436
Energy & Fuels 1996, 10, 436-442
Structural Characterization of Coal Tar Pitches Obtained by Heat Treatment under Different Conditions Yolanda Martı´n, Roberto Garcı´a,* Rube´n A. Sole´, and Sabino R. Moinelo Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain Received October 11, 1995. Revised Manuscript Received November 29, 1995X
Several chromatographic procedures were developed by the authors for the structural characterization and fractionation of pitches. Six coal tar pitches obtained by heat treatment under different conditions, together with a typical mesophase petroleum pitch, chosen for comparison, were studied following these procedures. The results gave an insight into the kind and extension of the chemical transformation occurring in pitch structure as a consequence of heat treatment. A good LC fractionation in compound classes (confirmed by FT-IR and HPLC analyses) of the toluene-soluble materials was achieved. The thermal behavior of the pitches was studied by thermogravimetric analysis and differential thermogravimetry, establishing a correlation between the softening point and the temperature of maximum weight loss rate. When toluene-soluble fractions are considered, HPLC and LC results indicate that the increasing heat treatment temperature gives rise to a decrease in the content of unsubstituted planar catacondensed compounds, with a corresponding increase in the peri-condensed compounds concentration and a general increase in the condensation degree of polyaromatic structures. The distillation of the parent tar, followed to obtain the soft coal tar pitch CTSP, promotes an extensive removal of alkyl- and aryl-substituted aromatic compounds which are subsequently present in small quantities in all the studied pitches. Petroleum pitch PP consists of a high proportion of substituted cata-condensed compounds while coal tar pitch CTP-2, with similar SP and QI content, contains peri-condensed compounds and β-resins as its main components.
Introduction The main source of binder pitch used in the manufacture of anodes for aluminum production is coal tar produced as a byproduct in coke ovens. Typical aluminum industry requirements for binder pitch are centered on properties such as softening point (SP), viscosity, quinoline- and toluene-insoluble (QI and TI) and β-resin contents, coking value, etc.1 For example, the current trend is to use pitches with high SP values (≈130 °C), which possess a high coking value and give rise to low matter losses and high electrode yields, with the additional advantage of promoting low emissions of potentially carcinogenic or mutagenic polyaromatic hydrocarbons (PAHs). However, in some cases, pitches with similar values of these empirical properties display different behaviors.2,3 The influence of the chemical composition on the properties of the manufactured anodes is not well understood as yet. Then, there is a need for improved methods to characterize chemical composition which may also play a key role in the performance of the final carbon product.4-7 Particularly, a more detailed knowledge is required to be able Abstract published in Advance ACS Abstracts, January 15, 1996. (1) McCormick, R. L.; Jha, M. C. Energy Fuels 1994, 8, 388-394. (2) Couderc, P.; Hyvernat, P.; Lemarchand, J. L. Fuel 1986, 65, 281287. (3) Twigg, A. N.; Taylor, R.; Marsh, K. M.; Marr, G. Fuel 1987, 66, 28-33. (4) Zander, M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1989, 34 (4), 1218-1222. (5) Zander, M.; Collin, G. Fuel 1993, 72, 1281-1285. (6) Marsh, H.; Latham, C. S. In Petroleum-Derived Carbons; Bacha, J. D., Newman, J. W., White, J. L., Eds.; ACS Symp. Ser., Vol. 303; American Chemical Society: Washington, DC, 1986; Chapter 1, pp 1-28. (7) Kershaw, J. R.; Black, J. T. Energy Fuels 1993, 7, 420-425. X
0887-0624/96/2510-0436$12.00/0
to predict the characteristics and performance of the carbon products. Coal tar pitches are extremely complex mixtures of polynuclear aromatic compounds (PACs), structurally related heterocyclic compounds, and derivatives such as phenols, ketones, amides, and amines.5,8-11 Although the number of pitch components is high, they belong to relatively few classes of compounds in terms of functionality and type of PACs, making the pitch behave as a homogeneous material,5,8 but its thermal behavior changes as a consequence of the different concentration of the several classes of compounds. The existence of pitches with similar empirical properties but displaying different performance indicates the importance of structural characterization. Chromatographic and spectroscopic techniques have been utilized to extensively characterize these coal derivatives.8,12-26 Reaction stud(8) Zander, M. Fuel 1987, 66, 1459-1466. (9) Cerny, J.; Mitera, J.; Vavrecka, P. Fuel 1989, 68, 596-600. (10) Zander, M. Fuel 1991, 70, 563-565. (11) Granda, M.; Mene´ndez, R.; Moinelo, S. R.; Bermejo, J.; Snape, C. E. Fuel 1993, 72, 19-23. (12) Bartle, K. D. Rev. Pure Appl. Chem. 1972, 22, 79-113. (13) Bartle, K. D.; Collin, G.; Stadelhofer, J. W.; Zander, M. J. Chem. Tech. Biotechnol. 1979, 29, 531-551. (14) Greinke, R. A.; O’Connor, L. H. Anal. Chem. 1980, 52, 18771881. (15) Greinke, R. A. Fuel 1984, 63, 1374-1377. (16) Zander, M. Fuel 1987, 66, 1536-1539. (17) Mulligan, M. J.; Thomas, K. M.; Tytko, A. P. Fuel 1987, 66, 1472-1480. (18) Zander, M. Fuel Process. Technol. 1988, 20, 69-80. (19) Moinelo, S. R.; Mene´ndez, R. M.; Bermejo, J. Fuel 1988, 67, 682-687. (20) Snape, C. E.; Kenwright, A. M.; Bermejo, J.; Ferna´ndez, J.; Moinelo, S. R. Fuel 1989, 68, 1605-1608. (21) Herod, A. A.; Stokes, B. J. Fuel Process. Technol. 1990, 24, 4551.
© 1996 American Chemical Society
Structural Characterization of Coal Tar Pitches
ies using model polyaromatic compounds have shown the importance of molecular rearrangement, side chains cleavage, and dehydrogenative polymerization on the carbonization process.27-30 However, due to the compositional complexity there is not a unique analytical technique which allows the complete structural characterization of pitches and results obtained by different approaches must be considered in order to acquire a global picture of the composition and its implications on the behavior of these materials. Solvent separation methods (toluene, quinoline) are considered an important tool in the characterization of pitches and other coal liquids, giving rise to less complex fractions amenable to analysis by techniques unsuitable for the bulk material. The composition of the soluble fractions could be related to pitch behavior, but also the characterization of the insoluble fractions, although less abundant, is important as their content constitutes one of the parameters considered in industrial use. Due to their different nature, the combination of different analytical techniques is needed for the characterization of these fractions. In this study, several pitches were characterized using a set of analytical techniques, which included solvent fractionation and characterization of the whole pitches and their soluble and insoluble fractions by chromatographic and spectroscopic techniques. The main objective of this work is to ascertain the relationship between chemical structure and thermal behavior by determining the changes in composition promoted by heat treatment. Stress is put on the analysis of the toluenesoluble (TS) fractions of the pitches using chromatographic techniques (LC fractionation and HPLC) and Fourier transform infrared spectroscopy (FTIR). Data are reported on the thermal behavior of pitches, studied by thermogravimetric analysis (TGA) and differential thermal gravimetry (DTG). Experimental Section Samples Studied. The operating conditions (pressure, P, and temperature, T) of the distillation used to obtain the pitches and some characteristics of the samples studied, namely the softening point (SP) and the QI and TI contents, are summarized in Table 1. This table also includes the pitch yields expressed as a weight percentage of the parent tar. The elemental analysis data of the pitches are shown in Table 2. The origin of the samples is as follows: (1) a soft pitch, CTSP (40 °C, SP), obtained by coal tar distillation at 350 °C; (2) five coal tar pitches (CTP-1 to CTP-5) with SP ranging from 65 to 195 °C, obtained from the aforementioned soft pitch, by vacuum heat treatment or distillation at different temperatures (Table 1); (3) a petroleum pitch, Ashland 240 (PP), typical mesophase pitch, selected for comparison. (22) Boenigk, W.; Haenel, M. W.; Zander, M. Fuel 1990, 69, 12261232. (23) Alvarez, R.; Dı´ez, M. A.; Garcı´a, R.; Gonza´lez de Andre´s, A. I.; Snape, C. E.; Moinelo, S. R. Energy Fuels 1993, 7, 953-959. (24) Mene´ndez, R.; Granda, M.; Bermejo, J.; Marsh, H. Fuel 1994, 73, 25-34. (25) Cebolla, V. L.; Weber, J. V.; Swistek, M.; Krzton, A.; Wolszczak, J. Fuel 1994, 73, 950-956. (26) Blanco, C. G.; Domı´nguez, A.; Iglesias, M. J.; Guille´n, M. D. Fuel 1994, 73, 510-514. (27) Lewis, I. C. Carbon 1980, 18, 191-196. (28) Lewis, I. C. Carbon 1982, 20, 519-529. (29) Lewis, I. C. J. Chim. Phys. 1984, 81, 751-758. (30) Lewis, I. C.; Singer, L. S. In Polynuclear Aromatic Compounds; Ebert, L. W., Ed.; Adv. Chem. Ser., Vol. 217; American Chemical Society: Washington, DC, 1986; Chapter 16, pp 269-285.
Energy & Fuels, Vol. 10, No. 2, 1996 437 Table 1. Production Conditions and Characteristics of the Samplesa CTSP CTP-1 CTP-2 CTP-3 CTP-4 CTP-5 P (mmHg)b T (°C)b t (h)b SP (°C) K.-Sarnov TI (%) QI (%) β-resin (%) pitch yield (%)
690 350 8.0 35
80 250 4.0 62
80 320 2.0 105
80 345 2.5 125
115 370 2.0 160
125 380 2.0 190
6.7 3.2 3.6 70
11.7 3.5 8.2 n.d.
20.9 4.1 16.8 39
25.5 4.9 20.6 38
40.7 10.1 30.6 35
45.7 14.6 30.6 29
PP
108 2.8 2.4 3.6
a P, pressure of the distillation used to obtain the pitch. T, temperature of the distillation used to obtain the pitch. t, time at distillation temperature. n.d. ) not determined. b In the case of CTSP these conditions refer to the distillation of the parent tar. CTP-1 to CTP-5 were produced by vacuum distillation of CTSP at the conditions shown.
Table 2. Elemental Analysis of Pitches CTSP CTP-1 CTP-2 CTP-3 CTP-4 CTP-5 C (%) H (%) H/C at. ratio
94.61 5.72 0.73
93.87 5.38 0.69
92.93 5.36 0.69
94.63 4.63 0.59
95.05 4.40 0.56
95.25 3.90 0.49
PP 91.11 6.70 0.88
Fractionation by Solvents. The pitch samples were fractionated using toluene and quinoline as solvents, according to the standard methods ASTM D4312 (toluene) and ASTM D2318 (quinoline), respectively. LC Fractionation of the Toluene-Soluble Fractions. The TS fractions of the pitches were further fractionated into compound classes by liquid chromatography (LC) on SiO2. Glass columns with a height of 60 cm and an internal diameter of 1 cm were utilized for this fractionation, using SiO2 (Kieselgel 60, 70-230 mesh, Merck) as stationary phase, previously activated at 220 °C overnight. The sequence of eluents used is as follows: (1) 70 mL of n-hexane; (2) 80 mL of n-hexane/toluene (1/1 v/v); (3) 70 mL of methanol; (4) 50 mL of chloroform, and (5) 100 mL of THF. The fractions recovered in each case were (1) saturates (50 mL); (2) aromatics I (40 mL); (3) aromatics II (60 mL); (4) polars I (70 mL); (5) polars II (20 mL), and (6) high molecular weight compounds (130 mL). In the present study, the fraction of saturated compounds was negligible for all samples, so five fractions were obtained in each case, their nature being assessed by FT-IR and HPLC. HPLC Analysis of the TS. The TS and their subfractions were analyzed by HPLC. The analyses were carried out using a Waters system incorporating two columns (300 mm × 7.5 mm of internal diameter) packed with polystyrene/divinylbenzene copolymer of different nominal pore size (500 and 100 Å, respectively) and connected in series. A UV detector operating at 254 nm was used. The mobile phase was dichloromethane/ methanol (9/1 vol) at a flow rate of 1 mL min-1. The columns were calibrated using a series of reference standard polyaromatic compounds consisting of substituted and unsubstituted cata- and peri-condensed compounds with a number of their heteroatomic analogous; Figure 1 gives the elution volumes for the different types of compounds studied,23 based on previous work by Lafleur and Wornat.31 Four groups can be differentiated: 1. Three groups of cata-condensed compounds: Cata1, mainly composed by heteroaromatics and compounds substituted with alkyl, aryl, and heteroatomic (OH, N, Ar-O-Ar, etc.) groups; Cata2, consisting of alkyl- and aryl-substituted compounds, and Cata3, with unsubstituted and planar compounds; 2. One group of peri-condensed compounds, referred as Peri. The size exclusion effect is also reflected in each group of compounds, with higher molecular weight compounds appearing at longer elution times. (31) Lafleur, A. L.; Wornat, M. J. Anal. Chem. 1988, 60, 1096-1102.
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Figure 2. DTG profiles of pitches.
Figure 1. Elution volumes of HPLC compound classes. TGA/DTG Analysis. Thermogravimetric analysis and differential thermal gravimetry were carried out on the pitches (sample amount ca. 17 mg) using a SETARAM equipment, Model TGA24, with a heating rate of 10 °C/min up to 900 °C in a nitrogen flow rate of 50 mL/min. FT-IR Spectroscopy. FT-IR analyses of the original pitches and their QI and TI fractions were performed by adding a known amount of sample to KBr before preparing the pellet, as previously described.32 In the case of TS and their fractions, a film of sample was created by placing a drop of dichloromethane solution of each sample on a KBr pellet, evaporating the solvent afterwards. All the spectra were corrected for scattering using two baselines (4000-1800 and 1800-450 cm-1).
Results and Discussion Whole Pitches. In general, an increase in the condensation degree of the complex aromatic structures present in the pitches is expected when a higher temperature is applied in the distillation process. The general characteristics of the whole samples studied confirm this trend (see below). As mentioned above, pitches CTP-1 to CTP-5 were obtained by thermal treatment or distillation of pitch CTSP at different temperatures and times. The increasing heat treatment temperature gives rise to higher volatile matter release as shown by the lower pitch yield (Table 1). In accordance with this, the SP increases from 62 °C for pitch CTP-1, obtained at 250 °C, to 190 °C for CTP-5, produced at 380 °C. The QI and TI contents follow the same trend, reaching levels markedly higher than those of the mesophase petroleum pitch PP (Table 1). The elemental analysis data (Table 2) reveal a slight increase of the carbon content together with a more remarkable decrease in the hydrogen content as the heat treatment temperature increases, revealing the dehydrogenation reactions inherent to the aromatic condensation and the cleavage of aliphatic side chains,29 both reported as key free-radical processes involved in the thermal treatment of polynuclear aromatic compounds.27-30 (32) Moinelo, S. R.; Garcı´a, A. Fuel 1987, 66, 1715-1719.
The thermal behavior of coal tar pitches up to 900 °C was studied by TGA. The thermal treatment temperature reached during pitch production significantly affects the weight loss percentage, which drops from ca. 75% for CTSP to ca. 40% for CTP-5. Petroleum pitch PP shows a behavior very similar to that of CTP-3 at temperatures lower than 300 °C; above this temperature, thermal decomposiion develops faster, reaching the same weight loss as CTP-2 (ca. 62%), which displays also a similar softening point (Table 1). Figure 2 shows the corresponding DTG profiles, in which a progressive decrease with temperature of the weight loss interval, with a simultaneous displacement of the temperature for maximum weight loss rate (Tmax) from 363 (CTSP) to 501 °C (CTP-5), can be observed, indicating a decreasing content of the lower molecular weight compounds. CTSP and CTP-1 display a low Tmax, while, in the case of the pitches obtained under more severe conditions, Tmax takes higher values, with a less resolved peak at ca. 520 °C, which is most intense for CTP-4 and CTP-5. The profile displayed by PP resembles that of CTP-3 (Figure 2), but without significant shoulders, although its SP is more similar to that of CTP-2 (Table 1). The variation of the thermal behavior of the different coal tar pitches is represented in Figure 3, by means of the plot of Tmax vs SP. It reveals a good linear correlation for coal tar pitches, suggesting that the thermal behavior under 200 °C, reflected in the SP, is related to the chemicophysical changes occurring in the whole temperature range up to 900 °C, revealed by the TGA and DTG traces (Figure 2). Although petroleum pitch (PP) appears slightly far from the correlation, the discrepancy is not significant taking into account the different nature of this sample, indicated by the insoluble material contents, especially in the case of β-resins (Table 1). In order to assess the influence of the pitch origin, several samples previously studied,33,34 namely nine binder and one impregnating coal tar pitches and three petroleum pitches, are also included in Figure 3. To warrant a correct comparison, the considered Tmax values for these samples are those obtained in TGA experiments performed with the same (33) Martı´nez-Alonso, A.; Bermejo, J.; Granda, M.; Tasco´n, J. M. D. Fuel 1992, 71, 611-617. (34) Bermejo, J.; Granda, M.; Mene´ndez, R.; Tasco´n, J. M. D. Carbon 1994, 32, 1001-1010.
Structural Characterization of Coal Tar Pitches
Figure 3. Variation of Tmax with the SP. In this study: (b) coal tar pitches (CTSP and CTP-1 to CTP-5); (9) petroleum pitch (PP). In refs 32 and 33: (O) binder coal tar pitches; (4) impregnation coal tar pitch; (0) petroleum pitches.
Figure 4. Material released in TGA experiments (expressed as a proportion of the parent tar).
heating rate (10 °C/min), which has a significant influence on the thermal behavior of pitches.34 The 13 samples match reasonably well the correlation found for the coal tar pitches in this study, confirming that the SP is a parameter appropriate for predicting the thermal behavior of pitches. However, clear differences are observed between coal and petroleum-derived pitches: generally speaking, coal tar pitches match better the correlation, with Tmax slightly lower than predicted; in contrast, petroleum pitches show higher Tmax, approaching that of pitch PP. It has been reported33,34 that the thermal behavior of pitches involves both devolatilization (endothermal) and polymerization (exothermal) effects. In the present study, although differential thermal analysis (DTA) curves did not reveal a good separation between both kinds of reactions, two temperature intervals can be roughly considered: up to 430 °C for devolatilization and above 430 °C for polymerization. Taking this temperature as the edge, the proportion of material simply devolatilized and that released during aromatic condensation reactions can be estimated. In Figure 4, these estimations are corrected considering the yields of the pitches (Table 1). The proportion of material in the TGA residue together with that released at the temperature interval of 430-900 °C remains approximately constant for all the coal tar pitches, while the amount of sample released below 430 °C markedly decreases when increasing heat treatment temperature. From this observation it can be inferred that condensation of aromatic structures is not the main effect promoted by an increase in the temperature of the heat treatment of pitch CTSP, followed for the preparation of the rest of the samples (CTP-1 to CTP-5). At least, the proportion of compounds which experience condensation reactions during the TGA experiment remains constant. In
Energy & Fuels, Vol. 10, No. 2, 1996 439
contrast, the loss of volatile material is evident and accounts for the decrease in pitch yield. CTSP was obtained from the parent tar at atmospheric pressure, while the rest of the coal tar pitches were produced from CTSP under vacuum. While the process temperature can be considered similar (with the exception of CTP-1, in which case it is significantly lower), reduced pressure turns out to be decisive in altering the structure of the pitch by favoring the release of volatile matter. The heat treatment temperature has a remarkable influence on the structure and physical properties of pitches obtained from coal tar under vacuum. The greater release of volatile compounds together with the condensation and cleavage of aliphatic chains promoted by distillation at higher temperatures is clearly shown by the analytical techniques applied to the whole pitches. The acquisition of more extensive information on the nature of the chemical structures present in these samples requires the application of solution state analytical techniques to the study of the TS fractions. Toluene Solubles. As TS content is always higher than 50% in these samples, its chemical composition plays a very important role in the behavior of the whole pitches. The suggested chemical alterations of pitch structure during thermal treatment can be more extensively determined by LC fractionation and the analysis of TS fractions by HPLC. The nature of the LC fractions was assessed by FTIR spectroscopy. As an example, FT-IR spectra corresponding to pitch CTP-1 and CTP-5 are shown in Figure 5. Based on the band assignments in these spectra, the general characteristics of the fractions can be outlined as follows: 1. Saturates (Sat): as expected, the proportion of this fraction was negligible in the pitches studied. 2. Aromatics I (Ar-I) was mainly composed of PAH of relatively low molecular weight (MW), with the main features in the FT-IR spectrum being centered at 3050 (aromatic C-H), 1600-1500 (aromatic C-C) and 700900 cm-1 (aromatic substitution). 3. Aromatics II (Ar-II) was composed of aromatic compounds, but also amines as the sharp peak at 3400 cm-1 (arising from NH groups) indicates. 4. Polars I (Pol-I) consisted of polar compounds with OH involved in hydrogen bonds (broad band centered at ca. 3300 cm-1) and NH (shoulder at 3400 cm-1) groups. 5. Polars II (Pol-II) showed the presence of free OH groups (band centered at 3500-3400 cm-1). 6. High molecular weight compounds (HMW): the spectrum (not shown) again displayed bands corresponding to functional groups with O and N. The comparison of the FT-IR spectra of similar LC fractions reveals the following general observations: (i) the bands corresponding to heteroatomic functional groups (N groups in fraction Ar-II, and O groups in ArII, Pol-I and Pol-II) display significantly lower intensities in pitch CTP-5, and (ii) a loss of aliphatic hydrogen (bands in the 3000-2900 cm-1 area) is also observed when moving from CTP-1 to CTP-5. The HPLC profiles of these fractions confirm the findings of FT-IR spectroscopy. Figure 6 shows the chromatograms corresponding to the LC fractions of the TS of pitches CTP-1 and CTP-5. The whole HMW fractions elute in Cata1 region, corresponding to het-
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Figure 6. HPLC profiles of LC fractions of the TS of pitches CTP-1 (a) and CTP-5 (b).
Figure 5. FT-IR spectra of the whole pitches and their LC fractions: (a) CTP-1; (b) CTP-5.
eroatomic compounds (Figure 1). Pol-I and Pol-II are also centered in region Cata1 but include some alkyland aryl-substituted (region Cata2) and planar (Cata3) cata-condensed compounds. Clearly differentiated are fractions Ar-I and Ar-II which elute primarily in the region Cata3 and in the region of peri-condensed compounds (Peri). Furthermore, when similar fractions
of both pitches are compared, it arises that a higher concentration of peri-condensed compounds of higher condensation degree is present in the fractions from pitch CTP-5. The proportions of the LC fractions corresponding to the TS of the six coal tar pitches are shown in Table 3. Sample recoveries were extremely good, greater than 95% in most of the cases. The proportion of sample not recovered but retained in the stationary phase was added to the HMW fraction. The results are corrected for the proportion of the TS fractions and Table 3 also includes the QI and β-resin (quinoline soluble but toluene insoluble, calculated by difference between TI and QI contents). Changes in TS nature principally affect fraction Ar-I. The proportion of this fraction decreases considerably from CTSP to CTP-5, as heat treatment becomes more severe. Increases in the proportions of β-resin and QI fractions are also observed when moving from CTSP to CTP-5. These observations indicate again the existence of a progressive increase in the condensation degree of the polyaromatic units. According to the TGA/DTG results of the whole pitches (Figure 6), the increasing loss of volatile compounds can also account for the decreasing content of Ar-I fraction, mainly constituted by relatively low molecular weight compounds. Polar compounds take part in this phenomenon to a lesser extent, suggesting that Pol-I and Pol-II fractions are composed mainly of heterocyclic compounds rather than heteroatomic-substituted pol-
Structural Characterization of Coal Tar Pitches
Energy & Fuels, Vol. 10, No. 2, 1996 441
Table 3. LC Fraction Distribution of the Pitch TS Fractions toluene solubles
toluene insolubles
Ar-I Ar-II Pol-I Pol-II HMW β-resin QI
CTSP
CTP-1
CTP-2
CTP-3
CTP-4
CTP-5
PP
40.8 25.4 8.6 13.6 4.8 3.6 3.2
34.4 21.7 11.1 15.8 5.3 8.2 3.5
25.2 21.8 5.1 16.5 10.5 16.8 4.1
19.4 23.0 10.8 16.2 5.1 20.6 4.9
12.4 21.5 4.6 13.8 7.0 30.6 10.1
6.6 22.0 5.3 14.6 6.3 30.6 14.6
30.1 32.8 4.2 16.6 13.5 0.4 2.4
Figure 7. HPLC composition of LC fractions of the coal tar pitches.
yaromatics (i.e., phenols, amines). The functional groups of the latter would be more susceptible to reaction, giving rise to the release of the heteroatoms in the form of volatile compounds (H2O, NH3) and resulting in nonpolar compounds which would elute in the aromatic fractions during the LC fractionation, with a subsequent decrease in the content of polar fractions. The LC fractions Ar-II, Pol-I, and Pol-II, display contents that remain almost constant during the thermal treatment (Table 3). However, the nature of some of these fractions experiences a progressive alteration. Figure 7 shows the proportions of the HPLC fractions (Cata1, Cata2, Cata3, and Peri) of the LC fractions. The aromatic fractions Ar-I and Ar-II display a progressive increase in the proportion of peri-condensed compounds
with a simultaneous decrease in the content of compounds eluting in region Cata3. In the case of fractions Pol-I and Pol-II, the main feature promoted by the increasing heat treatment temperature consists of a slight decrease in the proportion of heteroatomic compounds included in region Cata1. Finally, the nature of HMW fraction remains almost unchanged, with heteroatomic compounds eluting in region Cata1 being the main constituents. The HPLC procedure was also applied to the whole TS fractions and the proportions of the different classes of compounds are shown in Table 4. Again, the proportions of β-resins and QI are included. Although the contents of Cata1 and Cata2 fractions decrease slightly, the main feature promoted by the increasing heat treatment temperature is the decrease of the proportion of Cata3 fraction, constituted by planar cata-condensed compounds, complemented by the increasing content of β-resins and QI fractions. The insignificant changes in the contents of fractions Cata1 and Cata2 suggest that alkyl- and aryl-substituted compounds, normally eluting in these fractions in the HPLC analysis (Figure 1) and presumably the more susceptible to thermal cleavage to provide a route for subsequent polymerization,29 are extensively removed during the heat treatment of the parent tar at 350 °C (Table 1). As a consequence, these kinds of compounds are expected to be present in small quantities in the coal tar pitches studied. Thus, the main structural difference displayed by the TS fractions of the coal tar pitches is the increasing condensation degree and the decreasing content of volatile polyaromatic compounds from CTSP to CTP-5, principally reflected in Cata3, β-resin, and QI contents. In this sense, the HPLC elution profiles of both the TS and their LC fractions (Figures 6 and 8) reveal that the condensation degree of the peri-condensed compounds (elution volume >20.4 mL) increases from CTP-1 to CTP-5. Consequently, polymerization and condensation seem to occur at all levels, affecting both aromatic and polar compounds but being quantitatively reflected only in the proportions of Ar-I, β-resins and QI. Comparison between Petroleum (PP) and Coal Tar (CTP) Pitches. Two of the key parameters considered by pitch users in specific applications are SP and QI content. However, sometimes pitches with similar values of these parameters do not perform with the same efficiency. Structural characteristics are believed to be responsible of these differences. This is the case of mesophase pitch PP and heat treated coal tar pitch CTP-2. Both materials show similar values of SP and QI content (Table 1), but the liquid chromatography results indicate significant structural differences (Tables 3 and 4). Pitch PP is constituted by a higher proportion of catacondensed compounds, mainly substituted with hydro-
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Table 4. HPLC Compound Class Distribution of the Pitch TS Fractions toluene solubles
toluene insolubles
Cata1 Cata2 Cata3 Peri β-resin QI
CTSP
CTP-1
CTP-2
CTP-3
CTP-4
CTP-5
PP
12.1 16.9 32.5 31.7 3.6 3.2
15.4 16.1 23.6 33.2 8.2 3.5
16.6 14.9 11.8 35.8 16.8 4.1
13.8 13.9 10.6 36.2 20.6 4.9
10.1 10.2 7.4 31.6 30.6 10.1
8.2 9.2 6.0 31.4 30.6 14.6
17.4 31.6 17.3 30.9 0.4 2.4
advisable in order to assess more exactly the influence of chemical structure on pitch behavior, and this will constitute one of the main points of future work. Also, the suitability of the chromatographic procedures developed in this work will allow to study the kinetics of pitch polymerization following the evolution of the different classes of PAHs. Conclusions
Figure 8. HPLC profiles of pitch TS: peri-condensed region.
carbon groups, presumably alkyl groups (Cata2). In contrast, CTP-2 consists of peri-condensed compounds as major components, but the principal difference with PP rests on the insoluble materials, especially β-resins, whose content is much higher in CTP-2 (Table 4). LC chromatography reveals that the TS fraction of pitch PP as compared to CTP-2 shows a higher content of the aromatic nonpolar fractions (Table 3). The alkylsubstituted polyaromatic compounds present in PP in higher proportion experiences side chain cleavage at the early stages of carbonization35 giving rise to radicals which participate in polymerization or condensation reactions. The resultant highly condensed compounds, being very stable and less reactive, are responsible of the higher Tmax observed in DTG (Figure 2), thus compensating the contribution of insoluble polyaromatic compounds (β-resins and QI) which are more resistant to thermal decomposition and more abundant in CTP2. The content of β-resins in CTP-3 (Table 1) is high enough to cause a Tmax similar to PP. In CTP-4 and CTP-5 the contents of both β-resins and QI material were considerably increased by heat treatment (Table 1) and shift the Tmax to significantly higher temperatures (Figure 2). The differences observed between the kind of compounds present in these two pitches are significant enough to account for their different behavior when used in the several applications of pitch materials. The characterization of a more complete set of samples is (35) Solomon, P. R.; Hamblen, D. G. In Chemistry of Coal Conversion; Schlosberg, R. H., Ed.; Plenum Press: New York, 1985; Chapter 5, pp 121-251.
The analytical system developed has proven to be effective, especially for the study of the structural evolution of pitches under heat treatment, allowing the discrimination between different classes of compounds (aromatic and polar, cata- and peri-condensed). The increasing heat treatment temperature gives rise to higher volatile matter releases and lower pitch yields. The SP and the QI and β-resin contents also increase, reaching levels markedly higher than those of the mesophase pitch, according to the well-reported existence of a progressive polymerization or condensation in the structure of the pitch, also shown by the decreasing H/C atomic ratio. However, TGA/DTG results indicate that the loss of volatile matter, increasing with the increasing temperature of the heat treatment of the pitch, has a more important influence. Alkyl- and arylsubstituted compounds are extensively removed during the distillation of the parent tar, being present in small proportions in the coal tar pitches studied. The results obtained in this study suggest the suitability of the newly-developed analytical system for the investigation of the kinetics of polyaromatic hydrocarbons polymerization during the heat treatment of pitches. This work is currently being carried out by the authors. The mesophase pitch (PP) and one of the heat-treated coal tar pitches (CTP-2) show similar SP and QI contents, but chromatographic results indicate significant structural differences, namely higher contents of nonpolar cata-condensed polyaromatic compounds present in PP. Acknowledgment. The authors thank the Commission of the European Communities (DG XII-G) for financial support (research project No. CI 1*-CT92-0028) and USIMINAS (Ipatinga, Brazil) and UNICAMP (Campinas, Brazil) for sample supply. Y.M. and R.G. acknowledge the Asturian Foundation for Scientific and Technological Research (FICYT) and the Spanish Ministry of Education and Science (MEC), respectively, for funding their research at the Instituto Nacional del Carbo´n. EF950208J
