A Study of the Polymerization and Condensation Reactions during the

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A Study of the Polymerization and Condensation Reactions during the Heat Treatment of Pitches under Gas-Blowing Conditions Yolanda Martıń,† Roberto Garcıá,† Pat Keating,‡ Colin E. Snape,‡ and Sabino R. Moinelo*,† Instituto Nacional del Carbo´ n, CSIC, Apdo. 73, 33080 Oviedo, Spain, and University of Strathclyde, Department of Pure and Applied Chemistry, Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1XL, U.K. Received July 1, 1999. Revised Manuscript Received October 27, 1999

A coal tar pitch and a petroleum pitch were heat treated under gas-blowing conditions (air and argon) at 300, 350, and 425 °C in order to investigate the chemical transformations involved. The evolution profiles of four classes of polycyclic aromatic hydrocarbons (PAH) in the toluenesoluble (TS) fractions were determined using an HPLC method. The influence of the type of gas on heat-treatment characteristics of the pitches was clearly identified. The evolution of the different classes of PAH present in the TS of a coal tar pitch to yield toluene-insoluble (TI) material is more significant and faster under air-blowing than under argon-blowing. The presence of oxygen promotes a general increase in the reactivity of the TS, especially in the case of the peri-condensed compounds. The topology of the latter strongly determines their higher increase of reactivity with oxygen. The reactivity of the cata-condensed PAH is significantly higher in the case of the petroleum pitch due to the higher presence of alkyl substituents. It is therefore concluded that the chemical composition of the raw pitches influences their behavior during the carbonization under gas-blowing conditions. The evolution of cata- and peri-condensed compounds during the heat-treatment of the pitches under gas-blowing conditions did not always fit simple first-order kinetics. A model for the overall reaction pathway was proposed, in which β-resins constitute an intermediate product in the transformation of TS to quinoline-insoluble (QI). A good correlation between some thermal parameters obtained by TGA/DTG and the chemical composition of the pitches expressed in terms of their HPLC compound class distribution was found.

Introduction Pitches are very important precursors for the manufacture of carbon materials (CM). The chemical composition of pitches, their physical properties, and the mechanisms of the reactions involved in their transformation into the final products strongly determinate the properties and applications of the resultant CM. For these reasons, the chemistry of pitch carbonization has been studied extensively.1-5 However, the current knowledge about the structural composition of pitches is still insufficient to fully understand their thermal reactivity. Pitches comprise complex mixtures of polynuclear aromatic hydrocarbons (PAH) and heterocyclic compounds. A number of investigations on pitch carbonization have * Corresponding author. † Instituto Nacional del Carbo ´ n, CSIC. ‡ University of Strathclyde. (1) Lewis, I. C. Carbon 1982, 20, 519-529. (2) Zander, M.; Haase, J.; Dreeskamp, H. Erdo¨ l Kohle Erdgas Petrochem. 1982, 35, 65-69. (3) Tillmans, H. In Petroleum-Derived Carbons; Bacha, J. D., Newman, J. W., White, J. L., Eds.; ACS Symposium Series, Vol. 303; American Chemical Society: Washington, DC, 1986; Chapter 16, pp 215-233. (4) Lewis, I. C. Fuel 1987, 66, 1527-1531. (5) Lewis, I. C.; Singer, L. S. In Polynuclear Aromatic Compounds; Ebert, L. B., Ed.; Advances in Chemistry Series 217; American Chemical Society: Washington, DC, 1988; Chapter 16, pp 269-285.

pointed out that it can be considered as divided in two types of processes.1-5 The first process involves the removal of light hydrocarbon components of the pitch and a slight polymerization of the PAH, and the second type refers to the condensation of the aromatics rings giving rise to polyaromatic compounds of higher molecular mass. Unfortunately, the individual reactions taking place during pitch carbonization are not fully understood yet. Therefore, a deeper knowledge of the thermal reactivity of pitches and its relationship with pitch composition will improve the pitch and CM production processes and allow the development of new pitch-based materials. Several procedures have been established to modify pitch properties in order to optimize their utilization as CM precursors.6-14 Heat-treatment (HT) of pitches (6) Mochida, I.; Tamaru, K.; Korai, Y.; Fujitsu, H.; Takeshita, K. Carbon 1983, 21, 535-541. (7) Hein, M. Erdo¨ l Khole Erdgas Petrochem. 1990, 43, 354-358. (8) Rhee, B.; Chung, D. H.; In, S. J. Carbon 1991, 29, 343-350. (9) Maeda, T.; Zeng, S. M.; Tokomitsu, K.; Mondori, J.; Mochida, I. Carbon 1993, 31, 407-412. (10) Zeng, S. M.; Maeda, T.; Tokomitsu, K.; Mondori, J.; Mochida, I. Carbon 1993, 31, 413-419. (11) Miyake, M.; Ida, T.; Yoshida, H.; Wakisaka, S.; Nomura, M.; Hamaguchi, M.; Nishizawa, T. Carbon 1993, 31, 705-714. (12) Kanno, K.; Yoon, K. E.; Fernańdez, J. J.; Mochida, I.; Fortin, F.; Korai, Y. Carbon 1994, 32, 801-807.

10.1021/ef9901443 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/26/2000

Polymerization and Condensation in Pitch Heat Treatment

Figure 1. Schematic of the heat treatment experimental setup. (1) Reactor; (2) gas inlet; (3) steel stirrer; (4) reactor cap; (5) stirring head; (6) temperature programmer (Eurotherm 902); (7) refrigerant; (8) collection flask; (9) tubular oven; (10) stirring engine; (11) distilled matter exit; (12) collecting traps. Table 1. Properties and Analytical Data of Original Pitches CTP

PP

SPa (°C) TIb (wt %) QIc (wt %) β-resin (wt %)

50.5 30.8 18.7 12.1

108 4.9 0.2 4.7

C (wt % db) H (wt % db) N (wt % db) O + Sd (wt % db)

93.2 4.2 0.9 1.7

90.0 5.6 0.2 4.2

H/C

0.54

0.74

a

SP: softening point. b TI: toluene-insoluble content. c QI: quinoline-insoluble content. d By difference. Table 2. Heat Treatment Experimental Conditions pitch

gas

gas flow (mL/min)

temperature (°C)

residence time (min)

CTP CTP CTP CTP PP

air air air argon air

600 600 600 600 600

300 350 425 350 350

180 180 120 180 180

under air-blowing conditions is a common industrial practice to raise the softening point (SP) and the coking residue and limit the growth and coalescence of mesophase. Consequently, air-blown coal tar pitches are very useful precursors of isotropic carbon fibers. Considerable effort has been dedicated to gaining insight into the influence of oxidative HT on the reactions involved in the carbonization process.15,16 Gentle pitch oxidation can induce dehydrogenative polymerization,17 although the detailed mechanism of the air-blowing reaction is not yet clear. However, some kinetic models describing the carbonization of different kind of pitches (13) Matsumoto, M.; Higuchi, M.; Tomioka, T.; Sunago, H. Fuel 1994, 73, 237-242. (14) Zeng, S. M.; Maeda, T.; Mondori, J.; Tokomitsu, K.; Mochida, I. Carbon 1993, 31, 421-426. (15) Yang, C. Q.; Simms, J. R. Carbon 1993, 31, 451-459. (16) Yanagida, K.; Sasaki, T.; Tate, K.; Sakanishi, A.; Korai, Y.; Mochida, I. Carbon 1993, 31, 577-582. (17) Barr, J. B.; Lewis, I. C. Carbon 1978, 16, 439-444.

Energy & Fuels, Vol. 14, No. 2, 2000 381

under several types of gas-blowing conditions (air, hydrogen, carbon dioxide, and nitrogen) have been proposed.18-23 Choi et al.18,19 postulate the existence of two fractions of different thermal reactivity, although information about their chemical nature is not provided. Hu¨ttinger et al.20-23 work from the point of view of mesophase formation, with a kinetic model which introduces polyaromatics and mesogenic aromatics as intermediate fractions, but without distinguishing fractions of different reactivity in the original pitch. The activation energies of the global reactions involved, leading to the formation of toluene- or pyridine-insoluble (TI and PI) material and mesophase, depend not only on the chemical composition and concentration of pitch, but also on the type and flow rate of the blowing gas and the HT temperature. Some success has been achieved in the determination of the pitch behavior during its thermal treatment. For example, Boenigk and Niehoff24 have developed a method capable of providing a complete pyrolysis profile of a pitch using a high-vacuum distillation. However, no simple method or technique has been described to assess the thermal reactivity of the single components of a given pitch, and only thermal analysis (TGA/DTG/ DTA) provides some information about the global thermal behavior. Recently, a new HPLC procedure for pitch characterization was developed.25-27 It allows the quantitative determination of the composition of the pitch toluenesoluble (TS) fraction in terms of different classes of PAH. Four classes of PAH are clearly distinguished: (i) catacondensed compounds substituted with heteroatomic and/or alkyl and aryl groups (Cata1); (ii) cata-condensed compounds with naphthenic groups and/or substituted with alkyl and aryl groups (Cata2); (iii) unsubstituted and planar cata-condensed compounds (Cata3); and (iv) peri-condensed compounds (Peri). The elution of these fractions in this chromatographic system is believed to be ruled by the interaction between the π-electrons of both the PAH and the stationary phase: a better delocalization of the PAH π-electrons (lower ionization potential) gives rise to a higher elution volume.27 In contrast to other solvent and chromatographic separation techniques, each of the PAH classes distinguished by this HPLC methodology is homogeneous in terms of overall thermal reactivity and different from the other ones.28 The main effects of the carbonization process on the pitch composition are the release of volatile matter and the polymerization reactions. The latter proceed through the formation of free radicals with the ionization potentials being an indication of the tendency of (18) Choi, J. H.; Kumagai, H.; Chiba, T.; Sanada, Y. Extended Abstracts of 21st Biennial Conference on Carbon 1993, pp 286-287. (19) Choi, J. H.; Kumagai, H.; Chiba, T.; Sanada, Y. Carbon 1995, 33, 109-114. (20) Hu¨ttinger, K. J.; Wang, J. P. Carbon 1991, 29, 439-448. (21) Hu¨ttinger, K. J.; Wang, J. P. Carbon 1992, 30, 1-8. (22) Hu¨ttinger, K. J.; Wang, J. P. Carbon 1992, 30, 9-15. (23) Hu¨ttinger, K. J.; Bernhauer, K.; Christ, K.; Gschwindt, A. Carbon 1992, 30, 931-938. (24) Boenigk, W.; Niehoff, A. Proceedings of the 1992 International Carbon Conference, Essen (Germany), 1992, pp 157-159. (25) Alvarez, R.; Dıéz, M. A.; Garcıá, R.; Gonza´lez de Andre´s, A. I.; Snape, C. E.; Moinelo, S. R. Energy Fuels 1993, 7, 953-959. (26) Martıń, Y.; Garcıá, R.; Sole´, R. A.; Moinelo, S. R. Energy Fuels 1996, 10, 436-442. (27) Martıń, Y.; Garcıá, R.; Sole´, R. A.; Moinelo, S. R. Chromatographia 1998, 47, 373-382. (28) Martıń, Y. Ph.D. Thesis, University of Oviedo, Spain, 1997.

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Figure 2. Evolution of the different pitch fractions, during the heat treatment of the coal tar pitch (CTP) at 300 °C under airblowing conditions. The first point, at time ) -100 min, indicates the composition of the original pitch.

specific compounds to polymerize. Topology seems to play a key role in aromatic polymerization as it is intimately related to the delocalization of the PAH π-electrons and, then, to the stability of the intermediate radicals.28 For this reason, the HPLC class separation for PAH developed by the authors is especially interesting for thermal reactivity studies, being possible to monitor the actual chemical changes associated with polymerization and condensation reactions in the HT of pitches. Also, kinetic calculations for classes of compounds homogeneous in terms of thermal reactivity can be deduced on the basis of the results of the HPLC methodology. In this work, the kinetics of the reactions involved in the HT of pitches under different gas-blowing conditions are investigated. The evolution of the different classes of PAH present in the TS fractions of pitches during the HT in the presence of air or argon is traced, providing information about their behavior promoted by the process conditions. Residual pitch and volatile matter are considered separately, clearly distinguishing between the reacting fractions and the distilled material. The TS fractions of the pitches have also been analyzed using a traditional normal phase HPLC methodology (elution in terms of number of aromatic rings). This has the evolution of some individual PAH during the HT experiments to be determined, so confirming the results obtained from the HPLC separation in terms of classes of compounds. Cata- and peri-condensed compounds display a different thermal reactivity depending on the nature of the original pitch and the reaction conditions. The kinetics of the reactions experienced by these two types of compounds are studied here. The relationship between

pitch composition (in terms of the different classes of PAH of their TS) and some representative thermal parameters (temperature for maximum mass-loss rate, mass-loss percentage, and activation energy), calculated by thermogravimetric analysis (TGA) and differential thermogravimetry (DTG), is also investigated. Experimental Section Samples Studied. A coal tar pitch (CTP) and a petroleum pitch (PP) whose characteristics are summarized in Table 1, were subjected to different heat treatments under the experimental conditions listed in Table 2. A schematic diagram of the HT apparatus is shown in Figure 1.28 The apparatus is comprised of a 400 cm3 stainless steel batch reactor with a stirrer and a temperature controller and programmer and incorporates a gas inlet and an exit for the distilled material, which is collected in a group of ice-cooled traps. For each experiment, the pitch (30 ( 0.01 g) was loaded in the reactor and heated at 3 °C/min up to the final temperature (300, 350, or 425 °C). At this point, considered to be zero time, gasblowing (air or argon) and stirring were started at a constant flow of 600 cm3/min and a constant rate of 200 rpm, respectively. Hence, negative times in the plots presented below refer to the heating period, from ambient temperature up to the final temperature. When the latter is reached, positive residence time starts. The soaking times ranged between 15 and 180 min. No sampling was made during the experiments in order to prevent any concentration change derived from the removal of any material from the reactor. Thus, the concentration data of the fractions at different residence times correspond to different tests. For example, in the case of CTP at 300 °C and air-blowing conditions, five tests were conducted with 0, 30, 60, 120, and 180 min of residence time, respectively. At the end of each test, the reactor was immediately cooled by



Figure 3. Evolution of the different pitch fractions, during the heat treatment of the coal tar pitch (CTP) at 350 °C under airblowing conditions. The first point, at time ) -116 min, indicates the composition of the original pitch. immersion in water and ice. The resultant pitches and the distilled matter collected in the cool traps were recovered and weighed. Standard Characterization. The CTP and PP and the pitches obtained under the different experimental conditions were characterized in order to determine their SP and the toluene- and quinoline-insoluble (TI and QI) contents according to ASTM D4312 (toluene) and ASTM D2318 (quinoline), respectively. HPLC Analysis. The TS fractions from the pitches and the distillates collected in the experiments were analyzed by HPLC, following the methodology described elsewhere.26,27 Briefly, the HPLC analyses were carried out using a HewlettPackard HP1100 system consisting of two columns (PLgel, 300 mm × 7.5 mm, i.d.) packed with poly(styrene/divinylbenzene) copolymer of two different nominal pore sizes (500 and 100 Å, respectively) and connected in series. A diode-array detector operating at 254 nm was used. The mobile phase was dichloromethane (DCM)/methanol (9/1 vol) at a flow rate of 1 cm3/ min. Also, conventional HPLC analysis was carried out using a proprietary electron-deficient, nitroaromatic-bonded silica column manufactured by Shandon (Hypersil CTA normal phase column, 25 cm × 4.6 mm, 5 µm particles). The system also was comprised of an Applied Chromatography Systems Model 352 gradient elution pump, a Waters 486 UV detector at 254 nm, and a PC-based software package for peak integration. The volumetric flow rate was set at 0.6 cm3/min, and a standard mixture of 16 PAH was used to optimize the separation achieved with n-hexane and DCM mixtures, the most satisfactory gradient elution scheme being 10% v/v DCM in n-hexane for 20 min, then 10 to 100% DCM in 60 min, and finally 100% DCM for 20 min.29,30 (29) Sirkecioglu, O.; Andre`sen, J. M.; McRae, C.; Snape, C. E. J. Appl. Polym. Sci. 1997, 66, 663-671. (30) McRae, C.; Sun, C.-G.; Snape, C. E.; Fallick, A. E. Org. Geochem. in press.

Evolution of the Pitch Fractions. To understand the changes that a pitch undergoes when heated in the presence of air or argon, the concentrations of TI and the different classes of PAH in the TS fraction at any residence time was expressed as a percentage of the initial pitch. These concentrations included only the material still in the reactor and excluded any of the distillate collected at the end of each HT experiment that was subtracted from the amount of the initial pitch. TGA/DTG Analysis and Calculations. TGA and DTG were carried out on the pitches (approximately 17 mg) using a SETARAM equipment, Mod TGA24, with a heating rate of 10 °C/min up to 900 °C under a nitrogen flow rate of 50 cm3/ min. Activation energy (E) values have been calculated from DTG curves over the temperature range 250-600 °C. According to Raspopov et al.,31 the evaluation of kinetic parameters in complex substances such as pitches from TGA and DTG data requires two assumptions: (i) the global mass loss process (devolatilization) can be represented as a combination of successive individual stages, taking place in short intervals of temperature, each characterized by a different rate constant, and (ii) each stage is defined by a first-order kinetic equation with respect to the amounts of volatile matter. The thermograms were analyzed using the mathematical model described by Doyle and Gorvachev,32 which was previously used for the kinetic studies of the pyrolysis of oil shales32-34 and pitches.31 For the rate of mass loss during TG experiments at constant heating rates, eq 1 must be accomplished, x being the fractional mass loss (eq 2) and w0, wt, and wf the weights of sample at (31) Raspopov, M. G.; Balykin, V. P.; Kharlampovich, G. D. Solid Fuel Chem. 1986, 20 (11), 108-113. (32) Skala, D.; Kopsch, H.; Sokic, M.; Neumann, H. J.; Jovanovic, J. Fuel 1987, 66, 1185-1191. (33) Williams, P. F. V. Fuel 1985, 64, 540-545. (34) Ballice, L.; Yu¨ksel, M.; Saglam, M.; Schulz, H.; Hanoglu, C. Fuel 1995, 74, 1618-1623.

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Figure 4. Evolution of the different pitch fractions, during the heat treatment of the coal tar pitch (CTP) at 425 °C under airblowing conditions. The first point, at time ) -140 min, indicates the composition of the original pitch.

Figure 5. Evolution of the different pitch fractions, during the heat treatment of the coal tar pitch (CTP) at 350 °C under argonblowing conditions. The first point, at time ) -116 min, indicates the composition of the original pitch. initial time, any time, and final time, respectively; k is the rate constant. Assuming first-order kinetics, eq 3 can be

deduced in which q symbolizes the heating rate, T the Kelvin temperature, and A, E, and R the usual Arrhenius equation



Figure 6. Evolution of the different pitch fractions, during the heat treatment of the petroleum pitch (PP) at 350 °C under air-blowing conditions. The first point, at time ) -116 min, indicates the composition of the original pitch. terms. The integration of eq 3 results in eq 4.

dx/dt ) - k f(x)

In first-order kinetics, f(x) ) 1 - x

(1)

x ) (w0 - wt)/(w0 - wf)

(2)

q (dx/dT) ) A[exp(-E/RT)] (1 - x)

(3)

ln[-ln(1 - x)/T2] ) ln[AR/q (E + 2RT)] - E/RT

(4)

The activation energy was calculated from eq 4 by plotting ln[-ln(1 - x)/T2] versus 1/T. E can be determined from the slope (E/R).

Results and Discussion HPLC Characterization. Figures 2-6 show the evolution of the TI (β-resins and QI) and the classes of PAH in the TS during the heat treatment of the CTP and the PP under different conditions used. The data for the different components correspond to their residual concentrations at the specified residence times, after discounting the fractions present in the distillates at each time, i.e., the material released from the reactor and collected in the cooled traps. Implicitly, it is assumed that this material was simply distilled off from the reactor without reacting. Thus, only the evolution of compounds involved in thermal reactions are being considered. In HT experiments of CTP at 300 and 350 °C under air-blowing conditions (Figures 2 and 3), during the preheating stage, i.e., before the gas-blowing starts (negative residence times), only the transformation of Cata3 to Peri compounds can be observed. After gasblowing starts (zero minutes), the concentrations of the Cata and Peri fractions decrease, reaching lower final

values at 350 °C, with the increase in the QI content being greater at this temperature. The β-resins fraction, however, shows an initial increase in concentration, followed by a decrease after 60 min. This confirms that β-resins are intermediate products in the reaction of TS material to yield TI products. At 425 °C (Figure 4), the Cata3 fraction has almost disappeared at zero time, and the Cata2 fraction has also reacted, with slight increases in the Peri, β-resin, and QI fractions being observed. On the other hand, the increase in β-resins is restricted to the preheating period, before air-blowing starts. Overall, the evolution at this temperature magnifies what is observed at lower temperatures, with the conversions starting earlier, during the preheating period. In the HT experiment of CTP at 350 °C under argonblowing conditions (Figure 5), only the transformation of Cata3 to Peri compounds can be observed during the preheating stage, similar to the air-blowing HT. After gas-blowing starts (zero minutes), the most significant difference between the air and the argon experiments involves the content of peri-condensed compounds, whereas the evolutions of the different classes of catacondensed show similar decreases. The fast decrease in the content of the Peri fraction accompanied by an increase in the TI content is more significant in the presence of air than in the argon-blowing test. This indicates that air induces a higher thermal reactivity for the peri-condensed compounds, probably due to their topology or higher molecular size. They usually have lower ionization potentials (IP)1 than the analogous (same number of aromatics rings) cata-condensed compounds35,36 (Table 3). The reactions of PAH in the HT

386


Table 3. Ionization Potential (IP) of Some Polynuclear Aromatic Hydrocarbons (PAH)

Martıń et al. Table 4. Contents of the Peri1 and Peri2 Fractions during the HT of the Coal Tar Pitch at 350 °C under Gas Blowing soaking time (min) air argon

Peri1 Peri2 Peri1 Peri2

0

30

60

90

120

180

6.0 16.0 5.9 14.2

7.0 19.7 6.9 18.1

4.8 8.2 5.6 16.8

1.2 1.3 4.9 16.9

1.3 1.3

1.0 1.5 3.8 15.8

a Using Schmidt’s empirical relationship of IP ) [5.9 + 2.88(HOMO)]31

which are intermediates in the condensation reactions. A more efficient delocalization of π-electrons occurs in the peri-condensed compounds (the unpaired electron is stabilized by resonance), providing a higher stability of these oxiradicals. However, the details of the mechanism of the polymerization/condensation reactions of PAH in the presence of oxygen are not clear yet, although it is believed that the decomposition of the oxiradicals promotes the condensation of the aromatic rings by dehydrogenative polymerization. The evolution of the different pitch fractions during the preheating stage of CTP up to 425 °C, before the gas-blowing starts, indicates that the contents of the Cata2 and Cata3 fractions decrease, while the proportion of peri-condensed compounds increases (Figure 4). It can be suggested that, in the absence of oxygen or any other blowing gas, cata-condensed compounds are more thermally reactive and that polymerization/ condensation proceeds thermally. Therefore, the relative reaction rates of the cata- and peri-condensed compounds are strongly influenced by the operating conditions. The chromatographic system used permits the division of the peri-condensed compounds into two groups (Peri1 and Peri2), with the retention volume increasing with the size and condensation degree.25-27 The Peri2 fraction groups the largest and most highly condensed ring systems. During the HT of the CTP under airblowing at 350 °C, both groups display an initial increase in concentration until 30 min; then the content of Peri2 decreases faster than that of Peri1 (Table 4). When the CTP is heated in the presence of argon, the same behavior can be observed. However, the decrease in concentration after 30 min is less significant, especially in the case of fraction Peri2. It can be deduced that the largest and most condensed peri-condensed compounds show the highest reactivity to oxygen, in accordance with their IP values (Table 4). The HT of the PP at 350 °C under air-blowing gave rise to the results presented in Figure 6. In this case, the most reactive compounds in the presence of oxygen are those included in fractions Cata1 and Cata2. The increase of the content of peri-condensed compounds during the early stages of air-blowing is due to the marked diminution of the content of cata-condensed PAH.1,2 A PP contains a higher concentration of alkyl chains than a CTP that tends to react with oxygen, giving rise to alcoholic groups which subsequently form methylene and ethylene bridges between the polyaromatic units. Then, the formation of intermolecular crosslinks is promoted instead of an increase in the degree of aromatic condensation.10 This structural composition

of pitches can be rationalized in terms of the formation of free radicals in the presence of oxygen, oxiradicals,

(35) Dias, J. R. In Handbook of polycyclic hydrocarbons. Part A.; Elsevier: New York, 1987; pp 101-123. (36) NIST Chemistry Web Book, http://webbook.nist.gov/chemistry/.



Figure 7. Evolution of the individual PAH, grouped as cata- and peri-condensed compounds, in the HT of CTP under air-blowing conditions. Table 5. Concentrations of Some Individual PAH in the Residual Pitch during the HT of CTP under Air-Blowing Conditions T (°C) 300

350

residual time (min)

0

60

120

0

60

benz[a]anthracene chrysene fluoranthene pyrene

1.24 1.24 4.84 3.92

1.25 1.25 4.03 3.11

0.05 0.10 0.08 0.00

0.95 0.89 0.68 0.68

0.14 0.19 0.12 0.00

is a general characteristic of petroleum pitches, in which oligomeric structures predominate over large polycyclic aromatic moieties. HPLC Analysis of Individual Compounds. Some of the samples were analyzed by conventional HPLC (using the Hypersil column, an electron-deficient,

425 90

0

30

60

0.09 0.08 0.12 0.12

1.15 1.13 4.53 3.62

0.40 0.25 0.33 0.00

0.01 0.01 0.01 0.00

nitroaromatic bonded silica column), to obtain a separation of the PAH in terms of number of aromatic rings. This has allowed the evolution of some individual PAH (naphthalene, fluorene, acenaphthene, acenaphthylene, anthracene, phenanthrene, fluoranthene, benz[a]anthracene, chrysene, pyrene, benzo[k]fluoranthene, dibenz-

Figure 8. First-order kinetics plots for the HT of CTP at 300 °C under air-blowing conditions. In the case of QI fraction, ordinates are ln([X]/[X]0).

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Figure 9. First-order kinetics plots for the HT of CTP at 350 °C under air-blowing conditions. In the case of QI fraction, ordinates are ln([X]/[X]0).

Figure 10. First-order kinetics plots for the HT of CTP at 425 °C under air-blowing conditions. In the case of QI fraction, ordinates are ln([X]/[X]0). Table 6. Rate Constants of the Pitch Fractions in the HT of CTP and PP under Different Gas-Blowing Conditions CTP T (°C)

300

air 350

kCata × 103 (min-1) kPeri × 103 (min-1) kTS × 103 (min-1) kQI × 103 (min-1)

12.994 7.666 10.009 1.964

27.721 24.764 25.963 9.411

425

argon 350

PP air 350

37.896 26.343 29.272 17.719

9.419 1.626 4.689 0.360

7.707 5.366 6.157 10.618

Table 7. Activation Energies of the Different Pitch Fractions in the HT of CTP under Air-Blowing Conditions E (kJ mol-1)

Cata

Peri

TS

QI

27.94

31.68

27.69

57.41

[a,h]anthracene, benz[a]pyrene, benzo[b]fluoranthene, benzo[g,h,i]perylene, and indeno[1,2,3-c,d]pyrene) over the residence time of some of the HT experiments to be investigated, and to draw comparisons with the groups of cata- or peri-condensed compounds to which they belong determined using the PL gel columns.

Table 8. Values of SP, TI Content, and some Thermal Parameters (Tmax, Rr, and E) from the TGA/DTG Experimentsa sample CTSP CTP-1 CTP-2 CTP-3 CTP-4 CTP-5 C350A-2 C350A-6 C425A-1 C425A-2 C350Ar-4

SP (°C) TI (wt %) Tmax (°C) Rr (wt %) E (kJ/mol) 35 62 105 125 160 190

6.7 11.7 20.9 25.5 40.7 45.7

365 380 408 449 455 501

24.6 30.4 33.5 42.2 52.7 60.4

39.4 40.3 46.3 55.1 58.4 63.8

173 >280 191 >280 195

50.1 93.4 53.1 92.5 57.5

450 493 473 494 478

60.9 75.4 66.4 77.8 65.7

54.2 56.9 60.2 66.5 58.2

a CTSP to CTP-5: coal tar pitches from a previous study.26 C350A-2 and C350A-6: coal tar pitches prepared from sample CTP at 350 °C under air-blowing during 30 and 120 min, respectively. C425A-1 and C425A-2: coal tar pitches prepared from sample CTP at 425 °C under air-blowing during 15 and 30 min, respectively. C350Ar-4: coal tar pitch prepared from sample CTP at 350 °C under argon-blowing during 60 min.



Figure 11. First-order kinetics plots for the HT of CTP at 350 °C under argon-blowing conditions. In the case of QI fraction, ordinates are ln([X]/[X]0).

Figure 12. First-order kinetics plots for the HT of PP at 350 °C under air-blowing conditions. In the case of QI fraction, ordinates are ln([X]/[X]0).

As an example, the concentrations of some individual PAH in the residual pitch during the HT of CTP under air-blowing conditions are listed in Table 5. Both cata(benz[a]anthracene and chrysene) and peri-condensed compounds (fluoranthene and pyrene) are included. According to the previously described evolution of Cata and Peri fractions under the same conditions (Figures 2-4), the concentrations of these selected PAH continuously decrease as residence time increases. The addition of the concentrations of the different individual PAH grouped in cata-(naphthalene, fluorene, acenaphthene, acenaphthilene, anthracene, phenanthrene, benz[a]anthracene, chrysene, and dibenz[a,h]anthracene) and peri-condensed compounds (fluoranthene, pyrene, benzo[k]fluoranthene, benz[a]pyrene, benzo[b]fluoranthene, benzo[g,h,i]perylene, and indeno[1,2,3-c,d]pyrene) gives rise to the plots in Figure 7. The small differences with the evolution profiles derived from the results obtained with the PLgel columns (Figures 2-4) can be attributed to the compounds not evaluated in the HPLC analysis performed with the Hypersil column (mainly highmolecular-mass compounds).

Kinetic Aspects of the Evolution of cata- and peri-Condensed Compounds. Figures 2-4 show the evolution of the different components for the HT of CTP under air-blowing conditions. As described earlier, the behavior of the β-resins fraction clearly indicates that this is an intermediate product in a consecutive reaction pathway, suggesting the following sequences:

In contrast, for the plots corresponding to the argon-

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Figure 13. Variation of the TGA residue (Rr) with the composition of pitches.

Figure 14. Variation of Tmax with the composition of pitches.

blowing experiment of the CTP (Figure 5) and the airblowing experiment of the PP (Figure 6), the concentration of β-resins continuously increases throughout the whole treatment, with the peri-condensed compounds fraction showing an induction period typical of an intermediate product. This means that in these two cases some cata-condensed compounds react to form peri-condensed ones, at least, during the first part of the isothermal stage (0-30 min interval). However, for purposes of simplicity in the calculation of rate constants, the same reaction scheme previously outlined will be considered. In an attempt to interpret the concentration changes for these fractions, zero-, first-, and second-order kinetics were considered. The corresponding kinetic plots showed that the best fits are obtained when first-order irreversible reactions are considered for all the transformations included in the above kinetic model. Then, equations 5-8 must be accomplished.

ln

( (

) ) ( ) ( )

[Cata]0

ln

[Cata]

[Peri]0

ln ln

[Peri]

[TS]0 [TS]

[QI]0 [QI]

) kCatat

(5)

) kPerit

(6)

) kTSt

(7)

) -kQIt

(8)

where [X] ) concentration of fraction X (Cata, Peri, TS, QI) in the residual pitch at time t, and [X]0 ) concentration of fraction X (Cata, Peri, TS, QI) in the initial pitch (t ) 0). The plots of these logarithmic data versus the residence time are shown in Figures 8-12. They should be



Figure 15. Variation of E with the composition of pitches.

linear with the respective rate constants being calculated from their slopes. However, in the case of the HT of CTP under air-blowing conditions (Figures 8-10), it can be observed that the overall reaction paths are divided in two clearly differentiated time regions. At 60 min, the concentrations of the cata- and peri-condensed compound fractions (and consequently, the TS fraction) reach minimum values which remain almost constant during the rest of the experiment. Similar deviations from linearity were described elsewhere18,19 in the HT treatment of pitches under gas-blowing conditions and were attributed to the presence of compounds of different thermal reactivity. However, if a homogeneous thermal reactivity is assumed for the whole Cata and Peri fractions, the maximum in TI concentration (βresins + QI) reached at 60 min of residence time can be probably responsible for the discontinuity in the kinetics. There could be a major phase change occurring in the system, with the formation of a solid (sol-gel) dispersed phase, which could introduce a sudden mass transfer limitation to further TI formation. This discontinuity is also observed in the HT experiment of CTP under argon-blowing conditions, for the Cata and the TS fractions (Figure 11), but not in the HT of PP under air-blowing conditions (Figure 12). The peri-condensed compound fraction has a different evolution profile, but, as has been already mentioned above, this fraction behaves as an intermediate product, especially in the case of the PP. The values of the rate constants calculated from Figures 8-12 are listed in Table 6 for the following time intervals (in all cases, correlation factors are g0.86): • 0-60 min for the HT of CTP under air-blowing conditions (Figures 8-10); • 0-60 min for the Cata and TS fractions in the HT of CTP under argon-blowing conditions; the whole interval for the QI material, and the 60-180 interval for the Peri fraction (Figure 11); • the whole interval for the HT of PP under airblowing conditions, with the exception of the Peri fraction, for which only the 60-180 min interval is considered (Figure 12).

As far as the CTP experiments are concerned, the four fractions considered show an enhancement of their reactivity with increasing temperature and when the HT is carried out under air-blowing conditions. The most significant increase corresponds to the catacondensed compound class, which also displays the highest reactivity. The PP fractions are less reactive than their CTP counterparts under the same conditions. In the case of HT of CTP under air-blowing conditions, the values of the rate constants at different temperatures (Table 6) allow the calculation of the activation energies for the reaction of the different fractions by means of Arrhenius plots. The lower value obtained for the Cata fraction (Table 7) indicates that, under the same conditions, this fraction is more thermally reactive in the presence of oxygen than the Peri one. Nevertheless, the presence of oxygen promotes a higher reactivity increase for the Peri fraction, although without reaching the levels observed for the cata-condensed compounds. Finally, the activation energy for the thermal reaction of the TS fraction is similar to that of cata-condensed compounds, while the formation of QI material presents the highest value. Relationship between the HPLC and TGA/DTG Data. TGA and DTG are techniques generally used for the characterization of pitches, allowing the determination of the coking value, the temperature intervals in which mass loss occurs, and the temperature at which the mass loss is maximum. It is also possible to obtain the activation energy of the processes occurring in a specific temperature interval providing useful information in relation to the physicochemical phenomena taking place in it (polymerization, dealkylation, condensation, etc.). A previous TGA study26 showed a good correlation existed between the temperature for maximum mass loss rate (Tmax) and the SP of a group of coal tar pitches. A similar relationship was found between the content of TI of a pitch and the corresponding proportion of material in the TGA residue (Rr). Overall, the study confirmed that the SP and the TI content are appropriate parameters for predicting the thermal behavior of pitches. Table 8 lists the values of Tmax, Rr,

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and activation energy (E) for some coal tar pitches previously studied26 and several samples from this study prepared under gas-blowing at different temperatures. In general, these thermal and kinetic parameters increase with the SP and TI content of the pitches as expected. Tmax, Rr, and E data are plotted against the structural composition of pitches, expressed in terms of the peri/(cata + peri) ratio in Figures 13-15 where “peri” denotes the content of peri-condensed compounds of the TS of the pitch and “cata” the corresponding content of cata-condensed PAH. Good correlations between Tmax, Rr, E and the peri/(cata + peri) ratio are evident, confirming the suitability of the methodology of characterization in terms of PAH classes for studying thermal reactions on pitches. At the relatively high heating rate used for the thermal experiment (10 °C/ min), the volatilization of the lighter compounds probably dominates over the polymerization/condensation reactions. Therefore, the values of these thermal and kinetic parameters increase with the content of pericondensed compounds. Since the HPLC compound class distribution describes the chemical composition of pitches, and Rr, Tmax, and E depend on it, the HPLC separation can be considered a good characterization methodology for predicting the thermal behavior of pitches. Conclusions The HPLC methodology developed by the authors has proved a suitable tool for following the polymerization/ condensation reactions involved in the heat treatment of pitches. Although the changes of cata- and pericondensed compounds with the reaction time are not clearly described by simple first-order kinetics, the

Martıń et al.

global process of polymerization and condensation of pitch components under gas-blowing conditions follows the general reaction mechanism in which cata- and pericondensed compounds yield TI material simultaneously. However, the relative reaction rates of these processes are effectively influenced by the experimental conditions. Clear differences were observed between air- and argon-blowing conditions and between the coal tar and petroleum pitches. The thermal reactivity of pericondensed compounds increases in the presence of air in relation to cata-condensed ones, giving rise to TI compounds. The topology of peri-condensed compounds determines the higher reactivity with oxygen, as a result of the more efficient delocalization of π-electrons, making easier the formation of oxiradicals, which are considered as intermediates in the polymerization/ condensation reactions. As well as the presence of air, the reactivity of these compounds increases with HT temperature and their initial concentration. The different thermal reactivity of the petroleum pitches arises from their higher content of alkyl side chains, and this results in the formation of cross-links that accounts for the increase of the TI concentration. Acknowledgment. The authors thank the Commission of the European Communities (DG XVII\D2) for financial support (Contract No. 7220-PR/043). Y.M. acknowledges the Foundation for Scientific Research and Technology (FICYT) of Asturias, Spain, for funding their research at the Instituto Nacional del Carboń. The supply of one of the samples by Aceralia (Asturias, Spain) is gratefully acknowledged. EF9901443

A Study of the Polymerization and Condensation Reactions during the

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