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Energy & Fuels 1993, 7, 334-336
Hydrogenation of Coal Tar Pitch with Tritiated Hydrogen Catalyzed by Metal Carbonyl Complexes. Estimation of Hydrogen Mobility of Coal Tar Pitch in Catalytic Systems Atsushi Ishihara, Xiangsheng Wang, Hiroaki Shone,? and Toshiaki Kabe' Department of Chemical Engineering, Tokyo University of Agriculture and Technology, Nakamachi, Koganei, Tokyo 184, Japan, and Mineral Fiber Research Laboratory, Nitto Boseki Co. Ltd., Higashi Gonome, Fukushima, Fukushima 960, Japan Received May 11, 1992. Revised Manuscript Received December 7, 1992 Hydrogenation of coal tar pitch, which has been used as one of precursors of carbon fiber, has been considered to be an effective and important method for modification of the carbonization characteristics and the fluidity of pitch. Although the hydrogenation of coal tar pitch using a Ni-Mo/AlzOs catalyst' and lithium in ethylenediamine2 has been investigated, there were some unsolved problems that it is very inconvenient to separate the catalyst from the hydrogenated pitch when a solid catalyst was used and that the yield of pitch would decrease due to hydrocracking when molybdenum-based alumina catalysts were used. On the other hand, it has been well-known that transition metal complexes such as metal carbonyls serve as highly-dispersed catalysts effective for coal liquefaction.- However,there has been no examplewhere such metal carbonyls were used as catalysts for the hydrogenation of coal tar pitch. We have already reported the coal liquefaction6,' and the hydrogenation and carbonization of pitchsvg using a tritium tracer method which was effective for tracing quantitatively the mobility of hydrogen in coal or pitch. In this Communication,we investigated the abilityof metal carbonyl complexes for the hydrogenation of pitch by estimating hydrogen mobility of pitch in catalytic systems.. It was found that, in the hydrogen addition and exchange reactions of coal tar pitch with tritiated molecular hydrogen, ruthenium carbonyl showed the highest catalytic activity among transition metal complexes (Mo(CO)~, Fes(CO)12, R u ~ ( C O ) ~and Z , Ru(acetylacetonate)3) and molybdenum-based supported catalysts. The pitch used in this study was a coal tar pitch supplied from Nitto Boseki Co. Ltd. Data for its elemental analysis and solvent fractionation are shown in Table I. All metal carbonyl complexes used were commercial reagents supplied from Strem Chemicals. Commercial Ni-Mo/AlZOs catalyst (Moos, 15.8 wt %; NiO, 2.9 w t %; and specific surface area, 255 m2/g)and Co-Mo/A1203 catalyst (Moos, 12.3 wt 5% ; COO,3.8 wt 5% ; and specific surface area, 274 (1) Mochida, I.; Ueno, I.; Korai, Y. J. Jpn. Pet. Inst. 1981, 30, 31. (2) Yamada, Y.; Shiraishi, M.;Furuta, T. Bull. Chem. SOC.Jpn. 1984, 57, 3027. (3) Watanabe, Y.: Yamada, 0.. Fuiita, K.: Takeaami, - Y.; Suzuki, T. Fuel 1984, 63, 752. (4) Suzuki,T.;Ando,T.;Yamada,O.; Watanabe,Y. Fuel 1986,65,786. (5) Hirschon. A. S.:Wilson. R. B.: Laine. R. M. PreDr.-Am. Chem. Soc.,'Diu. Pet. Chem.'1987,32, 269.' (6) Kabe, T.;Horimatau,T.;Ishihara, A.; Kameyama, H.; Yamamoto, K. Energy Fuels 1991, 5, 459. (7) Kabe, T.; Ishihara, A.; Daita, Y. Ind. Eng. Chem. Res. 1991, 30, 1755. (8)Okuyama, S.;Shono, H.; Ishihara, A.; Kabe, T. J.Jpn. Pet. Inst. 1990, 33, 181. (9) Wang, X . ; Matsumoto, M.; Shono, H.; Ishihara, A.; Kabe, T. J. Jpn. Pet. Inst. 1991, 34, 314.
Table I. Elemental Analysis and Solvent Fractionation of Raw Pitch elemental analysis ( % ) solvent fractionation (wt %) C H N O(difOa HS HIS-BS BIS-THFS THFIS 93.6 4.5 0.9 1.0 15 52 6 27
Sulfur content, which was determined by XPS, was less than 0.3%.
m2/g) supplied from Nippon Ketjen Co. Ltd., were dried at 450 "C for 2 h and used directly without grinding. The hydrogenation of coal tar pitch was carried out in a 350mL stainless autoclave with a glass liner equipped with a mechanical stirrer assembly. After coal tar pitch (30 g) and a given amount of catalyst were added into the autoclave and the atmosphere was immediately substituted for gaseous hydrogen, tritium-labeled hydrogen a t an initial pressure of 60 kg/cm2 was charged, heated a t a heating rate of 10 "C/min with stirring (500 rpm) and maintained at reaction temperature 350 "C for 120 min. The initial radioactivity in the gas phase was controlled at about 1 000 000 dpm. After the reaction, the gas was analyzed by an on-line gas chromatograph, and a portion was burned to measure the radioactivity of water produced. Solid and liquid products were oxidized to water to measure their radioactivities. The radioactivity was measured with a liquid scintillation counter. The reaction mixture was separated by distillation and extraction. In this study, naphtha and light oil were the distillate fractions of under 200 and 200-350 "C, respectively. Hydrogenated pitch represents the distillation residue which was separated into hexane-soluble (HS), hexane-insoluble benzene-soluble (HIS-BS), benzeneinsoluble tetrahydrofuran-soluble (BIS-THFS), and tetrahydrofuran-insoluble (THFIS) fractions by the usual solvent fractionation methods. The hydrogen transferred from gas phase to coal tar pitch was classified into the hydrogen addition and hydrogen exchange. The amount of hydrogen added was equal to the amount of decrease of hydrogen in the gas phase after the reaction. The amount of hydrogen exchanged between gas phase and pitch was calculated on the basis of eq 1, in which the extent of hydrogen exchange Hex
=
Hini&pitch %tal
- Hadd
between gaseous hydrogen and hydrogenated pitch is calculated by subtracting the amount of hydrogen, which was added to pitch directly from the gas phase, from the amount of hydrogen corresponding to the amounts of tritium transferred to pitch. In eq 1, He, is the amount
0887-0624/93/2507-0334$04.00/0 0 1993 American Chemical Society
Energy & Fuels, Vol. 7, No. 2, 1993 336
Communications
Table 11. Effects of Various Catalysts on Hydrogenation of Coal Tar Pitche product distribution (wt %) hydrogenated pitchd run 1 1 3 4 5 6 7
catalyst* none Ni-Mo/Al203 (2.06 g) Co-Mo/AlzOs (1.95 g) FedC0)lz (0.51 g) Mo(CO)G(0.7 g) R ~ d C 0 ) i z(0.089) Ru(acac)3(0.15 g)f
gas
1.0 1.3 0.9 1.2 0.8 1.7 3.8
liquidc 9 23 17 14 11 13 16
HS 15 5 5 6 5 8 2
HIS-BS 30 37 44 48 56 54 55
BIS-THFS 3 14 10 10 5 9 7
hydrogen transferred THFIS 42 20 22 21 22 15 16
Hadd(Aadd) 0.15 0.32 (57) 0.29 (47) 0.32 (56) 0.30 (57) 0.42 (730) 0.35 (540)
Hex (Aex) 0.17 0.37 (67) 0.35 (60) 0.25 (27) 0.30 (49) 0.41 (649) 0.29 (324)
0 Pitch 30 g, 350 "C, 120 min. b Amounta of catalysts are given in the parentheses. Naphtha (350 "C). e Hsdd and Hexhave been explained in text. Aadd: Specific activity of catalyst based on hydrogen addition (g/mol); Aex: specific activity of catalyst based on hydrogen exchange (g/mol); specific activities are given in parentheses and calculated by the following equation; specific activity = (Heat - HJ Wcat,where Hcatis amount of hydrogen transferred with catalyst (g) (Haddor Hex),Hnois amount of hydrogen transferred without catalyst (Haddor He, in run l),and Wcat is amount of active metals in catalyst (mol). facac = acetylacetonate.
of hydrogen exchanged between gas phase and pitch(g); Hinitis the initial amount of hydrogen in the gas phase(g); Hadd is the amount of hydrogen added from gas phase to pitch(g); &,itch is the radioactivity in pitch and liquid products (dpm); R ~ isMthe total radioactivity in a system (initial radioactivity in gas phase). The XPS spectra of tetrahydrofuran-insoluble fractions (THFIS) of hydrogenated pitches were measured by using a Shimadzu ESCA-850 with a Mg KCYray. Etching, i.e., sputtering by argon ion, was carried out by an ion gun equipped with ESCA-850 (condition: 2 kV, 20 mA). The binding energies (BE) were referenced to the 0 1s band at 531.6 eV due to oxygen in pitch. The results for the hydrogenation of coal tar pitch using molybdenum-based supported catalysts and several organometalliccomplexes are summarized in Table 11. When supported catalysts were used (Ni + Mo = 3.0 mmol in run 2; Co + Mo = 2.7 mmol in run mmol in run 3), the amounts of hydrogen added from gas phase to pitch and exchanged between gas phase and pitch increased as compared with those in the absence of catalyst (run 1). Although THFIS fraction decreased, the hydrocracking into lighter fractions such as gas, liquid, and HS decreased yields of HIS-BS and BIS-THFS fractions which are important for the carbon fiber production. On the contrary, with the use of Fe3(C0)12 (Fe = 3.0 mmol in run 4) and Mo(CO)6 (Mo = 2.7 in run 5), the cracking into the lighter fractions was inhibited while the amounts of hydrogen transferred were similar to those of supported catalysts. The most active organometallic complex for hydrogen transfer from gas phase to pitch was found to be Rus(C0)ln (run 6). When 0.37 mmol of ruthenium was added to pitch, the amounts of hydrogen added and exchanged were 0.42 and 0.41 g, respectively. The use of Ru&0)12 gave lower yields of light fractions and higher yield of HIS-BS fraction than supported catalysts. From these results, it is suggestedthat metalcarbonyls, especially Ru3(CO)12, give higher hydrogenation activity and lower hydrocracking activity relative to the commercially available molybdenum-based supported catalysts. The specific activities (seethe footnote in Table I) of hydrogen addition (A& and hydrogen exchange (Aex)for Rus(CO)12 were 730 and 649,respectively,which were about 10times higher than those of the molybdenum-based supported catalysts. When Ru(acac)s was used (Ru = 0.37 mmol in run 7), the yields of gas and liquid fractions were higher than those of metal carbonyls. However, Aadd and A,, for Ru(acac)s were 540 and 324, respectively, indicating that ruthenium has higher catalytic activity for hydrogen transfer than iron and molybdenum.
1
E.T 0.0
I
J
724
712 Binding Energy/eV
I
\
700
Figure 1. Fe 2p spectra of THFIS from run 4 at various etching times.
I
210
I
230 220 Binding Energy / eV
Figure 2. Mo 3d spectrum of THFIS from run 5 at 7.5 min of etching.
X-ray photoelectron spectroscopy (XPS) has been utilized to determine the state of active metal remained in pitch after the hydrogenation. In Ni-Mo/AlzOa and Co-Mo/AlzOa catalysts picked out after the reaction, the peaks of molybdenum were observed at 229.0 (Mo 3d5p) and 231.8 (Mo 3d3p) eV. This indicates that Mo(V1) in Ni-Mo/AlaOs and Co-Mo/AlzOs catalysts were reduced to Mo(I1)-Mo(1V)during hydrogenation of pitch. It seems that these supported catalysts have higher catalytic activity for hydrocracking than catalysts derived from metal carbonyls. On the other hand, metal (0)species were detected when metal carbonyls were used. Figure 1shows the Fe 2~312spectra of THFIS in run 4. Although the pitch surface did not show the Fe spectrum, the characteristic peak of iron appeared at over 2.5 min of etching time. The binding energies observed were 706.9 and 720.0 eV, which are very close to that of Fe(0) (706.6 and 719.8 eV), suggesting that Fe(0) particles were produced from Fes(C0)12during the hydrogenation. Figure 2 shows the Mo 3d spectrum of THFIS in run 5 at etching time of 7.5 min. The characteristic peaks for Mo(0) species were observed at 230.8 (Mo d3p) and 227.4 (Mo d5p) eV. Although we have not obtained enough XPS data of
336 Energy &Fuels, Vol. 7,No. 2, 1993
ruthenium to discuss because of low content of ruthenium, it can be expected that low valent ruthenium specieswould be formed from Ru3(C0)12. In the case of Ru(acac)s, somewhat different species may have been obtained because the product distribution was slightly different from that with the use of RuQ(CO)IP. When IR spectra for pitch products were measured, no absorption assigned to carbonyl stretching in metal carbonyls was observed, indicating that metal carbonyls have been completely decomposed to metal particles. When XRD for THFIS of pitch hydrogenated by metal particles was measured, no peaks were observed, indicating that highly dispersed metal particles were formed from
Communications those complexes. These metal particles derived from metal carbonyls seems to be active for hydrogenation of aromatic rings in pitch rather than hydrocracking since the use of metal carbonyls inhibited the formation of light fractions. In summary, metal particles derived from ruthenium complexes, especially Ru~(CO)IZ, were found to be more active for the hydrogen transfer (hydrogen addition and exchange reactions) from gas phase to pitch than those from other metal complexes and the molybdenum-based supported catalysts. The use of metal carbonyls such as Mo(CO)6, Fe3(C0)12, and Ru&O)12 increased the yields of HS, HIS-BS, and BIS-THFS fractions and inhibited the hydrocracking of pitch to gas and liquid fractions.
