Characterization of Mesocarbon Microbeads and Their

192 石油学会誌 Sekiyu Gakkaishi, 42, (4), 192-200 (1999)

[Regular Paper]

Characterization of Mesocarbon Microbeads and Their Derivativesby IR, SEM and Raman Spectroscopy

Yan ZHANG†1), Satoru MURATA†1), Masakatsu NOMURA†1)*, and Chiharu YAMAGUCHI†2)

†1) Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871

†2) Research and Development Center, Osaka Gas Co., Ltd., 6-19-9 Torishima, Konohana-ku, Osaka 554-0051

(Received July 29, 1998)

Two kinds of mesocarbon microbeads (indicated MCMB-A and -B), prepared from different coal tar pitch, aswell as their derivatives obtained by butylation and solvent extraction subjected to ultrasonic irradiation, wereinvestigated by infrared (IR) spectroscopy, scanning electron microscopy (SEM) and Raman spectroscopy. ForIR spectroscopic analysis of these samples, MCMB-B and its derivatives both showed electron transition absorp-tions at near-IR region and aromaticities at C-H stretching region, stronger than those of MCMB-A and its deriva-tives. SEM observation suggested that MCMB-B seemed to be rich in free carbon, more than MCMB-A.These results indicated the difference in structural characteristics between two MCMB samples: it could be pre-sumed that MCMB-B would have degree of condensation and graphitization higher than those of MCMB-A.This could account for the experimental results previously reported that MCMB-B is inferior to MCMB-A regard-ing catalytic reactivity in butylation and solvent extraction. SEM images also indicated that MCMB had a sur-face layer of about 1-1.5μm. Raman spectra of the MCMB, before and after solvent extraction, suggested that

MCMB had a graphitized carbon layer on the surface, higher than the inner carbon.

1. Introduction

It is well known that mesophase transformation of

heavy hydrocarbon materials, such as pitches and bitu-

men, takes place in the heating process in the range of

350 and 500℃1)-5). By gradual heating of these mate-

rials, polycondensed aromatic hydrocarbons are formeddue to thermal decomposition and polymerization, fol-lowed by orientation of polycondensed aromatic hydro-carbons in a uniform direction. By the accumulationof polycondensed aromatic hydrocarbons oriented inlayers, the mesophase spherule is formed. Brooks andTaylor6) proposed the well-known lamella structure forthe mesophase spherule. Since then, a large numberof studies were made on the related subjects and at leasttwo additional types of mesophase spherules werereported, which are different in layer alignment fromthat found by Brooks and Taylor. Honda et al.7) havereported mesophase spherule with optical propertiesdifferent from those observed for the Brooks-Taylortype spherule and proposed their own structural modelhaving outer layers lying parallel to the spherule sur-face but having layer alignment similar to theBrooks-Taylor type spherule around the central regionof the spherule. Similar mesophase spherule was alsofound by Kovac and Lewis8). Huttinger9) disclosedanother mesophase spherule with all the layers lying in

concentric circles about the center of the spherule.Honda et al.2) used solvent fractionation to separate

mesophase spherule from heat-treated pitches. Theynamed these spherules, "mesocarbon microbeads"

(MCMB), which is known to be one of the more impor-tant carbonaceous materials. For example, MCMBhave been utilized to produce high density isotropiccarbon materials and to improve binder pitch10), whichafter calcination becomes applicable to electrodes oflithium ion battery11),12) Up to now, many studieswere reported on the chemical structure of MCMB,using solid state carbon-13 nuclear magnetic resonance

(13C-NMR), electron spin resonance (ESR) andothers13)-15). In order to obtain an insight into struc-tural features of MCMB, we examined the butylation ofMCMB with butyl iodide catalyzed by either potassiumor dibutylzinc, in a previous work. Both reactions

produced tetrahydrofuran (THF)-solubles in 86-95%yields. The detailed analyses of the MCMB and itsTHF-solubles of butylated products indicated thatMCMB molecules consist of highly condensed aromat-ic hydrocarbon (average ring size is calculated to be15-16) with exceedingly minor amounts of alkyl sub-stituents and oxygen-functional groups. Here, theauthors used two kinds of MCMB samples, MCMB-Aand -B. Although two MCMB samples showed dif-ferent properties, such as reactivity for butylation andsolvent extraction, the authors could not differentiatethe two in terms of chemical structure based on 13C-* To whom correspondence should be addressed.

石油学会誌 Sekiyu Gakkaishi, Vol. 42, No. 4, 1999

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NMR, gel permeation chromatography (GPC) andmicroanalysis data16). In the present study, MCMBand their related materials were investigated by IRspectroscopy and SEM. Raman spectroscopy wasalso used to examine the changes of structure whenMCMB samples were treated with organic solvents, inultrasonic irradiation. The results suggested that thesurface of MCMB-B seemed to be rich in free carbon,and MCMB-B might have degree of condensation and

graphitization higher than those of MCMB-A. On theother hand, free carbon may show higher degree of

graphitization and has some properties different fromthose of mesophase spherule. It was reported that amore highly graphitized carbon layer on the surface ofMCMB may cause reduced capacity or short cycle lifewhen used as the anode of secondary lithium-ion bat-

teries11). Consequently, the difference in free carboncontent between MCMB-A and -B may serve to pre-

dict, to a certain degree, their performance in the sameapplication.

2. Experimental

2.1. Preparation of SamplesTwo original samples (denoted MCMB-A and -B)

were repeatedly extracted by THF and quinoline toobtain two groups of samples, THF-soluble (THFS-A,-B) and THF-insoluble (THFI-A, -B), and quinoline-

soluble (QS-A, -B) and quinoline-insoluble (QI-A, -B),respectively. The extraction procedures were carriedout as follows: 50ml of THF or quinoline was added to1g of MCMB, and treated for 30min under ultrasonicirradiation at room temperature. After centrifugaliz-ing, the supernatant solution was decanted. These

procedures were repeated 10 times for THF-insolublesor 7 times for quinoline-insolubles, respectively.From the supernatant solution obtained after centrifu-

galizing, THF was removed by distillation, using arotary evaporator, and quinoline by distillation inreduced pressure. Then, both solubles and insolubleswere washed with water-ethanol mixture (in 1:1 vol-ume).

The butylation procedures of MCMB with butyliodide catalyzed by dibutylzinc (BZ method) aredescribed elsewhere16),17). A mixture of MCMB (0.5

g), dibutylzinc (10.5g) and butyl iodide (5ml) washeated in a 100ml flask at 180℃ for 4h. After the

end of reaction, remaining dibutylzinc was decomposed

at 0℃ by addition of ethanol, then butyl iodide and

ethanol being distilled off. Remaining solids werewashed with dilute hydrochloric acid (5wt%, 100ml)to remove the remaining zinc salts. In the present

work, butylated MCMB-A, -B (BZ-A, BZ-B) and theirTHF-insolubles (BZ-THFI-A, BZ-THFI-B) were also

studied.

2.2. Measurements of IR Spectra

IR spectra were obtained in the wavenumber ranges

of 4600-600cm-1 using a diffuse reflectance method

with a Shimadzu FT-IR-8100M FT-IR spectrometer.

Interferogrames of 500 time-scan were measured atleast 3 times to obtain a reproducible spectrum. The

sample was diluted with KBr, ratio of 1:100 in weight,

and ground to below 200 Tyler mesh (<75μm) in a

Wig-L-Bug shaker, then dried overnight at 100℃ in

vacuum. The spectra are shown without base-line

corrections, and the intensity of the absorption band is

calculated with the Kubelka-Munk function, f (R∝),

which is proportional to the absorptivity, if the light-scattering factor is independent of the wavenumber18).2.3. Observation with SEM

SEM was operated at an accelerating voltage of 20or 30kV. A secondary electron mode was applied forsurface observation. Sample surfaces were previouslysubjected to coating with gold layer.2.4. Raman Spectroscopy

Raman spectra were obtained on a JRS-S 1000 spec-trometer equipped with a CCD multichannel detector,using an argon-ion laser (λ=514.5nm). Spectra were

submitted to curve fitting procedures, in order to sepa-rate both graphite band (G band) at ca. 1580cm-1and disordered carbon band (D band) at ca. 1360

cm-1. The former increases with increasing graphiti-zation, and the latter increases with decreasing graphiti-

zation. The intensity of the D band was measured byits height relative to that of G band, being given by the

value of R. The bandwidths at half height of the two

peaks were indicated by WD and WG, respectively.

3. Results and Discussion

3.1. Solvent Extraction

There are two methods for separation of solvent sol-

uble and insoluble fractions: a decantation method after

centrifugalization and a filtration method using certain

pore filter. The solvent extraction results are usuallyaffected by the separation method. It is reported that,even by the filtration method, the smallest part of insol-

uble particles can pass through filters with a large pore

size, giving rise to errors in the results19). The main

purpose of the present study is to examine the surfacechange of MCMB after solvent extraction under ultra-

sonic irradiation and not to examine the solubility of

MCMB. We, therefore, adopted the decantation

method.

The results of extraction of MCMB and the elemen-

tal analyses of all materials are summarized in Tables

1 and 2, respectively. As mentioned above, these val-

ues were obtained as a result of repeating 7 or 10 times

the extraction procedure. It must be noted that the

supernatant solution after centrifugalization had consid-

erable coloring remaining, even after repeating extrac-


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tion 10 times, suggesting that the separation of solubleand insoluble materials could not be completely

attained from MCMB by this method. Insoluble

yields obtained by two solvents did not show any sig-nificant difference (Table 1). The solubles, however,showed greater H/C atomic ratio as compared with thatof the insolubles (Table 2). On the other hand,because the yield of solubles, separated by decantationmethod, is strongly affected by either the viscosity/specific gravity of solvents or the centrifuging frequen-cy/time, it is expected that this method should give riseto overestimation of extraction yields. Actually, partsof insolubles suspended in solution were incorporatedinto real solubles as confirmed by SEM observation

(Fig. 5b), which will be discussed later.3.2. IR Spectroscopy

Typically, IR spectroscopy is sensitive to the pres-ence of specific functional groups in organic materials.It can be noted that, basically, the functionalities foundin the IR spectra are in accordance with the compoundsidentified by pyrolysis capillary gas chromatography/mass spectrometry (Py-GC/MS), i.e. polycyclic aromat-ic hydrocarbons, certain polycyclic heteroaromaticcompounds and some of their alkyl derivatives20),21)Shown in Figs. 1 and 2 are the IR spectra of these sam-

ples in the region of 600 to 4600cm-1. It must betaken into account that all these samples essentially

give the same bands in the IR region, differing in theirintensities. In this study, the authors will mainly dis-cuss the following regions of interest: the aromatic C-H

out-of-plane bending modes between 910 and 665cm-1; the aromatic and aliphatic C-H stretching modesbetween 3100 and 2800cm-1; and near-IR regionbetween 4600 and 4000cm-1.3.2.1. Bands between 910 and 665cm-1

In MCMB, neither long aliphatic chains nor mineralmatter which produces absorbances in this region of thespectra are present. The pattern of bands between 910and 665cm-1, therefore, can be used for the estimationof the concentration of aromatic C-H bonds in MCMBand its related materials. This pattern is also used indetermining substituted positions on the aromatic ringby estimating the number of adjacent hydrogenatoms22). As shown in Figs. 1 and 2, four main peakswere observed in the spectra at ca. 870, 840, 820, and750cm-1. The band at 870cm-1 suggests the pres-ence of aromatic rings containing isolated C-H bonds.The two absorptions at 840 and 820cm-1 are attributedto two-adjacent aromatic C-H groups. The band at750cm-1 is considered to be given as an overlappedabsorption of three-adjacent (800-765cm-1) and four-or five-adjacent (770-740cm-1) aromatic C-H species.These patterns are schematically shown in Fig. 3.Fused rings follow the same general rules establishedwith isolated benzene molecules23). The chemical

Table 1 Results of Extraction of MCMB by THE and Quinoline

a) Extraction for 10 times. b) Extraction for 7 times.

Table 2 Elemental Analyses of Products Obtained by SolventExtraction and THF-insolubles of Butylated MCMB

a) Yield: 4.9% (MCMB base). b) Yield: 10.2% (MCMB base).

Fig. 1 Diffuse Reflectance IR Spectra of Samples from

MCMB-A

a) BZ-THFI-A, b) BZ-A, c) MCMB-A, d) THFI-A, e) THFS-A.


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composition of MCMB is considered to consist of

polynuclear aromatic hydrocarbons having a smallamount of aliphatic groups2). Four or five substitutedbenzenes illustrated in patterns a, b, c, and h in Fig. 3,therefore, correspond to extremely condensed polyaro-matic molecules and their alkyl substituents. Thehydrogen existing in these patterns mainly shows theband intensity at ca. 870cm-1. On the other hand,

patterns e, i, j and k involve three to five adjacent

hydrogen atoms, giving rise to higher absorbance at ca.750cm-1. Other patterns such as d, e, f and g showthe structural characteristic with lower degree of con-densation. The absorption of their hydrogen atoms,however, is mainly attributed to the bands at ca. 820,840, and 870cm-1. Here, quantitative analysis wasconducted by using clearly resolved two bands at 870cm-1 (isolated) and 750cm-1 (four or five adjacent), asapproximate parameters of the degree of condensationin samples24). The intensity of the band was deter-mined using the baseline method as reported by Guilenet al.24). The absorbances at ca. 750 (Abs750), 820

(Abs820), 840 (Abs840) and 870cm-1 (Abs870) wereobtained by drawing a linear baseline between 910 and665cm-1. Shown in Table 3 are the ratio of theabsorbance for these bands. The so-called ortho-andmeta-substitution indexes designated as I750 and I870 aredefined as Abs750/(AbS750+Abs820+Abs840+Abs870)×

100 and Abs870/(Abs750+Abs820+Abs840+Abs870)×

100, respectively21). The results suggested that THEsolubles contain the less fused structures and also themore polycyclic aromatic hydrocarbons containing fouradjacent aromatic C-H bonds. In other words, themore fused part of MCMB was found to be less THEsoluble.

For both MCMB-A and-B, the I750 reflecting thefour adjacent aromatic C-H concentration increasedfrom the original MCMB to their THE solubles andInsolubles, while the I870 reflecting the isolated aromat-ic C-H concentration decreased. As shown in Figs.1b and 2b, the intensity of these four bands substantial-

ly decreases in the spectrum of butylated MCMB,attributed to the replacement of the major portion of thearomatic hydrogen by butyl groups. The weakerabsorbances in this region for THE Insolubles of bu-tylated MCMB (Figs. 1a and 2a) indicated higherdegree of condensation of these components, whichwell correspond to their lower H/C atomic ratio as indi-cated in Table 2.3.2.2. C-H Stretching Region between 3100 and

2800cm-1The band at ca. 3050cm-1 is attributed to aromatic

Fig. 2 Diffuse Reflectance IR Spectra of Samples from

MCMB-B

a) BZ-THFI-B, b) BZ-B, c) MCMB-B, d) THFI-B, e) THFS-B.

Fig. 3 Scheme of the Substitution Patterns in Benzne Rings

Table 3 Aliphaticity, Ortho- and Meta-substitution Index ofMCMB, THE Solubles and Insolubles

a) Ian=Abs2920/(Abs3050+Abs2920)×100.

b) I750=Abs750/(Abs750+Abs820+Abs840+Abs870)×100.

c) I870=Abs870/(Abs750+Abs820+Abs840+Abs870)×100.


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C-H stretching vibrations. The aliphatic C-H stretch-ing region between 3000 and 2800cm-1 is consideredas the sum of contribution from three groups, CH, CH2and CH3 in this study. As can be seen from Figs. 1and 2, the aliphatic C-H stretching band at ca. 2920cm-1 appeared more intensively in the spectra of theTHF insolubles (Figs. 1d and 2d) compared with thespectra of the original MCMB (Figs. 1c and 2c), sug-

gesting that the relative content of aliphatic groups inTHF insolubles increased, in comparison with those ofthe original MCMB.

For the butylated MCMB, the aliphatic C-H stretch-ing bands between 3000 and 2800cm-1 were signifi-cantly increased by the introduction of butyl groups onaromatic rings. As shown in Figs. 1a and 2a, any

peak hardly appeared in the same region for THF-insol-uble of butylated MCMB, suggesting that these compo-nents hardly show any reactivity for butylation.

Absorption bands at ca. 3050 and 2920cm-1 can

give an evaluation of the concentration of aromatic andaliphatic C-H bonds for different samples. Brown andLadner25) have estimated the ratio of absorptivities foraromatic and aliphatic hydrogen from peak heights atnear 3030 and 2925cm-1, respectively. In this study,the absorbances at ca. 2920 (Abs2920) and 3050cm-1

(Abs3050) were obtained by drawing a linear baselinebetween 3100 and 2800cm-1 24). Shown in Table 3are the aliphaticities (Ial) of the original two MCMB,and their THF solubles and insolubles, which weredefined as Abs2920/(Abs2920+Abs3050). Although thealiphaticities of both MCMB-A and -B similarlyincreased and decreased in their THF-insolubles andsolubles, respectively, MCMB-A samples showed thehigher aliphaticities on the whole. Aliphatic groupsmay tend to prevent the aromatic sheets to stack. Thedifference in aliphaticity between two MCMB samplesthus, seems to bear out the foregoing results; the thick-ness of the stack and the number of the aromatic sheetsassociated in a stacked cluster of MCMB-B were

greater than those of MCMB-A as determined by X-raydiffraction16).

3.2.3. Near-IR Region between 4600 and 4000

cm-1

In general, the electronic transition for carbon mate-

rials in their absorption spectra is composed of the

intramolecular and intermolecular π-π* transitions orig-

mated from the isolated aromatic molecules and the

lamella structure, respectively. The absorption band

in high energy regions are usually strong in intensity

and broader in width. The tail of such electronic

absorption bands appears at energy regions lower than

6000cm-1. Nakamizo presumed that aromatic hydro-

carbons with more than 10-20 aromatic rings may

show electronic absorption peak (π-π* transitions) in

the near-IR region26). On the other hand, for aromatic

hydrocarbons of infinite size, such as graphite and

amorphous carbon, their transition energies approach

zero27), resulting in further intense and broad absorption

in the whole IR region of spectra of graphite and acti-

vated carbon28).

According to results reported in a previous work,

MCMB is believed to consist of highly condensed aro-

matic hydrocarbon with more than 15-16 aromatic

rings and to have highly stacked aromatic sheets16).

Hence, these materials should exhibit broad absorption

in near-IR region. After butylation, two kinds of

changes in structures took place in the butylated prod-

ucts. One of them is the decrease in the average size

of the aromatic ring of the macromolecules which

existed in the original MCMB, because they are

reduced to hydroaromatic and naphthenic structures

accompanying the reductive butylation29). Another is

that the butyl groups began to act as spacers between

stacked aromatic sheets. Such changes may reduce

both intermolecuiar and intramolecular π-π* transition

effects, leading to different absorption behaviorsamong MCMB and their butylated products. Asshown in Figs. 1 and 2, the absorption intensity of theoriginal MCMB (Figs. 1c and 2c) increased steeplywith wavenumber more than their butylated products

(Figs. 1b and 2b).Here, different absorption behavior between two

MCMB samples should be noticed. As shown inFigs. 2b-c and 1b-c, original and butylated MCMB-Bshowed electron transition absorptions in near-IRregion stronger than those of original and butylatedMCMB-A. This seemed to be corresponding to thefact that MCMB-B was inferior to MCMB-A in solu-bility to THE after butylation, i.e., butylated MCMB-Band -A yielded THF-insolubles, BZ-THFI-B and BZ-THFI-A, 10.2% and 4.9% (MCMB base), respective-

l y16). Two remarkable differences were foundbetween BZ-THFI-B and BZ-THFI-A. One is thatBZ-THFI-B showed significantly stronger absorptionintensity in near-IR region than BZ-THFI-A (Figs. 2aand 1a). Another is that the H/C atomic ratio of BZ-THFI-B is lower than that of BZ-THFI-A, as shown inTable 2. BZ-THFI-B, therefore, is suggested to have

condensed planar aromatic macromolecules larger andmore than those of BZ-THFI-A.

The broad absorption for either of THE insolubles

(Figs. 1d and 2d) was similar to that of the originalMCMB, suggesting that most of the highly polycon-

densed aromatic hydrocarbons of large size in the origi-

nal MCMB were concentrated in the insoluble. The

THE solubles also showed weaker broad absorption

band in the near-IR region, as shown in Figs. 1e and2e, attributed to a small amount of insolubles mixed

with the real THE solubles in the present study.

3.3. Scanning Electron Microscopic Observation

3.3.1. Size and Shape of MCMB

Shown in Fig. 4a is a typical SEM image, size and


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shape, of the spherules in MCMB-A. The size of

MCMB was about s to 50μm. The shape of MCMB

was spherical and a small number of fine particles wereirregular in shape. There were no significant differ-ences in size and shape, comparing MCMB-A and -B,and there was no crack observed, on both small and bigspherules.3.3.2. Interface and Spherule Surface Structure

of MCMBAs shown in Fig. 4a, the surface of MCMB-A

appears to be not so smooth. Two types of surfaceswere shown in Figs. 4b-d. These were designated,Type I and Type II, respectively. Type I (Fig. 4b)showed an obvious layer, and Type II (Figs. 4c and d)showed extremely ragged surface in which small sizespherical bodies seemed to gather on the surface oflarge size bodies. Such rough surfaces were usuallyobserved from the mesophase spherules formed by heattreatment when a small amount of free carbons existedin the pitch7). MCMB was separated from pitch

matrix containing free carbon particles (metaphase) bysolvent fractionation. Thus, MCMB obtained wasmixed with free carbon. Two types of surfaces wereobserved in both MCMB-A and -B, but it is impossibleto evaluate their contents with percentage values by

SEM observation. Honda et al.2) reported that free

carbon can be closely attached on the surface ofmesophase spherules, and it is difficult to separate freecarbon from mesophase spherules, even by solventextraction. As will be described below, after solventextraction, surface of spherules in insolubles becamevery smooth. In this case, the presence of Type II wasconspicuous for SEM observation. It was found thatthe amount of Type II was greater in QI-B and THFI-Bthan in QI-A and THFI-A. This suggests thatMCMB-B may be rich in free carbon, more thanMCMB-A.

Shown in Fig. 4e is a SEM image of the cross-sec-tion of a broken spherule in MCMB-A. The imageenlarged to a magnification of ×50000 at position A

shown in Fig. 4e is illustrated in Fig. 4f. Up to now,there has been no report of such SEM images ofmesophase spherule. Clearly, the image is different,and the surface layers (image indicated by arrow in Fig.4e) and inner carbons (Fig. 4f) are used to describethem. The surface layers seem to show a thickness ofabout 1-1.5μm and differ from inner carbons in SEM

image.

3.3.3. THE Insolubles and Solubles of MCMB

A SEM image of THFI-A is shown in Figs. 5a.

Most of spherules in this sample maintained their

spherical shape and the surfaces became exceedingly

Fig. 4 Scanning Electron Micrographs of MCMB

a) MCMB-A. b) The surface (Type I) observed in MCMB-A. c) The surface (Type II)

observed in QI-B. d) The surface (Type II) observed in MCMB-B. e) The surface layer

observed in MCMB-A (indicated by arrow). f) The enlarged image from position A.


198

smooth, indicating that the solvent treatment by ultra-

sonic irradiation at room temperature was mainly to

remove the surface layer of the spherules. Spherules

greater than 10μm showed shrunk cracks. On the

other hand, some particles broken off from original

spherules were also observed, the shape being irregular.

According to these observations, MCMB is easily

shrunk to form cracks, even in the conditions of this

study. The reason to form cracks may be attributed to

the ultrasonic effect, and the surface layer became thin-

ner after ultrasonic treatment in organic solvents.

Shown in Fig. 5b is the SEM image of THFS-A. It

was found that THE solubles exhibited exceedingly

fine particles and no spherule existed in these solids.

The same results were observed with butylated prod-

ucts of MCMB. However, a number of particles about

6μm were observed in THF solubles. Obviously, as

mentioned above, these particles must be attributed to

the insolubles accompanied by decantation method.3.4. Raman Spectra Profile

Shown in Fig. 6 are Raman spectra of the originalMCMB-A, -B and THFI-A. Spectral profile of threesamples showed two similar characteristic absorption

peaks found for other carbonaceous materials. It isknown that the Raman spectrum for natural graphiteconsists of a single band at ca. 1580cm-1 with narrow

band, which is attributed to the E2g in-plane vibrationsof the hexagonal layers. Polycrystalline graphite andcarbon materials, however, show additional Ramanband at ca. 1360cm-1, which was shown to arise from

graphite crystallites containing disorganized regionsnear crystal edges and lattice defects such as edge dis-location and lattice vacancies30),31). It was reportedthat the intensity ratio of the 1360cm-1 band (D band)to the 1580cm-1 band (G band), i.e. R=ID/Ic, not onlyincreases with decreasing graphitization, but also withincrease in the edge plane ratio of the crystal face31),and can be used as a measure of the concentration of

lattice defects in graphite structure32). For otherRaman parameters related to both G and D bands, the

peak position and bandwidth (WD) of G band arebelieved to relate to the graphitization degree; band-width of the D band (WD) correlates well with thedegree of disorder33). In the present study, the authorsfound that the peak position of G band for MCMB sam-

ples shifted ca. 15cm-1 to wavenumber (1595cm-1)which is higher than that of natural graphite (1580cm-1). Similar band positions were observed byAngell et al. in Raman spectra of mesophase pitches34),these indicating that MCMB has a degree of perfectionconcerning the aromatic hexagonal network lower thanthat of natural graphite.

Raman parameters of the original MCMB-A, -B andTHFI-A were given in Table 4. Comparing MCMB-A and THFI-A, the widths of both G and D bands, WGand WD, increased from 69 and 296cm-1 of MCMB-Ato 77 and 307cm-1 of the THFI-A, respectively. Atthe same time, the R value increased from 0.53 ofMCMB-A to 0.61 of the THFI-A. The differences inRaman results between the original MCMB-A andTHFI-A might be explained by the results of SEMobservation, i.e. MCMB-A has a surface layer of about1-1.5μm. After ultrasonic treatment in THF, the sur-

face layers became thin and some of spherules were

broken, leading to the increase in surface areas of inner

carbons in THFI-A sample. Raman spectroscopy can

only evaluate the surface structure to a depth less than 1

1-1.5μm. After ultrasonic treatment in THF, the sur-

face layers became thin and some of spherules were

broken, leading to the increase in surface areas of inner

carbons in THFI-A sample. Raman spectroscopy can

only evaluate the surface structure to a depth less than 1

μm. In this sense, Raman profile of original MCMB-

A mainly reflects the structure of surface layers, where-

as that of the THFI-A should provide information on

structure of the inner carbons. It appeared, therefore,

reasonable to presume that MCMB-A had graphitized

carbon layers on the surface, higher than that of the

inner carbons. This is supported by the results report-

Fig. 5 Scanning Electron Micrographs of THFI-A and THFS-

A Samples

a) THFI-A. b) The insoluble particles observed in THFS-A

(indicated by arrow).

Fig. 6 Raman Spectra of (a) MCMB-B, (b) MCMB-A and (c)

THFI-A


199

ed by Hara and co-workers11). On the other hand,

MCMB-A and -B showed no significant difference in

their Raman spectra, suggesting that the two MCMB

samples are largely identical but with minor differencesin Raman characteristics of their surface layers.

4. Conclusion

The differences in surfaces and components of the

two kinds of MCMB, as well as different solubilities oftheir butylated products, were clarified by IR spec-

troscopy, SEM observation and Raman spectroscopy.The results of this study are summarized below:

(1) MCMB showed a broad absorption in near-IRregion, suggesting that MCMB consists mainly oflarge size polycyclic aromatic hydrocarbons.MCMB-B showed a stronger broad absorption incomparison with that of MCMB-A, correspondingwell to the difference in amount of THF-insolublesof the two butylated MCMB. The THF-insolublesof butylated products showed the weakest absorp-tion in the whole IR region and the strongest broadabsorption in the near-IR region, indicating thatthese components are of less founctionalities andconsist mainly of free carbon existing in MCMB.The THF-insoluble of butylated MCMB-B has ahigher degree of condensation and larger averagering size than those of THF-insoluble of butylatedMCMB-A, respectively.

(2) SEM observation suggested that MCMB had a sur-face layer of about 1-1.5μm parallel to the spherule

surface. The surface layer of MCMB-B seemed tobe rich in the free carbons more than that ofMCMB-A.

(3) Raman spectra indicated that the insolubles,obtained from MCMB-A treated with organic sol-vents under ultrasonic irradiation, showed band-widths and R values greater than that of the originalMCMB-A, suggesting that the surface layer ofMCMB-A had graphiticity higher than the innercarbons.

(4) To the best of knowledge in the present study, IR,SEM and Raman spectroscopy are useful tools forinvestigation of materials such as MCMB, the struc-tural features of which can not be differentiated by

conventional means.

Acknowledgments

We would like to express our deep thanks to Mr. K.Kawauchi of JEOL, Ltd. for his help for the measure-

ments of Raman spectra.

Table 4 Raman Parameters of MCMB-A, -B, and THFI-A

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要旨

IR, SEMおよびラマン分光法を用いたメソカーボンマイクロビーズ

およびその関連物質のキャラクタリゼーション

張岩 †1), 村田聡 †1), 野村正勝 †1), 山口千春 †2)

†1) 大阪大学工学部応用化学科, 565-0871大阪府吹田市山田丘2-1

†2) 大阪ガス(株)研究開発部, 554-0051大阪市此花区酉島6-19-9

異なるコールタールピッチから調製された2種のメソカーボ

ンマイクロビーズ (MCMB-AとB) およびブチル化と超音波

照射下の溶媒抽出により得られた誘導体と抽出物を対象に, 赤

外分光法 (IR), 走査型電子顕微鏡 (SEM) およびラマン分光

法を適用してそれぞれの試料の分析を行った。IR分光法によ

り, MCMB-Bとその関連物質は対応するMCMB-Aとその関

連物質に比べて, 近赤外領域の電子遷移による吸収強度が高く,

芳香族性が高いことを示した。SEM観察から, MCMB-Bは

MCMB-Aと比べて, フリーカーボンが多く見られた。これら

の結果より, 両MCMBの構造特性の差が明確にされた。つま

り, MCMB-BはMCMB-Aより芳香環の縮合度と黒鉛化性が

高いことが示唆される。前報 (Zhang et al., Energy Fuels, 11,

433 (1997)) で報告したように, MCMB-Bはブチル化反応にお

いて反応性および有機溶媒への溶解性がMCMB-Aより劣って

いる事実がこれらの結果から説明できる。また, MCMB小球

体の横断面のSEM像から, MCMBには1～1.5μmの表面層が

観測され, さらに超音波による溶媒抽出を行った前後の

MCMBのラマンスペクトルの変化から, MCMBの表面層は内

部炭素より黒鉛化性が高いことが推測された。

Keywords

Mesocarbon microbeads, Solvent extraction, IR, SEM


Documents

Characterization of Mesocarbon Microbeads and Their