4nd International Conference May 21-23, 2013 Tatranské Matliare Renewable Energy Sources 2013 High Tatras, Slovak Republic

DETERMINATION OF TEMPERATURE REGIONS IN THERMAL DEGRADATION OF LIGNIN

Ház A., Jablonský M., Orságová A., Šurina, I. * Slovak University of Technology in Bratislava

Faculty of Chemical and Food Technology Institute of Polymer Materials

Department of Chemical Technology of Wood, Pulp and Paper Radlinského 9, 812 37 Bratislava, Slovakia

E-mail: Igor.Surina@stuba.sk

Abstract In this paper, industrial lignins were characterized by elemental analysis (EA) and thermogravimetry (TG). The aim was to find the main intervals of lignin degradation and maximum rate of their degradation. The results obtained can be helpful in finding the applicability of the materials at their thermal decomposition. The differences between individual lignins have been confirmed by EA and TG.

Keywords Lignin, elemental analysis, modification, thermal analysis

1 INTRODUCTION

Lignin is a thermoplastic 3D polymer of coniferylalcohol and its relative phenylpropene monomers of sinapylalcohol and p-cumarylacohol. Lignins are the most thermally stable component of biomass. Their 3D benzoic structure resists to thermal stress. Mainly under slow heating and atmospheric pressure, the conditions are formed for the growth condensation reactions. The yield of volatiles is relatively low; on the other hand, the yield of coal is high. The most important monomer products formed in the lignin thermolysis are phenols related to the parent C9–structure of lignins. The products of lignin thermolysis may undergo partial demethylization and the aliphatic chain may partially split so containing 1-3 C-atoms. Under heating, radicals of various structure and stability are formed. The radicals formed depending on reaction conditions (temperature, heating rate) are stabilized by consequent fragmentation or they participate in the growth reactions [1].

Lignosulfonates are the most accessible commercial lignin, which arise as a by-product from the manufacture of wood pulp. Lignosulfonates are water soluble anionic polymers. In practice, they are very frequently applied as a plasticizer in concrete, in the manufacture of dyes and leather processing. Oxidation of lignosulfonates can be used for the production of vanillin, which can be used directly in food processing [2].

Fig. 1 Thermolysis of lignin and coal formation [1]

Thermolysis of lignin is a complicated process, which combines a variety of degradation, but also condensation reactions. Region to 150°C is mainly linked with the loss of free and bound water. During the first stage of lignin heating in temperature range 100-180°C plasticization occurs (endothermic phase). In this stage, fission of weak ether, aryl-alkyl and phenyl glycosides bonds occurs. C-C bonds in aliphatic chain of phenylpropane units are

4nd International Conference May 21-23, 2013 Tatranské Matliare Renewable Energy Sources 2013 High Tatras, Slovak Republic broken down in the range from 240°C to 280°C provided that they either contain a hydroxyl group (breakdown arises after previous intramolecular dehydration of aliphatic hydroxyls) or a double bond. Ethers C-O-C and alkyl-aryl bonds are broken down at 270-300 ° C. The presence of free OH groups depressed this range for lower temperatures breakdown (150-270 C). Lignin is depolymerized with the release of monomeric phenols. Bonds CO-(C6H10O5)nH phenyl-glycoside linking lignin and hemicellulose are broken at 270-300 ° C (Fig. 2, aromatic ring 2. with no. 5).

Fig. 2 Scheme of a hypothetical fragment lignin macromolecule [1]

C-C single bonds splitting takes place in the temperature region exceeding 300°C, in a few parts of the activated alkyl C9 unit at 325-330°C (Fig. 2, second aromatic ring with no. 3), and the type of alkyl-aryl bonds at temperatures 320-325°C and in the bonds of type aryl-aryl at approximately 385°C (Fig. 2 between the aromatic rings 6 and 7).

The ether bonds splitting between the core and the CH3-group in methoxyl occurs in the temperature range 350-400°C, which is due to the sterical conditions in condensed system (aromatic rings 1 to 7). Temperature region corresponding to the endothermic effect at 400°C is characteristic for stronger links of guiacyl type, as well as for the bonds formed at radical recombination. The strong bonds include pinorezinol bonds (links between 5th and 6th benzene ring). Phenylcumarane ring is breaking down at 370-400°C, but with etherified hydroxyl its decomposition is carried out at a temperature of 400-420°C (between the aromatic rings with no. 1 and 2). With the temperature reached 395°C a splitting of carbon bonds in rings and also bonds C-C formed at radical recombination occur [1]. Further gradual increase in the temperature leads to starting to produce a greater proportion of small molecules (e.g., CO, CO2, and CH4).

2 MATERIALS AND METHODS

2.1 MATERIALS

The following industrial (commercial) samples of lignosulfonates were used: Borresperse N, Borrement Ca 120, Vanisperse CB (Borregard LignoTech), Marasperse N-22 (Daishowa Chemical Inc.), Orzan S (ITT Rayonier Inc.), Maratan SN (American Can Company), DP–02 and DP–03 (Biotech).

2.2 ELEMENTAL ANALYSIS

Carbon, nitrogen, hydrogen and sulphur content of the lignins, was determined by elemental analysis, using an analyser Vario Macro Cube, Elementar.

4nd International Conference May 21-23, 2013 Tatranské Matliare Renewable Energy Sources 2013 High Tatras, Slovak Republic 2.3 THERMOGRAVIMETRIC ANALYSIS

Thermogravimetric analysis is an analytical method to quantitatively monitor the weight change (increase, decrease) of the sample, depending on the temperature change. The sample mass depends on the capacity of the device. The module of our device operates with a capacity of the sample to 500 mg. In case of the module replacement, it can reach up to five grams. Analyzed samples are usually in the liquid or solid state.

Fig. 3 Scheme of thermogravimeter Mettler Toledo TGA/DSC 1 [3]

Thermogravimetric analysis (TGA) of lignins was carried out using Mettler Toledo TGA/DSC 1 instruments. The analysis was performed in reduction atmosphere keeping identical temperature regime for all samples. The atmosphere was ensured by argon of purity 4.6 and flow rate 50 ml/min-1. Argon plays the role of protection gas also in the balance space. The measurements were performed in the temperature interval of 30-800°C in three segments. At the beginning, the sample was conditioned at 30°C for 3 min. Subsequently, thermodynamic segment occurs increasing the temperature by 10 K/min. After reaching 800°C, the measurement was concluded at 800°C for 3 min.

3 RESULTS AND DISCUSSION

3.1 ELEMENTAL ANALYSIS

Elemental composition of investigated samples depends on the initial preparation of the material as well as possible additives which are industrially added to provide required properties and stabilization of the material. Changes in the composition of lignosulfonates are caused by a variety of incoming raw materials in their production but also by preparation methods. Differences represented by carbon content can be seen in Tab. 1. Maratan SN sample contains 65.26%, followed by sample Vanisperse value with 52.54%. The carbon content values of the other samples were in the range from 42.86% to 48.12%. Significant differences were observed in the case of determination of the sulfur content. The lowest content (2.96%) was determined for Vanisperse sample. Further group of samples contained sulfur in the range of 5.12 to 6.49% and the highest content was found in samples Maratan SN (8.73%) and DP-03 (7.15%). As the sulfur content can be one of the criteria for the use of commercial lignosulfonates in a wide range of applications, it should be as low as possible. In the processing of such materials (such as fuel) it is important not to release toxic emissions generated due to the sulfur content. Tab. 1 Elemental composition and ash content of lignosulfonates

Elemental analysis (% wt) Used lignosulfonates N C H S

Ash (%)

Vanisperse 0.12 52.54 4.08 2.96 25.48 Orzan S 0.09 45.42 5.05 5.15 10.83 Maratan SN 1.27 65.26 7.79 8.73 11.81 Borrement Ca120 0.14 46.63 5.35 5.62 13.13 Marasperse N 22 0.14 43.52 4.68 6.28 24.96 Boresperse N 0.14 44.11 4.65 6.49 23.34 DP-03 0.14 48.12 5.08 7.15 22.42 DP-02 0.16 42.86 4.38 5.12 19.64

4nd International Conference May 21-23, 2013 Tatranské Matliare Renewable Energy Sources 2013 High Tatras, Slovak Republic 3.2 THERMOGRAVIMETRIC ANALYSIS

Thermal degradation, major degradation temperatures, weight loss at 200-650°C and percentage of charred residue at 800°C are given for all studied lignosulfonates in Tab. 2. As an example, is shown in Fig. 4 (oxidative atmosphere) and in Fig. 5. (reductive atmosphere) thermogram of the sample Orzan S.

Tab. 2 Thermogravimetric analysis of various lignosulfonates

Major degradation temperature regions (°C) Lignosulfonates

0-100 100-200 200-470 470-650 650-800

% Degradation at 200-650°C

Charred residue % at 800°C

Vanisperse 80.2 140.9 348.2 - 690.2 30.6 57.2 Orzan S 80.5 195.7 258.7 - 699.0 41.7 39.5 Maratan SN 79.8 102.5 292.0 489.5 736.2 48.1 40.2 Borrement Ca120 96.1 102.7 296.4 474.1 710.9 34.7 49.9 Marasperse N 22 93.1 195.3 252.3-297.9 633.0 777.5 35.8 46.7 Boresperse N 83.4 195.0 303.2 - 690.5 33.2 49.5 DP-03 80.3 195.0 305.7 - 689.7 34.9 46.7 DP-02 80.6 190.8 292.7 485.9 736.5 33.3 52.9

All these industrial lignins have shown a wide variation in their thermal degradation phenomena [4, 5]. Lignin decomposition is a complex process involving several competing reactions. During the reactions, various bond fissions within lignin molecule occur at a wide ranges of temperatures based upon the bond energy [6]. The degradation of lignin takes place in a wide temperature range of 100-800°C but, the major degradation takes place between 200-700°C [6,7].

Fig. 4 Thermogravimetric diagram of weight loss in 5 steps

By comparing all samples in the first range 0-100°C, the loss of moisture content was observed, which was the highest for sample Vanisperse (3.73%), followed by sample Borresperse (2.32%) and the least moisture sample had DP-02 containing only 1.42% moisture. However, this was not kept in a second step and the sample with the largest weight loss in the temperature range 100-200°C was Orzan S (9.25%). All the other samples showed losses in the range of 3-5%. Weight loss in the first two steps are mainly associated to the bound and free water, but above 150°C breaking of alkyl-aryl ether bonds owing OH groups gradually occurs. The maximum values are mainly associated with the loss of moisture (step 1) in the range of 80.2-96.1°C, with a maximum of loss rate in Vanisperse reaching 9.44×10-3 min-1.

4nd International Conference May 21-23, 2013 Tatranské Matliare Renewable Energy Sources 2013 High Tatras, Slovak Republic After reaching 800°C, the sample Vanisperse (57.18%) and DP-02 (52.93%) had the smallest loss of their original weight, while the other samples have a significantly less of their original weight, in the range of 39.45-49.52%. This mass is associated predominantly with inorganic material (such as silicates, carbonates) and it can still contain carbon, which is also referred to as the fixed carbon. Another reason for the lower loss of original material lies in the stability of Vanisperse samples in the third interval of 200-470°C exhibiting the lowest decrease among all samples. The opposite case is represented by Maratan SN (40.22%) and Orzan S (39.45%) samples the lowest residues after the 800°C treatment. For both samples there was a massive degradation in the range 200-470°C, and also relatively high loss of material taken place also in the range 100-200°C. The fourth and fifth range from 470°C to 800°C is due in particular to the release of small molecules (e.g., CO, CO2, CH4, H2), but in the beginning of this range there is also a strong bonds break and release of organic compounds. In this section, samples have weight loss in the range of 8.15 to 13.07%.

Fig. 5 TG/DTG thermogram of Orzan S with major degradation temperatures

Weight loss of lignosulfonates in five degradation temperature regions is given for all studied lignosulfonates in Tab. 3. The most interesting field in terms of organic matter is just the part of the first steps up to 470°C.

The greatest weight loss in this part of the sample had Maratan SN (49.1%) and Orzan S (48.3%), followed by samples Marasperse N 22 (41.6%), Borrement Ca120 (40.8%), DP-03 (40.3%), DP-02 (38.9 %), Borresperse (38.8%) and the lowest loss of the sample reached the value for 32.3%Vanisperse. The highest rate in this area reached Orzan S with a loss rate of 27.4×10-3 min-1 at 195.4°C. The sample Vanisperse in terms of TGA curve shows particularly stronger bonds with the maximum loss at 348.2°C. The sample Orzan S is assumed to have mostly weaker bonds, as the two largest peaks are at temperatures of 195.7°C and 258.7°C. Tab. 3 Weight loss of lignosulfonates in five degradation temperature regions

Weight loss during degradation (%) SUM Lignosulfonates

0-100 °C 100-200 °C 200-470 °C 470-650 °C 650-800 °C (%) Vanisperse 3.7 3.3 25.3 5.3 5.2 42.8 Orzan S 1.7 9.3 37.3 4.4 8.1 60.8 Maratan SN 2.3 5.0 41.7 6.4 4.4 59.8 Borrement Ca120 2.3 3.8 34.7 5.0 4.4 50.2 Marasperse N 22 1.9 3.8 35.8 4.8 6.9 53.3 Boresperse N 1.5 4.1 33.2 4.7 7.0 50.5 DP-03 2.0 3.4 34.9 4.6 8.5 53.4 DP-02 1.4 4.3 33.3 5.0 3.1 47.1

4nd International Conference May 21-23, 2013 Tatranské Matliare Renewable Energy Sources 2013 High Tatras, Slovak Republic 4 CONCLUSION

Lignin is produced as a by-product in paper industry and can be found in black liquor. Lignin isolated from black liquor is known as lignosulfonate when sulfur-containing agents are used in delignification technology. In some case is the presence of sulfur in lignins is not desirable. Information of sulfur content can be achieved by elemental analysis. Both, lignin in black liquor and lignosulfonates have actually great importance as a source of raw material for chemicals with added value. One possible way to obtain these chemicals is their thermal degradation. Therefore, it is important to know the appropriate conditions for such kind of degradation.

Results obtained in this work show that all industrial lignosulfonates were obtained by technology using sulfur agents. Elemental composition was not the same for all the lignins. Maratan SN has higher sulfur and also higher carbon content. DP-02 has the lowest value for carbon content. Vanisperse shows the lowest sulfur content. Lignosulfonates with low sulfur content can be used in many applications where it is important to reduce the environmental impact resulted from their thermal decomposition.

In this work temperature intervals of thermal degradation, temperature of maximum rate of decomposition were determined. Vanisperse with the lowest sulfur content exhibited lower degree of degradation in the whole interval of temperatures. The highest weight loss (48.1%) was obtained for Maratan SN and the lowest value (30.6%) for Vanisperse in the temperature region of 200-650°C. Vanisperse and DP-02 had a higher amount of charred residue. The lowest amount of charred residue was observed in the case of Orzan S. The higher char content suggests that this product can be used as a fire retardant in composites.

Determined intervals of thermal degradation of lignosulfonates can be useful in many of their possible modifications.

5 ACKNOWLEDGEMENT

This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0850-11, and by the project PROLIGNO: Separation and study of lignin-type substances (Grant STU for young scientist).

6 REFERENCES

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[4] Grandmaison JL, Thibault J, Kaliaguine S. Fourier transform infrared spectrometry and thermogravimetry of partially converted lignocellulosicmaterials. Anal Chem 1987; 59:2153e7.

[5] TGA and macro-TGA characterisation of biomass fuels and fuel mixtures. Skreiberg, A., 2011, Fuel 90, p. 2182–2197.

[6] Jakab E, Faix O, Tiii F. Thermal decomposition of milled wood lignins studied by thermogravimetry/mass spectrometry. J Anal Appl Pyrolysis 1997; 40e41:171e86.

[7] TG-GC/MS, TG/MS and TG/FTIR applications on polymers and waste products. Reggers, G., 1997, Thermochimica Acta 295, p. 107-117.

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