3 rd. delignification. year. Chemical Processes

Transcription

1 New catalytic systems for oxygen delignification Basile GUENEAU Supervisors : Dr. N. Marlin, Pr. D. Lachenal 3 rd year Chemical Processes Oxygen delignification has become a conventional Background process in pulp manufacture. It considerably reduces the operating cost of the bleaching process and decreases the environmental impact of the bleaching effluent. Unfortunately the performance of oxygen delignification is limited by the fact that a part of the residual lignin is not reactive toward oxygen and that cellulose depolymerisation occurs. This project aims at finding an industrially usable catalyst that enhances oxygen selectivity and efficiency in an alkaline medium. Recent studies [1-7] on copper phenanthroline complexes have shown promising results on delignification efficiency, but cellulose was also more degraded. New complexes with different polypyridinyl ligands have been studied, in an attempt to find complexes which have more affinity toward lignin and less toward cellulose. The first part of this study aims at understanding better Introduction the interaction between the copper complexes and cellulose and the catalytic pathway. A fully bleached pulp (regarded as pure cellulose) was treated with oxygen and the copper complexes and analysed by several spectroscopic methods (UV-Visible, Electron Paramagnetic Resonance ). Cellulose oxidation was followed by viscosity measurement. In a second part, a lignin containing substrate consisting of a mixture of fully bleached pulp and mechanical pulp was used as model. The aim was to investigate the oxidation catalysis mechanism of native lignin. This model was chosen to approach the case of an unbleached kraft pulp (same kappa number), while playing with a higher molecular weight lignin, less susceptible to go into solution after oxidation. 461 rue de la Papeterie BP Saint Martin d'hères Cedex - France Tel 33 (0) Fax 33 (0)

2 Basile Gueneau Results Oxygen delignifications were carried out a.oxygen delignification using 3.8 liters tefloned pressurized autoclaves. The metal complex was preformed in distilled water by adding 22.1 mg of Cu(II) (as sulphate form [4]) and the quantity of ligand corresponding to a molar ratio (copper to ligand) of 1:2, according to the literature [1-7]. The complex was added to 50g of pulp. Then, 1g NaOH and 5 bars O2 pressure were applied at 10% pulp consistency. Temperature was increased to 100 C and maintained for 50 minutes. Catalysed oxidation of cellulose The fully bleached pulp was considered as cellulose model even though it still contains some hemicelluloses. Several ligands were selected (Figure 1). The choice was made among phenanthroline (no. 3) and terpyridine (no. 1) derivatives possessing functional groups susceptible to affect their affinity to lignin or cellulose. The aim was to determine the impact of those functional groups first on cellulose catalysed oxidation. The oxidation activity of the different copperligand combinations on the fully bleached softwood pulp n o 1 was assessed by the measurement of the cellulose viscosity (Figure 2). Corresponding values for the untreated pulp (a), oxygen treated pulp (b) and oxygen-copper treated pulp (c) are given for comparison. Figure 1: ligands used in the study 10 Yearbook 2010 LGP2

3 Chemical processes Papermaking and environment engineeering Paperphysics Converting Biomaterials Packaging Printing processes As shown in figure 2 and as already well-known, addition of Cu(II) without any ligand, in the oxygen delignification, reduced cellulose viscosity. Complexes n o 3, 5, 6, and 8 were the most active in cellulose oxidation. They are all of the copperphenanthroline family. However no catalytic activity was observed with the 2,9 dimethyl phenanthroline compounds (cuproines) (ligands no. 4, 7 and 9); Figure 2: catalytic activities of copper-ligand complexes on cellulose oxidation (pulp 1) Concerning the two terpyridine ligands, terpyridine (n o. 1) and tolylterpyridine (n o. 2), they did not show any additional catalytic effect when compared with the control using free Cu (II). From these results it could be concluded that dimethylation of phenanthroline at positions 2 and 9 decreased its catalytic activity on cellulose, which could be of interest provided that the complex is still active on lignin. Catalysed oxidation of native lignin Catalysed oxidation of native lignin was studied using some of the previous complexes. The pulp (pulp 3) was a mixture of mechanical pulp (TMP) and a fully bleached pulp of a high initial viscosity (19,6 mpa.s). By doing so, the lignin content becomes compatible with the requirement of the Kappa number standard. Catalytic activity was assessed by kappa number determination. Results are reported in figure 3 and compared with Kappa numbers of the untreated mixture (a) and of the oxygen treated mixture (b). 461 rue de la Papeterie BP Saint Martin d'hères Cedex - France Tel 33 (0) Fax 33 (0)

4 Basile Gueneau If the Cu-phenanthroline and Cu-terpyridine complexes (ligands nos 1 and. 3) slightly increased the delignification, better results were obtained with the 5,6 or 4,7 dimethylphenanthroline (ligands n o s. 5 and 6). On the contrary, the 2,9 dimethylphenanthroline (ligand n o. 7) inhibited the oxygen delignification since the Kappa number measured after oxygen treatment shifted back to the value of the untreated pulp. It must be noted that native lignin is of high molecular weight and not readily soluble, even after oxidation. The variation of Kappa b.uv-visible spectroscopic studies numbers are therefore of very low magnitude. Small differences of Kappa numbers may reflect a profound change in the extent of oxidation. UV-Visible spectra were recorded on (1:2) mixture of Cu(II) and ligands 1, 3, 6 or 9 (fig.1) in water at ph 7 and 11.5 (phosphate buffer). The results are summarized in table 1. The low intensity band at ca. 700 nm corresponds to Cu(II) d-d transitions. The λmax of the copper complexes formed with 3 and 6 is shifted from 702 nm to 670 nm on raising the ph from 7 to 11.5 (table 1). This is ascribed to the binding of a hydroxide ion on the copper centre [1], which is known to be a requirement for Figure 3: catalytic activities of copper-ligand combinations on lignin oxidation Table 1: λmax and corresponding absorbances of 0.5mmol/l copper-ligand complexes at ph 7 and Yearbook 2010 LGP2

5 Chemical processes Papermaking and environment engineeering Paperphysics Converting Biomaterials Packaging Printing processes a catalytic activity of copper-ligand complexes [1-3]. The absence of max shift for the copper terpyridine complex may explain the poor catalytic activity of both copper-terpyridine and copper-tolylterpyridine complexes on cellulose oxidation as shown in figure 2. The max of the copper complex of 9 was found to be significantly lower than for the others, whereas its intensity is much higher. As a matter of fact it does not correspond to a Cu(II) d-d transition (too intense and observed at too high energies), but rather arises from a charge transfer between the phenanthroline and a Cu(I) ion. It could thus be concluded that the copper ion remains at its (+II) redox state in the case of ligands 1, 3, 6, whereas it is reduced into Cu(I) in the case of 9. The increased steric bulk provided by the -methyl groups constrains the copper in a tetrahedral geometry, i.e. a geometry generally adopted by the Cu(I) ion in ligand 9. As a consequence the +I redox state is much more favoured for this complex, thus explaining these differences. This study showed that functionalisation of phenanthroline Conclusion had a significant effect on the catalytic activity of Cu(II)- phenanthroline complexes in the oxygen treatment of cellulose and lignin in pulp. The dimethylation at 2,9 positions seemed to stop the catalysed oxidation of substrates, whereas the dimethylation at 5,6 positions enhanced the oxidation of lignin. Then, 5,6 dimethylphenanthroline appeared to be a more promising ligand for the catalysis of pulp oxygen delignification. Some terpyridine complexes were also investigated (1:2 molar ratio between copper and ligand). Their catalytic action seemed to be handicapped by the fact that they could not fix any hydroxide anion in alkaline conditions. This fixation would be a prerequisite for the catalytic action. Terpyridine ligands being tridentate ligands, 1:1 molar ratio between copper and ligand will be tested. The methods used in this study should open the way to the discovery of new catalyst. References 1. Korpi, H. and al (2007), On situ prepared Cu-Phenanthroline complexes in aqueous alkaline Solutions and their use in the catalytic 461 rue de la Papeterie BP Saint Martin d'hères Cedex - France Tel 33 (0) Fax 33 (0)

6 Basile Gueneau oxydation of veratryl alcohol, in European Journal of Inorganic Chemistry pp Allix, M. Délignification à l oxygène du bois et des pâtes papetières en présence de catalyseur, Th D INPG, Grenoble, Coucharriere, C. Etude et mise au point d un système d activation du peroxyde d hydrogène en délignification et blanchiment des pâtes à papier, Th D INPG, Grenoble, Korpi, H. and al (2004), And efficient method to investigate metal-ligand combinations for oxygen bleaching, in Applied Catalysis A: General 268 pp Sippola, V.O. and Krause (2005), A.O.I, Bis(ophenanthroline)copper-catalysed oxidation of lignin model compounds for oxygen bleaching of pulp, in Catalysis today 100 pp Marlin, N. and al, Use of o-phenanthroline as catalyst in hydrogen peroxide stages th International Symposium on Wood, Fibre and Pulping Chemistry, Auckland, Petro, L. and al (2004), Parallel screening of homogneous copper catalysts for th oxidation of benzylic alcohols with molecular oxygen in aqueous solutions in Journal of Combinatorial Chemistry 6 pp Publications Gueneau, B.. and al, New catalytic systems for oxygen delignification th International Symposium on Wood, Fibre and Pulping Chemistry, Oslo, 2009 Gueneau, B.. and al, New catalytic systems for oxygen delignification th Brazilian Polymer Congress - 10th CBPol, Foz do Iguaçu, Yearbook 2010 LGP2