Ozonation of conventional kraft and SuperBatch residual lignins in methanol/water and water

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1 Holzforschung, Vol. 58, pp , 2004 Copyright by Walter de Gruyter Berlin New York Ozonation of conventional kraft and SuperBatch residual lignins in methanol/water and water Petri Widsten 1, *, Bo Hortling 2 and Kristiina Poppius-Levlin 2 1 Laboratory of Paper Technology, Dept. of Forest Products Technology, Helsinki University of Technology, Finland 2 KCL Science and Consulting, Espoo, Finland *Corresponding author. CSIRO Manufacturing & Infrastructure Technology, CRC Wood Innovations, PO Box 56, Highett, Melbourne, Victoria 3190, Australia Fax: q petri.widsten@csiro.au Abstract The chemistry of ozone bleaching of chemical pulps was explored by ozonating residual lignins isolated from conventional kraft and SuperBatch pulps in methanol/water medium to detect possible differences in the reactivity of the two types of lignin. SuperBatch lignin was ozonated also in water to study the effect of ozonation medium on the lignin reactivity. The residual lignins were found to display similar reactivities in methanol/water, implying that ozonation should result in equal delignification rates for both conventional kraft and SuperBatch pulps unless the rates of reagent diffusion in the pulps are different. The lignins were partly oxidized to volatile and nonvolatile low-molecular weight oxidation products by the so-called peeling mechanism, according to which oxidation products go into solution and insoluble reaction products resemble the starting lignins. The reaction products obtained upon ozonation of SuperBatch lignin in neutral water resembled those formed in methanol/water, but their yield was much lower. This is probably due to the better solubility of lignin in methanol/water than in water and/or higher degradability of lignin by ozone than by radicals formed as ozone decomposition products. Keywords: FTIR spectra; kraft lignin; ozone; residual lignin; SuperBatch. Introduction The need to comply with tightening environmental regulations favors the use of various oxygen-based chemicals in chemical pulp bleaching. Oxygen (O 2 ) delignification is already commonly used after kraft pulping, and the use of other oxygen-based bleaching agents with a high delignification capacity such as ozone and peracids is steadily increasing. A research area closely associated with the development of environmentally friendly bleaching methods for chemical pulps is the development of more easily bleachable chemicals pulps. The reasons for the differences in the bleachability of different chemical pulps are still not fully known. The reactivity of residual lignin toward bleaching agents depends on its chemical structure, the presence of lignin-carbohydrate linkages, and the accessibility of the bleaching chemicals in the fiber matrix. In addition, hexenuronic acids and other non-lignin structures formed during alkaline pulping or oxygen delignification cause brightness reversion and reduce bleaching efficiency by increasing reagent consumption during subsequent hydrogen peroxide or chlorine dioxide bleaching stages (Gellerstedt et al. 2003). The different modified kraft pulping technologies such as the SuperBatch technology (Mohr et al. 1994) enable pulping to low kappa numbers without significant losses in pulp yield and quality. However, the structure and reactivity toward oxygen-based reagents of the residual lignins present in these pulps may differ from those of residual lignin in conventionally cooked kraft pulps (Hortling et al. 1993; Froass and Ragauskas 1996; Ragauskas et al. 1999). It is therefore of interest to compare the reactivities of the residual lignins in conventional and modified chemical pulps toward different bleaching agents. Ozone is an effective, electrophilic delignification agent, whose main sites of attack in lignin are the electron-rich aromatic moieties and olefinic side-chain structures such as stilbenes and styrenes (Kratzl et al. 1976; Kaneko et al. 1980, 1983; Eriksson and Gierer 1985; Gierer 1986). The ozonolysis of lignin aromatic rings by 1,3-dipolar addition in protic, participating solvents such as water, alcohols, and carboxylic acids produces ozonides and hydroperoxides ( active oxygen ), which in turn give rise to muconic acid derivatives. The latter may undergo further oxidation by ozone to various lowmolecular-weight carbonyl compounds or cyclization to lactones. The ozonolysis of aromatic lignin units and further reactions of reaction products are illustrated in Figure 1. The addition of ozone across olefinic double bonds in lignin side-chains or muconic acid derivatives also generates carbonyl structures in lignin (Kratzl et al. 1976; Kaneko et al. 1980, 1983; Eriksson and Gierer 1985; Gierer 1986). Also depolymerization of lignin via cleavage of b-aryl ether linkages has been reported (Haluk and Metche 1986) to occur during ozonation. In addition to the reactions of ozone with lignin, also different oxygen radicals formed by decomposition of ozone and other pathways (Gierer 1997) such as hydroxyl (HO ) and superoxide (HO 2 /O 2y ) radicals bring about many oxidative changes in the lignin structure. As methanol effectively removes hydroxyl radicals from the reaction medium (Eriksson 1993), the ozonation products obtained in methanol/water medium should be representative of reactions of ozone with lignin rather than reac-

2 364 P. Widsten et al. Figure 1 Ozonolysis of an aromatic lignin unit and further reactions of the ring-opening product. tions of lignin and oxygen radicals. The use of pure water and higher ph instead of methanol/water at ph 3 should destabilize ozone, allowing hydroxyl and other radicals to contribute significantly to lignin oxidation. The aim of the present study, involving ozonation of lignins isolated from conventional kraft and SuperBatch pulps, was to determine whether the residual lignins in the two pulps show different reactivities toward ozone when reagent accessibility is not a limiting factor. In addition, the influence of ozone stability on lignin oxidation was studied by ozonating lignins in methanol/water (50/50, v/v) at ph 3 and neutral water. Materials and methods Lignin samples The residual lignins originated from a conventional pine (Pinus sylvestris) kraft pulp with a kappa number of 25.9 and from a SuperBatch pine (Pinus sylvestris) pulp with a kappa number of 20. The lignins were designated KRL and SRL, respectively. In addition, kraft lignin (KL) was isolated from the spent liquor of a conventional kraft cook of pine (Pinus sylvestris) to kappa number 31. The residual lignins were isolated from pulps by enzymatic hydrolysis and purified with a protease as described by Tamminen and Hortling (1999). Ozonation of lignins in methanol/water Lignin (100 mg, o.d. basis) was suspended in 120 ml of a methanol/water solution (50/50, v/v) prepared from p.a. grade methanol (Merck) and distilled water whose ph had been adjusted to 3.0 with sulphuric acid. Ozone was prepared from oxygen (O 2 ) with a GeBr. Herrmann Lab model ozonator, and the resulting oxygen ozone mixture was conducted into the lignin suspension at room temperature for 16 min under vigorous stirring. The ozone consumption was determined by iodometric titration of the residual ozone. The insoluble part (ZKL-AI, ZKRL-AI, ZSRL-AI) was recovered by filtration and the filtrate evaporated into dryness under reduced pressure. In the case of KL, the drying residue was fractionated into pentane-insoluble (ZKL-AII) and pentane-soluble (ZKL-AIII) parts, while the completely pentane-insoluble drying residues from KRL and SRL were divided into acetone-insoluble (ZKRL-AII, ZSRL-AII) and acetone-soluble (ZKRL-AIII, ZSRL-AIII) parts. Ozonation of lignins in neutral water Lignin (100 mg, o.d. basis) was suspended in 200 ml of distilled water. Ozone was generated from oxygen (O 2 ) using a Fischer Ozone Generator 500-M unit, and the resulting mixture of ozone and oxygen was bubbled into the vigorously stirred mixture at ambient temperature for 30 min at a flow rate of 18 l min y1 (1000 mg of O 3 h y1 ). The ozone consumption was not determined. The dissolved ozone and oxygen were then removed by running argon into the dispersion for 10 min. The insoluble part (ZKL-BI, ZSRL-BI) was recovered by filtration and the dissolved part (ZKL-BII, ZSRL-BII) by evaporating the filtrate into dryness under reduced pressure. Chemical analyses The total and conjugated phenolic hydroxyl groups of lignins were determined by the UV ionization difference method (Tamminen and Hortling 1999). The methoxyl group and nitrogen contents of lignins were determined at the Analytische Laboratorien, Gummersbach, Germany. The protein contents of lignins (0 7%) were calculated by multiplying the nitrogen contents by The carboxyl group contents of lignins were determined by conductometric titration from 4 to 10 mg of lignin dissolved in 0.5 ml of 0.1 M NaOH and diluted to 50 ml with deionized water. Nitrogen was run into the solution for 1 min to eliminate dissolved CO 2, after which the mixture was titrated first with 0.1 M HCl and then with 0.1 M NaOH under a continuous flow of nitrogen. The residual carbohydrates present in the lignins (2 10%) were determined by acid hydrolysis combined with anion exchange chromatography using pulsed amperometric detection according to Hausalo (1995). Spectroscopic analyses FTIR absorbance spectra of lignins were obtained from KBr pellets on a Nicolet 740 FTIR spectrometer. Standard protondecoupled 13 C-NMR spectra were recorded from 30 to 100 mg of lignin dissolved in d 6 -DMSO on a Varian Gemini-200 MHz FT spectrometer at ambient temperature operating at 50 MHz for carbon. The pulse delay was 1 s, a line broadening of 20 Hz was used for the Fourier transformation, and transients were collected. The chemical shifts were measured relative to DMSO at 39.6 ppm. Molar mass distribution measurements Molar mass distributions of lignins were determined on a Fractogel HW-55F gel in 0.5 M NaOH using mono-disperse Napolystyrenesulfonates as calibration compounds and a UV detector measuring at 280 nm (Hortling et al. 1990). Results and discussion Ozonation of lignins in methanol/water resulted in a partial dissolution of the lignins while the solutions gained a

3 Ozonation of residual kraft and SuperBatch lignins 365 Table 1 Ozone consumption of lignins and yields of insoluble (fraction I) and dissolved (fractions II and III) oxidation products. Lignin Reaction Ozone/lignin Ozone consumption of Yield (%) of lignin medium molar ratio lignin, mol O 3 /C 9 unit 1 fractions I III I II III Total KL CH 3 OH/H 2 O KRL CH 3 OH/H 2 O SRL CH 3 OH/H 2 O KL H 2 O 18.9 n.d SRL H 2 O 18.9 n.d Assuming a molar mass of 180 g mol y1 for a lignin phenylpropane (C9) unit. light yellow color. Evaporation to dryness of the solutions after ozonation gave yellow resinous residues with a pronounced acidic or peroxidic smell. These oxidation products (II- and III-fractions) consisted of oily, brown material, which on drying overnight in vacuo hardened and turned darker. These results are consistent with those of Soteland (1971), who showed the presence of active oxygen (ozonides and hydroperoxides) in the yellow drying residue of the material solubilized during ozonation of groundwood. On standing in a vacuum desiccator, the active oxygen disappeared and the residue turned dark and hard. The hardened residue was rich in carboxylic acid and ester groups. The presence of active oxygen in ozonated lignins was also reported by Katuscak et al. (1971a,b, 1972). During the ozonation in water of KL or SRL, most of the lignin remained insoluble and the solution acquired a light yellow color. The drying residue from the solution (IIfraction) was yellow, cotton-like and odorless. The lower yield (Table 1) of these highly oxidized lignins as compared to the ozonations in methanol/water can be explained by a better solubility of lignins in methanol than in water and/or by a higher degradability of lignin by ozone compared to oxygen-based radicals. Ozone decomposition to radicals should be much more extensive in neutral water than in methanol/water at ph 3. The yields of products from ozonation in methanol/ water indicate that the lignins were in part oxidized to volatile products such as CO 2, glyoxal, and methanol (Kratzl et al. 1976; Månsson and Öster 1988). The amount of volatile products formed was probably higher than indicated by the yields of the recovered (nonvolatile) ozonation products, because the total mass of lignin ozonation products (structures containing active oxygen or carbonyl groups) exceeds that of the original lignin (Katuscak et al. 1971a,b) and because methanol may become incorporated in the lignin structure as methoxyl groups (Katuscak et al. 1972). The yield of soluble lignin (fractions II and III) was much lower for KRL than for SRL, although both lignins consumed similar amounts of ozone. This indicates that the immediate oxidation products of KRL such as muconic acid derivatives were further degraded to volatile compounds to a larger extent than those of SRL. The ozone consumption of KL in methanol/water was approximately 2.7 times as high as that of the residual lignins, and the yield of soluble products (fractions II and III) from KL was nearly 50%. However, only some 10% of KL was oxidized to volatile products when the amount of oxygen introduced into the recovered lignin fractions is not taken into account. The relatively low proportion of the fraction ZKL-AI among the recovered lignins and the high ozone consumption of KL may be due to its relatively low molar mass (Figure 2) and high phenolic hydroxyl content (Table 2), rendering it more soluble in methanol/water than the residual lignins. The solubility of both kraft and residual lignins in water is very low at acidic or neutral ph values when their phenolic hydroxyl groups are undissociated. Also, ozone may cause more extensive cleavage of aromatic nuclei than hydroxyl, superoxide, and other radicals formed as ozone decomposition products. It is thus not surprising that the yields Figure 2 Weight-average molar mass and polydispersity of non-ozonated lignins and lignin ozonation products. For designations, see Materials and methods section.

4 366 P. Widsten et al. Table 2 Functional group contents 1 of lignins and lignin ozonation products per 180 g of sample. Lignin PhOH, PhOH, COOH OCH 3 total conjugated KL ZKL-AI ZKL-AII ZKL-AIII ZKL-BI ZKL-BII KRL ZKRL-AI ZKRL-AII ZKRL-AIII SRL ZSRL-AI ZSRL-AII ZSRL-AIII ZSRL-BI ZSRL-BII Values for untreated lignins and I-fractions are corrected for residual carbohydrates (2 10%) and proteins (0 7%). of insoluble ozonation products in water (ZKL-BI and ZSRL-BI) were much higher than those (ZKL-AI and ZSRL-AI) obtained in methanol/water. Changes in chemical structure and molar mass of lignins during ozonation The phenolic hydroxyl, methoxyl, and carboxyl group contents (Table 2) of the starting lignins were similar to those of the insoluble ozonation products (I-fractions) from ozonation in methanol/water, indicating that the aromatic units of the I-fractions are largely intact. The I-fractions also have higher molar masses than the corresponding non-ozonated lignins (Figure 2). However, the FTIR spectra of the residual lignins and the corresponding I-fractions (Figure 3) show that ozonation brought about an increase in the ratio of the bands assigned to quinones, olefinic double bonds, and a-carbonyl groups at cm y1 (Eriksson and Gierer 1985; Gierer 1986; Haluk and Metche 1986) to those from aromatic lignin units (Table 3). Of the side-chain structures conjugated to an aromatic unit a-carbonyl groups and olefinic double bonds the former can be expected to be relatively resistant to attack by ozone while the latter should show high reactivity toward ozone. The frequency of phenolic aromatic units conjugated to an olefinic double bond or a-carbonyl group remained roughly unchanged (Table 2). While aromatic units conjugated to an olefinic double bond were probably removed during the treatment, those with a-carbonyl groups may have been enriched in the I-fractions. The cleavage of olefinic side-chains may also result in new ring-conjugated carbonyl structures (Eriksson and Gierer 1985; Gierer 1986; Haluk and Metche 1986). The increase in the relative intensity of the band at cm y1 can thus be due to formation of olefinic double bonds by ring-opening reactions and/or quinones and an enrichment and/or formation of aromatic units conjugated to a carbonyl group. The FTIR spectra and chemical analyses also show that while the I-fractions retain a lignin-like structure, the lignins that passed into solution during ozonation consist of highly oxidized material low in aromatic units rich in carboxylic acids/esters, other carbonyl groups, and olefinic double bonds. Their low molar masses are consistent with this (Figure 2). These results show that lignin oxidation proceeded by the so-called peeling mechanism reported earlier (Lachenal et al. 1995), according to which layers of oxidized, degraded lignins are gradually peeled off during the treatment, leaving an insoluble fraction that largely resembles the starting lignin. As shown in Figure 2, the insoluble ozonation products have higher molar masses than the corresponding starting lignins, whereas the soluble oxidation products have very low molar masses. Figure 3 FTIR spectra of non-ozonated lignins and the products obtained from their ozonation in methanol/water. For designations, see Materials and methods section.

5 Ozonation of residual kraft and SuperBatch lignins 367 Table 3 Assignments of bands in the FTIR spectra (Bellamy 1958; Williams and Fleming 1980). Band Band maximum, cm y1 Main assignments Carboxylic esters and ab-unsaturated g-lactones (RCOOR) Carboxylic acids (RCOOH) a C_ O groups, quinones, olefinic C_ C, C_ O group in amides (protein contaminants) Olefinic C_ C conjugated to a C_ O group (muconic acid derivatives?) , 1600 Aromatic units, skeletal vibrations , 1270 Aromatic C O, stretching 7 852, 882 Alkyl peroxides?, epoxides?, olefinic C H? Figure 4 FTIR spectra of non-ozonated lignins and the products obtained from their ozonation in water. For designations, see Materials and methods section. A comparison of the FTIR spectra (Figure 4) and functional group contents of the lignins ozonated in water with those of the lignins ozonated in methanol/water shows that the solvent has little effect on the structure of the insoluble ozonation products. The 13 C NMR spectra (Figure 5) of SRL and ZSRL-BI show a decrease in intensity of the peaks from C3 and C4 of the aromatic units at ppm relative to the peaks from the other aromatic and olefinic carbons at ppm. This apparent increase in olefinic carbons relative to aromatic carbons is consistent with the FTIR spectra of the two lignin samples. The FTIR spectra of the lignins ozonated in different media look rather different in the region cm y1, but the bands at cm y1 and the chemical analyses show them all to have a high content of carboxylic acid/ester groups and olefinic double bonds and a low content of aromatic units. Conclusions Figure 5 13 C NMR spectra of SRL and ZSRL-BI. S, solvent; I, impurity. For designations, see Materials and methods section. Residual lignins isolated from conventional kraft and SuperBatch pulps exhibit similar reactivities toward ozone in methanol/water in terms of ozone consumption and solubility and chemical characteristics of the ozonated lignins. Judging by the reactivities of the isolated lignins, ozone should be an equally effective delignification agent for both conventional kraft and SuperBatch pulps when reagent accessibility is not a rate-limiting factor. The lignins are oxidized by a peeling-type mechanism in such a way that highly oxidized lignin passes into solution or is converted into volatile products while the lignin remaining insoluble resembles the starting lignin. Ozonation of lignins in neutral water proceeds in the same manner as in methanol/water, but the proportion of lignin solubilized or degraded into volatile products is consid-

6 368 P. Widsten et al. erably smaller. A better solubility of lignin and higher stability of ozone in methanol/water than in pure water are likely to account for this reactivity difference. References Bellamy, L.J. The Infra-Red Spectra of Organic Molecules. John Wiley & Sons, New York, Eriksson, T. Reactions of ozone and chlorine dioxide in bleaching. PhD thesis. Royal Institute of Technology, Stockholm, Eriksson, T., Gierer, J. (1985) Studies on the ozonation of structural elements in residual kraft lignins. J. Wood Chem. Technol. 5: Froass, P.M., Ragauskas, A.J. (1996) Chemical structure of residual lignin from kraft pulp. J. Wood Chem. Technol. 16: Gellerstedt, G., Li, J., Sevastyanova, O. (2003) The distribution of oxidizable structures in unbleached and bleached kraft pulps. Proc. 28 th EUCEPA Conference, Lisbon. pp Gierer, J. (1986) Chemistry of delignification. Part 2. Reactions of lignins during bleaching. Wood Sci. Technol. 20:1 33. Gierer, J. (1997) Formation and involvement of superoxide (O 2y / HO 2 ) and hydroxyl (OH. ) radicals in TCF bleaching processes: a review. Holzforschung 51: Haluk, J.P., Metche. M. (1986) Caractérisation chimique et spectrographique de la lignine du peuplier par acidolyse et ozonolyse. Cellul. Chem. Technol. 20: Hausalo, T. (1995) Analysis of wood and pulp carbohydrates by anion exchange chromatography with pulsed amperometric detection. Proc. 8 th ISWPC, Helsinki, vol. 3. pp Hortling, B., Ranua, M., Sundquist, J. (1990) Investigation of the residual lignin in chemical pulps. Part 1. Enzymic hydrolysis of the pulps and fractionation of the products. Nordic Pulp Pap. Res. J. 5: Hortling, B., Tamminen, T., Tikka, P., Sundquist, J. (1993) Structures of the residual lignins of modified kraft pulps compared with conventional and oxygen-bleached kraft pulps. 7 th ISWPC, Beijing, China, vol. 1. pp Kaneko, H., Hosoya, S., Nakano, J. (1980) Degradation of lignin with ozone. Mokuzai Gakkaishi 26: Kaneko, H., Hosoya, S., Nakano, J. (1983) Degradation of lignin with ozone. Reactivity of lignin model compounds toward ozone. J. Wood Chem. Technol. 3: Katuscak, S., Hrivik, A., Mahdalik, M. (1971a) Ozonation of lignin. Part I. Activation of lignin with ozone. Pap. Puu 53: Katuscak, S., Rybarik, I., Paulinyova, E., Mahdalik. M. (1971b) Ozonation of lignin. Part II. Investigation of changes in the structure of methanol lignin during ozonation. Pap. Puu 53: Katuscak, S., Hrivik, A., Katuscakova, G., Schiessl, O. (1972) Ozonation of lignin. IV. The course of ozonation of insoluble lignins. Pap. Puu 54: Kratzl, K., Claus, P., Reichel, G. (1976) Reactions of lignin and lignin model compounds with ozone. Tappi J. 59: Lachenal, D., Fernandes, J.C., Froment, P. (1995) Behaviour of residual lignin in kraft pulp during bleaching. J. Pulp. Pap. Sci. 21: Månsson, P., Öster, R. (1988) Ozonation of kraft lignin. Nordic Pulp Pap. Res. J. 3: Mohr, R., Germgard, U., Fiala, W. (1994) Lowest emissions from TCF pulp production with Super-Batch-kraft pulp process. Papier 48:V1 V6. Ragauskas, A.J., Lin, W., McDonough, T.J., Jiang, J.E. (1999) NMR studies. Part 5: Nature of residual lignin in kraft pulps. Tappi J. 82: Soteland, N. (1971) Attempts to characterize the oxidized lignin after ozone treatment of western hemlock groundwood. II. Norsk Skogindustri 25: Tamminen, T. Hortling, B. (1999) Isolation and characterization of residual lignin. In: Advances in Lignocellulosics Characterization. Tappi Press, Atlanta. pp Williams, D.H., Fleming, I. Spectroscopic Methods in Organic Chemistry. McGraw-Hill, UK, Received September 29, 2003