Production of pure cellulose from Kraft pulp by a totally chlorine-free process using catalyzed hydrogen peroxide

Industrial Crops and Products 49 (2013) 844–850

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Production of pure cellulose from Kraft pulp by a totally chlorine-free process using catalyzed hydrogen peroxide Satyajit Das, Dominique Lachenal ∗ , Nathalie Marlin Laboratoire de Génie des Procédés Papetiers (LGP2), Grenoble-INP Pagora, CS10065, 38402 Saint-Martin-d’Hères Cedex, France

a r t i c l e

i n f o

Article history: Received 11 April 2013 Received in revised form 21 June 2013 Accepted 29 June 2013 Keywords: Cellulose Kraft pulp Hydrogen peroxide Copper phenanthroline Chlorine-free process

a b s t r a c t A mixed hardwood Kraft pulp was used after oxygen delignification to produce pure cellulose for dissolving purposes. A totally chlorine-free bleaching and purification process was designed. It comprises an acid hydrolysis step in the presence of a metal chelating agent (EDTA) followed by a peroxide delignification stage under oxygen pressure catalyzed by a Cu(II)phenanthroline complex, a cold caustic extraction and finally, an ozone stage carried out under neutral conditions. High purity cellulose with less than 5% xylans could be produced. Both the catalyst and the ozone stage have an effect on the DP of the cellulose which then can be varied depending on the requirements. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The use of pure cellulose is experiencing a renewed interest for the synthesis of cellulose derivatives (ethers and esters) for additives and plastics and for the production of regenerated celluloses mainly for textile applications (Floe, 2011; Haemmerle, 2011; Sixta et al., 2012). This is caused by the search for products issued from biomass as substitutes for those made from fossil resources. In this context cellulose has a place of choice since it is the most abundant organic product on earth. To date dissolving grade pure cellulose for regeneration is extracted from wood by Kraft pulping of prehydrolyzed chips (prehydrolysis-Kraft process) (Peter and Lima, 1998; Sixta et al., 2006a) or by acid sulphite pulping of ordinary chips (Sixta et al., 2006b), followed by bleaching and purification stages, and also from cotton linters by appropriate purification treatments. To meet the future demand, wood is the raw material offering the greatest potential since the use of linters is limited by the hazards of the cotton cultivation. Several Kraft paper pulp producers see the production of pure cellulose for dissolution as a potentially profitable alternative to the production of paper pulp (Patrick, 2011). However, shifting from conventional Kraft to prehydrolysis – Kraft, represents a major and costly change (Patrick, 2011). Another drawback is the loss of cellulose which is indirectly due to the prehydrolysis step (Kämppi

∗ Corresponding author. Tel.: +33 4 76 82 69 48. E-mail address: [email protected] (D. Lachenal). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.06.043

et al., 2009). A more attractive way would be to purify a Kraft pulp by some appropriate treatments, which might then only necessitate some minor adjustments introduced in the bleaching line to get a more complete purification. Some purification treatments applied on a fully bleached pulp to remove the hemicelluloses have already been investigated (Ibarra et al., 2009, 2010; Janzon et al., 2006, 2008; Köpcke and Ibarra, 2008; Köpcke et al., 2010a,b). In those studies, carried out at laboratory scale, the bleaching process was therefore not modified. In one recent study however, the purification and bleaching operations were both considered and integrated in one single process (Gehmayr et al., 2011). Indeed, since bleaching is actually some kind of purification process, it seems justified to modify the bleaching process in such a way that it becomes an efficient purification process. Kraft pulps after cooking are brown and still contaminated by some lignin and hemicelluloses. The production of white fibres requires an additional bleaching step in order to remove coloured residual lignin. Chlorinated reagents (mainly chlorine dioxide) are used in that purpose. However, oxygenated reagents such as oxygen or hydrogen peroxide are now currently applied under alkaline conditions in partial substitution for the usual chlorinated agents responsible for the production of chlorinated organic compounds. Unfortunately, the combination of oxygen and hydrogen peroxide treatments is not sufficient to fully complete the bleaching. As a consequence, some chlorinated reagents are still needed and largely used. It has been proposed to add a catalyst in both the oxygen and hydrogen peroxide stages to enhance the delignification (Suchy and Argyropoulos, 2001). Among them one of the most promising is

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Fig. 1. Formation of Cu(phen)(OH)2 in alkaline solution from Cu(phen)2 ++ .

copper(II) phenanthroline which has been shown to be active on delignification in both treatments (Germer, 1995; Gueneau et al., 2009; Korpi et al., 2004; Sippola and Krause, 2005; Suchy and Argyropoulos, 2001). This catalyst is also active on cellulose which is reflected in a substantial loss of its degree of polymerization (DPv). Having some flexibility with the value of the DPv may be seen here as an advantage since some dissolving pulp grades require rather low DPv, whereas others need much higher DPv (FrassonManhaes, 2000). Bleaching does not affect the hemicelluloses substantially. As a consequence conventional bleached Kraft pulps typically still contain 15–25% hemicelluloses (in weight) depending on the wood species. Therefore some specific treatment has to be applied for their elimination at one point or another. The most efficient way is a cold caustic extraction performed at ambient temperature or slightly higher (CCE) (Sixta et al., 2006b). Cold caustic extraction dissolves the xylans, which possess some carboxyl groups, and does not produce any chemical reaction. Therefore, it does not affect the length of the cellulose chain. The required flexibility in the degree of polymerization of the cellulose chains has to be brought about by appropriate chemical treatments. Finally, some other impurities, or chemical groups, may require the use of other chemical treatments to be eliminated. As an example the hexenuronic acid groups (hexA), present on the xylans (Buchert et al., 1995a; Gustavsson and Ragnar, 2007) may cause the yellowing of cellulose under light or heat exposure (Buchert et al., 1995b). They are degraded neither by oxygen nor by hydrogen peroxide. In conventional industrial operation they are partly removed by a hot acid treatment (A), generally performed ahead of bleaching, and the rest is attacked by the chlorinated reagents during bleaching (Ratnieks et al., 2001). In the perspective of developing an environmentally friendly process, one chlorine free candidate to degrade these groups is ozone (Pouyet et al., 2013). This paper will try to rationalize the application of these different treatments for the production of pure cellulose from a conventional hardwood Kraft pulp. The aim is to develop an environmentally friendly process, which integrates both the bleaching and purification operations and offers some flexibility in the characteristics of the cellulose such as the length of the chains. 2. Experimental

formed in demineralised water at room temperature (Korpi et al., 2007). According to Korpi et al. (2007) the original Cu(phen)2 2+ complex may be converted to other species under alkaline conditions and for example loses one phen ligand as shown in Fig. 1. They have suggested that Cu(phen) (OH)2 was the active species in O2 delignification catalysis. Whatever the nature of the active species, it will be called Cu-phen in the following. The other chemicals used in the study are: veratryl alcohol (VA) (Aldrich, 96% purity) veratryl aldehyde (VAld) (Alfa Aesar, 99% purity), NaOH (Roth, 99% minimum purity), H2 O2 solution (Roth, at a concentration of 35%), MgSO4 (Janssen Chimica, 99.5% minimum purity), 2M H2 SO4 solution (prepared from commercial Carlo Erba product, 96% minimum purity), ethylenediaminetetraacetic acid, EDTA (Acros Organics, 99% minimum purity). Ozone was produced in a laboratory ozone generator (LN 103, Ozonia) from pure oxygen at a concentration of 50–60 mg/L. 2.2. HPLC analysis HPLC was used to follow the reaction of the veratryl alcohol with hydrogen peroxide in the presence of Cu-phen complex under alkaline conditions. HPLC analyses were conducted on a Spectra System with a Spectra-Physics 1500 pump instrument using a reverse-phase ODS Hypersil C18 column (250 mm × 4.6 mm, 5 ␮ particle size) and a UV 2000 Spectra-Physics detector set at 260 nm. A mix of water and acetonitrile (80:20 v/v ratios) was used as eluent. Pressure was around 1100 psi and the eluent flow was set at 1 mL/min. Acquisitions were treated with the Borwin software. External calibrations were carried out to quantify veratryl alcohol and veratryl aldehyde. For HPLC analyses, the reaction medium was cooled to room temperature before neutralization using one drop of HCl 1 M (pH 6–7). The solution was filtered through a 0.2 ␮m nylon filter before injection in the chromatography column. Fig. 2 gives an example of HPLC analysis after partial reaction of the veratryl alcohol in alkaline medium with hydrogen peroxide in the presence of the Cu-phen complex. Three main signals appear: the first one (peak 3) at around 2.5 min is attributed to phenanthroline, the second one (peak 1) at 6 min is attributed to the veratryl alcohol and the last one (peak 2) at 16 min to the veratryl aldehyde. Small peaks 4 and 5 are attributed to phenanthroline degradation products.

2.1. Chemicals 2.3. Reaction of hydrogen peroxide with veratryl alcohol The copper(II) complex was prepared using a 1:2 CuSO4 : 1,10-phenanthroline molar ratio. 1,10-phenanthroline (phen) was provided by Alfa Aesar (purity 99% minimum). CuSO4 was provided by Aldrich (purity 98% minimum). The Cu(phen)2 2+ complex was

All reactions were carried out using a two-neck round bottom flask (25 mL) in an alkaline aqueous solution (NaOH 0.05 M). Veratryl alcohol (typically 1.38 × 10−3 mol) was introduced first

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2.4.3. Hydrogen peroxide delignification under oxygen pressure without and with the Cu-phen complex, (P/O) and (P/O)cat The procedure was similar to peroxide delignification, except that the content of the polyethylene bag was introduced in an autoclave of the ERTAM rotating reactor and put under 5 bars oxygen pressure. No magnesium salt was added. The temperature and time programme was set as follows: 30 min for temperature rise to 90 ◦ C, 60 min reaction at 90 ◦ C and 20 min down to 50 ◦ C. 2.4.4. Cold caustic extraction (CCE) The pulp was placed in a beaker and mixed with a 8% caustic soda solution at a consistency of 10%. Mixing was assured by an overhead propeller rotating at 300 rpm. Duration was 1 h at room temperature. Then the pulp was washed with distilled water and filtered through filter no. 1 until neutral pH.

Fig. 2. Example of HPLC chromatogram after partial oxidation of VA by H2 O2 /Cuphen system.

in the flask. Then the aqueous solution of Cu-phen was added (6.88 × 10−5 mol) followed by the NaOH solution (6.88 × 10−4 mol) and finally by H2 O2 solution (6.88 × 10−4 mol). The reactions were performed either in the absence of oxygen (N2 bubbling) or under O2 atmosphere (O2 bubbling) at 80 ◦ C, using an oil bath, for 3 h under stirring. 2.4. Bleaching and purification treatments The pulp used for the experiments is an unbleached mixed hardwood Kraft pulps provided by Fibre Excellence, Saint-Gaudens mill, France (formerly Tembec group). The pulp was delignified with oxygen (Odel) in the laboratory under the following conditions: 10% consistency, 100 ◦ C for 1 h with 2% NaOH and 0.15% MgSO4 , 5 bars oxygen, in autoclaves placed in the oil bath of the ERTAM rotating reactor. After the reaction, the pulp is washed properly with distilled water. After delignification the pulp (Odel) kappa number was 9.0, DPv was 1400 (intrinsic viscosity 925 mL/g), and brightness 45.0% ISO. The Odel pulp was bleached and purified according to the following procedures: 2.4.1. Hot acid treatment (A), chelation (Q) and (AQ) stages The A stage was performed with sulphuric acid at pH 3, 10% consistency, 90 ◦ C for 3 h in a polyethylene bag. The Q stage was carried out with 0.5% EDTA at 90 ◦ C, 10% consistency, pH 5–6, for 30 min in a polyethylene bag. In the (AQ) stage the A treatment was performed as A but with 0.5% EDTA added. 2.4.2. Hydrogen peroxide delignification without and with Cu-phen complex, P and Pcat The P stage was run at 10% consistency, 90 ◦ C, for 2 h with 2% NaOH and 2% H2 O2 (on pulp) in a polyethylene bag. In Pcat 0.1% Cu-phen complex was added (expressed as phenanthroline dose on pulp). The Cu(phen)2 ++ complex was always added first followed by NaOH and H2 O2 in this order. After introduction of the reagents the polyethylene bag was placed in a thermostated water bath.

2.4.5. Ozone bleaching (Z) When the purpose is delignification, ozonation is normally carried out at acidic pH. Here the objective was to remove the hexenuronic acids which are the main contributors to the residual kappa number. Ozonation was performed at neutral pH. Working under neutral conditions has already been proposed when ozone is used as a final bleaching stage (Chirat and Lachenal, 1997). It was shown that ozone did work to as well as in acidic conditions and the industrial implementation is certainly much easier. Pulp was dewatered by centrifugation to a consistency of 35–40% and fluffed in a specially designed disintegrator. The fluffed pulp was introduced into a rotating glass reactor, slowly fed with a pre-decided amount of ozone, at room temperature. When the ozone had passed, the reaction was considered as completed. 2.4.6. SO2 washing After the bleaching and purification steps the pulp was washed with SO2 water prepared by passing SO2 gas through distilled water for 2–3 h (up to saturation). This treatment was supposed to reduce some residual oxidized structures and remove metal ions (particularly relevant here since a metal complex was introduced in the process). 2.5. Pulp characterization procedures Before and after treatment the pulp was characterized by using the following standard tests: - Pulp brightness: ISO 2470-2:2008. - Kappa number: ISO 302-2004. - Degree of polymerization (DPv): Tappi Test Method T230 om-99 (based on viscometry). - Pentosan content in pulp: Tappi Test Method T223 cm-01. This method is based on the quantification of furfural formed by hydrochloric acid hydrolysis of pulp. The furfural formed under these conditions essentially originates from pentoses issued from xylans. In this method 5-hydroxymethylfurfural formed from hexoses (cellulose and glucomannans) does not interfere. - Hexeneuronic acid groups in pulp: Tappi Test Method T282 pm07. - Alkali resistance and solubility of Pulp (R18 and S18 ): ISO 699/1982. The alkali resistance of the pulp (R18 ) is measured after its contact with a 18% NaOH solution for 1 h. The residual quantity is weighted. S18 is the solubilized part (S18 % = 100–R18 , %). - Transition metals (Fe, Cu, Mn), Mg, Ca and N contents were measured by CNRS (Vernaison-France). - Molecular mass distribution (MMD) of carbohydrates: The carbohydrate molecular mass distribution of the pulp samples were determined by size exclusion chromatography (SEC). Before chromatography analysis, lignin should be removed and the resulting

S. Das et al. / Industrial Crops and Products 49 (2013) 844–850 Table 1 Oxidation of veratryl alcohol (VA) to veratryl aldehyde (VAld) by H2 O2 /Cu-phen system. Molar ratio Cu-phen: H2 O2 : VA 1: 10: 20, NaOH 0.05 M, 80 ◦ C, 3 h. HPLC measurement.

NaOH + H2 O2 + VA NaOH + H2 O2 + VA + Cu-phen NaOH + H2 O2 + VA + Cu-phen + air NaOH + H2 O2 + VA + Cu-phen + O2 a a

VA loss %

VAld formed%

4.5 11.5 20.8 35.0

0.5 10.6 12.3 22.0

O2 bubling. % on original VA.

cellulosic substrate is derivatized into soluble cellulose tricarbanilate (CTC) by the reaction of phenylisocyanate on the alcoholic groups of the cellulose. SEC was performed in THF at 1 mL/min, 35 ◦ C., using a Viscotek TDA-302 apparatus equipped with 3 TSK ViscogelTM columns GMHHR -H (7.8 × 300). UV detection (at 260 nm) was installed. Other details (carbanilation protocol) and calculation procedure have been given by Mortha et al. (2011). 3. Results and discussion 3.1. Effect of copper(II) phenanthroline on peroxide delignification Copper(II) phenanthroline (Cu-phen) is known to enhance oxygen delignification (O) in alkaline medium It is admitted that the first step is the oxidation of the OH benzylic groups on lignin into ketone by the Cu-phen complex which is then reduced to a copper(I) species, which in turn is re-oxidized to the copper(II) phenanthroline by oxygen (Korpi et al., 2004; Sippola and Krause, 2005). Although the next step has never been clearly stated, there is plenty of evidence that these ketone groups will induce an easier cleavage of the residual ␤O4 linkages in the lignin molecules (Gierer and Ljunggren, 1979; Imai et al., 2007). The catalysis of hydrogen peroxide delignification (P) has been much less studied (Coucharriere, 2000; Marlin et al., 2005). One may make the hypothesis that the formation of ketone groups is still a prerequisite to improved delignification. As an assessment of the formation of carbonyl groups, the oxidation of veratryl alcohol (VA), a model for nonphenolic lignin unit containing a benzylic OH group, has been investigated by HPLC (Table 1). In theory this model should not be degraded by hydrogen peroxide under alkaline conditions, although it is never excluded that some radicals issued from peroxide decomposition may provoke some oxidation. Both the disappearance of VA and the formation of veratryl aldehyde (VAld) were followed. Table 1 clearly shows that the addition of Cu-phen in the H2 O2 reaction medium increased both the reaction of VA and the formation of VAld. It appears also that these reactions were more pronounced when oxygen was present, which was in line with the fact that Cu-phen is a good catalyst of oxygen delignification (Korpi et al., 2004). The fact that the quantity of VAld was less than that of the transformed VA must be the result of further oxidation to veratric acid or of VAld instability in the alkaline reaction medium. This suggests that (O/P) bleaching treatments will respond better to the addition of the catalyst than P alone. Cu-phen was added to various P stages performed on a mixed hardwood Kraft pulp, previously oxygen delignified. The objective was to bring some further delignification. Kappa number, hexA content, pentosan content, brightness and cellulose DP were measured. Results in Table 2 indicate first that a P stage, when applied directly after oxygen delignification, exhibited poor delignification selectivity and did not change the hexA content or the pentosan content. An acid treatment (A) carried out at pH 3 and 90 ◦ C, followed by a chelation stage with EDTA at pH 5.5 (Q) decreased the kappa number by 30% without any negative effect on cellulose. This effect resulted from the removal of half of the hexA present in the

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Table 2 Treatment of the oxygen delignified hardwood Kraft pulp by A (pH 3, 90 ◦ C, 3 h) with or without EDTA (Q)(0.5% on pulp) followed by hydrogen peroxide under various conditions (2% NaOH, 2% H2 O2 , on pulp, 90 ◦ C, with or without O2 (5 bars), with or without Cu(phen)2 2+ (0.1% phen on pulp).Other conditions in experimental section. Effect on the main pulp characteristics. Sample

Kappa number

DPv

Brightness %ISO

HexA (␮mol/g)

Pentosan %

Odel P AQ AQP (AQ)P (AQ)Pcat (AQ)(P/O) (AQ)(P/O)cat

9.0 7.0 5.9 4.9 4.5 3.1 4.3 2.7

1400 1100 1380 1200 1300 700 1360 950

45 60 52 72 75 76 78 80

54 50 26 25 25 24 20 18

23 22 22 21 21 20 20 20

oxygen delignified pulp. If one admits that a loss of 10 ␮mol hexA/g corresponds roughly to a kappa decrease of one unit (Pouyet et al., 2013), it can be concluded that the Kappa number drop in AQ was almost entirely explained by the removal of the hexA. After AQ, as expected, the P stage gave a better bleaching and led to less degradation of the cellulose. This was likely linked to the lower level in metal ions after AQ (Table 3). When the EDTA was directly introduced in A (AQ), the effect was even greater, which was in line with the further reduction in metal ions when EDTA was present during A (Table 3). Adding Cu-phen to P (Pcat) improved the delignification by 30–40%. At the same time the DP of cellulose was considerably reduced, which indicates that the copper complex induced cellulose oxidation, either by formation of carbonyl groups from the alcohol groups followed by ˇ elimination in the alkaline medium (Meller, 1960), or by the direct attack of some hydroxyl radicals generated from possible decomposition of H2 O2 by the copper complex. Globally the most interesting results in delignification and bleaching were obtained when oxygen was added to P to give (AQ) (P/O)cat. It is noticed that none of these treatments had any effect on the hemicelluloses content in pulp. Contrary to what was hoped and anticipated from the severe decrease of cellulose DP, the catalyst did not contribute to the oxidation of the xylans to an extent where some would have become soluble. 3.2. Removal of the hemicelluloses by cold caustic extraction Cold caustic extraction (CCE) was performed with a solution of 8% caustic soda for 1 h at room temperature. This process is known to solubilize most of the xylans present in a Kraft pulp (Sixta et al., 2006b) and is used to prepare pure cellulose after the prehydrolysis-Kraft pulping process. Residual xylans, DPv of cellulose, kappa number, brightness and hexA were measured after the CCE stage (Table 4). The pentosan content was efficiently reduced after the CCE step. Some decrease in kappa number was also observed. Considering the small variation in hexA content, one can conclude that the reduction in kappa number during CCE was mainly caused by some lignin removal. This was in line with the significant improvement in brightness obtained during the CCE stage. The increase in DPv during CCE was due to the dissolution of most of the xylans. It is interesting to note that since the hexA content in Table 3 Metal ions in pulp after treatments (in ppm). Nd: not detectable. A and (AQ) at pH 3, 3 h, 90 ◦ C. Q at pH 5.5, 90 ◦ C, 30 min. 0.5% EDTA. Sample

Cu

Mn

Fe

Mg

Odel A Q AQ (AQ)

5 4 4 4 <0.5

60 22 10 7 <5

32 5 5 3 nd

510 55 420 45 40

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Table 4 Effect of cold caustic extraction (CCE) and ozone treatment (Z) as purification steps. CCE: 10% consistency, 8% NaOH solution, room temperature, 1 h; Z: 0.2% ozone. Other conditions in experimental section. Sample

Pentosan %

Kappa number

DPv

HexA (␮mol/g)

Brightness %ISO

Odel (AQ) (AQ) (P/O) (AQ) (P/O)cat (AQ) (P/O)CCE (AQ) (P/O)cat CCE (AQ) (P/O) CCE Z (AQ) (P/O)cat CCE Z (AQ) CCE (AQ) CCE (P/O)

23 22 20 20 5.6 5.0 5.4 4.6 9.2 8.0

9.0 5.8 4.3 2.7 1.7 1.2 – – 3.9 2.0

1400 1380 1360 950 1400 1080 1300 600 1430 1200

54 26 20 18 16 12 6 2 20 19

45 53 78 80 82 85 90.5 91.5 59 82

Fig. 3. Molecular mass distribution of carbohydrates from Odel pulp, (AQ) (P/O) CCE and (AQ)(P/O) CCE Z treated Odel pulp.

pulp after CCE was not substantially changed, the xylans dissolved during CCE were not those containing the hexA groups. Finally, it appears that the hexA groups still in the pulp after CCE contributed to almost 100% of the kappa number actually measured on the pulp (10 ␮mol/g corresponding roughly to a kappa decrease of one unit). Therefore, the quantity of lignin after CCE must be very low. The next step should be the total degradation of the hexA groups still present in the pulp. One may wonder whether the CCE step would not be better positioned ahead of P where the lignin content is higher. Results in Table 4 show that the CCE treatment did not work as well when placed ahead of P. Delignification during CCE was also proportionally less. One explanation could be that the links between lignin and xylans (Choi et al., 2006) prevented the latter from dissolving easily. One other difference was the lower DPv after (P/O) when CCE was placed ahead of (P/O) which could be due to the introduction of new metal ions, as NaOH impurities, during CCE. What again had to be noticed here was that very few hexA groups were removed during CCE, while more than half of the xylans were dissolved. One has to explain why the extracted xylans were not those bearing hexA groups. It could be that the latter had an higher molecular size because they resisted the peeling reaction during the Kraft process. A last option would be to perform the CCE stage directly after oxygen delignification, ahead of (AQ). This case is under investigation but the above considerations indicate that the caustic soda concentration will have to be increased to achieve a satisfactory extraction of the xylans at this position. Table 4 also includes the effect of a final ozone stage (Z) in an attempt to get rid of the hexA groups (Pouyet et al., 2013). Most of them happened to be degraded. Ozone did not affect the content in pentosan and improved the brightness, while the DP was dramatically decreased during Z in the case when Cu-phen had been added in (P/O). This effect must be due to some reaction of residual Cu-phen complex or Cu-phen degradation products in pulp with ozone, as shown below.

Fig. 4. Molecular mass distribution of carbohydrates from Odel pulp, and (AQ) (P/O)cat CCE and (AQ)(P/O)cat CCE Z treated Odel pulp.

and (AQ) (P/O)cat CCE Z. The values indicate that there was virtually no copper present in the cellulose. Moreover the nitrogen content was roughly the same in both cases. Table 6 summarizes their main other characteristics. The R18 values were similar in both cases and higher than 96%. Figs. 3–5 gives the molecular weight distribution of the cellulose samples. Fig. 3 indicates that some depolymerization of cellulose has occurred during the (AQ) (P/O) CCE process, which did not reflect in the value of the average viscometric DPv as shown in Table 4. This degradation must have originated from the P/O stage since the CCE treatment should not cause any depolymerization of cellulose. This figure also confirms that no degradation occurred during the Z stage, located at the end of the sequence, where no lignin was present, and carried out at neutral pH, as already seen in other studies (Chirat and Lachenal, 1997). In Fig. 4 the curves corresponding to the case where the copper complex is added in (O/P) are given. The more extensive depolymerization occurring in (P/O) when the copper complex was added can be seen.

3.3. Quality of the cellulose produced After processing, the cellulose was washed with SO2 water as practiced in many mills after bleaching. Several characteristics have been measured. Table 5 gives the content in metal ions and in nitrogen in the two fully bleached and purified samples (AQ) (P/O) CCE Z Table 5 Metal ions and N content in cellulose after bleaching and purification. Cellulose was washed with SO2 water before determination (in ppm). sample

Fe

Cu

Mn

Mg

Ca

N

(AQ) (P/O) CCE Z (AQ) (P/O)cat CCE Z

<5 <5

<0.5 1.7

<5 <5

30 25

65 65

85 95

Fig. 5. Molecular mass distribution of carbohydrates from Odel pulp, and (AQ) (P/O) CCE Z and (AQ)(P/O)cat CCE Z treated Odel pulp.

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Table 6 Summary of the characteristics of the cellulose after bleaching and purification. Sample

Brightness % ISO

Pentosan %

HexA (␮mol/g)

DPv

Viscosity (mL/g)

R18 %

S18 %

(AQ)(P/O) CCE Z (AQ)(P/O)cat CCE Z Standard rayona Acetate (plastics)a Acetate (filter tow)a Nitrocellulosea Ethera

90.5 91.5 89–90 93 90–92 90–92

5.4 4.6

6 2

1300 600

875 440 300–500 500–750 500–750 500–650 250–750

96.7 96.5

3.3 3.5 3.5–4.0 1.5–2.0 2.0–2.5 2.0–2.5 2.5–3.0

a

From Frasson-Manhaes (2000).

Table 7 Effect of adding Cu(phen)2 ++ and phen in Z on the cellulose DPv. Pulp sample

(AQ) (P/O) CCE

(AQ) (P/O) CCE Z

(AQ) (P/O) CCE Z Cu-phen added in Z

(AQ) (P/O) CCE Z Phen added in Z

DP

1400

1300

980

1050

Both additives at 0.1% on pulp (expressed as phen). Z charge 0.2% on pulp.

In this case the ozone treatment reduced the molecular weight of the cellulose further, likely because of the presence of some residual complex in the pulp after CCE. Indeed, the results in Table 7 show that the (AQ) (P/O) CCE treated pulp was degraded during ozonation under neutral conditions when either Cu(phen)2 ++ or phen were purposely added to the pulp just before applying the ozone. Although the quantities of these latter compounds, still present in the pulp after (AQ) (P/O)cat CCE were likely lower than those originally introduced in (P/O) and tested here, the results in Table 7 at least suggest one possible origin for the degradation taking place during Z for the (AQ) (P/O)cat CCE treated pulp. Fig. 5 compares the molecular weight distribution profiles after the (AQ) (P/O) CCE Z and (AQ) (P/O)cat CCE Z processes. It is clear that the presence or not of the Cu-phen complex made it possible to produce cellulose with varying DP. The lowest values are those usually encountered in viscose manufacture. The highest ones are closer to the requirements for cellulose esters. The distribution curves exhibited a small shoulder in the lowest masses, indicating that some residual hemicelluloses were still present, which was in accordance with the values found with the pentosan measurement technique. Whether or not these hemicelluloses at this level of concentration may affect the downstream process remains to be checked. 4. Conclusion A totally chlorine-free process applied directly after conventional Kraft cooking was designed in order to produce a high purity cellulose from hardwood. After oxygen delignification a hot acid treatment in the presence of EDTA (AQ) could remove about half of the hexenuronic acid groups (hexA) and virtually all the transition metal ions from the pulp. Lignin removal was achieved during a hydrogen peroxide treatment catalyzed by Cu-phen (Pcat). The addition of the copper complex increased the delignification power of H2 O2 by 30–40% so that after Pcat the kappa number was essentially due to the residual hexA. None of these treatments had any effect on the hemicelluloses content in pulp. Most of the latter could be removed by a cold caustic extraction at room temperature (CCE). Placed at this point the CCE stage eliminated the last residues of lignin and improved the brightness. Positioning the CCE stage earlier in the process had less effect of the hemicelluloses dissolution. The residual hexA were easily degraded by an ozone treatment performed under neutral conditions with ozone charges as low as 1–2 kg/odp. After this sequence of treatments the cellulose exhibits the characteristics of high purity cellulose for dissolving purposes although some properties remain to be tested, depending on the final products which are targeted. Of particular interest is the fact that the DPv of cellulose is affected by the

addition of the copper complex and to a lesser extent by the ozonation stage, which broadens the scope of application of the cellulose. This approach seems particularly attractive by comparison with the Kraft cooking of prehydrolyzed wood chips which requires the development of a new technology and leads to substantial loss in cellulose during cooking. The process proposed here may also offer the possibility of recycling most of its effluent back to the recovery process of the Kraft mill, after recovery of the dissolved xylans.

Acknowledgment This project was conducted thanks to a grant from the French National Research Agency (ANR).

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