Volume 67, Issue 5 - Selected papers from the 12th European Workshop on Lignocellulosics and Pulp

Industrial Eucalyptus globulus wood chips were submitted to different autohydrolysis conditions followed by kraft cooking and soda-anthraquinone cooking. The autohydrolyzed wood chips were much easier to delignify than the control wood chips. Soda-anthraquinone cooking could be performed at a cooking temperature that was 20°C lower than that for the kraft cooking on control wood chips. Furthermore, the active alkali could be reduced. The resulting unbleached pulps reacted as well to oxygen delignification as the control pulps and could be further bleached to 90% ISO brightness with a D(EP)D sequence. The autohydrolysis liquors were investigated for their suitability as a source for the synthesis of alkylpolyxylosides (APX). These surface-active agents are synthesized through the reaction between the saccharides of the autohydrolysates and a fatty alcohol, with the former being the hydrophilic and the latter being the hydrophobic part of the molecule. The impact of the substances detected in autohydrolysates on the APX synthesis was studied. It was demonstrated that lignin dissolved during autohydrolysis should at least partially be removed before the production of APX.

The importance of Eucalyptus wood as a sustainable resource is well established. Mechanical pulp production is an energy-intensive process, and methods for decreasing energy demand are needed. In the present article, the structure of Eucalyptus hybrids and the produced pulps from the hybrids were assessed in terms of energy consumption, technical properties, and fiber morphology. The defibration and fiber development were investigated by traditional thermomechanical pulping in laboratory scale. There was no clear difference in the extent of defibration and fibrillation among the hybrids, with the exception of one hybrid (U2xGL1). Guaiacyl lignin contents of more than 7.2% had a minor influence on energy consumption, and this finding is different from the results of previous studies on chemimechanical pulping processes. However, a low amount of guaiacyl lignin (6.1% based on wood) decreased the energy demand. This assessment is of major importance concerning the energy-saving defibration during thermomechanical pulping production.

Xylans isolated from eucalypt kraft pulps have been characterized by modern analytical methods. The pulps were partially bleached (DE OP D) and fully bleached with either a final ClO 2 (DE OP DD) or an alkaline H 2 O 2 (DE OP DP) bleaching stage. Alternatively, xylan isolated from the DE OP D pulp was treated with ClO 2 or with H 2 O 2 under the same conditions as pulps in a final bleaching stage and were further characterized. 1 H nuclear magnetic resonance, size exclusion chromatography, UV-vis spectroscopy in cadoxen solution, and UV-resonance Raman spectroscopy were applied as analytical methods. The final ClO 2 stage generated new unsaturated moieties in xylan, whereas H 2 O 2 was very effective in the removal of xylan-related chromophores. The role of xylan to the delay of brightness development in the final ClO 2 stage was highlighted by means of a pretreatment with xylanase Pulpzyme HC before the final bleaching stage. By this approach, the final bleachability of the pulp was boosted and the brightness stability of the fully bleached pulp was improved.

Micropropagated hybrid aspen clones ( Populus tremuloides Michx.× Populus tremula L.) and a plus-tree (superior phenotypes selected) aspen ( Populus tremula L.) were grown under similar conditions in the central part of Latvia. After cutting at the age of 12 years, 64 sample trees were examined. The dimensions of the vessels and the content of cellulose, lignin, extractives and ash were determined. The cell walls were characterised by UV microspectrophotometry (UMSP) in the UV range. The dimensions of fibres obtained by kraft pulping were determined such as length, width, and their shape factors. Coarseness measurements were performed, and strength properties of the handsheets were tested. One of the clones (no. 44) had a significantly higher stem volume of 208 dm 3 and stem diameter of 17 cm at a height of 1.3 m. This clone had otherwise no notable differences to the other clones concerning the chemical composition, except a slightly higher lignin content (20.0% as compared with the mean value of 19.3% for other clones). Kraft cooking of hybrid aspen chips gave pulp yields in the range of 48.6–52.4%. Slightly higher strength properties (10–15%) were found for the handsheets prepared from a common aspen pulp (reference) as compared with those from hybrid aspen fibres.

In this article, a phenomenological model is proposed for alkali impregnation and hot water extraction of wood. Elementary reaction kinetics, ion exchange (IE) equilibrium, and mass transfer rates are considered in the model. IE is a consequence of the dissociation of the covalently bound uronic acid and phenolic groups in the wood fiber wall (FW). Due to this effect, the molality of cations is higher in the FW liquid than in the liquid external to the FW, whereas the opposite is true for anions. For the development of kinetic models under the participation of hydroxyl and hydrogen ions, the estimation of their molalities in these two liquid phases is crucial because of their ubiquitous presence in chemical reactions. This study applies a recently developed physicochemical modeling environment in which alkali impregnation and hot water extraction processes are considered. The simulations revealed that the IE effect is significant and affects the chemical reaction kinetics in both processes in focus. Furthermore, the IE effect does not remain constant but varies as a function of treatment time.

A combined pretreatment of sugarcane bagasse with glycerol and sulfuric acid was investigated based on a central composite rotatable experimental design. The following factors were varied: temperature (150–199°C), time (0.69–2.3 h), H 2 SO 4 concentration (0.0–1.1%), and glycerol concentration (55.4–79.6%). Xylans and lignin were considerably solubilized during pretreatment. Xylan solubilization, ranging between 6% and 94%, increased significantly with the increase of temperature, time, and H 2 SO 4 concentration and dropped with the increase of glycerol amount. Glycerol restricted the solubilization and full hydrolysis of xylans and the degradation of xylose. Lignin solubilization (20.6–49.4%) increased with the increase of all the experimental factors. Cellulose recovery, which was generally high, increased with the increasing of glycerol concentration and declined at high levels of the other factors. Recoveries above 97% were achieved at low H 2 SO 4 concentration and high glycerol load, whereas the lowest value (83.4%) was achieved in the longest-lasting experiment. The models based on the experimental results predicted the maximal lignin solubilization at 187.7°C, 2.3 h, 79.6% glycerol, and 0.64% H 2 SO 4 , whereas the highest yield of enzymatic hydrolysis can be expected at 194.1°C, 1.67 h, 79.6% glycerol, and 1.1% H 2 SO 4 . The optimal conditions were confirmed in control experiments. The synergistic effect of sulfuric acid and glycerol on the enzymatic hydrolysis of cellulose was demonstrated.

Raman spectroscopy is a technique that provides structural information on lignin and other components of wood and pulp in situ . However, especially lignin-containing samples may produce laser-induced fluorescence (LIF) that overlaps with Raman bands. In the worst case, this background signal can overwhelm the weaker Raman signal completely. In this study, the LIF of lignin was investigated with the excitation wavelength 532 nm applied in Raman spectroscopy to clarify the correlations between lignin structure and LIF intensity. Raman spectroscopic analyses with lignin model compounds illustrated that the 5-5′ structures induce LIF. It was also shown that the intensity of LIF was significantly less intense when the 5-5′ model compound was structurally rigid (as in dibenzodioxocin) compared with the flexible simple counterpart. The comparison between the free phenolic model compounds with the methylated analogue showed that the presence of the free phenolic structure was not a prerequisite for LIF. It was thus concluded that the conformation of the molecule is the key factor with respect to fluorescence. The role of conformational aspects was further investigated by comparing wood with chemical pulps and isolated lignins.

The oxidative modification of Biolignin (BL) has been investigated to make it more suitable as an adsorbent for transition/heavy metals. BL is a by-product of a wheat straw organosolv process for the production of pulp, ethanol, and pentoses (CIMV S.A. pilot plant, Levallois Perret, France). It was subjected to oxidation by a polyoxometalate (POM) H 3 [PMo 12 O 40 ], aiming at the increment of oxygen-containing adsorption-active sites. The POM oxidation of BL was performed under moderate conditions (1 bar, 60–90°C, and 200°C) with the co-oxidants O 2 or H 2 O 2 . The resulting lignin functionality and structure was evaluated by pyrolysis-gas chromatography/mass spectrometry, solid-state 13 C nuclear magnetic resonance, Fourier transform infrared, and chemical analysis. The condensation degree of BL and its COOH and aliphatic OH group contents increased significantly, whereas the polymer structure was maintained. Under optimal conditions with POM/H 2 O 2 , the sorption capacity of lignins toward Cd(II) and Pb(II) was increased threefold and twofold, respectively.

The solubility of lignosulfonates (LSs) in water is strongly dependent on other ions present in the water phase. The differences in the solubility might strongly influence the measurements of the physical and chemical properties of the LS molecules. A reduced solubility of the LS might also affect its utility in many practical applications. The understanding of the interaction between LSs and various salts is important for both practical and theoretical reasons. Therefore, the effect of salt concentrations on the LS has been investigated for 41 different salts with 14 different cations and 16 different anions. The observations cannot be explained by the common ion effect or the screening effects. On the contrary, it was found that the stability of LS solutions follows the Hofmeister series, with the exception of those ions that will chemically interact with the LS molecule. Moreover, the positions of phosphate (HPO 4 2- ) and sulfate (SO 4 2- ) ions were reversed.

The impregnation of wood pieces in the course of pulping can be divided into primary (advective mass transport) and secondary (diffusive mass transport) penetration. Little is known about the latter partly because of the difficulties in the determination of the relevant diffusivities for this system. In the present article, a precise experimental methodology has been developed to measure the concentration profiles of cations as a function of wood piece dimensions, impregnation time, temperature, and wood structure. The cation concentration can be measured at any position in the impregnated wood piece. The impregnation of Norway spruce wood samples with LiCl was investigated. The impregnated wood pieces were cut mechanically into cubes, which were cut in slices by means of a microtome, and the eluate of the 0.4-mm-thick slices in HNO 3 was analyzed by flame atomic emission spectroscopy. The method gave reasonable results, and defects in the wood piece (microcracks) were detectable. The preliminary results presented here have to be verified with more replicates due to the heterogeneity of wood.

Polycarboxylic acids (PCAs) aroused interest as crosslinking agents for natural polymers, such as cellulose, starch, chitosan, and hemicelluloses (HC), for improving their water resistance and imparting them new properties. Here, PCAs were studied for the crosslinking of HC films. HCs were isolated from gray alder ( Alnus incana L.) wood as a water-soluble fraction by sawdust extraction with 7% potassium hydroxide. Citric acid (CA), maleic acid (MA), and butane tetracarboxylic acid (BTCA) served as crosslinking agents in the presence of sodium hypophosphite as a catalyst. The crosslinking reaction was followed by the analysis of free and esterified COOH groups in HC films. At temperatures below 100–110°C, the HC esterification process with BTCA proceeded very slowly. At 140°C, most of the acids’ COOH groups reacted with HC during 1–1.5 h. BTCA had a higher esterification activity than CA and MA. The obtained films retained their form on immersing in water and subsequent drying in the open air. The water retention value of films followed the reaction kinetics, that is, decreased with increasing reaction time. Water-resistant films were obtained with no <10–11% of BTCA and 20% of CA based on the weight of HC. The water vapor sorption capacity of the films after modification with 10% BTCA decreased approximately twice in the whole range of the relative humidity of air compared with the case of unmodified HC films.

There is a need for new, antibacterial cellulose-based materials. Antibacterial cellulosic fibers with irreversibly attached polyvinylamine (PVAm) and polyacrylic acid (PAA) in multilayers were developed based on a water-based physical adsorption process. The antibacterial substance is thus prevented from leaching, in contrast to materials containing, for example, silver. It was shown on fibers from different sources that the antibacterial effect against both Escherichia coli and Bacillus subtilis can be tailored. The efficiency correlated with the initial fiber charge, which in turn correlated with the amount of adsorbed PVAm. In the case of highly charged fibers, the antibacterial efficacy was more than 99.9%. A sustainable antibacterial material can be obtained in a simple way based on a water-based process.

Multilayered thin films have been prepared by means of the “spin-assisted electrostatic layer-by-layer self-assembly” method, in which cellulose nanocrystals (CN) and cationic xylans (CX) are alternatively deposited up to 10 times on a rotating silicon wafer. The film growth process was studied and the thickness increment was found to be equal to 23 nm per bilayer. This value is relatively high in comparison with that of previous studies. Atomic force microscopy revealed that the surface of the films consists of thick layers of CX, which are deposited as a compact network of aggregates on the CN layers. After a few deposition cycles, structural color appears. When the film is submitted to enzymatic hydrolysis of xylans, the thickness of the film decreases and a visible color change is induced. The sensitivity of the test was evaluated in comparison with a usual colorimetric measurement, which relies on the detection of reducing sugars set free by enzymatic hydrolysis. The assay sensitivity was found to be similar to that of the colorimetric method. The presented new method is simple, fast, and easy to use. These findings show that the method based on multilayer thin films might open new opportunities to optimize the screening assays for xylanase discovery.

Highly developed nanoporous carbon materials have been prepared by a two-stage thermocatalytic process. In the first step, alder ( Alnus rhombifolia ) and birchwood ( Betula pendula ) were carbonized with and without a dehydration catalyst (H 3 PO 4 ); in the second step, the material was activated by means of NaOH. The dependence of the porous structure of activated carbons from process parameters was characterized by the novel limited evaporation technique. Specific surface areas, pore volumes, and radii were calculated according to the Derjaguin-Broekhoff-de Boer theory. The tests of activated carbons as electrodes in supercapacitors demonstrated their high potential for this application.

The effect of the hydrothermal modification (HTM) of the deciduous woods birch and aspen on their sorption behavior has been investigated by the vapor sorption method. An analysis of the experimental results was carried out based on the concept of Hansen solubility parameters (HSP), which takes into account the contribution of different forces – dispersion forces, dipole action, and hydrogen bonding – to the total cohesion energy. Sorption isotherms were measured concerning the vapors of water, methanol, and ethanol with unmodified and HTM woods at 160°C and 170°C during 3 and 1 h, respectively. The choice of sorbates was based on the parts of the hydrogen bonding and dispersion force to cohesion energy, in decreasing order of the former and increasing order of the latter. As a criterion of sorption, the value of the monolayer capacity was used, which was derived from the Brunauer-Emmett-Teller equation. Vapor sorption with unmodified and modified wood increased with increasing dispersion force component of the HSP of the sorbate. However, more substantial increase occurred for HTM wood, that is, wood surface became more hydrophobic. The reason for this observation is the change in the decreasing ratio holocellulose/lignin upon HTM. However, the chemical structure of lignin is also changed by HTM.