Characterization of meso-carbon microbeads prepared by emulsion method

Cbrbo~~Vol.?L). No. I. pp. 43-49. Prmtcd in Great Bntam

llwwl22319 I $3 00 + .lKI Copyright ‘CJ1990 Pergamon Prcs plc

1991

CHARACTERIZATION MICROBEADS PREPARED MASAYA

KODAMA,

TAKAYASU

OF MESO-CARBON BY EMULSION METHOD

FUJIURA, EIICHIIKAWA,

KENJIRO MEGURO,

KUNIO ESUMI,

and HIDEMASA

HONDA Department of Applied Chemistry, Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan (Received

17 November

1989; accepted in revised form 12 March 1990)

Abstract-This article describes the preparation of meso-carbon microbeads by the emulsion method from three kinds of bulk-mesophase pitches (coal-tar, FCC-decant oil, and naphtha-tar pitch), and their physicochemical properties. The meso-carbon microbeads were obtained as the spherical particles from respective pitches in yields of 65%-90%. The meso-carbon microbeads from the coal-tar and FCCdecant oil pitches had a larger amount of quinoline insolubles than those of starting pitches, while that from naphtha-tar pitch was approximately constant with heat treatment temperature. The coal-tar and FCC-decant oil based microbeads had an optically anisotropic texture with different domain sizes, whereas the naphtha-tar based microbeads exhibited an optically isotropic texture. The results of FTIR and ash content suggest that a permeation of silicone oil during heat treatment causes a distortion and expansion of the lamellar structure for the naphtha-tar based microbeads. Furthermore, it was found from FTIR and “C-NMR analysis that the coal-tar based microbeads had the highest aromaticity and that the naphtha based microbeads had the largest amount of aliphatic carbons, such as methyl. methylene, and alicylic groups. Kev WordsMeso-carbon tar-pitch. bulk-mesophase

microbeads, emulsion method, coal-tar pitch, FCC-decant oil pitch, naphthapitch.

1. INTRODUCTION

Brooks

and Taylor

phase formed

studied the carbonaceous

meso-

in the early stages of carbonization of pitches and proposed a now well-known model for the mesophase spherules, consisting of lamellar structure of oriented polycondensed aromatic hydrocarbons[ l-51. A large number of detailed studies concerned with the formation, growth, coalescence, and structure of the carbonaceous mesophase spherules has been reported[6-131. The mesophase formation has been recognized as a significant step for the preparation of carbon materials by the liquid phase carbonization of organic compounds. Honda and coworkers have used solvent fractionation to mesophase spherules from heat-treated pitches. They named these spherules meso-carbon microbeads (MCB)[14,15]. Furthermore, they have investigated the properties of MCB prepared from several raw materials (coal-tar pitch, straight asphalt, naphtha-tar pitch, etc.). They also attempted to utilize MCB to produce high density isotropic carbon materials and to improve binder pitch[16,17]. The MCB prepared by the conventional separation from heat-treated pitch have a number of deficiencies, including: a wide distribution of particle size, condensation of free carbon particles (metaphase) on the surface of spherules, and a low yield[l4]. To overcome these problems, a number of investigators have tried to modify the preparation of MCB to obtain spherules of better quality[l%201.

Mesophabe spnerules can usually be extracted as a solvent insoluble phase in mesophase pitch. In recent years, highly soluble bulk-mesophase pitches were developed for the purpose of spinning high tenacity and high modulus carbon fibers (HT and HMCF) from pitch[21-241. These fusible and soluble bulk-mesophase pitches are usually prepared by heat treatment of a hydrogenated isotropic pitch. They have softening points between 250 and 350°C. We have reported a new preparation method of MCB using soluble bulk-mesophase pitch. In this preparation method, which has been termed emulsion method, the molten pitch particles are spheroidized in a heat-stable liquid medium having less solubility for pitch. By the use of this emulsion method, MCB having a narrow size distribution and a satisfactory spherical shape are available in higher yields[25]. In the present study, we prepared three kinds of MCB using coal-tar, FCC-decant oil, and naphthatar pitches as raw materials and examined the physical and chemical properties of these microbeads by various analytical methods. 2. EXPERIMENTAL 2.1 Materials

The three sion method from coal-tar tha-tar pitch. pitches used

kinds of MCB were prepared by emulfrom bulk-mesophase pitches derived pitch, FCC-decant oil pitch, and naphAnalytical data for the bulk-mesophase are listed in Table 1. These bulk-meso-

44

M. KODAMAet al. Table 1. Analytical data for bulk-mesophase pitches used in the preparation of MCB

Bulk-mesophase

Pitches

Coal-tar pitch (C-pitch) FCC-decant oil pitch (F-pitch) Naphtha-tar pitch (N-pitch)

Quinoline Insoluble

Toluene Insoluble

Softening Point

(wt%)

(wt%)

(“C)

Ash (wt%)

0.9 20.0 1.4

82.3 76.4 27.5

307 290 284

0.64
phase pitches have low softening points and high solubilities in solvents such as quinoline and toluene. They were shown by polarized light microscopy to contain 100% of mesophase. The bulk-mesophase pitch particles were dispersed in a thermostable silicone oil and heat treated at prescribed temperatures and treatment times. After heat treatment, MCB were separated from silicone oil by centrifugation and were rinsed with benzene and acetone. The details of the procedure have been described in a previous paper[25]. The silicone oil used as the liquid medium consists of phenyl-methyl-polysiloxane structure (Toray Silicone Co.). The MCB prepared from coal-tar pitch based bulk-mesophase pitch (C-pitch) are called CMCB; those from FCC-decant oil pitch based bulkmesophase pitch (F-pitch) are F-MCB; and those from naphtha-tar pitch based bulk-mesophase pitch (N-pitch) are N-MCB. 2.2 Microscopy observations The optical structure of the mesophase was observed with a Leitz Ortholux microscope with xenon arc illumination using a reflected polarized light under crossed nicols. The samples were heat treated to 1,OOO”Cand then were embedded in a resin and polished by usual techniques. The shape and size of MCB were examined using a scanning electron microscope and an optical microscope. 2.3 Elemental analysis and determination of quinoline insolubles

The elemental analysis (C, H, N) was performed on a 2 mg sample using a Carlo Erba model 1106 analyzer with antipyrine (2,3-dimethyl-1-phenyl-Spyrazolone) as a standard compound. The determination of quinoline insoluble content was performed at 80°C in the manner described in the Japanese Industrial Standard (JIS)[26]. 2.4 Spectroscopic analysis The solid state “C-NMR measurements were made on a Bruker AC-200 superconducting spectrometer operating at 50.3 MHz for carbon with combined high-power decoupling and CP/MAS (cross-polarization and magic angle spinning) techniques. The NMR signals were accumulated 1,024 times at 3,250 Hz of sample spinning speed. Chemical shifts were referenced to tetramethylsilane (TMS). The infrared spectra were obtained over the wave-

Mesophase Content (%) 100 100 100

number range of 400-4,000 cm-’ using a diffuse reflectance method with a BIORAD DIGIRAB FTS60 FTIR spectrometer. 2.5 Nitrogen adsorption The adsorption of nitrogen on MCB at 77 K was measured with a Micromeritics-Shimadzu Surface Area Analyzer 2100 A. The MCB were pretreated by heating in vacuum at 120°C for 15 hours. The specific surface areas were evaluated by the BET method. 2.6 X-ray diffraction The x-ray diffraction pattern was measured using CuKa radiation. The procedures were performed by the standardized method prescribed by 117 Committee of Japan Society for the Promotion of Science[27]. 3. RESULTS AND DlSCUSSlON 3.1 Preparation of MCB The conditions for preparing MCB by the emulsion method can be described in terms of treatment temperature and time. Each pitch has a range of conditions which is capable of providing MCB with a satisfactory spherical shape. The spherical particles from C-pitch, F-pitch, and N-pitch were obtained at the following conditions: l C-MCB, 290-350°C within 60 min. (a recovery of pitch, 90%-85%) l F-MCB, 330-370°C within 30 min. (85%-75%) l N-MCB, 320-350°C within 30 min. (75%-65%)

The spheroidizing conditions and the yield of MCB differed for the different pitches. The C-pitch could be spheroidized over a wider range of temperature and time than the F-pitch and N-pitch. The differences in yield of MCB among the three pitches are due to the solubilities of the pitches in silicone oil at the respective temperatures. As a rule, a longer time was required at a lower temperature to obtain spherical particles, and the yield of MCB decreased with increase in temperature and time. The conditions for preparing spherical MCB are related not only to the softening point of pitch but to other factors as well, including: solubility of pitch in silicone oil, the molecular weight of pitch, and changes in the viscosity of the pitch and silicone oil. Figure 1 shows the scanning electron micrographs

Characterization

m

45

of meso-carbon microbeads

40Dm w

w


of three kinds of MCB prepared by the emulsion method. The particles had a satisfactory spherical shape and smooth surface. Neither cracks nor holes were observed. 3.2 Quinoline insoluble contents (Qf) of MCB Figure 2 shows the quinoline insoluble contents (QI) of MCB prepared from the three kinds of pitches treated at various temperatures (320-360°C) for 10 min. In this temperature range, the separated MCB are not always spherical in shape because of preparation at conditions which are out of the spheroidizing region. The QI values for the original mesophase pitches heat treated in N1 atmosphere using the same conditions

,,”

are also plotted

_.... ..__

. . . ...-

:__ .. pitch

in Fig. 2. As

r.

320

330

Temperature

340

q

?

350

360

(“C)

Fig. 2. Quinoline insoluble fractions (01) of bulk-mesophase pitch of MCB prepared at temperatures (320-360°C) for 10 min: 0. C-MCB; A, F-MCB; 0. N-MCB. Closed symbols; QI values of pitch heated in a furnace under a nitrogen atmosphere.

shown in Table 1, C-pitch and N-pitch contain 0.9 and 1.4 wt% of QI, and F-pitch contains 20 wt% before the heat treatment. Although the C-pitch and N-pitch have almost the same initial quantity of QI, the separated MCB exhibit distinctly different QI contents. Namely, the QI content of C-MCB is about 19 wt% at 320°C and increases slightly with temperature, while the QI content of N-MCB remains approximately constant at about 3 wt% with temperature. The slight increase in QI of N-MCB results from a slight solubility of the pitch in silicone oil. The QI content of the F-MCB increased during the heat treatment in silicone oil at 320°C similar to the C-MCB. However, unlike C-MCB, the QI of FMCB increased substantially at temperatures above 340°C and rose to more than 90 wt% at 360°C. This suggests that the molecular weight of mesophase increases as a result of chemical reactions such as polymerization during the heat treatment. It is apparent that the QI contents of separated MCB can vary depending upon the starting material and preparation conditions. Subsequent experiments were carried out using MCB prepared as follows: C-MCB(340”C--5 min). F-MCB(350”C-10 min), and N-MCB(330”C-10 min). 3.3 Elemental analysis The elemental analysis data for these MCB and the original bulk-mesophase pitches are shown in Table 2. Tl-e data for conventional MCB prepared from coal-tar pitch and asphalt pitch are added for comparison]28]. The bulk-mesophase pitches and MCB contain approximately 93%-95 wt% of carbon and 4%-5 wt% of hydrogen. The FCC-decant oil and naphtha-tar products are free from nitrogen. In the case of every kind of mesophase, the carbon contents slightly increased and the hydrogen contents decreased during preparation by emulsion method. The carbon increments of coal-tar. FCC-

M. KODAMAet al. Table 2. Elemental analysis of bulk-mesophase

pitches, MCB prepared by emul-

sion method, and conventional MCB N

C

H

(wt%)

0

s

H/C

C-pitch

93.34

3.93

1.35

0.93

0.43

0.505

C-MCB F-pitch F-MCB N-pitch N-MCB Type-M” Type-Ph

93.59 95.14 95.56 94.54 94.71 91.8 81.9

3.79 4.22 4.01 5.02 4.97 2.8 3.8

1.63 0 0.04 8

0.85 0.46 0.31 0.35 0.21 4.7’ 13.0’

0.14 0.18 0.08 0.08 0.06 -

0.486 0.532 0.504 0.637 0.630 0.37 0.56

0.7 1.3

a. Prepared by heat treatment (4~~C/90 min) of coal-tar pitch containing 3.4% of free carbon (methaphase). b. Prepared bv heat treatment f43OW60 min) of straight asphalt containing 5.4% of sulfur. . c. [lo0 - (C + H + N)]%

decant oil, and naphtha-tar mesophase were 0.23, 0.42, and 0.17 wt%, and the hydrogen increments of them were -0.14, -0.21, and -0.05 wt%. As a result, the atomic ratio of hydrogen and carbon (H/C) decreased. These differences in atomic contents and ratios between pitch and MCB increase in order of: F-MCB > C-MCB > N-MCB. On the other hand, compared with the conventional MCB which were prepared from isotropic pitches and separated as a quinoline insoluble matter, these MCB have higher carbon contents and lower contents of heterogeneous atoms such as nitrogen and oxygen.

The optically anisotropic spherules formed in the early stages of carbonization have a particular orientation of mesophase molecules. The Brooks-Taylors’ model, which has a representative structure of mesophase spherules, shows a well-defined extinction contour and pleochroism which are attributed to its molecular orientation[29]. In order to observe the mesophase structure of bulk-mesophase pitches and MCB prepared by emulsion method, they were oxidized in air at 300°C for 30 min and heat treated at 1,OOO”Cin Nz atmosphere. The bulk-mesophase pitches exhibited what is called fibrous texture, consisting of unidirectional arrangement of domains

H

having regular orientation of the mole~ules~30,31~. The flow textures of mesophase are dependent on the type of pitch, heating rate, and heat treatment temperature. The breadths of domains are different from each bulk-mesophase pitch. They are approximately l-10 pm (C-pitch), >lO pm (F-pitch), and >5 pm (N-pitch). F-pitch has larger oriented domains than C-pitch and N-pitch. The reflected polarized light micrographs of MCB are shown in Fig. 3. The C-MCB and F-MCB do not exhibit a definitive structure of molecular orientation; i.e., the domains that indicate the same color under crossed nicols aggregate at random in a cross sectional view of spherules. These results should be predictable from our preparation process. The arrangements of domains in both MCB appear to have some torsion compared with the bulk-mesophase pitches mentioned above. Since the mesophase texture of bulk-mesophase pitch is modified owing to a softening of pitch under the emulsion system, MCB show some complex domain arrangement. However, the C-MCB and F-MCB substantially succeed to the domain breadths of bulk-mesophase pitches; consequently, F-MCB have larger domains than CMCB. The average numbers of the domains per one cross sectional view of a particle are 3-10 (C-MCB) and l-5 (F-MCB).

:20&m.

Fig. 3. Polarized optical micrographs on cross section of MCB using reflected polarized light under crossed nicols. (a) C-MCB (34O”C, 5 min); (b) F-MCB (350°C 10 min); (c) N-MCB (330°C, 10 min).

Characterization

On the other hand, as shown in Fig. 3c, a most striking aspect of the present observations is that NMCB exhibit no pleochroism. Thus, its cross section seems to be isotropic. The remaining anisotropic domains (5 urn) of optically anisotropic as mentioned above, a transformation to isotropic phase appears to occur during the preparation process by emulsion method. This behavior will be discussed later. (If N-MCB has isotropic structure, the name of N-MCB is not necessarily appropriate; however, it will be called this for convenience’s sake.)

3.5 FITR and “C-NMR spectra The FTIR spectra of MCB are illustrated in Fig. 4. These are almost the same as those of the starting pitches, except for a new absorption band at l,OOO1,300 cm-’ of N-MCB. On the whole, three kinds

3500

3000

2500

2000

Wavenumber

1500

(cm-’

1000

500

1000

500

47

of meso-carbon microbeads

Table 3. The relative absorbance ratio of the aromatic CH band at 3,030 cm-’ and the aliphatic C-H band at 2,920 cm-’ observed by FTIR diffuse reflectance method, and the aromatic carbon fraction (fu) observed by W-NMR CP/MAS method FTIR spectra (&ar&a, = 3,030 cm-l/ 2,920 cm-‘) MCB Pitch Coal-tar FCC-decant oil Naphtha-tar

2.12 1.52 1.07

2.18 1.47 0.99

“C-NMR spectra

(fa =

C.&G, + Carl) Pitch MCB 0.954 0.932 0.862

0.958 0.946 0.870

of MCB indicate similar profiles; however, there are poorly characterized differences on the absorption bands at 700-900 cm-’ due to C-H out-of-plane vibration, and at l,lOO-1,400 cm-’ due to the functional groups including an oxygen and a nitrogen atom, such as C-O, O-H, and C-N. Table 3 shows the relative absorbance ratio of the aromatic C-H band at 3,030 cm-’ and the aliphatic C-H band at 2,920 cm-’ (KH_ar/KH_a,) of the pitches and MCB. (These values do not express the ratio of hydrogen atoms.) The ratio for C-MCB is twice that of N-MCB, while the ratio for the F-MCB falls between the two. There are no marked changes in the ratios for the pitches and the MCB resulting from the preparation process. In order to evaluate the carbon aromaticities of MCB, 13C-NMR spectra were taken using highpower decoupling and CP/MAS techniques. Figure 5 shows the r3C-NMR spectra of the different MCB

)

(b)

3500

3000

2500

2000

Wavenumber

1500

(cm-‘)

Fig. 4. FTIR spectra of pitch and MCB by diffuse reflectance method. (a) (i) C-MCB and (ii) F-MCB; (b) (iii) Npitch and (iv) N-MCB.

Fig. 5. ‘YJ-NMR spectra of MCB observed by high-power decoupling and CP/MAS methods at 3,250 Hz of sample spinning speed. (a) C-MCB, (b) F-MCB, and (c) N-MCB.

48

M. KODAMACY al.

materials The spinning side bands appear at 0, 60, 190, and 260 ppm. The centered strong band is assigned to aromatic carbons, and the weak bands between 10 and 40 ppm are due to aliphatic carbons. The aliphatic carbons evidently increase in order of: C-MCB < F-MCB < N-MCB. F-MCB and N-MCB show a peak at 20 ppm assigned to methyl carbon bonded to an aromatic ring. Additionally, N-MCB contain a distinct band due to a carbons (such as methylene and alicyclic) bonded to aromatic rings at 35 ppm, which is not apparent in C-MCB and FMCB. Thus, N-MCB appear to have a relatively large amount of aliphatic carbon groups as side chains on aromatic rings, while the C-MCB have only a small amount of alkyl groups. The aromatic carbon fractions (fa) estimated from the 13C-NMR spectra of pitch and MCB are shown in Table 3. The fu values range from 0.862 (approximately atomic ratio; Caar : C,, = 6:l) and 0.958 (23:l). N-pitch contains about 13% of aliphatic carbons, a large amount as bulk-mesophase pitch. Most aliphatic carbons remain after the preparation of N-MCB. The fa values of three kinds of mesophase slightly increased before and after the preparation by emulsion method. This result seems to be correlated to the decrease in H/C atomic ratio. It is suggested that the aromatization proceeded slightly during the heat treatment in silicone oil. 3.6 Nitrogen adsorption The specific surface areas evaluated by a nitrogen adsorption on MCB at 77 K were 0.38 (C-MCB), 0.21 (F-MCB), and 0.23 (N-MCB) m*/g. Moreover, the geometrical surface area estimated from the particle diameter of MCB is approximately 0.2 mYg. In general, the low molecular weight compounds such as benzene and iodine can be adsorbed from gaseous and liquid phase not only on external surface (periphery) of MCB but on internal surface (interlayer of the mesophase lamellar structure) at ordinary temperatures. This behavior should be called sorption rather than adsorption. Since the thermal motion of mesophase molecules is fixed at a low temperature of 77 K, that is, MCB have solid state, nitrogen molecules cannot enter the interlayer of lamellar structure[32]. Consequently, the BET surface area of MCB by the nitrogen adsorption is nearly equal to the geometrical surface area. 3.7 Internal structure of N-MCB N-MCB prepared from naphtha-tar buik-mesophase pitch seem to have optically isotropic texture, as described above. It appears that the optical texture of mesophase can be changed during a heat treatment in silicone oil. Figure 4b shows the FTIR spectra of N-pitch (iii) and N-MCB (iv). Compared with two spectra, clear differences are found between 1,050 and 1,300 cm-l; that is, new peaks at 1,070-90, 1,130, and 1,250 cm-’ appear. These peaks are probably attributed to absorption bands of organic silicone compounds: 1,070-90 cm- I, Si-

0; 1,130 cm-‘, Si-Ar; 1,250 cm-‘, Si-CH,. The silicone oil used is phenyl-methyl-polysiloxane structure, which has absorption bands at the above wavenumbers. The C-MCB and F-MCB exhibit no changes in FTIR spectra when compared to the pitches. Moreover, silicon was not detected in C-MCB by an electron probe microanaiyser (EPMA). Thus, a silicone oil adhering on the surface of MCB after rinsing by solvent is improbable. In addition, an ash content in N-pitch was 0.11 wt%, and that of N-MCB was 1.33 wt%. The ash contents in the other MCB were approximately constant. The increase of ash is attributed to the silicone oil in MCB, because it is partly fixed as ash by pyrolysis. From these results, the presence of silicone oil in N-MCB is confirmed and the silicone oil appears to permeate into the NMCB mesophase. Figure 6 illustrates the x-ray diffraction profiles at the angles between 15 and 30” of MCB. The low angle side of the peak of N-MCB is apparently broadened, indicating that the irregularities of lamellar structure in N-MCB are greater than those in C-MCB and F-MCB. N-pitch containing larger amounts of aliphatic carbons inhibits the stacking of molecules by steric hindrance; therefore, the relatively loose stacking of lameliae in N-pitch permits the silicone oil to permeate into it. As a result, the stacking of lamellae is distorted and interlamellar space is expanded by permeation of siiicone oil during the heat treatment. Consequently, a cross section of N-MCB appears to have optically isotropic texture.

1

15'

20”

25"

30'

28 Fig. 6. X-ray diffraction profiles of as-prepared MCB at the angles between 15 and 30” by CuKa radiation. (a) CMCB, (b) F-MCB, and (c) N-MCB.

Characterization

of meso-carbon microbeads

It is concluded that three kinds of MCB prepared by emulsion method from coal-tar, FCC-decant oil, and naphtha-tar pitches show different properties; C-MCB and F-MCB have larger amounts of quinoline insoluble fraction, while that of N-MCB is almost constant, compared with raw materials. Further, these MCB exhibit different optical textures and hydrogen and carbon aromaticities. Acknowledgement-We

thank Dr. Minoru Nakamizo and Mr. Yasuhiro Yamada for performing the spectroscopic experiments and elemental analysis. Further, we thank Dr. Minoru Shiraishi for kind direction about the polarized light microscope observation, and also Mr. Shigeji Hagiwara for performing the adsorption experiment.

49

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