Transformation of Iron Catalyst to the Active Phase in Coal

Jul 30, 1998 - The transformation of iron compounds (α-FeOOH, γ-FeOOH, ferrihydrite, limonite, and pyrite) into pyrrhotite (Fe1-xS) was systematical...
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Energy & Fuels 1998, 12, 897-904

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Transformation of Iron Catalyst to the Active Phase in Coal Liquefaction Takao Kaneko,* Kazuharu Tazawa, Toru Koyama, Kouichi Satou, Katsunori Shimasaki, and Yoichi Kageyama Takasago Research Laboratory, Nippon Brown Coal Liquefaction Co., Ltd., 2-3-1, Niihama, Arai-cho, Takasago, Hyogo, Japan Received December 21, 1997

The transformation of iron compounds (R-FeOOH, γ-FeOOH, ferrihydrite, limonite, and pyrite) into pyrrhotite (Fe1-xS) was systematically investigated through the sulfidation tests with elemental sulfur in the absence of feed coal under liquefaction conditions. The order of transformation temperature into pyrrhotite was as follows: ferrihydrite, FeOOH (250 °C) < limonite ore (300 °C) < pyrite ore (350 °C). The crystal growth of pyrrhotite proceeded in the following order: γ-FeOOH < limonite, R-FeOOH, ferrihydrite < pyrite (FeS2). Both transmission electron microscopy observations and corresponding X-ray diffraction data indicated that the ultrafine crystallites of Fe1-xS could be initially formed into the framework of iron oxyhydroxide particles at lower temperatures, followed by growing up to the large hexagonal crystal at higher temperatures through the disappearance of its framework. The presence of H2S is effective not only to maintain the sulfur-rich stoichiometry of Fe1-xS but also to suppress the crystal growth of pyrrhotite. A good correlation between the oil yield and the crystallite size of pyrrhotite was obtained for the fresh and the used catalysts, indicating the higher oil yield with smaller crystallite size. γ-FeOOH exhibited an excellent catalytic activity for the coal liquefaction due to the transformation into pyrrhotite with smaller crystallite size under the liquefaction conditions. The used catalyst, pyrrhotite in CLB-THFI, demonstrated a sufficient catalytic activity, although the oil yield decreased slightly as compared to that of the fresh catalyst. Catalyst deactivation through the deposition of coal mineral matters or organic residues appears to be considerably small.

Introduction The development of an effective, economical, and environmentally acceptable catalyst is the key to achieve a commercially viable goal in the direct liquefaction of coal. Iron-based compounds have been investigated as a disposal catalyst, owing to difficulties of recovering it from liquefaction residue. Several studies to increase the activity of iron catalysts have been reported on decreasing the particle size, increasing catalyst dispersion by loading it on coal surface, or modifying the catalyst composition by the addition of promoters.1-3 The need for added sulfur and the presence of ironsulfur compounds in the reaction products have led to the conclusion that an iron-sulfur compound (such as pyrrhotite, Fe1-xS) is the active phase in coal liquefaction.4-7 The initial dispersion of catalyst has a strong influence on the activity of the sulfided phases (1) Derbyshire, F.; Hager, T. Fuel 1994, 73, 1087-1092. (2) Pradhan, V. R.; Hu, J.; Tierney, J. W.; Wender, I. Energy Fuels 1993, 7, 446-454. (3) Weller, S. W. Energy Fuels 1994, 8, 415-420. (4) Cugini, A. V.; Krastman, D.; Lett, R. G.; Balsone, V. D. Catal. Today 1994, 19, 395-407. (5) Mochida, I.; Sakanishi, K. Adv. Catal. 1994, 40, 39-85. (6) Farcasiu, M.; Smith, C. Fuel Process. Technol. 1991, 29, 199208. (7) Bacaud, R.; Besson, M.; Djega-Mariadassou, G. Energy Fuels 1994, 8, 3-9.

formed under liquefaction conditions.8 However, the agglomeration of fine particles has been observed under liquefaction conditions when iron catalyst is converted to the active phase pyrrhotite in situ in the reactor, resulting in large crystal growth and loss of surface area. Therefore, many studies on iron-based catalysts have focused on developing methods to prevent agglomeration and crystal growth of pyrrhotite during coal liquefaction. Cugini et al. have reported that the presence of a carbonaceous support or coal itself for FeOOH tended to mitigate the particle size growth and favored the formation of smaller particle size iron sulfide catalysts.9 The adsorption of the FeOOH to the surface of coal may help to stabilize the iron compound so that it does not agglomerate during the transformation to pyrrhotite. Unfortunately, it is quite difficult to characterize the pyrrhotite formed at lower temperatures in the presence of coal due to the low iron concentration or the presence of iron species in coal. Also, Zhao et al. have proposed that the incorporation of a second element such as Si or Al to form binary ferrihydrite catalysts may hinder particle agglomeration at high temperatures and that these catalysts are as effective as or better than the (8) Darab, J. G.; Linehan, J. C.; Matson D. W. Energy Fuels 1994, 8, 1004-1005. (9) Cugini, A. V.; Krastman, D.; Martello, D. V.; Frommell, E. F.; Wells, A. W.; Holder, G. D. Energy Fuels 1994, 8, 83-87.

S0887-0624(97)00231-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/30/1998

898 Energy & Fuels, Vol. 12, No. 5, 1998

commercial 3 nm iron catalyst.10 Pradhan et al. showed that surface SO42- species effectively retarded the growth of R-Fe2O3 and it might result in the formation of an active pyrrhotite with smaller particle size under liquefaction conditions.11 Recently, Shah et al. pointed out that such a treatment to improve the dispersion of the catalyst could hinder the conversion of catalyst precursor into an active phase from the results obtained by in situ X-ray absorption fine structure (XAFS) spectroscopic studies.12 Moreover low-rank coal, such as brown coals, thermally initiated reactions can take place very rapidly at low temperatures, producing radical fragments. These radical fragments can either take the pathway of retrogressive reactions to form heavier materials or can be hydrogenated to form the desirable lighter products.5 Suzuki suggested that the role of the catalyst is to promote a direct hydrogen-transfer process from the gas phase to coal fragment radicals through the kinetic studies of coal liquefaction using iron carbonyl-sulfur catalyst.13 Ades et al. reported the dissociation of the hydrogen molecule on Fe1-xS clusters with a molecular orbital approach.14 It should be important that the catalyst precursor can be completely converted into active pyrrhotite at lower temperatures before the thermal decomposition of coal would take place during coal liquefaction. Huffman et al. reported that in situ XAFS spectroscopic studies showed the gradual conversion from iron oxyhydroxide to pyrrhotite and that the transformation temperature to pyrrhotite varied significantly with the catalyst system.12,15 In this paper, transformation of iron compounds (R-FeOOH, γ-FeOOH, ferrihydrite, natural pyrite, and limonite) to pyrrhotite (Fe1-xS) was systematically investigated through the sulfidation tests with elemental sulfur and without feed coal under liquefaction conditions. The transformation temperature of iron compound to pyrrhotite and the change in crystallite size or chemical composition of pyrrhotite were evaluated by powder X-ray diffraction (XRD) methods, and the sulfidation process was investigated by transmission electron microscopy (TEM). The effect of crystallite size of pyrrhotite on oil yield in the liquefaction of Yallourn coal was evaluated in relation to the residual activity of used catalysts, which are iron sulfides in coal liquid bottom recovered from the reactor after the liquefaction of coal. Experimental Section Feed Coal and Solvent. Experiments were conducted with Yallourn coal (Australian brown coal) and coal-derived solvent (180-420 °C boiling point) from a 0.1 t/d bench scale unit. Properties of the feed coal and solvent are presented in Table 1. (10) Zhao, J.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 38-43. (11) Pradhan, V. R.; Tierney, J. W.; Wender, I.; Huffman, G. P. Energy Fuels 1991, 5, 497-507. (12) Shah, N.; Zhao, J.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1996, 10, 417-420. (13) Suzuki, T. Energy Fuels 1994, 8, 341-347. (14) Ades, H. F.; Companion, A. L.; Subbaswamy, K. R. Energy Fuels 1994, 8, 71-76. (15) Huffman, G. P.; Ganguly, B.; Zhao, J.; Rao, K. R. P. M.; Shah, N.; Feng, Z.; Huggins, F. E.; Taghiei, M. M.; Lu, F.; Wender, I.; Pradhan, V. R.; Tierney, J. W.; Seehra, M. S.; Ibrahim, M. M.; Shabatai, J.; Eyring, E. M. Energy Fuels 1993, 7, 285-296.

Kaneko et al. Table 1. Properties of Feed Coal and Coal-Derived Solvent Yallourn Coal (As Received) proximate analysis (wt % dry) moist.

ash

VM

14.5

1.6

44.2

ultimate analysis (wt % daf) C

H

N

S

Odiff

H/C

66.7 4.7 0.5 0.3 27.8 0.846

Coal-Derived Solvent (bp 180-420 °C) ultimate analysis (wt %) C

H

N

S

Odiff

H/C

fa

87.9

9.4

0.7

0.1

1.9

1.283

0.52

Table 2. Properties of Iron Catalyst for Coal Liquefaction catalyst

particle size median (µm)a

crystallite size (nm)b

BET surface area (m2/g)

γ-FeOOH R-FeOOH ferrihydrite limonited pyritee

0.6 0.9 0.4 0.7 0.6

35 16 3-5c

67 61 300 47 15

a Average particle (grain) size measured by particle analyzer with laser method. b Crystallite size was determined by the Scherrer equation from the XRD peak (200). c Crystallite size was estimated from TEM photograph. d Iron ore (natural R-FeOOH) from Roveriver in Australia. Chemical analysis of finely pulverized limonite (wt %): Fe, 56.3; Si, 2.5; Al, 1.6; Ca, 0.7; Mg, 0.2; Na, 0.02. e Pyrite ore (natural FeS2) from Mt. Lyell in Australia. Chemical analysis of finely pulverized pyrite (wt %): Fe, 46.7; S, 46.9; Si, 1.1; Al, 0.5; Ca, 0.1; Mg, 0.1; Na, 0.01.

Preparation of Iron-Based Catalyst. R-FeOOH and γ-FeOOH were synthesized by neutralizing aqueous ferrous sulfate (0.6 M/L) with ammonia (2 M/L), followed by air oxidation at 40 °C for 8 h, by the reference in the literature.16 Precipitate was separated from the solution by filtration, washed with pure water, and then dried at 110 °C for 10 h. In the case of γ-FeOOH preparation, 1.7 mol % of (NH4)2HPO4 against ferrous sulfate was added to the precipitate before the air oxidation in order to suppress the formation of R-FeOOH. Both dried powders of R-FeOOH and γ-FeOOH were pulverized with the wet method in the coal-derived solvent by using a ball mill to give submicrometer average particle size prior to the use as a liquefaction catalyst. Ferrihydrite was synthesized by an alternative precipitation method from 0.36 M (NH4)HCO3 and 0.6 M Fe(NH4)(SO4)2 in aqueous solution at 8.0 of pH.16 The precipitate was separated from the solution by filtration and was washed with pure water and then ethanol, followed by substitution of coal-derived solvent for ethanol. Iron ores in Australia, which are limonite (Roveriver) and pyrite (Mt. Lyell), were also finely pulverized in the coalderived solvent by using a ball mill to submicrometer average particle size as a liquefaction catalyst. The limonite ore consisted of R-FeOOH, containing R-Fe2O3 less than 10 wt %, while the pyrite ore consisted of FeS2, although both iron ores contained as impurities a small amount of mineral matters such as Si, Al, Ca, etc. Properties of these catalysts are shown in Table 2. Sulfidation of Catalysts. Sulfidation tests for iron-based catalyst were carried out by using a microautoclave (100 mL inner volume), having an infrared-ray image furnace, with 0.0325 g (as Fe) of catalyst, 0.0373 g of elemental sulfur (S/Fe ) 2, atomic ratio) and 11.375 g of coal-derived solvent under simulated liquefaction conditions without feed coal at 10 MPa of initial hydrogen. The autoclave was heated from 200 to 450 °C at a 100 °C/min heating rate. The effect of the S/Fe atomic ratio on the transformation of γ-FeOOH into Fe1-xS was (16) Okamoto, S. Shin-Jikken Kagaku Koza; Japan Chemical Society; Maruzen Press; Tokyo, 1977; Vol. 8, pp 293-296.

Transformation of Iron Catalyst to the Active Phase examined under the same conditions. After the reaction, the autoclave was cooled rapidly to room temperature, and then the catalyst samples were washed with tetrahydrofuran (THF), followed by vacuum-drying at 110 °C for 2 h to remove the THF. Coal Liquefaction. Liquefaction tests were performed by using a large scale autoclave (5 L inner volume) under the standard conditions with 200 g (as daf) of dried Yallourn coal, 6.0 g (as Fe) of catalyst, 4.13 g of elemental sulfur (S/Fe ) 1.2, atomic ratio), and 500 g of coal-derived solvent at 7.5 MPa of initial hydrogen. The reaction was carried out at 450 °C for 1 h. Effects of the S/Fe atomic ratio on coal liquefaction were examined for γ-FeOOH, limonite, and pyrite catalyst in relation to the crystallite size of pyrrhotites. It must be kept in mind that H2S is a dangerous and toxic material, particularly when venting the autoclave after the coal liquefaction. After the reaction, the liquid products were roughly separated into the oil fraction (C5 ∼ 420 °C) and coal liquid bottoms (CLB; boiling point of above 420 °C) by distillation. The CLB was divided into solubles and insolubles by using the Soxhlet extraction method with THF, toluene, and n-hexane, respectively. Iron catalyst transformed into iron sulfide, such as pyrrhotite, was concentrated into the THF insolubles in CLB (CLB-THFI). As a used catalyst, the CLB-THFI, which was recovered from the reactor in a 0.1 t/d bench scale unit (BSU) was employed in this study. All catalyst samples were recovered as the residue in a THF extraction of the reactor products and carefully handled in nitrogen atmosphere to avoid any degradation of pyrrhotite upon exposure to air. Characterization of Catalysts. The particle size (grain size) distribution of dispersed catalyst in coal-derived solvent was measured by using a Shimadzu SALD-2000 particle size analyzer with laser diffraction method. The average particle size was defined in median, although the particle size distribution skewed toward smaller size for all samples in this study. Powder XRD was performed by using a Rigaku RINT-1500 X-ray diffractometer irradiating the Cu KR line at 40 kV and 200 mA. Catalyst samples for analysis were prepared by washing with THF to remove the solvent or by using the Soxhlet extraction method with THF after coal liquefaction, followed by vacuum-drying to remove the THF. The mean crystallite size of FeOOH or Fe1-xS was calculated with the Scherrer equation from a full width at half-maximum (fwhm) of the XRD peak (200). The approximate (1 - x) value of Fe1-xS was estimated from the d spacing of the XRD peak (202) with the following equation reported by Djega-Mariadassou17 or Lambert et al.:18 Fe (%) ) 45.212 + 72.86(d202 - 2.0400) + 311.5(d202 - 2.0400)2, where Fe (%) is the atomic percent of iron in Fe1-xS and d202 is the d spacing (Å) of the XRD peak (202) of Fe1-xS. TEM was obtained with a Hitachi H-8100 scanning transmission electron microscope at 200 kV. Sample preparation of TEM consisted of dispersing milligram quantities of the reaction product (THF insolubles) through the sulfidation test from 200 to 450 °C at 0 min holding time and at 450 °C for 30 min in electron microscopy grade acetone. Specific surface areas of catalyst samples were measured by Micrometrics ASAP-2400 from the adsorption isotherms of nitrogen at 77 K with the BET method.

Results and Discussion Properties of the Iron-Based Catalyst. Methods of preparation and the structural parameters of iron oxyhydroxides are well known in relation to the atmospheric rusting of iron and steels.19,20 Misawa et al. (17) Djega-Mariadassou, G.; Besson, M.; Brodzki, D.; Charcosset, H.; Huu, T. V.; Varloud, J. Fuel Process. Technol. 1986, 12, 143-153. (18) Lambert, J. M.; Simkovich, G.; Walker, P. L. Fuel 1980, 59, 687-690.

Energy & Fuels, Vol. 12, No. 5, 1998 899

Figure 1. Preparation scheme and the crystal structure of γ-FeOOH and R-FeOOH.

have proposed the mechanism of the formation of green rusts, Fe3O4, R-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH, and ferrihydrite in aqueous solution at room temperature.19 In this study, three types of iron oxyhydroxide, R-FeOOH, γ-FeOOH, and amorphous ferrihydrite, were synthesized by the precipitation method in aqueous solution, which is expected to be a low-cost process for the production of liquefaction catalyst. Figure 1 shows the preparation method and the crystal structure of γ-FeOOH and R-FeOOH. Linehan et al. have described the structures of the iron-oxygen building blocks that appeared in solution at different pH values.21,22 The double straight chains in γ-FeOOH are interconnected together to form a layered structure, the individual sheets of which are held together via hydrogen bonding, while R-FeOOH consists of crystallographical ordered arrays of double straight chain polymers. The structure of ferrihydrite is amorphous and is believed to consist of a highly disordered array of double straight chain polymers (protooxyhydroxide). The ferrihydrite prepared in this work was determined as two-line ferrihydrite from the number of peaks present in its XRD pattern.23 Both γ- and R-FeOOH were identified to be phase pure by XRD and to have the approximate theoretical atomic percent of iron by chemical analysis. As shown in Table 2, γ-FeOOH had 67 m2/g specific surface area and 35 nm crystallite size, which were slightly different from R-FeOOH. On the other hand, ferrihydrite had a high surface area of 300 m2/g and smaller crystallite size of about 3 to 5 nm, estimated by TEM photograph. The specific surface area of finely (19) Misawa, T.; Hashimoto, K.; Shimodaira, S. Corrosion Sci. 1974, 14, 131-149. (20) Yamashita, M.; Miyuki, H.; Matsuda, Y.; Nagano, H.; Misawa, T. Corrosion Sci. 1994, 36, 283-299. (21) Linehan, J. C.; Matson, D. W.; Darab, J. G. Energy Fuels 1994, 8, 56-62. (22) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory; VCH press: New York, 1991. (23) Matson, D. W.; Linehan, J. C.; Darab, J. G.; Buehler, M. F. Energy Fuels 1994, 8, 10-18.

900 Energy & Fuels, Vol. 12, No. 5, 1998

Kaneko et al. Table 3. Transformation of Iron Catalyst Precursor to Pyrrhotite (Fe1-xS) by Sulfidation with Elemental Sulfur (S/Fe ) 2), with Coal-Derived Solvent, and without Feed Coal

precursor γ-FeOOH

R-FeOOH

Figure 2. Change in XRD peak patterns with sulfiding temperature at 0 min of holding time: 0.0325 g (as Fe) of iron catalyst, 0.0373 g of elemental sulfur (S/Fe ) 2), and 11.375 g of coal-derived solvent under the liquefaction conditions without feed coal at 10 MPa of initial hydrogen and 100 °C/ min of heating rate.

pulverized limonite was 47 m2/g, remarkably higher than that of pyrite (15 m2/g). The secondary dispersion state of catalyst particles in the solvent may affect the temperature of transformation into pyrrhotite or the liquefaction activity. Therefore, the average particle sizes of all catalyst samples were kept nearly constant size of 0.4-0.9 µm in this study. Transformation to Pyrrhotite. The effect of temperature on the transformation of iron catalyst into pyrrhotite was examined through the sulfidation tests with elemental sulfur in the absence of coal at the holding time of 0 min. Figure 2 shows the changes in XRD patterns of catalyst samples (THF insolubles) at different sulfiding temperatures. The XRD patterns for FeOOH exhibited that the small peaks of Fe3S4 were observed at 250 °C with the complete disappearance of iron oxyhydroxide peaks. The ferrihydrate may be transformed into pyrrhotite at lower temperatures below 250 °C, although the phase transition could not be detected by XRD analysis due to the amorphous state of iron samples. The small peaks of Fe1-xS were observed at 300 °C for limonite. On the other hand, only a part of pyrite could be transformed into Fe1-xS at 350 °C, since the pyrite peak coexisted with pyrrhotite peaks. The order of transformation temperature to pyrrhotite is as follows: ferrihydrite, FeOOH (250 °C) < limonite ore (300 °C) < pyrite ore (350 °C). Huffman et al. have reported that conversion of 3 nm of iron catalyst into pyrrhotite proceeded rapidly at 110-180 °C.15 They claimed that the transformation of the larger particle size (>10 nm) catalyst proceeded somewhat sluggishly at 250-320 °C by in situ XAFS study in the presence of hexahydropyrene (HHP) and iron catalyst with elemental sulfur in the absence of coal. The transformation temperature into pyrrhotite could be dependent on the crystallite size or specific surface area of the iron compound precursor. In these conditions, the chemical composition of pyrrhotite may change in some extent, as generally indicated by (1 -x) in Fe1-xS formula. Power and Fine have reviewed the identification techniques for the pyrrhotites by powder XRD analysis, indicating a relation between the d spacing (202) of the powder XRD peak and (1 - x) value representing the chemical

ferrihydrite

limonite

pyrite

sulfiding temp (°C) (0 min holding time)

iron phase (from XRD data)

200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450 250 300 350 400 450

γ-FeOOH (Fe3S4) Fe1-xS (Fe0.854S) Fe1-xS (Fe0.892S) Fe1-xS (Fe0.889S) Fe1-xS (Fe0.911S) R-FeOOH (Fe3S4) Fe1-xS (Fe0.838S) Fe1-xS (Fe0.883S) Fe1-xS (Fe0.889S) Fe1-xS (Fe0.921S) amorphous amorphous Fe1-xS (Fe0.848S) Fe1-xS (Fe0.892S) Fe1-xS (Fe0.901S) Fe1-xS (Fe0.936S) R-FeOOH R-FeOOH Fe1-xS (Fe0.879S) Fe1-xS (Fe0.905S) Fe1-xS (Fe0.901S) Fe1-xS (Fe0.908S) FeS2 FeS2 . Fe1-xS (Fe0.898S) FeS2 , Fe1-xS (Fe0.925S) Fe1-xS (Fe0.921S) Fe1-xS (Fe0.928S)

crystallite size (nm) (XRD peak-200) 6 12 16 19 20 7 14 20 32 46 13 23 37 48 13 25 33 38 38 46 50 53

composition.24 The (202) peak of hexagonal pyrrhotite is a single sharp peak, the position of which varies continuously over the full range of its composition. Djega-Mariadassou et al. have determined the composition of iron sulfides after the liquefaction of coal by using powder XRD data.17 This method is strictly empirical but is straightforward and simple to use, although it should be noted that several stoichiometries may be encountered in the same sample after the sulfidation of iron compounds. Table 3 summarizes the change in iron phase and the crystallite size determined by corresponding XRD data (Figure 2) during the sulfidation of iron compound from 200 to 450 °C. It is worth noting that Fe3S4 can be initially formed from iron oxyhydroxide at 250 °C and then transformed to pyrrhotite. The calculated (1 - x) mean value varied between 0.838 at 300 °C for R-FeOOH and 0.936 at 450 °C for ferrihydrite, although the iron sulfides having the (1 - x) ranging between 0.875 (Fe7S8) and 1.0 (FeS) are referred to generally as pyrrhotites. It was estimated that pyrrhotite had a relatively sulfur-rich structure such as Fe7S8 at around 350 °C and changed to an ironrich structure such as Fe9S10 or Fe10S11 with an increase in the temperature due to the elimination of sulfur atoms as H2S. The chemical composition of pyrrhotite could depend on the temperature and the partial pressure of H2S or the amount of added sulfur in the reaction system. Crystal Growth of Pyrrhotite. Figure 3 demonstrates the increase in the crystallite size of Fe1-xS, calculated by the Scherrer equation, with an increase in the temperature as shown in Table 3. A remarkable increase in Fe1-xS crystallite size occurred for pyrite, (24) Power, L. F.; Fine, H. A. Minerals Sci. Eng. 1976, 8, 106-128.

Transformation of Iron Catalyst to the Active Phase

Energy & Fuels, Vol. 12, No. 5, 1998 901

Figure 3. Increase in crystallite size of pyrrhotite (Fe1-xS) calculated by the Scherrer equation: (a) γ-FeOOH, (b) R-FeOOH, (c) ferrihydrite, (d) limonite, and (e) pyrite.

whereas an increase in the crystallite size of Fe1-xS seemed to be considerably suppressed at the temperature over 350 °C for γ-FeOOH. The crystallite size of Fe1-xS from amorphous ferrihydrite increased gradually with increasing temperature as well as R-FeOOH. TEM photographs of iron compound precursors and the residue in a THF extraction of the reaction products through the sulfidation tests at each temperature are shown from Figure 4 to Figure 6. Both TEM observation and the corresponding XRD data (Table 3) indicated that fine crystallites of Fe1-xS could be initially formed into the framework (needle shape) of R-FeOOH particles at 250 °C and finally grew up to the large hexagonal crystal with the disappearance of its framework at the temperature above 400 °C as shown in Figure 4a. Figure 4b shows the pyrrhotites (darker particles) formed in the framework of R-FeOOH at 300 °C of the sulfiding temperature. This phenomenon of the remaining of the iron oxyhydroxide framework is well-known as one of the “topotaxy” effects on the dehydration of R-FeOOH to R-Fe2O3 or γ-FeOOH to γ-Fe2O3.25 Figure 5a indicated that the framework (leaf veins) of γ-FeOOH particles remained up to 450 °C without the large crystal growth of pyrrhotites. The agglomeration of fine particles of pyrrhotite may be suppressed through the layered structure of γ-FeOOH, due to the prevention of their free migrations. In contrast, amorphous ferrihydrite, which is a prototype of iron oxyhydroxide having a 3 nm particle size, resulted in a large crystal growth of pyrrhotites at high sulfiding temperatures as shown in Figure 5b. This would explain why the crystallite size of pyrrhotite formed from γ-FeOOH is smaller than that formed from R-FeOOH or ultrafine ferrihydrite. Limonite is mainly composed of R-FeOOH particles, existing in different morphology from synthesized R-FeOOH as shown in Figure 6a, although it was transformed completely to fine crystallites of pyrrhotite at 300 °C, which is 50 °C higher than that of synthesized R-FeOOH. The crystal growth of pyrrhotite formed from limonite seemed to be slightly smaller than that of synthesized R-FeOOH. Figure 6b and corresponding XRD data indicated that pyrite ore which consists of large FeS2 particles with low surface area could be partially transformed to Fe1-xS at 350 °C, with remark(25) Takada, T.; Iwase, K.; Hayashi, T. Powder Metallurgy; Interscience: 1961; pp 173.

Figure 4. TEM photographs of reaction products (THF insolubles) during the transformation of R-FeOOH into Fe1-xS by sulfidation with elemental sulfur under the liquefaction conditions without feed coal. Corresponding XRD data are summarized in Table 3.

able crystal growth of pyrrhotite. It is considered that fine particles of pyrrhotite may be initially formed on the surface of pyrite particles and can migrate freely on the surface, resulting in a agglomeration and larger crystal growth of pyrrhotite from lower temperatures around 350 °C. Thus, the crystallite size of pyrrhotite increased with an increase in the temperature, depending strongly on the crystal structure of iron compound precursor. It was ascertained that the crystal growth of pyrrhotite increased in the following order: γ-FeOOH < limonite,R-FeOOH, ferrihydrite, < pyrite (FeS2). Correlation between Pyrrhotite Crystallite Size and Surface Area. The specific surface area of pyrrhotite was determined from the adsorption isotherms of nitrogen measured by the BET method. Figure 7 shows the correlation between crystallite size and specific surface area of pyrrhotite. The specific surface area of pyrrhotite was inversely proportional to the crystallite size for γ-FeOOH catalyst. As the geometrical specific surface area of individual particles should be inversely proportional to the particle diam-

902 Energy & Fuels, Vol. 12, No. 5, 1998

Figure 5. TEM photographs of reaction products (THF insolubles) during the transformation of (a) γ-FeOOH and (b) ferrihydrite into Fe1-xS.

eter, the linear relationship suggests that pyrrhotite particles produced from γ-FeOOH may be weakly aggregated to each other with high dispersion. BET surface area is just the sum of only external surface areas of nonporous pyrrhotite particles. The surface area of pyrrhotite formed from limonite was considerably higher than that from γ-FeOOH catalyst. The mineral matters, such as porous SiO2 and Al2O3 contained in limonite ore, may contribute to the total BET surface area. The surface area of pyrrhotite from pyrite was only about 5 m2/g, extremely lower due to the large crystal growth during the phase transformation. It is difficult to determine the change in the surface area of the catalyst in the presence of coal due to the occurrence of mineral matters and organic residues in the recovered catalyst after coal liquefaction. Therefore the dependence of the surface area of the catalyst on the activity in coal liquefaction may be difficult to quantify. Effect of S/Fe Atomic Ratio on Pyrrhotite Crystallite Size. The chemical composition or crystallite size of pyrrhotite could depend on the temperature and

Kaneko et al.

Figure 6. TEM photographs of reaction products (THF insolubles) during the transformation of finely pulverized iron ore into Fe1-xS: (a) limonite and (b) pyrite.

Figure 7. Correlation between the reciprocal of the crystallite size and specific BET surface area of pyrrhotite, which was produced through the sulfidation tests with elemental sulfur in the absence of feed coal at various temperatures: (a) γ-FeOOH, (b) limonite, and (c) pyrite.

the partial pressure of H2S or the amount of added sulfur in the reaction system.

Transformation of Iron Catalyst to the Active Phase

Figure 8. Effect of the S/Fe atomic ratio on the crystallite size of pyrrhotite recovered after coal liquefaction at 450 °C for 1 h with 200 g of Yallourn coal (as daf), 2.0 g as Fe (1 wt % daf) of iron catalyst, and 500 g of coal-derived solvent at 7.5 MPa of initial hydrogen: (a) γ-FeOOH, (b) limonite, and (c) pyrite. Table 4. Effect of S/Fe Atomic Ratio on the Transformation of γ-FeOOH to Fe1-xS by Sulfidation with Elemental Sulfur, with Coal-Derived Solvent, and without Feed Coal sulfiding temp (°C)

holding time (min)

S/Fe (atomic ratio)

350

0

450

0

450

30

2 4 2 4 2 4

iron phase (Fe1-xS) Fe0.892S Fe0.868S Fe0.911S Fe0.892S Fe0.951S Fe0.905S

crystallite size (nm) 16 19 20 23 36 27

Table 4 shows the effect of the amount of sulfur added to iron (S/Fe atomic ratio) on the transformation of γ-FeOOH into the pyrrhotites in the absence of coal. The (1-x) value of Fe1-xS was decreased by increasing the S/Fe atomic ratio from 2 to 4, indicating the formation of pyrrhotites with relatively sulfur-rich structure. On the other hand, the crystallite size of pyrrhotites was little affected for the 0 min holding time up at the temperature to 450 °C, but the increase in crystallite size for the 30 min holding time at 450 °C was remarkably suppressed by increasing the S/Fe atomic ratio. It appeared that the increase in the S/Fe atomic ratio could be effective not only to maintain the sulfur-rich structure but also to suppress the agglomeration of pyrrhotite particles at high temperatures and long residence time. It has been previously reported that the presence of a carbonaceous support or coal itself tended to mitigate the particle size growth of FeOOH and favored the formation of smaller particle size iron sulfide catalysts.9 The adsorption of the FeOOH on the surface of coal may help to stabilize the iron compound so that it does not agglomerate during the transformation to pyrrhotite. The effect of the S/Fe atomic ratio on the crystallite size of pyrrhotite in the presence of coal is worth further consideration. Next, the effect of the amount of added sulfur to iron catalyst (S/Fe atomic ratio) was examined through the liquefaction of Yallourn coal at 450 °C for 1 h by using a batch-type autoclave (5 L inner volume) in order to determine the crystallite size of pyrrhotite which may contribute to the liquefaction activity. Figure 8 shows the crystallite size of pyrrhotite calculated with the

Energy & Fuels, Vol. 12, No. 5, 1998 903

Figure 9. Effect of crystallite size of pyrrhotite on oil yields in the liquefaction of Yallourn coal. Liquefaction conditions: 3 wt % daf as Fe of catalyst and the addition of elemental sulfur (S/Fe ) 1.2) at 7.5 MPa of initial hydrogen and 450 °C for 1 h.

Scherrer equation from the XRD data for the THF insolubles recovered from the reaction products after the liquefaction of coal. The crystallite size of pyrrhotite formed from γ-FeOOH or limonite was decreased with the amount of added sulfur due to the increase in H2S concentration in the reactor. The results suggest that the crystal growth of pyrrhotite can be suppressed through the keeping of the sulfur-rich one by controlling the sulfur concentration in the reaction system. The increase in oil yields upon addition of elemental sulfur to Fe7S8 was reported in relation to the possible interactions between pyrrhotite and H2S for catalyzing hydrogenolysis/hydrogenation reactions during coal liquefaction.2 Montano et al. have proposed that the catalytic activity of Fe1-xS is the result of iron vacancies and that H2S is necessary to maintain the surface of Fe1-xS with iron-deficient sites.26 The presence of H2S is effective not only to maintain the iron-deficient and sulfur-rich stoichiometry but also to suppress the crystal growth of pyrrhotite under liquefaction conditions. However for pyrite catalyst, the crystallite size of pyrrhotite was not appreciably influenced by the amount of added sulfur. This result may reflect the difference in the transformation mechanism to pyrrhotite from pyrite (FeS2) and oxyhydroxide (FeOOH). Effect of Crystallite Size on Liquefaction Activity. Figure 9 shows the effect of crystallite size of pyrrhotite on oil yields in the liquefaction of Yallourn coal with 3 wt % daf (as Fe) of fresh or used catalyst together with the addition of elemental sulfur (S/Fe ) 1.2) at 450 °C for 1 h. Changes in crystallite size of pyrrhotite for fresh and used catalysts are shown in Table 5. The crystallite size of pyrrhotite for the fresh catalyst was determined after coal liquefaction by using an autoclave, since it was completely converted into pyrrhotite in situ in the reactor. As the used catalysts, THF insolubles in CLB (bp > 420 °C), which were recovered from the reactor in 0.1 t/d BSU after coal liquefaction under 1.0 vol % H2S in hydrogen, were employed as well as fresh catalysts. A good correlation between the oil yield (C5-420 °C) and the crystallite size (26) Montano, P. A.; Stenberg, V. I.; Sweeny, P. J. Phys. Chem. 1986, 90, 156-159.

904 Energy & Fuels, Vol. 12, No. 5, 1998

Kaneko et al.

Table 5. Oil Yield in Liquefaction of Yallourn Coal with Fresh or Used Catalyst

γ-FeOOH limonite pyrite

catalysta

crystallite size of pyrrhotiteb (nm)

oil yieldc (wt % daf)

fresh used (CLB-THFI) fresh used (CLB-THFI) fresh used (CLB-THFI)

34 42 41 50 63 66

49.3 44.6 45.0 40.7 42.4 35.7

a The CLB-THFI, recovered from BSU reactor after 200 h of operation, was employed as a used catalyst. b Crystallite size of pyrrhotite for fresh catalyst was calculated with the Scherrer equation from XRD analysis of the reaction product (THFI) after coal liquefaction by using an autoclave. c Oil yield: distillate (C5420 °C of boiling point) oil yield. Liquefaction conditions: 450 °C, 1 h, 200 g daf of Yallourn coal, 500 g of coal-derived solvent, 6.0 g of catalyst as Fe, 4.13 g of elemental sulfur (S/Fe ) 1.2), and 7.5 MPa of initial H2.

of pyrrhotite was obtained for fresh and used catalysts, indicating the higher catalytic activity with a smaller crystallite size. γ-FeOOH exhibited an excellent catalytic activity in the coal liquefaction on account of the transformation into pyrrhotite with smaller crystallite size under liquefaction conditions, corresponding to the results of the sulfidation tests without feed coal. Used catalyst, which is pyrrhotite in CLB-THFI, demonstrated a sufficient catalytic activity, although the oil yield decreased slightly as compared to that of the fresh catalyst. The fresh pyrite catalyst exhibited the relatively high oil yield despite the large crystallite size, although there is some uncertainty in the determination of crystallite size by XRD data with the Scherrer equation particularly in the case of a large crystal due to the existence of some strains of the lattice. The H2S generated from the fresh pyrite catalyst during the transformation from FeS2 into Fe1-xS may affect the initial liquefaction activity. Also, the fact that the pyrite-based catalyst was remarkably deactivated with a much smaller increase (from 63 to 66 nm) in the crystallite size may suggest that some factor other than crystallite size growth may be at play here. A linear correlation between the oil yield and the crystallite size of pyrrhotite among the fresh and used catalysts, except for fresh pyrite catalyst, suggested that the catalyst deactivation was caused mainly by the agglomeration or by crystal growth of pyrrhotite during coal liquefaction. The influence of the deposition of coal mineral matter or organic residue on the catalyst deactivation appears to be considerably small. Therefore, the bottom recycle of a used catalyst together with heavy residual products (CLB) would be one of the promising ways to increase oil yield with a successful reduction of the loading level of the fresh catalyst to feed coal. Conclusions The transformation of iron compounds (R-FeOOH, γ-FeOOH, ferrihydrite, limonite, and natural pyrite) into pyrrhotite (Fe1-xS) was systematically investigated

through the sulfidation tests with elemental sulfur and without feed coal under liquefaction conditions. The order of transformation temperature into pyrrhotite was as follows: ferrihydrite, FeOOH (250 °C) < limonite ore (300 °C) < pyrite ore (350 °C). The crystal growth of pyrrhotite proceeded in the following order: γ-FeOOH < limonite, R-FeOOH, ferrihydrite < pyrite (FeS2). TEM observation indicated that the ultrafine crystallites of Fe1-xS could be initially formed into the framework of iron oxyhydroxide particles at lower temperatures followed by growing up to the large hexagonal crystal at higher temperatures through the disappearance of its framework. The crystallite size of pyrrhotite increased with an increase in the reaction temperature, depending strongly on the crystal structure of precursor materials. The inverse proportional relationship between the specific surface area and the crystallite size of pyrrhotite suggests that pyrrhotite particles produced from γ-FeOOH may be weakly aggregated with each other with high dispersion, resulting in a high catalytic activity for coal liquefaction. The crystallite size was decreased with the amount of added sulfur and the crystal growth of pyrrhotite can be suppressed through maintaining the sulfur-rich structure by controlling the amount of added sulfur. The presence of H2S is effective not only to maintain the iron-deficient and sulfur-rich stoichiometry of Fe1-xS but also to suppress the crystal growth of pyrrhotite under liquefaction conditions. The effect of crystallite size of pyrrhotite on the oil yield in the liquefaction of Yallourn coal was evaluated. A good correlation between the oil yield (C5-420 °C) and the crystallite size of pyrrhotite was obtained for the fresh and the used catalysts, indicating the higher oil yield with smaller crystallite size. γ-FeOOH exhibited an excellent catalytic activity for the coal liquefaction due to the transformation into pyrrhotite with smaller crystallite size under the liquefaction conditions, corresponding to the sulfidation tests without feed coal. The used catalyst, pyrrhotite in CLB-THFI, demonstrated a sufficient catalytic activity, although the oil yield decreased slightly as compared to that of the fresh catalyst. Catalyst deactivation through the deposition of coal mineral matter or organic residue appears to be considerably small. The bottom recycle of a used catalyst together with heavy residual products could be one of the promising ways to increase the oil yield with a successful reduction in the loading level of fresh catalyst. Acknowledgment. This research was supported by New Energy and Industrial Technology Development Organization (NEDO) as a part of New Sunshine Projects in Japan. The authors are grateful to NEDO for the support and the permission of this work. We also thank Dr. T. Suzuki of Kansai University for many valuable suggestions. EF9702310