Energy & Fuels 1992,6, 352-356
352
Highly Active Coal Liquefaction Catalyst: Soluble Ruthenium Complexes as Catalyst Precursors Toshimitsu Suzuki,* Hiroshi Yamada,? Keishi Yunoki, and Hiromitsu Yamaguchi Department of Chemical Engineering, Faculty of Engineering, Kansai University, Suita, Osaka 564, Japan Received November 15,1991. Revised Manuscript Received May 5, 1992
Ruthenium complexes such as Ru(cyclooctadiene)(cyclooctatriene), Ru(a~etylacetonate)~, and R u ~ ( C O acted ) ~ ~ as excellent catalyst precursors for the hydroliquefaction of various coals in a non-hydrogen donor solvent 1-methylnaphthalene. For Australian Yallourn brown coal, addition of 0.02 wt % of a ruthenium catalyst to coal afforded coal conversion of 89.3% and oil yield of 45.9%; on increasing the catalyst level to 0.07 wt % ,coal conversion increased to 96.5% and oil yield to 57.8% at 673 K under an initial hydrogen pressure of 5.0 MPa. However, decreases in the reaction temperature to 623 K drastically decreased the amount of oil fraction (19.2%) without changing coal conversion significantly (85.6%). Subbituminous (Wyoming) and bituminous (Illinois No. 6 and Mi-&e) coals were also converted with high conversion under mild conditions (698K, P(H2) = 5.0 MPa) with the addition of small amounts of ruthenium complexes. Studies on the kinetics of the liquefaction reaction were carried out using Yallourn coal. Compared with the iron catalyst (Fe(CO)&) ruthenium complexes apparently more effectively promoted reaction paths from preasphaltene to asphaltene and asphaltene to oil. X-ray photoelectron spectroscopy of the ruthenium catalyst after liquefaction revealed the possibility of the existence of metallic ruthenium.
Introduction Recently, use of finely dispersed catalyst has been developed in coal liquefaction. Suzuki et al. reported that metal carbonyls such as Fe(C0)5 and MO(CO)~ were very active catalyst precursors for the hydroliquefaction of various ranks of coals.'~~Garg and Givens demonstrated an increase in the oil fraction with the addition of slurry-soluble iron n a ~ h t h a l e n e . ~Previous attempts to use metal carbonyls as a homogeneous catalyst in coal liquefaction were not succe~sful.~*~ Kamiya et al. employed ferrocene for the hydrogenolysis of Yallourn SRC (solvent refined coal) and demonstrated higher catalytic activity as compared to iron(II1) oxide with suflur.6 Takemura and Okada reported that nickel acetylacetonate exhibited high catalytic activity for the hydroliquefaction of brown coal at a low temperature.' All these efforts are mainly focused on improving the dispersion state of the catalyst and keeping good contact between coal and catalyst during liquefaction, above all at the initial stage of liquefaction. As shown previously, the catalyst derived from Mo(CO)~ and sulfur exhibited an excellent activity for the hydroliquefaction of various coals.2 Few attempts have been carried out to exploit the catalytic activity of second-row elements of group VIII in the periodic table. Sharma and Moffett reported catalytic activities of various organometallic compounds including triruthenium dodecacarbonyl (RU~(CO)~$) in the hydroliquefaction of coals at 653 K in tetrahydronaphthalene. However, they did not mention any specific activities of second-row group VI11 transition-metal elements.8 Ruthenium complexes have been known to have strong activity toward hydrogen-transfer reaction^.^ Recently, supported or bulk ruthenium catalysts have been used for hydrodenitrogenation reaction of quinoline.1° In ref 10, it is stated that ruthenium-containing catalyst showed higher selectivity for the hydrodenitrogenation reaction Present address: Bridgestone Tire and Rubber Co., Kodaira, Tokyo 177, Japan.
than C0-M0-A1203 catalyst. However, no attempt except that by Sharma and Moffet has been reported on the use of ruthenium complexes catalysts for direct coal liquefaction. The purpose of this study is to elucidate the characteristic feature of homogeneous ruthenium complexes as catalysts for direct coal liquefaction. We have employed (q4-cyclooctadienyl)(qs-cyclooctatrienyl)ruthenium, Ru3(CO),,, and ruthenium tris(acety1acetonate) (Ru(acacI3) as catalyst for zero- and trivalent ruthenium species. We have found that ruthenium complexes exhibit extremely high activity for the liquefaction of Yallourn brown coal.
Experimental Section Samples. Coal samples used in this study are as follows: Yallourn, Australia (C 68.2, H 4.5, S 0.1, N 0.6 daf %; ash 1.1d %);Wyoming(C 74.3, H 5.7, S 0.4, N 1.1daf %; ash 4.8 d %); Illinois No. 6 (C 77.3, H 5.4, S 3.8, N 1.4 daf 70;ash 11.2 d %); Mi-ike, Japan (C 83.9, H 5.4, S 1.9, N 1.4 daf %; ash 8.2 d %). All coal samples were dried and sieved to pass 100-mesh screen and stored under an argon atmosphere. (q4-Cyclooctadienyl)(q6-cyclooctatrieny1)ruthenium[Ru(COD)(COT)] was prepared from RuC13.3H20and cyclooctadiene according to the literature." (1) Suzuki, T.; Yamada, 0.;Fujita, K.; Takegami, Y.; Watanabe, Y. Fuel 1984,63,1706-1710. (2) Yamada, 0.;Suzuki, T.; Then, H. J.; Watanabe, Y. Fuel Process. Technol. 1985,11, 297-303. (3) Garg, D.; Givens, E. N. Fuel Process. Technol. 1983.7.59-70 1984. 8, 123-134 (4) Ross, D. S.; Low, J. Y. Annual Report to ERDA FE2202, 1976. (5) Cox, J. L.; Wilcox, W. A.; Roberta, G. L. In Organic Chemistry of Coal; Lareen, J. W., Ed.; ACS Symposium Series 77; American Chemical Society: Washington, DC, 1978; pp 186-203. (6) Kamiya, Y.; Nagae, S.; Yao, T.; Hirai, H.; Fukuehima, A. Fuel 1982, 61, 906-911. (7) Takemura, Y.; Okada, K. Fuel 1988,67, 1548-1553. (8) Sharma, K. R.; Moffett, G. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1982,27(2), 11-17. (9) Cole-Hamilton, D. J.; Wilkinson, G. J. Chem. SOC.,Chem. Commun. 1977, 59-60. (10) Hirschon, A. S.;Wilson, R. B. Jr.; Laine,R. M. Appl. Catal. 1987, 34,311-316. Hirschon, A. S.;Laine, R. M. Energy Fuels 1988,2,292-295.
0887-0624192/2506-0352~03.00/0 .. ~ .. _ ., . 0 1992 American Chemical Society I
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Highly Active Coal Liquefaction Catalyst
Energy & Fuels, Vol. 6, No. 4, 1992 353
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Table I. Effects of Ruthenium Complexes on the Hydroliauefaction of Yallourn Coal' run catalyst mmol conv oil AS PA H2abs 0.8 36.7 27.9 5.4 3.4 1 none 95.7 54.6 19.2 21.8 2.4 2 Ru(COD)(COT) 0.020 0.075 98.1 68.5 18.9 10.8 3.1 2.3 Ru(a~ac)~ 0.020 97.2 61.8 24.8 10.6
~~~
~
0.076
Ru3(CO)izb 0.021 R u ~ ( C O ) I ~ ~ 0.077 Ru recycle of run 3
98.2 93.5 96.8 92.3
64.0 24.2 10.0 53.1 23.3 17.1 57.8 20.2 14.5 52.0 23.8 16.5
3.2
2.7
- 0 00
2.8
2.2
Fe(C0)5-Sc 0.44-0.45 83.5 46.2 18.1 19.2 2.4 'Coal, 2.0 g; 1-methylnaphthalene,4.0 mL. 698 K, P(Hz)= 5.0 MPa, 60 min. All values are in wt % daf coal. b673 K. c698 K. 9
Other materials were purchased from commercial sources and used without purification. Procedure. Hydroliquefaction reaction was carried out in a batch 50-mL autoclave made of Hastelloy C. Into this, 2.0 g of coal and 4.0 mL of 1-methylnaphthalene (1-MN) were added The together with a certain amount of catalyst (0.02-0.08 "01). reactor was pressurized with hydrogen to 1.0-5.0 MPa at room temperature. The autoclave was heated to the desired temperature (325-698 K)with a preheated stainless-steelheat block equipped with a shaker. Nominal reaction time was estimated from the time when the reaction mixture reached 20 "C below the deaired temperature. During the reaction internal pressure increased to 3-11 MPa depending on the initial hydrogen pressure and the reaction temperature. After a given reaction time, the autoclave was cooled by blowing air and then the reaction mixture was Soxhlet extracted with THF. Coal conversion was calculated from the amount of residue after THF extraction. The extract was concentrated to 10 g and poured it into 200 mL of n-hexane to precipitate asphaltene (AS) and preasphaltene (PA). The precipitate was separated into AS and PA by Soxhelt extraction with benzene. The amount of the oil fraction was calculated from the difference between coal conversion and AS + PA fractions. The gas was collected into a gas buret and the composition was analyzed by gas chromatography (Porapack Q and activated carbon columns). The amount of hydrogen absorbed was calculated from pressure changes and gas composition.
01
02
03
04
Catalyst level Ru wt %
Figure 1. Effects of catalyst concentration on the hydroliquefaction of Yallourn coal. Coal, 2.0 g; 1-methylnaphthalene,4.0 mL; P(HJ = 5.0 MPa, 673 K, 60 min. Table 11. Composition (mol %) of Recovered Solvent after Liquefaction' run Naph TL 1-MT 5-MT 1-MN 1 5.7 0.1 0.1 tr 94.2 3 5 7 8 9
13.7 14.6 5.2 16.1 13.6
0.9 1.3 0.3 0.9 0.5
3.0 3.7 1.6 3.5 1.2
5.0
5.6 1.6 5.6 2.6
77.3 74.8 91.3 74.0 82.0
Conditions and run numbers are the same as Table I. Naph = naphthalene; TL = tetralin; 1-MT = 1-methyltetralin;5-MT = 5methyltetralin; 1-MN = 1-methylnaphthalene.
Results and Discussion Ruthenium Complex Catalyzed Liquefaction of Yallourn Coal. The results of hydroliquefaction of Yallourn coal using ruthenium complexes as catalyst are shown in Table I. Since the initial hydrogen pressure is not high (5.0 MPa) and a hydrogen nondonor solvent (1MN) was employed, without added catalyst conversion of Yallourn coal was very low (run 1,36.7%). With the addition of Ru(COD)(COT), coal conversion increased from 36.7 to 97.2% and yield of oil fraction increased from 27.9 to 54.6%. An increase in the catalyst concentration to 0.075 mmol in 2.0 g of coal (0.15 w t % Ru to coal), did not affect coal conversion but increased oil yield considerably. When the residue of run 3 after THF extraction was dried and it was used as a catalyst in the liquefaction of Yallourn coal, still high coal conversion (92.3%) was obtained, although the yield of oil fraction decreased from 68.5 to 52.0% (run 8). Ruthenium complexes employed would decompose to finely dispersed species and seem to deposit on the coal surface. Consequently, they exhibited good contact between coal and catalyst, and higher activity is attained. During liquefaction, solid coal matrix is digested and ruthenium species tend to coagulate each other to form larger particles. Therefore, the amount of effective catalyst would be reduced in the recycle run. This indicates that
it is essential to use highly dispersed ruthenium species to achieve very high catalytic activity. R u ~ ( C Oalso ) ~ ~shows similar catalytic activity as shown in runs 4 and 5. Addition of R ~ ( a c a c increased )~ coal conversion to the same level as in the case of Ru(C0D)(COT). However, the oil yield was slightly lower than that with Ru(COD)(COT). This contrasta with the finding that the catalytic activity of Fe(aca~)~-S was much smaller than that of Fe(C0)6-S in the liquefaction of Wandoan coal.12 The effects of catalyst level on the product distribution in the liquefaction of Yallourn coal are shown in Figure 1. A decrease in the catalyst level to 0.02 mmol(O.04 w t %) considerably decreased oil yield with a slight decrease in coal conversion. This amount of catalyst is surprisingly small as compared to the usually used iron catalyst; for comparison, the result of the most active iron catalyst is listed in Table I. In Table I1 is shown composition of a recovered solvent after the liquefaction of Yallourn coal. A part of the methylnaphthalene was hydrogenated to 1- and 5methyltetralin and hydrocracked to methane and naphthalene. With the addition of ruthenium complexes, the amount of naphthalene and 1- and 5-methyltetralin increased at 698 K. However, the amount of hydrogenated solvent did not increase significantly with an increase in the catalyst concentration. Examination of the Used Catalyst by XPS. In order to see the fates of ruthenium carbonyl used for the liquefaction of various coals, the residue after THF extraction was subjected XPS analyses. Among the coal samples examined, residues from Yallourn and Wyoming coals did not give well-defined spectra. The binding energy of Ru(3d3/J is 284 eV. This value is unfortunately quite close to the binding energy of C(1s) 285.0 eV, and Ru(3d) peak was masked with strong C(1s) peak. Therefore, Ru(3~,,~) binding energy was measured. Differences in the binding energy between Ru02 (462.3 eV) and ruthenium metal (462.1 eV) are very small. The measured binding energies of used catalysta are as follows:
(11) Itoh, K.; Nagashima, H.; Ohshima, N. J . Organomet. Chem. 1984, 272, 179.
(12)Suzuki, T.; Yamada, 0.; Takahashi, Y.; Watanabe, Y. Fuel Process. Technol. 1985, 10. 33-43.
Suzuki et al.
354 Energy & Fuels, Vol. 6, No. 4, 1992 loo
100
7
-8
80
g
60
i I
AS
40 0
ga 320
340
360
380
400
Temperature
420
‘C
401 -10
20
run 10 11 12 13 14 15 16
011
30
20
30
40
50
Figure.4. Effects of initial hydrogen pressure on the liquefaction of Yalloum coal. 673 K,Ru(acac), = 0.15 wt %. Other conditions are the same as shown in the caption to Figure 1.
-1
E 20
10
Hydrogen pressure MPa
1.
- 0 00
- 0 00
440
Figure 2. Effects of reaction temperature on the hydroliquefaction of Yalloum coaL Ru(aca& = 0.15 wt %; P(H2) = 3.0 MPa Other conditions are the same as shown in the caption to Figure 1001
20
40
50
Hydrogen pressure MPa
Figure 3. Effects of initial hydrogen pressure on the liquefaction of Yalloum coal. 648 K, Ru(aca& = 0.15 wt %. Other conditions are the same as shown in the caption to Figure 1. hydrogenolysis of benzyl phenyl ether with R u ~ ( C Oon )~~ active carbon (as a coal model compound),461.8 eV; Illinois No. 6 coal with R u ~ ( C O at ) ~425 ~ “C,461.6 eV. These values are close to the binding energy of ruthenium metal or possibly sulfide. Sulfur peak was observed for the 11linois No. 6 case due to high concentration of sulfur in the coal. However, oxygen peak was not detected in these cases. At present we assume that ruthenium species exist as metal or partidy sulfided form in the presence of sulfur. Further studies on the nature of ruthenium species are required. Effects of Reaction Temperature. Effects of reaction temperature on the liquefaction of Yallourn coal were examined and the results are shown in Figure 2. Reactions were carried out under the lower initial hydrogen partial pressure of 3.0 MPa in order to find out the mildest reaction conditions yielding reasonable amounts of products. At both 648 and 673 K, very high coal conversions (>90%) were obtained, and even at a lower temperature of 598 K, the coal conversion was moderately high (85%). However, on increasing the temperature to 698 K, coal conversion unexpectedly decreased to 90%. This is assumed to be due to the lower hydrogen partial pressure. At 698 K hydrogen-deficient conditions occurred at the initial stage of the liquefaction by rapid decomposition of coal, and consequently, retrogressive reactions of fragment radicals to give insoluble materials proceeded. The oil yield monotonously increased from 11 to 54% on increasing the temperature from 598 to 698 K. This indicates that production of oil requires higher reaction temperature, even in the presence of very active ruthenium catalyst. Ruthenium catalyst seems not to exhibit high activity for hydrocracking of coal or coal fragments at a low temperature (
