Effect of graphite or FeCl3-graphite intercalation compounds on the mesophase development in coal tar pitch

Carbon Vol. 34, No. 5, pp. 6W626,1996 Copyright 0 1996 ElsevierScienceLtd Printedin Great Britain.All rightsreserved 000%6223/96 $15.00 + 0.00

Pergamon SOOOS-6223(96)00002-4

EFFECT OF GRAPHITE OR FeCl,-GRAPHITE INTERCALATION COMPOUNDS ON THE MESOPHASE DEVELOPMENT IN COAL TAR PITCH E. ALAIN,

Laboratoire

D.

de Chimie Mintrale

BEGIN,

G.

FURDIN

and

J. F. MARECHE

AppliquCe, URA CNRS 158, Facultt Vandoeuvre les Nancy, France

des Sciences, BP 239, 54506

(Received 14 September 1995; accepted in revised form 17 November 1995)

Abstract-The influence of the addition of first stage FeCl,-graphite intercalated compounds (GIC) or pristine graphite to coal tar pitch (CTP) upon the formation and the development of mesophase has been studied at 400, 430 and 450°C. Differences in the final samples were principally observed and studied via polarized light microscopic examinations of partially carbonized portions of the green-cokes. The insolubility was also investigated. The addition of pristine graphite to CTP inhibits the development of the mesophase: its presence generates a physical barrier which hinders the mobility and the coalescence of mesophase spherules. In the case of the addition of GIC to the CTP, the Lewis acid FeC13 desorbed out of the graphene layers plays the role of a catalyst during the early stages of the mixture carbonization, by enhancing the rate of formation of the mesophase. Copyright 0 1996 Elsevier Science Ltd Key Words-Coal

1.

tar pitch, graphite intercalation compounds, mesophase, microscopy, insolubility.

insoluble particles (QI), graphite, mica and carbon blacks. The results are often confusing and in some cases, contradictory. Kinetic studies have shown that carbon blacks initially mixed with CTP reduced the activation energy of the mesophase formation [ 171. Brooks and Taylor [12] have observed by light microscopy the close association between mesophase spherules and QI, that has been also confirmed by other authors. Tillmans et al. [lS] have reported that the QI particles in pitch accelerate the mesophase formation as a result of the presence of abundant nuclei. These observations disagree with those of Romovacek et al. [19] who found that the QI particles retard the formation of mesophase which appears at higher temperature than without QI. Stadelhofer [20] found that the presence of up to 10 wt% QI had no accelerating effect on the rate of mesophase formation during the early stages of pitch carbonization; they explained this behaviour by dehydrogenating polymerizations which did not involve solid particles. Studies of mesophase growth in the presence of QI have largely concentrated on the way in which QI particles coat the surfaces of mesophase spherules, and affect the final material structure. Marsh et al. [21] have observed that QI were adsorbed on mesophase surfaces: QI not only inhibited the mesophase coalescence but also reduced its flow characteristics. Forrest and Marsh [22], using light microscopy, noted that the addition of 1 wt% of carbon blacks into CTP retarded the growth and the coalescence of the mesophase and promoted clustering of these units because of adhesion of carbon black particles. In their recent work, Taylor et al. [23] have found that QI particles did not nucleate the mesophase and that they cannot be considered as substrates on which the

INTRODUCTION

In a recent study [ 11, we prepared a mixture of coal tar pitch (CTP) and small particles of FeCl,-graphite intercalation compounds (GIC). These mixtures were carbonized in order to obtain new carbonaceous materials with specific adsorbent properties. The thermal stability of GIC [2-41 allowed the release of the Lewis acid FeCl, at high temperature; it provides a dehydrogenating activity during the mesophasic transformation, and also induces a significant gasifying effect [5,6] which is responsible for the development of the porosity. The final green-coke was “charged” with iron resulting from the desorption of the intercalated species [ 7,8]. The presence of iron should ensure specific adsorbent properties to the carbonaceous materials obtained after activation of the resulting green-cokes [9]. For a long time, it has been known that during pitch carbonization, the most important change of structure from isotropic pitch to coke (or green-coke) occurs with the growth of mesophase at the expense of isotropic pitch: the transformation is temperatureand time-dependent [ lO,ll]. The works of Brooks and Taylor [ 121 on the characterization of mesophase spherules formed in the 400-500°C range from a graphitizable pitch, have led to a large number of studies on the chemical structures, transformations and properties of the pitches carried out in and beyond the range of mesophase formation. Many authors have examined the influence of solid additives on the nucleation and growth of the mesophase spherules, and also the determination of whether such additives can be used to alter the properties of the resulting pyrolyzed materials [ 12- 161. They have examined the effects of quinoline 619

620

E.

ALAIN et 01.

mesophase layers can be aligned. However, Brooks and Taylor [11] first reported that the mesophase formed from coal tar pitch wets graphite and mica flakes and that the mesophase layers tend to align parallel to the layer planes of these substrate materials. From these results, it appears that the nature of the subtrate is an important factor for the formation and the growth of the mesophase during the carbonization and has a significant effect on the resulting green-coke. Some authors have oriented their research on the way of catalysing the formation of the mesophase [24-271. In the present work, we study the role of the presence of pristine graphite and FeCl,-GIC in CTP during its carbonization (at 400,430 and 450’C) to obtain more information on the mesophase formation. This comparative study should allow a better understanding of the new results obtained after the pyrolysis of such mixtures [7]. In the first part, we follow the formation of mesophase with an optical microscope and determine how the mesophase domains interact with the different substrates or in the absence of a substrate. The second part is related to the measure of the insolubility of residual materials obtained at different steps of the carbonization in order to correlate this with the microscopy observations.

2. EXPERIMENTAL 2.1 Materials The elemental composition (wt%) of the coal tar pitch (from HGD, Atochem, France) used in this study is given in Table 1. The polycrystalline Madagascar natural graphite used here, is a dry milled graphite (called UF4) and the particle sizes are between 2 and 10 LLrn. The first stage FeCl,-GIC was prepared according to the well-known two temperature method [1,28], with freshly twice-sublimed FeCl, in a chlorine atmosphere. CTP-GIC or CTP-UF4 mixtures were prepared in a specially designed reactor under nitrogen atmosphere, as already described [ 11; 8 wt% of GIC or UF4 were added to the CTP. Carbonizations were carried out in an open reactor under nitrogen atmosphere. The heating rate was 10 ‘C/min and the final temperatures were 400, 430 and 450°C (usual temperature range for the mesophase formation [ 127). The reactor is kept at the final temperature for 2 hours. This reactor was a cylindrical quartz tube (36 mm in diameter and 500 mm in length) into which was Table 1. Elemental analysis of the coal tar pitch (wt%); the oxygen wt% is calculated by diflerence C

H

N

S

92.40

4.60

1.05 0.59

0 (difference)

C:H

Ash

QI

1.36

1.67

0.30

2.7

introduced a smaller Pyrex tube (20 mm in diameter and 170 mm in length) containing the CTP or the mixture (about 60 mm in height) so as to always use the same experimental conditions 1291. It was placed in a vertical position in the furnace and the NZ Row rate was 20 l/h. 2.2 Microscopy Optical microscopy was usually used to characterize the carbonaceous mesophase in a qualitative manner. Whitakker and Grindstaff [30] first reported quantitative measurements of average sphere size and concentration (spheres/mm2 in photomicrographs) during a study of mesophase development in coke feed-stocks. We chose the method of Chwastiak rt al. [ 311 for describing the formation and the development of the mesophase spherules. This quantitative microscopy method consists of measuring the mesophase area on photomicrographs of polished specimens to determine the vol.% of mesophase. Specimens were prepared for optical microscopy using standard metallographic techniques. Each sample was mounted in an epoxy resin. The hardened resin block was polished with finer and finer grades of metallographic abrasive paper with water lubricant. A final polish was applied with cloth discs and diamond suspensions. Samples were examined by polarized reflected light with an Olympus BH2 optical microscope. Photomicrograph analysis allows us to obtain the average size of the mesophase spherules, the density of mesophase and the domain size of the anisotropic mesophase. However, we note that the measured diameter on the micrographs is not the real diameter of the mesophase spherules. Indeed, the polished section contains mesophase spherules which are diametrically cut with the same probability, so the real diameter of the spherules is necessarily greater than the measured diameter. The following statistical equation [32] must be used: (&,a~= dmeasuredx 4/n Generally, the size distributions of the mesophase spherules are determined with more than 300 measurements on the micrographs. 2.3 Insolubility measurements It is often admitted that insolubility methods do not reflect real mesophase contents 133-351. But in our case, these measurements have been made for a comparative study of the carbonization of CTP, and mixtures of CTP--UF4 or CTP-GIC. The insolubility measurements are only used to appreciate the “degree of polymerization” at the different temperatures. The solvent used is tetrahydrofuran (THF). 3. RESULTS 3.1 Polarized light microscopy Pitch is a solution of mesophase forming (mesogens) in a solvent of non-mesophase material [ 361.

material forming

Effect of graphite or F&l,-graphrte The carbonaceous mesophase initially develops as small anisotropic spherules formed by the accumulation of oriented polycondensed aromatic hydrocarbons in layers. As the heat treatment progresses, the spherules grow and then coalesce to form large bulk anisotropic regions that separate from the isotropic pitch phase and slowly settle. This is generally thought to be the result of polymerization reactions, which tend to build up the molecular weight so as to satisfy the average molecular structural requirements for liquid crystal formation. The general scheme of nucleation and growth of mesophase spherical bodies is observed by optical microscopy with the increase of temperature, for the CTP at 430 and 450°C (Fig. l(a) and (b)). Two polarized micrographs of the CTP-GIC 8% mixture at 430 and 450°C are shown in Fig. 2(a) and (b). Figure l(a), (b) and Fig. 2(a) illustrate the Brooks and Taylor model [ 121 for a mesophase spherule, as confirmed by Dubois et al. [13] and Honda et al [ 371; the constituent lamellar molecules of the sphere are somewhat parallel to an equatorial plane, such a

Fig. 1. Photomicrographs

intercalation compounds

621

manner that the edges of the lamellar molecules are perpendicular to the spherule surface. Figure 2(b) shows that the CTP-GIC 8% mixture heat-treated for 2 hours at 450°C is converted to a coalesced mesophase state. The CTP heat-treated at 430°C presents similarly sized spherical bodies which are covered by fine irregular particles of insoluble matter (QI) (this situation was confirmed by SEM observations). The changes in the extinction contours are more complex. Many crosses and nodes as classified by White and Price [38] are observed (Fig. l(a)). On the other hand, the CTP-GIC 8% mixture heat-treated at the same temperature presents a clean surface of spherical bodies of various sizes (Fig. 2(a)). Moreover, the GIC similarly to the particles of carbon black [39], forms chains and clusters which are randomly dispersed within the plastic matrix of pitch. The presence of anisotropic zones around these clusters (as indicated in Fig. 2(a)) can also be observed. CTP-UF4 8% mixtures were carbonized under the

of the CTP: (a) carbonized at 430°C; (b) carbonized at 450°C.

E. AL.AIN er ul.

622

Fig. 2. Photomicrographs

of the CTP-GIC

8% mixture: (a) carbonized at 430°C; (b) carbonized at 450°C

same conditions with the aim of determining the role of the graphite on one hand and the role of the Lewis acid FeCl, (provided by its release out of the GIG during the heat treatment) on the other. Figure 3(a) and (b) represent this sample carbonized at 430 and 450°C respectively. The observations show analogies with those observed in the case of the CTP-GIC mixtures: the graphite agglomerates and the mesophase spherules show clean margins of various sizes, but they are much smaller than these corresponding to the CTP-GIC mixture. The major difference is that the mesophase does not coalesce at 450°C (Fig. 3(b)) and seems to not have evolved much as compared to the 430°C experiment (Fig. 3(a)), where the magnification scale is twice as large than on the others figures. Table 2 gives a summary of all the observations made on the three samples: CTP, CTP-GIC 8% and CTP-UF4 8% mixtures carbonized at 400, 430 and 450°C. According to these photomicrographs, we can deduce the following data: the sphere density

(spheres/mm”), the vol.% mesophase, the mean diameter of the spherules (@ in pm), the associated standard deviation (0 in pm), and finally the maximal and minimal diameters which define the limits of the size distribution of the mesophase spherules for each sample at the different temperatures (see Table 3). The smaller spherule density observed in the case of the CTP-GIC carbonized at 430°C in comparison with the CTP, can be explained by the presence of GIC clusters (the clusters of graphite in the case of the CTP-UF4 mixture carbonized at the same temperature are not sufficiently dense to allow making a comparison). The decrease of the spherule densities for the CTP-UF4 mixture and CTP from 430 to 450°C corresponds to the growth and the coalescence of the mesophase spherules. The vol.% of mesophase is systematically lower when graphite or GIC is mixed with the CTP for each carbonization temperature. Nevertheless, the mean diameter of the mesophase spherules is very close in the case of CTP and CTP-GIC 8% mixture heat treated at 430-C (about 13 pm). but the size

Effect of graphrte

Fig. 3. Photomicrographs

Table 2. Qualitative

Carbonization temperature (“C) 400

of the CTP-UF4

observations

or FeCl,-graphite

8% mixture:

intercalation

(a) carbonized 450°C

compounds

at 430°C (scale twice as large); (b) carbonized

using light optical microscopy of the CTP, CTP-GIC at 400,430 and 450°C

CTP-GIC

CTP Optically

isotropic

430

Many individual spheres with Ql particles attached to the periphery

450

Amsotropic domains + larger mesophase spherules

623

8%

8% and CTP-UF4

CTP-UF4

CTP 430°C CTP-GIC 8% 430°C CTP-UF4 8% 430°C CTP 450°C CTP-UF4 8% 450°C

1047 323 1205 508 731

8% carbonized

8%

No mesophase spherules; GIC uniformly dispersed in the pitch matrix

No mesophase spherules; graphite uniformly dispersed in the pitch matrix

GIC agglomerates + mesophase spherules of various stze with clean surfaces

Graphite agglomerates + very small spherules with clean margins

Bulk mesophase

Clusters of graphite + larger mesophase spherules

Table 3. Quantitative data deduced from the optical microscopic observations upon the CTP, CTP-GIC 8% and CTP-UF4 8% carbonized at 430 and 450°C Spheres/mm*

at

Vol.%

@ (pm)

u (pm)

(@,,,; @,,,)

15.8 8.3 4.6 30 5.3

13 13,5 6 21 8

4.6 12 3.5 28 5.5

(4.5; 27) (4.5; 67) (2; 13) (4.5; 179) (4.5; 40)

624

E.

ALAINetal.

distribution is much broader with the CTP-GIC mixture (the maximum diameter is three times higher). In the case of the CTP-UF4 mixture carbonized at 430°C the mesophase spherules are not as well developed and their mean diameter is two times lower than those of the CTP or the CTP-GIC mixture. This differences remains at 450°C: the vol.% of mesophase and the mean diameter of the spherules are smaller than those of the CTP heated at the same temperature.

3.2 Discussion In a very general description of the conversion process, the heat treatment results in the volatilization of lower molecular weight components (inhibitors of mesophase formation or non-mesogens) and thermal polymerization of the more reactive species. As a result of chemical polymerization reactions increasing the average molecular weight of the pitch, structural order (a physical transformation process) has been detected by polarized light microscopy [40]. Greinke and Singer 1411 have shown that the molecular weight distribution of the mesophase does not change during the transformation and that the coexisting phases contain molecules of similar size but in different proportions. This suggestion does not agree with the fact that mesophase formation involves polymerization reactions. Indeed, a small molecule has a greater probability of being found in the isotropic phase than in the mesophase and a larger molecule has a greater probability of being found in the mesophase than in the isotropic phase. The environment of any given molecule is then critical. In their early pioneering work on the structure of mesophase, Mochida et LJ/. [42] suggested that the spherule growth occurs by the incorporation of smaller molecules from the isotropic matrix into the spherules at a rate adequate to maximize their diameter. In contrast however, Htittinger and Wang [43] suggested that spherule growth occurs by the diffusion of mesogenic aromatics (large molecules thermally formed after distillation of the volatiles from a sequence of several polymerization reactions) through the boundary layers. The growth of mesophase from the pitch phase is thus dependent upon a rather critical set of conditions involving the sizes of molecules. the more or less planar state of the configuration, the effective concentration of such molecules, the heat treatment temperature and the fluidity of the system. As we have seen, the carbonization of pitches has remained a large problem because pitches are made of mixtures of many different aromatic and heterocyclic molecules. In these conditions, the different behaviour of the mesophase growth in the case of the carbonization of the CTP, the CTP-UF4 8% and CTP-GIC 8% mixtures will be studied in the following. QI are generally present in the material from which the pitch is produced and the most common type in

coal tar pitches arises mainly from the coke ovens [21]. As concerns CTP, we have observed that QI appear on the margins of the spherules: these QI particles in the original pitch inhibit the coalescence and growth of mesophase spherules. Further, it may be worthwhile to mention that some large nonspherical mesophases are also observed at 450°C due to the incomplete coalescence of the mesophase spherules. In the case of the presence of graphite or GIC in the initial CTP, we can see that these particles have an important contribution in the formation and the development of the mesophase. First, we have seen that graphite inhibits the development of the mesophase, that is characterized by the small mean diameter and the narrow size distribution of the mesophase spherules. However, the surfaces of the mesophase spherules are regular and smooth, as explained by the absence of QI. The inclusion of particulate matter often leads to attractive interactions between the particles which agglomerate to form blocks: a similar behaviour is observed in the case of CTP-UF4 (as has also been shown in an other study [44]) and CTP-GIC mixture carbonizations. The pitch matrix does not seem to wet the graphite when the mesophase appears and we can assume that the QI particles assemble with the graphite. However, the wetting of a solid by a liquid is often characterized by the contact angle: the interpretation of this wetting is also difficult because of the role of the porosity of the substrate must be taken into account [45]. Moreover, the flow characteristics of the mesophase are important factors on the nucleation and the coalescence of the anisotropic phase [46]. The presence of a dispersed phase increases the viscosity and may introduce nonNewtonian character to the flow behaviour [47]. In the present system such as CTP-UF4, the fluidity is certainly too low to enable the constituent molecules to orientate to form a liquid crystal. The presence of GIC in the CTP has a double effect during the carbonization: first, it influences the growth and the coalescence of the mesophase spherules; and secondly, it increases the rate of formation of mesophase. Indeed, at 45O“C the bulk mesophase is achieved contrary to the case of CTP carbonization (see Table 2). The GIC may act as a catalyst, allowing the beginning of mesophase formation at a lower temperature as confirmed at 430°C. At this temperature the size distribution is three time greater in the presence of GIG. The effect of the Lewis acid F’eCI, desorbed from the graphite during the heat treatment not only annuls the inhibiting effect due to the presence of graphite, but also increases the mesophase formation. In a recent study 171, the catalytic activity of the Lewis acid during the carbonization of a CTP-GIC mixture was observed: it was characterized by a dehydrogenating effect and an increase of the green-coke yield. Another parameter is the viscosity which raises a complex problem because the effect of a dispersed

Effect of graphite

or FeCl,-graphite

phase is dependent on many factors including the volume fraction, the particle size distribution and the strength of particle-particle interactions. The fact that QI particles may agglomerate with graphite favours the coalescence of the spherules; such an interpretation should be confirmed with the use in the same experimental conditions of a filtered coal tar pitch from which QI particles would be removed. But the presence of the GIC must be taken into account: it may catalyse the polymerization reactions to obtain mesogens because of the Lewis acid desorption out of the graphite (the heavy and aromatic molecules are condensed, aided by the catalyst, into dimers or trimers). Then the mesogens can be included in the mesophase spheres already formed [41] and leading to larger spherules. When graphite agglomerates are present, these bigger spherules have a greater probability of encountering each other because the collision probability is greater. Another important observation is the presence around the GIC of anisotropic zones. We can assume that the GIC more than graphite (there are just little anisotropic regions in the case of the CTP-UF4 mixture) is an adequate substrate for the organization of basic structural units (BSU) [48,49]. This fact was confirmed by electron microscopy: the BSU of the coke seemed to be oriented in a direction parallel to the aromatic layers of the GIC (study made upon the green-coke obtained after pyrolysis of the CTP-GIC 8% mixture at 75O”C, then steam activated [SO]). Observations of the mesophase spherules have been also made by SEM. The important fact is that GIC particles are included inside the mesophase spherules for the CTP-GIC 8% mixture pyrolysed at 430°C. This can be explained by the desorption of the Lewis acid out of the GIC which accelerates polymerization reactions near the GIC particles; then, those generated large molecules lead to the formation of the mesophase spherules around these GIC particles. More details will be published in a forthcoming paper [ 511 and these observations, never seen in literature, confirms the main role of the Lewis acid during the heat treatment.

3.3 Insolubility

measurements

Though the mesophase content in pitch does not correspond to the insoluble portion in a pitch, the THF insolubility value (THFI) can give an indication about the mesophase formation process of the pitch. While all primary particles (QI) are insoluble, the mesophase may be partly soluble and there may be species in the isotropic matrix which are insoluble in THF. THFI is thus not an absolute measure. Lewis [ 521 presented a review on the chemistry of carbonization and showed the importance of dehydrogenative polymerization on the carbonization process. The dehydrogenative polymerization reactions increase the average molecular weight with increasing carbonization temperature and then decrease the pitch solubility in solvents such as pyridine, toluene and THF.

intercalation

compounds

625

In the case of the three samples studied in the present work, the different heat treatments invariably result in an increase in the THFI contents with increasing temperature. This is due to the removal of relatively lower molecular weight components from the precursor pitch, as well as to polymerization and condensation reactions taking place between the various planar aromatic molecules present in this pitch. Such reactions obviously lead to the formation and growth of mesophase spherules as we have already indicated in Section 3.2. The THFI results are presented in Table 4. The fact that graphite inhibits the development of the mesophase is confirmed by the lower value of the THFI contents observed at 400 and 430°C. In Table 4, we have of course taken into account the insolubility of the graphite in the THF. We also remark that a same THFI value does not correspond to the same vol.% of mesophase (Table 4 in comparison with Table 3). The hypothesis of a greater viscosity in the CTP-UF4 mixture can be confirmed by the presence of mesogens (assimilated with THFI) in approximately the same quantity as in the CTP sample. But in the case of the CTP-UF4 mixture, mesogens cannot be organized to give rise to new or bigger mesophase spherules because of the high viscosity: indeed the size distribution of the mesophase spherules is lower in that case (Table 3). The presence of the GIC enhances the formation of mesogens and thus the development of mesophase: the THFI contents of the materials from the CTP-GIC 8% mixture show a significant increase in comparison with those of the CTP. This rise reaches 36% at 430°C and 63% at 450°C. The action of the GIC can be assumed to be thermally activated.

4. CONCLUSIONS Mixtures of CTP-GIC 8% utilized as new original materials for carbonaceous adsorbents show a different behaviour during carbonization compared to the CTP or the CTP-UF4 mixture. This study has shown the catalytic activity of the Lewis acid desorbed out of the graphite during the carbonization of a CTP-GIC 8% mixture. In spite of an inhibiting effect caused by the presence of the graphite particles (the mesophase growth is slower in the case of the carbonization of a CTP-UF4 mixture compared to the carbonization of a CTP), the presence of the GIC increases the rate of the mesophase formation. The bulk mesophase is reached at 45O’C in opposition to the case of CTP carbonization for which large anisotropic domains are observed at the same temperature. Such a difference of behaviour must be characterized by a variation of the texture in the final green-coke, and work is in progress in this direction. A future paper will present a kinetic study of the CTP-GIC 8% mixture carbonization, and first results confirm the activity of the Lewis acid which is

Table 4. THFI contents

after carbonization at 400.430 CTP

CTP

by about

of CTP, CTP-UF4 and 450°C

and CTP-GIC

CTP-GIC

UF4 8%

mixtures

8%~

400 ‘C

43O’C

450~ c

400’ c

430 c

450 ‘C

400’-c

430°C

450 c

35

5 I .9

61

35.1 29.5”

52.1 4X.6”

64.X 61.7”

47.8 44.2”

7.18 70”

99.7 99.7”

’ THFI contents taking into account the materials after carbonization. characterized

et ul.

E. ALAIN

626

by a decrease

of the activation

energy

40 kJ/mol.

Acknowled~ements~The authors would like to acknowledge Mr R. Javier of the Centre de Pyrolyse de Marienau for helpful discussions about polishing and optical microscopic observations, Dr J. Pajak for the insolubihty measurements. and Drs E. McRae and M. Nutz for heloful discussions. Partial financial support was provided from PICS 119: Carbochimie and Environnement-CNRS ECOTECH, France and ADEME, France.

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the nresence

of graphite

or GIG particles

in

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