MECHANISTIC BEHAVIOR OF LIGNIN MODEL COMPOUNDS: INVESTIGATION ON THE LOWER BLEACHABILITY OF SODA/ANTHRAQUIONE (SAQ) PULP (original) (raw)
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Industrial & Engineering Chemistry Research, 2013
We have studied the structural modifications of lignins from sisal and flax during their soda-anthraquinone (AQ) pulping and subsequent totally chlorine-free (TCF) and elementary chlorine-free (ECF) bleaching. For this purpose, residual lignins were isolated from pulps, analyzed by Py-GC/MS, 2D-NMR, and GPC, and their characteristics were compared to the "milled-wood" lignin of the raw materials. Soda-AQ pulping caused a preferential removal of S-lignin and cleavage of β−O−4′ linkages, but the structure of the residual lignin remained relatively similar to native lignin. TCF bleaching barely affected the lignin structure, and noticeable amounts of β−O−4′ linkages still occur in these pulps. In contrast, ECF bleaching caused strong modifications in the lignin structure with the complete removal of lignin markers in ECF-bleached sisal pulp. However, residual lignin was still present in ECF-bleached flax pulp, with a predominance of G-and H-lignin units and the presence of β−O−4′ linkages.
THE CHEMISTRY OF BLEACHING AND POST-COLOR FORMATION IN KRAFT PULPS
In this review, the chemical structure of the residual lignin in softwood and hardwood is presented as far as we know it. With this knowledge as a starting point, the basic principles in bleaching of kraft pulp with bleaching agents such as chlorine dioxide, oxygen and hydrogen peroxide is discussed with an emphasis on differences between the behaviour of guaiacyl (softwood) and syringyl-guaiacyl (hardwood) lignin. The importance of other pulp constituents such as hexenuronic acid on the outcome of bleaching operations and various possibilities for removing this structure from the pulp is discussed. In the final part of the presentation, the post-color formation by storage or heat treatment in fully bleached chemical pulp is described and possible means of reducing this nondesirable effect are suggested.
INCLUSION OF A PRESSURIZED ACIDOLYSIS STAGE IN CHEMICAL PULP BLEACHING
Hardwood soda-AQ pulps are believed to be rich in benzyl sugar ethers (BSE) that can be partially cleaved by aqueous acidic treatments. The aim of this investigation was to evaluate the effect of acidolysis on final bleached brightness for kraft and soda-AQ (SAQ) hardwood pulps. The increase in final brightness due to acidolysis at 110 °C was twice as high for a eucalyptus SAQ pulp as compared to the kraft pulp. An oxygen delignified maple C-SAQ pulp (carbonate pre-treated SAQ) was acidolyzed at 120 °C and pH 2.6 for 30 min. When 1.60% ClO2 + 0.25% H2O2 on pulp was used in DEPD final bleaching of the control sample a brightness of 91.5% was achieved. When only 1.00% ClO2 + 0.25% H2O2 on pulp was used for the acidolyzed sample a brightness of 92.0% was attained. Analyses of the maple pulp after the acidolysis showed no major change in lignin content, brightness, or pulp yield. The minor changes suggest that a facile reaction such as benzyl ether cleavage was responsible for the improved bleachability. Preliminary research involving a lignin model compound and commercial birch xylan showed that lignin-carbohydrate condensation products were generated under SAQ cooking conditions. Furthermore, a fraction of these lignin-carbohydrate moieties were subsequently cleaved by acidolysis at pH 2.5 and 105 °C.
BioResources, 2010
The peroxymonocarbonate mono-anion (HCO 4 ─) is generated in solutions containing bicarbonate anions (HCO 3 ─) and hydrogen peroxide (H 2 O 2). The mono-anion is believed to have a pK a value of ca. 10 and as such would start dissociating to the di-anion (CO 4 2─) at pH ca. 8. The mono-anion should demonstrate electrophilic properties, while the dianion should be a nucleophile. Results that appear to be due to electrophilic reactions of HCO 4 ─ were presented in Part 1 of this series for lignin model compounds (LMCs) and chemical pulps. Some evidence was also observed for nucleophilic reactions with LMCs in the pH range of 8.8 to 9.5. Results are now being presented for mechanical pulp brightening, where nucleophilic reactions were observed. Hydrogen peroxide decomposition in the HCO 3 ─ solutions was significant on some occasions, and Fe catalyzed decomposition was the most significant contributor in both pulp slurries and pulp-free solutions.
On the importance of oxidizable structures in bleached kraft pulps
After cooking, kraft pulps always contain not only residual lignin but also significant amounts of hexenuronic acid and other non-lignin structures oxidizable by permanganate under the standard kappa number determination conditions. These here referred to as false lignin. Like ordinary lignin, the false lignin also consumes bleaching chemicals, thus increasing both the production costs and the environmental impact of bleach plant effluents. The false lignin also has an effect on pulp properties such as brightness stability. This necessitates the development of efficient experimental routines for the determination of false lignin in different types of unbleached and bleached kraft pulps, together with studies of its formation, chemical behaviour, and ultimate fate.
On the Behaviour of Spruce Thermomechanical Pulp Lignin during Hydrogen Peroxide Bleaching
Holzforschung, 1994
Lignins were isolated from spruce TMP and hydrogen peroxide bleached TMP (BTMP) by means of enzymatic hydrolysis with cellulases in yields of 67 and 78%, respectively. In addition, the TMP lignin, TMP EL, was subjected to a treatment with bleaching agent to obtain a bleached lignin, TMP ELB. The three lignin samples were all characterized by thioacidolysis and 13 C-NMR spectroscopy. Comparison of these lignin samples provided evidence about the behaviour of lignin towards alkaline hydrogen peroxide bleaching. It has been demonstrated that the bleaching does not affect the overall structure of lignin. The only one major change of the lignin moiety is elimination of coniferaldehyde end structures, which would be an important contribution to the brightening of pulp.
Holzforschung, 2007
Structural modification of eucalypt pulp lignin was investigated in a totally chlorine-free (TCF) bleaching sequence including a laccase-mediator stage. This stage was applied after two oxygen delignification stages, and was followed by an alkaline peroxide stage. After two oxygen delignification stages, two more stages with a laccase mediator and alkaline peroxide were applied. The residual lignins were enzymatically isolated from the different pulps and analyzed by spectroscopic techniques and analytical pyrolysis. The latter revealed high amounts of syringyl units ()70%) in the lignins. 13 C-1 H heteronuclear single quantum correlation (HSQC) NMR indicated high amounts of b-O-49 inter-unit linkages ()75% sidechains). Changes in lignin composition and inter-unit linkages were demonstrated in the course of the bleaching sequence. Moreover, oxidative modification of the major syringyl units was shown by C 2,6 -H 2,6 HSQC correlations and by the presence of oxidized pyrolysis markers in pyrograms. The existence of both C a keto and carboxyl groups in the residual lignin, together with normal (C ahydroxylated) units, was revealed by heteronuclear multiple bond correlation (HMBC) between aromatic H 2,6 and side-chain carbons. These C a -oxidized structures represent nearly 60% of total units in the lignin isolated from the enzymatically treated pulp. Analysis of residual lignin after the final peroxide stage compared with a simple alkaline treatment revealed that most of the oxidatively altered lignin was removed by the alkali used in the peroxide stage. Thus, the kappa number decreased and the final residual lignin was more structurally related to that found before the oxidative stages, although it contained less resinols and more carboxyl group-bearing units. However, the action of peroxide is necessary to attain the high brightness required ()90% ISO).
Holzforschung, 2000
Summary Residual lignins were isolated from unbleached and oxygen-bleached Eucalyptus kraft pulps by acid hydrolysis. The structural changes and degradation of residual lignin occurring during kraft pulping and oxygen bleaching were followed and identified by elemental analysis, residual carbohydrate analysis, molecular mass distribution, as well as qualitative and quantitative solution 13C NMR. The dissolved lignins in the kraft cooked and oxygen bleached liquors were also studied and compared with the corresponding residual lignins. Milled wood lignin treated under acid hydrolysis conditions served as a reference for the structural comparison. The results show that etherified syringyl structures were quite resistant towards degradation in the oxygen bleaching, causing little depolymerisation in residual lignin and a small increase in carboxylic acid content, but producing appreciable amounts of saturated aliphatic methylene groups.