446
Energy & Fuels 1993,7, 446-454
Activity and Characterization of Anion-Modified Iron(II1) Oxides as Catalysts for Direct Liquefaction of Low Pyrite Coals V. R. Pradhan,+ J. Hu, J. W. Tierney, and I. Wender* Chemical and Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received October 26, 1992. Revised Manuscript Received February 24, 1993
In previous papers, we reported on the catalytic activity of finely divided sulfated iron oxides for the direct liquefaction of coals containing between 0.2 and 2.5 w t 7% of pyrite (Energy Fuels, 1991, 5,497-507). The addition of small amounts of sulfated iron oxide catalysts (3500-7000 ppm relative to coal) significantly improved coal conversions and oil yields in the direct liquefaction of these coals. However, the large amounts of inherent pyrite present clouded the interpretation of catalyst composition, dispersion, and activity of the added sulfated iron oxide catalysts. In this paper, we address the quantification of catalyst activity and characterization, before and after reaction, of some previously reported and some newly synthesized anion-modified iron oxide catalysts by using two coals with very low inherent contents of pyrite. Information obtained on these anion-modified iron oxide/oxyhydroxide catalytic systems includes: physicochemical characterization of the sulfated Catalysts before coal liquefaction; activity of small amounts of iron added as FeOOH/SOr, Mo/ FeOOH/SO4,FezO$SOr, and Mo/Fe2O$SOr Catalystsfor direct liquefaction of a low-pyritebituminous (Blind Canyon) and asubbituminous coal (Wyodak);andquantification of dispersion and composition of these catalysts after coal liquefaction and their transformation and sintering behavior under coal liquefaction conditions. The initially added sulfated iron oxides are completely converted to highly dispersed pyrrhotites (-20 nm) essentially all as Fe& in about 30 min under liquefaction conditions. These pyrrhotites maintained their state of high dispersion for long reaction times (120 min) without apparent agglomeration. The ability of sulfated iron oxide catalysts to form highly dispersed pyrrhotites under coal liquefaction conditions and to maintain them with little or no agglomeration renders these catalysts well suited for direct coal liquefaction reactions at low catalyst concentrations.
Introduction Iron, because of its low cost, activity, and environmental acceptability, has been perceived as a potential catalyst for the first stage of coal liquefaction, namely coal dissolution. A chief objectiveof various methods of adding iron catalysts is to provide high catalytic surface area and fine particulate size. The initial dispersion of the precursor has a strong influence on the activity of the sulfided phases formed under liquefaction conditions. Means must also be sought to prevent agglomeration of catalyst particles so as to maintain their state of high dispersion.' In the presence of enough sulfur, iron catalysts form pyrrhotites (Fel-xS) which, along with hydrogen sulfide, function as catalysts for the hydrogenation and hydrogenolysis reactions which occur during the hydroliquefaction of coal.26 Application of finely divided and chemically modified powdered solid iron oxide based catalysts used in this work shows considerable pr0mise.6'~ The objective of this research has been to use low-sulfur, low-pyrite coals to verify the catalytic effects of small * To whom the correspondence should be addressed.
+ Present address: HRI, Inc., Princeton, NJ 08540. (1) Derbyshire, F. J. Catalysis in Coal Liquefaction: New Directions for Research; IEA Coal Research London, 1988; IEA CR-08. (2) Trewhella, M. J.; Grint, A. Fuel 1987,66,1315-1319. (3) Montano, P. A.; Granoff, B. Fuel 1980,59, 214-217. (4) Baldwin, R. M.; Vinciguerra, S. Fuel 1983, 62, 498-501. (5) Ogawa, T.; Stenberg, V. I.; Montano, P. A. Fuel 1984,63, 16601663. (6) Derbyshire, F.; Hager, T. Dispersed Catalysts for Coal Dissolution. Prep. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1992,37, No. 1,312-319. (7) Derbyehire, F. J., Energy Fuels 1989,3, 273-277.
0887-062419312507-0446$04.00/ 0
amounts of iron added to the liquefaction reactor as sulfate and molybdate anion-promoted oxides or oxyhydroxides. We have reported on the use of sulfate-promoted iron and tin oxides for the direct liquefaction and coprocessing of Argonne Illinois No. 6 coal with tetralin and with Maya ATB heavy oil, respecti~ely.~8J~>m The following topics will be discussed (i) activity of small amounts of iron and molybdenum added as sulfated (8)Charcoeset, H.; Genard, A. Catalysis in Direct Coal Liquefaction. In Synthetic Fuels from Coal; Romey, I., Paul, P. F. M., Imarieio, G., Eds.; Kluwer Academic: Norwell, USA, 1987. (9) Beeeon, M.; Bacaud, H.; Charcoeeet,H.; Cebolla-Burillo,V.; Oberaon, M. Fuel Process. Technol. 1986,12,91-109. (10) Bacaud, R.; Beeeon, M.; Charcoeeet, H.; Oberson, M.; Vinh Huu, T.; Varloud, J. Fuel Process. Technol. 1986,14,213-220. (11) Charcoeset, H.; Bacaud, R.; Beseon, M.; Jeunet, A.; Nickel, B.; Oberson, M. Fuel Process. Technol. 1986,12,189-201. (12) Marriadaeeou, D. G.; Charcoeset, H.; Besson, M.; Varloud, J. Fuel Process. Technol. 1986,12, 143-153. (13) Andres,M.;Charcoeeet,H.;Chicke,P.;Davignon,L.;Marriada~~~u, G.; Joly, J. P.; Pregennain, S. Fuel 1982,62, 69-72. (14) Eklund, P. C.; Stencel, J. M.; Bi, X.X.;Keogh, R. A.; Derbyshire, F. J. Prepr. Pap. Am. Chem. Soc., Diu. Fuel Chem. 1991,36, No. 2,551558. (15) Tanabe, K.; Hattori, H.; Yamaguchi, T.; Iizuka, T.; Matauhashi, H.; Kimura, A.; Nagase, Y. Fuel Process. Technol. 1986,14, 247-260. (16) Kotanigawa, T.; Yokoyama,S.; Yamamoto, M.;Maekawa, Y. Fuel 1989,68,618-621. (17) Yokoyama, S.; Yamamoto, M.; Maekawa,Y.; Kotanigawa, T. Fuel 1989,68,531-533. (18) Pradhan, V. R.; Tiemey, J. W.; Wender, 1.Prepr.Pap-Am. Chem. SOC.,Diu. Fuel Chem. 1990,35, No. 2,793-800. (19) Pradhan, V. R.; Huffman, G. P.; Tierney, J. W.; Wender, I. Energy Fuels 1991,5,497-507. (20) Pradhan, V. R.; Herrick, D. E.; Tierney, J. W.; Wender, I. Energy Fuels, 1991,5, 712-720.
0 1993 American Chemical Society
Anion-Modified Zron(IIl) Oxides as Catalysts Table I. Ash-Free Elemental Analyses (in Weight Percent) of Coals pyritic coal carbon hydrogen nitrogen oxygen sulfur sulfur Wyodak 75.0 5.4 1.1 18.0 0.5 0.17 Blindcanyon 81.6 6.2 1.4 10.3 0.5 0.02 Table 11. Preparation Conditions for Catalysts Used in This Study catalyst hydrolysis catalyst0 designation starting saltb DH Fez03 I iron(II1)nitrate 8.6 FezOdSO, I1 iron alum 4.5 Mo/FezOs/SO, I11 iron alum 4.5 FeOOH/SO4 IV iron alum 4.1 FezOa/MoO4 V iron(II1)nitrate 8.5 Fe20$WO4 VI iron(II1)nitrate 8.6 Mo/FeOOH/SO4 VI1 iron alum 4.5 a All the catalysts except IV and VI1 were calcined at 600 "C; Catalysts IV and Vn were used without calcination. bCatalysts synthesized from iron alum were prepared using the homogeneous precipitation; all other were prepared using the heterogeneous precipitation method.
oxides for direct liquefaction of low-pyrite coals (hvbC Blind Canyon 0.01 w t % pyrite, and subbituminous Wyodak0.17 % pyrite), (ii) synthesis and physicochemical properties of sulfated and other anion-modifiediron oxide/ oxyhydroxide catalysts before use in coal liquefaction reactions, (iii) quantificationof dispersion and composition of iron phases after coal liquefaction reactions, and (iv) transformation and sintering behavior of these catalysts under coal liquefaction conditions. Experimental Section Starting Materials. High-volatilebituminous (hvbC) Blind Canyon DECS-17coal, obtained from the Penn StateCoal Sample Bank, and Wyodak subbituminous coal, obtained from the Argonne Coal Sample Bank, were used in this study. Elemental analyses of the coals are given in Table I. Tetralin (99+% pure) was obtained from the Fisher Scientific Co. Starting materials used for catalyst preparation were iron alum [F~Z(SO~)S(N)~)ZSOI.~~HZO], ferric nitrate, urea, and 28% ammonia water, all purchased from the Aldrich Chemical Co. Ammonium heptamolybdate and ammoniummetatungstate were purchased from the Sigma Chemical Co. and from Strem Chemicals, Inc. respectively. Catalyst Preparation andcharacterization. The sulfated oxides and oxyhydroxides of iron were prepared from either the sulfate or nitrate salts precipitated with either what is termed as heterogeneousprecipitation (usingNH4OH as the precipitating agent and adding it dropwise so that the pH of the solution changes with time) or by homogeneous precipitation (using urea as the precipitating agent so that the local concentration of OHions and solution pH is uniform duringprecipitation). The result of the precipitation reaction (hydrolysis)is the formation of iron oxyhydroxide (FeOOH) with small residual amounb of sulfate anion adsorbed on its surface. The presence of sulfate anions during precipitation has been reported to bring about surface charge modification of the precipitated particles; this affects the chemistry and kinetics of the precipitation/crystallization.29The catalysta used in this study and conditions of their preparation are listed in Table 11. In a bimetallic catalyst, MO/Fe20s/SOI, molybdenum was introduced onto the sulfated iron oxyhydroxide by an incipient wetness impregnation technique using a solution of ammomium heptamolybdate. For a molybdate anion promoted iron oxide, Fe~Os/Mo04,5 wt 5% of molybdate anion was introduced into a FeOOH (21)Matijevic, E.;Schneiner,P. J. ColloidZnterface Sci., 1978,63 (3), 231-237.
Energy & Fuels, Vol. 7, No. 4, 1993 447 precipitate by contacting it with a solution of ammonium heptamolybdate. It was then calcined in air at 500 OC. Similarly, for a tungstate anion promoted iron oxide catalyst, FeOOH precipitates were treated with a solution of ammonium metatungstate [(N&)sHzWlzO401. To characterize the size and structure-related properties of these catalysts, the following measurements were made: BET-surface area, sulfur content, thermogravimetry (TGA), acidity measurements, thermal stability measurements, X-ray diffraction, and electron microscopy. Residues of coal liquefaction experiments were also analyzed usinga Philips X-ray diffractometer and a JEOL 2000 FX STEM (100 kV beam) with an energy-dispersive X-ray spectrometer (EDX) to determine composition and dispersion information about catalytic phases formed under liquefaction conditions. Reaction Studies. Tetralin was the reaction solvent (3:l by weight to coal) and elemental sulfur ( 2 1 by weight to catalyst) was used for insitu catalyst sulfidation. Althougha donor solvent such as tetralin tends to mask catalytic effects in coal conversion, ita presence is necessary in direct coal liquefaction to ensure complete conversion of the iron catalyst precursor to its sulfide via HzS formation in 5-6 min at 400 OC at 1000 psig of cold Hz. Coal liquefaction reactions were carried out in both a 300-cm3 stainless steel autoclave and a 27-cm8 tubing bomb microreactor at 400 "C and 1000 psig of ambient HZ (1800 paig at reaction temperature). The heatup times in the 300-cm* stainless steel autoclaves were relatively longer (30-35 min) as compared to those in the 27-cm8tubing bomb microreactors (2-3 min). Coal conversions were determined using Soxhlet extraction with methylene chloride; soluble products were recovered by rotary evaporation at 45 OC under vacuum. Pentane soluble8(oils)were determined by adding 40 volumes of n-pentane to the methylene chloride solubles and using Soxhlet extraction with n-pentane. Pentane-insoluble but methylene chloride soluble material was referred to as asphaltenes. Total conversion of coal and the conversions to product fractions were defined on an ash-free basis as shown below (repeatability of our experimental results, determined by the duplicate runs, was f 2 % for total coal conversion and f2.5% for oils): coal conversion:
YT = 100(1- Y); Y = (W, - w,
conversion to asphaltenes:
YA = loo(wA/w,)
conversion to gases (+losses): Yo = 1oo(YT- Y,), conversion to oils:
- Wd)/ w,
where YM= W,/ W,
Yo = lOO(Y, - YA)
where WI is the weight of CHzClz insoluble products, WCis the weight of catalyst used, W d is the weight of ash in coal, Wis the moisture- and ash-free weight of starting coal, WAis the weight of n-pentane-insoluble products, and WMis the weight of methylene chloride soluble products.
Results and Discussion C a t a l y s t S y n t h e s i s and Characterization. The catalysts used are listed in Table I11 with relevant physicochemical properties. T h e presence of sulfate anions during precipitation was necessary t o form nano-sized oxide particles, found resistant to agglomeration at high temperature. Resulfation (using 1.0 N HzS04) of iron oxyhydroxide precipitates, already containing some sulfate, did not influence either its particle size or activity for coal liquefaction. The measure of initial dispersion of the catalysts was obtained using XRD line broadening measurements and transmission electron microscopy (TEM). The crystallite
448 Energy & Fuels, VoZ. 7,No.4,1993
Pradhan et al.
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sizes of sulfated iron oxyhydroxides could not be determined by X-ray diffraction due to their very low bulk crystallinity. These oxyhydroxides, after calcination,gave rise to catalyst I1 (high bulk crystallinity). Use of TEM revealed that catalyst IV contained very small needleshaped elongated, thin crystallites with average dimensions of 30 X 3 nm (Figure la). Nitrogen-porosimetry measurements were carried out on all catalysts. As shown in Figure l b for catalyst IV, a macroporous distribution of pores was obtained, indicating that individual fine particulates of the catalyst come together forming very thin void channels (about 20-30 nm average dimensions). Most of the BET surface area is therefore external and easily available for reactions. A scanning electron micrograph
(Figure IC),indicating the surface morphology of catalyst IV, also shows an open porous structure. All calcined iron oxides, completely crystalline after calcination, were about 10-15 nm and were elongated in shape as determined by electron microscopy (Figure 2, a, b, and c). As indicated in Table 111, the uncalcined sulfated oxyhydroxides (such as catalyst IV) had BET specific surface areas higher (120-130 m2/g) than the calcined oxides (80-90 m2/g). Evidence that all of the sulfated iron oxides had low porosities (pore volumes smaller than 0.2 cm3/g)was obtained by calculating equivalent spherical diameters from their surface area values and comparing them with those determined by TEM; the values were within 10% of each other. The nature of the iron oxide surfaces was studied by FTIR and XPS. As shown in Figure 3a,the FTIR spectrum of catalyst IV, obtained at 450 "C under vacuum, shows an S=O band (1440 cm-1) similar to the S=O bond formed in the bidentate chelating complex (of sulfate anion with the oxide surface) proposed by Tanabe et al.15 Upon adsorption of pyridine, this band shifted to about 1500 cm-l and new bands, corresponding to coordinatively bonded pyridine (Lewis acid sites on the catalyst surface) and to pyridinium ions (Bronsted acid sites) appear in the spectrum (Figure 3b). XPS of the sulfated and molybdated iron oxides indicated that almost 95% of the S and of the Mo, both in +6 oxidation states,were on the catalyst surface. For the unsulfated FeOOH, phase transition from
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Anion-Modified Iron(III) Oxides as Catalysts
Energy & Fuels, Vol. 7, No. 4,1993 449
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an0
wn
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Figure 3. (a, top) An FTIR spectrum of catalyst IV a t 450 O C under vacuum and (b, bottom) Lewis and Bronsted acid sites by pyridine adsorption on catalyst I V FTIR a t 400 "C under vacuum.
Figure 2. Transmission electron micrographsof (a, top) catalyst 11, (b, middle) catalyst V, and (c, bottom) catalyst VI.
an amorphous to a crystalline phase occurred at about 400 "C;for the sulfated and the molybdated iron oxyhydroxides, this phase transition occurred at about 600 "C. Reaction Studies. Direct coal liquefaction reactions were carried out both in horizontally shaken tubing bomb microreactors and a well-stirred 300-cm3batch autoclave. Blank (thermal) runs were carried out using a brand new tubing bomb reactor to determine the catalytic activity of inherent mineral matter present in the coals. Runs were also made with only elemental sulfur added to the coal liquefaction reactor (in the absence of added catalyst) for comparison. In all experiments the catalysts were simply
\
mixed with the coal-tetralin slurry by manual stirring after the catalysts were preheated in an oven at 450 "C for 1h. Direct coal liquefaction experiments with sulfated iron oxides, presonicated in the reaction solvent for 1h, were also carried out; this treatment of sulfated catalysts is responsible for breaking the physical agglomerates of catalyst (about 1pm size,as determined by light scattering) into smaller particles which formed stable colloidal suspensions in tetralin, therefore increasing the extent of initial catalyst dispersion. 1. Comparison of Catalytic Activities of Sulfated Iron Oxides and Oxyhydroxides with Other Finely Divided Catalyst Precursors. Use of catalysts IV and VI1 (both uncalcined) resulted in total coal conversion levels similar to those obtained with their calcined forms (catalysts I1 and 111),although higher oil yields were obtained with the calcined forms (Figure 4). The higher activity of calcined sulfated iron oxides for oil production is probably related to their calcination treatment at 500 "C for 3 h. The higher water content of the oxyhydroxides may make them more susceptible to sintering during transformation to pyrrhotites; some evidence for this was obtained when the particle sizes of pyrrhotites derived from catalyst IV were compared with those formed from the calcined catalysts I1 and 111.
Pradhan et al.
450 Energy & Fuels, Vol. 7, No. 4, 1993 E Z %Coal Conversion I %Oils Yield
Catalyst VI1 Catalyst 111
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Fe(CO)S
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Figure 4. Activities of sulfated iron oxidesJoxyhydroxidesin comparison with soluble organometallic precursors for hydroliquefadion of Blind Canyon DECS-17 coal at 400 O C , 1000 psig (cold)of H2,in a 300-cm3stirred batch autoclave, 1200rpm, 60 min, 3:l tetralin-to-coal,0.35 wt % Fe relative to coal. 90
40
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80
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PPM molybdenum (with 2500 ppm Iron)
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% 011Yields
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Figure 6. Conversionof DECS-17 and yield of oils as a function of loading of molybdenum, added as catalyst 111, at 400 "C, 100 psig (cold) of H2,in a 27-cm3batch tubing bomb reactor, 150 cpm, 60 min, 3:l tetralin-to-coal. e23 %Coal Conversion I %Yield of Oils
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Figure 5. Conversion of DECS-17 and yield of oils as a function of loading of iron, added as catalyst IV, at 400 OC, loo0 psig (cold)of Hz,in a 27-cms batch tubing bomb reactor, 150 cpm, 60
min, 3:l tetralin-to-coal.
Figure 7. Activities of iron(II1) oxides, modified by small amounts of different anions, for hydroliquefaction of DECS-17 coal at 400 "C, loo0 psig (cold) of Hz,in a 300-cm9stirred batch autoclave, 1200 rpm, 60 min, 31 tetralin-to-coal, 0.35 w t % Fe relative to coal.
Pyrrhotites formed from catalyst IV were larger (average size 30 nm) than those formed starting with catalyst I1 (average size 20 nm). Activities of sulfated iron oxides were compared with those of organometallic precursor complexes such as Fe(CO)s and Mo(CO)~ as well as with a finely divided (30 A) iron oxide catalyst; catalyst I1 was more active than Fe(C0)s at the same iron loading. Interestingly, 500 ppm of Mo (added as either Mo(COIs or molybdenum naphthenate) relative to coal resulted in about the same conversion levels as 3500 ppm of Fe added as catalyst 11. 2. Coal Conversion and Oil Yields as a Function of Catalyst Concentration. Coal conversions were carried out using the smallest catalyst loadings that gave meaningful results. As pointed out, most coals contain considerable amounts of iron (as FeS2) so that it is difficult to obtain reliable data on the effect of small catalyst loadings of iron on coal conversion levels. The Blind Canyon coal studies contained only 0.02 wt % of pyrite and the Wyodak, only 0.17%. Uncalcined catalyst IV, which was as active as the catalyst I1 (obtained after hightemperature calcination), was employed as the catalyst for runs carried out to determine the effect of small loadings of iron (100&5000 ppm with respect to coal) on coal conversion and on oil yields (Figure 5). With the Blind Canyon coal, a thermal blank conversion
of 60% (maf), baaed on the solubilityof products in CH2C12, was obtained with a 22 % yield of n-pentane solubles. With 3500 ppm of Fe as catalyst 11, total coal conversion increased to 77 % (maf) with over a 35 % yield of oils. The highest coal conversion, 87 % (maf) with about 48% oils, was achieved with 1 wt % iron (catalyst IV) relative to coal. The effect of Mo loadings between 20 and 200ppm (with respect to coal) added with 2500 ppm of iron as catalyst I11is shown in Figure 6. Amounts of Mo added to catalyst I1enhanced total coal conversion from about 72 % to about 80% ;the yield of light oils, however, rose from 32 % without Mo t o about 46% with 100 ppm of Mo, both with 2500 ppm of Fe. On increasing the Mo loading to about 200 ppm, total coal conversion was not much affected (87% maf); the yield of oils changed from 46% with 100 ppm Mo to about 52% at 200 ppm. Molybdenum in catalyst I11probably forms MoS2, which has a strong hydrogenation and hydrogenolysis function and thus more oil is produced at the expense of asphaltenes; the overall conversion of coal is only slightly affected. 3. Effect ofDifferentAnionic Modifications ofIron(III) Oxides on Coal Liquefaction Activity. As shown in Figure 7, iron oxides modified with 5 wt % each of molybdate (Moo4)and tungstate (WOd anions were as active as the
Anion-Modified Iron(II0 Oxides as Catalysts sulfated iron oxide catalysts in terms of overall coal conversion levels. All runs indicated in Figure 7 were made with addition of elemental sulfur to the reactor. Addition of elemental sulfur alone in a blank run (no iron catalyst added) resulted in 66% (maf) total coal conversion with a 27% oil yield. Iron oxides modified with either tungstate or molybdate anions resulted in slightly higher oil yields than the sulfated iron oxide; higher activity could be due to the formation of highly active and well-dispersed MoSz and WSZformed during coal liquefaction. These new types of anionic modifications of iron oxides do indeed bring about significant enhancement in coal liquefaction activity, primarily by increasing the extent of their initial catalytic dispersions and an ability of preserving this state of higher dispersion under reaction conditions. Molybdenum, introduced into the reactor either as Mol Fe20dS04 catalyst or in its anionic form, FezOdMoO4, is certainly more active at lower loadings (50-200 ppm relative to coal) as compared to molybdenum used alone (in any suitable precursor form). For the direct coal liquefaction reactions we carried out, no significant differences in the performances of Mo/Fe203/S04 and FezOJMoO4 were obtained. 4. Catalyst Presulfidation and Use of Resulting Pyrrhotites as Catalysts. To establish the activity of highly dispersed pyrrhotites formed from finely divided sulfated iron oxides, we carried out presulfidation, instead of in situ catalyst sulfidation, of different sulfated iron oxides and oxyhydroxidesby reacting the initial sulfated catalysts with 2:l (by weight) of elemental sulfur in the presence of tetralin at 400 "C and lo00 paig (cold) Hz for 30 min in a 300-cm3batch autoclave. The resulting sulfides were washed with methylene chloride and dried at 90 "C under vacuum. X-ray diffraction and electron diffraction both indicated that all sulfated iron oxides were converted to Fe& with traces of FellSlz. X-ray line broadening measurements and transmission electron microscopy indicated a crystal size of about 20 nm for these preformed pyrrhotites, which interestingly contained about 3-5 w t 7% of carbon derived from the solvent, tetralin. Tetralin was found to have been hydrocracked, hydrogenated, and dehydrogenated during presulfidation of the sulfated c a t a l y ~ t S . ~About ~ J ~ 2 5 4 0 % of the tetralin was converted to naphthalene, decalin, 1-methylindane,and other lighter products (as determined from the GC-MS analysis). The preformed pyrrhotites were then employed as catalysts for direct liquefaction of Blind Canyon coal at 400 "C at 0.25-0.35 wt % of iron loading relative to coal both with and without additional elemental sulfur. The results of these experiments are shown in Figure 8. The preformed Fe& catalyst was almost as active as catalyst IV at 0.35 wt 7% iron loading. Further addition of elemental sulfur to this Fe& catalyst improved its activity, yielding 42% of oils as compared to 30% with larger amounts of Fe& in the absence of added sulfur with 2500 ppm of iron. Increase in oil yields upon addition of elemental sulfur to Fe7S8 indicates possible interactions between pyrrhotites and H2S for catalyzing hydrogenolysislhydrogenation reactions during coal liquefaction. The presence of HzS also ensures that the iron-deficient and sulfur-rich stoichiometry of pyrrhotites is maintained. The starting sulfated iron oxides were also used as catalysts for two successive liquefaction runs, i.e., FezO3/
Energy &Fuels, Vol. 7, No. 4, 1993 461 % Coal Conversion
I% Oils Yield
Fe7SgcS 25W pprn
3wo Fe7S8 ppm
re
FeOOH/SO4+S 3m ppm h FeOOH/SO4+S ZSW ppm
r.
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90 .80
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70
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60
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--
6 5 Fez03 + S I Fe203/S04
+S
eZa Mo/Fe~Og/S04 + S
50 40
30 20
10
0
Temperature,
OC
Figure 9. Effect of reaction temperature on the activities of sulfated iron oxidesfor hydroliquefaction of an Argonne Wyodak coal at lo00 psig (cold) of H2,in a 27-cm9 batch tubing bomb reactor, 150cpm,60 min,3 1 tetralin-to-coal,0.36wt % Fe relative to coal.
SO4 was used as catalyst for one liquefaction run and the spent catalyst (iron sulfides) recovered from this run was placed into the liquefaction reactor for the second consecutive run. This spent catalyst did not do as well in terms of converting coal to liquids as the fresh sulfated iron oxide, although it brought about noticeable rises in conversion levels as compared to those obtained in a noncatalytic liquefaction (Figure 8). 5. Effect of Reaction Temperature. Catalysts I, 11, and I11 were used for liquefaction of Wyodak coal (0.17 wt 5% pyrite) at 375,400, and 425 "C. As shown in Figure 9, the activities of these catalysts for the production of oils from coal increased proportionately on going from 375 to 425 "C except for the catalyst I, an unmodified iron oxide, with which oils decreased somewhat in going from 375 to 400
"C.
Sulfated catalysta resulted in higher yields of oils than the unsulfated ones at all temperatures. Fifty ppm of Mo relative to coal, present as the bimetallic catalyst 111, exhibited a maximum catalytic effect on oil formation a t 375 "C. Quantification of Dispersion and Composition of Catalysts after Coal LiquefactionReactions. In order to study what happened to the initially added iron catalyst precursors after coal liquefaction, we carried out reactions
Pradhan et al.
452 Energy & Fuels, Vol. 7, No. 4,1993
Table IV. Results of Catalyst Transformation and Sintering Studies on Model System of Active Carbon reaction time, min Oa
30
120
iron phases detected Fe203, FesO,, FeS2, Fe1-S FeS2, Fe1,S Fe1,S
av particle size (XRD), (Fe/S) atomic nm ratio 14 16 16
1.4 1.0 0.9
a 0 minutes of reaction means ‘heatup” time (35 min from room temperature to the reaction temperature)of the 300-cm3 autoclave.
times) and the extent of catalyst sintering (i.e., the degree of catalyst dispersion) of the iron-containing phases that result from transformations of sulfated iron oxides during coal liquefaction. In general, the stoichiometry (or composition) of the iron phases formed during liquefaction is controlled by the following schematic representation: Fe20$S0,
+ S + H2[Fe(SH), or FeOHSH]
. i
t
t
Figure 10. Transmission electron micrographs of the direct coal liquefaction residues, showing iron-containing particles of pyrrhotites obtained from (a, top) sulfated iron oxide precursor and (b, bottom) an Fe(CO)5 precursor.
of Blind Canyon coal with catalyst I1 and with Fe(C0)S precursors in separate experiments. The insoluble residues obtained in these experiments, containing the transformed iron catalysts, were characterized by X-ray diffraction and STEM coupled with an energy-dispersive X-ray detector (EDX). As expected, all the initially added iron for both precursorswas completelyconverted to pyrrhotites, highly dispersed in the insoluble organic matter in the residues of liquefaction. A comparison,by TEM, of the dispersion of iron-bearing particles (mainly pyrrhotites) in the liquefaction residues obtained from catalyst I1 and from the soluble organometallic precursor, Fe(C0)5, is shown in Figure 10, a and b. It is apparent from these micrographs that pyrrhotites formed starting with Fe(C0)5 are larger in size (30-50nm) than those resulting from catalyst I1 (15-20 nm) showing that sulfated iron oxides are more resistant toward hightemperature sintering than the products formed from the solubleprecursors such as Fe(C0)s. Sulfur-to-iron atomic ratios of between 1.12 and 1.14 were obtained for pyrrhotites obtained from these experiments. An unsulfated iron oxide (30 A in size) was more susceptible to agglomeration than a sulfated catalyst (according to the results of diffraction studies and particle size studies using an electron microscope). Catalyst Transformation and Sintering Studies. We attempted to investigate the rate of transformation (i.e., compositions of sulfides formed at different reaction
-
Fe#,
+ H2S
While one sees pyrrhotite at the end of the coal liquefaction reaction as the predominant iron phase, there may exist intermediate phases of iron and sulfur such as indicated in this equation. Formation of Fe(SH)2starting from iron oxyhydroxides and H2S has been proposed by Baruah et a1.22 A goal of our catalyst transformation studies was to determine atomic Fe/S ratios and grain sizes of the iron phases in coal liquefaction residues using X-ray diffraction and electron microscopy. Even a lowpyrite coal such as Blind Canyon DECS-17 contains 3600 ppm of iron in various forms. These amounts of inherent iron, while low for liquefaction catalysis, could interfere with characterization of the small amounts of added iron catalysts after reaction. The amounts of iron added in the form of the sulfated catalysts in our experiments were of the same order of magnitude as the amounts present in low-pyrite coals. We therefore chose to use active carbon instead of coal to avoid any interference due to mineral matter in the coal. The composition of the catalysts (sulfated and sulfided phases) and their sizes were studied at different reaction times using the following model system: Active carbon + tetralin + Fe2Os/SOa catalyst (under our “normal” coal liquefaction conditions). The reactions were carried out under identical conditions as coal liquefactionwith addition of elemental sulfur.When the reactions were carried out with a sulfated iron oxide for different reaction times, it was found that the initial catalyst takes about 5 min at reaction temperature (400 “C) to be completely converted, through pyrite to pyrrhotites. The grain size changes occur primarily during the phase transformation of oxide to sulfide. No further changes (increases) in the grain size of iron-containing phases were observed even when the reaction was carried out for 2 h. The results of catalyst size (dispersion) and composition studies obtained using the above model system, followed by examination of the residues by XRD, TEM, elemental analysis, and EDX, are shown in Table IV and Figure 11 (comparison of the EDX X-ray counts for Fe and S in the residues). We also carried out presulfidation of the sulfated iron catalysts by reacting them with sulfur in the presence of tetralin at 400 “C and 1000psig (cold) pressure of H2 for 30 min of reaction time. Several sulfated and Mo(22) Baruah, M.K.Fuel Process Technol. 1992,31,115-126.
Anion-Modified Iron(III) Oxides as Catalysts Reaction time
=0
Energy & Fuels, Vol. 7, No. 4,1993 453
min.
Fe203 Fe304
FeS2 F e1,,s Davg(XRD)
= 14 nm ( F e / S ) a t o m i c = 1.4 Reaction time
=
30 min.
Reaction time
=
120 min.
= 16 nm ( F e / S ) a t o m i c = 0.88 Davg
(XRD)
r
II
I .
i e
I " , rV
Figure 11. Comparison of the EDX X-ray energy counts for different sulfided samples during the transformation of iron(II1) oxides to sulfides.
promoted sulfated iron. catalysts were treated in this manner. These presulfided catalysts were also active for direct coal liquefaction (Figure 8). Figure 12,a, b, and c, show single hexagonal crystallites of FeTSa and its electron diffraction spot pattern, respectively. There was no apparent increase in the grain size during this high-temperature transformation. Attempts were made to characterizethe Mo phases in the liquefaction residues derived from catalytic runs employing 111, but neither X-ray diffraction nor electron microscopy revealed useful information about the dispersion and composition of Mo phases. We speculate that, as Mo is well dispersed on the surface of Fe20dS04 (Moos crystallites are as small as 3-5 nm), Mo forms highly dispersed sulfides under coal liquefaction conditions. These sulfides, of extremely fine size and small concentrations in the overall catalyst, are beyond the detection limits of techniques discussed in this paper. Conclusions The followingconclusionscan be drawn from this work: 1. Sulfated iron oxides (and oxyhydroxides) and molybdenum-promoted sulfated iron oxide, Mo/Fe20dS04, were active for converting low-pyrite coals to liquids. Coal conversionlevels above 75% ! (maf) were obtained (thermal conversion was 66 ?6) with iron loadings between 2500 and 5000 ppm relative to coal at 400 "C. In the bimetallic catalyst, 20-200 ppm of Mo relative to coal along with 2500 ppm Fe enhanced the yield of light oils. Since not much iron was present in the starting coals, all enhanced
Figure 12. (a,top; b, middle) Transmission electron micrographs and (c, bottom) an electron diffraction pattern for Fe& formed by the sulfidation of catalyst I1 at 400 "C, 1000psig (cold) of Hz, in a 300-cm3 stirred batch autoclave, 1200 rpm, 30 min.
conversions are likely to be due to the small amounts of added iron and molybdenum catalysts. 2. All three anions (sulfate, molybdate, and tungstate) modified the physicochemicalproperties of iron(II1)oxides similarly and were about equally active for direct liquefaction of the low-pyrite Blind Canyon coal but slightly higher yields of oils were obtained with iron oxides promoted by molybdate or tungstate.
454 Energy & Fuels, Vol. 7, No. 4, 1993
3. Uncalcined sulfated iron oxyhydroxides were almost as active as the sulfated iron oxides which had been calcined at 500 "C for 3 h; however, slightly higher amounts of oils were obtained on a reproducible basis from use of the calcined oxides rather than the uncalcined oxyhydroxides. We believe that, although initially both oxides and oxyhydroxides are highly dispersed, sulfated oxides resist sintering at high temperatures. The uncalcined sulfated oxyhydroxides have residual moisture which is probably responsible for sinteringor agglomeration of these catalysts under coal liquefaction conditions. 4. Iron, added to the coal liquefaction reactor in the form of sulfated oxides, is completely converted to highly dispersed pyrrhotites within a few minutes at 400 "C. These have a composition of Fe& (hexagonal crystallites) and an average particle size of about 20 nm. No apparent growth in the size of pyrrhotite particles was observed
Pradhan et al. with reaction time. The presulfided iron oxides, Fe&, are also active for direct liquefaction of coal in the presence of added sulfur. 5. This type of catalyst system, based on anion-modified iron oxides, have an initial fine size and an unique ability to maintain this fine size at higher temperatures without significant sintering or agglomeration,making them highly dispersed active catalysts for coal liquefaction.
Acknowledgment. The authors gratefully acknowledge the funding support from the U.S. Department of Energy under award DE-FC22-90PC90029, The financial support from the Exxon Educational Foundation is also acknowledged. The contributions of coal samples by the Argonne Premium Coal Sample Bank and the Penn State Coal Sample Bank are acknowledged.
