LCC

STUDY OF THE ACTION OF AUTO-HYDROLYSIS OF SOFTWOOD CHIPS ON LIGNIN AND LIGNIN - CARBOHYDRATES COMPLEXES (LCC) Claire Monot1*, Christine Chirat1, Xueyu Du2, Jiebing Li2 1

LGP2 - Grenoble INP - Pagora - 461 rue de la Papeterie, CS 10065, 38402 Saint-Martin d'Hères France; 2 KTH - Department of Fibre and Polymer Technology, Teknikringen 56-58, SE-10044, Stockholm - Sweden (*[email protected])

ABSTRACT Several pulp and paper biorefinery projects are under development. In particular, the extraction of hemicelluloses using an autohydrolysis step before the production of cellulose fibers is studied. The extracted hemicelluloses could constitute a major source of sugars, oligomers and polymers for the production of bioproducts, bio-fuels or biomaterials. It has been shown that this hemicelluloses extraction could improve the manufacturing process of cellulosic fibers, by facilitating the cooking, as well as oxygen delignification and bleaching. The first part of our study compares different types of cooking (kraft, soda, soda/AQ) on prehydrolysed and control softwood chips. The prehydrolysis would enable to apply a sulfur free cooking on softwood wood chips, which is hardly possible on control wood chips. To understand the reasons for this better cooking ability of softwood chips, some lignin analyses were carried out. The molecular mass distribution obtained for prehydrolysed and control softwood lignins indicated that only a slight depolymerisation of the lignin occurred during the autohydrolysis treatment. 13C NMR analysis of the isolated lignins indicated that after autohydrolysis the lignin contained slightly more aromatic carbons and free phenolic groups and less secondary hydroxyl groups and aliphatic carbons than the control lignin. Lignin carbohydrate complexes (LCC) were also isolated using a method described in [1]. I. INTRODUCTION A biorefinery consists in the production of materials and energy. A pulp biorefinery produces fibers, chemicals (tall oil and turpentine), ethanol, and energy needs for the process. In modern kraft mills, extra energy is produced and sold to the market. The common process in kraft mills uses Na2S and NaOH as reagents, at a temperature between 160 and 170°C during a couple of hours. During this process, the main part of lignin and hemicelluloses of wood are dissolved, which form the effluent of the process called black liquor. This effluent is concentrated and burned in a recovery boiler to produce the energy. During this stage, chemicals are recovered to be used again in the process. However the calorific potential of the hemicelluloses is much lower than that of lignin and a better valorization of the hemicelluloses should be looked for. Autohydrolysis was chosen to extract part of the hemicelluloses prior to alkaline cooking, and valorize them either by fermentation into ethanol or to produce chemicals such as surface active agents [2, 3]. It has been shown that prehydrolysis improves cooking and bleaching [4, 5, 6]. Therefore, sulfur free cooking (like soda cooking) could be considered. The first objective of this study is to develop a sulfur free cooking of prehydrolysed softwood chips. The advantage of this type of cooking is that the black liquor will then not contain sulfur and could be used for another valorization such as gasification [7]. The second objective is to understand the easier ability of the wood to be delignified after a prehydrolysis. Lignins of control and prehydrolysed chips were extracted and compared for their chemical composition and molecular weight distribution. The quantification of the linkages between lignin and carbohydrates (LCC) was also carried out, using a method developed by [1]. II. EXPERIMENTAL Sulfur free production process - Mixed softwood chips, kindly provide by Fibre Excellence Tarascon, were used in this study. The composition is 35% Sylvestre Pine, 24% Black Pine, 18% Alep Pine, 16% Spruce, 7% Douglas fir. The autohydrolysis step was performed in autoclaves placed in a rotating oil bath to adjust the desired temperature. Autohydrolysis consists in an acidic hydrolysis without addition of any external acid. Acetic acid released from the wood when it is heated in water is the acid source. The operating conditions were: liquor to wood ratio of 4, the temperature was 170°C, the time at temperature was 60 minutes. Cooking was performed in the same rotating oil bath as the prehydrolysis. Two types of cooking were carried out: kraft cooking, according to the common industrial process, and soda - anthraquinone cooking. The operating conditions were the same for both types of cooking: liquor to wood ratio of 4, the temperature was 170°C, the time at temperature was 110 minutes. The amounts of NaOH and Na2S are indicated in the text. The percentage

of anthraquinone (AQ) varied between 0.1 and 0.2% of the dry mass of wood. After cooking, the pulp obtained was characterized by its yield and its kappa number, which is proportional to the lignin content, to evaluate the performance of cooking. The kappa number is determined according to the T 236 cm-85 standard. Isolation and analysis of lignin and lignin - carbohydrates complexes (LCC) - Extractives were first eliminated from the wood chips prior to isolation and analysis by acetone extraction. The isolation of lignin was performed according to Björkman [8]. The method that was used for the isolation of LCC is described in [1]. To perform 13 C NMR, 300 mg of lignin were dissolved in 4 mL of DMSO-d6. An acetylation was performed on both types of lignin to better determine the different types of hydroxyl groups using the method described in [9] replacing hydrochloric acid by ethanol and the ice - water mixture by diethyl ether. The molecular mass distribution was realized on a size exclusion chromatography thanks to the dissolution of lignin in DMAc/LiCl (0.5%). The standard used is polystyrene. Analysis of wood - The analyses of different components of wood were performed, especially to know their proportion in the wood. The determination of the percentage of lignin was carried out as described in the Tappi Standard T 222 om-02. The saccharides percentages were determined according to T 249 cm-09 and using a high - performance anion exchange chromatography with pulse amperometric detection (HPAEC PAD) in a Dionex ICS 5000 apparatus. III. RESULTS AND DISCUSSION Sulfur free production process First of all, an autohydrolysis was performed on the softwood chips and gave a yield of 80.1%. The results presented in Table 1 show that the percentages of lignin and cellulose in the prehydrolysed wood were higher than the ones in control wood, which is due to the solubilization of a part of the hemicelluloses during this step. Table 1. Relative proportions of lignin and carbohydrate content in the control and prehydrolysed chips Control chips Prehydrolysed chips Variation in composition due to the autohydrolysis, %

Lignin

Cellulose

31.4 36.7

39.6 50.2

+ 16.8%

+ 26.7%

Hemicelluloses* GGM Xylans 17.7 11.2 9.0 4.1 - 49.9%

- 63.4%

*: acetyl and methylglucuronic acid groups are not included

Two types of cooking, kraft and soda - AQ, were carried out on both types of wood chips. The yields and the kappa number are given in Table 2. Table 2. Kappa numbers and yields obtained after kraft and soda - AQ cooking of control and prehydrolysed wood chips Control softwood Prehydrolysed softwood

Kraft (Na2S 8.1%, NaOH 18.9%) Yield* Kappa number 44.0 24.4 34.5 27.9

Soda - AQ (NaOH 27%, AQ 0.1%) Yield* Kappa number 46.0 46.6 37.1 31.7

*: yield = yield after prehydrolysis x yield after cooking

The yields of prehydrolysed (PH) chips are lower than the ones of control chips because the yield of the prehydrolysis is taken into account. It can be seen that even if there is a higher lignin content in the prehydrolysed chips, the final lignin content is quite the same after a kraft cooking, which implies that a higher delignification rate was obtained.

Figure 1. Yield versus kappa number for the soda AQ cooking of the softwood prehydrolysed and control chips (the first number above the point indicates the NaOH %, and the second the AQ %) and kraft control (the number indicates the NaOH %).

After a soda - AQ cooking, the prehydrolysed wood chips showed a much higher delignification ability than the control chips: the kappa number after PH soda cooking was in the same range as the one obtained after PH kraft treatment, whereas the control wood chips exhibited a much higher kappa number of 46.6. Additional experiments were made by varying the NaOH and AQ charges, Figure 1. As expected, a reduction in the alkali charge led to increased kappa number, and increased yield. A previous study showed that pulps from PH wood chips were also very easy to delignify in a subsequent oxygen bleaching stage and that it could be advantageous, from a yield point of view, to stop the cooking at a higher kappa, as the oxygen delignifiation is more selective than the end of a cooking [4]. One could then imagine a process using PH - soda with a high kappa number (65 in this case), followed by oxygen delignification to obtain low kappa numbers. Analyses of lignin and lignin - carbohydrates complexes Reasons for the better delignification ability of the prehydrolysed softwood chips to soda AQ cooking could be either that the lignin has been modified [10], or that some lignin carbohydrate complexes have been hydrolysed during the acidic treatment. Lignin isolation was performed using a mixture of water - dioxane and hydrochloric acid. The yield of this isolation was between 70 and 75% for both types of wood. Molecular mass distribution and 13C NMR analysis were carried out on the two lignins. The molecular weight distribution of prehydrolysed wood lignin was slightly shifted to the small masses which indicate that only a slight depolymerisation of the lignin occurred during the prehydrolysis, Figure 2.

Figure 2. Molecular mass distributions of control and prehydrolysed softwood chips The results of 13C NMR showed that lignin of autohydrolysed wood contained more aromatic carbons and free phenolic groups and less secondary hydroxyl groups and aliphatic carbons (between 15 and 20% less) than the lignin of control wood, Table 3. Whether these differences would be enough to explain the efficiency of the cooking of prehydrolysed wood chips is not sure. For the other functional groups the differences were not significant. Table 3. Contents of functional groups of acetylated lignin (for an aromatic unit) Group/aromatic unit Aromatic OCH3 Primary OH Secondary OH Phenolic OH Aliphatic C=O of RCOOR’ Cβ of β-β and β-5 Cβ of β-O-4 Overall OH Condensed aromatic C Protonated aromatic C Aliphatic C

Control softwood chips lignin 0.86 (± 0.043) 0.63 (± 0.032) 0.32 (± 0.016) 0.29 (± 0.015) 0 0.03 (± 0.0015) 0.15 (± 0.008) 1.24 (± 0.062) 1.88 (± 0.094) 2.32 (± 0.116) 1.78 (± 0.089)

Prehydrolysed softwood chips lignin 0.89 0.61 0.27 0.36 0.003 0 0.22 1.24 1.95 2.44 1.39

LCC were isolated from control and prehydrolysed wood chips, and the yields of each type of LCC are given in Table 4. LCC1, LCC2 and LCC3 fractions contain respectively mainly LCC from glucan - lignin, glucomannan - lignin and xylan - lignin. The results obtained for the control wood used in this study were compared to that of spruce wood obtained at KTH. Higher yields could be obtained with spruce, but the general tendencies were about the same at least for LCC1 and LCC2: about 53 - 54% of the LCC can be attributed to glucan - lignin LCC and 33 - 36% to the glucomannans - lignin LCC. For the xylan - lignin LCC fraction (LCC3) there were more differences between the wood species: 9% for the mixed softwood and almost 14% for the spruce. One reason for that could be that the spruce wood contained more xylans than our mixed softwood species. The proportions of LCC2 and LCC3 decreased after a prehydrolysis, whereas LCC1 increased, which is normal given the fact that the prehydrolysis eliminated mainly hemicelluloses. The proportion of lignin in LCC1 was

about the same for the control and prehydrolysed wood chips, and the relative change in carbohydrate in LCC1 was the same as the increase in cellulose content in wood chips, which implies that the cellulose lignin bonds do not seem to have been significantly affected by the prehydrolysis. The relative change in carbohydrate content in LCC2 was 45%, which is of the same order than the reduction in GGM in wood chips due to the autohydrolysis. The lignin content in LCC2 was however higher than for the LCC2 in control wood. The reduction in LCC3 (lignin - xylan) was particularly high. A thorough analysis of the carbohydrate composition of the LCC fractions is now needed to investigate the effect of prehydrolysis further. Table 4. LCC yields for control and prehydrolysed softwood chips (the number between brackets indicates the relative contribution of a given type of LCC: yield LCCi/Yields (LCC1 + LCC2 + LCC3)) Control wood Lignin content in LCC, % Data from spruce wood (KTH study, [1]) Lignin content in LCC, % Prehydrolysed wood Lignin content in LCC, % Relative change of carbohydrate content in LCC% due to autohydrolysis, % Relative change of lignin in LCC due to autohydrolysis

LCC1 42.4 (54.4%) 19.7 49.5 (53.1%) 19.3 55.9 (69.1%) 20.9 + 25.1%

LCC2 28.3 (36.3%) 41.1 30.9 (33.2%) 29.2 23.0 (28.4%) 58.7 - 45%

+ 26%

+ 9.4%

LCC3 7.3 (9.3%) 52.0 12.8 (13.7%) 42.7 2.0 (2.5%) Non determined

Total 78.0 93.2 80,9

-

V. CONCLUSIONS The implementation of a prehydrolysis before the cooking on softwood allowed a better delignification of the wood. Thus, it could be possible to replace the traditional kraft cooking by a sulfur free cooking, a soda anthraquinone cooking for instance. To understand the influence of prehydrolysis on cooking, a study of lignin and lignin - carbohydrates complexes (LCC) was performed. The size exclusion chromatography and 13C NMR did not show big differences between the lignins of control wood and of prehydrolysed wood. Yields of LCC indicated a loss of lignin - hemicelluloses complexes, particularly for the xylan - lignin complexes. Lignin cellulose complexes did not seem to be degraded by the prehydrolysis. The next steps will be to continue the analyses on LCC and to show that cellulose of suitable properties, either for paper application or for textile application, can be obtained from these pulps. V. ACKNOWLEDGEMENT The authors would like to thank Institut Carnot Energie du Futur for the funding of this study. VI. REFERENCES [1] Du, X.; Gellerstedt, G.; Li, J. Universal fractionation of lignin - carbohydrate complexes (LCCs) from lignocellulosic biomass: an example using spruce wood. The Plant Journal, 2013, 74, 328–338. [2] Sanglard, M.; Chirat, C.; Jarman, B.; Lachenal, D. Biorefinery in a pulp mill: simultaneous production of cellulosic fibres from Eucalyptus globulus by soda/anthraquinone cooking and surface-active agents. Holzforschung, 2013, 67, 481–488. [3] Boucher, J.; Chirat, C.; Lachenal D. Simultaneous production of ethanol and softwood kraft pulp: extraction, concentration, secondary hydrolysis and fermentation step. Paptac Electronic Proceedings 17th ISWFPC, Vancouver, 2013, 2556–2563. [4] Chirat, C.; Boiron, L.; Lachenal, D. Bleaching ability of pre - hydrolysed pulps in the context of a biorefinery mill. Tappi Proceedings International Pulp Bleaching Conference, Portland, USA, October 5-7, 2011, 12–18. [5] Ragauskas, A.J.; Nagy, M.; Kim, D.H.; Eckert, C.A.; Hallett, J.P.; Liotta, C.L. From wood to fuelsIntegrating biofuels and pulp production. Industrial Biotechnology, 2006, 1, 55–65. [6] Chirat, C.; Lachenal, D.; Sanglard, M. Biorefinery in a kraft pulp mill: extraction of xylans from hardwood chips prior to the production of cellulosic paper pulp. Process Biochemistry, 2012, 47, 381–385. [7] Roubaud, A.; Chirat, C.; Huet, M.; Monot, C.; Lachenal, D. Development of a new pulp production process and black liquor gasification. Récents Progrès en Génie des Procédés, 2013, 104, [sfgp2013120618]. [8] Björkman, A. Studies on finely divided wood. Part I. Extraction of lignin with neutral solvents. Sven. Papperstidn. 1956, 59, 477–485. [9] Thring, R.W.; Chornet, E.; Bouchard, J.; Vidal, P.F. Evidence for the heterogeneity of glycol lignin. Ind. Eng. Chem. Res. 1991, 30, 232–240. [10] Leschinsky, M.; Zuckerstätter, G.; Weber, H.K.; Patt, R.; Sixta, H. Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 1: Comparison of different lignin fractions formed during water prehydrolysis. Holzforschung, 2008, 62, 645–652.