Molybdenum Hexacarbonyl as a Catalyst Precursor for Solvent-Free

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Energy & Fuels 1996, 10, 612-622

Molybdenum Hexacarbonyl as a Catalyst Precursor for Solvent-Free Direct Coal Liquefaction Robert P. Warzinski* and Bradley C. Bockrath Pittsburgh Energy Technology Center, U.S. Department of Energy, Pittsburgh, Pennsylvania 15236 Received December 15, 1995. Revised Manuscript Received February 13, 1996X

The use of molybdenum hexacarbonyl, Mo(CO)6, an effective catalyst precursor for coal liquefaction even in the absence of added liquids or special impregnation procedures, was studied in solvent-free microautoclave experiments. The activity of the catalysts formed from Mo(CO)6, both for liquefaction of a Blind Canyon bituminous coal and for methanation of CO in the absence of coal, was a function of activation conditions and reaction parameters. The catalysts were characterized by X-ray diffraction and other methods. Decomposition of Mo(CO)6 in the absence of sulfur led to an active form of molybdenum carbide. The use of H2S, the preferred sulfur source, promoted the decomposition of Mo(CO)6, led to MoS2, and promoted coal liquefaction. Pressure/temperature data recorded throughout the microautoclave experiment were used to determine the onset of catalytic activity for methanation of the CO released from Mo(CO)6 upon decomposition in the absence of coal. Methanation activity was seen at temperatures as low as 280 °C. However, a sharp increase in rate occurred at 340 °C. When the catalyst was formed in the presence of coal, rapid hydrogen uptake began at a slightly higher temperature (370 °C). The conversion of coal to products soluble in tetrahydrofuran correlated strongly with the initial rate of gas uptake. The cyclohexane conversion of coal was also catalyzed.

Introduction Various transition metal carbonyls of the formula Mx(CO)yswhere M is Cr, Fe, Co, Ni, Mo, Ru, Rh, W, Reshave been used as catalyst precursors in laboratoryscale investigations of direct coal liquefaction.1-7 Under appropriate conditions, most of these compounds formed highly dispersed catalysts that were active for coal liquefaction. The first reported use of the simple carbonyls in direct coal liquefaction experiments of the type reported in this paper was by Sharma and Moffett in 1982.1 They surveyed a number of organometallic compounds with two coals. Table 1 summarizes the data from this report as well as the results of a similar survey later published by Suzuki et al.2 The data in this table represent the increase in liquefaction conversions caused by the presence of the catalyst; thermal (no added catalyst) conversions have been subtracted from the catalytic results. The thermal conversions can be found in the table footnotes. The data in bold print highlight those instances where the results for particular compounds are poorer than others in the same group. Nearly all of the metal carbonyls included in Table 1 performed well in the liquefaction of the coals shown. Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Sharma, R. K.; Moffett, G. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1982, 27 (2), 11-17. (2) Suzuki, T.; Yamada, O.; Then, J. H.; Ando, T.; Watanabe, Y. Proc. Int. Conf. Coal Sci. 1985, 205-208. (3) Yamada, O.; Suzuki, T.; Then, J. H.; Ando, T.; Watanabe, Y. Fuel Process. Technol. 1985, 11, 297-311. (4) Suzuki, T.; Yamada, H.; Yunoki, K.; Yamaguchi, H. Energy Fuels 1992, 6, 352-356. (5) Suzuki, T. Energy Fuels 1994, 8, 341-347. (6) Warzinski, R. P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 503-511. (7) Artok, L.; Davis, A.; Mitchell, G. D.; Schobert, H. H. Energy Fuels 1993, 7, 67-77. X

This article not subject to U.S. Copyright.

For two coals, the results with ammonium heptamolybdate (AHM), a commonly used liquefaction catalyst precursor, are given for comparison. Except for Cr(CO)6 and in some instances W(CO)6, all of the metal carbonyls provided a significant increase in the conversion of these coals relative to the thermal tests or to the tests with AHM. The tests with the low-sulfur Wandoan coal showed that, except for Co2(CO)8 and Ni(CO)4, the presence of sulfur improved the performance of these compounds. W(CO)6 appeared to be most sensitive to the presence of sulfur. Several aspects of the reactivity of Mo(CO)6 are also of interest. For the Illinois No. 6 coal, Mo(CO)6 provided higher conversions to pentane-soluble products at equal metal loadings than the other compounds used with this coal. In subsequent tests with the Wandoan coal, Mo(CO)6 in the presence of added sulfur performed just as well as the other metal carbonyls in relation to conversion to tetrahydrofuran (THF)-soluble products at onetenth the concentration of the other metals. In this case, all of the metal carbonyls tested provided nearly complete conversion (90+%) of the Wandoan coal to THF-soluble product. The utility of Mo(CO)6 in the presence of sulfur as a catalyst precursor for the liquefaction of coal also has been evidenced in our previous work and in the work of others.7,8 The effectiveness of this system for promoting reactions of coal model compounds has also been demonstrated.9 In our laboratory, Mo(CO)6 has been used to study the influence of a catalyst on the liquefaction of coal.6,10 This work has shown that the inherent (8) Warzinski, R. P.; Lee, C.-H.; Holder, G. D. J. Supercrit. Fluids 1992, 5, 60-71. (9) Suzuki, T; Yamada, H; Sears, P. L.; Watanabe, Y. Energy Fuels 1989, 3, 707-713. (10) Warzinski, R. P. Proc. Int. Conf. Coal Sci. 1993, 2, 337-340.

Published 1996 by the American Chemical Society

Molybdenum Hexacarbonyl

Energy & Fuels, Vol. 10, No. 3, 1996 613

Table 1. Comparison of the Effectiveness of Various Simple Metal Carbonyls for Increasing Conversion of Coal Catalytic Minus Thermal Conversions

compound Cr(CO)6 Fe(CO)5 Co2(CO)8 Ni(CO)4 Mo(CO)6 Ru3(CO)12 W(CO)6 Re2(CO)10 AHMg

South Barber #8 71% Ca C6H6b

York Canyon 79% Cc C6H6b C5b 4 14 30

18 7 23 5

29 31 30 30 2

3 6 10

llinois No. 6d 77% C, 4% S THFb C5b

Wandoane 77% C, 0.2% S THFb C5b

Wandoan + Se 77% C, 0.2% S THFb C5b

20 35 36 37 35

4 18 16 10 36

36 45 44 24f

10 26 27 4f

45 42 43 41f

27 28 29 18f

35

11

3

-8

45

35

8

aSee ref 1. Subbituminous coal, 6 h at 380 °C in 3:1 tetralin:coal under hydrogen in a 1-L rocking autoclave. Catalyst loading 1% based on maf coal. 61% thermal conversion. bC6H6, benzene solublity; THF, tetrahydrofuran solubility; C5, pentane solubility. cSame as in footnote a only with bituminous coal. Thermal conversions: 44% C6H6; 18% C5. dSee refs 2 and 3. 1 h at 425 °C in 2:1 1-methylnaphthalene: coal under hydrogen in a microautoclave. Catalyst metal loading 1% based on coal. Thermal conversion: 57% THF; 22% C5. eSee refs 2 and 3. Same reaction conditions and catalysts loading as in footnote d. Thermal conversions: 49% THF, 22% C5. fMo loading reduced to 0.1% based on coal. gAmmonium heptamolybdate (AHM) added at 1% based on maf coal.

volatility of Mo(CO)6 permits it to form an active liquefaction catalyst in the presence of sulfur with no special preparation, impregnation, or dispersion techniques. The liquefaction of coal is effectively accomplished by the simple direct addition of Mo(CO)6 to the liquefaction reactor even in the absence of any added solvents or vehicles. The work reported here describes the activation, reactivity, and characterization of the catalyst formed from Mo(CO)6 and discusses the role of this catalyst in the solvent-free liquefaction of a lowsulfur bituminous coal. Experimental Section Materials. The Mo(CO)6 was used as received from Strem Chemical Company. Purity was given as 98+%, and moisture was the only major contaminant. Ammonium tetrathiomolybdate (ATM) was purchased from Alfa Products and used as received. Elemental analysis of the ATM showed that it contained some oxygen; however, the S:Mo ratio was 4:1. In all the experiments with coal, the DECS-17 coal from the Penn State Coal Sample Bank was used. The coal was -60 mesh and was riffled prior to use. The elemental analysis (on a dry basis) provided with the coal was as follows: 76.3% carbon, 5.8% hydrogen, 1.3% nitrogen, 0.4% sulfur (0.02% pyritic sulfur), 6.6% ash, and 9.7% oxygen (by difference). The asreceived moisture content was 3.7%. Microautoclave Experiments. Experiments with catalyst precursors and/or coal were performed according to previously described procedures8,11 in 316-stainless steel microautoclaves of approximately 46 cm3 internal volume. Separate microautoclaves were used for thermal and catalytic experiments to avoid residual catalytic effects. Catalyst precursors and/or coal was added to the microautoclaves without any special impregnation or mixing procedures. Specific amounts are noted in the text. Unless otherwise indicated, all the experiments were conducted for 1 h at reaction temperature using a slow heat-up and rapid cooldown. The products were recovered according to the referenced procedures.8,11 Elemental analyses of the products were performed at Huffman Laboratories in Golden, Colorado. Electron spectroscopy for chemical analysis (ESCA) and X-ray diffraction analysis were performed at the Pittsburgh Energy Technology Center (PETC). Utilizing Pressure/Temperature DatasCompensating for the Nonisothermal Nature of the Microautoclave System. The pressure and temperature data collected during microautoclave experiments were used to follow the change (11) Warzinski, R. P.; Holder, G. D. Fuel 1992, 71, 993-1002.

in the total number of moles of gas in the system with time. This provided information concerning the transformation of Mo(CO)6 to an active catalyst in regard to methanation of CO in the absence of coal and the temperature of the onset of this catalytic activity. Similar information on gas uptake was also obtained from experiments with coal. Determining the amount of gas in the system, especially during the heat-up period, from the pressure and temperature data was complicated by the fact that a significant portion of the total gas volume in the reactor system is outside the heated portion of the microautoclave. The cooler region consisted of the internal volumes of the pressure transducer, connecting tubing, and associated valves and was about 12 cm3. During heat-up and reaction, the gas in the cooler region is at a higher density than the gas in the microautoclave. Therefore, the uncorrected value calculated using the ideal gas law for the apparent number of moles of gas decreases as the temperature inside the microautoclave increases. This is illustrated in Figure 1a for a test in which only 0.161 mol of H2 at an initial pressure of 7.0 MPa was used in the microautoclave. Shown in this figure are the apparent change in the gas content of the reaction system as a function of time and the time/internaltemperature history of the reactor during slow heat-up to 425 °C, a 60-minute soak at this temperature, and rapid cool-down. Instead of remaining constant, the calculated gas content decreased as the temperature of the reactor increased and finally leveled out at the lower value when the temperature stabilized. As expected, after cooling to room temperature, the apparent number of moles of gas returned to the initial value. To compensate for the nonisothermal behavior, a correction equation was developed by fitting the percentage apparent change in the number of moles of gas, (no - n)/no, where no is the initial moles of gas in the system, to the change in temperature, T - To. These data and a representative fit for the test shown in Figure 1a are contained in Figure 1b. Because the correction is sensitive to the relative volume of gas heated, a new correction equation was developed for each microautoclave used. A second- or third-order polynomial correction was normally sufficient. Figure 1c contains the data in Figure 1a after applying the correction equation to the number of moles of gas calculated by the ideal gas law as a function of time (data are collected every 10 s during an experiment). Deviations in the gas content using this procedure are typically less than 1 mmol. Periodic tests with H2 or H2/3% H2S were performed to verify that an adventitious change had not reduced the precision of the correction. If the deviations exceeded 1 mmol, a new correction equation was developed. In experiments with Mo(CO)6 and/or coal, differences in gas content data for repeat experiments are normally less than 5 mmol.

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Warzinski and Bockrath Table 2. Analysis of Products Produced from Mo(CO)6 and ATM atomic S/Mo ratio temp, oC precursor elemental ESCA elemental composition 175 250 250 375 375

Figure 1. Illustration of a mathematical expression to compensate for the nonisothermal nature of the microautoclave system: (a) apparent change in gas content during a test with only hydrogen (7.0 MPa initial cold pressure) in the system as a function of time and temperature (data calculated using the ideal gas law); (b) apparent percentage change in the number of moles of gas as a function of the change in temperature and the curve representing the polynomial equation (given in the figure) describing this relationship; (c) representation of the data after applying the correction equation to the results calculated using the ideal gas law.

Results and Discussion Conversion of Mo(CO)6 to an Active Catalyst. Mo(CO)6 is a sublimable solid that decomposes without melting at 150 °C.12 We have observed the conversion

Mo(CO)6 Mo(CO)6 ATM Mo(CO)6 ATM

2.5 2.0 2.3 2.3 2.3

1.3 1.8 1.8 1.9

MoS2.0C0.3H1.7O1.1N0.1 MoS2.3C0.3H2.4O2.3N0.3 MoS2.3C0.3H0.9O0.3 MoS2.3C0.3H2.7O1.8N0.1

of Mo(CO)6 in H2S to a MoS2-containing compound in a high-pressure, windowed cell. A description of this system has been published.8 In this experiment, 0.37 mmol of Mo(CO)6 and 4.4 mmol of H2S were added to the cell. At 90 °C and 7.0 MPa, a brown coating started to cover the Mo(CO)6 particles and the interior surfaces of the cell. At 110 °C, the coating on the window prevented further visual observation. Inspection of a sample of the brown, mirrorlike coating on the window by ESCA after the experiment indicated that it was similar in composition to MoS2. Therefore, in the absence of a liquid phase and in the presence of H2S, the initial reactions involved in the transformation of Mo(CO)6 to a MoS2-containing catalyst can take place between Mo(CO)6 vapor and H2S at temperatures near 100 °C. The observed sublimation and migration of Mo(CO)6 and subsequent deposition of a MoS2-containing compound by this means could serve to effectively form a highly dispersed catalyst on coal during heat-up to liquefaction conditions. To determine the fate of Mo(CO)6 in the microautoclave liquefaction system, experiments were performed at several temperatures with Mo(CO)6 and the results compared to those obtained from similar experiments with ATM, a nonvolatile liquefaction catalyst precursor. In these experiments, 0.9 g of the catalyst precursor and 9.1 MPa of a H2/10% H2S gas mixture were charged to the microautoclave. The products from reactions with Mo(CO)6 and ATM were recovered as methylene chlorideinsoluble and THF-insoluble residues, respectively. The S:Mo atomic ratios of the products determined from both elemental analysis and ESCA are contained in Table 2 along with the elemental composition. At 175 °C, the lowest temperature investigated, approximately 45% of the carbonyl reacted to form a product with a S:Mo ratio of 2.5. Unreacted Mo(CO)6 was also observed. An experiment with ATM was not performed under these conditions. At 250 °C, no unreacted Mo(CO)6 was observed. At this temperature, 97% of the carbon monoxide in the carbonyl was detected in the product gas. The products formed from Mo(CO)6 and ATM at 250 °C have slightly different S:Mo ratios. However, at 375 °C, the products were similar in composition and close to the expected value of 2:1 for MoS2. ESCA analyses of the samples indicated a lower S:Mo ratio on the surface. The ESCA analyses also showed the presence of a small amount of SO42on the surface of these samples. Direct oxygen analyses of these samples at Huffman Laboratories confirmed the presence of oxygen. X-ray diffraction analysis was also performed at PETC to determine the degree of crystallinity of the products from Mo(CO)6 and ATM. At 175 and 250 °C, the products were essentially amorphous compounds. (12) CRC Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC: Boca Raton, FL, 1988; B-108.


Figure 2. Corrected pressure/temperature data to monitor the decomposition and activation of Mo(CO)6 in the microautoclave with H2/10% H2S (7.8 MPa initial cold pressure): (a) reaction at 425 °C for 1 h; (b) reaction in (a) compared to an experiment at 250 °C for 1 h and an experiment in which a slow heat-up to 325 °C was performed followed by immediate quenching. The numbers in parentheses in part b are the relative percentages of CO, CO2, and CH4 (plus a small amount of higher hydrocarbon gases), respectively, at the end of each experiment shown.

At 375 °C the development of some crystallinity was observed. Similar work on the transformation of Mo(CO)6 and ATM under liquefaction conditions was reported by Artok et al.7 Results with ATM are in agreement with those in Table 2; however, they reported lower S:Mo ratios (1.1-1.7) when using Mo(CO)6. One possible reason may be that in our experiments, the H2S:Mo ratio in the microautoclave system was initially about 6.3:1; whereas in the work by Artok et al. it was reported as 2.5:1. Under typical coal liquefaction conditions in our system, at 1000 ppm Mo, the H2S:Mo ratio was initially about 150:1. Experiments with Mo(CO)6 were also conducted in the microautoclave in which the pressure and temperature data were used to determine the variation in the amount of gas present as a function of time and temperature. The data were corrected as described in the Experimental Section to compensate for the nonisothermal nature of the microautoclave system. Three experiments were performed using 3.7 mmol of Mo(CO)6 under an initial pressure of 7.8 MPa H2/10% H2S: one at 425 °C for 1 h, one at 250 °C for 1 h, and one that was quenched immediately upon reaching 325 °C. A slow heat-up to reaction temperature was used. Figure 2a contains the data for the 425 °C experiment, which is typical of the conditions used for liquefaction studies. Figure 2b contains the heat-up and reaction period data for all


three experiments as well as the results of the hydrocarbon gas analyses performed on a gas sample collected after the reactions were quenched. These results, in parentheses, represent the amounts of CO, CO2, and hydrocarbon gases (mainly methane). The time/temperature history of the experiments are also shown in both figures. Interpretation of the complex pattern of behavior seen in these figures is made easier by recalling that MoS2 is a potent water/gas shift catalyst13 and methanation catalyst.14 The data in Figure 2a show that gas first evolved (section A) and then was consumed (section B) during the heating period. The gas production in section A started near 160 °C, which is close to the reported decomposition temperature of 150 °C for Mo(CO)6 under its own vapor pressure.12 The amount of gas in the microautoclave continued to increase until, at 280 °C, the trend reversed and a gradual decrease was observed. As the reactor temperature approached 340 °C, a marked increase in the rate of gas consumption occurred. The data from the 250 °C, 1 h experiment in Figure 2b show that at this temperature the gas content continued to increase to a level of 22 mmol and remained there for the duration of this experiment. This is equal to the maximum amount of CO that could have been liberated from the Mo(CO)6 charged. The gas analysis showed that CO accounted for 96% of the carbon-containing gases produced. Even after 1 h, only 4% CO2 is detected, likely due to a small amount of water/gas shift taking place (Mo(CO)6 contained about 2% water as the major contaminant). The water/gas shift reaction is shown below.

CO + H2O f CO2 + H2

(1)

Figure 2b also shows data from another experiment that was terminated immediately upon reaching 325 °C. This experiment was performed to investigate the cause of the abrupt halt in the increase in gas content at 280 °C and the onset of the gradual decrease. Analysis of the recovered gases showed that some methanation, shown below, and water/gas shift conversion had occurred, although carbon monoxide accounted for 94% of the carbon-containing gases produced.

CO + 3H2 f CH4 + H2O

(2)

The methanation reaction accounts for the observed reduction in the net moles of gas in Figure 2a. The water/gas shift reaction does not change the molar balance of gases and therefore would not be observed in this treatment of the data. (The reaction of H2S with Mo(CO)6 to form MoS2 would also not change the molar balance of gases, since one mole of H2 would be produced for each mole of H2S consumed.) In section B of Figure 2a, a rapid decrease in the amount of gas commences near 340 °C. This is due to the increased rate of methanation. Gas analysis data taken at the end of the 425 °C experiment and contained in Figure 2b show that all of the CO liberated from Mo(CO)6 was utilized with a 90% selectivity toward methane formation. A fairly uniform rate of gas consumption (0.4 mol mol-1 (Mo) min-1) occurred in the range 370410 °C. (13) Hou, P.; Wise, H. J. Catal. 1983, 80, 280-285. (14) Hou, P.; Wise, H. J. Catal. 1985, 93, 409-416.

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Warzinski and Bockrath

Table 3. Crystallite Dimensions and Elemental Composition of Products Produced from Mo(CO)6 crystallite size (Å) temp (oC)

residence time (h)

(002) plane

(110) plane

elemental composition

250 325 425

1 0 1

20 32

40 58 65

MoS2.1C0.8H2.0O0.4 MoS2.0C0.3H1.0O0.2 MoS2.0C0.2H0.7O0.2

The data in Figure 2a also show that after the reaction was quenched, the number of moles of gas present was lower than at the end of the reaction prior to quenching owing to the condensation of water vapor formed as a result of the methanation reaction. The amount of water indicated in Figure 2a (23 mmol) is in reasonable agreement with that expected based upon the amount of Mo(CO)6 charged and the methane formed (20 mmol). The total number of moles of gas consumed, 55 mmol, is also close to the net 57 mmol hydrogen demand of the system (59 mmol to form the methane indicated less the 2 mmol produced in the water/gas shift reaction). It is apparent that the microautoclave data, after appropriate correction, provide a convenient means of following the decomposition and activation of Mo(CO)6. Regarding methanation of CO, the onset of catalytic activity occurs near 340 °C under conditions identical to those used for the liquefaction of coal. The most probable source of error in this analysis procedure would be due to the escape of condensable gases (water and Mo(CO)6 vapor) into the colder region of the system. The above data show that for this system, this does not appear to be a major problem. Table 3 contains information concerning the elemental composition and crystalline structure of the methylene chloride-insoluble products from the three experiments represented in Figure 2b. The elemental compositions of the catalysts formed show that the disulfide was the predominant phase, even in the short-residence time experiment at 325 °C. Contamination of the catalysts with C, H, and O was similar to that observed in Table 2. The dimensions of the MoS2 crystallites obtained by X-ray diffraction increased with reaction temperature. The crystallites in the 250 °C material were essentially single-layer MoS2. At the higher temperatures, some stacking occurred. The effects of other reaction parameters on the decomposition and activation of Mo(CO)6 have also been investigated. By use of the temperature and pressure data as before, the activation of Mo(CO)6 in the H2/10% H2S environment is compared in Figure 3a to similar experiments conducted in H2 with CS2 as the sulfur source and in H2 alone. In Figure 3b, the activation in H2/10% H2S is compared to experiments using N2 and N2/10% H2S. All of the experiments were performed with 3.7 mmol Mo(CO)6 with 7.0 MPa of either H2 or N2. A slow heat-up to reaction temperature was used. In all five cases shown in parts a and b of Figure 3, the liberation of CO is observed as Mo(CO)6 decomposes. The largest and most uniform release of CO to the gas phase occurred when H2S was present in either H2 or N2. The data in Figure 3a also show that the most active methanation catalyst was formed in the presence of H2 without an added sulfur source. This is evidenced by the sharp decrease in gas content that began at 365 °C. The rate of decrease in the temperature range 380-

Figure 3. Effects of other reaction parameters on the decomposition and activation of Mo(CO)6: (a) experiments with H2 alone, with H2S, and with CS2; (b) comparison of the reaction in H2/10% H2S with N2 and N2/10% H2S.

410 °C was 50% greater (0.6 mol mol-1 (Mo) min-1) than for the catalyst formed in the presence of H2/10% H2S. When N2 was used, no significant drop in gas content occurred, either with or without H2S. The results from X-ray diffraction analyses of most of the samples collected from the above experiments are contained in Table 4 along with the temperatures of the onset of gas uptake, both initial and rapid, and the rates of initial rapid gas uptake. In the presence of H2/10% H2S, the strongest peak in the X-ray diffraction pattern was from MoS2. The average crystallite size was 75 Å with a stacking height of 30 Å, similar to the data in Table 3 for the compound prepared at 425 °C. The MoS2-containing catalyst formed in the presence of N2/ 10% H2S had similar dimensions. When either N2 or H2 alone was used, the strongest peak was from Mo2C. In these cases, the crystallites were more uniform in size with an average diameter of between 14 and 20 Å. By reference to the data in Figure 3a, the carbide formed in the presence of H2 appears to be a more active catalyst for methanation than the sulfided species. This agrees with published results obtained at 350 °C and 101.3 KPa.15 Although the carbide-containing catalyst exhibited a higher methanation rate, the catalyst formed in the presence of H2S became active with respect to methanation at temperatures at least 25 °C lower. Mo(CO)6 As a Catalyst Precursor for the Liquefaction of DECS-17 Coal. We have previously shown that Mo(CO)6, even in the absence of added liquids or special impregnation procedures, was an effective catalyst precursor for the liquefaction of Illinois No. 6 coal.8 (15) Saito, M.; Anderson, R. B. J. Catal. 1980, 63, 438-446.



Table 4. Observations on the Activity and Structure of Catalytic Species Formed from Mo(CO)6 in Different Environments H2/10% H2S onset of initial gas uptake, °C onset of rapid gas uptake, °C initial rate of rapid gas uptake, mol mol-1(Mo) min-1 major peak in X-ray spectrum crystallite size, Å (plane)

N2/10% H2S

N2

280 340 0.4 MoS2 30 (002) 75 (110)

MoS2 28 (002) 70 (110)

Mo2C 14 (111) 19 (220)

H2

H2/CS2

365 0.6

330 350 0.2

Mo2C 14 (111) 19 (220)

ND ND ND

Figure 4. Effect of molybdenum concentration (added as Mo(CO)6) on the liquefaction conversions of the DECS-17 coal: (O) THF conversion; (0) cyclohexane conversion. Data at 10 ppm are for results for the raw coal (no added catalyst).

Figure 5. Effect of catalyst, initial gas composition, and form of sulfur addition on the liquefaction conversions of the DECS17 coal.

Under the rapid heat-up conditions employed in this earlier work, effective conversion of this coal at 425 °C required a molybdenum concentration of at least 1000 ppm based on dry ash-free (daf) coal. This earlier work also revealed the advantages of using a slow heat-up for effective conversion of the Mo(CO)6 to an active catalyst. In the current work, the DECS-17 coal was chosen for study owing to its low levels of pyritic iron, a known native liquefaction catalyst precursor. A series of experiments was performed with this coal to determine the effect of the catalyst concentration under slow heatup reaction conditions. Figure 4 shows the effect on the conversion of the DECS-17 coal at 425 °C of simply adding various amounts of Mo(CO)6 powder along with coal to the microautoclave. These experiments were performed using 3.3 g of coal with an initial charge of 7.2 MPa H2/3% H2S. Duplicate experiments were performed with the raw coal (indicated in Figure 4 at 10 ppm) and with 100 ppm added molybdenum. Six replicate experiments were performed at a level of 1000 ppm added molybdenum. The bars associated with the data points in Figure 4 indicate the range of values obtained. No bars are shown if they are within the size of the symbol. Good conversions to both THF- and cyclohexane-soluble products are noted at catalyst concentrations of 500 ppm Mo (based on daf coal) and above. A pronounced catalytic effect is noted even at molybdenum loadings of 50 to 100 ppm. There does not appear to be much additional benefit of using catalyst loadings above 1000 ppm under these reaction conditions. This concentration of catalyst was used in the work described below. To extend the work shown in Figure 3 to coal, experiments were performed using the DECS-17 coal

(3.3 g) with various combinations of Mo(CO)6 (1000 ppm Mo based on daf coal), H2 (7.0 MPa), H2/3% H2S (7.2 MPa), CS2 (0.3 g), and N2 (7.0 MPa). As with the experiments with Mo(CO)6 alone, shown in Figure 3, all of these experiments were performed at 425 °C. The reagents used and the conversion results are shown in Figure 5. Bars that represent the range of values obtained about the average are shown for the experiments performed more than once. The first set of data illustrates the high conversions obtained with this coal using only Mo(CO)6 with H2/3% H2S. Comparing the first three sets of data shows the importance of adding sulfur and adding it as H2S in this system. A drop in both THF and cyclohexane conversions occurred when H2S was eliminated. However, the conversions were still relatively high. Compared to no added sulfur (third set of data), use of CS2 (second set of data) had no benefit. The fourth and fifth sets of data represent thermal conversions in the absence of catalyst. A small improvement was observed when H2S was present, even in the absence of catalyst. Finally, the last two sets of data represent experiments performed in N2. The presence of Mo(CO)6 had no effect in the absence of H2. When DECS-17 coal is present, the Mo(CO)6 sublimes and disperses onto the coal from the gas phase either prior to being sulfided or as it is converted to the sulfide phase. The previously described experiments in the view cell support this assumption. At the levels of Mo used in the experiments represented in Figure 5 (1000 ppm), the fate of the Mo(CO)6 could not be determined using X-ray crystallography on the THF-insoluble residues owing to the small amount present and to interferences from the coal mineral matter in the residue. However, the residue from a similar experiment at 425 °C in which Mo(CO)6 was used at a higher level (10 000 ppm) could be examined after removal of the mineral

618



Table 5. Number of 1 h Replicate Experiments Performed with DECS-17 Coal reaction temp, °C

thermal

catalytic

325 350 375 400 425

2 4 3 4 4

2 4 2 2 6

matter with hydrofluoric acid. In this case, X-ray diffraction analysis identified molybdenum disulfide in the THF-insoluble product with a crystallite size of approximately 80 Å in the (110) plane and a relatively low degree of stacking. When coal was absent, the MoS2 crystallites formed from Mo(CO)6 under similar conditions (Table 4) had a similar dimension in the (110) plane but exhibited a higher degree of stacking. To determine the effect of Mo(CO)6 on the liquefaction of DECS-17 coal as a function of temperature, liquefaction experiments were performed at 25 °C intervals from 325 to 425 °C in the presence and absence of this compound. Samples of 3.3 g of the coal and 7.2 MPa of H2/3% H2S were used in all of these experiments. In the catalytic experiments, Mo(CO)6 was used at a level of 1000 ppm Mo (based on daf coal). Residence times at the reaction temperature of both 1 and 8 h were used. Table 5 summarizes the number of replicate experiments that was performed in each case at the 1 h residence time. Only one 8 h experiment was performed at each condition. The conversion data for the thermal and catalytic experiments are shown in parts a and b of Figure 6, respectively. As with Figure 4, the symbols represent the average conversion values and the bars associated with the symbols for the 1 h experiments indicate the range of values obtained. No bars are shown if they are within the size of the symbol. The data show that greater variability in conversion values was associated with specific conditions. For example, the greatest variabilities were associated with determinations of cyclohexane conversion for the catalytic experiments. It was also noted that the filtration of the THF solution from the catalytic experiment at 375 °C was much more difficult than for the same product at the other temperatures, resulting in greater variability than for the same determination at the other temperatures. In the presence of the catalyst, the maximum conversion of the DECS-17 coal to THF- and cyclohexanesoluble products is 94% and 81%, respectively, as evidenced in the 8-hour 400 °C and 425 °C data in Figure 6b. After 1 h, the THF conversion at these temperatures is complete; however, conversion to cyclohexane-soluble product still occurs. Retrograde reactions are not evident in the presence of catalyst; however, the data in Figure 6a show that these reactions have a significant impact at temperatures above 375 °C when catalyst is not present. The decrease in conversion with increasing temperature is most evident in the 8-hour experiments without catalyst. To better illustrate the effect of the catalyst, Figure 7 contains the differences obtained by subtracting the thermal conversions from the corresponding catalytic conversions. Regarding THF conversions, little or no catalytic effect is observed at 325 °C after 1 h. However an effect is observed at this temperature at the longer residence times. For the 1 h experiments, as the reaction temperature increases, there is a corresponding

Figure 6. Effect of temperature and time on the liquefaction conversions of DECS-17 coal: (a) thermal (no added catalyst) experiments; (b) catalytic experiments. A sample of 1000 ppm Mo was added as Mo(CO)6. Symbols are defined as follows: (O, b) THF conversion; (0, 9) cyclohexane conversion. The open symbols represent 1 h experiments and the filled symbols 8 h experiments.

Figure 7. Catalytic minus thermal liquefaction conversions for the DECS-17 coal. Symbol definitions are the same as in Figure 6.

increase in the additional amount of THF conversion because of the effect of the catalyst. The catalytic effect levels off as the maximum total conversion is approached (400 °C). The greatest increment to the catalytic effect on THF conversion in the 1 h experiments occurs between 350 and 375 °C. The influence of the catalyst is less for the higher temperature, 8 h experiments owing to the relatively higher thermal conversions.



Figure 8. Average gas production for DECS-17 coal for 1 and 8 h thermal (T) and catalytic (C) experiments at different reaction temperatures. Bars are defined in the figure.

Approximately 20-30% more conversion occurs when the catalyst is present in the 8 h experiments. Different trends are observed for cyclohexane conversion. For the 1 h experiments, no activity is observed until the reaction temperature exceeds 375 °C, at which point the conversion attributed to the catalyst suddenly increases. A much smaller increment in conversion is noted between 400 and 425 °C. For the 8 h experiments, no catalytic activity is observed at 325 °C. However, above this temperature, the catalyst has an effect that increases with temperature. The conversions noted above are calculated by difference using the weights of the insoluble residues collected and thus do not differentiate between the yields of liquid and gaseous products. Figure 8 shows the average production of gaseous products for the 1 and 8 h thermal and catalytic experiments at the various temperatures. It is apparent that most of the gases produced are the result of thermal chemistry. The largest increase in gas production as a function of temperature occurs between 400 and 425 °C for the 1 h reactions. Only the production of butane and, to a lesser degree, propane appears to be influenced by the presence of the catalyst. The increased production of these species points to increased cracking activity, possibly of hydroaromatic ring structures. The total pressure within the microautoclave was recorded at 10 s intervals during each experiment. These data were converted to estimates of the number of moles of gas in the microautoclave using the same procedures described above for experiments with Mo(CO)6 alone. An example of this treatment for the 1 h experiments at 425 °C is shown in Figure 9. The data from five thermal and five catalytic experiments (including the corresponding data from the 8 h experiments) were averaged to obtain the results shown. In addition to the thermal and catalytic data, this figure also contains data labeled "Catalytic-Thermal", which represents the changes in gas content due to the presence of the catalyst and is obtained by subtracting the thermal data from the corresponding catalytic results. The thermal history of the experiments, averaged from the same data, is also shown in this figure. In both the thermal and catalytic experiments, the number of moles of gas initially increased because of the native water in the coal. The total amount of water

Figure 9. Effect of adding 1000 ppm Mo as Mo(CO)6 on the changes in gas content in the microautoclave as a function of time and temperature.

present in this coal would amount to 6.7 mmol. The value observed in Figure 9 was lower than this probably because of condensation of some of the water vapor in the cooler portion of the system. Without a catalyst, the onset of gas production occurred at 320 °C. By reference to the data in Figure 8, the gas produced is primarily CO2. More rapid gas evolution began at 390 °C. Shortly after the reactor reaches 425 °C, gas evolution ceased and a gradual uptake of gas occurred for the duration of the experiment. With a catalyst, no gas evolution is apparent after the initial liberation of water. A small amount of gas uptake was noticed near 340 °C, which increased in rate at 375 °C. If the thermal reactions associated with the production or consumption of gas in this system are assumed to occur in the same fashion when a catalyst is present, the catalytic-thermal data in Figure 9 represent the effect of the catalyst in this system. These data show that the catalyst formed from Mo(CO)6 started to facilitate hydrogen uptake near 325 °C. A dramatic increase in this activity occurred near 370 °C. A limit on hydrogen uptake due to the presence of the catalyst is observed in the data in Figure 9. This is evidenced by the decreasing slope of the line representing the catalytic-thermal data. Based on the data shown in Figure 9, the maximum hydrogen uptake due to the

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Figure 10. Effect of adding 1000 ppm Mo as Mo(CO)6 on changes in gas content in the microautoclave as a function of time and temperature.

catalyst for this coal is about 0.017 mol (0.011 g H2/g daf coal). Calculating the maximum hydrogen uptake from the data obtained after quenching the reactions (not shown in Figure 9) results in a value of 0.017 g H2/g daf coal. This latter value is identical to that obtained using the actual gas quantity and composition information obtained after the microautoclave experiments. For comparison purposes, the total hydrogen uptakes from the catalytic and thermal experiments based on the microautoclave gas quantity and composition data were 0.033 and 0.015 g H2/g daf coal, respectively. Taken together, the data from these 10 experiments shown in Figure 9 as 2 average trend lines provide a compelling illustration that, with appropriate care, a global aspect of the complex chemistry of the liquefaction process can be followed with a high degree of precision. Coal is commonly regarded as a quite variable feedstock and liquefaction as composed of a complicated assortment of processes. Despite this inherent profusion, the present results lend support to the belief that this knotty mixture of physical events and chemical reactions can be observed, modeled, and rationalized as a reproducible sum of phenomena. In particular, the catalytic effects observed here are taken as a consistent, reproducible record of performance. Figure 10 summarizes catalytic-thermal gas-phase data for the 1 h experiments at all the different reaction temperatures. At least two sets of data were averaged to obtain each of the results shown. The average thermal history for the 425 °C experiments is also shown. The lower-temperature experiments follow the same portion of the heat-up profile from room temperature to reaction temperature. Little influence of the catalyst on the rate of hydrogen uptake is noted at 325 °C. A slightly higher rate is observed at 350 °C. At even higher temperatures a pronounced increase in the rate of hydrogen uptake is noted. A summary of the initial rates of gas uptake at each reaction temperature is presented in Table 6. Parts a and b of Figure 11 compare two measures of catalytic activity: the initial rate of gas uptake and the total amount of hydrogen uptake, both corrected for thermal contributions, to the liquefaction conversions caused by the catalyst (thermal contribution subtracted) in the 1 h experiments. The initial rates of gas uptake

Figure 11. Effect of catalyst on liquefaction conversions (catalytic minus thermal) as functions of (a) the initial rate of gas uptake due to the catalyst and (b) the total hydrogen uptake due to the catalyst: (O, b) THF conversion; (0, 9) cyclohexane conversion. Open symbols represent data from experiments represented in Figure 7, and filled symbols represent catalytic data from experiments in Figure 4 minus the respective thermal conversions.) Table 6. Rates of Initial Gas Uptake Caused by the Presence of a Catalyst During the Liquefaction of DECS-17 Coal reaction temp (oC)

rate of initial gas uptake (mmol g-1 (daf coal) min-1)

325 350 375 400 425

0.007 0.018 0.084 0.127 0.238

were determined from the microautoclave temperature and pressure data. The total hydrogen uptake was calculated from the gas analyses and the amounts of gas charged and recovered from the system. The data set was derived from the experiments conducted at different temperatures with 1000 ppm Mo (open symbols) and the experiments conducted at 425 °C with different concentrations of catalyst (closed symbols). The conversion data from these experiments were also used in Figures 7 and 4, respectively. Regarding THF conversion, the best correlation (r2 ) 0.99) is observed when the initial rate of gas uptake (plotted on a logarithmic scale in Figure 11a) is used as the measure of catalyst activity. A correlation with a good but lower sample coefficient of determination (r2


) 0.93) was obtained from the correlation between conversion and the total hydrogen uptake (Figure 11b). Artok et al. reported a similar correlation relative to total hydrogen uptake (plotted on a linear scale) for experiments with another sample of the same coal.7 Regarding cyclohexane conversion, the strengths of the associations depicted in parts a and b of Figure 11 are less, r2 values in both cases being near 0.70. As shown in Figure 7, the catalyst only influenced cyclohexane conversions above a temperature of 375 °C in the 1 h experiments. If only the data for experiments at temperatures above 375 °C are used, the r2 values for the correlations of cyclohexane conversion with the initial rate of gas uptake and the total hydrogen uptake are 0.79 and 0.88, respectively. Again, the results of Artok et al. are similar.7 Conclusions The results presented above confirm that Mo(CO)6 forms a finely divided active catalyst for coal liquefaction even in the absence of added liquids or special impregnation procedures. The decomposition of Mo(CO)6 and the liquefaction of coal in the presence of this compound were both facilitated by the presence of H2S. Under these conditions, an active MoS2-containing phase was formed. The coal was found to influence the structure of the catalyst formed. With coal, the MoS2 crystallites produced at 425 °C had an average size of 80 Å with a low degree of stacking. Under similar conditions without coal, the average size was 75 Å and the stacking dimension was about 30 Å. The form of added sulfur was also found to be important. Substituting CS2 for H2S adversely affected Mo(CO)6 decomposition, as determined by the liberation of CO, and the liquefaction conversion of the DECS-17 coal. In the absence of added sulfur, Mo(CO)6 formed a carbide-containing phase that was even more active with respect to methanation than the sulfide phase. However, this increased activity was not noted in liquefaction experiments with this low-sulfur coal. It is possible that the above-mentioned influence of the coal on the structure of the catalysts formed also influenced these results. In H2S, Mo(CO)6 begins to decompose and form a MoS2-containing phase near 100 °C. In microautoclave experiments with Mo(CO)6 without coal in an H2/H2S mixture, the onset of activity of the MoS2-containing catalyst for the formation of methane from the CO liberated from Mo(CO)6 as it decomposed began near 280 °C and increased dramatically at about 340 °C. The situation was different when Mo(CO)6 was used in coal liquefaction. In this case, the relatively small amounts of Mo(CO)6 charged with the coal to the autoclave precluded observation of the onset of activity for CO conversion. However, the onset of a catalytic effect on hydrogen uptake in the experiments with coal became detectable at about 325 °C and was more pronounced on reaching 370 °C. Concerning the net change in gas observed in the microautoclave, the time profiles in Figure 9 for thermal and catalytic reactions appear as reflections of each other over the period of initial heat-up. At the moment thermal reactions begin to generate gas, a net uptake of hydrogen was observed if the catalyst was present. This close correlation in time of gas-generating and gas-


consuming reactions is taken as evidence that an active catalyst has been formed before the onset of coal thermolysis and that the catalyst is able to supply the hydrogen required as a consequence of the decomposition of coal. In this view, gas-generating reactions are a signal for the onset of the larger group of thermal reactions that could exert hydrogen demand. The initial rates of net gas uptake (Table 6) then would reflect to a large degree the hydrogen demand placed on the catalyst by the thermal reactions of the coal. This view of one of the many roles played simultaneously by MoS2 is an outgrowth of the concept put forth by Bearden and Aldridge16 of “catalytically controlled free radical reactions”. In addition to the onset temperatures for catalytic activity being higher when coal was present, the rates of hydrogen uptake were also slower for reactions with coal compared to the methanation of CO. Again, the differences in the structures of the catalysts formed in the presence and the absence of coal may explain some of this behavior. Also, the presence of coalderived products may alter the performance of the catalyst. Another view of catalyst performance was given by the results of solvent analysis of the products. The overall effect of the catalyst formed from Mo(CO)6 was made evident by changes in the product distribution. The catalyst facilitated dissolution of DECS-17 coal to THF-soluble products, the effect increasing with reaction temperature. At the long reaction time (8 h), there was a significant effect even at the lowest temperature investigated, 325 °C. At the higher temperatures, the catalyst doubled the yield of THF-soluble product found after 1 h. Retrogressive behavior that limits conversion and even causes it to decline was particularly evident at higher temperatures in the thermal reactions. The small amount of catalyst used here (1000 ppm Mo) was sufficient to overcome this effect. The importance of the ability of the catalyst to rapidly supply hydrogen to the reacting coal was also evident from the strong correlation of the initial rate of catalyst-assisted gas uptake to the improvement in THF conversion caused by the catalyst. This aspect of catalyst activity appeared to be even more important than the total hydrogen utilized as a result of the catalyst being present. A second role of the catalyst was made evident by the unique pattern observed in the improvement in yield of lighter, cyclohexane-soluble products. As made clear by plotting the difference between catalytic and thermal conversions, the onset of catalytic influence occurred at a higher temperature for the latter role. At the shorter reaction times, temperatures above 375 °C were required. At long reaction times this activity was observed even at 350 °C. Also, no strong correlation to either initial or total hydrogen utilization was observed for the catalyst-assisted cyclohexane conversions, further highlighting a separate reaction sequence for these reactions. The separate roles and their differing temperature dependencies may be easily rationalized on the basis that THF conversion is catalytically assisted mainly by prevention of retrogressive reactions while cyclohexane conversion is catalytically assisted by the well-known repertoire of hydrogenation, heteroatom removal, and cracking activities commonly associated with MoS2. An (16) Bearden, R.; Aldridge, C. L. Energy Prog. 1981, 1, 44-48.

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unresolved issue at present is whether changes in catalyst structure conceivably induced by the higher reaction temperatures may also play a role. Finally, this work has shown that with an appropriate empirical correction, microautoclave pressure and temperature data can be used to extract useful information from the changes in the amount of gas in the system during an experiment. Of particular importance was information gained on the point at which catalytic activity was initially observed. This information provided the basis for choosing a slow heat-up regime for these experiments to prevent confusion between the effects due to catalyst activity vs those due to catalyst activation. At the same time, such data added greatly to the information obtained on the catalyst activity itself and, separately, the hydrogen demands of thermal


reactions that were satisfied by the catalyst. Use of this technique to expand kinetic studies of working catalysts is in progress. Acknowledgment. The authors thank Richard Hlasnik and Jerry Foster for performing the microautoclave work, Sidney Pollack and Elizabeth Frommell for X-ray diffraction analysis and interpretation, and John Baltrus and Rodney Diehl for obtaining the ESCA results. Reference in this report to any specific product, process, or service is to facilitate understanding and does not imply its endorsement or favoring by the United States Department of Energy. EF950258O

Molybdenum Hexacarbonyl as a Catalyst Precursor for Solvent-Free

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