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Thermal behaviour of extrographic fractions of coal tar and petroleum pitches
ELSEVIER
FuelVol. 76, No. 2, pp. 179-187, 1997 Copyright © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/97/$17.00 + 0.00
PII: S0016-2361(96)00173-1
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches Jenaro Bermejo, Marcos Granda, Rosa Men6ndez, Roberto Garcia and Juan M. D. Tascbn Instituto Nacional de/Carb6n, CSIC, Apartado 73, 33080 Oviedo, Spain (Received 8 June 1996) Two coal tar pitches and a petroleum pitch were fractionated by extrography on a preparative scale. The chemical composition of the fractions was determined by elemental analysis, FT-i.r., 1H n.m.r, and solidstate 13C n.m.r, spectroscopies and number-average molecular weight, ~tn, and their thermal behaviour was studied using thermal analysis techniques (t.g., d.t.g, and d.t.a.). The fractions were also carbonized in a horizontal furnace, and the resulting cokes were characterized by optical microscopy. Exo- and endothermic effects shown by the d.t.a, curves of extrographic fractions of the pitches were associated with the physicochemical phenomena expected to occur during the pyrolysis of their respective constituents. It was found that the composition of a pitch and the reactivity of its constituents are reflected in the degree of overlap between the endothermic effect of volatilization and the exothermicity of polymerization. The study of the optical texture of the cokes in relation to the chemical and structural composition of the pyrolysed samples provided new evidence on the relations between composition, reactivity and thermal behaviour of pitches. Copyright © 1997 Elsevier Science Ltd. (Keywords: pitch; thermal analysis; extrography)
Thermal analysis (t.g., d.t.g., d.t.a.) is useful to characterize pitches as it easily allows distinction between pitches of different origin or allocated for different uses 1'2, or even between pitches of the same origin but prepared by different procedures 2'3. However, the interpretation of thermal effects detected by d.t.a, is not yet straightforward, despite a number of studies carried out either with pure aromatic compounds used as model substances 4- 13 or with pitch fractions obtained by extraction with different solvents ~'14. Extrography, in its different modes 15-21, is a simple and suitable technique for obtaining fractions enriched in compounds of different functionality and molecular size. In comparison with the whole pitch, extrographic fractions with a simpler, more uniform and better known composition 19'22'23 should be more appropriate for further studies on the relations between composition and thermal behaviour of pitches, and also for the abovementioned interpretation of thermal effects. In the present work, three commercial pitches differing in origin or preparation method were fractionated by extrography into classes of compounds, and the fractions were characterized by elemental analysis and FT-i.r., 1H n.m.r, and 13C n.m.r, spectroscopies. The thermal behaviour of these fractions was studied by thermal analysis (t.g., d.t.g, and d.t.a.) and by carbonization in a horizontal furnace followed by optical microscopy of the pyrolysis products. The aim was to gain further knowledge on the relation between the chemical composition of pitches and their pyrolysis behaviour. Another objective was to contribute to establishing relations between the
exo- and endothermic effects detected by d.t.a, and the physicochemical phenomena with which they are associated; this could provide invaluable help in characterizing pitches by thermal analysis. EXPERIMENTAL Materials Two coal tar pitches (T and V) and a petroleum pitch (P), all of them commercial, were selected for this study. Their elemental analyses and principal characteristics are given in Table 1. Pitch T was obtained by distillation until 90°C softening point of a coal tar previously heattreated at 380°C for 11 h. As specified by the producer, pitch V was obtained by vacuum distillation of a coal tar. The petroleum pitch was A240 from Ashland. Methods The pitches were fractionated by extrography on preparative and semipreparative scales using respectively 20g and 4g. Pitch samples, <0.20mm in particle size, were solubilized or suspended in dichloromethane, and silica gel (0.063-0.200mm) was added to the solution. The amount of silica gel was 200 g when 20 g of pitch sample was used, or 40 g with a 4 g pitch sample, and the volume of solvent was matched to the amount of sample to be fractionated. For 20g of pitch sample, 200, 850, 600, 700, 700 and 800 mL of solvent was needed to elute fractions I to VI respectively. The percentage distributions of fractions obtained with 4 and 20 g of the same pitch were practically identical, as well as the predominant
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Thermal behaviour of extrographic fractions of coal tar and petroleum pitches: J. Bermejo et al. Table 1 Characteristics of the pitches Elemental analysis (wt% daf) Pitch T V P
C
H
N
S
Oa
Ash (wt%)
93.4 92.5 92.0
4.5 4.4 6.0
0.9 1.1 1.0
0.4 0.7 0.5
0.8 1.3 0.5
0.1 0.2 0.0
H/C atomic ratio
SPb (°C)
QI c (wt%)
0.58 0.57 0.78
90 98 120
7.5 4.1 0.5
a Determined by difference bSofteningpoint (Kraemer-Sarnow) cQuinoline-insolubles functionalities (FT-i.r.) and composition of the volatile part of the fractions as determined by g.c. 23. A seventh fraction (Fr-VII) was obtained from the residual material retained on the silica gel, by Soxhlet extraction using pyridine. FT-i.r. spectra of the fractions were obtained using KBr pellets in a Perkin-Elmer 1750 Fourier transform infrared spectrometer. Molecular weights were determined by v.p.o, in a Knauer apparatus with dichloromethane as solvent, at 30°C, and standardized with benzyl(diphenylglyoxal). V.p.o. results are given based on extrapolation to infinite dilution. 1H n.m.r, spectra were obtained in a Bruker WM250 spectrometer from samples dissolved in CDC13 using tetramethylsilane as standard. Solid-state 13C n.m.r, analyses were carried out in a Bruker MSL 100 (25 MHz) spectrometer using crosspolarization (CP) with and without dipolar dephasing (DD). In DD experiments, a contact time of 5 ms was used with a DD time of 50 #s and an acquisition time of 250 ms. Simultaneous t.g.-d.t.g.-d.t.a, measurements were conducted in a Stanton-Redcroft STA-781 thermoanalyser interfaced with a CETA data acquisition and processing system. Samples of pitches and their fractions, with a mass of ,,~15mg and a particle size of <0.212 mm, were placed on platinum crucibles 5 mm in diameter and 5 mm in height, a-Alumina was used as reference for d.t.a, measurements. Temperature was measured with P t - R h thermocouples located in contact with the bottom of the Pt crucibles. Pyrolysis experiments were carried out under 99.995 oVopure argon at a flow of 15 mL min- 1 using a heating rate of 10 K min -1 over the temperature interval 20-960°C. About 0.4 g of each of the fractions was placed in Pyrex tubes (50 mm length x 5 mm diameter) which were flushed with nitrogen. The sample tubes were held vertically in a graphite pot which was positioned in the centre of a horizontal furnace. The pitch fractions were heated under nitrogen at 4 Kmin -1 to 500°C and kept at this temperature for 1 h. The tubes were removed from the furnace and rapidly cooled under nitrogen. Each heat-treated fraction was mounted in epoxy resin and then polished and examined using a Leitz microscope fitted with crossed polars and one wave-retarder plate to examine their optical texture. A series of colour micrographs were taken for each heated pitch fraction at × 60 magnification.
spectra of the fractions showed that the major classes of compounds present in the different fractions separated by extrography agreed with those previously reported19,22,24: Fr-II, aromatics and neutral heteroaromatics with S and O (MW ~<300)24; Fr-III, aromatics and neutral heteroaromatics with S, O (MW > 300) 24 and neutral nitrogen heteroaromatics; Fr-IV, aza-compounds; Fr-V, more condensed aza-compounds22; Fr-VI and Fr-VII, highly polar compounds. The recovery amounts to ~90 wt% (or 95 wt%, taking into account that QI is irrecoverable) for the coal tar pitches and 100wt% for the petroleum pitch. The distribution among fractions of pitches T and V differs clearly in the amounts of fractions IV and V, in which aza-compounds concentrate according to their molecular weight and aromatic condensation 22. For pitches not submitted to thermal treatment, fraction IV can also contain a small amount of monophenols 19. The petroleum pitch shows a rather different fractionation behaviour, with the contents of fractions II, III and IV totalling >85 wt% of the pitch. The higher H/C ratio and lower heteroatom content of pitch P (Table 1) suggest the occurrence of less polar and aromatic constituents which are more soluble in the extrographic solvents, giving rise to more abundant and heavier fractions II and III than those of coal tar pitches 24. Table 3 shows the results of elemental analysis of the fractions of pitch T, indicating the hydrogen and heteroatom distribution. They are in close agreement with those obtained for the fractions of a similar coal tar pitch 22. Unlike size exclusion chromatography, where the components are separated according to their molecular weight, extrographic separation depends strongly on the polarity of the constituents. Therefore N and O atoms concentrate in fractions eluted by the highly polar solvents. However, polarity is due not only to the functionality but also to the size and aromaticity of the components. Consequently a wide distribution of molecular weight is found in extrographic fractions of pitches 24, with the number-average molecular weight (_~rn) increasing with the number of the fraction. This is the reason why the H content decreases with increasing number of the fraction, whereas the decrease in C
Table 2 Distribution of the fractions obtained by extrography from the pitches (wt% relative to pitch) Pitch
Fr-I
Fr-II
Fr-III
Fr-IV
Fr-V
Fr-VI
Fr-VII
Total
T V P
0.1 0.0 0.1
34.2 35.0 42.7
19.1 17.3 26.9
10.2 18.8 15.7
11.6 6.9 3.4
9.7 8.7 8.1
3.9 4.3 3.1
88.8 91.0 100.0
Table 3 Characteristics of the extrographic fractions of pitch T obtained by fractionation of 20 g of sample Elemental analysis (wt% daf) Fraction
C
H
N
S
O4
H/C atomic ratio
92.0 92.8 90.8 87.2 91.4
5.0 4.6 4.4 4.3 3.1
0.1 1.3 2.5 4.4 2.0
0.5 0.9 0.7 1.4 0.4
2.4 0.3 1.6 2.7 3.1
0.65 0.59 0.58 0.59 0.41
Fractionation by extrography; composition of fractions
Fr-II Fr-lI! Fr-IV Fr-V Fr-VI
The results of the fractionation of the pitches into classes of compounds are shown in Table 2. FT-i.r.
a Determined by difference b Number-average molecular weight
RESULTS AND DISCUSSION
180
Fuel 1997 Volume 76 Number 2
/~fnb 270 420 530 590 -
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches." J. Bermejo et al. Table 4 IH and 13Cn.m.r, data (%) for the extrographicfractions of
pitch T Fraction IH n.m.r. Hat Hal Hat/Hal H~, 2
H,~ H;~ H~ ~3Cn.m.r. Ca[
Car Car/Cal Cqa
Fr-II
Fr-III
Fr-IV
Fr-V
85.2 14.8 5.8 3.7 10.1 1.0 0.0
76.5 23.5 3.2 4.0 15.6 3.8 0.1
79.1 20.9 3.8 1.7 15.0 3.7 0.5
81.9 18.1 4.5 3,8 12.0 1.7 0.6
6.8 93.2 13.7 35.4
3.6 96.4 26.8 -
4.0 96.0 24.0 44.8
5.0 95.0 19.0 54.8
Fr-VI
2.0 98.0 49.0 45.6
a Quaternary carbon
content is due to its replacement by heteroatoms. The result of both decreases is an almost constant H/C ratio in fractions III, IV and V. It is worth noting the coincidence of the highest concentrations of N and S in fraction V, which suggests that S is mainly present in polar components rich in N, O or both 25'26. The results of 1H n.m.r, and solid-state 13C n.m.r, in Table 4 help to provide some insight into the average structure of the constituent compounds of fractions. The analytical data of fraction II point to an average hydrocarbon molecule with four or five rings. Half of the molecules have a methyl substituent, whereas one molecule in four bears a methylene bridge. According to Tables 3 and 4, the average molecule of fraction III should be an eight-ring aromatic structure, half of the molecules having a methylene bridge, a CH3 and a CHzCH3 substituent. Furthermore, one third of the molecules would contain a pyrrolic N atom. The average molecule of Fr-IV would have a structure with nine or ten aromatic rings bearing a methyl group. More than 90% of the molecules would also have a basic N atom; one third of them would have a CH2CH 3 substituent, and a quarter of them would have a methylene bridge. The degree of aromatic condensation of the rings suggests the occurrence of one or two A r - A r ' bonds per molecule. According to the elemental analysis and/f/n data of Fr-V, the average formula of this fraction would be C43H26Nl.8OS0.25. Of the 43 C atoms, 23-24 would be quaternary, whereas among the 2 6 H atoms, 21-22 would be aromatic, three Ha and one H~,2. Therefore the main characteristics of this fraction are the high content of heteroatoms, especially N, and a degree of ring condensation (Cq) higher than those of the other fractions. Finally, the available analytical data for fraction VI indicate that its components are very rich in aromatic carbon; however, their structures are not so highly condensed as could be presumed from their low hydrogen contents.
Therrnogravimetry The t.g. curves of the two coal tar pitches and their extrographic fractions are compared in Figure 1. It is evident that the weight losses of pitch V and its fractions are smaller than those of the respective samples from
pitch T, especially for Fr-II. It could seem surprising a priori that all fractions obtained from a thermally treated pitch undergo larger weight losses during pyrolysis than the corresponding fractions from a pitch prepared by vacuum distillation of a tar. However, there is a plausible explanation for this, based on two facts. First, pitch V has a softening point higher (by 8°C) than that of pitch T. Second, the polymerization promoted by thermal treatment of pitch T induces an increase in its softening point. Hence to keep this characteristic at a similar value for both pitches, an amount of light components greater than that present in the vacuum pitch must be present in the thermally treated one to compensate the effect of the heavier components that are more abundant in the latter pitch. The weight losses of the fractions increase in decreasing order of fraction number except for Fr-VII (obtained by Soxhlet extraction with pyridine), which loses more weight than Fr-VI does. However, an important part of this weight loss takes place at very low temperatures, which suggests incomplete solvent elimination or the presence in this fraction of light and highly polar compounds. Figure 2 shows d.t.g, curves of pitches T and V and their fractions. It clearly shows the weight loss of fraction
I00
~ o 80
~
Fr_.~Vl
I /
N
Fr-IV
\
, 0
,
200
~
~
4.00
F.~_,,
600
800
Temperer ure ('~)
I00 Fr-Vl
~ V FrV -I
80g ~, 6o3
.E -~ 40-
Pitch V
20-
Fr-II
® I
I
200
400
I
6oo Temperature (*C)
I
I
see
looo
Figure 1 T.g. curves of pitches and their extrographic fractions:
a, pitch T; b, pitch V
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181
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches." d. Bermejo et al. VII at low temperature. Weight loss also occurs in the other polar fractions, although to a smaller extent, suggesting that losses could be due partly to distillation of light polar compounds concentrated in these fractions. It can also be seen that the fractions of pitch T are richer in light, volatile compounds than the respective fractions of pitch V. However, the d.t.g, curves of fractions II! and IV, in which heavy neutral aromatics are present22, show higher maximum weight loss rates for pitch T than for pitch V. These differences are in agreement with the lower softening point of T and the polymerization of this pitch during the thermal treatment. The d.t.g, curves indicate that the weight loss up to 400°C is basically due to the
Fr-Vll Fr-Vl
--
Fr- I V ~ Fr- II
distillation of constituents of Fr-II (MW < 300) and of the light components of fractions III, IV and V, whereas the weight losses at higher temperatures are mainly due to constituents of Fr-III (hydrocarbons with MW > 300), with smaller contributions from the heaviest fractions. Table 5 gives the extrographic yields of the various fractions and their coke yields, as well as the percentages of material irreversibly retained on the silica gel, Ur (which includes the QI fraction). If it is considered that the weight loss during pyrolysis of the retained material must be very small, the weighted sum of the coke yields of the fractions, E (EYi. CYi/100), is nearly equal to the coke yield of the pitch. This suggests the absence of significant interactions among pitch components in t.g. pyrolysis, under the experimental conditions used here. This last result is in contrast to previous findings from a study of the pyrolysis of coal tar extracts I, where the weight loss of the toluene-insoluble (TI) fraction (mixture of 13-resin and QI fraction) was much lower than the weight loss of the fl-resin, and similar to that of the primary QI. The reason for this behaviour could lie in the presence of QI and its high concentration in the TI fraction (60 wt%).
Pitch T
Differential thermal analysis Figure 3 shows the d.t.a, curves of pitch T and its
Fr- III
cEln
I-E3
/
I®
o
I
2OO
k~J
]500 pg/min
I
I
400
I
600
800
I000
Temperature (*C) Fr-Vll Fr-Vl
Table 5 Results of extrographic fractionation of pitches T and V and pyrolysis of their fractions
-
Fr-III
Fr-
r~
Pitch
II
V
500 I~g/rnin
I
200
I
I
400 600 Temperature (*C)
I
800
I
1000
Figure 2 D.t.g. curves of pitches and their extrographic fractions: a, pitch T; b, pitch V
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Fuel 1997 Volume 76 Number 2
fractions. The respective curves of fractions from pitch V are not shown, because they are similar to those of pitch T (except for Fr-III, where significant differences arise that will be discussed later). The large weight loss of Fr-II (Figure 2) must be associated with the wide and deep endothermic peak in the d.t.a, curve of this fraction between 200 and 400°C. This is followed by an exothermic peak due to polymerization, the maximum of which extends to 500°C. The large magnitude of this peak of Fr-II is quite surprising in comparison with the equivalent polymerization exothermic peaks of the rest of the fractions, which according to their coke yield undergo more extensive polymerization. This can be explained by taking into account that the composition of Fr-II (Mn < 300)22 suggests extensive vaporization of its components, which takes place previously to (and does not overlap with) polymerization. Thus the thermal
Sample
EY a (wt%)
CY b (wt%)
EY. CY/100
Pitch T Fr-II Fr-III Fr-IV Fr-V Fr-VI Fr-VII Ur c
100.0 34.2 19.1 10.2 11.6 9.7 3.9 11.2
40.0 1.2 30.5 47.9 50.5 73.5 57.0 -
40.0 0.4 5.8 4.9 5.8 7.1 2.3 11.2
Pitch V Fr-II Fr-III Fr-IV Fr-V Fr-VI Fr-VII Ur c
100.0 35.0 17.3 18.8 6.9 8.7 4.3 9.0
45.0 10.9 45.2 54.5 63.2 85.8 63.3 -
45.0 3.8 7.8 10.2 4.4 7.5 2.9 9.0
a Extrographic yield b Coke yield by t.g. (960°C, 10 Kmin -x) c Unrecovered
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches."J. Bermejo et al.
Pitch
T
t3~
Exo
2~V
Endo
0
I
I
I
I
200
400
600
800
I000
Temperoture (*C) Figure 3
D.t.a. curves of pitch T and its extrographic fractions
effect of polymerization is clearly isolated in the d.t.a. curve of Fr-II from the opposite effect of vaporization. For the rest of the fractions, vaporization occurs at higher temperatures, overlapping with polymerization. For instance, in the curve of Fr-III the endothermic effect is detected between 450 and 500°C (notice in Figure 2b that the maximum rate of weight loss coincides with this temperature interval), a temperature range in which a strong polymerization process must take place; therefore the two effects overlap strongly. Nevertheless, it can clearly be observed that the exothermic effect extends to 550°C, which is the highest temperature observed for any fraction. The temperature of this maximum coincides with the hardening temperature 27 and consequently indicates the end of the liquid-phase polymerization. Therefore this temperature could be taken as a rough measure of the carbonization rate. According to this, fraction III should be the least reactive, in complete agreement with the composition of Fr-III, in which aromatic hydrocarbons with M W > 300 are predominant22, exhibiting a high thermal stability as corroborated by the optical texture of the corresponding
coke obtained after pyrolysis at 500°C (Figure 6b), which will be discussed below. In the d.t.a, curves of fractions IV and V the endothermic effect associated with weight loss is almost totally compensated by the exothermic effect of polymerization. The latter process begins at lower temperatures as a consequence of the higher reactivity of the N-rich fractions28-30 (Table 3). Both curves exhibit a sharp endothermic effect near 800°C, which could be associated with the high N content of these fractions, being due probably to the cleavage of C - N bonds 31. The highly polar fraction Fr-VI yields a d.t.a, curve showing a rapid sequence of sharp endo- and exothermic peaks attributable to consecutive cracking and polymerization reactions. This fact reveals that a significant number of components of fraction VI are unstable at temperatures between 480 and 500°C, which is corroborated by an appreciable weight loss (Figure 2a). Finally, the curve of Fr-VII shows as a major feature a broad exothermic peak extending between 250 and 500°C, indicative of a continuous change in the molecular structure and size of its most representative components.
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Thermal behaviour of extrographic fractions of coal tar and petroleum pitches: J. Bermejo et al. On the other hand, all the d.t.a, curves exhibit a final exothermic effect near 900°C, attributable to polymerization and aromatization reactions leading to the large macromolecules typically present in pitch cokes. In previous pitch characterization studies by thermal analysis 3'23, the d.t.a, curves of thermally treated pitches were found to differ clearly from those of pitches obtained by vacuum distillation of tars; the difference lay in the line shape in the 550-850°C interval. The same difference can be observed in the present work by comparing the d.t.a, curves of pitches V and T and their Fr-III (Figure 4): pitch V and its Fr-III give rise to a W-shaped profile, whereas a V-shaped profile is obtained for pitch T and its Fr-III. Therefore the difference between the d.t.a, curves of pitches V and T can be attributed to components of fraction III, mainly aromatic hydrocarbons with -Mn ~ 4 2 0 (Table 3).
Pitch T
Different structures could bring about different polymerization pathways for the formation of the large planar aromatic macromolecules that constitute pitch cokes. Structural differences between the aromatics present in both fractions are expected to exist, since part of those present in pitch T come from the polymerization that occurs in the thermal treatment of the tar. In summary, the thermal effects evidenced by the d.t.a. curves of extrographic fractions of coal tar pitches seem to be closely associated with the physicochemical phenomena expected to take place during pyrolysis of their principal constituents. This conclusion is supported by the results obtained from the pyrolysis of the extrographic fractions of petroleum pitch P, the d.t.a. curves of which are shown in Figure 5. The curves of fractions II and III are similar to those of the same fractions of pitch T, in spite of the differences between the d.t.a, curves of the two pitches. This is because the major components of these fractions in both pitches are aromatic hydrocarbons. It seems that components of fractions IV and V are responsible for the differences in the pyrolysis behaviour of the two pitches, especially those of Fr-IV if the small amount of Fr-V (Table 2) is taken into account. As can be seen in Figure 5, the sequence of endo- and exothermic peaks between 450 and 500°C, characteristic of pitch P, can be attributed mainly to the constituents of its Fr-IV. The d.t.a, curves of Fr-IV and Fr-V reveal the complexity of the pathways
Pitch p
Q
Endo
0
7'00
400
I
@
600
8 0
I000
Temperature (*C)
Fr- III T
E
Fr- V
to
¢-~
Fr-Vl
o
o
zro
~o
~o
8;0
,ooo
T e m p e r a t u r e (*C)
Figure 4
184
D.t.a. curves: a, pitches T and V; b, Fr-[II of pitches T and V
Fuel 1997 Volume 76 Number 2
0
I
I
I
I
200
400
600
800
Temperature (°C)
Figure 5 D.t.a. curvesof pitch P and its extrographicfractions
tO00
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches: J. Bermejo et al.
Figure 6 Opticalmicrographs of cokesfrom extrographic fractions of pitch T: a, Fr-lI; b, Fr-III; c, Fr-IV; d, Fr-V; e, Fr-VI; f, Fr-VII
in the molecular growth of their components. In coal tar pitches, Fr-IV consists mainly of basic nitrogen compounds with a minor amount of heavy aromatic hydrocarbons of low thermal reactivity22. In petroleum pitches, this fraction consists of the same classes of compounds, but having a larger molecular size and a different molecular structure. Aryl-aryl groups bearing alkyl substituents and hydroaromatic and naphthenic structures are frequently present among the heavy components of petroleum pitches 26'32-34. This class of compounds with a thermal stability lower than that of planar aromatics could account for the thermal behaviour shown by the d.t.a, curve of Fr-IV of pitch P.
Finally, the d.t.a, curves of fractions IV and V of pitch P also show an endothermic effect near 850°C, although less intense than that discussed for the corresponding fractions of coal tar pitches. The d.t.a, curves of the fractions are still complex. However, from the sequence and intensity of the thermal effects, especially in the 300600°C interval, together with the chemical composition of the fractions, it is possible to obtain qualitative information about the thermal reactivity of the main constituents of the fractions. This information can be confirmed by the pyrolysis rate and the optical texture of the cokes obtained by carbonization in the horizontal furnace. However, it is necessary to take into account
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Thermal behaviour of extrographic fractions of coal tar and petroleum pitches: J. Bermejo et al. that the results of both kinds of tests can be significantly affected by the different experimental conditions used in the two tests.
Optical texture of cokes Cokes of the pitch fractions were obtained by pyrolysis at 4Kmin -l up to 500°C with a soaking time of 1 h. Under these conditions the parent pitch was completely converted into an anisotropic coke. Figure 6 shows optical micrographs of the pyrolysis products of the extrographic fractions of pitch T. Fraction II produced an anisotropic coke with optical texture of flow domains (Figure 6a). Nevertheless, some small isotropic zones showing mesophase spheres were also found. Conversely, the pyrolysis product of fraction III consisted mainly of large mesophase spheres in an isotropic matrix (Figure 6b), indicating that the carbonization of this fraction needed a higher temperature or longer soaking time to form coke. Fractions IV and V gave highly anisotropic cokes, with optical texture of flow domains and elongated flow domains (Figure 6c and d). Finally, the cokes from the highly polar fractions VI and VII showed textures of small domains (Figure 6e) and small domains with mosaic (Figure 6f), respectively. The molecular structures of fractions IV and V seem to be optimal in favouring the development of large and well-oriented structures upon carbonization. When the reactivity or the molecular weight increased, as supposedly occurs with fractions VI and VII, less-structured cokes were obtained. The low carbonization rate of fraction III is in accord with its d.t.g, and d.t.a, curves, where Fr-III exhibited the highest temperature for the exothermic peak at 550°C, indicating the hardening of the system27. These results present some interesting features. The existence of isotropic zones in the pyrolysis products of fractions II and III, considering that under identical pyrolysis conditions the parent pitch gave an anisotropic coke, demonstrates the existence of interactions among the different kinds of components of pitches during carbonization in a horizontal furnace. This is in contradiction to the absence of interactions deduced from the t.g. tests, showing once again the strong influence of the pyrolysis conditions on the amount and characteristics of the pyrolysis products. The carbonization rate of the fractions with high contents of basic nitrogen (fractions IV and V) is higher than that of fractions rich in aromatic hydrocarbons (fractions II and III), which suggests a higher thermal reactivity. However, fractions IV and V give rise to highly structured cokes, a feature typical of systems with low viscosity during the stage of mesophase formation and development, attributable to high thermal stability of the system in this temperature interval. The higher reactivity of fractions rich in basic nitrogen could be due to the presence of amines (450 of total N in coal tar pitches), which are efficient rr-radical precursors, by this means promoting thermally induced polyme_rization 35. Furthermore, fractions IV and V exhibit Mn values sufficiently higher to indicate the presence therein of a substantial amount of high-molecular-weight compounds able to form mesophase after a short polymerization time24. It is also plausible that the higher carbonization rate of fractions IV and V compared with fractions II and III is due to the probable presence of highly reactive oligo-aryl structures 36. The Cq content of the extro-
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Fuel 1997 Volume 76 Number 2
graphic fractions (Table 4) is compatible with the presence of these structures. Compounds with oligoaryl structures undergo cyclodehydrogenation reactions to form large, planar and thermally more stable aromatic structures that favour the formation of highly anisotropic carbons 36 such as those obtained from fractions IV and V. CONCLUSIONS Extrography can be easily adapted to the preparative scale by appropriately changing the amounts of pitch and eluents. In this way, the field of application of this technique is significantly widened. Thermal effects evidenced by the d.t.a, curves of extrographic fractions of pitches seem to be closely associated with the physicochemical phenomena expected to take place during the pyrolysis of their main constituents. Differences in the profiles of d.t.a. curves of pitches from thermally treated or vacuumdistilled coal tar are attributable to differences in the composition of fraction III, mainly consisting of aromatic hydrocarbons and neutral heteroaromatic compounds with Mn ~ 420. The composition of a pitch and the reactivity of its constituents are reflected in the degree of overlap between the endothermic effect of volatilization and the exothermic one of polymerization. The smaller the degree of overlap, the larger the content of thermally stable, light components. The study of the optical texture of pyrolysis products in relation to the chemical and structural composition of the parent samples provides new evidence on the relations between the composition, reactivity and thermal behaviour of pitches. The lowest reactivity among pitch constituents corresponds to medium-molecular-size neutral aromatics, whereas mixtures of basic nitrogen compounds and other N/O mixed heterocycles with Mn between 500 and 600, such as those present in fractions IV and V, seem to be optimal for obtaining a wellstructured coke under the experimental conditions used. Conversely, an excessive increase in the reactivity of pitch constituents brings about a decrease in coke optical texture. ACKNOWLEDGEMENT Financial support from DGICYT (project PB87-0456) is gratefully acknowledged.
REFERENCES 1 2 3 4 5 6 7 8
Martinez-Alonso, A., Bermejo, J., Granda, M. and Tascbn, J. M. D., Fuel, 1992, 71, 611. Weber, J.-V., Swistek, M., Darif, M., Schneider, M., Fixari, B., Wolszczak, J. and Lauer, J.-C., C.R., Acad. Sci. Paris Ser. II, 1990, 311, 1171. Bermejo, J., Granda, M., Menrndez, R. and Tascbn, J. M. D., Carbon, 1994, 32, 1001. Lewis, I. C. and Edstrom, T., in Proceedings of the 5th Conference on Carbon, Vol. 2. Pergamon Press, New York, 1963, pp. 413-430. Lewis, I. C. and Edstrom, T., J., Org. Chem., 1963, 2,8, 2050. Fitzer, E. and Mueller, K., Ber. dt. Keram. Ges., 1971, 48, 269. Fitzer, E., Mueller, K. and Schaefer, W., in Chemistry and Physics of Carbon, Vol. 7, ed. P. L. Walker, Jr. Dekker, New York, 1971, pp. 237-383. Patrick, J. W., Yearb. Coke Oven Mgrs' Ass., 1976, 201.
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches. J. Bermejo et al. 9 10 11 12 13
14 15 16 17 18 19 20 21
Katime, I., Cesteros, L. C. and Ochoa, J. R., Thermochim. Acta, 1982, 59, 25. Guet, J. M. and Tchoubar, D., Carbon, 1985, 23, 273. Bacon, N. A., Barton, W. A., Lynch, L. J. and Webster, D. S., Carbon, 1987, 25, 669. Rustschev, D. D. and Shopov, G. K., in Proceedings, "Carbon '86', DKG, Baden-Baden, 1986, pp. 103-104. Martinez-Alonso, A., Bennejo, J. and Tasc6n, J. M. D., J. Therm. Anal., 1992, 38, 811. Alula, M., Cagniant, D. and Lauer, J. C., Fuel, 1990, 69, 177. Bliimer, G. P., Kleffner, H. W., Lficke, W. and Zander, M., Fuel, 1980, 59, 600. Moinelo, S. R., Men6ndez, R. and Bermejo, J., Fuel, 1988, 67, 682. Alula, M., Cagniant, D. and Gruber, R., Fuel Process. Technol., 1988, 20, 81. Alula, M., Diack, M., Gruber, R., Kisch, G., Wilhem, J. C. and Cagniant, D., Fuel, 1989, 68, 1330. Granda, M., Bermejo, J., Moinelo, S. R. and Men6ndez, R., Fuel, 1990, 69, 702. (2ern~,,J., Pavlikov~t, H. and Machovi~, V., Fuel, 1990, 69, 966. Beilina, N. Yu., Kozhueva, E. N., Golubkov, O. E., Ostrovskii, V. S. and Shipkov, N. N., Khim. Tverd. Topl., 1990, 24 (5), 132.
22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
Granda, M., Men6ndez, R., Moinelo, S. R., Bermejo, J. and Snape, C. E., Fuel 1993, 72, 19. Granda, M. Ph.D. Thesis, University of Oviedo, 1992. Men6ndez, R., Granda, M., Bermejo, J. and Marsh, H., Fuel, 1994, 73, 25. Zander, M., Fuel, 1991, 70, 563. Kershaw, J. R. and Black, K. J. T., Energy Fuels, 1993, 7, 420. Bermejo, J., Menbndez, R., Figueiras, A. and Granda, M., Fuel, 1995, 74, 1792. (~ern~,,J., Fuel, 1989, 68, 402. Zander, M., Erd6l Kohle, Erdgas, Petrochem., 1989, 42, 344. Zander, M. and Alscher, A., in Extended Abstracts and Programme, 'Carbone'90". GFEC, Paris, 1990, pp. 302-303. Isaacs, L. G., Carbon, 1970, 8, 1. Dickinson, E. M., Fuel, 1980, 59, 286. Zeng, S. M., Maeda, T., Tokumitsu, K., Mondori, J. and Mochida, I., Carbon, 1993, 31, 413. Mochida, I., Fujimoto, K. and Oyama, T., in Chemistry and Physics of Carbon, Vol. 24, ed. P. A. Thrower. Dekker, New York, 1994, pp. 111-212. Zander, M., Fuel 1987, 66, 1459. Zander, M., Erd6l Kohle, Erdgas, Petrochem., 1985, 38, 497.
Fuel 1997 Volume 76 Number 2
187
FuelVol. 76, No. 2, pp. 179-187, 1997 Copyright © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/97/$17.00 + 0.00
PII: S0016-2361(96)00173-1
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches Jenaro Bermejo, Marcos Granda, Rosa Men6ndez, Roberto Garcia and Juan M. D. Tascbn Instituto Nacional de/Carb6n, CSIC, Apartado 73, 33080 Oviedo, Spain (Received 8 June 1996) Two coal tar pitches and a petroleum pitch were fractionated by extrography on a preparative scale. The chemical composition of the fractions was determined by elemental analysis, FT-i.r., 1H n.m.r, and solidstate 13C n.m.r, spectroscopies and number-average molecular weight, ~tn, and their thermal behaviour was studied using thermal analysis techniques (t.g., d.t.g, and d.t.a.). The fractions were also carbonized in a horizontal furnace, and the resulting cokes were characterized by optical microscopy. Exo- and endothermic effects shown by the d.t.a, curves of extrographic fractions of the pitches were associated with the physicochemical phenomena expected to occur during the pyrolysis of their respective constituents. It was found that the composition of a pitch and the reactivity of its constituents are reflected in the degree of overlap between the endothermic effect of volatilization and the exothermicity of polymerization. The study of the optical texture of the cokes in relation to the chemical and structural composition of the pyrolysed samples provided new evidence on the relations between composition, reactivity and thermal behaviour of pitches. Copyright © 1997 Elsevier Science Ltd. (Keywords: pitch; thermal analysis; extrography)
Thermal analysis (t.g., d.t.g., d.t.a.) is useful to characterize pitches as it easily allows distinction between pitches of different origin or allocated for different uses 1'2, or even between pitches of the same origin but prepared by different procedures 2'3. However, the interpretation of thermal effects detected by d.t.a, is not yet straightforward, despite a number of studies carried out either with pure aromatic compounds used as model substances 4- 13 or with pitch fractions obtained by extraction with different solvents ~'14. Extrography, in its different modes 15-21, is a simple and suitable technique for obtaining fractions enriched in compounds of different functionality and molecular size. In comparison with the whole pitch, extrographic fractions with a simpler, more uniform and better known composition 19'22'23 should be more appropriate for further studies on the relations between composition and thermal behaviour of pitches, and also for the abovementioned interpretation of thermal effects. In the present work, three commercial pitches differing in origin or preparation method were fractionated by extrography into classes of compounds, and the fractions were characterized by elemental analysis and FT-i.r., 1H n.m.r, and 13C n.m.r, spectroscopies. The thermal behaviour of these fractions was studied by thermal analysis (t.g., d.t.g, and d.t.a.) and by carbonization in a horizontal furnace followed by optical microscopy of the pyrolysis products. The aim was to gain further knowledge on the relation between the chemical composition of pitches and their pyrolysis behaviour. Another objective was to contribute to establishing relations between the
exo- and endothermic effects detected by d.t.a, and the physicochemical phenomena with which they are associated; this could provide invaluable help in characterizing pitches by thermal analysis. EXPERIMENTAL Materials Two coal tar pitches (T and V) and a petroleum pitch (P), all of them commercial, were selected for this study. Their elemental analyses and principal characteristics are given in Table 1. Pitch T was obtained by distillation until 90°C softening point of a coal tar previously heattreated at 380°C for 11 h. As specified by the producer, pitch V was obtained by vacuum distillation of a coal tar. The petroleum pitch was A240 from Ashland. Methods The pitches were fractionated by extrography on preparative and semipreparative scales using respectively 20g and 4g. Pitch samples, <0.20mm in particle size, were solubilized or suspended in dichloromethane, and silica gel (0.063-0.200mm) was added to the solution. The amount of silica gel was 200 g when 20 g of pitch sample was used, or 40 g with a 4 g pitch sample, and the volume of solvent was matched to the amount of sample to be fractionated. For 20g of pitch sample, 200, 850, 600, 700, 700 and 800 mL of solvent was needed to elute fractions I to VI respectively. The percentage distributions of fractions obtained with 4 and 20 g of the same pitch were practically identical, as well as the predominant
Fuel 1997 Volume 76 Number 2
179
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches: J. Bermejo et al. Table 1 Characteristics of the pitches Elemental analysis (wt% daf) Pitch T V P
C
H
N
S
Oa
Ash (wt%)
93.4 92.5 92.0
4.5 4.4 6.0
0.9 1.1 1.0
0.4 0.7 0.5
0.8 1.3 0.5
0.1 0.2 0.0
H/C atomic ratio
SPb (°C)
QI c (wt%)
0.58 0.57 0.78
90 98 120
7.5 4.1 0.5
a Determined by difference bSofteningpoint (Kraemer-Sarnow) cQuinoline-insolubles functionalities (FT-i.r.) and composition of the volatile part of the fractions as determined by g.c. 23. A seventh fraction (Fr-VII) was obtained from the residual material retained on the silica gel, by Soxhlet extraction using pyridine. FT-i.r. spectra of the fractions were obtained using KBr pellets in a Perkin-Elmer 1750 Fourier transform infrared spectrometer. Molecular weights were determined by v.p.o, in a Knauer apparatus with dichloromethane as solvent, at 30°C, and standardized with benzyl(diphenylglyoxal). V.p.o. results are given based on extrapolation to infinite dilution. 1H n.m.r, spectra were obtained in a Bruker WM250 spectrometer from samples dissolved in CDC13 using tetramethylsilane as standard. Solid-state 13C n.m.r, analyses were carried out in a Bruker MSL 100 (25 MHz) spectrometer using crosspolarization (CP) with and without dipolar dephasing (DD). In DD experiments, a contact time of 5 ms was used with a DD time of 50 #s and an acquisition time of 250 ms. Simultaneous t.g.-d.t.g.-d.t.a, measurements were conducted in a Stanton-Redcroft STA-781 thermoanalyser interfaced with a CETA data acquisition and processing system. Samples of pitches and their fractions, with a mass of ,,~15mg and a particle size of <0.212 mm, were placed on platinum crucibles 5 mm in diameter and 5 mm in height, a-Alumina was used as reference for d.t.a, measurements. Temperature was measured with P t - R h thermocouples located in contact with the bottom of the Pt crucibles. Pyrolysis experiments were carried out under 99.995 oVopure argon at a flow of 15 mL min- 1 using a heating rate of 10 K min -1 over the temperature interval 20-960°C. About 0.4 g of each of the fractions was placed in Pyrex tubes (50 mm length x 5 mm diameter) which were flushed with nitrogen. The sample tubes were held vertically in a graphite pot which was positioned in the centre of a horizontal furnace. The pitch fractions were heated under nitrogen at 4 Kmin -1 to 500°C and kept at this temperature for 1 h. The tubes were removed from the furnace and rapidly cooled under nitrogen. Each heat-treated fraction was mounted in epoxy resin and then polished and examined using a Leitz microscope fitted with crossed polars and one wave-retarder plate to examine their optical texture. A series of colour micrographs were taken for each heated pitch fraction at × 60 magnification.
spectra of the fractions showed that the major classes of compounds present in the different fractions separated by extrography agreed with those previously reported19,22,24: Fr-II, aromatics and neutral heteroaromatics with S and O (MW ~<300)24; Fr-III, aromatics and neutral heteroaromatics with S, O (MW > 300) 24 and neutral nitrogen heteroaromatics; Fr-IV, aza-compounds; Fr-V, more condensed aza-compounds22; Fr-VI and Fr-VII, highly polar compounds. The recovery amounts to ~90 wt% (or 95 wt%, taking into account that QI is irrecoverable) for the coal tar pitches and 100wt% for the petroleum pitch. The distribution among fractions of pitches T and V differs clearly in the amounts of fractions IV and V, in which aza-compounds concentrate according to their molecular weight and aromatic condensation 22. For pitches not submitted to thermal treatment, fraction IV can also contain a small amount of monophenols 19. The petroleum pitch shows a rather different fractionation behaviour, with the contents of fractions II, III and IV totalling >85 wt% of the pitch. The higher H/C ratio and lower heteroatom content of pitch P (Table 1) suggest the occurrence of less polar and aromatic constituents which are more soluble in the extrographic solvents, giving rise to more abundant and heavier fractions II and III than those of coal tar pitches 24. Table 3 shows the results of elemental analysis of the fractions of pitch T, indicating the hydrogen and heteroatom distribution. They are in close agreement with those obtained for the fractions of a similar coal tar pitch 22. Unlike size exclusion chromatography, where the components are separated according to their molecular weight, extrographic separation depends strongly on the polarity of the constituents. Therefore N and O atoms concentrate in fractions eluted by the highly polar solvents. However, polarity is due not only to the functionality but also to the size and aromaticity of the components. Consequently a wide distribution of molecular weight is found in extrographic fractions of pitches 24, with the number-average molecular weight (_~rn) increasing with the number of the fraction. This is the reason why the H content decreases with increasing number of the fraction, whereas the decrease in C
Table 2 Distribution of the fractions obtained by extrography from the pitches (wt% relative to pitch) Pitch
Fr-I
Fr-II
Fr-III
Fr-IV
Fr-V
Fr-VI
Fr-VII
Total
T V P
0.1 0.0 0.1
34.2 35.0 42.7
19.1 17.3 26.9
10.2 18.8 15.7
11.6 6.9 3.4
9.7 8.7 8.1
3.9 4.3 3.1
88.8 91.0 100.0
Table 3 Characteristics of the extrographic fractions of pitch T obtained by fractionation of 20 g of sample Elemental analysis (wt% daf) Fraction
C
H
N
S
O4
H/C atomic ratio
92.0 92.8 90.8 87.2 91.4
5.0 4.6 4.4 4.3 3.1
0.1 1.3 2.5 4.4 2.0
0.5 0.9 0.7 1.4 0.4
2.4 0.3 1.6 2.7 3.1
0.65 0.59 0.58 0.59 0.41
Fractionation by extrography; composition of fractions
Fr-II Fr-lI! Fr-IV Fr-V Fr-VI
The results of the fractionation of the pitches into classes of compounds are shown in Table 2. FT-i.r.
a Determined by difference b Number-average molecular weight
RESULTS AND DISCUSSION
180
Fuel 1997 Volume 76 Number 2
/~fnb 270 420 530 590 -
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches." J. Bermejo et al. Table 4 IH and 13Cn.m.r, data (%) for the extrographicfractions of
pitch T Fraction IH n.m.r. Hat Hal Hat/Hal H~, 2
H,~ H;~ H~ ~3Cn.m.r. Ca[
Car Car/Cal Cqa
Fr-II
Fr-III
Fr-IV
Fr-V
85.2 14.8 5.8 3.7 10.1 1.0 0.0
76.5 23.5 3.2 4.0 15.6 3.8 0.1
79.1 20.9 3.8 1.7 15.0 3.7 0.5
81.9 18.1 4.5 3,8 12.0 1.7 0.6
6.8 93.2 13.7 35.4
3.6 96.4 26.8 -
4.0 96.0 24.0 44.8
5.0 95.0 19.0 54.8
Fr-VI
2.0 98.0 49.0 45.6
a Quaternary carbon
content is due to its replacement by heteroatoms. The result of both decreases is an almost constant H/C ratio in fractions III, IV and V. It is worth noting the coincidence of the highest concentrations of N and S in fraction V, which suggests that S is mainly present in polar components rich in N, O or both 25'26. The results of 1H n.m.r, and solid-state 13C n.m.r, in Table 4 help to provide some insight into the average structure of the constituent compounds of fractions. The analytical data of fraction II point to an average hydrocarbon molecule with four or five rings. Half of the molecules have a methyl substituent, whereas one molecule in four bears a methylene bridge. According to Tables 3 and 4, the average molecule of fraction III should be an eight-ring aromatic structure, half of the molecules having a methylene bridge, a CH3 and a CHzCH3 substituent. Furthermore, one third of the molecules would contain a pyrrolic N atom. The average molecule of Fr-IV would have a structure with nine or ten aromatic rings bearing a methyl group. More than 90% of the molecules would also have a basic N atom; one third of them would have a CH2CH 3 substituent, and a quarter of them would have a methylene bridge. The degree of aromatic condensation of the rings suggests the occurrence of one or two A r - A r ' bonds per molecule. According to the elemental analysis and/f/n data of Fr-V, the average formula of this fraction would be C43H26Nl.8OS0.25. Of the 43 C atoms, 23-24 would be quaternary, whereas among the 2 6 H atoms, 21-22 would be aromatic, three Ha and one H~,2. Therefore the main characteristics of this fraction are the high content of heteroatoms, especially N, and a degree of ring condensation (Cq) higher than those of the other fractions. Finally, the available analytical data for fraction VI indicate that its components are very rich in aromatic carbon; however, their structures are not so highly condensed as could be presumed from their low hydrogen contents.
Therrnogravimetry The t.g. curves of the two coal tar pitches and their extrographic fractions are compared in Figure 1. It is evident that the weight losses of pitch V and its fractions are smaller than those of the respective samples from
pitch T, especially for Fr-II. It could seem surprising a priori that all fractions obtained from a thermally treated pitch undergo larger weight losses during pyrolysis than the corresponding fractions from a pitch prepared by vacuum distillation of a tar. However, there is a plausible explanation for this, based on two facts. First, pitch V has a softening point higher (by 8°C) than that of pitch T. Second, the polymerization promoted by thermal treatment of pitch T induces an increase in its softening point. Hence to keep this characteristic at a similar value for both pitches, an amount of light components greater than that present in the vacuum pitch must be present in the thermally treated one to compensate the effect of the heavier components that are more abundant in the latter pitch. The weight losses of the fractions increase in decreasing order of fraction number except for Fr-VII (obtained by Soxhlet extraction with pyridine), which loses more weight than Fr-VI does. However, an important part of this weight loss takes place at very low temperatures, which suggests incomplete solvent elimination or the presence in this fraction of light and highly polar compounds. Figure 2 shows d.t.g, curves of pitches T and V and their fractions. It clearly shows the weight loss of fraction
I00
~ o 80
~
Fr_.~Vl
I /
N
Fr-IV
\
, 0
,
200
~
~
4.00
F.~_,,
600
800
Temperer ure ('~)
I00 Fr-Vl
~ V FrV -I
80g ~, 6o3
.E -~ 40-
Pitch V
20-
Fr-II
® I
I
200
400
I
6oo Temperature (*C)
I
I
see
looo
Figure 1 T.g. curves of pitches and their extrographic fractions:
a, pitch T; b, pitch V
Fuel 1997 Volume 76 Number 2
181
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches." d. Bermejo et al. VII at low temperature. Weight loss also occurs in the other polar fractions, although to a smaller extent, suggesting that losses could be due partly to distillation of light polar compounds concentrated in these fractions. It can also be seen that the fractions of pitch T are richer in light, volatile compounds than the respective fractions of pitch V. However, the d.t.g, curves of fractions II! and IV, in which heavy neutral aromatics are present22, show higher maximum weight loss rates for pitch T than for pitch V. These differences are in agreement with the lower softening point of T and the polymerization of this pitch during the thermal treatment. The d.t.g, curves indicate that the weight loss up to 400°C is basically due to the
Fr-Vll Fr-Vl
--
Fr- I V ~ Fr- II
distillation of constituents of Fr-II (MW < 300) and of the light components of fractions III, IV and V, whereas the weight losses at higher temperatures are mainly due to constituents of Fr-III (hydrocarbons with MW > 300), with smaller contributions from the heaviest fractions. Table 5 gives the extrographic yields of the various fractions and their coke yields, as well as the percentages of material irreversibly retained on the silica gel, Ur (which includes the QI fraction). If it is considered that the weight loss during pyrolysis of the retained material must be very small, the weighted sum of the coke yields of the fractions, E (EYi. CYi/100), is nearly equal to the coke yield of the pitch. This suggests the absence of significant interactions among pitch components in t.g. pyrolysis, under the experimental conditions used here. This last result is in contrast to previous findings from a study of the pyrolysis of coal tar extracts I, where the weight loss of the toluene-insoluble (TI) fraction (mixture of 13-resin and QI fraction) was much lower than the weight loss of the fl-resin, and similar to that of the primary QI. The reason for this behaviour could lie in the presence of QI and its high concentration in the TI fraction (60 wt%).
Pitch T
Differential thermal analysis Figure 3 shows the d.t.a, curves of pitch T and its
Fr- III
cEln
I-E3
/
I®
o
I
2OO
k~J
]500 pg/min
I
I
400
I
600
800
I000
Temperature (*C) Fr-Vll Fr-Vl
Table 5 Results of extrographic fractionation of pitches T and V and pyrolysis of their fractions
-
Fr-III
Fr-
r~
Pitch
II
V
500 I~g/rnin
I
200
I
I
400 600 Temperature (*C)
I
800
I
1000
Figure 2 D.t.g. curves of pitches and their extrographic fractions: a, pitch T; b, pitch V
182
Fuel 1997 Volume 76 Number 2
fractions. The respective curves of fractions from pitch V are not shown, because they are similar to those of pitch T (except for Fr-III, where significant differences arise that will be discussed later). The large weight loss of Fr-II (Figure 2) must be associated with the wide and deep endothermic peak in the d.t.a, curve of this fraction between 200 and 400°C. This is followed by an exothermic peak due to polymerization, the maximum of which extends to 500°C. The large magnitude of this peak of Fr-II is quite surprising in comparison with the equivalent polymerization exothermic peaks of the rest of the fractions, which according to their coke yield undergo more extensive polymerization. This can be explained by taking into account that the composition of Fr-II (Mn < 300)22 suggests extensive vaporization of its components, which takes place previously to (and does not overlap with) polymerization. Thus the thermal
Sample
EY a (wt%)
CY b (wt%)
EY. CY/100
Pitch T Fr-II Fr-III Fr-IV Fr-V Fr-VI Fr-VII Ur c
100.0 34.2 19.1 10.2 11.6 9.7 3.9 11.2
40.0 1.2 30.5 47.9 50.5 73.5 57.0 -
40.0 0.4 5.8 4.9 5.8 7.1 2.3 11.2
Pitch V Fr-II Fr-III Fr-IV Fr-V Fr-VI Fr-VII Ur c
100.0 35.0 17.3 18.8 6.9 8.7 4.3 9.0
45.0 10.9 45.2 54.5 63.2 85.8 63.3 -
45.0 3.8 7.8 10.2 4.4 7.5 2.9 9.0
a Extrographic yield b Coke yield by t.g. (960°C, 10 Kmin -x) c Unrecovered
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches."J. Bermejo et al.
Pitch
T
t3~
Exo
2~V
Endo
0
I
I
I
I
200
400
600
800
I000
Temperoture (*C) Figure 3
D.t.a. curves of pitch T and its extrographic fractions
effect of polymerization is clearly isolated in the d.t.a. curve of Fr-II from the opposite effect of vaporization. For the rest of the fractions, vaporization occurs at higher temperatures, overlapping with polymerization. For instance, in the curve of Fr-III the endothermic effect is detected between 450 and 500°C (notice in Figure 2b that the maximum rate of weight loss coincides with this temperature interval), a temperature range in which a strong polymerization process must take place; therefore the two effects overlap strongly. Nevertheless, it can clearly be observed that the exothermic effect extends to 550°C, which is the highest temperature observed for any fraction. The temperature of this maximum coincides with the hardening temperature 27 and consequently indicates the end of the liquid-phase polymerization. Therefore this temperature could be taken as a rough measure of the carbonization rate. According to this, fraction III should be the least reactive, in complete agreement with the composition of Fr-III, in which aromatic hydrocarbons with M W > 300 are predominant22, exhibiting a high thermal stability as corroborated by the optical texture of the corresponding
coke obtained after pyrolysis at 500°C (Figure 6b), which will be discussed below. In the d.t.a, curves of fractions IV and V the endothermic effect associated with weight loss is almost totally compensated by the exothermic effect of polymerization. The latter process begins at lower temperatures as a consequence of the higher reactivity of the N-rich fractions28-30 (Table 3). Both curves exhibit a sharp endothermic effect near 800°C, which could be associated with the high N content of these fractions, being due probably to the cleavage of C - N bonds 31. The highly polar fraction Fr-VI yields a d.t.a, curve showing a rapid sequence of sharp endo- and exothermic peaks attributable to consecutive cracking and polymerization reactions. This fact reveals that a significant number of components of fraction VI are unstable at temperatures between 480 and 500°C, which is corroborated by an appreciable weight loss (Figure 2a). Finally, the curve of Fr-VII shows as a major feature a broad exothermic peak extending between 250 and 500°C, indicative of a continuous change in the molecular structure and size of its most representative components.
Fuel 1997 Volume 76 Number 2
183
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches: J. Bermejo et al. On the other hand, all the d.t.a, curves exhibit a final exothermic effect near 900°C, attributable to polymerization and aromatization reactions leading to the large macromolecules typically present in pitch cokes. In previous pitch characterization studies by thermal analysis 3'23, the d.t.a, curves of thermally treated pitches were found to differ clearly from those of pitches obtained by vacuum distillation of tars; the difference lay in the line shape in the 550-850°C interval. The same difference can be observed in the present work by comparing the d.t.a, curves of pitches V and T and their Fr-III (Figure 4): pitch V and its Fr-III give rise to a W-shaped profile, whereas a V-shaped profile is obtained for pitch T and its Fr-III. Therefore the difference between the d.t.a, curves of pitches V and T can be attributed to components of fraction III, mainly aromatic hydrocarbons with -Mn ~ 4 2 0 (Table 3).
Pitch T
Different structures could bring about different polymerization pathways for the formation of the large planar aromatic macromolecules that constitute pitch cokes. Structural differences between the aromatics present in both fractions are expected to exist, since part of those present in pitch T come from the polymerization that occurs in the thermal treatment of the tar. In summary, the thermal effects evidenced by the d.t.a. curves of extrographic fractions of coal tar pitches seem to be closely associated with the physicochemical phenomena expected to take place during pyrolysis of their principal constituents. This conclusion is supported by the results obtained from the pyrolysis of the extrographic fractions of petroleum pitch P, the d.t.a. curves of which are shown in Figure 5. The curves of fractions II and III are similar to those of the same fractions of pitch T, in spite of the differences between the d.t.a, curves of the two pitches. This is because the major components of these fractions in both pitches are aromatic hydrocarbons. It seems that components of fractions IV and V are responsible for the differences in the pyrolysis behaviour of the two pitches, especially those of Fr-IV if the small amount of Fr-V (Table 2) is taken into account. As can be seen in Figure 5, the sequence of endo- and exothermic peaks between 450 and 500°C, characteristic of pitch P, can be attributed mainly to the constituents of its Fr-IV. The d.t.a, curves of Fr-IV and Fr-V reveal the complexity of the pathways
Pitch p
Q
Endo
0
7'00
400
I
@
600
8 0
I000
Temperature (*C)
Fr- III T
E
Fr- V
to
¢-~
Fr-Vl
o
o
zro
~o
~o
8;0
,ooo
T e m p e r a t u r e (*C)
Figure 4
184
D.t.a. curves: a, pitches T and V; b, Fr-[II of pitches T and V
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I
I
I
I
200
400
600
800
Temperature (°C)
Figure 5 D.t.a. curvesof pitch P and its extrographicfractions
tO00
Thermal behaviour of extrographic fractions of coal tar and petroleum pitches: J. Bermejo et al.
Figure 6 Opticalmicrographs of cokesfrom extrographic fractions of pitch T: a, Fr-lI; b, Fr-III; c, Fr-IV; d, Fr-V; e, Fr-VI; f, Fr-VII
in the molecular growth of their components. In coal tar pitches, Fr-IV consists mainly of basic nitrogen compounds with a minor amount of heavy aromatic hydrocarbons of low thermal reactivity22. In petroleum pitches, this fraction consists of the same classes of compounds, but having a larger molecular size and a different molecular structure. Aryl-aryl groups bearing alkyl substituents and hydroaromatic and naphthenic structures are frequently present among the heavy components of petroleum pitches 26'32-34. This class of compounds with a thermal stability lower than that of planar aromatics could account for the thermal behaviour shown by the d.t.a, curve of Fr-IV of pitch P.
Finally, the d.t.a, curves of fractions IV and V of pitch P also show an endothermic effect near 850°C, although less intense than that discussed for the corresponding fractions of coal tar pitches. The d.t.a, curves of the fractions are still complex. However, from the sequence and intensity of the thermal effects, especially in the 300600°C interval, together with the chemical composition of the fractions, it is possible to obtain qualitative information about the thermal reactivity of the main constituents of the fractions. This information can be confirmed by the pyrolysis rate and the optical texture of the cokes obtained by carbonization in the horizontal furnace. However, it is necessary to take into account
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Thermal behaviour of extrographic fractions of coal tar and petroleum pitches: J. Bermejo et al. that the results of both kinds of tests can be significantly affected by the different experimental conditions used in the two tests.
Optical texture of cokes Cokes of the pitch fractions were obtained by pyrolysis at 4Kmin -l up to 500°C with a soaking time of 1 h. Under these conditions the parent pitch was completely converted into an anisotropic coke. Figure 6 shows optical micrographs of the pyrolysis products of the extrographic fractions of pitch T. Fraction II produced an anisotropic coke with optical texture of flow domains (Figure 6a). Nevertheless, some small isotropic zones showing mesophase spheres were also found. Conversely, the pyrolysis product of fraction III consisted mainly of large mesophase spheres in an isotropic matrix (Figure 6b), indicating that the carbonization of this fraction needed a higher temperature or longer soaking time to form coke. Fractions IV and V gave highly anisotropic cokes, with optical texture of flow domains and elongated flow domains (Figure 6c and d). Finally, the cokes from the highly polar fractions VI and VII showed textures of small domains (Figure 6e) and small domains with mosaic (Figure 6f), respectively. The molecular structures of fractions IV and V seem to be optimal in favouring the development of large and well-oriented structures upon carbonization. When the reactivity or the molecular weight increased, as supposedly occurs with fractions VI and VII, less-structured cokes were obtained. The low carbonization rate of fraction III is in accord with its d.t.g, and d.t.a, curves, where Fr-III exhibited the highest temperature for the exothermic peak at 550°C, indicating the hardening of the system27. These results present some interesting features. The existence of isotropic zones in the pyrolysis products of fractions II and III, considering that under identical pyrolysis conditions the parent pitch gave an anisotropic coke, demonstrates the existence of interactions among the different kinds of components of pitches during carbonization in a horizontal furnace. This is in contradiction to the absence of interactions deduced from the t.g. tests, showing once again the strong influence of the pyrolysis conditions on the amount and characteristics of the pyrolysis products. The carbonization rate of the fractions with high contents of basic nitrogen (fractions IV and V) is higher than that of fractions rich in aromatic hydrocarbons (fractions II and III), which suggests a higher thermal reactivity. However, fractions IV and V give rise to highly structured cokes, a feature typical of systems with low viscosity during the stage of mesophase formation and development, attributable to high thermal stability of the system in this temperature interval. The higher reactivity of fractions rich in basic nitrogen could be due to the presence of amines (450 of total N in coal tar pitches), which are efficient rr-radical precursors, by this means promoting thermally induced polyme_rization 35. Furthermore, fractions IV and V exhibit Mn values sufficiently higher to indicate the presence therein of a substantial amount of high-molecular-weight compounds able to form mesophase after a short polymerization time24. It is also plausible that the higher carbonization rate of fractions IV and V compared with fractions II and III is due to the probable presence of highly reactive oligo-aryl structures 36. The Cq content of the extro-
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graphic fractions (Table 4) is compatible with the presence of these structures. Compounds with oligoaryl structures undergo cyclodehydrogenation reactions to form large, planar and thermally more stable aromatic structures that favour the formation of highly anisotropic carbons 36 such as those obtained from fractions IV and V. CONCLUSIONS Extrography can be easily adapted to the preparative scale by appropriately changing the amounts of pitch and eluents. In this way, the field of application of this technique is significantly widened. Thermal effects evidenced by the d.t.a, curves of extrographic fractions of pitches seem to be closely associated with the physicochemical phenomena expected to take place during the pyrolysis of their main constituents. Differences in the profiles of d.t.a. curves of pitches from thermally treated or vacuumdistilled coal tar are attributable to differences in the composition of fraction III, mainly consisting of aromatic hydrocarbons and neutral heteroaromatic compounds with Mn ~ 420. The composition of a pitch and the reactivity of its constituents are reflected in the degree of overlap between the endothermic effect of volatilization and the exothermic one of polymerization. The smaller the degree of overlap, the larger the content of thermally stable, light components. The study of the optical texture of pyrolysis products in relation to the chemical and structural composition of the parent samples provides new evidence on the relations between the composition, reactivity and thermal behaviour of pitches. The lowest reactivity among pitch constituents corresponds to medium-molecular-size neutral aromatics, whereas mixtures of basic nitrogen compounds and other N/O mixed heterocycles with Mn between 500 and 600, such as those present in fractions IV and V, seem to be optimal for obtaining a wellstructured coke under the experimental conditions used. Conversely, an excessive increase in the reactivity of pitch constituents brings about a decrease in coke optical texture. ACKNOWLEDGEMENT Financial support from DGICYT (project PB87-0456) is gratefully acknowledged.
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