₂ delignification of kraft pulps from Norway spruce was shown having a significant impact on the reactivity of the residual pulp lignin as revealed from their responses to sulfite treatments at pH 7.5. A substantial higher ratio of lignin sulfonation to the phenolic hydroxyl group content of residual pulp lignin was observed for the O₂ -delignified kraft pulps (～ 0.8) as compared to a value of ～ 0.3 for the unbleached samples and ～ 1 for the spruce wood lignin. Under the prevailing sulfite treatment conditions, the sulfonation would be largely attributed to the phenolic lignin component and the etherified structures containing an α -carbonyl or -unsaturated group. The contribution from the latter units, evaluated by a borohydride pretreatment of pulps prior to the sulfite treatment, can only account for approximately 15% of the sulfonation observed for the O₂ -delignified sample. Thus, the nature of phenolic structures in the O₂ -delignified pulps was more similar to that of the wood lignin than that of the kraft pulps.     

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.     

Structure and Reactivity of Spruce Mechanical Pulp Lignins Part III. Bleaching and Photoyellowing of Isolated Lignin Fractions

Organosolv lignin fractions were used as substrate for further investigations on the effect of bleaching and photoyellowing on lignin. These lignins were extracted from spruce TMP, bleached TMP (BTMP) and bleached and yellowed TMP (YBTMP). Furthermore, lignin fractions from TMP were also directly submitted to bleaching conditions and lignin fractions from BTMP to irradiation in the solid state. These lignin samples were all analyzed by thioacidolysis and FT-IR spectroscopy. The bleaching did not substantially change the lignin polymer but considerably destroyed some chromophoric structures such as coniferaldehyde end-groups. On the other hand, the photoyellowing resulted in marked degradations of lignin, shown by a decrease in the amount of β-O-4 and β-1 interunit bonds and a substantial increase in vanillin and vanillic acid end-groups and in catechol units.     

Structure and Reactivity of Spruce Mechanical Pulp Lignins Part I. Bleaching and Photoyellowing of in situ Lignins

This paper reports the structural investigation of a series of in situ spruce lignins in wood, thermomechanical pulp (TMP) and the corresponding bleached (BTMP) and photoyellowed (YBTMP) samples. This was achieved by using an original two-step degradative technique, thioacidolysis followed by Raney nickel desulfuration. The determination of thioacidolysis monomeric and dimeric products allowed an estimate of various lignin building units and interunit bonds, respectively. It was observed that the thermomechanical and bleaching treatments did not affect spruce lignin structure to an appreciable extent. On the contrary, the photoyellowing treatment caused marked structural changes in lignin, particularly when run in severe conditions. These changes were essentially a decrease in the amount of β-O-4 and β-1 interunit bonds and a relative increase in catechol units and in vanillin end-groups.     

Characterization of kraft pulp delignification using sodium dithionite as bleaching agent

Abstract

The delignification of kraft pulp with sodium dithionite was studied to remove lignin content in the pulp. The sodium dithionite dissolves the chromophoric groups and residual lignin present in the pulp. The increase in dosage of sodium dithionite and reaction temperature of delignification has positive effect on kappa reduction of pulp. X ray diffraction was used to determine the crystallinity index of bleached pulp. The crystallinity increased from 83.3% for unbleached pulp to 86.7% after delignification. Fourier transform infrared spectroscopy shows the reduction in hydrogen bonding in bleached pulp and also the conversion of cellulose I to cellulose II. FT-Raman spectra shows that the fluorescence observed in the spectra of unbleached pulp reduced significantly in comparison to the spectra of bleached pulp resulting in removal of residual lignin and chromophoric groups present in the pulp. Scanning electron imaging shows the smoothening of fiber surface after bleaching. The delignification reaction followed first-order kinetics and activation energy is 33.57kJ/mol.     

麦草浆二氧化氯与氧气脱木素效率及相应漂白流程的对比

对麦草浆二氧化氯与氧气脱木素效率及相应的漂白流程进行了对比研究。结果表明,ClO2脱木素(D0)对木素的修饰和改性效果好于氧脱木素(O),在脱木素率低于及接近氧脱木素率的条件下,D0浆的可漂性均大于O浆。在D0段和O段脱木素率相同的条件下,采用D0A/QP、D0EP漂白流程漂后纸浆白度分别比OA/QP漂白流程高11.2和10.3个百分点。D0段废液和相应D0A/QP漂白流程三段混合废水的CODCr负荷均比O段和OA/QP漂白流程混合废水的CODCr负荷轻。D0段和O段废液的紫外光谱都呈现出木素苯环的特征吸收。纸浆的FT-IR谱图分析表明,D0浆和O浆中的羟基和羰基数量增多,随着漂白的进行,羰基等主要发色基团逐渐消除,纸浆白度得到提高。纸浆纤维形态的SEM分析显示,D0段和相应的D0A/QP漂白流程对纤维表面的破坏和损伤较O段和相应的OA/QP漂白流程轻。     

氧脱木质素对稻草SFP浆性能的影响

亚硫酸盐―甲醛―蒽醌(SFP-AQ)法是适合于草类原料的一种制浆方法。文章研究了氧脱木质素对稻草SFP浆的卡伯值、白度、粘度及强度性能的影响。结果表明,较高的用碱量有助于残余木质素的脱除,在合适的工艺条件下,稻草SFP浆单段氧脱木质素的脱木质素率可达50%以上。在氧脱木质素用碱量3.5%、氧压0.4 MPa、温度110℃、时间60 min或90 min的条件下,稻草SFP浆的卡伯值较低、白度较高,而同时具有良好的粘度和强度性能。     

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.     

Xylanase effects on pulp delignification

Xylanase effects on softwood pulp delignification were investigated experimentally and using mathematical models. The effect of xylanase molecular size on pulp delignification was investigated. As xylanase molecular weight decreased from 67 kDaltons to 20 kDaltons, lignin removal from pulp increased from 30 wt% to 48 wt%, respectively. The rate of xylanase-aided pulp delignification was studied using a batch system. The rate-data was fitted to a mathematical model of the batch system that enabled estimation of process parameters including xylanase and lignin effective diffusivities, lignin mass transport coefficient, and effective particle diameter for mass transport. Parameter values thus obtained were used to simulate the semi-batch delignification process, which predicted 84.6% of lignin available to UGA xylanase (MW=39,000 Daltons) would be removed in 2.5 h.     

XYLANASE EFFECTS ON PULP DELIGNIFICATION

Xylanase effects on softwood pulp delignification were investigated experimentally and using mathematical models. The effect of xylanase molecular size on pulp delignification was investigated. As xylanase molecular weight decreased from 67 kDaltons to 20 kDaltons, lignin removal from pulp increased from 30 wt% to 48 wt%, respectively. The rate of xylanase-aided pulp delignification was studied using a batch system. The rate-data was fitted to a mathematical model of the batch system that enabled estimation of process parameters including xylanase and lignin effective diffusivities, lignin mass transport coefficient, and effective particle diameter for mass transport. Parameter values thus obtained were used to simulate the semi-batch delignification process, which predicted 84.6% of lignin available to UGA xylanase (MW=39,000 Daltons) would be removed in 2.5 h.     

Effect of Kraft Cooking Conditions on the Chemical Composition of the Surface and Bulk of Spruce Fibers

By varying cooking temperature, alkali charge, ionic strength, and cooking time in Kraft pulping of spruce chips, pulps ranging between kappa numbers 20–80 were obtained. The unbleached Kraft pulp fibers were subjected to mechanical peeling in order to separate the surface material from the bulk of the fibers and the carbohydrate composition and lignin content of the two fractions were analyzed. As expected, the lignin and xylan contents were higher on the fiber surface than in the fiber wall. The percentage of xylan on the fiber surface was fairly constant, independent of different pulping conditions or degree of delignification. The lignin proportion on the fiber surface gradually decreased with decreasing kappa number. At a given kappa number, pulping at a higher temperature resulted in less lignin on the fiber surface, probably because of the higher solubility of lignin at higher temperature. Cooking at lower alkali charge also resulted in lower lignin content on the fiber surface at a given kappa number. In this case, there was more time available for degradation of the surface lignin since the lower alkali charge resulted in longer cooking time needed to reach a certain kappa number.     

Dissolved Lignin and Other Aromatic Substances in Thermomechanical Pulp Waters

Abstract

Dissolved aromatic substances in water suspensions of unbleached and peroxide‐bleached spruce thermomechanical pulp (TMP) were isolated by sorption on XAD‐8 resin and fractionated into lignin and oligomeric aromatic substances (OAS). The fractions were characterized by chromatographic (GC and HP‐SEC) and spectrometric (FT‐IR, ¹³C‐NMR, GC‐MS) methods, as well as by GC after direct on‐line pyrolysis and thermally assisted hydrolysis and methylation. The dissolved lignin in unbleached TMP water was structurally similar to spruce milled‐wood lignin (MWL), but its average molar‐mass was lower and the mass distribution more narrow. The oligomeric aromatic substances included phenolic dimers, trimers, and tetramers that were structurally different from those of MWL. After peroxide bleaching the amount of dissolved semipolar (MTBE) extractives and oligomeric aromatic substances was much lower. The amount of dissolved lignin from wood fibers was, on the contrary, much higher. The lignin released from TMP after bleaching was extensively oxidized and had a slightly higher average molar‐mass than lignin released from unbleached TMP.     

Tetrahydrofurfuryl Alcohol (THFA) Pulping of Rice Straw

Abstract

A 80–95% solution of tetrahydrofurfuryl alcohol (THFA) added with 0.15–0.5% catalytic hydrochloric acid (HCl) was used to pulp rice straw. The pulping conditions applied for organosolv digestions of the straw at atmospheric pressure and 120°C cooking for 4 h. The characteristics of the digestion, chemical properties of the resulting pulp, and the handsheet physical properties were evaluated. As for the pulp yields, the method has high delignification specificity, at kappa number 20, the yield was ca. 60%, about 15–20% higher than the traditional alkaline pulping method. Furthermore, with increasing THFA concentrations, efficacies of delignification also increased. Increasing the catalyst dosage also caused an increase in delignification. Delignification rate of 120°C cooking are not so appreciable, but high yield were retained. If cooking temperatures were increased to 130 and 150°C, although even higher delignification rates were achievable, the yield decreased as well. During the cooking the dissolution of carbohydrate was low, at most 23%, consisting of mostly hemicelluloses, which was as high as 78% of the dissolved carbohydrates. The optimal conditions of the THFA/HCl cooking applied 95% THFA, 0.50% HCl, temperature of 120°C, and cooking time of 240 min. Residue lignin in the resulting pulp was low, and can be bleached to high brightness easily with a conventional bleaching sequence. If, however, energy and operation efficiency was a primary consideration, then a procedural heating scheme could be employed. The physical properties of the THFA pulp handsheets were inferior to those of the kraft pulp. The main reason was the damage to cellulose sustained during the acidic cooking condition.     

Reactions of Conventional Kraft and Superbatch Pulp Residual Lignins with Peroxyformic Acid

Abstract

The residual lignins KRL and SRL, isolated from a conventional kraft pulp and a SuperBatch pulp respectively, were reacted with peroxyformic acid (PFA). The reagent consumptions were determined and the products fractionated according to their solubility. Both lignins consumed a roughly equal amount of PFA, which was nearly 50% higher than that consumed by kraft lignin. The undissolved fractions of PFA-treated residual lignins (yield >80%) contained less aromatic units, phenolic hydroxyl and methoxyl groups, and considerably more carboxyl groups (including those of formate esters) than the untreated lignins. The extent of these modifications was approximately equal for KRL and SRL, which together with the similar PFA consumptions and product yields of KRL and SRL indicates that the two residual lignins showed no essential difference in reactivity toward PFA. The effect of fiber wall morphology on delignification efficiency during PFA delignification is suggested to be small because of the cleavage of lignin-carbohydrate linkages occurring under acid conditions. The part of KRL remaining insoluble after PFA-treatment had a 40% higher molar mass than KRL while in the case of SRL there was hardly any such difference in molar mass. The lignins solubilized during the PFA treatments represented the most highly degraded part of the lignins, having very low molar masses and being richer in carboxyl groups and lower in aromatic units, phenolic hydroxyl, and methoxyl groups than the undissolved lignins.     

Effects of the Preferential Oxidation of Phenolic Lignin Using Chlorine Dioxide on Pulp Bleaching Efficiency

Chlorine dioxide is widely used for pulp bleaching because of its high delignification selectivity. However, efficient and clean chlorine dioxide bleaching is limited by the complexity of the lignin structure. Herein, the oxidation reactions of phenolic (vanillyl alcohol) and non-phenolic (veratryl alcohol) lignin model species were modulated using chlorine dioxide. The effects of chlorine dioxide concentration, reaction temperature, and reaction time on the consumption rate of the model species were also investigated. The optimal consumption rate for the phenolic species was obtained at a chlorine dioxide concentration of 30 mmol·L⁻¹, a reaction temperature of 40 °C, and a reaction time of 10 min, resulting in the consumption of 96.3% of vanillyl alcohol. Its consumption remained essentially unchanged compared with that of traditional chlorine dioxide oxidation. However, the consumption rate of veratryl alcohol was significantly reduced from 78.0% to 17.3%. Additionally, the production of chlorobenzene via the chlorine dioxide oxidation of veratryl alcohol was inhibited. The structural changes in lignin before and after different treatments were analyzed. The overall structure of lignin remained stable during the optimization of the chlorine dioxide oxidation treatment. The signal intensities of several phenolic units were reduced. The effects of the selective oxidation of lignin by chlorine dioxide on the pulp properties were analyzed. Pulp viscosity significantly increased owing to the preferential oxidation of phenolic lignin by chlorine dioxide. The pollution load of bleached effluent was considerably reduced at similar pulp brightness levels. This study provides a new approach to chlorine dioxide bleaching. An efficient and clean bleaching process of the pulp was developed.     

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.     

Quantification of Lignin and Hexenuronic Acid in Bleached Hardwood Kraft Pulps: A New Calibration Method for UVRR Spectroscopy and Evaluation of the Conventional Methods

Abstract

Lignin content determination is an important task when pulp bleaching is studied. However, none of the conventional methods were developed for bleached pulps and therefore they are accurate mainly for unbleached pulps. This article describes a new, rapid method to determine lignin and hexenuronic acid contents of bleached hardwood kraft pulps based on UV resonance Raman (UVRR) spectroscopy. The lignin contents of pulp samples were determined from the aromatic band heights of the UVRR spectra. Therefore the measurements gave the content of aromatic lignin in pulp, and did not include extensively oxidized lignin structures. The method was applicable for hardwood kraft pulps with lignin content less than 1%. The measured lignin content correlated linearly with the kappa number. The lignin content (% on pulp) equaled 0.15κ+0.16. The constant 0.16 was presumably caused by the incomplete oxidation of the lignin in the kappa number determination. Klason lignin or total lignin determinations were not accurate for these kinds of pulps. Hexenuronic acid content was simultaneously determined from the UVRR band height of unsaturated C?O and C?C structures. The linear correlation of this band with hexenuronic acid indicated that the content of other unsaturated structures was constant in all the pulps, was proportional to the hexenuronic acid content of the pulps, or was insignificantly low. When compared to conventional methods, the UVRR spectroscopic method is fast, requires little sample and pretreatment, and the procedure has good repeatability. In addition, the accuracy of this technique increases with decreasing lignin content (<1%) making it a very attractive method for bleaching studies.     

The Influence of Sulfidity in Kraft Pulping on the Nature of Residual Lignin

The influence of sulfidity in kraft pulping of Norway spruce chips on the characteristics of residual lignin has been examined at two levels 25 and 40%. Comparing with the reference kraft cook at similar kappa number, the high sulfidity pulp lignin displayed a significantly lower phenolic hydroxyl group content, and a lower response to O₂ delignification, but it gave a slightly higher yield of nitrobenzene oxidation products, and was more responsive to a neutral sulfite treatment.     

The effects of lignocellulosic fiber surface area on the dynamics of lignin oxidation and diffusion

A novel preliminary experimental approach was developed to determine the effect of maximizing pulp fiber surface area for the regulation of the oxidation and diffusion of lignin through the carbohydrate matrix. The approach entailed liquid nitrogen freezing of mature black spruce kraft pulp fibers and subsequent mechanical grinding to a fine powder to test the effect of enhanced pulp fiber surface area on the efficiencies of lignin oxidation and lignin diffusion. It was found that the liquid nitrogen ground pulp samples provided higher optical reflectance (brightness) after both oxygen and hydrogen peroxide oxidation, which appears to be from chromophoric (chemical) differences and not light scattering differences as supported by UV/Vis spectroscopy. BET (absorption) experiments indicated a dramatic difference in the surface area of the pulp as a result of the grinding, although no differences existed among the pore sizes between the samples, as determined from SEM experiments. Lignin diffusion experiments demonstrated that differences did exist in the kinetics of lignin diffusion into a bulk solution between a control and liquid nitrogen ground samples. Lignin uptake into bulk solutions was enhanced by the liquid nitrogen/pulp‐grinding technique albeit not from chemical changes in the lignin because the subsequent NMR spectra did not point to any major chemical differences as a result of liquid nitrogen freezing and grinding. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 177–181, 2004