Influence of Granular Carbons on the Thermal Reactivity of Pitches

22

Energy & Fuels 2004, 18, 22-29

Influence of Granular Carbons on the Thermal Reactivity of Pitches A. Meńdez, R. Santamarıá, M. Granda,* and R. Meneńdez Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080-Oviedo, Spain Received May 6, 2003. Revised Manuscript Received September 9, 2003

Mixtures of various pitches and granular carbons were pyrolyzed at 430 °C in order to study the influence of different granular carbons on pitch pyrolysis behavior. The toluene-insoluble content was used to monitor the degree of polymerization of the pitches during thermal treatment. The influence of the presence of granular carbons on mesophase development was studied by optical microscopy. Important differences in the pyrolysis behavior of the pitches were observed with the different granular carbons used. The differences were more pronounced for green petroleum coke and graphite. Green petroleum coke accelerated condensation and polymerization reactions. Graphite, however, slowed polymerization of the pitch components, causing mesophase to form and grow on the surface of the graphite particles.

1. Introduction Coal tar pitches are excellent precursors of carbon composites matrixes because of their low cost, their ability to generate graphitizable carbons, and their relatively high carbon yield on pyrolysis.1 However, for certain specific applications, commercial coal tar pitches need to be modified by thermal or air-blowing treatment so that some of their properties can be improved2,3 and their environmental impact can be reduced. The preparation of carbon materials from pitches involves a carbonization process, during which pitches undergo important transformations through polymerization and condensation reactions that lead to the development of mesophase.4-6 Pitch pyrolysis behavior depends on pitch composition, which is related to its origin and the pretreatments (air blowing or the thermal treatment) to which it has been subjected. Consequently, different pitches give rise to carbon matrixes with different microstructure and properties. What is more, variations in the processing conditions may modify the characteristics of the final material for the same pitch precursor.7 Pitch pyrolysis behavior is also greatly influenced by the presence of carbons used as reinforcing material. The influence of insoluble solid additives such as natural graphite,8 mica,9 carbon blacks,10 and silica11 on mesophase development has * Corresponding author: Dr. Marcos Granda, Instituto Nacional del Carboń, CSIC, Apartado 73, 33080-Oviedo, Spain. Tel: (+34) 985 11 90 90. Fax: (+34) 985 29 76 62. E-mail: [email protected]. (1) Savage, G. Carbon-Carbon Composites; Chapman and Hall: London, U.K., 1992; pp 156-191. (2) Blanco, C.; Santamarıá, R.; Bermejo, J.; Meneńdez, R. Carbon 2000, 38, 517-523. (3) Fernańdez, J. J.; Figueiras, A.; Granda, M.; Bermejo, J.; Parra, J. B.; Meneńdez, R. Carbon 1995, 33, 1235-1245. (4) Brooks, J. D.; Taylor, G. H. Chemistry and Physics of Carbon; Marcel Dekker Inc.: New York, 1968; pp 248-286. (5) Lewis, I. C.; Kovac, C. A. Carbon 1978, 16, 425-429. (6) Walker, P. L.; Marsh, H. Chemistry and Physics of Carbon; Marcel Dekker Inc.: New York, 1975; pp 230-285. (7) Granda, M.; Santamarıá, R.; Meneńdez, R. Chemistry and Physics of Carbon; Marcel Dekker Inc.: New York, 2003; pp 263-330.

been investigated, although their effects are not fully understood. In short, a better knowledge of the interactions of the pitch with solid particles is important for developing new carbon materials that can overcome the limitations of traditional C/C composites. Over the years, carbon-fiber-reinforced composites (C/C composites) have shown their outstanding properties in many applications for which they cannot be replaced by any other material. However, these composites have failed in some cases, mainly due to their high cost. The use of expensive raw materials (i.e., carbon fiber prepregs or carbon fiber matrixes) together with very expensive processes (i.e., time- and energyconsuming CVD processes, slow graphitization treatments, large densification steps) make C/C composites high-cost materials unaffordable for most of the conventional applications. The combination of granular carbons as reinforcement and pitches as matrix precursors could overcome these problems and produce materials with good properties at a relatively low cost. For example, such materials could be used as carbon brakes in many conventional applications.12 This work, which is part of a broader study dealing with the process of the preparation of granular reinforced carbon composites for brakes, focuses on the effect of different types of granular carbon on the pyrolysis behavior of pitches of different composition. Four granular carbons (anthracite, foundry coke, graphite, and green petroleum coke) were used so that their influence on the pyrolysis behavior of four pitches (a commercial impregnating pitch, the same pitch but air-blown and (8) Alain, E.; Be´gin, D.; Pajak, J.; Furdin, G.; Mereˆche´, J. F. Fuel 1998, 77, 533-541. (9) Marsh, H.; Forter, J. M.; Herman, G.; Iley, M. Fuel 1973, 53, 234-242. (10) Forrest, M.; Marsh, H. Fuel 1983, 62, 612-615. (11) Obara, T.; Yokono, T.; Sanada, Y.; Marsh, H. Fuel 1985, 64, 995-998. (12) Blanco, C.; Santamarıá, R.; Bermejo, J.; Meneńdez, R. Carbon 2000, 38, 1043-1051.

10.1021/ef030107i CCC: $27.50 © 2004 American Chemical Society Published on Web 11/12/2003

Granular Carbons and the Thermal Reactivity of Pitches

Energy & Fuels, Vol. 18, No. 1, 2004 23

two other pitches from the same parent pitch obtained by thermal treatment) could be studied. The pitch/ granular carbon mixtures were pyrolyzed at 430 °C for 1 or 2 h ,and the pyrolysis behavior of each pitch in the corresponding mixtures was studied from the evolution of the toluen-insoluble content (TI) and the optical texture of the pyrolysis products. 2. Experimental Section 2.1. Raw Materials. Four pitches were used in the study. D0, a commercial impregnating coal-tar pitch supplied by Industrial Quı´mica del Naloń S. A., G18 obtained by air blowing of D0 for 18 h, and D3 and D5, obtained by subjecting D0 to thermal treatment for 3 and 5 h, respectively. The detailed experimental conditions for the preparation of G18, D3 and D5 are given elsewhere.13 The granular carbons used were: green petroleum coke (PC) supplied by REPSOL YPF, foundry coke (FC) supplied by Industrial Quı´mica del Naloń S. A., graphite (GR) supplied by ISMAF and anthracite (AT) from the North of Spain. All the carbons were ground and sieved below 100 µm. 2.2. Characterization of Raw Materials. 2.2.1. Softening Point. The softening point of the pitches (SP) was measured using a Mettler Toledo FP90 following the ASTM D3 104 standard procedure. A small cup with a pierced bottom was filled with approximately 0.5 g of pitch. The cup was placed in the Mettler furnace and preheated to 20 °C below the expected SP. The oven temperature was then increased at a rate of 2 °C min-1 until a drop of pitch flowed through the hole, this representing the softening temperature of the pitch. 2.2.2. Solubility of Pitches. The solubility of the pitch in toluene was determined using the Pechiney B-18 (PT-7/79 of STPTC) standard procedure. Two grams of pitch, sieved to below 0.4 mm particle diameter, and 100 mL of toluene were placed in a 500 mL flask. The solution was heated to the boiling point and maintained under reflux for 30 min. Filtering was performed in a No. 4 porous ceramic plate. The residue was washed with hot toluene and acetone for complete removal of toluene-soluble. 2.2.3. Mesophase Content of Pitches. The mesophase content of the pitches was determined by optical microscopy. The pitch samples were embedded in an epoxy resin, polished, and examined using a microscope fitted with a polarizer and a onewave retarded plate to create interference colors. The microscope was equipped with oil-immersion objectives (20×, 50×, and 100×). The mesophase content of the pitches was determined from the analysis of 500 points, statistically selected using a point-counter coupled to the microscope. 2.2.4. Elemental Analysis. The carbon, hydrogen, nitrogen, and sulfur contents of the pitches and granular carbons were determined using a LECO-CHNS-932 microanalyzer. The oxygen content was determined by means of a LECO-VTF900 furnace coupled to the same microanalyzer. 2.2.5. Ash Content and Moisture. The ash and moisture contents were determined for each carbon in accordance with UNE 32001 and UNE 32002 standards, respectively. 2.2.6. Mineral Matter Composition. Ash analysis was performed by X-ray fluorescence spectrometry (XRF) in a Siemens SRS3000 spectrometer, on fused glass disks. Normative analysis14 was calculated from the data of ash composition and mineral identification, to estimate the concentrations of the major crystalline phases identified by XRD. 2.2.7. Carbon Yield. Four grams of sample (pith < 0.4 mm and granular carbon < 0.1 mm) were placed in a ceramic crucible and introduced into a horizontal tube furnace. The (13) Meńdez, A.; Santamarıá, R.; Meneńdez, R.; Bermejo, J. J. Anal. Appl. Pyrolysis 2001, 58-59, 825-840. (14) Martıńez-Tarazona, M. R.; Martıńez-Alonso, A.; Tascoń, J. M. D. Erdo¨ l, Erogas-Petrochem. 1993, 46, 202-209.

Figure 1. Scheme of the reactor used for the pyrolysis of pitches. Table 1. Main Properties of Pitches D0, D3, D5, and G18 elemental analysis (wt %) pitch

C

H

N

S

O

D0 D3 D5 G18

92.2 93.3 93.7 92.2

4.5 4.0 3.8 4.2

1.1 1.1 1.1 1.2

0.6 0.5 0.4 0.6

1.6 1.1 1.0 1.8

C/Ha SPb Mes.c 1.7 1.9 2.1 1.8

97 169 336 180

0 30 50 0

TId

CYe

21.8 51.6 65.0 46.7

37.6 61.3 60.7 53.7

a Carbon-to-hydrogen atomic ratio. b Softening point (°C). c Mesophase content by optical microscopy (vol %). d Toluene-insoluble content (wt %). e Carbon yield (wt %).

samples were carbonized under nitrogen flow (73 mL min-1) by increasing the furnace temperature to 1000 °C at a rate of 1 °C min-1, and maintaining this temperature for 30 min. The carbon yield (CY) was calculated from the weight of the carbonaceous residue. 2.3. Preparation of Pitch/Granular Carbon Mixtures. The mixing process of the two components (pitch and granular carbon) was carried out at an initial pitch/granular carbon ratio of 30/70 in weight. The mixtures were heated to a temperature about 100 °C higher than the softening point of the pitch in order to ensure a good impregnation of the carbon particles by the pitch. In each case, 50 g of mixture was prepared in a 0.5 L stainless steel reactor and heated in an electrical furnace under continuous stirring in a nitrogen atmosphere. Once the mixing process was completed, the mixture was ground and sieved to below 1 mm. 2.4. Pyrolysis of Pitch/Granular Carbon Mixtures. The thermal treatment of the mixtures was carried out at 430 °C for 1 h (D5 mixtures) or 2 h (D0, D3, and G18 mixtures) under nitrogen in the experimental device shown in Figure 1. The stainless steel autoclave 2.5-L had 6 holes in the base into which the glass tubes containing the different mixtures were placed. The autoclave was heated in an electrical furnace in a nitrogen atmosphere. It was thus possible to perform the pyrolysis of each individual pitch and of the corresponding pitch mixtures with the different granular carbons in the same run, thus ensuring identical processing conditions. 2.5. Characterization of Pyrolyzed Pitch/Granular Carbon Mixtures. The different pyrolyzed mixtures were characterized in accordance with their pyrolysis yield, tolueneinsoluble variation, and the optical texture of the pitch, using the methods described above.

3. Results and Discussion 3.1. Characterization of Raw Materials. Table 1 shows the main properties of the pitches used in this study. These were D0, a commercial impregnating coal tar pitch; G18, obtained by air-blowing of D0 at 275 °C for 18 h; and D3 and D5, which were obtained by the

24

Energy & Fuels, Vol. 18, No. 1, 2004

Me´ ndez et al.

Table 2. Main Characteristics of Granular Carbons: AT, GR, FC, and PC immediate analysis (wt %) granular carbon

moist.a

ashb

C

H

N

S

O

dHec

ACO2d

AN2e

CYf

AT GR PC FC

3.4 0.1 0.1 0.1

11.4 11.5 1.4 5.8

92.5 98.8 87.9 97.8

1.9 0.1 3.0 0.1

1.1

1.1 0.2 5.9 0.5

3.4 0.9 1.4 0.4

1.70 2.35 1.50 2.00

273.6 0.7 173.8 12.2

49.0 1.6 8.6 8.6

90.6 97.3 89.5 98.1

1.8 1.2

a Moisture content. b Ash content. c Real density (g cm-3). d Specific surface area measured by CO adsorption (m2 g-1). e Specific surface 2 area measured by N2 adsorption (m2 g-1). f Carbon yield (wt %).

thermal treatment of the parent pitch, D0, at 430 °C in a nitrogen atmosphere for 3 and 5 h, respectively. The softening point, SP, carbon yield, CY, and tolueneinsoluble content, TI, increased with pitch treatment as a result of the distillation of the lighter compounds and the formation of larger molecules through polymerization and condensation reactions. Differences were observed in the properties of pitches obtained through both treatments, possibly related with their different molecular structures.2,15,16 Thus G18 presents a higher SP than D3 (180 versus 169 °C), but at the same time the TI (46.7 versus 51.6%) and the CY contents (53.7 versus 61.3%) are lower. One of the most significant differences between the four pitches is to be found in their mesophase content. Whereas thermally treated pitches, D3 and D5, contain 30 and 50 vol % of mesophase, respectively, D0 and the air-blown G18 are entirely isotropic. The presence or absence of mesophase might be important when pitches are mixed with granular carbons, as a good mixture homogeneity might be difficult to achieve. The characteristics of the four granular carbons used are shown in Table 2. Some properties might not be very relevant to the study of their interactions with the pitch during pyrolysis, e.g., the moisture present in the anthracite (3.4 wt %), while others, such as ash content, might be determinant. Graphite and anthracite present the highest ash content: 11.5 and 11.4 wt %, respectively. By contrast, green petroleum coke exhibits the lowest ash content, with only 1.4 wt %. Besides total ash content, the chemical nature of this inorganic matter is also of major importance. The mineral matter present in natural graphite is usually very rich in SiO2 (74.1 wt % in this particular case); it is mainly mineral matter (clay in this case), which is not closely connected to the carbon structure, that can be considered a rather inert component. For anthracite the case is similar; its ash composition can be related with minerals such as clay (SiO2: 33.4%; Al2O3: 24.1%) and pyrite (Fe2O3: 12.2%; S: 7.0%). Petroleum coke, however, contains very little ash (1.4 wt %) but, as is well-known, some of the metals it contains are very active as catalysts (V: 800 ppm; Ni: 400 ppm; Fe: 300 ppm; Na 100 ppm) and are well integrated in the organic matrix. Consequently they may play a very active role during the pyrolysis of the pitch, even if the amounts of these metals are relatively low. Another factor that may affect the pyrolysis behavior of the pitches is the BET area of the granular carbons that are in contact with them. GR and FC exhibit a very low surface area, meaning they do not have any porosity. In the case of PC, there is some (15) Barr, J. B.; Lewis, I. C. Carbon 1978, 16, 439-444. (16) Zeng, S. M.; Maeda, T.; Tomitsu, K.; Mondori, J.; Mochida, I. Carbon 1993, 31, 413-419.

Table 3. Variation of Toluene-Insoluble Content with Pitch Pyrolysis pitch

yielda

TIb

p-TIc

∆TId

D0 D3 G18 D5

79.4 92.5 84.0 97.3

21.8 51.6 46.7 65.0

53.4 67.2 61.9 71.3

144.7 30.1 32.6 9.7

a Pyrolysis yield (wt %). b Toluene-insoluble content of pitches (wt %). c Toluene-insoluble content of pyrolyzed pitches (wt %). d Toluene-insoluble content variation during pyrolysis (%).

microporosity, as reflected in the differences of the areas measured by N2 and CO2. AT presents the highest porosity (49 m2 g-1 measured in N2) with a high contribution of micropores (273.6 m2 g-1 measured in CO2). Porosity had little influence during the mixing of the pitch with the carbons, as pitch is a viscous liquid that will not go deep into the pores. The presence of these pores during pyrolysis might be relevant as some of the evolving gases might still be retained inside them. Table 2 also shows important differences in the density of the carbons, ranging from 2.35 g cm-3 for GR to 1.50 g cm-3 for AT. This parameter will influence the surface of the carbon available for contact with the pitch, as the mixtures are prepared on a weight basis. Carbon yield (CY) is also an important parameter to be considered, as it gives some indication of the extent of the modifications undergone by the carbons during the thermal treatment. These modifications may affect the interactions of the carbons with pitch. GR and FC exhibit very high CY as might be expected. On the other hand, AT and PC have a CY close to 90 wt %, indicating that they change during thermal treatment. As a result, they can be expected to interact more intensely with the pitch when they are pyrolyzed together. Finally, the heteroatom content of the carbons should be considered, with PC showing a significant amount of sulfur (5.9 wt %) and anthracite the highest oxygen content (3.4 wt %). 3.2. Pyrolysis of Pitches. Before studying the pyrolysis of the pitch in the presence of the various carbons, the pitches were pyrolyzed separately for comparative purposes. 3.2.1. Pyrolysis Yield. In general, the thermal treatment of pitches results in the devolatilization of lower molecular weight compounds and the formation of larger structures through polymerization and condensation reactions. As a result of these transformations, the average molecular weight of the pitch increases. The pitches were pyrolized at 430 °C for various times (2 h for G18, D3, and D0 and 1 h for D5) in accordance with section 2.4 of the Experimental Section. D5 was treated for only 1 h to avoid total conversion to semicoke as its mesophase content was already very high. The pyrolysis yield of the different pitches is shown in Table 3. Pitch D5 shows the highest yield due to its lower content in light compounds and its lower reactivity, due to the fact



Figure 2. Mesophase development on D0 (a, b, c, and d) and G18 (e and f).

that it had been more severely treated on being obtained, and the fact that it was thermally treated for only 1 h. The D0, D3, and G18 pitches were pyrolyzed for the same time, 2 h, which makes a comparative study of these three pitches possible. D3 shows the highest yield, whereas D0 pitch shows the lowest, as it had not been previously treated and it contains a large amount of volatiles. The pyrolysis yield of G18 is lower than that of D3 since some of the compounds formed during air blowing are unstable and decompose on pyrolysis, resulting in a lower carbonization yield. 3.2.2. Toluene Solubility. The molecular size, shape, polarity, and intermolecular interactions determine the toluene-insoluble content of pitches. Normally, when no heteroatoms have to be considered, this parameter increases with the mean molecular size and can be used as an indicator of the degree of polymerization achieved by the pitch during pyrolysis. For this reason it is worthwhile examining the evolution of the tolueneinsoluble content, TI, during pyrolysis. Table 3 summarizes the toluene-insoluble content of pitches before and after pyrolysis at 430 °C. D0 shows the highest increase, as it was not pretreated. In the case of D3 and D5, the most reactive molecular sites had already polymerized during the preliminary thermal treatment used in their preparation. Consequently, these pitches were less reactive in the second pyrolysis at 430 °C. On

the other hand, G18 shows a toluene-insoluble increase during pyrolysis that is slightly higher than D3. This might be due to the presence of more reactive structures. 3.2.3. Mesophase Evolution. Optical microscopy observation of the pyrolyzed pitches shows the differences in mesophase evolution in relation with the characteristics of the pitches. D0 develops a high number of homogeneous size spheres with QI particles on their surface (Figure 2a). In the case of D3 and D5, which have an initial mesophase content of 30 and 50%, respectively, the isotropic and anisotropic phases show a different behavior. Pyrolysis reactions in the isotropic phase result in the formation of new QI free spheres with a high internal order, as shown in Figure 2b (Brooks and Taylor structures). These new spheres coalesce easily due to the absence of QI particles on their surface (Figure 2c). The coalescence of spheres is difficult to observe in a coal-tar pitch containing QI. However, the initial mesophase present in these pitches increases in size by coalescence with other spheres, QI particles continuing to surround the spheres (Figure 2d). When the air-blown pitch, G18, was pyrolyzed, the mesophase developed was very different from that of the commercial and thermally treated pitches. All the new spheres were surrounded by QI with a disordered internal structure (Figures 2e and 2f). This could be due to the high viscosity of the isotropic and anisotropic

26


Figure 3. Variation of pitches solubility during pyrolysis in the presence of various carbons.

phases that may hinder the molecular rearrangement of the mesophase. 3.3. Pyrolysis of Pitch/Granular Carbon Mixtures. Pitch/granular carbon mixtures were pyrolyzed at 430 °C for 2 h for the D0, D3, and G18 mixtures and 1 h in the case of D5. Before and after thermal treatment, the toluene-insoluble content of the pitches in the mixtures was determined so that the influence of the different granular carbons on the thermal polymerization of the pitches could be studied. In all cases, it was assumed that granular carbons were completely insoluble in toluene. Figure 3 shows the toluene-insoluble content of D0, D3, and G18 in the corresponding mixtures after py-

Me´ ndez et al.

rolysis (stripped region), together with the toluenesoluble (TS-pyr) and volatile release (Vol-pyr). The toluene-insoluble values are split into the content of TI before (TI-in) and after (TI-pyr) pyrolysis. The values obtained for the single pitches are also included in the figure for comparative purposes. The initial tolueneinsoluble content of the pitches increases just because they mix with the different granular carbons, as can be seen in Figure 3, on comparing the values of TI-in of the individual pitches with the values obtained with the various carbons as it was discussed in a previous work.17 A considerable amount of toluene-soluble compounds present in the pitches become insoluble due to their interaction with the granular carbons for no other reason than that of being mixed at a relatively low temperature (100 °C above the softening point of the pitches). The toluene-insoluble content of the pitches increases with pyrolysis due to the polymerization and condensation reactions and the release of volatile components. These two values are greatly influenced by the type of granular carbon in the mixture, and both are shown in Figure 3 for the various pitches. The most noticeable results shown in Figure 3 relate to the variation in the TI content after pyrolysis (TI-pyr) when the pitches are pyrolyzed with the PC. It can be seen from Figure 3 that very little soluble material remains after pyrolysis, in contrast with the results obtained for the pitch on its own or with the other carbons, indicating that the reactivity of the pitches is greatly enhanced by the presence of granular PC. The reason PC has such a catalytic effect is unclear, but it could be due to the presence of very active mineral matter,18 or to the organic phase itself, i.e., the organic phase might contain free radicals in the constituent polycyclic aromatic hydrocarbons (PAH) of the coke which act as initiators of pyrolysis reactions.19 Another general fact that might be inferred from Figure 3 is related with the release of volatiles during pyrolysis. The presence of granular carbons reduces the total amount of volatiles that are released, anthracite being the carbon with the most pronounced effect. A possible explanation may be the greater surface area of anthracite (Table 2) which retains the volatiles within the pores of the granules, and that the volatiles can polymerize and incorporate into the carbon material. By means of optical microscopy of the pyrolyzed mixtures it was possible to study the effect of the granular carbons on mesophase development where important differences were observed, depending on the granular carbon used. Figure 4 shows pyrolyzed mixtures of D3 with foundry coke and anthracite. In the mixture with anthracite (Figure 4a) the initial mesophase present in D3 coalesced during the mixing process. This phenomenon only occurred with anthracite and was attributed to the strong interaction between the QI particles and the anthracite grains that attract the QI from the surface of the mesophase spheres. New mesophase spheres of different sizes formed during the pyrolysis, which developed without any primary QI particles. With foundry coke (Figure 4b), the initial (17) Meńdez, A.; Santamarıá, R.; Granda, M.; Morgan, T.; Herod, A. A.; Kandiyoti, R.; Meneńdez, R. Fuel 2003, 82, 1241-1250. (18) Bernhaver, M.; Braun, M.; Hu¨ttinger, K. Carbon 1994, 32, 1073-1085. (19) Lewis, I. C. Carbon 1982, 20, 519-529.



Figure 4. Optical texture evolution of pitch D3 in the presence of (a) AT; (b) FC.

Figure 5. Optical texture evolution in the presence of PC of pitches: (a) and (b) D0; (c) and (d) D3; (e) and (f) G18.

mesophase present in D3 did not coalesce during the mixing process, as the QI particles remained on the surface of the spheres. During the pyrolysis reaction, the spheres initially present in the pitch increased in size due to the incorporation of molecules from the polymerized isotropic phase. No new mesophase spheres were formed. Still more interesting results are obtained when GR and PC are used. Some representative optical images of pyrolyzed mixtures with green petroleum coke are shown in Figure 5. The optical texture of the matrix is predominantly that of small mosaics or even isotropic.

In the mixtures with D0 (Figure 5a and 5b) and D3 (Figure 5c and 5d), the pitch was totally converted to mesophase, giving rise to a texture of small mosaics that are marked in the corresponding figure. It would even be more accurate to say that the pitch evolved to coke or semi-coke, instead of mesophase, as the pitch became almost insoluble in toluene (Figure 3). This transformation confirms the enhanced reactivity of the pitch due to the presence of the PC particles. Similar results were obtained for G18, but in this case the texture is both small mosaics and isotropic in character (Figure 5e and 5f).

28


Me´ ndez et al.

Figure 6. Optical texture evolution in the presence of GR of pitches: (a) D0; (b) D3; (c) D5; (d) G18.

One of the most interesting results reported previously was the catalytic effect on the pitch pyrolysis of green petroleum coke, which is reflected both in the high rate of TI and the mesophase formation. Two possible explanations were offered: the activity of the metals present as mineral matter or the type of aromatic structure contained in the green petroleum coke. These structures could have played a part in the free radical reactions, promoting the formation of free radicals in the pitch and thereby increasing the reaction rate. The green petroleum coke was carbonized alone at 1000 °C, its organic structure being profoundly modified, while the ash content remained at similar levels. The carbonized petroleum coke was mixed with the D0, D3, and G18 pitches in a pitch/coke proportion of 30/70. The mixtures were pyrolyzed at 430 °C for 2 h and the toluene-insoluble content of the pitches was analyzed after treatment. In these experiments the TI contents obtained were similar to the values obtained for the pitches in the mixtures with anthracite, graphite, or foundry coke and, consequently, they were lower than the values obtained for the green PC. Optical microscopic examination of the pyrolyzed mixtures with the carbonized petroleum coke also revealed a lower mesophase content than in the mixtures with the parent green petroleum coke. These results show that the catalytic effect of green petroleum coke disappears when the latter is carbonized. Whether the deactivation is due only to a modification of the molecular organic structure or to the modification of the metal environment cannot be conclusively derived from these experiments. A completely different behavior is observed in the pyrolysis of the pitches when GR is the granular carbon used. Figure 6 shows some representative optical micrographs of graphite mixtures pyrolyzed at 430 °C. The granular carbon delays the formation of mesophase in D0, D3, and D5 (Figures 6a, 6b, and 6c, respectively) but not in G18 (Figure 6d). After the pyrolysis of these mixtures, D0 remains entirely isotropic and no meso-

phase is observed in the pitch. In D3 and D5 the initial mesophase spheres present in the pitch appear to have coalesced, but in this case some new mesophase has formed. Significantly, the new mesophase does not appear as single mesophase spheres but as drops attached to the surface of the GR particles (Figure 6b and 6c, M). The behavior of G18 is completely different. In this case, the development of mesophase is not hindered by the presence of the carbon, and the mesophase does not grow on the surface of the particles but forms free mesophase spheres (Figure 6d). It is not easy to find an explanation for such different behavior in the various pitches, but some reasons can be found in the bibliography. A number of rheological studies of pitches containing graphite powders have suggested the existence of strong van der Waals interactions between the large pitch aromatic molecules and the graphene layers.20 Other studies on pitch behavior in the presence of graphite particles from the glass transition analysis have revealed that there is interaction between large pitch aromatic molecules and the graphite.13 Yet other studies have shown that while thermally treated pitches are mainly composed of large planar aromatic molecules, in air-blown pitches the aromatic units eventually cross-link. Consequently, if the larger aromatic structures of the pitch interact strongly with the graphite it can be expected that in the case of D3 and D5 the mesophase will appear on the surface of the graphite. To obtain a better understanding of the interaction of graphite with the aromatic structures of pitches, thermogravimetric studies of model polycyclic aromatic hydrocarbons (PAHs) mixed with different granular carbon materials were performed. Coronene was selected, as it represents a relatively large size planar PAH. Figure 7 shows the thermogravimetric curves of coronene and mixtures of coronene with graphite and (20) Nutz, M.; Furding, G.; Medjahdi, G.; Mareˆche´, J. F.; Moreau, M. Carbon 1997, 35, 1023-1029.



Figure 7. TG curves of coronene and mixtures of coroneneGR and coronene-FC.

Pitch/carbon systems have been used to prepare carbon composites. These materials are being tested for use as brake pads, and preliminary results show that the frictional behavior of the composites is mainly controlled by the granular carbons. Low friction coefficients have been obtained from graphite composites that show poor mechanical properties, whereas the anthracite shows very high and unstable values of friction coefficients but good mechanical properties. Intensive research is being carried out in order to determine the frictional performance of these composites and to try to correlate this behavior with the properties of the mixtures studied in this work. The contribution of each component (pitch or carbon) to the properties of the composite is being evaluated so that more complex systems, in which various carbons will be mixed together to enhance performance, can be prepared.

foundry coke (1wt % of coronene). Two different phenomena can be observed in the curves. The temperature at which weight loss starts is lower when carbons are present, independently of the type of carbon. Certain physical effects due to the presence of the particles might be responsible for this. More significant is the variation in the final weight loss of the three samples. Coronene evaporates completely when it is heated alone or mixed with FC and no residue is left. But in the presence of GR, nearly 30% of the coronene remains as carbon residue when it is heated to 600 °C. A feasible explanation for this could be the strong van der Waals interactions between the planar molecules of the coronene and the basal plane of the graphite. The planar molecules of the pitches, present mainly in thermally treated pitches, possibly interact in the same manner. This would explain the growth of mesophase on the graphite surface, where the planar molecules are to be found and the considerable increase in the TI content despite the fact that little or no mesophase has formed. The molecules attached to the surface become insoluble, although they are still isotropic. The behavior of the airblown pitch, G18, is different, as it is not made up of planar molecules but 3D cross-linked structures.

Conclusions The pyrolysis behavior of pitches is influenced considerably by the presence of granular carbons, the effect differing according to the type of carbon and the characteristics of the pitch. The presence of carbons increases the carbon yield of the pitches during pyrolysis, this effect being the even more noticeable when the anthracite, the carbon with highest surface area, is used. Green petroleum coke accelerates the polymerization and condensation reactions of parent, thermally treated, and air-blown pitches significantly during their pyrolysis, as shown by the increase in toluene-insoluble content and the development of mesophase. Graphite has an interesting effect on the pyrolysis of thermally treated pitches. The graphene layers seem to interact strongly with the larger and more planar aromatic structures present in the thermally treated pitches. These interactions between the graphene layers and the large aromatic pitch structures explain why in the thermally treated pitches mesophase forms as small drops on the graphite surface. EF030107I

Influence of Granular Carbons on the Thermal Reactivity of Pitches

Recommend Documents