Preparation of carbonaceous spheres from suspensions of pitch materials

Carbon.

Vol. 30, No. 5. pp. 781-786,

Printed

in Great

1992 Copyright

Britain.

%5.OfJ + .OO 000%6223/92 0 1992 Pergamon Press Ltd.

PREPARATION OF CARBONACEOUS SPHERES FROM SUSPENSIONS OF PITCH MATERIALS SEONG-HOYOON,YANG-DUKPARK,'~~~ISAOMOCHIDA Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka, Japan, 8 16 ‘Research Institute of Science and Industry, Pohang, Kyungsangbukdo, Korea, 790-330 (Received 26 November 1991; accepted in revisedform

27 Jam1ar.v 1992)

Abstract-Carbonaceous spheres were prepared from mesophase pitch, isotropic pitch, and fractioned portions of mesophase pitch. The sequential procedure is: suspension in lean suspension medium, removal of dissolving solvent, filteration, and oxidative stabilization. The shapes of spheres obtained depended on the type of solvent, suspension agent, and suspension medium. The carbonaceous spheres prepared from THF soluble fractions (THFS) exhibited relatively narrow distributions of sizes and perfect spheres in shape. The size of spheres could be controlled by the concentration of the soluble fraction of pitch in solution. The spheres prepared from the quinoline-soluble fractions (QS) were reduced in size compared with those from THFS, and flake-like materialsappeared due to the aggregation ofspheres upon the removal of quinoline. A specific polymeric suspension agent, polyvinylalcohol (PVA), was stable up to 290°C and was employed as a surface active agent in the suspension medium of glycerine. Key Words-Pitch,

carbonaceous sphere, suspension.

1.INTRODUtXION

method for the preparation of mesocarbon microbeads (MCB) from a quinoline soluble mesophase pitch of 100 wt%. Because surfactants were not applicable in such an emulsion system, it is difficult to prevent coagulations or flocculations of MCB. Authors have already reported the preparation of carbonaceous spheres from commercial pitches by a suspension method[ 141. In the present study, the suspension of pitches or their fractions was examined in water or other media with the aid of non-ionic polymer suspension agents to prepare carbonaceous spheres (beads). Maximum temperature allowed to the polymer suspension agent (polyvinylalcohol: Mw = 14,000, 100% hydrolysed) was also investigated to find the relevant surface active agent for the suspension of the whole mesophase pitch. Examples of controlling the diameters of spheres were also reported.

Carbonaceous mesophase spheres, which have been recognized as the intermediate stage in carbonization leading to anisotropic coke[ I], have been claimed to be excellent raw materials for column packing in liquid chromatography[2,3,4], and for making high-density carbon materials with high tensile strength[ 51. The carbonaceous spheres have been conventionally prepared by extraction from pitches containing mesophase spheres[6,7,8], or by carbonization of pitch or polyvinylchloride with polyethylene under high pressure[9,10,11]. Neither method is satisfactory because of the low yields and accordingly, unacceptably high expences. The authors[ 121 have reported a selective method of preparing spheres from commercial pitches. The procedure was carbonization under vacuum, and introducing many nuclei for spheres by precipitation through reduction of solubility by cooling. Addition of poor solvent for a pitch may enhance such precipitation by introducing an increased number of spheres. Such a procedure appears similar to a suspension method widely used in the preparation of polymer microbeads in the polymer industry. Recently, Honda et a/.[ 131 reported an emulsion

2. EXPERIMENTAL

2.1 Materials Some properties of starting pitches are summarized in Table 1. A petroleum-derived mesophase pitch (PP) and a coal-tar derived isotropic pitch (IP)

Table I. Some analytical properties of raw pitches Solubilities (wt%) Code

Raw material

ACa(vol%)

PP IP

FCC-DO coal-tar

100 0

SPb(OC) THFS 256 258

aAnisotropic contents. bSoftening point. 781

42 65

THFI-QS

QI

H/C

47 25

I1 10

0.55 0.58

782

S.-H. YOON et al. Table 2. Conditions of sus~nsion

of pitches or pitch solutions

Materials Suspension medium (vol%)

Conditions

Code

Raw material

Suspension agent WW

s- 1 s-2 s-3 s-4

THFS THFS THFS THFS

PVA(0.5) PVA( 1.O) PVA(2.0) PVA(3.0)

water water water water

THF THF THF THF

1.0 1.0 1.0 1.0

25-85” 25-85 25-85 25-85

s-5 s-6

THFS THFS

PVA( 1.O) PVA( 1.O)

water water

THF THF

2.0 4.0

25-85 25-85

1

30-30 30-30

s-7

QS

MeOH

quinoline

10.0

25

1

30

s-8

QS

T&on X-100(0.5) X-100(0.5)

quinoline

10.0

25

I

30

s-9

QS

X-100(0.5)

MeOH/water 50:50(v01/v01) water

quinoline

10.0

s-10 s-11

PP IP

PVA( I .O) PVA( 1.O)

glycerine glycerine

Concentration of solution (wt%)

Solvent of solution

Temperature (“C)

Pressure of reaction (k/cm*)

1 1 I 1 1

Reaction time (min.) 30-30” 30-30 30-30 30-30

25

1

30

310 290

30 30

30 30

“After treatment at 25°C for 30 min followed by the treatment at 85°C for 30 min.

were used. The samples were fractionated by solvent extraction with tetrahydrofuran (THF) and quinoline (Q). Commercially available polyvinylalcohol (Mw = 14,000, 100% hydrolyzed; PVA) and Triton X-100@ were used as suspension agents. The solutions of THF soiuble in THF and Q-soluble fractions in quinoline were abbreviated as THFSo and QSo, respectively. Distilled water (resistivity: above IO6 n . cm--‘) and commercially available methanol were used without further purification.

Original pitches without fractionation were suspended in glycerine containing 1 wt% of PVA in an autoclave equipped with a four-bladed turbo-type agitator. After agitation for 30 min, the spheres were filtered. Details of experimental procedures are summarized in Table 2. 2.3 Stab~l~zat~u~ ofp~tch spheres Table 3 summarizes the conditions of oxidative stabilization of the spheres. Pitch spheres were stabilized in air at 300°C at a heating rate of l”C/min or S”C/min, according to the melting point and reactivity of the pitch used. Spheres were also stabilized in 30% HNO, aqueous solution at 50°C for 1 h.

2.2 Preparative of ~arbo~a~eo~sspheres THFS dissolved in THF was poured into a PVA solution of water (0.5 - 3.0 wt%) in a Pyrex flask equipped with a four-bladed turbo-screw. The mixture was agitated at 1500 rpm for 30 min, and then was heated to 85°C to remove THF from the water. Suspended pitch spheres were filtered with a G-4 glass filter and rinsed twice with methanol. QS dissolved in quinoline was also poured into distilled water, methanol, or water-methanol mixture. All the media contained 0.5 wt% Triton X-100@ of a suspension agent. After 30 min agitation, the spheres were filtered.

Stabilized spheres were moulded into a disk under 400 kg . f/cm* at room temperature and carbonized at 1000°C for 1 h at the heating rate of S”C/min. Shapes and diameters of carbonized spheres were observed with a Jeol-JSM 25s scanning electron microscope (SEM).

Table 3. Stabilization conditions of the pitch spheres

~~

__

Code

Heating rate (Qmin.)

s- I s-2 s-3 s-4 s-5 s-6 S-7 s-8 s-9 s-10 s-l 1

2.4 carbonization and characterization of stabilized spheres

Temperature (“C)

Time (min.)

Atmosphere

1 I

300 300 300 300

30 30 ::

1

::

30 33:

air air air air air air air air air air 30 wt% HN03

I 1

1 : 55 -

300 300 300 270 50

:: 30 60

Preparation of carbonaceous spheres from suspensions of pitch materials

Fig. 1. SEM photographs

783

of carbonaceous spheres derived from THFS of PP. (a) s-l, (b) s-2, (c) s-3, (d) s-4 (experimental conditions, see Table 2).

3. RESULTS

3.1 Carbonaceous spheresfrom THFS Figure I shows the SEM photographs of carbonaceous spheres prepared from THFS of PP. The concentration of PVA influenced the shapes and diameters as shown in Fig. 1. Spheres prepared with 1 wt%

PVA, as shown in Fig. I b, exhibited relatively narrow distributions of diameter of ca. 2 pm. However, as shown in Fig. la, spheres prepared in the medium of 0.5 wt% PVA appeared to have some coagulum due to the instability of the suspension system. When the concentration of PVA increased to 2 and 3 wt%, the

Fig. 2. SEM photographs ofcarbonaceous

excess PVA was precipitated on the surface of the carbonaceous spheres, as shown in Figs. lc and d. Figure 2 shows the effects of THFS concentration on the size of spheres. Average diameter of spheres increased with an increase of concentration of the THFS in the solvent.

3.2 Carbonaceous microparticlesfrom QS SEM photographs of carbonaceous particles obtained from QS in various suspension media are shown in Fig. 3. Carbonaceous particles obtained from QS appeared much smaller in diameter (ca. 300

spheres derived from THFS of PP. (a) s-5, (b) s-6 (experimental conditions, see Table 2).

784

S.-H. YOONet al.

nm), and were coagulated into irregular flakes. The particles precipitated appeared relatively fine in comparison with the spheres from THFS. The extent of coagulation was strongly affected by the suspension media. Figure 3a shows carbonaceous flakes that were prepared using water as a suspension medium for flocculated coagulum. This may indicate that collapsed spheres were formed from the flake-like materials because of the removal of the residual quinoline in the suspended pitch. Though methanol

extracted the quinoline more rapidly than water, it also failed to suppress the aggregations of spheres (Fig. 3~). This failure in formation of the separated spheres may be caused by too high an extraction rate of alcohol. The rapid extraction of quinoline with methanol would solidify the surfaces of the suspended spheres. The solidified surface would prevent the diffusion ofquinoline in the particles into the medium. A mixture of water and methanol (50:50 ~01%) allowed the Ieast extent of coagulation, as shown in Fig. 3b.

3.3 Carbonaceous spheres from whole PP and IP Carbonaceous spheres prepared from PP in glycerine at 310°C exhibited irregular shapes of large sizes, as shown in Fig. 4. The decomposition of PVA would introduce poor suspension and result in the formation of the irregular shapes. However, uniform circular-shaped spheres were obtained from IP in glycerine at 290°C. Thus, PVA was found to be app&able up to 290°C as a suitable surface active agent.

Fig. 3. SEM photographs of carbonaceous particles derived from QS of PP. (a) s-7, (b) s-8, (c) s-9 (ex~~rnen~l conditions, see Table 2).

Fig. 4. SEM photographs of carbonaceous spheres derived from PP and IP pitches. (a) s-10, (b) s- I t (experimental conditions, see Table 2).

785

Preparation of carbonaceous spheres from suspensions of pitch materials

3.4 Stabilization and carbonization ofpitch spheres The spheres prepared from THFS were stabilized at 300°C for 30 min in air. The heating rate should be slower than l”C/min to avoid their adhesion. Particles prepared from QS showed higher oxidation activity than those from THFS, the heating rate being SC/min. Mesophase pitch spheres prepared from the whole PP were stabilized at 270°C for 30 min in air at the heating rate of S”C/min. However, spheres obtained from IP were stabilized in 30 wt% HN03 to avoid some difficulties appearing in air. The condition was 50°C for 1 h. Figure 5 shows an SEM photograph of the fractured surface of high-density carbon material prepared from stabilized QS spheres. The artifact exhibited a bulk density of 1.76 g/cm3 and compressive strength of 1.96 Ton/cm’ after carbonization at 1000°C for 1 h. No crack was observable in the fractured surface under SEM, indicating dense packing, sufficient adhesion, and uniform shrinkage of the spheres. 4. DISCUSSION

In the present study, there were satisfactory results in preparing carbonaceous spheres of uniform shape and narrow distribution in diameter. The carbonaceous spheres are stabilized in air or in oxidizing medium to be moulded. They were carbonized into a carbon artifact showing high density and high compressive strength, which are comparable to those found in the literature] 15,161 The mechanism of sphere formation may be valuable for discussion. Figure 6 illustrates a schematic mechanism of sphere formation through the suspension. Pitch or its fractioned portion dissolved in the solvent are poured to be suspended in water or in mixed medium, which can extract the dissolving sol-

Fig. 5. SEM photograph of fractured section of carbon artifact prepared with stabilized particles derived from QS of

PP.

suspension medium

suspension agent

i_i

i, .dispersion of solution droplet

2.solwnt diffusion into suspension media GSicle solidification

( carbonaceous spheres)

Fig. 6. Schematic diagram of sphere formation mechanism in a pitch suspension system.

vent, but hardly dissolve the pitch materials. The pitch material in the suspended droplet is solidified through the extraction of solvent or solvent-like components in the pitch itself by using water or added methanol. After solidification of the pitch materials, the solvent in water can be removed by distillation or by employment of other means to reduce the dissolving ability of medium containing the solvent extracted. The reduction in solubility of mixed media will prevent adhesion. Particle size of the sphere can be controlled by varying pitch concentration of the suspended droplet. Both are easily ~ontrollabIe by the extent of agitation and concentration of the poured solution. More severe agitation and lower concentration provide smaller diameters. The separation of the extracted solvent from the suspension medium is a key to fixing the shape ofthe spheres. THF is easily separated by distillation, providing independent spheres of narrow distribution of diameters. The larger diameter seems to result from a relatively high diffusion rate of THF into the suspension medium. Reorganization of the pitch molecules could be produced during the extraction of THF at room temperature, allowing the formation of perfect spheres even for the THFS, only a limited portion of the pitch. Quinoline is very difficult to remove from water; hence the spheres derived from QS tend to adhere, allowing formation of flake-like materials. The low solubility and high softening point of QS tend to solidify the surface of particles at an early stage, giving smaller spheres. Quinoline in the suspension medium or remaining in the particles causes

786

S.-H. YOON et al.

the aggregation. Efficient extraction of quinoline from the quinoline-water mixed medium suppresses the aggregation. High tem~rature is required to removequinoiine from suspension medium. It issometimes a tough problem to avoid the decomposition of suspension aid polymers. The decomposition makes the polymer lose the function of the suspension, and the degraded polymer precipitates on the surface of the spheres. The specific polymer of PVA was found to be applicable up to 290°C although its decomposition temperature is known under 250°C. A simple and efficient procedure for the preparation of carbonaceous spheres is described here, assuring high yield and controlling the shape and diameter. These results can be applicable to practical purposes.

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