Energy & Fuels 1996,9,753-759
763
Role of Iron in Dry Coal Hydroconversion Ana M. Mastral,* M. Carmen Mayoral, M. Teresa Izquierdo, and B. Rubio Instituto de Carboquimica, Apdo. 589, 50080-Zaragoza, Spain Received August 3, 1994@
The behavior of two different catalytic precursors based on iron (FeSOc'lHzO and Fez03) in direct hydroconversion of two coals are studied in this paper. Coal itself was the catalytic support of the dispersed iron sulfide (from iron sulfate); when the catalyst precursor was iron oxide (from red mud), coal and catalyst were directly mixed as powders. The reaction conditions were: 10 MPa (H2, cold) initial pressure, 30 min, and reaction temperatures of 300,350,400, and 425 "C, reaching 450 and 500 "C for the high-rank coal. The results from Mossbauer spectroscopy demonstrate that pyrite in all the runs,inherent to coal or added as catalyst precursor, is converted into pyrrhotite to a variable extent according to the previous iron distribution and the iron chemical state in the catalyst precursors as well as the CS2 addition. Important chemical and physical transformations of catalysts are observed by XRD and SEM-EDX during the reaction. The catalytic performance seems to be due to the transformation of pyrite into pyrrhotite, to the H2S homogeneous catalysis and, when red mud was the catalytic precursor, to an acid behavior. The red mud exhibited a behavior which deserved particular study for its dependence on composition and acid characteristics.
Introduction Dissolution catalysts are used t o promote the breakdown of coal structure to liquid products. Vernon1 proposed a mechanism to explain the promotion of bond cleveage in the presence of molecular hydrogen, in which the catalyst does not participate directly in bond cleavage, because it seems to be dependent upon the level of thermal energy input. This could explain why, for a given coal, different catalysts have been found to show evidence of liquefaction activity over the same range of temperature.2 This threshold temperature will depend upon the type and distribution of connecting linkages and is expected to differ from coal to coal and t o show a systematic change with coal rank.3 In order to influence directly reactions which are taking place within the coal particles, which at least at the onset of reaction are solid, dissolution catalysts are required to attain some degree of intimate contact with the coal. Because of that, such catalysts have been added in various forms, as in solvent mixtures or impregnated upon the coal. Normally, these catalysts are used at concentrations up to a few p e r ~ e n t . ~ , ~ Catalyst activity is determined mainly by the degree of dispersion and the composition of the catalyst under a given set of reaction conditions. These factors are related to each other and t o the composition and mode of addition of the catalyst precursor. Although the catalyst dispersion and composition may be altered upon reaction, the nature and extent of these changes will be determined to some degree by the situation prevailing on the onset of reaction. @Abstractpublished in Advance ACS Abstracts, July 15, 1995. (1)Vernon, L. W.Fuel 1980,59, 102-106. (2) Charcosset, H.; Bacaud R.; Besson M.; Jeunet A.; Nickel B.; Oberson M. Fuel Process. Technol. 1986, 12, 189-201. (3) Derbyshire, F. J.; Davis, A.; Rin, R. Energy Fuels 1989,3,431437. (4) Derbyshire, F. J. Catalysis in coal liquefaction; IEA Coal Research London, 1988.
Several studies have demonstrated the advantages of adding the precursor by impregnation over its addition in the form of particulates for liquefaction in the presence of s01vent.~~~ However, the distinction between these modes of addition tends to disappear when very small particles are i n ~ o l v e d . ~ In fundamental research, a great deal of effort has been given to investigate the use of iron compounds, and in particular iron sulfides as liquefaction catalyst. There has been a continuing level of activity in this area, some of which has been concerned with trying to improve the performance of iron catalyst through modifying their chemical and physical Much of the interest on iron, apart from its low cost, stems from the fact that iron pyrite is the most catalytically active constituent (or, more accurately, catalyst precursor) of coal mineral matter. However, since the content and catalytic activity of coal pyrite can vary widely and are not controllable, stable process operation cannot be based upon an indigenous mineral-matter catalyst? The bulk of the available evidence indicates that pyrrhotite is the active form of the iron catalyst under liquefaction conditions. Coal provides a support effect during sulfidation and prevents, to some extent, the agglomeration of catalyst particles.1° The presence of a carbonaceus support to impregnated iron tends to mitigate particle size growth and favors the formation of smaller particle (5) Derbyshire, F. J.; Stansberry, P. G.; Terrer, M. T.; Mastral, A. M. The mobile phase in coals: its nature and modes of release; F'roj. DoE-PC-60811-9,10,1986. (6) Mastral, A. M.; Rubio, B. Liquefaction low-seuerity conditions. The catalyzed depolimerization as a pretreatment for coal liquefaction; Contrato EN 3V-O04E(A),CSIC-CE Comisi6nXI, New Energy Vectors, Final Report, April, 1990. (7) Andres, M.; Charcosset, H.; Chiche, P.; Davignon, L.; DjegaMariadassou, G.; Joly, J. P.; Pregermain, S. Fuel 1983, 62, 69-72. (8) Pregermain, S. Fuel Process. Technol. 1986,12, 155-162. (9) Davidson, R. M. Mineral effects in coal conversion; IEA Coal Research London, 1983. (10) Bacaud, R.; Besson, M.; Djega-Mariadassou,G. Energy Fuels 1994, 8, 3-9.
0887-0624/95/2509-0753$09.00/0 0 1995 American Chemical Society
754 Energy & Fuels, Vol. 9, No. 5, 1995 Table 1. Characterization of Coals ultimate analysis proximate analysis ( w t %, dry basis) (wt %, daf basis) coals C H S 13 67.4 7.6 B 25 79.4 5.3 DECS-17 75.9 5.7
Stoudrvl
7.2 1.2 0.4
So, 5.9 0.7 0.4
SDir ash volmatter fxedC 1.43 13.6 35.4 50.8 39.5 51.7 0.51 8.8 43.3 43.2 0.02 8.5
size iron sulfide catalyst that are likely to have higher surface area.ll Kotanigawa12 in 1987 showed that, in the presence of sulfur and water, iron oxides added as catalyst precursors will react to form FeS04 as well as iron sulfides. The formation of sulfate is interesting since Hattori13reported that ita presence increased the cracking activity of Fez03. This was attributed to the formation of a superacid.14 The bifunctional catalyst FeS~.Fez03(S04)~gives an excellent result for coal liquefaction under the experimental conditions,15J6and the hydroliquefaction activities occur preferentially via reaction with molecular hydrogen rather than through participation of the donor solvent. The sulfate anion treatment promotes the catalytic activity of iron and tin oxides for direct coal liquefaction reactions, principally by causing reduction in their average particle diameter and subsequent increase in the specific surface area available for catalysis thereby enhancing catalyst dispersion. The sulfate group probably inhibits agglomeration of the metal oxide catalyst at high temperatures." The most modern techniques, such as the Mossbauer spectroscopy, SEM-EDX, and XRD, have been used for the study and measurement of the size and distribution of these catalytic species, in the initial form as well as their evolution with reaction.18J9 The performance of the coal hydrogenation catalyst is also observed through the characterization of the obtained solid and liquid products.
Experimental Section Coals. Two coals S13 (Andorra-Arifio, Spain), with ASTM rank SubC, and B25 (Bagworth, UK), with ASTM rank hvCb, were studied. The characteristics of these coals and DECS17 coal (used in the Results and Discussion section for comparation) are summarized in Table 1. Catalysts. Two catalytic precursors were studied: iron(11) sulfate heptahydrate (Merck, with purity higher than 99.5%) and iron(II1) oxide (as red mud composed of 36.5% Fe203, 5.3% CaO, 8.5% Si02, 13.5% Ti02 and 23.8% A l 2 0 3 , courtesy of Deustche Montan Technologie). (11)Cugini, A. V.; Kratsman, D.; Martello, D. V.; Frommell, E. F.; Wells, A. W.; Holder, G. D. Energy Fuels 1994,8, 83-87. (12) Kotanigawa, T.; Yokoyama, S.; Yamamoto, M.; Maekawa, Y. Fuel 1987,66, 1452-1453. (13) Hattori, H.; Yamaguchi, T.; Tanabe, K.; Yokoyama, S.; Umematsu, J.; Sanada, Y. Fuel Process. Technol. 1984,8, 117-122. (14)Yokoyama, S.; Yamamoto, M.; Maekawa, Y.; Kotanigawa, T. Fuel 1989,68, 531-533. (15)Kotanigawa, T.; Yokoyama, S.; Yamamoto, M.; Maekawa, Y. Fuel 1989,68, 618-621. (16)Yokoyama, S.; Yamamoto, M.; Yoshida, R.; Maekawa, Y.; Kotanigawa, T. Fuel 1991,70, 163-168. (17) Pradhan, V. R.; Henik, D. E.; Tierney, J. W.; Wender, I. Energy Fuels 1991,5 , 712-720. (18)Huffman, G. P.; Ganguly, B.; Zhao, J.; Rao, K. R. P. M.; Shah, N.; Feng, Z.; Huggins, F. E.; Mehdi, M.; Lu, F.; Wender, I.; Pradhan, V. R.; Tierney,J. W.; Seehra, M. S.; Ibrahim, M. M.; Shabtai,J.; Eyring, E. M. Energy Fuels 1993,7 , 285-296. (19)Rao, V. U. S. Energy Fuels 1994,8, 44-47.
Mastral et al. Table 2. Mtissbauer Parameters of Hydrogenation Residues Using RM and IS as Catalytic Precursors T iron distribn Ha," xb % net % o/o Featcconversn oils HzSd coal prec ("C) compd of iron (%) S13 RM 400 Fel-,S 74 271 0.101 47.33 74 16 1.4 spmoxe 26 72 268 0.108 47.13 72 19 1.6 S13 IS 400 Fel-,S . .. 28 FeSz 78 270 0.10247.29 70 14 1.6 B25 RM 400 Fel-,S spm Ox 23 87 278 0.083 47.82 70 13 0.2 B25 IS 400 Fel-,S 13 FeSz 80 269 0.10647.18 75 B25 RM 425 Fel-,S 23 3.8 20 spm Ox 93 280 0.079 47.93 79 20 0.3 B25 IS 425 Fel-,S FeSz 7 a Average magnetite hyperfine field (kOe). Atomic percentage % HzS in of Fe in Fel-,S. Number of vacancies of Fe in Fel,S. afier-reaction gas analysis. e Superparamagnetic oxohydroxides.
The iron sulfate was dispersed on the coal surface as an intermediate oxo-thiosalt (IS) prepared by bubbling HzS for 15 min through an aqueous FeSOc7HzO solution in the presence of air. This method of dispersion in which the coal is the catalytic support has been widely used p r e v i o ~ s l y . ~ ~ ~ ~ For the iron(II1) oxide, the red mud (RM) was directly mixed with coal as a powder by hand stirring. The catalyst loading was 5 wt % Fe on daf basis with both precursors. Hydrogenation Procedure. The hydrogenation experiments were carried out in a 160 cm3 shaken tubing bomb reactor in the absence of solvent. The coal load was 10 g (daf basis) in each run. A stoichiometric amount of CS2 was added to the low sulfur content bituminous B25 coal when RM is used as precursor. As fixed conditions, the initial cold Hz pressure was 10 MPa and 30 min of inmersion in a preheated sand bath a t the fixed reaction temperature. The coal was treated at a wide range of temperatures: 300, 350, and 400 "C for blank tests, 425 "C for catalyzed runs, and also 450 and 500 "C for the bituminous coal. Products Workup. The gases were vented from the cold reaction vessels into a gas sampling bag for GC analysis. The reactor content was quantitatively transferred into a Soxhlet extraction thimble for extraction with tetrahydrofurane (THF) for 24 h. The THF was removed from the extract in a rotary evaporator. The THF solubles were fractionated with hexane into oils and asphaltenes. Yields. The obtained yields are calculated as follows:
conversion (%) = 100[(loaded coal weight),, solid residue weightY(1oaded coal weight),,,
basis basis
(1)
The weight of the initially added RM is subtracted from the solid residue weight, regardless of the final species of iron: Fe could be present after reaction as I1 and I11 oxides, and monoand disulfides (Table 21, and the difference in weight between them and its influence in conversion calculations can be considered negligible with respect to the bulk red mud composition., More details of the experimental procedure are described elsewhere.20 Products Characterization. Residues of coal hydrogenation experiments were analyzed using Miissbauer spectroscopy at room temperature (in a constant acceleration spectrometer of standard design with radioactive source of 50-100 mCi of W o in a Pd matrix), scanning electron microscopy (SEMI with energy dispersive X-ray (EDX) in an electron microscope IS1 DS-130 with a S i n i detector and processor 8000-11 Kevex, and X-ray photoelectronic spectroscopy (XPS)in a Perkin-Elmer (20)Mastral, A. M.; Rivera, J.; Prado, J. G.; Rubio, B.; Ferro, M. A.; Izquierdo, M. T.; Mayoral, M. C.; Maldonado, F. J.; Pardos, C. Valorization of coal conversion by swelling measuring; Contract No. 7220/EC/755, ECSC, Final Report, June 1993.
Role of Iron in Dry Coal Hydroconversion PHI-5600 MultiTechnique system. X-ray diffractograms were obtained in a Rigaku Geiflex device.
Results and Discussion Initial Results. This work is part of a broader studyz0 on dry catalytic hydrogenation of a set of 25 coals from diverse mining areas. Seven of them were hydrogenated with iron catalysts, and the data were analyzed as a function of coal rank, hydrogenation temperature, sulfur content in the reactor, and catalyst precursors.21 In this previous work, the extent of conversion and product distribution depended upon the intrinsic characteristics of each coal, mainly in the sulfur and volatile matter content. Nevertheless, the behavior of the group of coals could be divided into two different trends, based upon their rank. The addition of catalyst over the low-rank coals showed a slight effect over the runs performed a t 350 'C. At 400 "C, those coals reached maximun levels of conversion (80%)with both precursors, IS and RM. Although RM achieved higher coal conversion, it was made mainly in terms of gas production. IS precursor, on the other hand, exhibited more hydrogenation activity, in terms of higher production of THF-solubles, and oil fraction (20%) as an indication of the degree of stabilization for the thermally generated radicals. For bituminous coals, the reaction conditions that presented the highest degree of conversion (80%) were 400-425 "C treated with CS2 addition in catalyzed runs. Higher temperatures (450 "C), for both coals, caused the increment in hydrocracking reactions extent, involving an increase in light fraction production (gas and oil) at the expense of asphaltene yield. Higher severity conditions (500 "C) indicated significant retrogressive reactions with a dramatic decrease in soluble products. It is commonly believed that the active iron-bearing catalyst phase in coal liquefaction processes is some form of pyrrhotite (Fel-,S), generated in situ under reaction conditions. The precursor to that active catalyst can be either the dispersed iron sulfide, which decomposes to pyrrhotite, or the iron oxide or oxohydroxide phase in red mud which must undergo transformation to the active phase in the presence of some form of sulfur. The sulfur can be from the intrinsic S of the coal as well as added S, both of them producing H2S in reaction environment. The different catalytic performance showed by the two different precursors suggested a further study of them. In order to establish the physical relationship between catalysts and coal surfaces following dispersion, the interfaces were studied by scanning electron microscopy in a previous work.21 In the present paper this study is broadened by the characterization of catalyst precursors evolution in two different coals residues: subbituminous S13 and bituminous B25. It as well seemed interesting t o perform a deeper study of red mud precursor throughout certain activity tests. Iron Dispersion. The atomic absorption measurements of samples before and after iron sulfide dispersion confirm the quantitative 5 wt % addition of iron. The dispersions of the ,913 raw coal and iron-dispersed coal were characterized by SEM-EDX and XRD techniques in a qualitative way: (21) Mastral, A. M.; Mayoral, M. C.; Palacios, J. M. Energy Fuels 1994,8,94-98.
Energy & Fuels, Vol. 9,No. 5, 1995 755 The Ka line profile of iron shows that pyrite appears usually in this coal as very small particles ( ~ 0 . 5pm) embedded in the coal matrix and very rarely in big nodules, although local analysis with EDX identifies the iron as iron sulfate, at least on the surface layer. After dispersion, the Ka line profile of iron indicates a great increase in iron amount, mainly in a highly dispersed way as small particles, with the sporadic presence of nodules presumably from the original pyrite. The analysis obtained from EDX and Mossbauer spectroscopy of the subbituminous coal sample with iron dispersed differs: while EDX local analysis points out an external composition of iron monosulfide, the Mossbauer spectroscopy gives a bulk composition of disulfide. Both samples were analyzed by XRD and XPS in order t o characterize the particle size and composition of iron. The comparison between XRD diffractograms of samples corresponding to the subbituminous coal before and aRer catalyst dispersion does not present big differences. In a first approach, both of them have more than a 30% of amorphous material and similar mineral matter composition, although the dispersed sample presents peaks of siderotil (iron sulfate) and an increase in pyrite presence. This intensification in pyrite peaks does not involve an iron addition but a sulfidation of the original iron sulfate throughout H2S bubbling during dispersion. The lack of iron increase detection by XRD can be due to a high dispersion degree of the added iron, or even to a low-bulk crystallinity of the iron species. The bituminous coal and the corresponding dispersed sample allowed better characterization with those analysis. The diffractogram of the dispersed sample presents clearly defined pyrite peaks (XRD line broadening gives a mean diameter of 800 A),although the XPS spectra show the iron as iron oxide. The iron addition efficacy has been proved not only by atomic absorption but also by conversion results compared t o blank tests.21 The problems in the characterization and quantification of the dispersion have only arisen with the high oxygen content coal, and it is well-known that the initial surface oxidation of pyrite is very rapid and unavoidable even in carefully handled samples, resulting in the loss of the pyritic iron 2p signal in XPS analysis.22 Application of M6ssbauer Spectroscopy to the Study of Hydrogenation Residues. In RM, the iron oxide particles are small enough t o behave superparamagnetically and give rise to a spectrum that is a superposition of broadened six-peak magnetic hyperfine (66% Fe corresponding to a-FezOa) and quadrupole (doublet 34% Fe corresponding to FeOOH) spectra. So, the addition of RM over coal implies the presence of iron(II1). The residue of the 350 "C hydrogenation of S13 coal and RM as the precursor confirmed that all the hematite has disappeared, with the subsequent formation of pyrrhotite (53% of iron) and the intermediate specie magnetite in a 10% of iron, not completely reduced at these low-severity condition^.^^ In spite of pyrrhotite being the thermodynamically stable iron species at liquefaction conditions, the complete reduction of iron is not reached at 400 "C, although at 425 "C its presence is higher as can be seen in Table 2. The (22) Kelemen, S. R.; Gorbaty, M. L.; George, G. N.; Kwiatek, P. J. Energy Fuels 1991, 5 , 720-723. (23) Wang, L.; Cui, 2.;Liu, S. Fuel 1992, 7 1 , 755-759.
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756 Energy & Fuels, Vol. 9, No. 5, 1995
Table 3. Experimental Runs with Different Precursors extent to which iron was reduced to pyrrhotite was (400 "C, 10 MPa of H2 Cold,30 min) similar at 400 "C for both coals studied with RM as coal precursor 70conversion 70asphaltenes % oils precursor. With IS as precursor, the low-rank coal hydrogenation subbituminous 51.8 21.62 9.7 red mud 78.1 38.85 14.6 residue shows a reduction to pyrrhotite in the same 29.64 9.2 60.3 Fez03 extent (72% iron) than the sulfide produced from RM 53.6 Ti02 22.64 11.8 precursor at the same conditions. The bituminous coal, 54.1 Fez03 + Ti02 22.65 10.9 on the other hand, once hydrogenated at 400 "C with bituminous 58.1 33.89 8.6 55.71 14.1 (CS2 added) red mud 75.7 both precursors, produced two samples which differ in 57.1 36.54 2.5 Fez03 pyrrhotite by lo%, while the magnitude of iron transTi02 56.8 34.61 9.9 formation into pyrrhotite for the residues obtained with 57.8 36.22 9.8 Fez03 + Ti02 RM as precursor is similar t o the low-rank coals. The IS precursor tends to be more easily reduced to pyrrhoThere is a clear dependence of the amount of iron in tite with the high-rank coal. This trend is also evident Fel-,S and the total amount of sulfur present in the at 425 "C (Table 2), a temperature which implies a system e n v i r ~ n m e n t :higher ~~ HzS partial pressure higher degree of iron reduction to pyrrhotite. yields pyrrhotite with more iron deficiency. For the low In this way, apart from temperature, degree of sulfur content bituminous coal, although the IS precurdispersion, and particle size, the extent to which an iron sor shows a higher tendency to be reduced to pyrrhotite, species on the coal surface reduces to pyrrhotite under the number of vacancies is lower, corresponding to the hydrogenation conditions relies as well upon the rank low volume percentage of HzS in the gas formation. It of coal. The low-rank coal, with higher oxygen content, must be noted that with this precursor, no CS2 was develops a surface oxidation of added iron which proadded because the necessary proportion of S was present duced the troubles of detection as mentioned above for with the actual precursor. Moreover, there can be found IS precursor in XPS measurement. The reason for this no clear tendency between the HzSpartial pressure and phenomenon lies in the origin of ferrous sulfate in this percentage of conversion and oils production due to the coal and the mechanism of pyrite decompositi~n.~~ overlaping of heterogeneous (iron precursor) and homoOwing to the weathering of coal, pyrite reacts slowly geneous (HzS pressure) catalysis mechanism. with moisture and forms sulfates on the surface. The Red Mud. Apart from iron oxide, the red mud thickness of this sulfate layer would depend on the composition comprises other transition metal oxides temperature and on the exposure time of the pyrite. The whose activity in coal hydrogenation was tested. Sevsulfate layer separates the iron sulfide from the reaceral experiments were done with different mixes of the tants in the surroundings, and sulfide decomposition main constituents of RM (Table 3). A great interest was would depend on the removal of S2- ions from its put on the role of titania from red mud for catalytic surface. The ferrous sulfate covering the sulfide surface liquefaction of model compounds.27Nevertheless, as can effectively blocks the removal of sulfur ions from that be seen in Table 3, neither titanium nor the titanium surface. Hence, the decomposition of iron sulfide will and hematite mixture develops any catalytic activity not be possible until sulfates decompose and expose the over coal conversion into THF solubles, and as a pyrite to the surrounding atmosphere. This effect seems measure of proper hydrogenation, into oils. This beto result in an additional difficulty in reaching the havior for titanium was expected since in previous pyrrhotite state. The fact that the presence of coal does worksz1 it was pointed out that titanium was not not affect the activation energy of the disulfide into reduced to the sulfide under reaction conditions. pyrrhotite does not necessarily imply that these moieties The special behavior of red mud, apart from its own cannot interact with FeSz and/or its products of decomcontent of iron oxide, can be due to several factors. position.25 Binary metal oxides have been shown to have surface The results in Table 2 show that net conversion does areas and acid strengths greater than the component not present a relationship with increasing Fe1,S formed. ~xides,~ and ' if the active catalytic sites are associated it has been demonstrated that not In previous with or derived from defect structures in the iron oxide/ only the quantity of the produced Fe1-3 but also the oxohydroxide network, the crystallite structure of the crystal pattern and crystal structure of Fel-,S and the precursor powder, in addition t o particulate size and partial pressure of HzS have an influence on the surface area, also plays a siccant role in determining conversion of coal liquefaction. whether an iron-based powder will act as an active Montano and Granoe6 reported that the conversion catalyst .2g of coal liquefaction increased linearly with the number On the other hand, the possible dispersion effect of of vacancies ( X ) in Fel-,S with four different kinds of the solid mass added as precursor must be taken into coal under the same liquefaction conditions. However, account. The mass transport and diffusional phenomena effects were tested by doing blank tests with the WangZ3did not find a clear connection between conversion and X, and it is not clear in the present work also. same mass amount of Sic as that added as RM in On the other hand, and as an evidence of hydrogenation catalyzed runs. The results showed that the addition activity, the percentage of obtained oils is not related of a solid as a dispersant under the reaction conditions studied did not imply an enhancement in conversion or with the Fe1-3 percentage or with the number of vacancies. product distribution, so this possible effect of RM was rejected. (24) Bommannavar, A. S.; Montano, P. A. Fuel 1982,61,523-528. (25)Thomas, G.; Padrick, T. D.; Stohl, F. V.; Stephens, H. P. Fuel 1982, 61, 761-764. (26)Montano, P. A.; Granoff, F. B. Fuel 1980,59, 214-219.
(27)Pratt, K. C.; Christoverson, V. Fuel 1982, 61, 460-462. (28)Matson, D. W.; Linehan, J. C.; Darab, J. G.; Beuhler, M. F. Energy Fuels 1994, 8, 10-18.
Role of Iron in Dry Coal Hydroconversion
I
a
b
c
d
e
Energy & Fuels, Vol. 9, No. 5, 1995 757
1
4 4 I-NaphtylmeUlyl)bibcnzynzyl
Rraction pathway
Roduas obsnved
a
methylbybend, naphtnlenc, teaalin
b
methylnaphIalenc, bikncyl
C
bencylnaphtnlene, ethylbenme
d
naphtyltolylm*h9nc, toluene
e
naphtyltolylethanc,benzene
Figure 1. Possible products from the hydrocracking reactions of 441-naphthylmethy1)bibenzyl. Table 4. Results of Reactions of Iron Precursors with Naphthylbibenzylmethane in the Presence of Sulfur at 400 "C, 1 h expt prec wt%* %conv %selb a % s e l b % s e l d 3.0 43.2 0.0 none 56.8 45.4 1 RMc 10 7.3 49.7 4.9 2 RM 20 8.5 56.5 3.8 39.7 3 WRMd 84.5 10 22.7 6.1 9.4 4 WRM 20 34.1 88.4 5.5 6.1 5 Fez0se 10 35.3 86.5 7.8 5.7 6 mix!' 20 38.2 88.8 7.6 3.6
* Weight percentage of iron precursor with respect to naphthylbibenzylmethane. Selectivity is defined as the amount of organic products from a, b, or c cleavages in the model compound divided by the total amount of organic products. Red mud. Washed red mud. e Hematite (BASF).f 50%hematite, 25%4203, 15% TiO2, 10% Si02.
Naphthylbibenzylmethane (4-(l-naphthylmethyl)bibenzyl) (Figure 1)has been used as a model compound for studying catalytic activity toward bond scission reactions relevant to coal liquefaction. The model compound catalytic test procedure was outlined by Farcasiu and Smith.29130It could be attractive for this application because of the possibility of various types of C-C bond cleavage (denoted a-d, Figure 1)in the compound and because of the presence of both monocyclic and bicyclic aromatic units. This compound is supposed to be able to react by radical, ion-radical, or ionic mechanism and in this way it could be possible to distinguish among the various mechanism routes. This method has been used for preliminary evaluation of pure and heterogeneous iron-bearing powders as precursors for coal liquefaction c a t a l y s t ~ . ~ ~ - ~ l Results of model compound test runs designed t o evaluate the relative activities of the various ironbearing precursor powders are presented in Table 4. At the reaction conditions used for this investigation (1h at 400 "C in 9,10-dihydrophenantrene, 10% elemental sulfur), and with no catalyst precursor added, almost none of the model compound (3%) was converted into product species. The activity of different nonporous catalysts is influenced by the chemical nature of their active sites and by the actual number of active sites available to the substrate. In order to eliminate the (29) Farcasiu, M.;Smith, C. Energy Fuels 1991,5 , 83-87. (30) Farcasiu, M.;Smith, C.; Pradhan, V.; Wender, I. Fuel Proc. Tech. 1991,29,199-208. (31)Tang, Y.;Curtis, C. W. Energy Fuels 1994,8 , 63-70.
influence of the second parameter and concentrate the research on the chemical nature of the active sites in differentiron compounds, a large excess of catalyst was used. Analysis of the products from the test runs in which elemental sulfur and 10% and 20% RM (experiments 1 and 2, Table 4) indicates that the powder slightly improved selectivity of the bond b cleavage relative to control runs in which no iron-bearing species was present. The expected acid behavior of RM could be screened by the presence of basic cations in its chemical composition, such as calcium or sodium. M e r exhaustive washing with distilled water the chemically adsorbed sodium content can be reduced, as can be seen through XPS analysis of both samples. There is a dramatic difference between the nature of catalytic sites if the reactions are performed in the presence of 20% washed red mud (experiment 4, Table 4). The model compound consumption is significantly higher even when the load of precursor was only 10%. The selectivity of bond b scission, similar for both loads, denotes a possible ionic (acid) mechanism, similar to the behavior showed by strong acids (trific acid or acidic zeolite).30 The experiments performed with pure hematite in 10 w t % load (experiment 5 ) and 20%load of a mixture of 50% hematite, 25%Al203,15% TiO2, and 10% Si02, close to RM composition, involved the same high selectivity and model compound consumption as that obtained with 20% WRM. In previous works, a simple dissolutiodprecipitation process was used t o activate the red mud.27,32The explanation offered for the improved activity of the red mud in the hydrogenation of naphthalene was the increased surface area of the activated sample: from 64 m2*g-lof original RM to 155 m2-g-l of activated red mud. Na is a well-known syntering agent for A l 2 0 3 , and those authors considered that its removal could have taken part in that effect. In contrast to these reports, other works33showed that the magnitude of surface area is not a crucial factor in determining the efficiency of Fez03 catalysts, as has been observed as particle size decrease^.^ In the present work, the effect that the sodium removal (confirmedby ESCA) seems to confer to RM is not to avoid the syntering of the solid, because an increase in precursor load (as done in experiment 2 compared to 1)would have involved significant enhancement in selectivity. Since the washed RM of this work only implied a surface area increase from 35 m2*g-lof original RM t o 41 mz*g-l,the effect of surface area that Pratt27pointed out is not as significant for the red mud activation as the Na removal. Hydrogenation Runs with Washed Red Mud (WRM).The obtained results with WRM in the activity test suggested the performance of new hydrogenation experiments with coal. None of the previouly studied coals21were used, because all of them had high levels of sulfur, pyritic as well as organic, that could interfere in catalyst activity investigations. The selected coal was (32) Llano, J. J.;Rosal, R.; Sastre, H.; Diez, F. V. Fuel 1994,73, 688-694.
(33)Kamiya, Y.;Nobusawa,T.; Futamura, S. Fuel Proc. Tech. 1988,
18,1-10,
758 Energy & Fuels, Vol. 9, No. 5, 1995
Mastral et al. 9'
e
2
B
"
7
"
6
51
30 Arm = 525 0 178
176
1
174
172
170 BlMq
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8
12
10
16
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% wt precursor
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Figure 2. Net conversion and product distribution of the hydrogenation of DECS-17coal with red mud and washed red mud as catalyst precursor. DECS-17, a bituminous one from the Penn State Coal bank, often used in this kind of research34because of its composition (Table 1). The experimental runs were carried out as described before, and the RM and WRM addition ranged from 0 t o 15 wt % (Figure 2). The conversion and product distribution enhancement when 9 and 15% of WRM was added compared to RM precursor results is slightly enough to be under experimental error, but when the percentage is decreased to 5% (that implies 1.2% Fe over daf coal), the increase in conversion is high enough t o consider the red mud washing as an improvement in catalyst activity. Those results, in which the main subject is the Na elimination from RM, point out the importance of the chemical state of the catalytic precursor a t the first stages of reaction: the H2S generation under reaction enviroment when the RM has been washed is rapidly involved in Fez03 sulfidation instead of in the neutralization of basic sites. When the RM is not washed, both reactions can compete delaying the catalyst ability to stabilize by hydrogenation the fast thermally generated radicals. The behavior of WRM as a function of percentage addition with DECS-17 coal provides mechanistic information about catalytic phenomena; from a process point of view, it seemed to be interesting to contrast both RM and WRM in experimental runs with the loading previouly used and over the B25 bituminous coal. This addition of 5% Fe over coal daf implies around 17 wt % RM over coal weight. So, it is evident that under these conditions, the catalyst precursor RM has been added in an excess enough to develop an activity similar to WRM. Similar results were obtained using the activation method described by Pratt:35 Activated red mud was shown to be significantly superior to red mud as a catalyst for the hydrogenation of polynuclear aromatics. However, for liquefaction of both high- and low-rank coals in poor and good hydrogen donor solvents, activated red mud showed little improvement over red mud. Sulfated Oxides. In the literature it is possible to find works in which the sulfated oxides described in the Introduction are prepared prior t o reaction as well as generated in situ under reaction conditions. Kotani~~
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(34) Stansberry, P. G.;Wann, W. P.; Stewart, W. R.;Yang, J.;Zondlo, A. H.; Stiller, A. H.; Dadybujor, D. B. Fuel 1993, 72, 793-769. (35)Eamsiri, A.; Jackson, W. R.; Pratt, K. C.; Christov,V.; Marshall, M. Fuel 1992, 71, 449-453.
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Figure 3. SzPXPS spectrum of red mud treated at 400 "C,30 min, 100 kg.cm-2 HZinitial pressure, with HzO. gawa12 purposed that the behavior of the iron-sulfur system in the presence of water and hydrogen under coal liquefaction conditiones develops the Fe203*S042formation. In that work the initial iron compounds were heated to 450 "C at 2.5 "C min-l. Identification and distribution of the sulfur in the reaction system can be made using infrared spectroscopy, XPS, and thermal analysis measurements. With the same method, later works14identified the sulfate formation on the red mud surface. In a first approach, the reaction conditions performed in this paper suggest that the same phenomenon could have happened in the system in which red mud is the catalytic precursor. Several solid residues obtained by S13 coal hydroconversion in a previous workz1 make evident the high degree of iron syntering along with heating treatment. That implies that in the reaction system there is not any compound able t o prevent iron from syntering, one of the roles presented by sulfated oxides in the bib1iography.l' This type of catalyst system based on anio-modified iron oxides has an initial fine size and unique ability to mantain this fine size at higher temperatures without significant syntering or agglomeration, making them highly dispersed active catalysts for coal liquefaction. Iron, added to the coal liquefaction reactor in the form of these sulfated oxides, is completely converted to highly dipersed pyrrhotite within a few minutes at 400 0C.36 Nevertheless, other instrumental techniques were performed to study the formation of the sulfate over the red mud used in this work: infrared spectroscopy and
XPS. The sample t o test was the solid obtained in the red mud treatment with HzO and elemental sulfur in a reactor with 10 MPa of initial hydrogen pressure, fastheated to 400 "C, and the temperature kept for 30 min. The S(2p) spectra obtained from that sample, shown in Figure 3, confers to the sample a surface sulfur composition of both sulfide and sulfate, which indicates that sulfides can be easily oxidized. By cross-section ratios (02-/Fe2+,S6+/Fe2+, and S2-/Fe2+)to the intensity data form O(ls), Fe(2p312)and S(2p) binding energies, Kotanigawa15 estimated the surface composition of the catalyst studied. This calculation, even with the limitation of the analytical method, cannot be used over the samples obt,,ned with red mud, due to the nonstoichiometric initial presence of oxygen as oxide from its composition. Nevertheless, the technique is useful in this case because, apart from detecting the presence of (36) Pradhan, V. R.; Hu, J.;Tierney, J. W.; Wender, I. Energy Fuels
1993, 7, 446-454.
Role of Iron in Dry Coal Hydroconversion sulfate, it also shows that the iron sulfide is more likely to be monosulfide than disulfide: the binding energy of the FeSz should be higher than 161.7 eV, obtained for this sample. This result confirms the iron assignment to pyrrhotite suggested by the Mossbauer spectroscopy and EDX to the solid residues obtained in this work.21 Kayo3? pointed out that Fe203*S042-,regarded as the superacid, has an asymmetric stretching band of the S-0 at 1375 cm-l in infrared spectroscopy, although FeS04 has a broad band in the region of 1235-1100 cm-l. Infrared study reveals that sulfate ion interacts with iron oxide by a different state form of iron sulfate, shifting its characteristic bands. In this way, infrared spectroscopy has been used to identify the sulfate oxide formation.15 The sample tested by XPS in this work again presents problems in its characterization by IR: there is an absence of any peak at the expected wavenumber because of the complexity of the sample.
Conclusions Iron dispersed onto a coal surface as iron disulfide shows a different state and behavior depending on the (37) Kayo, A.; Yamaguchi, Y.;Tanabe, K.J. Cutal. 1983,83,99106.
Energy & Fuels,Vol. 9,No. 5, 1995 759 type of coal: over the subbituminous coal, the detection of added iron by XPS and XRD appears to be masked by the rapid oxidation by the heteroatoms present in coal composition. On the other hand, apart from the better characterization, the iron dispersed on the highrank coal presents a greater disposition to be reduced to pyrrhotite under hydrogenation conditions. The extent of reduction to pyrrhotite and the number of its vacancies do not present a direct relationship to the conversion degree achieved in dry hydrogenation or t o the selectivity to oil production for both catalytic precursors, iron sulfide and iron oxide from red mud. Although the results obtained with red mud characterization indicate a possible acid behavior, which comprises free radical as well as cracking mechanisms, the iron oxide does not seem to develop the sulfate ion chemisorption that would confer to the solid the superacid characteristics.
Acknowledgment. The authors thank Dr.M. Farcasiu and Dr. F. Huggins for their scientific contribution and the European Community (Project 7220/EC/755) and the Spanish CICYT (Project PB-413)for the financial support of this research. EF940157A
