Energy & Fuels 2001, 15, 449-454
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Characterization and Thermal Polymerization of Eucalyptus Tar Pitches Marcos J. Prauchner,† Vaˆnya M. D. Pasa,*,† Choyu Otani,‡ and Satika Otani‡ Departamento de Quı´mica, Universidade Federal de Minas Gerais, Av. Antoˆ nio Carlos, 6627, Belo Horizonte, Brazil, and Departamento de Fı´sica, Instituto de Tecnologia Aeroespacial, Sa˜ o Jose´ dos Campos, Brazil Received September 6, 2000. Revised Manuscript Received December 18, 2000
Differently from fossil pitches, wood tar pitches have been very little studied so far as precursors of advanced carbonaceous materials (ACM). However, the development of applications for biopitches is important to increase the revenue of the charcoal manufacturing industry and to stimulate the use of biomass, thereby answering the appeals of environment preservation. This work consists of a pioneer study on Eucalyptus tar pitch, its chemical characterization, and pretreatment aiming toward the production of carbon fibers. The pretreatment is made to adjust the pitch properties, which are important for the subsequent steps in material processing and for the final product performance. Fourier transform infrared spectroscopy (FTIR), solid state 13C NMR, and elemental analysis show that biopitches are mainly constituted of interlinked phenolic rings, which are highly substituted and oxygenated. Pretreatment involved thermal polymerization. The changes in pitch polymerization degree, structure, and properties during pretreatments were assessed using differential scanning calorimetry (DSC), gel permeation chromatography (GPC), thermogravimetry (TG), 13C NMR, and elemental analysis. Moreover, the softening points (SP) and acetone-insoluble contents (AI) were determined. The results showed that polymerization was more effective at higher temperatures (about 250 °C) and it was followed by increases in glass transition temperature (Tg), SP, AI, thermal stability, and coke yield. During polymerization, side chains were released giving rise to an increase in pitch aromaticity. The possibility of adjusting wood pitch properties lends them a good perspective as precursors of ACM.
Introduction Advanced carbonaceous materials (ACM) are constituted basically of the chemical element carbon and display specific properties suitable for designed applications. Studies from all over the world have pointed to fossil pitches (coal tar and petroleum pitches) as having one of the greatest potentialities as precursors of ACM. The manufacture of ACM from pitches is important considering the economic and technological viewpoints, since it is possible to obtain materials with a large number of applications1-4 from an abundant and cheap raw material. However, wood tar pitches have been poorly studied so far as precursors of ACM. The reason is the existence of this material being practically limited to Brazil, where the tropical climate favors forests growth. This country * Author to whom correspondence should be addressed. CEP: 31270 901. Fax: (055) 31 3499 5700. E-mail:
[email protected]. † Universidade Federal de Minas Gerais. ‡ Instituto de Tecnologia Aeroespacial. (1) Handbook of Polymer-Fibre Composites; Jones, F. R., Ed.; Longman Scientific and Technical: Essex, U.K., 1994. (2) Gill, R. M. Carbon Fibres in Composites Materials; Butterworth Books: London, 1994. (3) Otani, S.; Otani, C. Anais do V Encontro de Carboquı´mica; Belo Horizonte: MG, Brazil, 1992; 01. (4) Gautier, S.; Frackowiak, E.; Conard, J.; Rouzand, J. N.; Be´guin, F. In Extended Abstracts 23rd Biennial Conference on Carbon; Pennsylvania, 1997; Vol. 2, p 148.
has an important siderurgical activity, which produces about 6.3 million ton/year of pig-iron. About one-fourth of this production uses charcoal as thermo-reducer, consuming around 6.6 million tons of charcoal in Brazil every year. About 67% of this charcoal is obtained from planted Eucalyptus forests.5 In the slow pyrolysis of Eucalyptus for charcoal production, tar is recovered by washing and condensing the vapors generated (about 600 kg per ton of charcoal produced) in masonry ovens. Wood tar has been fractionated to separate fine chemical products such as phenyl, guaiacyl, and siringyl derivatives.6,7 The fusible residue of Eucalyptus tar distillation (about 50%, depending on the distillation profile) is called Eucalyptus tar pitch. Pitch properties can be correlated to the conditions of wood pyrolysis, and tar recovery and distillation. The Brazilian potential production of Eucalyptus pitch is about 250 000 ton/year, considering only planted forests. The development of ACM from wood tar pitches is important to increase the revenue of the charcoal making industry and to stimulate the use of biomass, (5) Statistical YearbooksAssociac¸ a˜ o Brasileira de Carva˜ o Vegetal; Belo Horizonte: Brazil, 1999. (6) Stuckenbruck, P.; Neto, F. R. de A.; Carazza, F. Anais do II Encontro de Carboquı´mica; Salvador, Brazil, 1989; 03. (7) Capanema, E. A. Dissertac¸ a˜ o de Mestrado; Universidade Federal de Minas Gerais: MG, Brazil, 1993.
10.1021/ef000196o CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001
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thereby answering the appeals for environment preservation.8 The main representatives of ACM are the carbon fibers. These fibers can be produced from pitches by a multistep process.1,9 First of all, the pitch needs to undergo a pretreatment to adjust its properties which influence subsequent steps of spinning, fiber stabilization, carbonization and graphitization, and the final product structure and performance.3,10 For wood tar pitches, the pretreatment aims mainly to adjust their softening point (SP), viscosity, and thermal stability. These adjustments are carried out through the increase of the molar masses of the molecules by heat treatments. They are necessary mainly because the reactions used to stabilize the as-spun fibers (to make them thermosetting) occur only at temperatures higher than crude pitch SP. As a result, fiber fusing would occur prior to stabilization. However, the pretreatment made it possible to raise pitch SP to a value sufficiently high to make fiber stabilization possible, although not so high as to make the pitch unspinnable. This spinnability loss probably takes place because the temperature for adequate spinning viscosity becomes so high that it is near the pitch degradation temperature. Furthermore, the thermal stability of the material is improved during the pretreatment, which is desirable for the production of ACM. This work discusses the results obtained in a pioneer study on Eucalyptus tar pitch. The material was chemically characterized by Fourier transform infrared spectroscopy (FTIR), solid state 13C NMR, and elemental analysis. Moreover, some correlations between pretreatment parameters (time and temperature) and the properties of the resulting materials are established. It is very important to define these correlations as well as possible to make the process reproducible and repeatable. To assess the changes generated by the polymerization, both crude and treated pitches were analyzed by means of differential scanning calorimetry (DSC) to determine glass transition temperatures (Tg). Gel permeation chromatography (GPC) was used to estimate the weight-average molar masses (M h w) and molar mass distributions. Thermogravimetry (TG) was used to study the thermal stability of the materials. 13C NMR and elemental analysis permitted us to assess the chemical changes. In addition, SP and acetone-insoluble content (AI) were determined aiming to evaluate polymerization degree. Experimental Section Pitch Sample. Wood chips (30-40 cm long, 15-24% moisture) of planted Eucalyptus forests (Minas Gerais, Brazil) were submitted to slow pyrolysis to produce charcoal in industrial masonry ovens with a maximum pyrolysis temperature of about 400-500 °C (12-14 °C/h). Simultaneously, the smoke was washed and condensed, producing Eucalyptus tar. The precursor pitch was obtained by vacuum distillation of the generated tar in a pilot plant batch. A 3000-L boiler was (8) Biomass for Energy and Environment, Agriculture and Industry in Europe; Grassi, G., Bridgwater, T., Eds.; Edizione Esagono: Milano, Italy, 1992. (9) Donnet, J.-B.; Bansal, R. C. Carbon Fibers; Marcel Dekker: New York, 1990. (10) Otani, S. Dissertac¸ a˜ o de Mestrado; Escola Polite´cnica da Universidade de Sa˜o Paulo: Sa˜o Paulo, Brazil, 1991.
Prauchner et al. used, coupled to a fractionation column of 4 drilled plates. Homogenization of the mixture was accomplished through a centrifuge pump which recirculates the material available in the bottom of the boiler. The cut temperature was 180 °C at 30-38 mmHg (corresponding to 260 °C at 760 mmHg). The distillation time was about 8 h, and the pitch yield was about 50% (w/w). Heat Treatment. The pitch (about 400 g) was heat-treated in a 1000-mL kettle vessel connected to a vigreaux column and using an electric mantle. The pitch was homogenized using a mechanical stirrer, and the bulk temperature was measured. Softening Point (SP). SPs were determined using the “ring and ball method” following the ASTM D2398-73 Standard Test. In accordance with the norm, the tests were carried out in duplicate. If the measurements presented a difference larger than 1 °C, the analyses were repeated. Acetone-Insoluble Content (AI). AI was determined according to DIN 53.700 norm. Powdered samples (60 mesh) were wrapped in a quantitative filter paper and extracted with acetone in a Soxhlet extractor until complete extraction of the soluble fraction. Subsequently, acetone was evaporated, the residue weighed, and compared to the initial mass. The tests were carried out in triplicate. If the measurements fell in a range larger than 2%, they were repeated. Differential Scanning Calorimetry (DSC). DSC was used to determine Tg. The analyses were carried out in a Shimadzu DSC-50 Calorimeter following the ASTM D 341882 norm. The powdered samples (about 10 mg) were weighed into an aluminum pan, placed in the DSC cell, and heated at a rate of 20 °C/min under a helium dynamic atmosphere (50 mL/min). In accordance with the norm, the determinations were made in duplicate and they did not differ by more than 2.5 °C. Gel Permeation Chromatography (GPC). GPC analyses were used to determine the molar mass distribution and the weight-average molar masses of the soluble fraction of pitches in tetrahydrofuran (THF). Pitches with lower polymerization degree (the crude one, for example) showed almost total solubility. Only those pitches treated at 250 °C for 6 and 8 h showed significant insoluble content. Analyses were carried out in a Shimadzu LC-10AD Liquid Cromatographer coupled with a Shimadzu UV-Vis SPD-10AV Detector at 254 nm and a Shimadzu C-R7A Chromatopac Integrator with a software for GPC calculations. The experiments were based on techniques developed for lignin analyses.11 The elution was carried out in THF at 30 °C and a flow rate of 1 mL/min, using two coupled columns of polystyrenedivinyl benzene gel (Shim-pack GPC-8025 and Shim-pack 803). Injections of 20 µL were made with the samples dissolved in the eluant (2 mg/mL). The calibration curve was built using polystyrene standards. Thermogravimetry (TG). The analyses were carried out in a Shimadzu TGA-50 thermogravimetric analyzer. Basically, the ground samples (about 5 mg) were weighed into a platinum pan, placed into the TG cell, and heated at 10 °C/min under a nitrogen dynamic atmosphere (150 mL/min). TG curves also gave the derivative thermogravimetry (DTG) curves. Solid-State 13C NMR. 13C NMR analyses were carried out in a Varian INOVA-300 spectrometer at 75.4 MHz with a RT CP/MAS probe adequate for solid-state analyses. The spectra were obtained using single pulses (direct polarization) and magic-angle spinning (SP/MAS technique). The rotor (7 mmzirconia) was spun at 5.9 kHz at the magic-angle (54°44′). The spectra were acquired using spectral width of 50 kHz, acquisition time of 0.05 s, pulse of 90° (6 µs), delay time of 100 s, 400-2300 scans per spectrum, and high-power proton decoupling. Chemical shifts were referred to the methyl carbon (11) Johnson, D. K.; Chum, H. L.; Hyatt, J. In LigninsProperties and Materials; Gassler, W. G., Sarkanen, S., Eds.; American Chemical Society: Washington, DC, 1989; p 109.
Eucalyptus Tar Pitches
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Table 1. Results of Elemental Analyses and Thermogravimetry for Eucalyptus Tar Pitches treatment time (h)/temperature (°C) pitch properties
crude
2/250
4/250
8/250
C (%) H (%) O (%) H/C ratio O/C ratio coke yield at 650 °C (%)a
68.0 6.3 24.4 1.1 0.27 33
70.0 5.9 22.9 1.0 0.25 36
71.6 6.0 22.3 1.0 0.22 41
73.8 6.2 18.8 1.0 0.19 50
a
Coke yield by TG (650 °C, 10 °C/min).
Figure 2. Solid state 13C NMR spectrum for crude Eucalyptus tar pitch. Table 2. Assignments for the Main FTIR Absorptions of Crude and Pre-treated Eucalyptus Tar Pitches15,16 wavenumber (cm-1)
Figure 1. FTIR spectrum for crude Eucalyptus tar pitch. resonance of solid hexamethyl benzene (17.3 ppm relative to liquid TMS). Fourier Transform Infrared Spectroscopy (FTIR). Infrared analyses were carried out in a Perkin-Elmer FTIR SPECTRUM 1000 Spectrometer. Samples were prepared as KBr pellets with pitch concentration of 1% m/m. Elemental Analysis. The samples were analyzed in a Perkin-Elmer 2400 Elemental Analyzer to determine carbon, hydrogen, and nitrogen content. The oxygen content was determined by difference and taking the ash content into account (around 1%).
3450-3400 2940-2835 1740-1700 1510 and 1600 1460-1440 1365-1355 1275-1200
1115-1110 1030
