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Energy & Fuels 1991,5, 712-720
Finely Dispersed Iron, Iron-Molybdenum, and Sulfated Iron Oxides as Catalysts for Coprocessing Reactions Vivek R. Pradhan, David E. Herrick,+ John W. Tierney, and Irving Wender* Chemical and Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received April 10,1991. Revised Manuscript Received June 24, 1991
We have recently reported on the catalytic activity of sulfated iron and tin oxides for the direct liquefaction of coal (Energy Fuels 1991, 5 , 497-507) and on the activity of the soluble precursor, Fe(CO)5,for coprocessing of Illinois No. 6 coal with Maya ATB residuum (650 O F + ) (Energy Fuels 1990,4,231-237). This paper addresses the activity and characterization of finely dispersed ironand molybdenum-containing catalysts based on the soluble precursors Fe(C0)5and Mo(CO)~,and on the finely divided (average crystallite size of 30-80 A) sulfated metal oxide superacids such as FezO3/S0?- and SnOz/S04" for coprocessing reactions. The catalysts were characterized and tested for activity with various coals and Maya ATB heavy oil (650 O F ) in coprocessing reactions. The following catalyst-coal combinations are reported: Fe(C0)5 with three premium Argonne coals; Mo(CO)~and Mo naphthenate with Illinois No. 6; mixtures of Fe(C0)5and Mo(CO)~with Illinois No. 6; sulfated iron and tin oxides with Illinois No. 6; and a new catalyst, Mo-promoted sulfated iron oxide, with Illinois No. 6 coal. The use of a newly synthesized bimetallic catalyst, M O / F ~ ~ O ~ / S O ~ ~ - , consisting of 50 ppm Mo and 3500 ppm iron, gave a 78% conversion of Illinois No. 6 coal to methylene chloride soluble products with a selectivity to oils of 80 wt % at 400 "C. The following order of catalyst activity (the yield of n-pentane-soluble products is referred to here as "activity") was observed for coprocessing reactions carried out with Illinois No. 6 coal and Maya ATB oil at 400 OC: Mo/ Fez03/S042-> Fez03/S042-,Fe(C0)5/Mo(CO)6> Mo(CO)~> Fe(C0)5. The addition of elemental sulfur to the coal-oil mixture prior to the coprocessing reactions did not show any notable effect on conversions. Both hydrodenitrogenation (-40%) and hydrodesulfurization ( - 6 0 % ) were obtained with iron-molybdenum bimetallic catalysts based on sulfated oxides. We believe that the sulfate group in these catalysts helps to prevent sintering or agglomeration of catalysts at high temperatures. The high surface acidity of the catalyst may influence the nature of the reactions that occur in the early stages of coprocessing reactions but the catalyst activity is mainly due to the easy accessibility of the dissolved coal, heavy oil, and Hzto the small catalyst particles.
Introduction In coal-oil coprocessing reactions, as in direct coal liquefaction, supported metal catalysts can deactivate rapidly and suffer from diffusional limitations. It is well recognized that unsupported, well-dispersed catalysts derived from finely divided solids or soluble precursors can offer efficient contact of coal solvent slurries with the surface of catalyst^.^ recently published an investigation of the Herrick et activity of a coprocessing catalyst produced from a soluble iron pentacarbonyl precursor using Illinois No. 6 coal and Maya (atmospheric tower bottoms) residuum as the substrate. Small amounts of Fe (0.5 w t % based on the coal-oil mixture), added as Fe(C0)5,increased coal conversion to methylene chloride solubles from 39% (no added catalyst) to 82%. The iron in the precursor was converted to pyrrhotite, Fel-& The pyrrhotite particles formed in the initial stages of reaction had a mean crystallite size of about 12 nm as measured by a number of techniques such as scanning and transmission electron microscopy, X-ray diffraction, and Mossbauer spectroscopy. We have recently studied the activity of sulfated iron and tin oxides in direct coal liquefaction'. The superiority of these catalysts over those derived from the soluble organometallic precursors led us to investigate the activity
* To whom correspondence should be addressed.
Present addreas: E. I. duPont deNemom and Co., P.O. Box 200, Laplace, LA 70068.
of sulfated metal oxides in coal-oil coprocessing reactions. It is of interest that some early coprocessing studies' used a proprietary iron-containing catalyst to promote coal-oil coprocessing reactions. These reports and our prior work in coprocessirqf stimulated us to employ the sulfated metal oxides of iron and tin as catalysts for coprocessing reactions. We have also used an entirely new catalyst based on a superacid promoted by small amounts of molybdenum, M O / F ~ ~ O ~ / S O ~ ~ - . This paper extends our coprocessing studies with the same 650 O F + residuum to (i) the use of an Fe(CO)5precursor for coprocessing of three coals of different ranks, (ii) the activity of small amounts of Mo(CO)~and combined Fe(C0)5-Mo(CO)6catalysts for coprocessing, (iii) the activity and properties of sulfated oxides of iron and tin, which have been found to be excellent catalysts for the direct liquefaction of coal,' mainly because of their small particle size and surface acidic character, and (iv) the activity of sulfated iron oxide promoted with small amounts (20-100 ppm) of molybdenum. (1) Pradhan.V. R.;Huffman, G. P.;Tierney,J. W.;Wender, I. Energy Fuels 1991, 5, 497-507. (2) Herrick, D. E.; Tierney, J. W.; Wender, I.; Huffman, G. P.; Huggins, F. E. Energy Fuels 1990,4, 231-237. (3) Derbyshire, F. J. 'Catalysis in Coal Liquefaction: New Directions for Research": IEA CR-08 IEA Coal Research. London, U.K., 1988. (4) Fouda, S. A.; Kelly, J. F. Presented at thehtemational Conference
on Coal Science, Maastricht, The Netherlands, 1987. (5) Herrick, D. E. Ph.D. Thesis, University of Pittsburgh, 1990.
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Energy & Fuels, Vol. 5, No. 5, 1991 713
Catalysts for Coprocessing Reactions
The use of soluble precursors containing iron in coal liquefaction has been studied extensively. Examples of such precursors are ferrocene, ferric acetylacetonate? and carbonyl complexes of iron such as Fe(CO)+' These precursors generally produce catalysts with high activity for coal conversion under donor solvent liquefaction conditions, especially in the presence of added sulfur; this catalytic activity at relatively low concentrations is attributed mainly to their high degree of dispersion. Pyrrhotites are the principal species formed from the precursor when sulfur is present in the reaction mixture.8 Garg and GivensBimpregnated coal with 1% Fe and 200 ppm Mo from ferrous sulfate and ammonium molybdate, respectively. They found that, although iron was active for direct coal conversion, addition of small amounta of Mo increased conversion of asphaltenes to oils, as well as maintained the quality of the solvent. Suzuki and otherdo employed a combination of Fe(C0l6 and Mo(CO)~(1%Fe, lo00 ppm Mo, based on coal) in the liquefaction of a high-volatile bituminous coal and obtained large increases in conversion and oil yields compared to the case where no Mo was added. They attributed this large increase to the ability of the Mo catalyst to hydrogenate aromatic compounds. Yamada and others" also used the Fe(CO)6-M0(CO)6system in combination with sulfur to convert the metals to their sulfided form. They found that, without sulfur present, the activity of the catalyst was much less (72% conversion vs 95%). Previous work in this laboratory2 using Fe(CO)c, as a catalyst precursor concentrated on determination of the catalyst dispersion and composition at various stages of the coprocessing reaction. A goal of the present work was to further investigate the use of Fe(CO)5in coprocessing in two ways. First, Fe(CO)6 was added to coprocessing reactions with two additional Argonne coals to determine the effectiveness of this catalyst with other ranks of coal. Second, the effect of adding very small amounts (300m2/g) as shown in Table 111. The high catalytic activity of the Sn02/Fez03/S042-catalyst correlates well with the increase in the sensitivity of Fe203 doped with SnOz as a combustible gas 9ens0r.l~Both of these properties of the sulfated iron oxides are functions of their fine particle sizes and high specific surface areas. It is likely that, with increase in the specific surface area and decrease in the average particle size of the oxides upon addition of small amounts of the sulfate group, conversion of the oxides to active catalytic sulfide phases, especially nonstoichiometric sulfides of iron, is facilitated.18 More
10
,
20
30
40
50
60
70
80
90
I
ppm M o (+3500 ppm Fe)
Figure 6. Activity of Mo/Fe203/S0:- catalysta for coproceeeing at 400 and 425 "C, 10oO psig of cold H2, 1h.
of the active catalyst surface of these sulfides becomes available for reaction. There is also a likely contribution from the ability of the sulfate group in preventing sintering or agglomeration of these particles at coprocessingreaction temperatures. (v) Activity of M O / F ~ ~ O ~ Catalysts / S O ~ ~ at 400 and 425 "C. The sulfated iron oxides (corresponding to about 20-100 ppm of Mo concentration with respect to the coal-oil mixture) resulted in substantial improvement in the selectivity to lighter products (oils). A series of experiments were carried out to determine the coprocessing conversion of Illinois No. 6 with Maya ATB at two different reaction temperatures (400and 425 "C) and for different amounts of molybdenum loadings on Fe203/ SO:-. The results of experiments carried out at 400 and 425 "C are shown in Figure 6. Total coal conversions, as well as selectivities to oils, increased for both temperatures with increase in the amount of Mo in the sulfated iron oxide catalyst (20-100 ppm Mo with respect to the coal-oil mixture). Conversions as high as 84 wt % with selectivity values to oils of over 80 wt 90were obtained at about 100 ppm Mo + 3500 ppm Fe added in the form of M O / F ~ ~ O ~ / S O Surprisingly, ~~-. we obtained slightly lower values for both coal conversion and selectivity to oils in almost all reactions carried out at 425 "C as compared to those at 400 "C. This apparent decrease in coprocessing conversion may be attributed to increased coking reactions occurring because of the presence of an acidic functionality in the catalyst, which could be active in the early stages of the reaction. The coprocessing residues obtained at 425 "C had a rocklike appearance in contrast to those at 400 "C, which were powdery. All of the above reactions were carried out without adding an external source of sulfur since Illinois No. 6 coal and the heavy oil (Maya ATE!) together have enough sulfur (18)Montano, P.A.; Bommannnvar, A. 5.;Shah,V.&el 1981,60,703.
718 Energy & Fuels, Vol. 5, No. 5, 1991 Table VI. Hydrocracking of Maya ATB Resid Alone with Different Catalysts Before Hydrocracking Original Maya resid % asphaltenes = 20 % oils' = 80
After Catalvtic Hvdrocrackinn % yields temp, catalvst emdoved OC coke nases asuhaltenes oils Fe(CO)&+ MO(CO)~ 400 5 10 8 77 Mo/Fe203/SOf 400 3 11 4 82 Mo/Fe209/SOf 425 2 09 8 81 Oils are the part of products soluble in n-pentane.
as pyrite and organic sulfur compounds to sulfide the added metals during reaction. Nevertheless, a run was made with addition of elemental sulfur (10% in excess of that required for complete sulfidation of both the Fe and Mo in the catalyst) to the coal-oil mixture prior to reaction at 400 "C with 3500 ppm of iron and 100 ppm of Mo. No effect of the added sulfur was observed on either overall coal conversion (86 wt %) or the selectivity to lighter oils (82 wt %). To obtain information on the extent of coking and asphaltene formation during coprocessing reactions that is contributed solely by the heavy oil (Maya ATB), reactions were carried out with heavy oil in the absence of any coal. The results of these resid-only reactions are presented in Table VI. The starting Maya ATB contains about 20 wt % of asphaltic material (n-pentane insolubles). When it was treated with the Mo/Fe203/S04" catalyst (50 ppm Mo and 3500 ppm Fe) at 400 "C for 60 min, the asphaltic fraction was reduced to 5 wt % . A similar reaction employing iron and molybdenum carbonyl precursors (together) at the same metal loadings with respect to heavy oil resulted in a product consisting of about 12 wt % of asphaltic material. A run carried out with Maya ATB resid alone at 425 "C resulted in almost the same amount of CH2C12insolubles (coke) as that carried out at 400 "C. It seems, therefore, that when coprocessed with Illinois No. 6 coal the resid behaves differently from what it does when processed alone. Decrease in conversion levels which we obtained with increase in temperature from 400 to 425 OC can therefore be attributed to coking effects of high-rank coals in the presence of heavy oils. Similar effects have also been reported by Fouda et al.19 Product Characterization. (i) Elemental Analyses of Methylene Chloride-Soluble Products. The CH2C12 solubles of the coprocessing product from using the Fe-Mo carbonyl precursor were analyzed for C, H, N, and S contents. As shown in Table VII, improvement in the H/C atomic ratios is obtained for the soluble products by using either the sulfated iron oxides containing small amounts of Mo or iron-molybdenum carbonyl precursors as catalysts. At the same time, about 40%+ of HDN and 60%+ of HDS were obtained. The catalyst to coal nitrogen ratios were very small (Fe/N or Mo/N < 0.2). The small values of these ratios indicate that nitrogen removal from coal is not solely based on an adsorption (on the acid sites of the catalyst) mechanism; some other mechanism must also be operative under reaction conditions. The reasons for this heteroatom removal obtained with the sulfated metal oxides need further investigation. We plan to use model compounds to elucidate the nature of these reactions. There is about a 25% increase in the H/C ratio over the feed H/C when the Fe(CO)~/MO(CO)6(0.5% Fe/500 ppm (19) Fouda, S. A.; Kelly, J. F. Paper presented at the Direct Coal Liquefaction Contractor's Review Meeting (DOE) Pittsburgh, 1987.
Pradhan et al. Table VII. Elemental Analysis of CH2C12Solubles Obtained from Coprocessing of Illinois No. 6 with Maya ATB and Different Catalvsts' Before Coprocessing % N = 0.70 % S = 4.8 H/C (atomic) = 1.20
After CoDrocessine catalvst emdoved - Mo/Fe203/S0, Fe(C0I6 + Mo(CO),
Fe + Mo, Dum -. 3500 + 50 5000 + 500
% N
%S
0.55 0.43
2.01 2.94
H/C
(atomic) 1.47 1.57
'All runs were made with Illinois No. 6 coal at 425 OC, lo00 psig of H2 for 60 min and with a 20 wt % coal180 wt % Maya resid mixture.
Mo) catalyst is used. This, in addition to the higher oil yield obtained by using the carbonyl precursor, demonstrates the ability of the M~(Co)~-derived catalyst to upgrade the coprocessing product. (ii) X-ray Diffraction and Electron Microscopy of Residues. X-ray diffraction (XRD) was performed on the CH2C12-insolublefraction of coprocessing runs carried out with Illinois No. 6 coal with added 0.5 wt % Fe (from Fe(CO),) and 500 ppm Mo (from Mo naphthenate and also from Mo(CO),). The XRD results showed that pyrrhotites were formed from the Fe(CO),, but no Mo-containing compound was detected, probably due to its very high dispersion coupled with its low concentration. There was no evidence that the Mo compounds interfere with formation of pyrrhotites from the iron precursor. Transmission electron microscopy (TEM) was employed to determine the size range of the Fe- and Mo-containing particles produced from the Fe(C0)5and MO(CO)~ precursors. To eliminate interference of the iron and other mineral matter in the coal, a model system was used which consisted of activated carbon with Mo or Fe-Mo deposited on it. This mixture was produced by heating activated carbon (with a very low iron content), toluene (solvent), and Mo(CO)~with or without Fe(CO), in an autoclave to decompose the precursor into small particles, some of which would end up on the carbon support. Similarly, for electron microscope studies, a M o / F ~ ~ O ~ / Scatalyst O~~was deposited on an activated carbon matrix using tetralin as the dispersion medium. The TEM mode image and the EDX spectrum of the catalyst thus produced are shown in Figures 7 and 8. Figure 7 essentially shows that the catalyst particles on active carbon have sizes ranging from 10 to 30 nm. The EDX spectrum in Figure 8 shows the presence of small amounts of Mo and sulfur in the catalyst. We have also carried out some catalyst sintering and transformation studies using the above model system in the presence of an excess of sulfur. Our measurements of the particle sizes of the iron-sulfur inorganic phases (generated at various stages of reaction) by XRD and STEM indicate that iron, introduced either as Fe(CO)6or as Fe203/S042-catalyst, forms Fe& particles having an average size of about 20 nm. The TEM/STEM images of the Mo on activated carbon revealed that the Mo-containing particles were very small, on the order of 50 nm or less. The larger carbon support decorated with smaller Mo catalyst particles is shown in Figure 9. Energy-dispersiveX-ray analysis coflirmed that the smaller, dark particles contained Mo; the more diffuse carbon matrix appears light in color since carbon does not diffract electrons strongly. Although the use of a few micrographs is not a rigorous method of measuring particle sizes, it does indicate that the catalyst produced from the Mo(CO)~precursor is highly dispersed, so that a small amount of catalyst pro-
Energy & Fuels, Vol. 5, No. 5, 1991 719
Catalysts for Coprocessing Reactions I
I I
Figure 9. Transmission electron micrograph of Mo deposited .on activated carbon by thermal decomposition of Mo(CO)& Figure 7. Transmissionelectron micrograph of F e M o deposited on activated carbon by thermal decomposition of Mo/Fe203/S04Z.
I
-
-
.. . ..
Figure 8. EDX spectrum of F e M o containing particle deposited on activated carbon by thermal decomposition of Mo/F%O,/SO,%.
vides a large amount of surface area. The result is a large increase in conversion and oil yield with 500 ppm Mo added as MO(CO)~. Particle sizes of catalyst samples containing both iron and molybdenum appeared similar to those with molybdenum alone. In the F e M o sample (Figure 10) a ratio of 10Fe:lMo was used as a catalyst precursor. Both metals were detected by energy-dispersive X-ray analysis (EDX), but the metals appeared to be intimately mixed rather than present in separate particles. It is possible that the two metals form separate particles, but the resolution of
I
Figure 10. Transmission electron micrograph of FeMo deposited on activated carbon by thermal decomposition of Fe(CO)5-Mo(CO),.
the X-ray analysis is not sufficient to distinguish between the small particles of different composition very close to each other. Conclusions The following conclusions can be drawn from the experimental investigation reported in this paper:
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Energy & Fuels 1991,5,720-723
(1)In catalytic coal-oil coprocessing, three Argonne coals of different ranks resulted in much higher conversions in the presence of an Fe(C0I6 catalyst precursor (at 0.5 wt 9c Fe) than in its absence (thermal background). The high conversions and oil yields were due chiefly to the departure from bulk properties, especially in surface energetics, as the particle size of the Fe catalyst produced in situ from Fe(CO), was reduced to less than about 12 nm. (2) The effect of the Mo catalyst, added as Mo(CO)~, plus an Fe(CO), precursor, was to increase conversions while also increasing the hydrogen content of the liquid product. This is in contrast to the Fe catalyst, which was active for conversion, but not as active as Mo for hydrogenation. (3) The soluble catalytic precursors of Fe and Mo, added together, are converted to fine grain sizes (20-50 nm) of sulfided phases such as Fel-,$ and probably MoS2, respectively, under reaction conditions. (4) The sulfated oxides of iron and tin were active catalysts for coprocessing reactions both at 400 and 425 "C. These catalysts gave greater conversions and oil yields at 400 "C than a t 425 "C, probably due to coking reactions of the heavy petroleum-derived oils coprocessed with bituminous coals at the higher temperature. ( 5 ) Incorporation of small amounts of molybdenum (2G100 ppm with respect to the reaction mixture) on the sulfated iron oxides enhanced total coal conversions as well as the oil yields in coprocessing. It is likely that the hydrogenation function of Mo helps improve product quality over that obtained with sulfated iron oxides in the absence of Mo. The amount of molybdenum used in our catalytic systems was limited to 100 ppm on the higher side keeping in mind its relative cost as compared to iron and its economically viable use as a disposable catalyst.
(6) The sulfated iron oxides (with or without Mo) were found to be more active for conversion of coal (in coprocessing) to lighter products at 400 "C than the soluble precursors of the same metals (Fe(C0I6 and Mo(CO),) added at the same metal loadings. (7) The catalytic activity of the sulfated oxides is the result of several factors: the fine grain size is undoubtedly of the greatest importance but it is possible that the superacidity of the starting catalysts may play a part during the initial phases of the reaction. In the case of Mo/ Fe2O3/S0t-,the hydrogenation ability of Mo comes into play. (8) There is still much to be learned about the mechanisms involved in these types of reactions. With these superacidic sulfated metal oxides as catalysts, the chemistry of the reactions in coal liquefaction could be different from what is known so far (such as possible involvement of radical ion intermediatesm). In any case, these finely divided solid catalyst precursors show great promise for application in hydroprocessing reactions.
Acknowledgment. We gratefully acknowledge the contributions of the Argonne Coal Sample Bank for providing the coal samples, C. van Ormer and J. R. Blachere for the transmission electron microscopy, the donation of Maya crude by CITGO, and funding support from the U.S. Department of Energy under grant No. DE-FC2288PC8806. Registry No. Fe(C0)5, 13463-40-6; MO(CO)~,13939-06-5; Fe203,1309-37-1;Sn02, 18282-10-5;Mo, 7439-98-7. (20) Farcasiu,M.; Smith, C.; Wender, I.; Pradhan, V. R. Submittedfor publication in Energy Fuels.
Surface Composition of Iron and Inorganic Sulfur Forms in Argonne Premium Coals by X-ray Photoelectron Spectroscopy S. R. Kelemen,* M. L. Gorbaty, G. N. George, and P. J. Kwiatek Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received April 15, 1991. Revised Manuscript Received June 19, 1991
X-ray photoelectron spectroscopy (XPS) has been used in the study of the surface composition of iron and inorganic sulfur forms in coals from the Argonne Premium Sample Program. The concentration of iron at the coal surface can be very different than the bulk average. XPS and elemental analysis of selected particle size distributions show that the differencea at the surface cannot be explained solely on the basis of particle size effects. In every case the XPS iron 2p and sulfur 2p results indicate the presence of iron oxides or oxyhydroxides. No evidence was found for the presence of pyritic iron at the surface of any sample other than Illinois No. 6 coal. The results indicate that the initial surface oxidation of pyrite is very rapid and unavoidable even in carefully handled samples. Sulfate sulfur was identified as a surface oxidation product after several days' exposure to ambient air.
Introduction X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES)spectroscopy are two techniques which have been applied recently for the 0887-0624/91/2505-0720$02.50/0
direct speciation and approximate quantification of organicdy bound forms in carbonaceous materials.1-8 XPS (1) George, G. N.; Gorbaty, M. L. J . Am. Chem. SOC. 1989,111,3182.
0 1991 American Chemical Society
