Ind. Eng. Chem. Res. 1993,32, 1723-1726
1723
Carbonization Behavior of Pitches Containing Fine Molybdenum Particles Atsushi Ishihara, Xiangsheng Wang, Hiroaki Shono,t and Toshiaki Kabe' Department of Chemical Engineering, Faculty of Technology, Tokyo University of Agriculture & Technology, Nakamachi, Koganei City, Tokyo 184, Japan, and Mineral Fiber Research Laboratory, Nitto Boseki Co. Ltd., Higashi Gonome, Fukushima, Fukushima 960,Japan
In the carbonization of coal tar pitch and naphthalene pitch containing fine molybdenum particles, it was found by using a tritium tracer method that the fine molybdenum particles added into pitch in advance can selectively catalyze the dehydrogenation and polycondensation of saturated hydrocarbons below 700 "C,and thereby the carbonization yield of coal tar pitch containing partially saturated structures or aliphatic side chains in its component molecules increased. Further, it could be inferred that the fine molybdenum particles mainly accelerate the release of the hydrogen which is difficult to isotopically exchange - with water in the preparation of tritiated pitch in the presence of Pt/A1203.
Introduction It has been well-known that carbonization behavior of pitches is influenced by the presence of particulate matters in the pitches, such as carbon blacks, natural graphite and mica, carbon felt, and silica (Forrest and Marsh, 1983). Kuo et al. reported that particulate matters (diameter < 1pm) in pitch significantly influenced the carbonization behavior and reduced the size of the optical texture in resultant coke (Kuo et al., 1987). Further, it has been clarified that most of the elements in the periodic table are effective as a catalyst on graphitization of carbon (Oya, 1980; Ishikawa et al., 1965). However, few works have been done to investigate the effect of such metal elements or compounds on dehydrogenation and polycondensation during low-temperature carbonization of heavy hydrocarbons. Recently, we have reported the effects of various catalysts on the hydrotreatment of coal tar pitch for high performance carbon fibers (Wanget al., 1992). The results have showed that fine metal particles derived from metal carbonyls have higher activity for hydrogen transfer from gas phase to pitch molecules and can inhibit the formation of light fraction by hydrocracking, as compared with conventional supported catalysts. In the course of our study, we were interested in what behavior the fine metal particles incorporated into pitches will show in the Carbonization of the pitches, and how they influence the carbonization of the pitches. In the present research, the carbonization of pitches containing fine molybdenum particles has been investigated to estimate the effect of fine metal particles on carbonization and to clarify the mechanism of the process. The tritium tracer technique and elemental analysis were used as major means for this research.
Experimental Section Materials. The materials used in this study were coal tarpitch and naphthalene pitch supplied from Nitto Boseki Co. Ltd., which are all starting materials for highperformance carbon fiber. Their characteristics are listed in Table I. Commercial Mo(C0)a was used as a precursor for the preparation of fine molybdenum particle-containing pitches. Nitrogen (99.99 ?4 ) was supplied from Tohei Chemicals Co. Ltd. + Nitto Boseki Co. Ltd.
Table I. Properties of Pitches. elemental analysis (wt % ) properties 0 FC SP QI TI ash pitch C H N (dim C/H ( w t % ) ("C) (wt%) ( w t % ) ( w t % ) CTP 93.6 4.5 0.9 1.0 1.73 56.4 88.8 9.2 30.7 0.07 NP 95.4 4.0 0.1 0.4 1.99 92.5 239.0 42.62 75.31 30(ppm) a CTP, coal tar pitch; NP, naphthalene pitch; FC, fixed carbon; SP, softening point; &I, quinoline-insolublefraction; TI, tolueneinsoluble fraction. Table 11. Radioactivity Distributions after the Isotope Exchange and Hydrogen Exchange Ratios.
CTP
661 600
41.6
NP 893 800 77.7 Initial radioactivity in water (Amb,) waa 5 816 300 dpm/g. "Radioactivity in pitch after reaction (Ap&. c R a x = (Hmm/ Hp&(Api~Wpi&Am~ W-w), where& = hydrogenexchange ratio (%), H , t , = amount of hydrogen in tritiated water (g), W p i a= weight of pitch (g), H P i =~ amount of hydrogen in pitch(g), and Wwab,= weight of tritiated water (g).
Preparation of Tritium-LabeledPitches. The tritium-labeled pitches were prepared by isotope exchange of pitches with tritiated water catalyzed by Pt/A1203. Thirty g of pitch, 60 g of water, and 2.0 g of Pt/A1203 (Pt 3.0 w t %) were charged into a 350-mL autoclave. The autoclave was replaced with nitrogen and then heated to 350 "C at rate of 10 "C/min and held at this temperature for 3 h. After the reaction, tritiated water was carefully removed, and the pitch was dried under reduced pressure (3 Torr) for 24 h to remove the remaining tritiated water. Since pitch was not dissolved in water, water could be removed with this method. The catalyst was picked out after the reaction. Pitches were analyzed and measured for their elemental compositionsand radioactivities. The tritiated water separated from the reaction mixture was distilled to measure the radioactivity. The experimental radioactivity distribution and the hydrogen exchange ratios calculated from the experimental tritium concentrations in pitches are shown in Table 11. The hydrogen exchange ratio means the ratio of exchangeable hydrogen to total hydrogen in pitch. Pt/&o3 catalyst was prepared by volume impregnation with an aqueous solution of HzPtCLy6Hz0, followed by drying in air at 110 "C (8 h) and calcined at 600 "C (4 h) in flowing nitrogen gas.
0 1993 American Chemical Society 0888-5885/93/2632-1723$04.~0/0
1724 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
Preparation of Pitches Containing Molybdenum Particles. The pitches containing molybdenum particles were prepared by two methods: method 1,by mechanically mixing of pitch with Mo(CO)~in an inert atmosphere at room temperature; method 2, by mixing of pitch and Mo(Cole at 250 "C with an autoclave. In more details, tritiated pitch prepared in the above section and Mo(CO)~ were added into a 350-mL stainless autoclave equipped with a stirrer assembly. The autoclave was charged with nitrogen gas with an initial pressure of 5.9 MPa, and then was heated to 250 "C and held at this temperature for 2 h. It was confirmed by FTIR measurement (Nippon Bunkou FTIR-5300) of the pitch that Mo(CO)6completely decomposed in this procedure. Carbonization Procedure. The carbonization of tritium-labeled pitch with and without molybdenum particles was carried out with a covered vertical quartz tube under a flow of pure nitrogen gas (0.4 L/min). One gram of sample was used in each run. The rate of heating was programmed at 15 "C/min. The period of annealing at each temperature was 1 h. The carbonized products were analyzed and measured for their elemental compositions and radioactivities. Radioactivity Measurements. For the measurement of radioactivities of the products, liquid scintillation counting was applied (Kobayashi and Maudsley, 1974; Horrocks, 1974;Heyden and Son, 1977). Specificactivities of SHcontained in pitches tritiated with water, pitches before and after carbonization, and tritiated water were measured with a liquid scintillation counter according to the same procedures described in a previous paper (Ishihara et al., 1993). Typical procedures were as follows: The pitch was oxidized by an autocombustion system (Aloka ADC-113R) into water to measure the radioactivity. Each water sample (0.1 g) was dissolved into 14mL of a scintillatorreagent (Monophase S; Packard Japan), and the radioactivity of the obtained solution was measured with a liquid scintillation counter (Aloka LSC1050). The elemental analysis was measured with Yanako MT-2. The relative errors for the elemental analysis and radioactivity measurement were 3 7% and 5 % ,respectively. Calculation of Carbonization Yield and Residual Ratios of Hydrogen and Tritium. The carbonization (coke) yield was calculated according to the following equation
where YCarbn is the carbonization (coke) yield, Wbeforeis the weight of pitch containing molybdenum before carbonization (g), R M is~ the ratio of molybdenum in pitch (7% 1, and W&, is the weight of coke containing molybdenum after carbonization (g). In this calculation, it was assumed that the molybdenum particles added into pitch at the preparation of molybdenum-containing pitch did not vaporize. Residual ratio of hydrogen was the ratio of an analytical value (7% H) at each run to that of a starting meterial (7% H). Residual ratio of tritium was the ratio of a tritium concentration of pitch (dpm/g) a t each run to that of a starting material (dpm/g). Results Figure 1shows the effect of content of molybdenum on carbonization yield when molybdenum-containing pitches prepared by the two methods mentioned above were carbonized a t lo00 "C for 1h. In the case of coal tar pitch, when the initial content of molybdenum was 0.6 w t % ,the carbonization yield reached the highest value with two
Naphthalene Pitch
-
1
1
I"
Coal Tar Pitch
u 30 0.0
1.o 2.0 Content of M o (Who)
Figure 1. Effect of content of molybdenum on coke yield of pitches (0, method 1; 0 , method 2).
preparation methods. This result suggeststhat the volatile low molecular hydrocarbons in the coal tar pitch could be catalytically carbonized by fine molybdenum particles. Further, the molybdenum-containing coal tar pitch prepared by method 2 showed higher carbonization yield than one prepared by method 1for all contents of molybdenum. It is considered that since Mo(CO)~begins to decompose at 150 "C and pitch becomes a liquid state at about same temperature, most of the Moparticles were quantitatively incoroporated into pitches in preparation method 2. By contrast, in the case with preparation method 1,because the preparation was carried out at room temperature, a part of Mo(CO)~would not decompose and probably partially vaporize during the following carbonization. However, in the case of naphthalene pitch, the increase of carbonization yield by adding molybdenum was scarcely observed as shown in Figure 1. This would be due to the differences in structure and composition between the two pitches. Further, in order to estimate the effect of fine molybdenum particles on dehydrogenation during Carbonization of pitch, the tritium-labeled coal tar pitch without molybdenum (TCTP) and with molybdenum (Mo-TCTP, Mo 2.1 wt 7% ) prepared by method 2 were carbonized up to lo00 "C with 100 "C intervals. The changes in relative concentration of hydrogen or tritium in the coke as well as carbonization yield with temperature are shown in Figure 2. The change of hydrogen or tritium concentration in the coke with temperature was represented with relative percentage of residual hydrogen or radioactivity in coke, making the concentration of hydrogen or tritium in the starting pitch 100% . The weight loss commenced below 300 "C and terminated at about 700 "C for both TCTP and Mo-TCTP (Figure 2a). The dehydrogenation (Figure 2b) and detritiation (Figure 2c) commenced below 400 "C and continued up to 1000 "C. As compared with the case of TCTP, the Mo-TCTP showed a higher carbonization yield at each temperature. Moreover, the rate (slope of curve) of dehydrogenation in the carbonization of MoTCTP was faster than that in the carbonization of TCTP below 700 "C while it became nearly the same with that of TCTP above 700 "C. It is considered that, in the presence of molybdenum, the carbonization yield would increase because low molecular weight hydrocarbons and saturated branches of aromatics in the coal tar pitch could be catalytically dehydrogenated and polymerized by fine metal particles to produce larger molecules. Further it was suggested that the fine molybdenum particles act as a catalyst promoting selective dehydrogenation below 700 "C during carbonization of coal tar pitch. As regards the rate of detritiation, it showed the same tendency as the dehydrogenation (Figure 2c). However, the promotion effect of molybdenum particles on detritiation was much smaller than that of dehydrogenation. Further, the release of tritium in the cases of both without and with molyb-
Ind. Eng. Chem. Res., Vol. 32, No. 8,1993 1725
"0
"
I
"
'
'
'
1
cI
0
0
"
"
"
200
'
"
"
400 600 800 Temperature (O C)
1000
Figure 2. Changes of hydrogen and tritium concentrations in coke and coke yield with temperature (coal tar pitch).
denum particles was faster than that of hydrogen (Figure 2b,c). The results imply that, during the carbonization of pitch, molybdenum particles would mainly promote the release of the hydrogen which is difficult to exchange with tritiated H2O in the presence of Pt/A1203. In addition, these indicate that the hydrogen exchangeable with tritiated HzO in the presence of Pt/A1203is easier to release independent of molybdenum particles than other hydrogen difficult to exchange with tritiated H2O. The carbonization of tritiated naphthalene pitch was performed without (TNP) and with molybdenum particles (Mo-TNP, Mo 2.1 wt %). The naphthalene pitch containing molybdenum particles was prepared by method 2. The results were quite different from those of coal tar pitch. The increase of carbonizationyield by molybdenum particles was hardly observed, and the promotion effect of molybdenum particles on dehydrogenation was much smaller than that in the case of coal tar pitch.
Discussion It has been generally accepted that a pathway of transformation from a given hydrocarbon to carbonaceous material is indefinite depending on various factors such as the presence or absence of a catalyst, carbonization conditions, etc. In particular, a carbonization reaction pathway of hydrocarbon in the presence of a catalyst could be entirely different from one in the absence of a catalyst. In the present work, it was found that the fine molybdenum particles derived from Mo(C0)s can selectively catalyze the dehydrogenation of coal tar pitch below 700 "C, and hence can enhance the yield of resultant coke. The results can be summarized as follows: (1) The addition of molybdenum increased the carbonization yield of coal tar
pitch (Figure 1 or Figure 2a); (2) the rate of dehydrogenation of coal tar pitch with molybdenum was faster than that without molybdenum below 700 O C (Figure 2b); (3) the rate of detritiation of coal tar pitch was faster than that of dehydrogenation in both the presence and absence of molybdenum, as Figure 2b was compared with Figure 2c; (4)the promotion effect of molybdenum particles on detritiation of coal tar pitch was much smaller than that on dehydrogenation (Figure 2b,c); ( 5 ) the influence of molybdenum on carbonization of naphthalene pitch was much smaller than that on the carbonizationcoal tar pitch, and the rate of detritiation of naphthalene pitch was very close to that of its dehydrogenation as compared with the case of coal tar pitch. On the basis of these findings, we can make several responsible propositions to the carbonization mechanism of coal tar pitch. The average structure of coal tar pitch has been estimated by many workers using various methods, and the result showed that coal tar pitch generally consists of polyaromatic molecules connected by methylene group or phenylene, with various alkyl substitution side chains and naphthene groups (Zander, 1987;Qian and Ling, 1990). In the absence of a catalyst, the carbonization of coal tar pitch startswith the formation of radicals produced by the scission of alkyl side chains or by break of saturated rings. When a catalyst is used, the break of C-C bond could be inhibited and the selective dehydrogenation and polycondensationof saturated rings and alkyl side chains could be accelerated. As compared with coal tar pitch, naphthalene pitch mainly consists of polynuclear aromatics without substituents or saturated rings, and most of the hydrogens in molecules are aromatic. Thus, the influence of molybdenum particles on the carbonization of naphthalene pitch should be much smaller than that on the carbonization of coal tar pitch. For better understanding of the results, Scheme I was illustrated by using 1,2,3,4-tetrahydro-2-methylnaphthalene as a possible model compound. It has been determined by using similar model compoundsthat the location of tritium in the product of isotope exchange reaction catalyzed by platinum catalyst is the benzyl position as well as the aromatic position (Garnett and Kenyon, 1971; Keith and Garnett, 1975;Hodges and Garnett, 1969;Asante and Stock, 1986). Thus, hydrogensat benzyl and aromatic have positions in 1,2,3,4-tetrahydro-2-methylnaphthalene been tritiated in Scheme I. Routes I, 11,and I11 in Scheme I represent dehydrocondensation, cracking, and dehydrogenation, respectively, which are the possible reactions during carbonization of pitches (Alonso et al., 1992). Possible mechanisms are as follows: Molybdenum particles would promote not detritiation but the selective dehydrogenation in route I. This is consistent with the fact that the fine molybdenum particles mainly accelerate the release of the hydrogen which is difficult to isotopically exchange with water in the preparation of tritiated pitch in the presence of Pt/A1203. This leads to result 2. As a result of selective dehydrogenation promoted by molybdenum, route I1was inhibited and hence the carbonization yield of coal tar pitch increased. This can explain result 1. On the other hand, detritiation of coal tar pitch through routes I1 and I11 should proceed independent of molybdenum since these reactions may be easier to occur thermally than those in the route I. This will lead to results 3 and 4 and is consistent with the fact that the hydrogen in pitch exchangeable with tritiated H2O in the presence of Pt/A1203is easier to release independent of molybdenum particles than other hydrogen difficult to exchange with tritiated H2O. Some facts allow us to believe that the molybdenum added into pitches was highly dispersed in the pitch. After the hydrogenation of coal tar pitch using metal carbonyls
1726 Ind. Eng. Chem. Res., Vol. 32, No. 8,1993
Scheme I. Carbonization Mechanisms of Coal Tar Pitch I-
I PH
TH
>
T
1% $1
CH,
Y
HT
T
H
T
H
H(r1
-
H
T
H
T
H
H
T
H
T
H
Further condensation
Aromatics and C,-C5hydrocarbons
H
condensation of naphthene rings and aliphatic chains in pitch. As a result, the carbonization yield of coal tar pitch containing Mo increased in comparison with the carbonization in the absence of molybdenum particles.
Literature Cited Alonso, A. M.; Bermejo, J., Granda, M.; Taacon, J. M. D. Suitability of Thermogravimetry and Differential Thermal Analysis Techniques for Characterization of Pitches. Fuel 1992, 71,611417. Asante, K. 0.; Stock, M. L. A Selective Method for Deuterium Exchange in HydroaromaticCompounds. J. Org. Chem. 1986,51, 5452-5454.
Crook. M., Johnson, P., Eds. Liquid Scintillation Counting; Heyden: London, 1977; Vol. 4. Forrest, M.; Marsh, H. Theoretical and Experimental approaches to the Carbonization of Coal and Coal Blends. Fuel 1983.62612619.
Figure 3. Transmission electron micrograph of molybdenumcontaining coal tar pitch carbonized at 1M)O ‘C.
as catalyst precursor, the sizesof metal particles remained in the hydrogenated pitches have been observed to be
