Energy & Fuels 1994,8, 341-347
341
Development of Highly Dispersed Coal Liquefaction Catalysts? Toshimitsu Suzuki Department of Chemical Engineering, Kansai University, Suita, Osaka, Japan 564 Received July 12, 1993. Revised Manuscript Received November 30, 1 9 9 P
The development of iron pentacarbonyl derived coal liquefaction catalyst is summarized. Iron pentacarbonyl derived catalyst is one of the most active catalysts for the hydroliquefaction of a variety of coal samples. When low sulfur content coal was employed, the addition of an equivalent amount of sulfur to iron was necessary in order to achieve high activity. The active species of iron carbonyl derived catalyst is attributed to pyrrhotite, some of which is of nanosize order as determined by XRD and Mossbauer spectroscopic investigation. Kinetic studies of coal liquefaction using iron carbonyl sulfur catalyst showed that the role of catalyst is to promote direct hydrogen-transfer process from gas phase to coal fragment radicals. This was also supported by the results of the liquefaction of coal in hydrogen donor solvent tetralin.
Introduction In order to reduce the cost of a coal-derived liquid fuel to the current petroleum price, much effort must be devoted to improve the coal liquefaction processes under consideration or the previously tested ones. Milder reaction conditions would greatly contribute to a reduction in the cost of coal liquefaction processes. However, under milder reaction conditions, several undesirable problems such as a lower coal conversion,lower yield of low molecular weight fractions, and coking in the reactor would appear. One possibility to overcome this is the development of a highly active catalyst for coal liquefaction. Overview of Coal Liquefaction Catalyst. Recently much attention has been focused on the use of highly dispersed catalyst for coal hydro1iquefaction.l Although, a large number of discussions have been reported on the mechanism of catalyzed coal liquefaction reactions, exact roles of coal liquefaction catalyst have not been well elucidated. Until now, understanding the role of coal liquefaction catalyst involved many contradictory arguments. Moritomi et al. reported that the role of catalyst is hydrogenation of condensed aromatic compounds resulting in an increase in the hydrogen-donating ability of the solvent.2 Charcosset et al. reported the importance of molecular hydrogen as compared to hydrogen donor solvent in the presence of a well-dispersed ~ a t a l y s t . ~ Vernon reported the importance of molecular hydrogen in the thermal cracking reaction of coal model compound 1,2-di~henylethane.~ Our understanding in catalyzed coal liquefaction mechanisms is as follows: (i) Thermal scission of covalent bonding in coal macromolecular structure occurs to give f This article was part of the section on Proceedings of an ACS Division of Fuel Chemistry Symposium on Iron-Based Catalysts for Coal Liquefaction, published in Energy F u e l s 1994, J a n / F e b issue. 0 Abstract published in Advance ACS Abstracts, January 15, 1994. (1)Fine Particle Catalyst Testing US DOE Advanced Research Liquefaction. Symposium on iron-based catalysts for coal liquefaction. Prep. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1993,38(1), 1-234. (2)Moritomi, K.;Nagaishi, H.; Naruse, M.; Sanada, Y.; Chiba, T. J. Fuel SOC.Jpn. 1983,62, 264-262. (3) Charcosset, H.;Bacaud, R.; Besson, M.; Jeunet, A.; Nickel, B.; Oberson, M. Fuel Process. Technol. 1986, 12, 189-201. (4)Vernon, L. W.Fuel 1980,59, 102-106.
Scheme 1. Coal Liquefaction Scheme and Functions of the Catalyst Required in the Particular Steps
Thermal decomposition H
abstraction (a,b)
Hydrocracking (b,c)
free radicals. In this stage, added catalyst scarcely assists coal degradation reaction. (ii) Free radicals thus formed would be stabilized by hydrogen abstraction from either hydrogen donor substances or a molecular hydrogen activated on a catalyst. (iii) Hydrogenation of aromatic moieties in the coal-derived initial products and hydrocracking of the coal derived initial products gradually decreases their molecular weight to give lighter oil fraction. This process is schematically represented in Scheme 1. From these view points, the following four conditions would be required for an efficient coal liquefaction catalyst: (a) to keep good contact between catalyst and solid coal; (b) high activity toward hydrogen transfer to both coal fragment radicals and aromatic moieties; (c)high activity for cracking and hydrocracking of C-C bonds; (d) low cost of the catalyst and easy recovery or recycle use of the catalyst. Among them, points a and b are the most significant characteristics required for a catalyst in the early stage of the coal liquefaction course shown in Scheme 1. A catalyst which fulfills all the requirement shown above cannot be found hitherto. Among many catalysts previously investigated, ZnClz or SnClz is superior in functions a and c to other metal species, on the other hand, iron has been used for its low cost (d). Co-Mo and Ni-Mo catalyst employed for the hydrodesulfurization of petroleum fractions are extremely active in functions b and c. However, such a supported catalyst provides a poor contact between catalysts and coal (a) and in addition deactivation of catalyst occurs.
0887-0624/94/2508-0341$04.50/0 0 1994 American Chemical Society
342 Energy & Fuels, Vol. 8, No. 2, 1994 As early as 1941 the idea to improve contact between catalyst and coal was proposed. Accordingto the method, iron sulfide was prepared in situ from an aqueous solution of FeS04and Na2S comixed with coal and vehicle oil slurry. Precipitated iron sulfide was well dispersed into coal and oil slurry, and then the byproduct NazS04 was washed out by hot water. A high level of catalyst was loaded in coal, in order to avoid treating a large amount of coal. The catalyst-loaded coal was diluted with untreated coal. Thus, the catalyst was dispersed into coal without handling excessive amounts of coal in laborious catalyst loading process. By this method fair to good yield was a ~ h i e v e d . ~ In 1951, Weller and Peripetz first reported that the distribution of the catalyst is very important for high activity.6 Impregnation with FeS04 greatly enhanced coal conversion as compared to the addition of powdered FeS04 or cogrinding of FeS04 with coal. They tried to add an aqueous solution of FeS04 in order to exclude the impregnation step but no or little catalytic activity of iron was observed. In several paths involved in coal liquefaction, hydrogen transfer to coal fragment radicals is the most important path in order to obtain higher coal conversion.I A certain amount of coal fragment radicals exists in the solid coal matrix or appear as partially fused macromolecular structures whose mobility is restricted, a t the early stage of liquefaction. Under such conditions, contact between catalyst and solid coal is a very important factor to achieve efficient hydrogen transfer from catalyst to free radicals. From a practical view point, it is desirable not to employ complicated and energy-consuming processes as pretreatment of coal. Impregnation or ion-exchange processes involving large amounts of water to treat coal are not practical processes, even if they increase coal conversion. A certain organometallic complex of transition metal shows solubility in organic solvents and exhibits activity in the hydrogenation of carbon-carbon double bond. McCabe reported that the use of coal anchored C02(C0)~ significantly increase conversion as compared to neat COZ(co)s.8 Lee et al. studied several metal carbonyls as catalysts for upgrading of Synthoil oil at 100 "C, 700 psi using a flow reactor. However, they did not observe significant catalytic activity with metal carbonyls tested (Cr, Co, Mo, W, and Mn). They concluded that metal carbonyls are volatile, in the course of the reaction (in a flow system) a certain amount of catalyst was lost, and as a result high activity to hydrogenation was not ~ b t a i n e d Attempts .~ to use homogeneous transition-metal complexes involving C02(C0)8and Fe&0)12 as a catalyst were not successful.1° We have noticed that iron carbonyl decomposes to give fine particles of iron metal at an elevated temperature. This prompted us to use iron carbonyl as a catalyst precursor of coal liquefaction at a high temperature, because iron carbonyl itself is not a good catalyst for the hydrogenation of aromatic moieties. Iron carbonyl is freely soluble in common organic solvents, and therefore the catalyst precursor may penetrate into pore structures of ( 5 ) Morikawa, K. J. Jpn. Pet. Inst. 1975, 18, 377-382.
(6) Weller, S.; Peripetz, M. G . Ind. Eng. Chem. 1951, 43, 1243. (7) Neavel, R. C. Fuel 1976,55, 237-241. (8) McCabe, M. V.; Orchin, M. Fuel 1976, 55, 266-269. (9) Lee, L. J.; Burkhouse, D. W. DOE Annual Report (1975-1976), 1976, FE2202-12. (10) Cox, J. L.; Wilcox, W. A.; Roberta, G. L. In Organic Chemistry of CoaE;Larsen,J. W., Ed.;ACS Symposium Series 77;American Chemical Society: Washington, DC, 1978; pp 186-203.
Suzuki Table 1. Hydroliquefaction of Mi-ike Coal with Various Iron Catalysts. run
catalyst
1 2 3 4 5 6 7 8
none none redmudc FeAA-A1Et3d FeAA FeC13 Fe(C0)5 Fe(CO)5
mmol
1.0 1.0 1.0 1.0 1.0 1.0
temp, "C convnb oilb ASb PAb H$ 425 445 425 425 425 425 425 445
96.0 97.2 98.7 64.2 97.4 93.0 100 100
38.8 49.8 48.2 25.3 40.1 50.8 45.4 57.5
30.4 33.5 35.4 26.9 38.6 29.9 39.8 32.5
31.7 14.0 15.1 12.0 18.7 12.3 14.8 10.0
0.4 0.4 1.1 1.7 0.6 0.6 1.9 1.7
Coal 2.0 g, tetralin 4.0 mL, P(H2)5.0 MPa; reaction time 60 min. Convn = conversion; oil = pentane soluble; AS = asphaltene; PA = preasphaltene. Hz = Hz transferred to coal, all daf % to coal. Contains 44.6 w t % Fez03. Al/Fe = 311, FeAA = Fe(acac)s, AlEta = Al(CzH5)s.
coal below the decomposition temperature and above the decomposition temperature the precursor decomposes to a finely dispersed iron species on the solid coal. Thus, good contact between the catalyst and solid coal could be attained. This paper mainly summarizes our results in the development of dispersed coal liquefaction catalysts.
Results of Coal Liquefaction with Organometallic Compounds Experimental Procedure. Hydroliquefaction of coal was carried out in a batch 50-mL microautoclave under various reaction conditions. Typically, 2.0 g of coal and 4.0 mL of a vehicle oil (1-methylnaphthalene or tetralin) were charged with a certain amount of catalyst. The autoclave was heated to the desired reaction temperature and shaked under 5-7.9 MPa initial hydrogen pressure. Hydrogen consumption was calculated from the amount of gas in the autoclave after the reaction and it was corrected with the composition of the recovered gas. The reaction mixture was separated into preasphaltene, asphaltene, and oil fractions.llJ2 Survey of Dispersed Iron Catalyst. Lower valent transition-metal complexes are reported to be active in the catalytic hydrogenation of organic compounds. Thus, we screened several iron-containing compounds as catalyst for coal hydroliquefaction. Catalysts screened were Fe(acac)3,Fe(acac)sreduced with Al(CzH& (black solution), FeC13 impregnated to coal from diethyl ether solution, Fe(C0I5, and red mud as a reference. Since the liquefaction reaction was carried out in tetralin using Japanese bituminous coal (C 83.9 % 1, no significant superiority with Fe(C0)b to other iron species in conversion was observed as shown in Table 1. As seen in run 1, in the absence of a catalyst already 96% conversion was obtained, and it indicated the powerful capability of hydrogen donor solvent. However, good to fair increases in oil and asphaltene fractions were observed with an addition of Fe(C0)5. The larger amount of absorption of molecular hydrogen with Fe(C0)5and Fe(acac)3-A1(CzH& catalysts must be noted.ll Effects of Reaction Conditions. Since the effect of catalyst is less pronounced in tetralin, the non-hydrogendonating solvent 1-methylnaphthalene was employed for evaluating catalytic activity of iron carbonyl based catalyst. The results of the liquefaction of Illinois No 6 coal (C (11) Suzuki, T.;Yamada, 0.; Fujita,K.; Takegami, Y.; Watanabe, Y.
Chem. Lett. 1981,1467-1468; Fuel 1984,63, 1706-1709.
(12) Suzuki, T.; Yamada, 0.; Fujita, K.; Takegami, Y.; Watanabe, Y.
Ind. Eng. Chem. Process Des. Deu. 1985, 24, 832-836.
Energy & Fuels, Vol. 8, No. 2, 1994 343
Highly Dispersed Coal Liquefaction Catalysts
Table 2. Hydroliquefaction of Illinois No. 6 Coal Using Iron Pentacarbonyl.
run 1 2 3 4 5 6 7 8 9 10 11 12 13 14
catalystb none Fe(C0)5 Fe(C0)b none Fe(C0)s Fe(C0)5 none Fe(C0)S Fe(C0)b Fe(CO)& Fe(C0)g none Fez03 Fe(CO)a
H2, MPa 5.0 5.0 5.0 5.0 5.0 5.0 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9
temp, OC 425 425 425 460 460 460 460 460 460 460 460 480 480 480
time,min 60 60 10 20 20 40 20 10 20 20 20 10 10 10
conv, % 56.7 92.9 88.7 54.9 84.2 83.9 69.7 93.5 95.0 94.8 93.3 61.2 83.7 93.9
gas, %
oil, % 22.8 33.1 13.0 31.5 40.9 47.3 37.2 34.5 53.1 56.2 39.0 34.9 43.4 50.4
3.7 6.3 1.6 4.2 5.2 6.7 6.4 5.1 5.6 4.6 5.2 5.3 6.3 6.4
AS, % 19.2 39.0 31.3 15.8 25.5 23.3 20.4 33.7 30.5 26.2 31.1 16.2 25.4 29.5
PA, % 11.0 14.5 42.8 7.6 17.8 13.3 12.1 20.2 11.4 10.2 18.0 10.1 14.9 14.0
H2: % 0.5 2.2 0.8 0.8 2.0 2.4 1.4 1.9 3.0 2.7 2.4 1.2 2.2 2.8
Fe 0.4 mmol(l.l wt % coal). Hydrogen absorbed from gas phase. Solvent 0 Coal 2.0 g, solvent 1-methylnaphthalene 4.0 mL; 75 daf coal. decalin. e Fe 0.20 mmol (0.55 wt % coal).
76.8 7% ) under various reaction conditions are summarized in Table 2.12 Since we have known that Fe(C0)5 exhibited higher catalytic effects than those of conventional iron catalyst previously reported, we wanted to carry out the liquefaction reaction at a higher reaction temperature in a short reaction period. A t an initial hydrogen pressure of 5.0 MPa, coal conversion was not high as expected (compare runs 2 and 5 ) . Increasing the initial hydrogen pressure to 7.9 MPa resulted in an increase of coal conversion and oil yield, at a reaction time of only 20 min (run 9). At 480 OC a reaction time of 10 min is sufficient to obtain oil yield of 50%. Conventional Fez03 catalyst afforded smaller coal conversion and oil yield of 43 % (see run 13). The effect of catalyst concentration on the product distribution was examined. The catalyst level above 1.1wt % to coal as Fe did not show any change, but a decrease to 0.55 wt % of coal slightly lowered the yields of oil and asphaltene (run 11). Even in averyshort reaction time (10 min), conversions to the THF-soluble fraction were almost constant and very high (over 93%) in the temperature range of 425-480 "C (compare runs 3,8, and 14). However,the asphaltene and oil yields increased with an increase in the reaction temperature. This suggests that conversion of THF-soluble products to oil and asphaltene fractions cannot be promoted with the iron catalysts at a lower temperature. Liquefaction of Low Sulfur Coals. We have extended iron carbonyl based catalysts to the liquefaction of Australian low-rank coals (Yallourn, C 68.2, S 0.1%; Wandoan, C 76.8, S 0.3%). The results are summarized in Table 3.13 Much lower conversions were observed in the absence of catalyst as compared to Illinois No 6 coal. Use of Fe(C0)5 increased conversion, but the increases are not as important as in the case of Illinois No. 6 coal. Addition of 1-2 equiv of sulfur to iron pentacarbonyl greatly enhanced the catalytic activity. Several iron carbonyl derivatives also enhanced coal conversion and oil yield markedly with sulfur. Iron tris(acety1acetonate) is soluble in 1-methylnaphthalene, however, as shown in run 7, it did not gave high coal conversion, and the yield of oil fraction was low, even in the presence of sulfur. Use of red mud increased coal conversion to the same level as Fe(C0)b-S catalyst but the yield of oil fraction was low. Kamiya et al. used ferrocene as a catalyst for the hydrogenolysis of SRC from Yallourn coal and claimed (13)Suzuki, T.; Yamada, 0.; Takahashi, Y.; Watanabe, Y. Fuel
Processing Technol. 1985, I O , 33-43.
Table 3. Hydroliquefaction of Australian Coals Using Iron-Sulfur Catalysts. run
Fe S mmol
catalyst
convn gas
oil, %
AS, PA, H z , ~ % % %
22.3 35.5 48.6 47.3 47.7 46.2 27.7 39.3 42.1
12.6 21.0 24.8 27.5 25.8 27.7 25.2 28.5 29.8
5.9 21.8 11.5 12.4 11.1 12.8 20.7 16.7 13.4
12.9 25.3 30.8 34.6 41.8 36.8
5.4 14.4 22.9 17.0 21.6 19.3
3.4 0.8 14.8 1.6 19.9 2.3 21.1 2.2 17.4 3.3 19.7 2.9
Wandoan Coal 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15
none Fe(C0)5 Fe(CO)5-S Fe(CO)5-S Fez(C0)g-S Fe3(C0)12-S Fe(acac)s-S redmud-S FezSz(C0)a
0 0 1 0 1 1 2 2 1 1
none Fe(C0)5 Fe(C0)s Fe(CO)5-S Fe(CO)s-S Fe(C0)b-S
0 1 2 1 1 1
1 1 1 1
48.6 7.8 85.2 6.9 94.4 9.5 94.7 7.5 92.7 8.1 1 94.8 7.9 1 81.6 8.0 1 93.2 8.7 1 93.3 8.0 Yallourn Coal 0 36.7 15.0 0 69.7 15.2 0 92.5 18.9 0.5 91.5 18.8 1 97.7 16.9 2 96.0 20.2
0.2 1.8 2.8 2.6 2.6 2.9 2.4 2.4 3.0
a Coal 2.0 g; 2-methylnaphthalene 4.0 mL; Hz pressure 5.0 MPa (425 "C), 60 min. Hz absorbed 5% daf coal.
slightly higher catalytic activity than Fez03 catalyst^.'^ Recently, Ikura used ferrocene and Fe(acac)s as a catalyst for coal-oil coprocessing,and stated slight advantage with ferrocene. However, our findings suggest that iron species soluble in a coal liquefaction vehicle do not always act as good catalyst for 1iq~efaction.l~ An investigation of the role of sulfur in the hydroliquefaction of Yallourn coal with Fe(CO)5catalyst was carried out. Increase in the conversion with Fe(C0)5 was less pronounced as compared to the case of Wandoan coal, but increasing the amount of the catalyst to 5.6 w t 7% (2.0 mmol) to coal markedly increased conversion and yield of oil. Addition of sulfur dramatically increased conversion of coal and oil fraction. The optimum ratio of S/Fe was around 1. This suggests that iron sulfide is the active species. Iron species contained in the residues from the liquefaction reaction was examined by X-ray diffraction. Pyrrhotite has been proposed to be one of the active species of iron-sulfur based catalyst by Montano.16 In our case, a liquefaction residue from Fe(CO)5-S catalyst system clearly showed intense peaks attributed to Fel,S in the XRD pattern. On the other hand, that obtained in ~
~~~
~
~~
~~
(14) Kamiya, Y.; Nagae T.; Yao, T.; Hiarai, H.; Fukushima, A. Fuel
1982,61, 906-911. (15) Ikura, M.; Kelley J. F. 1991Znt. Conf. Coal Sci., Proc. 1991,774778. (16) Montano, P. A.; Granoff,B. Fuel 1980,59,214-216. Bommannarar, A.; Montano, P. A. Fuel 1982,61, 1288-1290.
344 Energy & Fuels, Vol. 8, No. 2, 1994
Suzuki
Table 4. Hydroliquefaction of Wyoming Coal in Donor or Nondonor Solvent. naphe run 1 2 3 4 5 6 7 8
catalyst none(N2) none none RUB(CO)I~ Fe(C0)s-S Fe(CO)S-Se Mo(C0)gS Rus(CO)iz
mmol
solvent
convn
oil
AS
PA
Hzb
mol%
mmol
TL,dmol%
80.7 90.5 60.7 93.4 92.9 90.1 92.4 92.5
38.5 55.4 41.3 59.3 67.3 61.0 64.0 63.2
20.1 26.3 13.8 23.5 20.4 23.2 21.7 20.1
22.1 8.5 5.6 10.7 4.6 5.9 7.0 9.2
2.3 (2.3) 2.0(1.8) 0.5 2.0 2.3 (0.7) 2.2 (0.6) 2.3 (1.2) 2.2 (0.7)
39.2 27.8
12.0 8.0
0.04 0.45 0.80 0.04 0.04
TL TL MN MN TL TL-N TL TL
13.2 21.7 16.0 9.6
3.7 7.4 5.2 2.5
58.6 70.3 (0.3) (3.9) 81.3 76.2 85.6 89.1
0 Coal 2.0 g; solvent 4.0 mL (30.2 mmol); P(H2) 5.0 MPa (425 OC), 60 min. TL = tetralin, MN = 1-methylnaphthalene, TL-N = tetralinnaphthalene mixed solvent. Values in 5% daf coal unless otherwise stated. Numerals in parentheses indicate hydrogen transferred from tetralin. The amount of naphthalene after reaction in mol % and absolute amount. Numerals in the parenthese indicate amounts of 1- and 5-methyltetralin. e Tetralin (30.4 mmol) and napyhthalene (3.9 mmol) were used as a solvent.
*
the absence of sulfur exhibited the peaks ascribed to Fe304. These findings indicate that the active form of Fe(C0)5 based catalyst is essentially the same as that of conventional iron oxide or iron sulfide based catalyst. Studies of the active form of Fe(CO)~j-basedcatalyst were done using Mossbauer spectroscopy with the catalyst recovered after the reaction with coal model compound^.'^ Mossbauer spectra of FeS2 or Fe203-S based catalyst after a hydrocracking reaction of diphenylmethane exhibited clear sextet signals ascribed to Fe1,S. On the other hand, Fe(CO)a-S-based catalyst exhibited rather broad doublet signals in the center of the spectra, in addition to the sextet signals ascribed to Fel-,S. The doublet absorption is probably attributed to highly dispersed paramagnetic iron species, so called superparamagnetic species. Very recently Wender and Huffman used Fe(C0)5 as a catalyst for coprocessing and carried out detailed characterization of iron species. They concluded that Fe(C0)5 decomposes to finely dispersed pyrrhotite, Fe&, and iron oxides. The size of pyrrhotite was estimated to be less than 1 pm by scanning electron microscope (SEM). Mossbauer observation showed the existence of pyrrhotite of much smaller sizes (size 10-30 nm). The larger size obtained with SEM is attributed to the agglomeration of finely dispersed species during reaction.18 In the hydrogenation of pyrene or phenanthrene, Febased catalyst exhibited 30-50 % higher conversions than Fez03 or FeS2 catalyst (conversion 20-23% for FeS2 or Fe203-S and 30% for Fe(C0)5), even with a smaller amount of Fe(C0)5 (FeS2,Fe203-S 1.0 mmol and Fe(C0)5 0.25 mmol). 4,5-Dihydropyrene or 9,lO-dihydrophenanthrene is the main product of the hydrogenation, respectively, in all cases. However, Fe(CO)s-based catalyst promoted further hydrogenation to tetra- and hexahydopyrene and tetra- and octahydrophenanthrene. This indicates again superiority of dispersed catalyst to other iron species in the hydrogenation of condensed aromatic compounds. In these reactions, Fe(C0)5 and Fe(C0)5-S catalysts exhibited similar activity. This shows that metallic iron generated from Fe(C0)5 acts also as an active catalyst. The result obtained with a very large amounts of Fe(C0)5 (Table 3, run 12) indicating promoted conversion in the liquefaction of Yallourn coal can well be interpreted considering that a certain amount of metallic iron may act as an active catalyst in the early stage of liquefaction, before it is oxidized to FesOd with water from coal. (17) Suzuki,T.;Yamada, H.;Watanabe. Y. Energy Fuels 1989,3,707713.
(18)Herric, D. E.;Tierney,J. W.; Wender, I.; Huffman, G . P.;Huggins, 1990, 4 , 231-236.
F.E.Energy Fuels
On the hydrogenolysis of diphenylmethane or 1,2diphenylethane slight catalytic activities of Fe(C0)b and Fe(C0)5-S catalysts were observed as compared to the uncatalyzed run. All these results suggest that the hydrogenation and hydrogenolysis activities of metallic iron and iron sulfides based catalyst are in the same level and that only dispersion of the ctalyst is a measure of the activity in the absence of water in the reaction system. Effects of Catalyst in Hydrogen Donor Solvent. The role of catalyst in the liquefaction of coal in a hydrogen donor solvent is of interest to examine. When tetralin was employed as a solvent, naphthalene was formed after the reaction. The amount of naphthalene decreased in the presence of catalyst. Most investigators believed this to be due to the rehydrogenation of naphthalene in the presence of catalyst.2 The results of liquefaction of Wyoming coal (C 74.3% ) in the presence of various catalyst are shown in Table 4. A large amount of naphthalene was produced in the liquefaction under nitrogen atmosphere. Even in the absence of catalyst, under hydrogen pressure, decrease in the amount of naphthalene was observed. Use of catalyst greatly decreased the amount of naphthalene in the course of reaction(from 8.0 to 3.7 mmol) with an apparent increase in the amount of hydrogen transferred from gas phase. This seems to indicate that an added catalyst hydrogenates naphthalene, formed after the hydrogen transfer to coal fragment radicals, to tetralin during the liquefaction. If the hydrogen transfer from tetralin to coal fragment radicals is the primary reaction, 4.3 mmol of naphthalene (differences in 8.0 and 3.7 mmol) was estimated to be hydrogenated to tetralin in the presence of the iron catalysts. However, as shown in run 6 in Table 4,7.4 mmol of naphthalene was found after the reaction with tetralin-naphthalene mixed solvent (tetralin 30.4 mmol and naphthalene 3.9 mmol). Assuming that in this reaction the same amount of naphthalene was produced from tetralin as in the reaction with tetralin alone (run 5) and added naphthalene was not hydrogenated during the reaction, the hypothetical amount of naphthalene in the run with mixed solvents is calculated as 7.6 mmol. The difference between calculated and observed value, only 0.2 mmol, is the net amount of naphthalene hydrogenated. Such findings strongly suggest that hydrogenation of naphthalene scarcely occurred during coal liquefaction. The reason for this is difficult to explain at present. One possibility is that polar substances from coalderived liquid are strongly adsorbed on the active site of catalyst, and consequently hydrogenation of naphthalene would be suppressed. On the other hand, hydrogen transfer from molecular hydrogen activated on the cat-
Highly Dispersed Coal Liquefaction Catalysts
Energy & Fuels, Vol. 8, No. 2, 1994 345
Scheme 2. Kinetic Model of Liquefaction Reaction (Model Used for the Previous Calculation)
- -
Scheme 3. More Probable Reaction Scheme for Coal Liquefaction Insoluble matwlals
kr
Coal
&
PA
kz
AS
011
lb
Coal
14
alysts to coal fragment radicals containing polar group would proceed smoothly. Thus hydrogen abstraction from tetralin competes to the hydrogen transfer from molecular hydrogen. As a result, the amount of naphthalene would be reduced in the presence of active catalyst. Such decrease in the amount of naphthalene is only significant in the presence of dispersed catalyst. Similar discussion was reported by Charcosset et aL3 Details of studies will be published in a near future.
Kinetic Studies of Fe(C0)s-Based Catalystslg Additional investigation of the nature of Fe(C0)5 based catalyst was carried out by kinetic treatment of coal liquefaction. A kinetic model used in our study is a combined parallel and consecutive reaction paths shown in Scheme 2, where first-order rate constants were estimated by computer curve-fitting methods. In the calculation, the following simultaneous differential equations were numerically integrated to minimize the sum of square of the residuals between calculated values and observed ones.20 d[C]/dt = kl[C-CI] - k4[C-CIl - k,[C-CI]
(1)
d[PAl/dt = kl[C-CI] - k,[PA] - k,[PA]
(2)
d[ASl/dt = k2[PAl + k,[C-CI] - k3[ASl
(3)
+ k,[C-CII + kJPA1
(4)
d[OilI/dt = kJAS1
[Cl = 100 - [PA1 - [AS1 - [Oill
(5)
Here [CII means the amount of inconvertible organic materials in coal. We have made the assumption that the amount of [CII depends only upon the reaction temperature with a particular coal brand. From the material balance in the calculation (eq 51, [Cl at reaction time t is automatically determined. Therefore, once CI is settled to a certain amount for the selected coal at a certain reaction temperature, best fittings to [PA], [AS], and [Oill confine the amount of unreacted coal [Cl at t. This made the calculation erroneous in certain cases. If we adjust the [CI] value to get better fitting between observed and calculated values, the assumption that [CII is solely depend on the nature of coal constituent cannot hold. The same kinetic model that involves gas as a product but does not contain preasphaltene was investigated in the catalyzed hydroliquefaction of Wyodak coal by Pradhan et a1.21 They also observed similar disagreement between observed and calculated results for unreacted coal against reaction time in the presence of a smaller amount of catalyst. Since they carried out liquefaction in tetralin (19) Suzuki, T.; Ando, T.; Watanabe, Y. Energy Fuels 1987,1, 295300. (20) Radomymnskim B., Szczygiel, J. Fuel 1984,63, 744. (21) Pradhahn, V. R.; Holder, G. D.; Wender, I.; Tierney, J. W. Ind. Eng. Chem. 1992,31,2051-2056.
k’
+
k , t l
I
.
k.
,r
,
I,’
Radical pool
_ _ _ _ _ _ks,- _ _ _ - _ _ _ _ _ _ _ _ _ _ _
- - kl
\i*
b’
PA
AS
011
Scheme 4. Kinetic Model for Coal Liquefaction Used To Calculate Better Fitting Insoluble mstwlals
- tI k7
Coal
k’
\
14
,,r______________________
‘; ;
Radical pool ‘L
- - -\* k.
ki
ka
zk’ PA
- - - - -ks- - - - - - - - -
AS
.4
.I
Oil
/
and they counted preasphaltene fraction as unreacted coal, the difference in the observed and calculated values for the ultimate conversion is not so significant as in our case. However, observed conversions at time 30 and 60 min are much smaller than calculated values. These discrepancies are exactly due to the same phenomenon that we are discussing here. We previously introduced reversible reaction between preasphaltene and insoluble coke precursor. However, preasphaltene is one of the stable intermediate products. This indicates that considerable amounts of hydrogen transfer to coal fragment radicals have been completed for the formation of preasphaltene. Thus it is unlikely that in the absence or with a smaller amount of catalyst, once formed, preasphaltene tends to recombine to give coke precursor. This is because, as described in a previous paper, the reaction of preasphaltene separated from different runs under similar reaction conditions afforded only a small amount of THF-insoluble materials.19 Recently, we have introduced a new kinetics model involving radical pool shown in Scheme 3.22 A similar kinetic model has been proposed by Jackson and c o - ~ o r k e r s . ~ ~ However, the kinetic model (Scheme 3) contains many rate constants to be determined compared to the limited numbers of observed results (oil, AS, PA, and unreacted coal). This made the results of the simulation by computer not unique. A slightly simplified model shown in Scheme 4 was introduced here and better fitting was obtained without arbitrary coincidence. In this kinetic model, the “radical pool” is not dependent upon reaction conditions except reaction temperature. Above a certain reaction temperature, weak covalent bonds in coal dissociate rapidly to give free radicals most of which exist in a solid coal matrix or as partially fused macromolecular state. This process is independent of the absence or presence of the catalyst and is supported by the results in tetralin; that above 90% coal conversion was obtained in the absence of catalyst and hydrogen. If sufficient amounts of catalyst are employed in the absence of abundant hydrogen donor substances, hydrogen transfer over the catalyst proceeds smoothly to give lighter fractions (increases in kl,124, and (22) Suzuki, T.; Yamada, H.; Yunoki, K.; Yamaguchi, H. Energy Fuels 1992,6, 352-356. (23) Jackson, W. R.; Marshall, M.; Rash, D.; Redlich, P; Watkins, I. D.; Larkins, F. P. 1985 Int. Conf. Coal Sci., R o c . 1985, 31.
346 Energy & Fuels, Vol. 8, No. 2, 1994
Suzuki 100
I
a
a 80
c
.
eo .......... ............................ ............... ...............
....
40
'.
Q
40
,.e. .,."O
aQ
-m
dp
-s
0 U a,
-
2o
.... . .i,
o
-n
! 20
0
40
80
80
(II
. :
0
.....
0
.................................................................
0
P)
0
20
40
80
80
0
Ill
b
100
E
b
a
P
E (D
80
E!
-*
(10
? .............
.................0. 40
....................................................................... 20
r"""
........................................................................................... .......................... 0
R............ 20
40
80
60
Time min
0 0
20
40
(10
80
Time min
Figure 1. Observed and simulated results of the liquefaction of Yallourn coal (a) according to kinetic model shown in Scheme 2 and (b) according to kinetic model shown in Scheme 4. Dotted lines indicate simulated results, and symbols indicate observed results: 0 ,unreactedcoal; 0 ,oil; A,asphaltene; 0,preasphaltene. Yallourn coal 2.0 g, 1-methylnaphthalene4.0 mL, P(H2) 5.0 MPa (400 "C), Fe(CO)a 0.40 mmol, S 0.40 mmol.
Figure 2. Observed and simulatedTGults of the liquefaction of Yallourn coal (a) according to kinetic model shown in Scheme 2 and (b) according to kinetic model shown in Scheme 4. Dotted lines indicate simulated results, and symbols indicate observed results 0 ,unreacted coal; 0,oil; A,asphaltene; 0,preasphaltene. Yallourn coal 2. Og, 1-methylnaphthalene 4.0mL, P(H2)7.9 MPa, (425 "C),Fe(C0)5 0.40 mmol, S 0.40 "01.
k5). If supply of hydrogen to radical pool is not enough, certain part of the radicals repolymerize to insoluble materials. A considerable part of insoluble material gradually decomposes to free radicals again. Through this step the rate of coal conversion would be varied. This method of adjusting coal conversion is quite different from that of simply changing CI depending on the reaction conditions using kinetic model shown in Scheme 2. Results of curve fitting are shown in Figure 1,a (Scheme 2) and b (Scheme41, by applying kinetic models in Schemes 2 and 4 in the hydroliquefaction of Yallourn coal with a smaller amount of Fe(CO)5-S catalyst (0.4 mmol) A very good fitting for unreacted coal was obtained for the case using Scheme 4. Although at a higher initial hydrogen pressure a better fitting was obtained in the kinetic model shown in Scheme 2 (Figure 2a), improvement of fitting is remarkable as shown in Figure 2b (Scheme 4). In these calculations (Scheme 41, for the values of kl' + k4' for entries l', 2', and 3' in Table 5, the same values are employed. The value (kl' + k4') taken here is obtained by the first-order plot of unreacted coal vs time in the hydroliquefaction of the same coal giving the highest rate
at the same temperature, that is for this particular coal sum of kl, k4, and k5 in entry 2 was employed. The first order rate constants (based on Scheme 2, entries 1-6, and Scheme 4, entries 1'- 4') are summarized in Table 5. Among the six rate constants, much larger values for kl, kq, and k5, for the model shown in Scheme 2, are noticeable in all the experiments. Such a tendency is more pronounced in the caae of low-rank coal (Yallourn). This indicates that direct depolymerization of this coal into asphaltene and oil fractions predominate over the consecutivepathways via preasphaltene intermediate. This seems to strongly reflect the chemical structure of lowrank coal. Yallourn coal is believed to contain small amounts of condensed aromatic ring structure and to have large amounts of ether linkages, which would easily cleave thermally. Thus even at an early stage of the liquefaction smaller molecular weight fragment radicals would be formed. Increasing the amount of catalyst markedly increased all the rate constants (compare entries 1and 2). Among the rate constants, kp and k~ increased by a factor of 11 and 13,respectively, with a2.5-fold increase in the loading
.
Energy & Fuels, Vol. 8, No. 2,1994 347
Highly Dispersed Coal Liquefaction Catalysts
Table 5. Rate Constants for Hydroliquefaction of Various Coals with Fe(CO)5 Catalyst. rate constant, min-1 entry
coal
1 Y L 2 Y L 3 YL 4 YL’
5 w 6 1’ 2’ 3’ 4’
MKe
catalmt,mmol P(H21,MPa convn,” % 0.40 1.00 0.40 0.40 0.40 0.40
5.0 5.0 7.9 5.0 5.0 5.0
83.5 97.7 89.4 87.8 87.4 96.8
kl’ k,’ kl Kinetic Model Scheme 2 0.0213 0.0740 0.0380 0.0230 0.0700 0.1300 Kinetic Model Scheme 4 0.201 0.040 0.250 0.201 0.035 0.335 0.170 0.070 0.243 0.116 0.041 0.312
k2
ka
kd
ks
ke
0.0007 0.0078 0.0070 0.0024 0.0120 0.0050
0.0007 0.0110 0.0062 0 0.0100 0.0038
0.0366 0.0760 0.0590 0.0340 0.0300 0.0100
0.0178 0.0550 0.0380 0.0160 0.0330 0.0700
0.0018 0.0049 0.0037 0 0.0080 0.0033
0.0076 0.0127 0.0042 0.0044
0.0105 0.0182 0.0081 0.0012
0.125 0.175 0.0056 0.0044
0.225 0.225 0.246 0.210
0 0.60 0.006 0.085 0.0120 0.204 O.OOO4 0.695
k7
k8
0.035 0.050 0.057 0.045
a Coal 2.0 g, 1-methylnaphthalene4.0 mL, temperature 420 ‘C. YL = Yallourn coal, WD = Wandoan coal, MK = Mi-ike coal. Entries 1’-4‘ are recalculations of entries 1-4. b Conversion at 60 min. At 400 ‘C.
of Fe(CO)s. On the other hand, kl, kq, and k6 increased by factors of about 3. From these findings, we concluded that an abundant catalyst promotes the consecutive pathways more effectively than the direct formation of asphaltene and oil fractions from coal. Such a conclusion is not consistent with previous findings showing that Fe(CO)5 based catalyst is not highly active for the hydrogenation of condensed aromatic compounds or hydrocracking of C-C bond in the studies of coal model compounds. Using the present model (Scheme4), we see the increases of rate constants kl to k,j (see entries 1’and 29, with an increase in the amount of catalyst, but the increases are not as notable as the case of the previous model. The most significant differences in entries l‘and 2’are observed in the decrease of k, when larger amount of catalyst was employed. This clearly shows that prompt hydrogen transfer to radical pool favored by a larger amount of catalyst prevents repolymerization of coal fragment radicals and promotes forward reaction giving PA, AS, and oil. A similar observation was also noticed for the reaction under a higher hydrogen partial pressure (entry 3’). In this case, direct conversion of coal to oil (k4’) increased considerably, indicating that rapid hydrogen transfer reaction to a smaller coal fragment radical did occur. Yallourn coal is very young coal and unit structures are small. Therefore, AS and oil could be simultaneously produced from this coal. In bituminous coal like Mi-ike,
the reaction proceeds mostly through a consecutive pathway, preasphaltene and asphaltene as intermediates, as indicated by the larger value of kl and smaller value of k4. In such a case, the kinetic model shown in Scheme 2 can be applied without large discrepancy. Such a conclusion clearly shows that the model employed here (Scheme 4) describes coal liquefaction mechanism more accurately than Scheme 2. Conclusion. Iron pentacarbonyl (Fe(C0)6) and iron pentacarbonyl with molecular sulfur provided an active catalyst for coal hydroliquefaction. Active species was characterized to be pyrrhotite in highly dispersed state by Mbsbauer spectroscopy. Liquefaction with the dispersed iron catalyst in tetralin and tetralin-naphthalene mixed solvent suggests that the most significant role of the catalyst is to activate molecular hydrogen and transfer it to coal fragment radicals. Kinetic treatment of catalyzed coal liquefaction strongly supported above mechanism of hydrogen-transfer reaction. Consequently, retrogressive reactions of thermally decomposed coal fragment radicals leading to an insoluble fraction are suppressed and increased coal conversion and yield of oil fractions are obtained.
Acknowledgment. The author thanks the Ministry of Education, Science and Culture for Grant in Aid for priority area “Energy” 03203254 and 04203101.
