Carbonization of aromatic hydrocarbons—V

Carbon, 1976, Vol. 14, pp. 341-344

Pergamon Press.

Prmted m Great Britain

CARBONIZATION OF AROMATIC HYDROCARBONS-Vi’ MICROSCOPIC FEATURES OF CARBONS OBTAINED BY THE AID OF CATALYSTS ISAOMOCHIDA,EIICHI NAKAMURA,$ KEIKO MAEDA and KENJ~ROTAKESHITA ResearchInstitute of Industrial Science, Ky&hu University, Fnkuoka, Japan 812

TORU HOSHINO ApplicationDepartmentElectron Optics Division,JEOL Ltd., 1418NakagamiAkishima,Tokyo, Japan 196 (Receiued 23 April 1976) Abstract-Microscopic observation of carbons obtained from pure aromatic hydrocarbons by the aid of carbonization catalysts was carried out to clarify the microstructure of these carbons of different features. Reflected polarized-light microscopy distinguished needle, mosaic and isotropic cokes, former two of which were produced with aluminumchloride and the last with potassium. High resolution microscopy revealed that these carbons calcined at 1250”had different degree of layered structure, corresponding to the crystallographic parameters of these samples graphitized at 2500°C.The reasons for the carbons produced with potassium to be non-graphitizable are discussed from the macro- and micro-features of the carbons. 1. INTRODUCTION

Two factors are considered to determine the structural aspects of the carbon such as graphitizability and degree of orientation of microcrystallites at the carbonization step. One of them is concerned with the starting material, and the other is with the carbonization conditions[l]. Among latter factors, the solvent [2], the co-carbonizing material[3], kind and pressure of the atmosphere gas [4], the heat-treatment conditions [5,6] and the carbonization catalyst [2,6] are known to have significant effects on the structure. The present authors have reported that the carbonizing catalysts could control the properties of the carbon and proposed a possible mechanism for the carbonization reaction [2,6]. In conclusion, aluminum chloride gave anisotropic needle-like cokes from naphthalene and anthracene and an anisotropic mosaic coke from pyrene, whereas, potassium, sodium or ferric chloride yielded isotropic cokes from the same sources. The different features of these carbons have been assumed to be brought about by the different carbonization intermediates which were produced under the influence of the catalyst. In the present study, further microscopic observation was carried out to investigate the microstructure of these carbons and to reveal their different properties from the viewpoint of their structural differences. For such a purpose, reflected-polarized-light, scanning and highresolution electron microscopy can provide qualitatively important information. Especially, a high-resolution electron microscope can make observable aspects of the microstructure of carbons which have never been resolved by X-ray analysis[7]. The samples used in the present study are an anisotropic needle-like coke, an anisotropic mosaic coke and an isotropic coke obtainable

from the aromatic hydrocarbons by the aid of carbonization catalysts. 2. EXPERIMENTAL 2.1 Materials Naphthalene, anthracene and pyrene obtained from Tokyo Kasei Co. were used without further purification. Aluminum chloride, sodium and potassium were obtained from Kishida Kagaku Co. 2.2 Carbonization Aluminum chloride of powder form was added to the hydrocarbon in a Pyrex tube (30 x 300 mm), and then the mixture, after shaking under nitrogen, was heat-treated up to 600°C at the rate of 15O”/hrunder a nitrogen flow. In the case of alkali metal catalyst, the sliced metal was added to the suspension of hydrocarbon in a small amount of n-hexane, and the samples were heat-treated by the same manner as in the case of aluminum chloride. The samples were held at 600°C for 2hr. The carbon samples thus prepared were heat-treated at 1250 and 2500°C for 0.5 hr under an argon flow. Details of the carbonization and graphitization were described in previous papers[2,6]. 2.3 Microscopic observation The optical structure of carbons was observed with a Leitz Ortholux microscope using reflected polarized light under crossed nicols. Scanning electron micrographs were made by means of a JEM-lOOC-SEG-ASID microscope. Small specimens (5 x 10x 5 mm) were coated with gold and were examined at magnification of 200 and 2000. High-resolution electron micrographs were obtained using a JEM-1OOC electron microscope. Specimens were prepared by grinding followed by dispersion with chloroform, and examined at direct magnification of 300,000. 3. RESULTS Polarized light photomicrographs of carbons obtained

-‘TPart1V of this series: Carbon 14, 123 (1976). fOn leave of absence from Idemitsu Kosam Co,

in the presence of aluminum chloride or alkali metals are 341

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MOCHIDAet al.

shown in Fig. 1. In spite of the same source of aromatic hydrocarbons, the catalysts gave different optical features to the carbons. The carbons obtained with alkali metals were isotropic regardless of the starting materials as reported before[2] (Fig. le, f). In contrast, carbonization with aluminum chloride produced an anisotropic needle coke from naphthalene (Fig. la, b) and a mosaic one from pyrene (Fig. lc, d). To compare these carbons from a morphological viewpoint, photographs taken by means of a scanning electron microscope are presented in Fig. 2. Distinct di!TerenGecould be observable among these carbons. The anisotropic cokes regardless of the needle or mosaic

Fig. 2. Features of carbon revealed by a scanning electron microscope. (a) Carbon from anthracene with AlCl, (SOO°C), x200 and (b) x2000; (c) carbon from anthracene with potassium (SWC), x200 and (d) x2000; (e) carbon from pyrene with potassium (SWC), x2009. structure consisted of piles of thin flakes (Fig. 2b) which formed a rough surface in overall appearance. In a marked contrast, the isotropic carbons obtained with

Fig. 1. Features of carbons revealed by polarized light microscope (x200). (a) Carbon from napbthalene with AlCl, heat-treated at 600°C and (b) at 1250°C; (c) carbon from pyrene with AlCI, heat-treated at 600°Cand(d) at 1250°C;(e) carbon from naphthalene with potassium heat-treated at WC; (f) carbon from pyrene with potassium heat-treated at 600°C.

alkali metals consisted of thick grains (Fig. 2c), sizes of which were five times larger than those of the former coke (Fig. 2a). The surfaces of the latter coke were smooth without pores or crevices on one grain. Such morphologies may correspond to the values of their surface area shown in Table 1, where the isotropic carbons had quite small areas, whereas anisotropic ones had large areas. Photographs taken by means of a high-resolution electron microscope are shown in Fig. 3, comparing the microstructures of these carbons calcined at 1250°C.The

Table 1. Some properties of carbons obtained with aluminum chloride or potassium

cot

LCI

SS Structurei

Antbracene

6.721

770

450 Flow

Pyrene

6.744

450

170 Mosaic

Hydrocarbon

K

AU,

Catalyst

cot 6.76ll ( 6.87 6.76ll I 6.87

Lt

SS Structure$

23

5 Isotropic

38

12 Isotropic

The mole ratio of catalyst/hydrocarbon was unity except for the case of pyrene-AK% system, where the mole ratio was one-tenth. t& the sample was heat-treated at 2500°C. $BET surface area; m*/g.The sample was heat-treated at 500°C. OOptical-microscopicfeature. pwo-component peak.

Carbonizationof aromatichydrocarbons-V

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clear differences were again present in their lattice fringe structures of carbon layers. A lot of long and wide fringes with regular intervals between layers forming Rakes were observed in the carbon obtained from naphthabne with aluminum chloride (Fig. 3a). The size of a fringe structure which may correspond to L, or L, measured by X-ray diffraction reached up to 200-IOOO A. The carbon obtained from pyrene with aluminum chloride showed distinct fringe strn&nes, however their length and width were rnnch smaller than those of the n~hthaleae carbon (Fig. 3b). In marked contrast to these anisotropic carbons, isotropic one from anthracene with sodium did not show layer structures except for those limited in the edge area. These fringes were rather circular and similar to those of the graphitized carbon black@, 91. Crystallographic parameters obtained by means of powdered X-ray diffraction were summarized in Table 1. The table indicates that anisotropic carbons were graphitizable soft carbons regardless of their needle or mosaic structures, however isotropic carbons were non-~~hiti~ble hard carbons. 4. DISCUSSION Two kinds of classification as for the structure of the carbon are quite important to estimate its properties. One of them is concerned with the crystallinity, and the other with the degree of o~enta~on of its mi~ro~rystalIites~The t&t criterion produces a distinction between graphjtizable and non-graphitizable cokesl81, and the latter one between needle-like and mosaic cokes in the evaluation of its quality as electrode cokes, Although these classifications of coke can be achieved by means of optical microscopy and X-ray diff~~~o~~~~ for the samples heat-treated at ~~~t~z~ng temperatures, further information on their microstructure is aiso essential to elucidate why these carbons are so. In this sense, an electron microscope is a quite useful tool. Ban[9], Oberlinf7J, Jenkins [IO], Fourdeux [I 11, Harsh [If, Evans 1131 and Johnson Il4] reported interesting information on the microstruct~e of carbons by means of a high resolution electron microscope. An electron microscope has revealed distinct differences among the carbons obtained from the same source with different catalysts in the present study. As for the first criterion, the isotropic coke of the present study has quite small nnntber of fringe structures and only in limited areas. It showed less graphitizabiiity than the glassy carbon abtained from polyfurfuryl alcohol[2]. The less fringe structure of the former carbon in comparison with the photographs of the latter carbon[IO] may correspond to the less ~aphitizability. The anisotropic one consists, on the whole, of fringe structures, although the length of the fringe structures depends on the quality of the anisotropic cokes, Such a generalization is similar to that proposed by Oberlin et al. [7]. Scanning microscopy and surface area measurements both indicated that the anisotropic carbon had a lot of pores, but that the isotropic one was of low surface area and a bulky form without pores. Kawamura and CARBON Vol. 14 No. 6-C

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Tsuzuku[lS] and Kamiya and Suzuki[l6] discussed the effect of porosity on the ~ap~tizab~ity of the hard carbon. Graphi~~tion of the hard carbon may require the accumulation and relaxation of the stress due to the thermal expansion at the graphitization treatment. The latter process can take place at the grain boundaries or around pores, where the deformation brought about the alignment of carbon layers in a crystallite is possible. Thus the porosity of the certain degree may help the ~aphitization. The former process may be related to real density. The density of the carbon obtained with alkali metals was much less than that of one obtained with aluminum chloride [2], probably decreasing the degree of stress accnmulation. Possible reasons for the carbons to be non-~aphitizable can be enumerated as follows. (1) A three dimensional sp’ network which prohibits the transformation of the carbon-carbon bond into sp* hybridized bond of the graphite. (2) Lack of layered structures which should be formed by virtue of free motion of condensed molecules during the ~rbo~~tion stages. The restricted displacement of carbon atoms in the solid phase at the graphitization stage does not lead to the formation of the graphite. (3) Lack of the stress, of which accumulation and relaxation help the formation of the graphite phase. In addition to these reasons, eatalystsllll] and high pressure [ 18] during the heat-trea~ent may intIuence the graphitization; however these factors need not be taken into account in the present case. The present data shown in Table 1 and Figs. 2 and 3 suggest that the isotropic non-graphitizability of the carbon obtained with alkali metal is due to the reasons (2) and (3). In relation to the latter reason, the carbon ob~ned with alkali metal pretreated in the solvent which had a rather high surface area is fairly graphitizable[l9]. Although no evidence supporting the reason (1) was obtained in the present study, it could not completely be ruIed out yet. it requires further microscopic study on the cokes which should have .$ bonds in their struct~e[20]. It may be interesting to discuss that relation will be present between the second criterion and microstructures revealed by a high-resolution electron microscope, although these concepts deal with the problems of the different crystallographic sizes. The pyrene carbon of a mosaic structure had shorter fringe structure than the naphthalene carbon of a needle structure. The sizes of anisotropic domains observable by an optical microscope are diierent for a mosaic and a needle coke, however the

microstrncture within an anisotropic domain has not been investigated except for X-ray diffraction study, which indicates only a small diierence in their crystal sizes 16,211. A systematic observation by a high-resolution electron microscope may be necessary to resolve the details, although the present study indicated the different features of needle and mosaic cokes in their microstructure.

1. Kimura H., Sauada Y. and Honda E., J. Fuel Sot. Japan 49, 764 (1970). 2. Mochida I., Nakamura E., Maeda K. and Takeshita K., Carbon 13,489 (1975). 3. Japanese Pat. 43162 (1973); Isaacs C. G., Carbon 6, 765 (1968); Evans S. and Marsh M., ibid. 9,749 (1971). 4. Ohtam S. and Oya A., Kogyo Kagaka Zasshi 72, 317, 323 (1%9); ibid. 73, 1110(1970). 5. Fitzer E., Mueller K. and Shaefer W. Chemistry and Physics of Carbon 7, 237 (1971). 6. Mochida I., Kudo K., Fukuda N., Takahashi R. and Takeshita K., Carbon 13, 135 (1975). 7. Oberlin A., Ten-ire G. and Boulinier 3. C., Tanso 1975 (No. 80), 29; ibid 1975(No. 83), 153. 8. Frankho R. E., Proc. Roy. Sot. AX& 1% (1951). 9. Heidenreich R. D. and Hess W. hf., J. Appi. Cryst. 1,1(1968); Ban L. L. and Hess W. M., 9th and 10th Conference on Carbon (1%9,1971);Symposiumon petroleum derived carbon (1975). 10. Jet&ins G. and Kawamura K., Proc. Roy. Sot. London A327, 501 (1971). 11. Fourdeux A. and Ruland W., Con@. Rend. 273,Cl021 (1971). 12. Marsh P. A., Voet A., Mullen T. J. and Price L. D., Carbon 9, 797 (1971):Doonet J. B.. Voet A.. Dauksch H.. Ehrburaer P. and Mar& P., Carbon il, 430 (i973). 13. Evans E. L., Jenkins J. L. and Thomas J. M., Carbon 13,321 (1972). 14. Johnson D. J., Tomizuka I. and Waft O., Carbon 13,321 (1975). 15. Kawamura K. and Tsuzuku T., Carbon 12,352 (1974). 16. Kamiya K. and Suzuki K., Carbon 13, 317 (1975). 17. Yokokawa C., Hosokawa K. and Takegami Y., Carbon 4,459 (1966);Jackson P. W. and Morioram J. M., Nature 218,83 (1968);Fitzer E. and Kegel B., Carbon 6,433 (1968);Ohtani S. and Ova A.. Bull. Cbern. Sot. Janan 45.623 (1972):Ohtani S., Oya A. and Akagami J., Carbon 13, 353 (197.5). 18. Kamiya K., Ioagaki M., Mizutani M. and Noda T., Buff.Cbem. Sot. Japan 41,2169 (1968);Kamiya K., Ipagak M. and Noda T., High temperature and Highpressure 51331(1973);Iuagaki M., Hayashi S. and Naka S., ibid. 6,485 (1974);Inagaki M., Horii K. and Naka S., Carbon 13,97 (1975). 19. Mochida I., Nakamura E., Maeda K. and Take&a K., Carbon 14,123 (1976). 20. Otani S., Oshima T., Oya A. and Ota E., Tanso 1974(No. 77), 45. 21. Mochida I., Ogawa M. and Takeshita K., Bull. Chem. Sot. Japan 49, 514 (1976).