Bo Hortling

Molar mass determination of lignins by size-exclusion chromatography: towards standardisation of the method

Abstract The reactivity and physicochemical properties of lignins are partly governed by their molar mass distribution. The development of reliable standard methods for determination of the molar mass distribution is not only relevant for designing technical lignins for specific applications, but also for monitoring and elucidating delignification and pulping processes. Size-exclusion chromatography (SEC) offers many advantages, such as wide availability, short analysis time, low sample demand, and determination of molar mass distribution over a wide range. A collaborative study has been undertaken within the “Eurolignin” European thematic network to standardise SEC analysis of technical lignins. The high-molar-mass fraction of polydisperse lignins was shown to be the main source of intra- and interlaboratory variations, depending on the gel type, elution solvent, detection mode, and calculation strategy. The reliability of two widespread systems have been tested: one based on alkali and a hydrophilic gel (e.g., TSK Toyopearl gel) and the other based on THF as solvent and polystyrene-based gels (e.g., Styragel). A set of practical recommendations has been deduced.

Proposed mechanism of the enzymatic bleaching of kraft pulp with xylanases

The modification of fibre surface by an enzymatic pretreatment was studied by analyzing the hydrolysis products released from the pulps and the molar mass distribution of the alkali-extracted residual lignin. In kraft pulp, the reprecipitated or readsorbed alkali-resistant xylan appears to form a physical barrier to the extraction of residual lignin from the fibres. The treatment of kraft pulp with hemicellulases removes some of this xylan and renders the fibre structure more permeable. The increased permeability allows the passage of lignin or lignin-carbohydrate molecules in higher amounts and of higher molecular masses in the subsequent chemical extraction. The improved extractability of lignin leads to higher brightness of pulp, or can alternatively be exploited to reduce the consumption of bleaching chemicals.

Cellulose crystallinity and ordering of hemicelluloses in pine and birch pulps as revealed by solid-state NMR spectroscopic methods

Solid-state 13C NMR spectroscopy was used to determine the degree of cellulose crystallinity (CrI) in kraft, flow-through kraft and polysulphide–anthraquinone (PS–AQ) pulps of pine and birch containing various amounts of hemicelluloses. The applicability of acid hydrolysis and the purely spectroscopic proton spin-relaxation based spectral edition (PSRE) method to remove the interfering hemicellulose signals prior to the determination of CrI were also compared. For softwood pulps, the spectroscopic removal of hemicelluloses by PSRE was found to be more efficient than the removal of hemicelluloses by acid hydrolysis. In addition to that, the PSRE method also provides information on the associations between cellulose and hemicelluloses. On the basis of the incomplete removal of xylan from the cellulose subspectra by PSRE, the deposition of xylan on cellulose fibrils and therefore an ordered ultrastructure of xylan in birch pulps was suggested. The ordered structure of xylan in birch pulps was also supported by the observed change of xylan conformation after regeneration. Similarly, glucomannan in pine pulps may have an ordered structure. According to the 13C CPMAS measurements conducted after acid hydrolysis, the degree of cellulose crystallinity was found to be slightly lower in birch pulps than in the pine pulps. Any significant differences in cellulose crystallinity were not found between the pulps obtained by the various pulping methods. Only in pine PS–AQ pulp, the degree of cellulose crystallinity may be slightly lower than in the kraft pulps containing less hemicelluloses.

Study of Residual Lignin in Pulp by UV Resonance Raman Spectroscopy

Summary A major problem in the development of new bleaching technologies for pulp is the difficulty of analysing the changes induced by bleaching chemicals in the structures of residual lignin. Separation and isolation of the residual lignin before the analysis may modify the constituents of interest, whereas if the pulp is analysed directly, the responses from cellulose and hemicelluloses will overlap the signal from the residual lignin. In this study, UV resonance Raman spectroscopy, a powerful technique for detecting trace components in complex mixtures, was applied to detect the changes in the residual lignin content after bleaching stages. The resonance Raman technique was found to be highly sensitive and selective for lignin structures allowing the detection of trace amounts of lignin after the final bleaching stages. Furthermore, it enabled rapid and easy determination of hexenuronic acid content. UV resonance Raman spectroscopy would appear to be a technique of great potential for pulping and bleaching research.

13C CPMAS NMR investigations of cellulose polymorphs in different pulps

In order to obtain information about the crystallinity and polymorphs of cellulose, and the occurrence of hemicelluloses in pulp fibers, wood cellulose, bacterial cellulose, cotton linters, viscose, and celluloses in different pulps were investigated by solid state 13C CPMAS NMR spectroscopy. A mixed softwood kraft pulp and a dissolving-grade pulp were treated under strongly alkaline and acidic conditions and the effect on cellulose crystallinity was studied. The presence of different crystalline polymorphs of cellulose and the amounts of hemicelluloses are considered.

Solid State NMR Studies on Cellulose Crystallinity in Fines and Bulk Fibres Separated from Refined Kraft Pulp

Summary Solid state NMR spectroscopy has been used in this work to investigate crystallinity of cellulose in spruce wood before and after kraft pulping and TCF bleaching. Effects of refining of the spruce kraft pulp in water and in weak alkali have been studied by determination of the crystallinity of isolated fines and corresponding bulk fibres in order to find out whether the crystallinity in the fibre surface material and inside the fibre wall differ from each other. Also the possible effect of enzymatic endo-1,4-β-galactanase treatment on the crystallinity of some refined pulps has been investigated. It was found that the Iα crystalline form of cellulose predominates over the Iβ form in native spruce and vice versa in all the pulps studied. In pulping part of the cellulose Iα is converted to the more stable Iβ form mainly by heat. Any measurable changes in the degree of crystallinity or in the relative proportions of different crystalline forms of cellulose could not be seen in pulps after refining, TCF-bleaching or galactanase treatment. However, the degree of crystallinity was found to be considerably lower in the fines compared to the corresponding bulk fibres. Therefore it is suggested that the cellulose crystallinity is lower or the size of crystallites smaller on the fibre surface than inside the fibre wall. Slightly higher contents of lignin and hemicelluloses, mainly xylan and mannan, were also found in the fines compared to the bulk fibres.

Carbohydrate structures in residual lignin-carbohydrate complexes of spruce and pine pulp

Abstract Residual lignin carbohydrate complexes (RLCC) were isolated enzymatically from spruce and pine pulp. The RLCCs contained 4.9–9.4% carbohydrates, with an enrichment of galactose and arabinose compared to the original pulp samples. The main carbohydrate units present in all studied RLCCs were 4-substituted xylose, 4-, 3- and 3,6-substituted galactose, 4-substituted glucose and 4 and 4,6-substituted mannose. These units were assigned to carbohydrate residues of xylan, 1,4- and 1,3/6-linked galactan, cellulose and glucomannan. RLCCs of surface material and the inner part of spruce kraft pulp fiber were compared to obtain information on the heterogeneity of layers of the fiber wall. The 1,4-linked galactan was the major galactan in RLCC of fiber surface material of spruce kraft pulp. Towards the inner part of the fiber, the proportion of 1,3/6-linked galactan increased relative to 1,4-linked galactan. This finding is presented for the first time. 1,3/6-Linked galactan structures are suggested to have a role in restricting lignin removal from the secondary fiber wall. RLCCs of three different alkaline pine pulps were studied before and after oxygen delignification to evaluate differences resulting from the cooking method. The pulps were conventional kraft pine pulp (PCK), a polysulfide/anthraquinone pine pulp (PPSAQ) and a soda/anthraquinone pine pulp (PSoAQ); all were cooked to approximately kappa number 30. Small differences were found in the carbohydrate structures of the unbleached pulps. The study indicated that the RLCC of unbleached PSoAQ pulp contained longer oligomeric carbohydrate chains and less branched 1,3/6-linked galactan residues than the RLCCs of unbleached PCK and PPSAQ pulps. The RLCC of the unbleached PSoAQ also contained more 1,4-linked glucose units suggesting a greater number of linkages of lignin to cellulose in the PSoAQ pulp than in the other two pulps. All RLCCs of oxygen-delignified pulps had more non-reducing ends and less 1,3/6-linked galactan than the corresponding RLCCs of the unbleached pulps. The RLCC of the oxygen-delignified PSoAQ pulp had a higher ratio of 1,4-galactan to 1,3/6-linked galactan and shorter xylan residues than the RLCCs of oxygen-delignified PCK and PPSAQ pulps.

A CP/MAS 13C-NMR study of cellulose structure on the surface of refined kraft pulp fibers

The average lateral fibril and fibril aggregate dimensions and the crystallinity of the cellulose in a spruce kraft pulp were investigated by CP/MAS 13C-NMR spectroscopy in combination with spectral fitting. Cellulose isolated by chlorite-delignification and acid hydrolysis from fines fractions enriched in surface material and long fiber fractions enriched in bulk material exhibited no major differences in either lateral dimensions or crystallinity index.

Characterization of Alkali Soluble Fraction of Steam Exploded Birch Wood

Birch wood chips, with or without peroxyacetic acid pretreatment, were subjected to steam explosion treatment at 160, 220 and 250°C for different periods of time to reach the same severity or P-factor values (1000, 15,000 and 96,000 min) at each temperature. The exploded pulp was subsequently extracted with water and a 0.4% alkali solution. The alkali soluble fraction was studied by means of elemental, methoxyl analysis, conductometric titration, 1 H-, quantitative 13 C-NMR and FTIR spectroscopy, and gel permeation chromatography methods. With an increasing P-factor value, the polysaccharide content of the alkali soluble fraction decreases rapidly, the syringylpropane unit and aliphatic OH-group concentration decreases, the phenolic OH-group concentration increases, lignin side chains are subjected to oxidation and partial destruction, and pseudolignin is incorporated into lignin macromolecules. The increase in temperature at equal severity of steam explosion has some effect on the removal of the syringylpropane units and on pseudolignin formation. Peroxyacetic acid pretreatment does not have a pronounced effect on the structure of alkali soluble compounds

Determination of Carboxyl and Non-Conjugated Carbonyl Groups in Dissolved and Residual Lignins by IR Spectroscopy

FTIR spectroscopy is a well known method for elucidating the structures of lignin samples. Determination of the ratio between the content of C=O groups and aromatic structures in lignin samples is an important aid to monitoring the degree of delignification during different pulping and bleaching procedures. In the present work a semiquantitative method is described in which calibration mixtures with eight different ratios of tartaric acid to milled wood lignin are prepared in order to account for the different responses of the aromatic band at 1510cm -1 and the C=O band at 1740cm -1 . From the calibration function obtained it is possible to calculate the results in mmoles of C=O/g aromatic lignin. The method is tested on residual lignins and spent liquor lignins from pulps with different degrees of delignification.