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Ind. Eng. Chem. Res. 1997, 36, 306-309
Iron-Based Catalysts Supported on Carbon Nanoparticles of Hollow Structure for Coal Liquefaction Kinya Sakanishi,* Hideki Taniguchi, Haru-umi Hasuo, and Isao Mochida Institute of Advanced Material Study, Kyushu University 86, Kasuga, Fukuoka 816, Japan
Recent studies have shown that NiMo catalysts supported on carbon nanoparticles (Ketjen Black: KB) are very effective for the conversion of low-rank coals to oil; however, because of the high cost of Mo, lower cost alternative metals are desired. This study describes the preparation and use of iron-based catalysts having performances very similar to those of NiMo/KB-based catalysts. The iron-based catalysts were prepared by impregnating soluble iron salts onto KB. Coal liquefaction studies were conducted at 450 °C, 60 min, 15 MPa H2 pressure, using solvent (tetralin)/coal/catalyst weight ratios of 1.5/1/0.03. Of the various iron salts investigated, catalysts prepared from iron(II) fumarate (Fe 15 wt % supported on KB) gave the highest oil yield (60%) in the liquefaction of Tanitoharum coal. This yield was similar to that obtained in experiments using 12 times as much Fe in the form of synthetic iron pyrite but was still less effective than the NiMo/KB catalyst. Using 3 times as much Fe fumarate/KB catalyst increased the oil yield to ca. 70% which was comparable to that achieved by NiMo/KB catalyst. Addition of Mo (2 wt %) or Ni (10 wt %) to the Fe/KB catalyst increased the oil yield to ca. 70% with the same weight of catalyst. Comparisons of the hydrogenation and hydrocracking activities of Fe-based KB catalysts and NiMo/KB catalysts for the conversion of coal macromolecules are discussed. Introduction Coal liquefaction to produce synthetic crudes is anticipated to be necessary early in the next century in some particular countries as the supply of conventional petroleum crudes will become limited and the worldwide demand for crude is continually growing in the developing countries including China and India because of their drastic motorization. However, the cost of the coal liquefaction processes developed so far is still too high for commercialization. To lower these costs, improvements are needed in the yield of oil from coal, the quality of distillate products, catalyst activity and stability for recycle, the amount of solvent required, and the use of milder reaction conditions (Zhang et al., 1995; Cugini et al., 1994; Mochida and Sakanishi, 1994; Mochida et al., 1986, 1994). The present authors have proposed the use of NiMo catalysts supported on a particular carbon black of Ketjen Black (KB) which consists of very fine hollow particles, around 30 nm, which provides a large surface for supporting Ni and Mo. These catalysts, being hollow, have a very low specific gravity and can potentially be recovered from the coal liquefaction products by gravimetric separation methods (Sakanishi et al., 1995, 1996a). Such catalysts were found to have excellent activity for the liquefaction of subbituminous coals and were indeed separable by flotation. Effective dispersion in slurry in the liquefaction reactor was achieved by high stirring speeds, and oil yields higher than 70% were obtained (Sakanishi et al., 1996b). Such catalysts, though appearing promising, still require high efficiency for catalyst recovery, and recycle as any loss of the catalyst even in a small quantity may increase the cost of the overall process. Iron-based catalysts were preferentially developed and utilized in the Bergius process in the very early coal liquefaction processes even though they were known to have lower catalytic activities than those of NiMo/Al2O3. This choice was made because of the limited availability and high cost of Mo, and iron-based catalysts were cheap S0888-5885(96)00319-3 CCC: $14.00
enough to be considered disposable in spite of their lower catalytic activity (Bergius, 1920; Varga, 1928; Wurtel, 1979; Yokoyama et al., 1968; Bammannavar and Montano, 1982, 1983). Pyrrhotite has been reported to be an active species (Montano and Granoff, 1980; Baldwin and Vinciguerra, 1983; Stenberg et al., 1983). Grinding it to finer particles, impregnation of iron species directly into coal macromolecules, preparation of nanoparticle iron catalysts, and combination use of iron- and Mo-based catalysts have been reported to increase the activity of the iron-based catalysts (Andres et al., 1983; Bacaud, 1991; Cugini et al., 1991; Taghiei et al., 1993; Eklund et al., 1991; Suzuki et al., 1989; Herrick et al., 1990; Cebolla et al., 1994; Ades et al., 1994). In the present study, iron-based catalysts were supported on carbon nanoparticles in an attempt to provide a high dispersion of active iron species which should exhibit higher liquefaction yields with a smaller amount of iron than fine pyrite particles and in addition provide a means of more facile catalyst recovery. Other metals such as Ni and Mo were also coimpregnated with Fe on Ketjen Black to promote the hydrogenation and hydrocracking activities to give higher yields of liquid products. Experimental Section Catalysts and Materials. Some properties of Ketjen Black (KB) carbon nanoparticles are shown in Table 1. Three kinds of iron salts of iron(II) fumarate (Fe-FM), iron(III) acetylacetonate (Fe-AA), and iron(II) sulfate (Fe-SL) were used as iron precursors in the present study. The catalysts prepared in the present study are listed in Table 2. The prescribed amount of KB was dispersed in methanol by ultrasonic irradiation, and an iron salt dissolved in methanol was added to the KBmethanol slurry with a small amount of nitric acid (1 vol %) as an additive. After the iron salt was completely dissolved, the slurry was well dispersed by ultrasonic irradiation, the slurry was then dried to obtain iron supported on KB. Combination metal catalysts, Fe/Mo, © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 307 Table 1. Some Properties of Ketjen Black KB EC600JD surface area (m2/g) volatile matter (%) pH particle size (nm) apparent density (g/L) ash (%) Ni (ppm) V (ppm) Fe (ppm) Cu (ppm) Mn (ppm)
1270 0.7 9.0 30 115 0.1 1.5 30 100 1 1
Table 2. Catalysts and Salts catalystsa
Fe salts
1. Fe(15)/KB 2. Fe(15)/KB 3. Fe(15)/KB 4. Fe(10)/KB 5. Fe(5)/KB 6. Ni(2)Fe(10)/KB 7. Ni(5)Fe(10)/KB 8. Ni(10)Fe(10)/KB 9. Mo(2)Fe(10)/KB 10. Ni(2)Mo(10)/KB 11. synthetic pyrite
Fe(III)-AA FeSO4 FeC4H2O4 FeC4H2O4 FeC4H2O4 FeC4H2O4 FeC4H2O4 FeC4H2O4 FeC4H2O4
a
Ni salt
Mo salt Figure 1. Effect of iron salts of Fe/KB catalysts on the liquefaction of Tanitoharum coal. Reaction conditions: catalyst, iron precursors FeC4H2O4, Fe(III)-AA, FeSO4; catalyst/solvent/coal ) 0.1/4.5/3.0; reaction temp, 450 °C; reaction pressure, 15 MPa; reaction time, 60 min; heating rate, 20 °C/min; stirring speed, 1300 rpm.
Ni(NO2)3 Ni(NO2)3 Ni(NO2)3 Ni(OAc)2
MoO2(II)-AA MoO2(II)-AA
FeS2
Numbers in parentheses indicate the weight percent of metals.
Table 3. Elemental Analyses of Tanitoharum Coal wt %, d.a.f. C
H
N
S
Oa
ash (wt %)
71.6
5.6
1.6
0.23
21.0
3.9
a
Difference.
Fe/Ni, and Ni/Mo-supported KB catalysts, were prepared by simultaneous impregnation of the two salts using methanol as the solvent: Fe-FM and molybdenum dioxyacetylacetonate (MoO2-AA), Fe-FM and Ni(NO3)2 and (Ni-Nt), and Ni-Ac and MoO2-AA, respectively. The catalyst precursors were dried at 120 °C for 12 h in vacuo. The catalysts were presulfided in flowing 5% H2S/H2 at 360 °C for 2 h prior to the reaction. Synthetic pyrite powder provided by NEDO (New Energy and Industrial Technology Development Organization, Japan) was used as a reference catalyst for comparison. The elemental analysis of Tanitoharum coal, an Indonesian subbituminous coal, is summarized in Table 3. Commercially guaranteed grade tetralin (TL) was used as a liquefaction (hydrogen-donating) solvent. Liquefaction Procedure. The liquefaction was carried out using a magnetically-stirred 50 cm3 autoclave at 450 °C. The stirring speed was varied in the range of 500-1300 rpm. The coal (3.0 g), the solvent (4.5 g), and catalyst (0.1 g) were charged into the autoclave, which was then pressurized with hydrogen to 9.3-11 MPa at room temperature after replacing the air with nitrogen gas. The heating rate to the reaction temperature was ca. 20 °C/min. After the reaction, the product remaining in the autoclave was recovered with THF and extracted in sequence with n-hexane, acetone, and THF after the evaporation of THF. The n-hexanesoluble (HS), n-hexane-insoluble-acetone-soluble (HIAcS), acetone-insoluble-THF-soluble (AcI-THFS), and THF-insoluble (THFI) substances were defined as oil (O), asphaltene (A), preasphaltene (PA), and residue (R), respectively. The gas yield (G) was calculated by the difference between weights of the initial raw materials and the recovered products. The reactions under the
Figure 2. Effect of Fe concentrations of the Fe/KB catalyst on the liquefaction of Tanitoharum coal. Reaction conditions: catalysts, Fe(X wt %)/KB, X ) 5, 10, 15; iron precursor, FeC4H2O4; catalyst/solvent/coal ) 0.1/4.5/3.0; reaction temp, 450 °C; reaction pressure, 15 MPa; reaction time, 60 min; heating rate, 20 °C/min; stirring speed, 1300 rpm.
same conditions were repeated three times to minimize the experimental errors. Results Catalytic Activities of Iron Supported on KB. Figure 1 illustrates the liquefaction yields of Tanitoharum coal at 450 °C, 15 MPa H2, 60 min, and 1300 rpm of high stirring speed with Fe (15 wt %)/KB catalysts prepared from different iron salts and synthetic pyrite. The Fe/KB catalyst prepared from iron(II) fumarate gave the highest oil yield (58.3%) among the Fe/KB catalysts. This compares favorably with the oil yield obtained with the synthetic pyrite (59.8%). However, the amount of iron needed in the Fe/KB catalysts was only one-twelfth of the amount of iron in the synthetic pyrite. A high dispersion of iron species supported on KB is suggested. Figure 2 shows the effect of different Fe loadings from iron(II) fumarate catalyst preparations on the liquefaction of Tanitoharum coal. The oil yield gradually decreased from 58.3 to 53.9% with decreasing loading of iron from 15 to 5%. Higher loadings of iron on the KB resulted in the reduced activity per mole of iron; however, the yields did increase with higher amounts of iron. Figure 3 illustrates the liquefaction yields of Tanitoharum coal with different catalyst/coal ratios using Fe (15 wt %)/KB catalysts prepared from iron(II) fumarate.
308 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 Table 4. Hydrogenation of 1-Methylnaphthalenea over Fe/KB and Mo/KB Catalysts catalysts
conv (%)
Fe(15)/KB Ni(2)Fe(10)/KB Mo(2)Fe(10)/KB Ni(2)Mo(10)/KB
2 4 7 50
a Reaction conditions: catalyst, 10 mg; 1-methylnaphthalene, 1 g; decalin (solvent), 9 g; reaction temp, 400 °C; reaction pressure, 9 MPa; reaction time, 60 min.
Figure 3. Effect of added amount of the Fe(15)/KB catalyst on the liquefaction of Tanitoharum coal. Reaction conditions: catalysts, Fe(15)/KB; iron precursor, FeC4H2O4; solvent/coal ) 1.5/1.0; reaction temp, 450 °C; reaction pressure, 15 MPa; reaction time, 60 min; heating rate, 20 °C/min; stirring speed, 1300 rpm.
of oil, again comparable to the yield obtained with the Ni(2)Mo(10)/KB catalyst. Hydrogenation Activity Measurement. Table 4 summarizes the activity of the catalysts examined in the present study for the hydrogenation of 1-methylnaphthalene at 400 °C, 9 MPa H2, and 60 min. The hydrogenation reactions were performed at a lower temperature and a lower pressure, because the hydrogenation conversion at 450 °C was above 90% with a significant amount of hydrocracked products on the NiMo/KB catalyst. NiMo/KB exhibited the highest hydrogenation activity among these catalysts, while all of the Fe-based catalysts exhibited low hydrogenation activities, even those in which Ni or Mo was added. This contrasts with the observed improvement of the liquefaction activity for Fe/KB catalysts with added Ni or Mo. Discussion
Figure 4. Effect of Ni or Mo addition to the Fe(10)/KB catalyst on the liquefaction of Tanitoharum coal. Reaction conditions: catalysts, Fe(10), Ni(2,5,10)Fe(10), Mo(2)Fe(10)/KB; Fe precursor, FeC4H2O4; Ni precursor, Ni(NO3)2; Mo precursor, MoO2-AA; catalyst/solvent/coal ) 0.1/4.5/3.0; reaction temp, 450 °C; reaction pressure, 15 MPa; reaction time, 60 min; heating rate, 20 °C/min; stirring speed, 1300 rpm.
The oil yield significantly increased from 58.3 to 69.2% by increasing the amount of catalyst added from 3.3 to 9.9 wt % based on the coal. It should be noted that the higher amount of added Fe/KB catalyst converted the asphaltene selectively to the oil without increasing the gas yield. This suggests that improved contact of the Fe/KB nanoparticle catalysts with the heavy products in the reaction slurry may enhance the efficient conversion of the asphaltene. Catalytic Activities of Fe/Ni and Fe/Mo Catalysts. Figure 4 shows the effect of Ni or Mo addition to the Fe (10 wt %)/KB catalyst on the liquefaction of Tanitoharum coal. The addition of 2 wt % of Ni or Mo to Fe/KB significantly increased the oil yield, especially Mo(2)Fe(10)/KB catalyst which produced a 67.9% of oil yield, although Mo(2)/KB gave a yield similar to that with the Fe(10)/KB catalyst. It is noted that the oil yield with the Mo(2)Fe(10)/KB catalyst is as high as the yield obtained with Ni(2)Mo(10)/KB catalysts. Figure 4 also illustrates the effect of Ni concentration on the liquefaction yield of Tanitoharum coal at 450 °C, 15 MPa H2, and 60 min for Ni-Fe/KB catalysts. The oil yield increased substantially with higher amounts of added Ni. The Ni(10)Fe(10)/KB catalyst yielded 69.8%
Low-cost Fe-based coal liquefaction catalysts have been recognized to be more feasible than Mo-based catalysts even from the beginnings of coal liquefaction (Bergius, 1920; Varga, 1928; Wurtel, 1979; Yokoyama et al., 1968). However, the inferior catalytic activity of Fe-based catalysts requires a large amount of disposal and reduces the recoverable oil yield. The active species of iron catalysts has been reported to be pyrrhotite (Montano and Granoff, 1980; Baldwin and Vinciguerra, 1983; Stenberg et al., 1983) formed during the coal liquefaction reaction independent of the iron source. Pyrite is one of the major iron catalysts which has been developed in large scale for coal liquefaction because it is effectively converted into pyrrhotite on heating to around 400 °C and has relatively high activity for the liquefaction reaction. Other iron sources such as oxohydroxyiron, iron sulfate, and carbonyl (Andres et al., 1983; Bacaud, 1991; Cugini et al., 1991; Taghiei et al., 1993; Eklund et al., 1991; Suzuki et al., 1989; Herrick et al., 1990) have been investigated in coal liquefaction as highly dispersed or fine particle catalysts; however, their cost is problematic. Repeated use of the catalyst appears essential to reduce the costs associated with manufacture and disposal. The present authors reported that carbon nanoparticles such as Ketjen Black (KB) are promising catalyst supports because they have extremely high surface area and low density, which enables the high dispersion of active metal species on their surfaces and provides a means for facile catalyst recovery, respectively. The demonstration of high activity and a recovery procedure for NiMo/KB catalysts has been previously reported (Sakanishi et al., 1995, 1996a,b). The present study revealed that Fe-based catalyst supported on KB gave much higher oil yield in the liquefaction of Tanitoharum coal than that with synthetic pyrite catalyst for the same number of moles of Fe. This high activity is attributed to the high dispersion of active metals on the KB carbon nanoparticles.
Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 309
Iron(II) fumarate was found to be the best catalyst precursor among the iron salts examined in the present study, probably due to its high solubility in methanol and facile decomposition during the presulfiding treatment which maintains high dispersion. The addition of Ni or Mo to Fe/KB catalyst substantially improved the yield of oil in the liquefaction of Tanitoharum coal, reaching the level in the oil yield comparable to that obtained with NiMo/KB catalysts. On the other hand, the hydrogenation activity for twomembered aromatic rings was quite low with these Febased/KB catalysts and was not enhanced significantly by the addition of Ni or Mo. It is suggested that high hydrogenation activity for two-membered aromatic rings may not be essential in coal liquefaction reactions. hydrogenolysis or hydrogenative capping of the fragments from coal molecules enhanced by the addition of Ni or Mo appears to accelerate oil production without excess gas production. A combination use of Mo- and Fe-based catalysts in the two-stage liquefaction consisting of the hydrogenation at lower temperatures in the first stage and the hydrocracking at higher temperatures in the second stage, respectively, is expected to improve the conversion of the heavy products into light distillates without excess production of hydrocarbon gases. Acknowledgment The authors are very grateful to Dr. D. Duayne Whitehurst for his useful discussion and comments on the paper. Literature Cited Ades, H. F.; Companion, A. L.; Subbaswamy, K. R. Energy Fuels 1994, 8, 71. Andres, M.; Charcosset, H.; Davignon, L.; Mariadassou, G. D.; Joly, J. P.; Pregermain, S. Fuel 1983, 62, 69. Bacaud, R. Fuel Process. Technol. 1991, 28, 203. Baldwin, R. M.; Vinciguerra, S. Fuel 1983, 62, 495. Bammannavar, A. S.; Montano, P. A. Fuel 1982, 61, 1288. Bammannavar, A. S.; Montano, P. A. Fuel 1983, 62, 932.
Bergius, F. English Patent 148436, 1920. Cebolla, V. L.; Diack, M.; Cagniant, D.; Oberson, M.; Bacaud, R.; Nickel-Pepin-Donat, B. Energy Fuels 1994, 8, 77. Cugini, A. V.; Utz, B. R.; Krastman, D.; Hickey, R. F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36 (1), 91. Cugini, A. V.; Krastman, D.; Martello, D. V.; Frommell, E. F.; Wells, A. W.; Holder, G. D. Energy Fuels 1994, 8, 83. Eklund, P. C.; Stencel, J. M.; Bi, X.-X.; Keogh, R. A.; Derbyshire, F. J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36 (2), 351. Herrick, D. E.; Tierney, J. W.; Wender, I.; Huffman, G. P.; Huggins, F. E. Energy Fuels 1990, 4, 213. Mochida, I.; Sakanishi, K. Catalysis in Coal Liquefaction In Advances in Catalysis; Eley, D. D., Pines, H., Haag, W. O., Eds.; Academic Press: New York, 1994; p 39. Mochida, I.; Sakanishi, K.; Korai, Y.; Fujitsu, H. Fuel Process. Technol. 1986, 14, 113. Mochida, I.; Sakanishi, K.; Sakata, R.; Honda, K.; Umezawa, T. Energy Fuels 1994, 8, 25. Montano, P. A.; Granoff, B. Fuel 1980, 59, 213. Sakanishi, K.; Hasuo, H.; Mochida, I.; Okuma, O. Energy Fuels 1995, 9, 995. Sakanishi, K.; Hasuo, H.; Kishino, M.; Mochida, I.; Okuma, O. Energy Fuels 1996a, 10, 21. Sakanishi, K.; Taniguchi, H.; Hasuo, H.; Mochida, I. Energy Fuels 1996b, 10, 260. Stenberg, V. I.; Ogawa, T.; Wilson, W. G.; Miller, D. Fuel 1983, 62, 1478. Suzuki, T.; Yamada, H.; Sears, P. L.; Watanabe, Y. Energy Fuels 1989, 3, 707. Taghiei, M. M.; Huggins, F. E.; Gangly, B.; Huffman, G. P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (1), 149. Varga, J. Brenn. Chem. 1928, 9, 277. Wurtel, H. E. Fuel Process. Technol. 1979, 2, 227. Yokoyama, S.; Yoshida, R.; Narita, H.; Kodaira, K.; Maekawa, Y. Fuel 1968, 65, 164. Zhang, T.; Jacobs, P. D.; Haynes, H. W., Jr.; Swanson, A. J. Fuel 1995, 74, 431.
Received for review June 10, 1996 Revised manuscript received October 7, 1996 Accepted October 24, 1996X IE960319L
X Abstract published in Advance ACS Abstracts, December 1, 1996.
