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Effect of Preparation Conditions on the Characterization and Activity of Aerosol-Generated Ferric Sulfide-Based Catalysts for Direct Coal Liquefaction R. K. Sharma, A. H. Stiller, and D. B. Dadyburjor* Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506-6102 Received October 30, 1995X
Direct liquefaction of coal was studied using ferric sulfide-based catalysts generated in an aerosol reactor. The catalyst materials were prepared under varying conditions of temperature, pressure, and precursor concentration. In some runs, Fe-Cu-S mixed-metal catalysts containing 10 and 40% of Cu (based on total metal) were also used. Characterization studies reveal that the catalysts consist of hollow particles 3-20 nm in diameter, aggregated in clumps. Liquefaction experiments were performed at 350-440 °C under a hydrogen pressure of 1000 psi(cold) and 30 min reaction time with tetralin or phenanthrene as solvent. The catalyst activity and selectivity to oil-range products increase with increase in temperature of the aerosol reactor. The selectivity improves slightly when the catalyst exposure to air is minimized. The catalytic effects are more pronounced with phenanthrene as solvent. At 400 °C, both the conversion of coal and the oil yield increase with increase in catalyst loading, but the effect is more pronounced at low loadings. The conversion also increases with the liquefaction temperature, from 50% at 350 °C to 87% at 440 °C, with 1.67% loading. The addition of Cu to the Fe-S system increases the selectivity slightly, but at the expense of conversion.
Introduction Iron-based catalysts are commonly used in direct coal liquefaction (DCL) since they show relatively good activity and are cheap and environmentally desirable.1,2 Work in our laboratory has focused on ferric sulfide, which is unstable at room temperature and disproportionates into pyrite (PY, FeS2) and nonstoichiometric pyrrhotite (PH, FeSx with x ≈ 1), the relative amount of each depending upon the time and temperature of disproportionation.3 The activity of these catalysts may be improved further by decreasing their effective size to improve the dispersion in the coal. Precipitation,4 impregnation,5 and chemical reactions in aerosol droplets6 are among the techniques used to produce the fine particles. The aerosol technique has the advantage that the reaction conditions can be precisely controlled to produce large amounts of fine particles of known composition. Gadalla * To whom correspondence should be addressed. Phone: (304) 2932111, x 411; Fax: (304) 293-4139. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Weller, S. W. Catalysis and Catalyst Dispersion in Coal Liquefaction. Energy Fuels 1994, 8, 415-420. (2) Dadyburjor, D. B.; Stewart, W. E.; Stiller, A. H.; Stinespring, C. D.; Wann, J. P.; Zondlo, J. W. Disproportionated Ferric Sulfide Catalysts for Coal Liquefaction. Energy Fuels 1994, 8, 19-24. (3) Stansberry, P. G.; Wann, J. P.; Stewart, W. R.; Yang, J.; Zondlo, J. W.; Stiller, A. H.; Dadyburjor, D. B. Evaluation of a Novel Mixed Pyrite/Pyrrhotite Catalyst for Coal Liquefaction. Fuel 1993, 72, 793796. (4) Pradhan, V. R.; Tierney, J. W.; Wender, I.; Huffmann, G. P. Catalysis in Direct Coal Liquefaction by Sulfated Metal Oxides. Energy Fuels 1991, 5, 497-507. (5) Liu, Z.; Yang, J.; Zondlo, J. W.; Stiller, A. H.; Dadyburjor, D. B. In-situ Impregnated Iron-Based Catalysts for Direct Coal Liquefaction. Fuel 1995, 75, 1-7. (6) Pratsinis, S. E. An Overview of Material Synthesis by Aerosol Processes. AIChE Symp. Ser. 1987, 85, No. 270, 57-68.
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and Hsuan7 used the aerosol technique to prepare NiFe2O4 particles. In our laboratory, ferric sulfidebased catalysts generated using the aerosol technique have shown8 a significant activity for coal liquefaction. The catalyst activity or selectivity may also be altered by incorporating a second metal in the iron sulfide lattice to alter the nature of active sites. Metals such as nickel, copper, cobalt, magnesium, and molybdenum have ionic radii which are close to that of iron and may easily be substituted in the lattice. Since the small catalyst particles are known to agglomerate under reaction conditions, the addition of a second metal may also be helpful in this regard.9-11 The conversion of coal is also governed by the type of solvent used in liquefaction. Tetralin has mostly been used as the solvent in the literature due to its strong hydrogen-donor properties. However, Stohl and (7) Gadalla, A. M.; Hsuan, F. Y. Preparation of Fine Hollow Spherical NiFe2O4 Powders. J. Mater. Res. 1990, 5, 12. (8) (a) Stiller, A. H.; Dadyburjor, D. B.; Stinespring, C. D.; Chadha, A.; Tian, D.; Martin Jr., S. B.; Agarwal, S. Preparation of Iron-SulfideBased Catalysts Using an Aerosol Technique. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40, 44-47. (b) Dadyburjor, D. B.; Stiller, A. H.; Stinespring, C. D.; Chadha, A.; Tian, D.; Martin Jr., S. B.; Agarwal, S. Use of an Aerosol Technique to Prepare Iron-Sulfide-Based Catalysts for Direct Coal Liquefaction In Advanced Techniques in Catalyst Synthesis; Moser, W. R., Ed.; Academic Press: New York, in press. (9) Zhao, J.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Binary Iron Oxide Catalysts for Direct Coal Liquefaction. Energy Fuels 1994, 8, 38-45. (10) Hager, G. T.; Compton, A. L.; Givens, E. N.; Derbyshire, F. J. The Effect of Promoter Metal Concentration on the Catalytic Activity of Sulfated Hematite for the Liquefaction of a Subbituminous Coal. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39(4), 1083-1087. (11) Dadyburjor, D. B.; Stiller, A. H.; Stinespring, C. D.; Zondlo, J. W.; Wann, J. P.; Sharma, R. K.; Tian, D.; Agarwal, S.; Chadha, A. Towards Improved Iron-Based Catalysts for Direct Coal Liquefaction. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39(4), 1088-1092.
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Figure 1. Schematic of apparatus for the preparation of aerosol catalysts.8
Diegert12 observed that catalytic effects in the presence of tetralin are small, since tetralin can supply almost all the hydrogen necessary for the liquefaction of coal. According to McMillen et al.,13 the role of the solvent is to stabilize the coal free radicals by donating hydrogen as well as to promote the cracking of coal molecules. Phenanthrene is known to be a poor hydrogen-donor solvent compared to tetralin;14 however, phenanthrene may enhance the cracking reactions or act as a hydrogen shuttler. The objective of the present work was to study the characterization and DCL activity of iron-based catalysts generated in an aerosol reactor. Varying conditions of temperature, pressure, and precursor concentration were used in the preparation of the catalyst. The effect of adding a second metal, copper, to the iron was also investigated. The liquefaction was performed at 350-440 °C with a hydrogen pressure of 1000 psia(cold). Tetralin and phenanthrene were used as solvents during the liquefaction runs to study the effects of facile hydrogen donation from the solvent. The products were analyzed in terms of asphaltene, oil, and gas fractions. In all cases, the results were compared with those from uncatalyzed (thermal) runs under similar conditions. Experimental Method Coal. The coal used in this study was a high-volatile-A bituminous coal from the Blind Canyon seam in Utah. The coal was received from the Pennsylvania State University Coal Bank and ground to -60 mesh under nitrogen. This coal, classified as DECS-6, is extremely low in iron and is therefore useful for liquefaction experiments involving iron catalysts. The proximate and ultimate analyses of this coal (performed at Galbraith Laboratories) indicate that it contains 49 wt % volatile matter and 51 wt % fixed carbon.15 Its nitrogen content is 1.5 wt % and it has less than 1 wt % sulfur. Aerosol System. The aerosol system was used to prepare the catalyst by reacting ferric acetate with H2S. Figure 18 shows a schematic diagram of the aerosol system. The system (12) Stohl, F. V.; Diegert, K. V. Development of Standard Direct Coal Liquefaction Activity Tests for Fine-Particle Size, Iron-Based Catalysts. Energy Fuels 1994, 8, 117-123. (13) McMillen, D. F.; Malhotra, R.; Tse, D. S. Cleavage of Benzylaromatics and Their Relevance to Coal Conversion. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 498-503. (14) Whitehurst, D. D.; Mitchell, T. D.; Farcasiu, M. Coal Liquefaction; Academic Press: New York, 1980. (15) Tian, D.; Sharma, R. K.; Stiller, A. H.; Stinespring, C. D.; Dadyburjor, D. B. Direct Liquefaction of Coal Using Ferric-SulfideBased, Mixed-Metal Catalysts Containing Magnesium and Molybdenum. Fuel, in press.
Figure 2. Details of aerosol reactor.8 consists of a pump, a stainless-steel nozzle, the reactor, and a scrubbing unit. The pump is a high-pressure pneumatic type (Haskel, Model M-71) capable of delivering up to 1 mL of liquid per cycle at 8000 psia at a frequency of 5-60 cycles/min. The nozzle is essentially a stainless-steel hollow-cone atomizer and is backed up by a pressure-check valve to generate an aerosol. The aerosol reactor is shown in Figure 2.8 The aerosol reactor consists of a 1 m long, 250 mm diameter stainless-steel cylinder with external band heaters. The temperature in the reactor is measured by a thermocouple extending radially to the center of the reactor. The reactor top is equipped with a pressure gauge, an inlet gas line to supply a mixture of 10% H2S in N2, and an exit gas line with a back-pressure valve. Another line at the bottom of the reactor is used to remove the catalyst-water suspension from the reactor. The scrubbing unit has two sealed containers in series containing aqueous sodium hydroxide solution for the removal of H2S from the exit stream. The gases leaving the second scrubber are monitored to ensure the complete removal of H2S. The second scrubber serves as a safety device before the gases are vented into a hood. The catalysts were prepared at temperatures ranging from 200 to 250 °C, and pressures of 100 to 200 psia. Precursor concentrations of 0.01 and 0.1 M were used based on our previous (nonaerosol) work15 which indicates that the weaker solutions result in finer catalyst particles. The ferric acetate solution was prepared by first reacting aqueous ferric chloride solution with ammonium hydroxide. The ferric hydroxide precipitate was filtered, washed, and reacted with excess acetic acid to form the ferric acetate. The pH of the solution was adjusted to 4 by adding ammonium hydroxide, to ensure a sufficient concentration of the sulfide ions from H2S for the reaction. The solution was diluted with water to the desired concentration, either 0.1 or 0.01 M. In the case of the FeCu-S system, the appropriate amount of cupric acetate solution (prepared similarly) was added to the ferric acetate solution. Values of the atom fraction of Cu (fCu, Cu/(Fe + Cu)) were 0.1 and 0.4. Before starting the preparation procedure, the aerosol reactor was first pressurized with N2 to 200 psia to check for any leakage. The aerosol reactor was then brought to atmospheric pressure. To start the run, a mixture of N2 and H2S was charged into the aerosol reactor to a predetermined “cold” pressure (i.e., at ambient temperature) depending upon the desired pressure at the final reactor temperature. (For example, initial cold pressures of 55 and 65 psia were used at 250 and 200 °C, respectively, to obtain a pressure of 100 psi.) The aerosol reactor was then heated to the required temperature. When the desired temperature was reached, the final
Preparation Conditions for Aerosol Ferric Sulfide Catalysts pressure adjustment was made, either by operating the backpressure valve at the reactor exit or by further addition of the H2S/N2 gas mixture. The acetate solution was now sprayed into the aerosol reactor, by forcing the solution through the nozzle at high pressure to generate fine droplets. The aerosol reactor temperature and pressure were monitored closely and could be controlled by adjusting the back-pressure valve and by varying the liquid flow rate. The initial products of reaction are ferric sulfide and acetic acid; in addition, steam is generated by the vaporization of water in the solution. The particles of ferric sulfide accumulated at the bottom of the reactor and disproportionated into pyrite and pyrrhotite. The acetic acid and steam were swept out with the exit stream, along with some of the H2S. Due to the continuous loss of H2S from the aerosol reactor, additional H2S was added intermittently. The exit gases were passed through the scrubbing unit, where the NaOH solution removed the H2S, before venting the gases into a hood. After the desired amount of solution was sprayed, the aerosol reactor was cooled and purged with N2. A quantity of water was sprayed into the aerosol reactor to slurry the iron sulfide product. The catalyst-water slurry was flushed out from the bottom of the aerosol reactor into a collection vessel filled with N2. After most of the available catalyst was recovered, the aerosol reactor was washed thoroughly with water. The recovered catalyst particles in the collection vessel were separated by centrifugation, washed, and dried at room temperature for over seven days in vacuum under N2. In all these steps, the catalyst was isolated from air. Since all the catalyst could not be recovered, it is difficult to calculate the yield of the product exactly, or to make an iron balance. However, based on our experience with other techniques for making the ferric sulfide, we expect the reaction to form ferric sulfide to be essentially complete. Further, a relatively small fraction of the solid is entrained out of the reactor and captured by the scrubbing unit. We estimate that 80-90% of the maximum available amount of catalyst is typically recovered. Catalyst Characterization. The catalysts were characterized in terms of specific gravity, surface area, and pyrrhotite/pyrite ratio (PH/PY). X-ray diffraction (XRD), Auger electron spectroscopy (AES), and electron dispersive X-ray (EDX) analyses of some samples were also carried out. It should be realized that exposure of the catalyst to air could not be prevented during the characterization measurements. The specific gravity and surface area were measured using a He pycnometer and a BET surface area analyzer, both from Micromeritics. The amounts of PH and PY were measured in terms of hydrochloric acid solubility and nitric acid solubility, respectively, in that order. The solutions from the acid dissolutions were analyzed by atomic absorption spectroscopy (AA) to obtain the amount of iron. These assignments of acid solubilities were confirmed by AES. The XRD patterns were obtained in the laboratory of Professor J. J. Renton at WVU. The diffractograms were obtained without any background subtraction. Identification of the peaks was achieved by using d-spacing values rather than by using relative peak heights, i.e., from the ASTM data files rather than from ASTM cards. For iron pyrites, the former technique is preferred, since peak intensities may vary due to absences of one or more species in the lattice, unless extreme care is taken to ensure that characterization-grade samples are being prepared. The particle size was calculated using transmission electron microscopy (TEM) in the laboratory of Professor G. P. Huffman at the University of Kentucky. The sample was prepared by suspending the catalyst in ethyl alcohol using a sample/alcohol weight ratio of about 1/800. The suspension was agitated in an ultrasonic bath for about an hour. A drop of the suspension was placed on thin carbon formvar (Ted Pella Inc., Redding, CA) predeposited on 200-mesh copper grids. The specimen
Energy & Fuels, Vol. 10, No. 3, 1996 759 was placed in the sample holder after the alcohol evaporated leaving the ultrafine particles on the grid. Micrographs were obtained using a Hitachi H800 NA microscope. The operating voltage was 200 kV. Six different areas of each sample were monitored and micrographs obtained at magnifications of 80 000. Liquefaction Run Procedure. A stainless-steel tubing bomb reactor with a volume of 27 mL was used for the liquefaction. The reactor was charged with 3 g of coal and 4.7 g of either tetralin or phenanthrene. In the catalytic runs, 0.05 or 0.25 g of catalyst was also added to the reaction mixture, corresponding to a loading of 1.67 or 8.3%, respectively, based on dry, ash-free (daf) coal. To ensure the presulfiding of the catalyst, 0.1 mL of CS2 was added to the reaction mixture. In our previous work,3,15 it was observed that the results are not affected by increasing the amount of CS2 above 0.1 mL. After loading, the reactor was purged and pressurized with hydrogen to 1000 psia(cold). The reactor was heated in a fluidized sand bath which was preheated to the desired temperature before the run. Run temperatures ranged from 350 to 440 °C. The reactor reached the desired temperature in less than 3 min. The run duration was 30 min, including the warm-up time. At the end of the run, the reactor was quenched in water. In some runs, the gaseous products were collected in a sampling flask and analyzed by gas chromatography. The amount of gas was evaluated on a hydrogen-free, ethane-equivalent basis; i.e., the response factors for various components of the gaseous product (other than hydrogen) were assumed to be the same as that for ethane. The solid and liquid products in the reactor were washed and extracted with tetrahydrofuran (THF) for 24 h. The THFinsoluble (TI) material was separated by filtration. The overall conversion of the original material was calculated from the amount of TI. After the removal of THF by rotary evaporation, the THF-solubles were extracted with hexane for 2 h. The (THF-soluble) extract was separated into hexane-insoluble (HI) and hexane-soluble fractions by filtration. The HI fraction represents asphaltenes and preasphaltenes. The conversion (X) and the yield of asphaltenes and preasphaltenes (A) were calculated as follows:
X ) (Fm - TI)/Fdaf
(1)
A ) HI/Fdaf
(2)
Here Fm and Fdaf represent the amount of feed on a moisturefree and a daf basis, respectively. When the gas yield (G) was determined independently from the gas analysis, the oil yield (O) was obtained by difference:
O)X-A-G
(3)
In runs where the gaseous product was not analyzed, the combined oil-plus-gas yield (OG) was obtained by difference:
OG ) X - A
(4)
All the runs were made in duplicate. Reproducibility has historically been better than (3%. Some of the (duplicate) runs were repeated occasionally, and the reproducibility was (again) within the limits above, which indicates that there are no reactor effects.
Results and Discussion Catalyst Characterization. Scanning electron microscopy of the products from the aerosol reactor has shown that they consist of shells of varying thickness.8a Figure 3 shows a TEM micrograph for a typical aerosol catalyst sample. The TEM operator noted that micrographs from other areas of the catalyst sample are similar, and indicated that the bulk of the particles are
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Figure 3. Transmission electron micrograph of an aerosol catalyst prepared at 200 °C, 200 psia, and a precursor concentration of 0.1 M. Table 1. Characterization of Aerosol Catalysts preparation conditions aerosol reactor
precursor concn (M)
T (°C)
P (psia)
fCua
0.1 0.1 0.01 0.01 0.01 0.1 0.1
200 200 250 200 200 200 200
200 100 100 200 100 200 200
0 0 0 0 0 0.1 0.4
characterization by surface areac specific gravityb (m2/g) PH/PYd 3.61 3.2 4.01 3.9 2.78 3.86 3.86
7.55 9.28 3.06 9.82 1.73 13.744 23.12
0.6 1.1 NA 0.1 NA NA NA
a Atom fraction Cu/(Cu + Fe). b He pycnometry. c N adsorption 2 BET. d AA.
in the range 3-20 nm. The presence of both fine particles as well as large agglomerates is indicated. As soft agglomerates are expected to break during the sonication of the TEM sample, the agglomerates may be expected to be hard. Table 1 shows the specific gravity, BET surface area, and PH/PY ratios of some catalysts. It should be realized that the catalyst properties in the table essentially represent the clusters or aggregates of the fine particles. From Table 1, it can be seen that the specific gravity varies between 2.8 and 4. Repeated measurements indicate a reproducibility of approximately (0.02 for this parameter; hence the changes in values with the changes in preparation are significant. Most iron sulfides have a specific gravity in the range of 4.3 (ferric sulfide) to 5 (pyrite). The present values are considerably lower. The difference may be due to the presence of sulfur or carbon impurities in these catalysts. Also, these catalyst particles may be hollow. According to Gadalla and Hsuan,7 the size and thickness of aerosol particles is determined by the relative rates of reaction over the catalyst surface and the evaporation of water. A high evaporation rate leads to small-diameter, large-thickness particles, as the aerosol droplets shrink rapidly due to the loss of water. On the other hand, a high reaction rate results in largediameter, small-thickness (hollow) particles, since a ferric sulfide crust is formed quickly at the surface of the droplet before a significant shrinkage in size occurs.
Similar observations were also made by Stiller et al.8 and Zhang et al.16 Since the rate of evaporation increases with an increase in temperature or a decrease in the water pressure in the aerosol reactor, whereas the rate of reaction increases with an increase in temperature and reactant concentration in the aerosol reactor, the ultimate size and thickness of the particles is determined by the preparation conditions. The results in Table 1 show that, with the precursor concentration of 0.1 M, the specific gravities of the ironalone catalysts are relatively low (at 3.2-3.6). This indicates the presence of large-diameter, small-thickness particles. The low specific gravities may also be due to the presence of impurities as mentioned earlier. When the catalyst particles are crushed, their specific gravities increase. This supports our assertion that the particles are hollow. As the precursor concentration decreases to 0.01 M, the specific gravities increase to 3.9-4.1 (except at the lowest conditions of pressure and temperature, which are discussed below). The increase may be due to both a decrease in the catalyst particle size and a decrease in the amount of impurities. The high specific gravity of the catalyst prepared at 250 °C indicates the presence of small particles. Table 1 also shows that the copper-containing catalysts have specific gravities which are slightly higher than that for the iron-alone catalyst prepared under similar conditions. The surface areas of the catalysts are between 1 and 23 m2/g. Repeated measurements indicate a reproducibility of approximately 0.1 m2/g, indicating that the changes noted in Table 1 are due mainly to differences in preparation procedures. The values of the surface areas are relatively low. This is probably due to agglomeration of the catalyst particles. Interestingly, the copper catalysts have relatively high surface areas, indicating a reduction in the extent of agglomeration in the presence of copper. For the iron-alone catalysts, there appears to be only a small dependence of the surface area on the preparation conditions. When the precursor concentration is high (0.1 M), the surface areas are somewhat higher, probably due to the small thickness of the particles. The extremely low surface area and low specific gravity at preparation conditions of the lowest temperature and pressure may be due to the decomposition of the reaction product at the surface of the particle to form an impermeable skin. The pyrrhotite/pyrite (PH/PY) ratios of the catalysts, determined using the acid dissolution technique, are around 1, consistent with those observed previously. The total iron content of these catalysts lies between 368 and 404 mg/g of catalyst. This range is below the stoichiometric iron content of ferric sulfide (538 mg/g) or pyrite (467 mg/g). The difference is again perhaps due to the presence of sulfur and other impurities. These results are consistent with AES and EDX analyses of the catalysts as described below. EDX analysis of the catalyst samples showed that their S/Fe ratios (bulk) are between 0.9 and 2.5. These values are similar to those for PH and PY which are 1 and 2, respectively. A S/Fe ratio of greater than 2 indicates the presence of elemental sulfur in the catalyst, which is consistent with the He pycnometry results. (16) Zhang, S. C.; Messing, G. L.; Huebner, W. YBa2Cu3O7-x Superconductor Powder Synthesis by Spray Pyrolysis of Organic Acic Solutions. J. Aerosol Sci. 1991, 22, 585-599.
Preparation Conditions for Aerosol Ferric Sulfide Catalysts
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Figure 4. X-ray diffraction of air-isolated samples of aerosol catalysts. Peaks of pyrite (PY), elemental sulfur (S), greigite (G) and monoclinic pyrrhotite (PH) are indicated. (a, top) Fe-S catalysts (b, bottom) Cu-Fe-S catalysts.
The AES analysis showed that the S/Fe ratios at the surface of the catalysts are between 2 and 3.6. Again, these values indicate the presence of both PH and PY, and elemental S. A comparison of the EDX and AES results shows that the S/Fe ratios at the catalyst surface are higher than those in the bulk. In other words, the sulfur appears to migrate preferentially to the surface of the catalyst. The results further indicate that the
S/Fe ratios are not significantly affected by the preparation conditions. Typical XRD results are presented in Figure 4. As mentioned earlier, the patterns are shown without any background subtraction, and peak positions (rather than relative peak areas) are used for identification of the species. Most of the peaks in the case of the iron-alone catalysts (Figure 4a) correspond to those of pyrite (FeS2),
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Figure 5. Effect of solvent on performance of catalyst 1 (aerosol reactor conditions: pressure, 100 psia; temperature, 200 °C; precursor concentration, 0.1 M) and catalyst 2 (100 psia, 250 °C, 0.01 M). Liquefaction conditions: 400 °C, 30 min, 1000 psia H2 (cold), phenanthrene solvent, 1.67% catalyst loading, air-isolated catalyst.
with a few S peaks, a few greigite (Fe3S4) peaks, and relatively few peaks of monoclinic pyrrhotite (Fe7S8), labeled PH in the figure. (Note that greigite can be considered a form of PH with x ) 1.33.) The XRD patterns from other Fe-S aerosol catalysts are similar, except that the pyrite peaks appear somewhat stronger for catalysts prepared at low precursor concentration. The presence of greigite, generally considered to be an unstable phase, has been noted in preliminary results with this aerosol reactor.8 The hypothesis is that aerosol catalysts may contain normally unstable phases due to the rapid quenching of the particles in the aerosol reactor. The small number of discrete PH peaks indicates that monoclinic pyrrhotite structures are too small to show a crystal pattern. As mentioned already, the results from both the acid dissolution technique and AES/EDX analyses have indicated the presence of pyrrhotite in these catalysts; i.e. the catalysts contain both greigite and monoclinic pyrrhotite. The results for Cu-iron sulfide catalysts (Figure 4b) are similar to those for iron sulfide catalysts. However, there are some additional peaks corresponding to chalcopyrite (CuFeS2), and the intensities of these peaks increase with the increase in copper concentration. The results indicate that the copper has indeed been incorporated in the catalyst structure. Earlier work, with a Ni-Fe-S catalyst (not prepared by an aerosol process) indicated11 that the Ni is present as (Fe,Ni)Sx, with x ) 0.89, i.e., a pyrrhotite-type alloy. In fact, if the chalcopyrite of the present Figure 4b can be written as Cu0.5Fe0.5Sx with x ) 1, the results are consistent with the previous work. The relatively large peaks of the chalcopyrite could indicate that the insertion of copper improves the formation of pyrrhotite-type structures. Catalytic Activity. A. Effect of Solvent. Figure 5 compares the results for the two solvents, phenanthrene and tetralin, for two different catalysts. The conversions are lower with phenanthrene. This is almost certainly due to the poor hydrogen-donor properties of
Sharma et al.
Figure 6. Effect of aerosol reactor temperature on catalyst performance. Other preparation conditions are as follows: aerosol reactor pressure 100 psia and precursor concentration 0.1 M. Liquefaction conditions are as in Figure 5.
phenanthrene compared to tetralin. As mentioned before, the role of the solvent is to stabilize the coal free radicals by donating hydrogen as well as to promote the cracking of coal molecules. The strong hydrogen-donor properties of tetralin enable it to supply almost all the hydrogen necessary for the liquefaction. Using tetralin, the conversions and oil yields for the two catalysts are very similar. However, the results with phenanthrene indicate that the conversion and oil yield are higher with catalyst 2 than with catalyst 1. This suggests that the choice of the solvent is important when identifying small differences in the activities of various catalysts. In the presence of tetralin, although the product yields were not different for the two catalysts, the compositions of asphaltenes and oils in the catalytic case may be different from those in the thermal case. In other words, the catalyst may promote the bond scission of thermally unreactive bonds, and alter the nature or type of the reaction products, compared to the thermal case. Stohl and Diegert12 also observed that the product yields from the liquefaction of DECS-17 coal were not affected by the addition of a sulfated catalyst. B. Effect of Preparation Conditions. Figure 6 compares the activities of two catalysts prepared at aerosol temperatures of 200 and 250 °C. The liquefaction was carried out at 400 °C using phenanthrene as solvent, with a catalyst loading of 1.67% based on daf coal. The results from the thermal base case are also shown. Both the catalysts show significant activity relative to the thermal case. The conversion with the catalyst prepared at 250 °C is about 4 percentage points higher than that with catalyst prepared at 200 °C, indicating that the higher preparation temperatures are beneficial to the catalyst activity. The oil yield for the catalyst prepared at the higher temperature is also 4 percentage points greater. This indicates that the improved activity translates directly into improved oil yield. The characterization of Table 1 indicates that the catalyst prepared at 250 °C has relatively small particles with a large thickness. Therefore, the improved activity may
Preparation Conditions for Aerosol Ferric Sulfide Catalysts
Figure 7. Effect of aerosol reactor pressure on catalyst activity. Aerosol reactor temperature, 200 °C; precursor concentration, 0.1 M. Liquefaction conditions are as in Figure 5.
Figure 8. Effect of catalyst loading on performance. Other liquefaction conditions are as in Figure 5. Catalyst preparation conditions are as follows: aerosol reactor pressure 100 psia; temperature 200 °C; and precursor concentration 0.1 M.
be due to the small size of the catalyst particles and the associated increase in the number of particles per unit weight of the catalyst. Figure 7 shows the effect of aerosol reactor pressure on the catalyst activity under similar reaction conditions. The pressure has virtually no effect on the activity. However, the oil yield increases from 17 to 20% as the pressure increases from 100 to 200 psi. The effect of precursor concentration on activity is also small (
