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Catalysis & DCL

Catalysis & DCL






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    Catalysis & DCL Catalysis & DCL Document Transcript

    • FuelVol. 75, No. 6, pp. 751-758, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved ELSEVIER 0016-2361(95)00294-4 0016-2361/96 $15.00+0.00 Direct liquefaction of coal using ferric-sulfide-based, mixed-metal catalysts containing Mg or Mo Dacheng Tian, Ramesh K. Sharma, Alfred H. Stiller, Charter D. Stinespring and Dady B. Dadyburjor Department of Chemical Engineering, West Virginia University, PO Box 6102, Morgantown, WV 26506-6102, USA (Received 6 March 1995; revised 17 July 1995) Direct liquefaction of coal was studied using ferric-sulfide-based mixed-metal catalysts containing magnesium or molybdenum as the second metal. The catalysts were mostly impregnated in situ on the coal, although physical mixtures of catalyst and coal were also used in some runs for comparison. The liquefaction was performed at 350-440°C under a hydrogen pressure of 6.9 MPa (cold). Tetralin and phenanthrene were used as solvents. The catalytic effects became more evident with phenanthrene as solvent. The activities of impregnated catalysts were 5-8% higher than those of the physical mixtures of catalyst and coal. The addition of magnesium was found to be not particularly beneficial to the activity and selectivity of the catalyst. The addition of molybdenum increased the catalyst activity by up to 8 wt%, resulting in conversions of >90 wt% at 400°C. The yield of the oil fraction also increased considerably in the presence of molybdenum, especially at 400 and 440°C. The activity of the catalyst decreased by ,-~5% when it was exposed to air. Copyright © 1996 Elsevier Science Ltd. (Keywords: coal liquefaction; catalysts; Fe-Mo) Iron-based catalysts are commonly used in direct coal a second species such as Si or A1 is introduced in the liquefaction (DCL), since they show relatively good catalyst. activity and are cheap and environmentally desirable 1,2. The incorporation of a second metal may be used to The authors' work on such catalysts has focused on the modify the nature of active sites on the catalyst surface use of ferric sulfide precursor. Ferric sulfide, Fe2S3, is and to alter the catalytic activity. Metals which have unstable at room temperature and disproportionates ionic radii within 10% of that of the primary metal may into pyrite (FeS2) and non-stoichiometric pyrrhotite readily be substituted in the lattice, following the so- (FeSx, x ~ 1). The relative amount of each of these and called Hume-Rothery rules. Previously8, we character- the nature of the pyrrhotite depends upon the time and ized the surface structure and the catalytic performance temperature of disproportionation 3. of the solid formed when Ni was incorporated into a Catalysts of small particle size (10-20 nm) improve the ferric-sulfide-based material. (The catalyst was mixed dispersion and increase the activity. However, small with, rather than impregnated into, the coal.) The particles generally agglomerate and grow, resulting rationale for using Ni was that Ni is not only sufficiently 45 in a loss of activity' . Cugini e t al. 6 observed that, for close to Fe in ionic size but is a substantial hydro- an impregnated catalyst, the presence of coal minimizes genation catalyst in its own right. It was found 8 that the the agglomeration of the particles. This indicates that conversion of coal increased from 62 wt% for the iron- the particle size of impregnated catalyst may be smaller alone catalyst to a maximum of 70 wt% when the Ni/ than that when the catalyst is prepared in the absence of (Ni + Fe) ratio was 0.5. The yield of the oil-plus-gas coal. fraction also increased, by about 3 percentage points. The authors have dev.eloped7 a technique for impreg- These increases can be considered relatively small, nating in s i t u the ferric sulfide precursor, which then considering the cost of the added metal. disproportionates to the pyrite-pyrrhotite mixture in the Hager et aL 9 used nickel, cobalt, tungsten and coal. The technique uses solutions, rather than an molybdenum as promoters in iron-based catalyst for incipient-wetness approach. The effects of catalyst the liquefaction of Black Thunder coal with tetralin as loading and of using different methods of introducing solvent. It was reported that, among all the metals the iron have been described. studied, molybdenum showed the highest increase in The addition of a second species to the catalyst may conversion and yields. Pradhan et aL 10 also observed that also be helpful. Zhao e t al. 5 reported that the agglom- the addition of molybdenum or tungsten up to 5 wt% eration of ferrihydrite catalyst particles is reduced when (based on iron) as a promoter to sulfated haematite Fuel 1996 Volume 75 Number 6 751
    • Direct liquefaction of coal." D. Tian et al. resulted in a significant increase in activity. In these ash (db). The elemental composition (wt%) is: C, 81.9; studies too, the catalysts were mixed with, rather than H, 6.3; N, 1.5; S, 0.9. impregnated into, the coal. The conversion of coal is also governed by the type of Catalyst solvent used in liquefaction. According to McMillen Catalysts with varying amounts of second metal were et al.ll, the role of the solvent is to stabilize the coal free- prepared. They are designated as A1-A3 (with magne- radicals by donating hydrogen as well as to promote the sium) and B1 (with molybdenum). cracking of coal molecules. The use of tetralin as solvent Catalysts of type A1 were not impregnated in situ. is widely reported in the literature, owing to the strong Instead, they were prepared by mixing stoichiometric hydrogen donor properties of tetralin. However, Stohl amounts of ferric chloride, sodium sulfide and mag- and Diegert4 observed that catalytic effects in the nesium acetate, all in aqueous solutions. In order to presence of tetralin are small, since tetralin itself can obtain small particles, weak solutions (0.01 M) of the supply almost all the hydrogen necessary for the starting reagents were used. The precipitate was filtered, liquefaction of coal. Phenanthrene is known to be a washed and dried under vacuum at 85°C. This catalyst poor hydrogen donor solvent compared with tetralin ~2. was mixed with coal at a loading of 8.4 wt% (based on Hence DCL conversions should be smaller in the daf coal). The Mg fractions (fMg, Mg/(Mg + Fe) ratio) presence of phenanthrene than with tetralin, and were 0.01, 0.05, 0.1 and 0.5. catalytic effects should be more pronounced in the Catalysts A2 and A3 were impregnated in situ on the presence of phenanthrene than with tetralin. However, coal. To do this, coal was first added to the dilute phenanthrene may enhance cracking reactions during aqueous ferric chloride solution. The slurry was mixed DCL. thoroughly before adding the dilute sodium sulfide and Anderson and Bockrath 13 observed that the activity of magnesium acetate solutions. The mixture was agitated iron-based catalysts was improved by the addition of for 2h and filtered. The impregnated coal was dried sulfur, especially when the S/Fe molar ratio of the overnight in N2 at 85°C under vacuum. The catalysts catalyst was <1. The effect of sulfur was attributed loadings were 1.67wt% (A2) and 8.5wt% (A3), again mainly to the formation of catalytically active pyrrhotite. based on daf coal. The values offMg were again 0.01, The objective of the present study was to investigate 0.05, 0.1 and 0.5. the performance of impregnated, iron-based, mixed- The molybdenum-containing catalysts were prepared metal catalysts in DCL using various solvents. The basic in the same way as the F e - M g - S catalysts, except that catalyst is an intimate mixture of pyrite and pyrrhotite ammonium heptamolybdate solution was used in place obtained by disproportionation of ferric sulfide. The of magnesium acetate solution. The catalyst loading was in situ impregnation technique used earlier for the iron- 1.67 wt%. The fMo ratios (Mo/(Fe + Mo)) were 0.05, alone catalyst was modified to generate mixed-metal 0.08, 0.1, 0.15 and 0.25. A few runs were also made with sulfides. The second metal was generally incorporated catalysts in which the ammonium heptamolybdate before disproportionation and impregnation. The solution was added to Fe-impregnated coal after the second metals used in this work were magnesium and filtration. In some cases, a small amount (5wt%) of molybdenum, both of which meet the Hume-Rothery ferrous chloride was added to the ferric chloride solution, criteria. Magnesium was selected to determine whether a to check if the extent of precipitation of MoS2 is altered metal with a significantly lower cost and less strategic by the addition of ferrous chloride. importance than Ni or Mo could be used in conjunction with iron to obtain comparable results. Molybdenum Liquefaction equipment and proeedure was selected because of the performance of the non- A stainless steel tubing bomb reactor with a volume of impregnated non-sulfide Fe-Mo catalysts described 27ml was used for the liquefaction. The reactor was above. Different concentrations of these second metals charged with 3 g of impregnated coal, 4.7 g of tetralin or were used, but the concentration of the second metal was phenanthrene, and 0-0.I ml CS2. After loading, the kept low in all cases, so that these catalysts would be reactor was purged and pressurized with hydrogen to economically and environmentally desirable. Tetralin 6.9MPa (cold). The reactor was heated in a fluidized and phenanthrene were used as solvents to investigate sand bath which was preheated to the desired tempera- the effects of facile hydrogen donation from the solvent. ture before the run. At the end of the run, the reactor was Runs were made at 350-440°C with a hydrogen pressure quenched in water. The run duration was 30 min or 1 h. of 6.9 MPa (cold). The products were analysed in terms In some runs, the gaseous products were collected in a of the asphaltene fraction and the oil-plus-gas fraction. sampling flask and analysed by gas chromatography. In all cases, the results were compared with those from The amount of gas was evaluated on a hydrogen-free, iron-alone catalysts prepared similarly. ethane-equivalent basis, i.e. the response factors for various components of the gaseous product (other than EXPERIMENTAL hydrogen) were assumed to be the same as that for ethane. Additional details on equipment and procedure Coal are available elsewhere 2. The coal used in this study was DECS-6, which is a The solid and liquid products in the reactor were high-volatile-A bituminous coal from the Blind Canyon washed and extracted with tetrahydrofuran (THF) for seam in Utah. The coal was received from the 24 h. The THF-insoluble material (TI) was separated by Pennsylvania State University Coal Bank and ground filtration. The overall conversion of the original material to <250 #m under nitrogen. DECS-6 was used in this was calculated from the amount of TI. After the removal work because of its extremely low pyrite content, thus of THF by rotary evaporation, the THF-solubles were allowing the effect of added iron catalyst to be obtained. extracted with hexane for 2 h. The extract was separated The coal is of 49wt% volatile matter (daf) and 6.3 wt% into hexane-insoluble (HI) and hexane-soluble (HS) 752 Fuel 1996 Volume 75 Number 6
    • D#ect fiquefaction of coaL D. Tian et al. fractions by filtration. The THF-soluble-hexane- RESULTS AND DISCUSSION insoluble fraction, i.e. the HI fraction, was defined as asphaltenes. The conversion (X) and the yield of Effect o f solvent asphaltenes (A) were calculated on dry, ash-free (daf) Figure 1 compares the results for the two solvents basis as follows: tetralin and phenanthrene. With tetralin, and in the absence of any catalyst, 57 wt% of the coal is converted. X = (F m - TI)/Fda f (1) On the other hand, with phenanthrene as solvent, the uncatalysed conversion is much lower. These results are A = HI/Fda f (2) consistent with those reported earlier4. where Fm and Fda f represent the amount of feed on the The yield of oil-plus-gas is ~7 wt% in each case. This moisture-free and daf bases, respectively. (The form of shows that the yield of asphaltenes is higher with tetralin Equation (1) is used so that the ash content does not have than with phenanthrene. Clearly the solvent has a to be subtracted separately from TI.) When the gas yield, considerable effect on the conversion and product slate G, was determined (independently) from the gas analysis, for the uncatalysed reaction. Because of the lower values the oil yield, O, was obtained by difference: of conversion and yield obtained in non-catalytic runs with phenanthrene, experiments using this solvent can O = X- A - G (3a) more easily differentiate between the activities of various In runs where the gaseous product was not analysed, the catalysts than can experiments with tetralin. combined oil-plus-gas yield (OG) was obtained by Catalytic results are also shown in Figure 1. For difference: simplicity, a second metal is not considered. With either solvent, the conversion increases when iron-alone OG = X - A (3b) catalysts of loading 1.67wt% (type A2) are used. The increase due to the catalytic activity is about 6 percentage Most of the runs were made in duplicate and the points in the case of tetralin and 23 percentage points in reproducibility was better than +3%. the case of phenanthrene. The yield of asphaltenes also increases correspondingly in each case. Catalyst A3, with Catalyst characterization the higher loading (8.4wt%), has even more dramatic Coal mixtures containing iron-molybdenum catalysts effect on the results: the conversions are 8 percentage were characterized in terms of pyrrhotite/pyrite ratio, points higher than those with A2, and the oil-plus-gas PH/PY, before and after reaction. The amounts of PH yields are about 5 percentage points higher. and PY were measured in terms of hydrochloric acid Significantly, the conversions and yields with the two solubility and nitric acid solubility, respectively, in that solvents in the catalytic runs are almost indistinguish- order. The solutions from the acid dissolution were able. In other words, this catalyst may make it analysed by atomic absorption spectroscopy (AAS) to economically advantageous to use a cheaper poor- obtain the amount of iron. The contributions of PH and hydrogen-donor solvent. Similar observations were PY from the raw coal alone were also obtained. These made when the second metal was added to the catalyst. could be subtracted from the values for the catalyst plus It is well known that the initial step in coal liquefaction coal mixtures, at least for the pre-liquefaction samples. is the thermal fragmentation of the coal molecules to X-ray diffraction (XRD) and Auger electron spectro- produce free radical species u. These free radicals must scopy (AES) of some samples were also carried out. be stabilized by hydrogen in order to prevent their 100 Conversion, Oil+Gas Yield 80 D I I, ~ Thermal v 1~7"~, ~ Catalyst A2 .o 60 ~, ~ Catalyst A3 + 40 O 2C ..r, 0~ O Tetralin Phenanthrene Figure 1 Effect of solvent type on conversion and yield for catalysts A2 and A3 withfMg = 0. Reaction conditions: 350°C, 1 h, 4.7 g solvent, 0.1 ml CS2 Fuel 1996 Volume 75 Number 6 753
    • Direct fiquefaction of coaL" D. Tian et al. recombination by the retrogressive reactions. The are probably due to better dispersion of the small catalyst necessary hydrogen may come from the solvent, the particles on the coal than in the physical mixtures of gaseous hydrogen or the coal itself. Tetralin is a good catalyst and coal. However, the yields of the oil-plus-gas hydrogen donor and can supply almost all the hydrogen fraction are somewhat lower with the impregnated necessary for the stabilization of the radicals, resulting in catalysts than those with A1. The improved conversion the higher coal conversions seen in Figure 1. The catalyst and lower oil-plus-gas yields for the impregnated in this case may have a role only in the hydrogenation of catalysts result in higher asphaltene yields for this the spent solvent by the gaseous hydrogen. mode of preparation. Phenanthrene on the other hand is a poor hydrogen In any case, the catalyst performance is not very donor solvent and consequently the conversion of coal is largely altered by the addition of magnesium, as lower than that with tetralin. However, it has been indicated by Figures 2 and 3. reported that phenanthrene is a good hydrogen acceptor In order to determine if the catalytic activities are and allows the transfer of hydrogen from the gas phase dependent on the amount of sulfur, runs were made at to the coal radicals in the presence of iron sulfide various CS2 levels using catalyst A3 withfM~ = 0.01. The catalyst 14. The increase in coal conversion in the catalytic conversion and oil-plus-gas yield increase with increasing runs is probably due to the faster hydrogen transfer CS2, but by a maximum of 1 percentage point, again processes from the gas phase. less than the expected error bars. This indicates that presulfiding does not improve the activity and selectivity Mixed-metal catalysts containing magnesium of these catalysts significantly, as observed previously3. Figure 2 shows the effect of magnesium fractionfMg on The negligible effect of CS: addition is consistent with conversion and product yield using the physically mixed observations of Anderson and Bockrath ~3, since the S/Fe F e - M g - S catalysts, type A1. At 350°C when fMg = 0, molar ratio for these catalysts is higher than unity 64wt% of coal is converted, and the oil-plus-gas yield (nominally 1.5, based on Fe2S3). (OG) is 15 wt%. The high conversions may be due to the presence of tetralin as the solvent. The addition of Mixed-metal catalysts containing molybdenum magnesium (fMg =0.01) can be seen to affect the These catalysts were tested at 350-440°C using catalytic activity positively, but the improvement (one phenanthrene as solvent. In preliminary runs, no percentage point) is well within the limits of error noted significant advantage in activity was gained when earlier. At higherfMg values, conversions are lower than catalyst loading was increased above 1.67wt%. Hence for the iron-alone catalyst 0cMg= 0), and oil-plus-gas the loading was kept constant at 1.67 wt% in all the runs. yields are little changed. These decreases are greater than At 350°C, the conversion of coal is 2-3 percentage points the error bars noted earlier. higher using the F e - M o - S catalyst withfMo = 0.05 than Figure 3 compares the activities of physically mixed with the iron-alone catalyst. Similarly, the conversion catalyst (A1) and impregnated catalyst (A3), both at to asphaltenes is 6-7 percentage points higher with the 8.4 wt% catalyst loading. The activities of catalysts A3 at F e - M o - S catalyst. These are significant differences. all the fMg ratios are 5-10 wt % higher than those of A 1. The results at 400°C, presented in Figure 4, are even These numbers are greater than the error bars, and more interesting. The conversion and yields with the F e - indicate that the impregnated catalysts are superior to M o - S catalysts are considerably higher than those with the physical mixtures of catalyst and coal. This result is iron-alone catalyst. A maximum conversion of 88 wt% is consistent with the observations of Weller l, among achieved when fMo = 0.15. The oil-plus-gas yield is a others. The higher activities of the impregnated catalysts maximum (25 wt%) when fMo is 0.1 or 0.15. The reason 70-- 60 "---o .o 50 0 ~ e~ 20 0 v 0 0 ~ 10 n o Conversion • Oil+Gas Yield I I // I I 0.00 0.05 0.10 0.5 Mg/(Fe + Mg) Figure 2 Effect of Mg/(Fe + Mg) ratio on conversion and oil-plus-gas yield for catalyst A1. Reaction conditions as in Figure 1, except that the solvent is tetralin 754 Fuel 1996 Volume 75 Number 6
    • Direct fiquefaction of coal." D. T/an et al. 100 AI A3 [ I,~'/A Conversion Oil+Gas Yield "O m >. ~3 .4- 50 -- // *O .o o =! 0.00 0.01 0.05 Mg/(Fe + Mg) Figure 3 Comparison of the performance of physically mixed catalyst (A1) and impregnated catalyst (A3). Reaction conditions as in Figure I except that the solvent is tetralin for a decrease in conversion at fMo = 0.25 is not clear. to an improvement in the catalyst activity. However, the One possibility for the lower activity atfMo = 0.25 may conversions of coal in these runs are not very different be the lower iron content of the catalyst. It is also from those with catalysts prepared without the ferrous possible that there is a maximum in the dispersion of Mo chloride addition. Further, the yield of gas appears to on the catalyst surface corresponding to fMo = 0.1-0.15. increase slightly, whereas the oil yield decreases. This Gardner et al. 15 observed a similar optimum in the Mo indicates that any early precipitation of the Mo sulfide concentration in their studies on pyrene hydrogenation (in the absence of ferrous chloride) has at best little effect over NiMo catalysts. on catalyst performance. These results show that the addition of molybdenum is In some instances the Mo source (ammonium hepta- beneficial to the activity and selectivity of the catalyst. molybdate) was added to the Fe-impregnated coal Runs were also made with catalysts prepared using a after filtration. The corresponding results are shown in mixture of ferrous and ferric chlorides instead of ferric Figure 5. The conversions and yields appear to be chloride alone. It was thought that the extent of MoS2 independent of the stage at which the molybdenum precipitation might be controlled by ferrous ions, leading solution is added to the coal. This would imply that the I 80 ¢> Thermal 60-- O + / 30'-- ,,~ 0 0 20 - Thermal 0 I I 0.0 0.1 0.2 Mo/(Fe + Mo) Figure 4 Effect of Mo/(Fe + Mo) ratio on conversion and yield for catalyst B1. Reaction conditions: 400°C, 0.5 h, 4.7 g phenanthrene, 0.1 ml CS2. Fuel 1996 Volume 75 Number 6 755
    • Direct fiquefaction of coaL" D. Tian et al. Mo added before filtration, air-exposed loo - I I Mo after filtration, air-exposed Mo before filtration, not exposed to air '~ 90 -- ~ 80 0 i 1I ~ 20 0 rj 1o i 0.0 0.05 0.10 0.15 Mo/(Fe + Mo) l~gm'e 5 Effects of order of Mo addition and of air exposure for F e - S and F e - M o - S catalyst on performance. Reaction conditions as in Figure 4 amount of molybdenum that is not on the coal but However, the oil-plus-gas yields in the catalytic runs at remains in solution (and therefore is lost during 400 and 440°C are also higher than in the non-catalytic filtration) is negligible. runs. Further, the gas yields in the F e - M o - S runs are In order to study the effect of exposure to air on the much smaller than those in the non-catalytic runs and the performance of the various F e - M o - S catalysts, runs iron-alone runs. Hence the molybdenum addition can be were made in which the catalyst was not exposed to air at seen to improve the oil yield significantly at 400 and any time during or after impregnation on coal. The 440°C. results from these runs are also presented in Figure 5. As An exact comparison of the present results with those seen in the figure, for iron-alone catalysts, the conversion in literature is difficult, due to the differences in the type increases by 3 percentage points when the catalyst is not of coal used as well as in the liquefaction conditions such exposed to air. Also, the oil-plus-gas yield increases by as reaction time and type of solvent. Cugini et al. 6 about 4 percentage points. These increases are consistent studied the liquefaction of Blind Canyon bituminous and significant. Similar observations can be made for the coal (DECS-6) at 425°C and 6.9 MPa H2 pressure for F e - M o - S catalysts, for which the increase in conversion 30min reaction time, using Panasol (a mixture of is greatest atfMo = 0.1. However, the increase in oil-plus- alkylated naphthalenes) as solvent. The conversion of gas yield appears smaller in this case than for the iron- coal in the absence of catalyst was ,,,58 wt%, with an oil alone catalyst. yield of 30 wt%. Using impregnated FeOOH as catalyst, The decrease in the activity of the catalysts on the conversion and yield increased to 85 and 41 wt% exposure to air may be due to structural changes and respectively. With molybdenum catalyst, the conversion oxidation of the small particles of the catalyst, and the increased further to 90 wt%, with a small increase in oil changes in the surface structure that follow. However, it yield. These values are similar to those in the present has not been possible to demonstrate this unequivocally study (Figure 6). Hager et al. 9 used iron-based mixed- for the impregnated samples, because of the difficulty in metal catalysts to study the liquefaction of a Wyodak locating the catalytic material in the coal matrix. Some Black Thunder subbituminous coal at 415°C with characterization results are given in the following tetralin as solvent. The activity of a catalyst containing subsection. Mo was 6 percentage points higher than for the iron- The effect of temperature on catalyst activity is alone catalyst. Zhao et al. 5 used a ferrihydrite catalyst to presented in Figure 6. Results of thermal (non-catalytic) study the liquefaction of a Blind Canyon bituminous runs are also shown for comparison. The catalysts were coal (DECS-17) at 400°C with tetralin as solvent. It was not exposed to air and the fMo values were 0, 0.1 and observed that the conversion increased from 81 wt% 0.15. The results from runs at fMo = 0.15 are not (without catalyst) to 85 wt%. The increase in oil yield included in the figure, since they were similar to those was <2 wt%. atfMo = 0.1. At 350°C, the addition of a small amount of Even though the above results cannot be compared molybdenum serves to double the difference between the directly with those from the present study, the activities catalytic and non-catalytic conversions. When the of the present mixed-metal catalysts are as high or as temperature is 400 or 440°C, the conversions with iron- better than those of the catalysts used. In particular, alone catalyst far surpass the non-catalytic conversions. for the studies where tetralin was used as solvent, the Using F e - M o - S catalysts, the conversions increase still results are comparable with the present results using further. The increase due to Mo addition at the higher phenanthrene. temperatures are less significant than at the lowest temperature, perhaps because the conversion with the Catalyst characterization iron-alone catalyst is so close to 100 wt% to begin with. The amounts of iron in the impregnated coal and the 756 Fuel 1996 Volume 75 Number 6
    • Direct fiquefaction of coal. D. Tian et al. Thermal fMo=O fMo=O.1 V-1. I~ Conv. lO0 Oil+Gas _ _ , Gas ~ 80 + ~) 60 "~ 40 ill- 20 0 350 400 440 Temperature (°C) Figure 6 Effect o f temperature on performance of F e - S and Fe M o - S catalysts. Reaction conditions as in Figure 4 except that the temperature is as indicated and the catalysts were not exposed to air liquefaction residue, and the corresponding PH/PY exposure of the catalyst to high temperatures and ratios, are presented in Table 1. The figures indicate hydrogen pressures during liquefaction. This is consist- that a very large fraction of the iron used is actually ent with earlier observations 2 with iron-alone catalyst. impregnated on the coal, but a not negligible fraction is An attempt was made to characterize the F e - M o - S lost during DCL. catalysts by X-ray diffraction (XRD) and Auger electron From the first column, the total amount of iron spectroscopy (AES). However, due to the presence of the detected in the impregnated coal is 8 - 9 m g g -1. This coal, the crystallinity of the samples was too low for the amount is only 6-9% less than the amount of iron used XRD pattern to be meaningful. Charging of the in the catalyst preparation. The difference is the amount impregnated coal was also a major difficulty for AES, of iron lost in the filtrate from catalyst preparation. In hence many samples could not be analysed. To remedy the second column, the amount of iron in the sample this, an F e - M o - S catalyst with fMo = 0.1 was impreg- after reaction is obtained per gram of residue. Hence this nated on comparable graphite particles. Spectra of figure is much higher than that in the previous column. catalyst-coal particles, where obtained, were compar- Multiplying this value by (1 - X) leads to a calculated able with those of the corresponding catalyst-graphite amount of iron per weight of coal after reaction, shown samples. Finally, some F e - M o - S catalysts were pre- in the third column. This figure can be compared with the cipitated during preparation in the absence of coal or value before reaction. The comparison shows that the graphite. decrease is ,,~25% atfM o = 0.05, 44% atfM o = 0.1 and The AES results are summarized in Table 2. Here 34% atfMo = 0.15. This indicates that some iron is lost 'precipitated' refers to the catalyst in the absence to the liquefaction products. of graphite (or coal), 'catalyst-graphite' refers to the PH/PY ratios are also shown in Table 1. These figures as-impregnated graphite, while 'THF-insoluble' refers to indicate that the pre-liquefaction values of the ratios the liquefaction residue. In some cases, the carbonaceous for catalysts increase with increase in Mo content, from material from the liquefaction residue was removed by 7.4 for iron-alone catalyst (fMo = 0) to ~15 when dissolution in N-methylpyrrolidone (NMP); the resulting fMo = 0.1-0.15. Interestingly, the high PH/PY ratios at material, presumably the catalyst and other inorganic fMo = 0.1-0.15 correspond to catalysts which have matter, was analysed by AES. Accordingly, in Table 2, higher activities than those with lower fMo ratios. The 'NMP-insoluble' refer to the residue after washing with PH/PY ratios in the residues obtained after liquefaction NMP. are much lower than in the impregnated coal before The results are shown as fractions of S and Mo relative liquefaction. The decrease is probably due to the to Fe plus Mo on the surface. They indicate that the Table 1 Total iron and PH/PY ratio of F e - M o - S catalyst Total Fe PH/PY ratio Before DCL After DCL After DCL Before After fMo (mgg -1 coal) (mgg i residue) (mgg -1 coal) DCL DCL 0.0 8.58 n.d. a n.d. 7.47 n.d. 0.05 7.74 46.07 5.82 9,76 1.22 0.1 8.6 39.82 4.78 15.39 1.05 0.15 8.18 42.12 5.36 14.18 1.05 Not determined Fuel 1996 Volume 75 Number 6 757
    • Direct liquefaction of coal." D. Tian et al. Table 2 Surfaceanalysis of Fe-Mo-S catalysts (fMo= 0.1) to the oil yield. Activity and selectivity to the oil Surface ratio fraction increase with temperature, at least up to 440°C. Catalyst type S/(Fe + Mo) Mo/(Fe + Mo) 5. With the precipitated F e - M o - S catalysts, the surface Precipitated 1.2 0.17 is Mo-rich and S-poor. On impregnation, the S and Catalyst-graphite 1.1 0.0 M o contents of the catalyst surface decrease appre- THF-insoluble 0.5 1.0 ciably. After DCL, carbonaceous material covers all NMP-insoluble 1.8 0.17 the Fe sites on the surface and a large fraction of the S sites on the surface. surface of precipitated catalyst is slightly S-poor, relative 6. Catalytic effects during D C L are more evident when to the bulk value of S / ( F e + Mo) = 1.4 (assuming Fe2S3 phenanthrene is used as solvent, relative to tetralin. with fMo = 0.1). On the other hand, the surface of the 7. Isolation of F e - M o - S catalysts from air improves fresh catalyst is enriched with Mo, relative to the their performance. expected value of 0.1. Both these effects are counter to those observed previously with Ni catalyst 8. Hence it is not surprising that the catalytic performances of the ACKNOWLEDGEMENTS F e - N i - S and F e - M o - S materials are different. This work was conducted under US Department of The presence of graphite decreases slightly the fraction Energy Contract No. DE-FC22-90PC90029 under the S/(Fe + Mo) on the surface, relative to the precipitated Cooperative Agreement with the Consortium for Fossil sample. However, the surface Mo ratio is considerably Fuel Liquefaction Science. The authors are grateful to diminished when the catalyst is impregnated on the A. Chadha for the AES analysis and to J. Yang for graphite, again compared with the precipitated sample. performing the AAS. (The value of zero for this entry in Table 2 indicates that the signal for Mo was below the detection limit. Typically, AES can detect species comprising 0.1% of REFERENCES the atoms on the surface. Recall that the bulk Mo loading is 0.1 of 1.67 wt%. Hence a small but detectable 1 Weller, W. Energy Fuels 1994,8, 415. S. signal would be expected if the Mo was present to the 2 Dadyburjor, B., Stewart, W. E., Stiller, A. H., Stinespring, D. C. D., Wann, J. P. and Zondlo, J. W. Energy Fuels 1994,8, 19 same extent on the surface as it was in the bulk.). 3 Stansberry, G., Wann, J. P., Stewart, W. R., Yang, J., Zon- P. In the THF-insoluble fraction, the Fe signal is dlo, J. W., Stiller, A. H. and Dadyburjor, D. B. Fuel 1993,72, negligible, but it reappears after the N M P wash. This 793 indicates the presence of significant carbonaceous 4 Stohl, F.V. andDiegert, K . V . EnergyFuels1994,8,117 material ('coke') on Fe during DCL, but not on Mo. 5 Zhao,J., Feng, Z., Huggins, F. E. and Huffman, G. P. Energy Fuels 1994,8, 38 The S signal is reduced after liquefaction, but to a lesser 6 Cugini,A. V., Krastman, D., Lett, R. G. and Balsone, V. D. extent than the Fe signal, and the S signal also reappears Catalysis Today 1994, 19, 395 after the N M P wash. 7 Liu, Z., Yang, J., Zondlo, J. W., Stiller,A. H. and Dadyburjor, D. B. Fuel 1996, 75, 51 8 Dadyburjor,D. B., Stiller, A. H., Stinespring, C. D., Zondlo, CONCLUSIONS J. W., Wann, J. P., Sharma, R. K., Tian, D., Agarwal, S. and Chadha, A. Am. Chem. Soc. Div. Fuel Chem. Preprints 1994, 1. Mixed-metal sulfide catalysts can be made with the 39, 1088 primary metal iron based on the disproportionation 9 Hager,G. T., Compton, A. L., Givens, E. N. and Derbyshire, of ferric sulfide. F. J. Am. Chem. Soc. Div. Fuel Chem. Preprints 1994,39, 1083 10 Pradhan,V. R., Herrick, D. E., Tierney,J. W. and Wender, I. 2. The catalysts can be precipitated separately and later Energy Fuels 1991,5, 712 physically mixed with the coal, or the catalysts can be 11 McMillen, F., Malhotra, R. and Tse, D. S. Am. Chem. Soc. D. impregnated in situ on the coal. DCL activities of the Div. Fuel Chem. Preprints 1991,36, 498 latter catalysts are higher, while oil yields are smaller. 12 Whitehurst,D. D., Mitchell, T. D. and Farcasiu, M. 'Coal Liquefaction', AcademicPress, New York, 1980 3. Mixed-metal catalysts containing magnesium provide 13 Anderson,R. R. and Bockrath, C. B. Fuel 1984,63, 329 no particular advantage over the iron-alone catalyst. 14 Wilson,M. A., Rottendorf, H., Collins, P. J., Vassallo, M. A. 4. The presence of 0.1-0.15 atom fraction of Mo in iron- and Barron, P. F. Fuel 1982,61,357 based catalysts results in an active catalyst for DCL. 15 Gardner,T. J., McLaughlin,L. I., Lott, S. E. and Oelfke,J. B. Am. Chem. Soc. Div. Fuel Chem. Preprints 1994,39, 1078 The addition of molybdenum is especially beneficial 758 Fuel 1996 Volume 75 Number 6