Energy & Fuels 1994,8, 94-98
94
Iron Catalytic Precursors in Dry Coal Hydroconversion A. M. Mastral" and M. C. Mayoral Znstituto de Carboqutmica, CSZC, Apdo 985, 50080-Zaragoza, Spain
J. M. Palacios Znstituto de la Catalisis, CSIC, Canto Blanco, Apdo 28049-Madrid, Spain Received July 12, 1993. Revised Manuscript Received November 7, 1 9 9 9
The catalytic activities of three catalytic precursors based on iron (FeS0~7H20,Fe203, and Fe(C0)B)in direct hydroconversion of seven coals covering a broad range of characteristics are reported in this paper. Coal itself was the catalytic support of the iron species dispersed when iron sulfate (IS) was the precursor. When the catalyst precursor was iron oxide (from red mud, RM), coal and catalyst were directly mixed as powders, and for the iron pentacarbonyl (IP) as precursor, this was directly embedded on coal in inert atmosphere. With five of the seven coals, which were high sulfur content coals, Illinois No. 6, Mequinenza, and three Andorra-Arifio, high conversions and THFsolubles were obtained even in the absence of catalyst at 10 MPa (Hz, cold) for 30 min. With the two other coals, the conversions, THF-solubles, and oils/asphaltenes ratios were enhanced by CS2 and 450 "C. After reaching a maximum addition. The temperatures studied were 300,350,400,425, in conversion percentages at 400 or 425 "C, depending on the coal, a decrease takes place at the highest temperature studied on the THF-solubles accompanied by a considerable increase in oils formation. It seems that part of the formerly released asphaltenes, major components of the THFsolubles, at 450"C are converted into oils and gas by hydrocracking reactions and into THF-insolubles by retrogressive reactions. The Mossbauer spectroscopy shows that pyrite is converted into pyrrhotite in all the processes, catalyzed or noncatalyzed, to a variable extent depending on the previous iron distribution, on the iron chemical stage in the catalyst precursors, and on the CSZaddition. The total sulfur content in the reactor is also an influencing factor. 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 the sulfated iron oxides formation.
Introduction
Coal hydrogenation consists of its transformation into progressivelylighter products by heating and by hydrogen transfer. In heat treatment there is a compromise: at low temperatures breaking of links does not occur, while at high temperatures the lighter hydrocarbon formation leads to poorer hydrogen efficiency and retrogressive reactions take place producing char and high-molecular-weight species which are very difficult to hydrotreat.' To avoid these problems, catalyzed coal hydrogenation has proven to be very useful but generally expensive. At the moment, important efforts are being done to advance the catalytic aspects of coal hydroconversion, and in this context, iron catalysis2 is studied in depth. It is known that the reactivity of some coals liquefied by the Farben process (1930s)Germany) was enhanced by Fe addition to the feed Later, in the 1970s, it was found that some minerals present in coal mineral matter served as hydrogenation catalysts, proposing that iron sulfide was the Abstract published in Aduance ACS Abstracts, December 15,1993. (1)Whitehurst, D. D.;Michell,T. 0.; Farcasiu, M. Coal Liquefaction; New York Academic Press: 1980; p 370 and references cited therein. (2) Huffman, G. P.; Farcasiu, M.; Wender, I. Iron based catalysts for coal liquefaction. Preprints of the ACS Diu. Fuel Chem. (Denver) 1993, 38 (l),and references cited therein. (3) Stephen, H. P. International Conference on Coal Science, Essen, 1981. (4)Probstein, P. Synthetic Fuels; McGraw Hill,1982;Chapter 6,p 490. (5)Cook, P. S.; Cashion, J. D. Fuel 1987, 66, 661-668.
The problem with the inherent mineral matter containing the iron species is related to the degree of dispersion. These species are usually found in an agglomerated state with large particle size,a very low specific surface area, and, consequently, low catalytic activity.* It is known that highly dispersed catalysts offer more efficient contact, and better conversion yields are reached with the same catalyst more uniformly di~tributed.~ Working on disposable catalyst coal liquefaction to identify the active species of coal mineral matter, increasing conversions10 with increasing mineral matter and a synergistic effect between pyrite and organic sulfur were found. The ironbased catalytic precursors which are converted to fine grain sizes of sulfided iron under hydrogenation conditions'l can be dispersed onto the coal surface in different ways. This dispersion may require previous d i s s o l ~ t i o n of ~~-~~ the precursor, but several compounds of interest as (6)Tarrer, A. R.; Guin, J. A.; Pitts, W. S.; Henley, J. P.; Prather, J. W.; Styles, G. A. Prep. Pap.-ACS Diu. Fuel Chem. 1976,21 (5),59-77. (7)Garg, D.; Given, E. N. Ind. Ing. Chem. Process Des. Deu. 1982,21, 113-117. (8)Derbyshire, F. J. Catalysis in coalliquefaction. IEA Coal Research 1988. (9)Bacaud, R.; Besson, M.; Djega-Mariadossou, G. Prep. Pap.-ACS Diu. Fuel Chem. 1993,38 (l),1-7. (10)Mukherjee et al. Fuel 1976,55,4-8. (11)Mastral, A. M.; Mayoral, C.; Izquierdo, M. T.; Pardos, C. Prep. Pap.-ACS Diu. Fuel Chem. 1993,38(l),124-129. (12)Mastral, A. M.; Rubio, B. Final Report CSIC t o EC, Non Nuclear Energy, New Energy Vectors, Contract E N 3V 044 E(A) March, 1991. (13)Suzuki, T.; Yamada, H.; Watanabe, Y. Energy Fuels 1989,3,707713.
0 1994 American Chemical Society 088~-062~/94/2508-0094$04.50/0
Energy & Fuels, Vol. 8, No. 1, 1994 95
Fe Catalytic Precursors
Table 1. Characterization of the Studied Coals* S9 Mequinenza S13 A-A S16 A-A 518 A-A B19 Illinois No. 6
coal proximate (% wt, dry) M.M. ash V.M. fix. c ultimate (% wt, daf) C H N Stot (dry) Sorg (daf) SPY (dry) maceral (% vol) vitrinite exinite inertinite rank (ASTM) a
B22 Zollverein
B25 Bagworth
31.07 23.99 41.00 35.00
23.48 13.60 35.37 50.79
21.60 12.75 33.89 53.01
27.51 17.17 33.05 49.77
17.78 13.79 32.66 53.54
12.08 12.07 25.88 60.95
13.19 8.80 39.51 51.68
65.29 7.79 0.57 9.72 11.5 0.67
67.45 7.63 0.40 7.23 5.87 1.43
64.82 6.67 0.64 7.32 5.63 1.95
67.40 6.85 0.48 7.19 2.84 3.34
76.98 5.60 1.45 4.05 2.62 2.05
83.30 5.20 1.71 0.99 0.72 0.44
79.40 5.32 1.34 0.74 0.51
90.9 0.4 8.7 SubC
71.8 3.1 25.1 SubC
74.7 1.6 23.7 SubC
75.4 1.0 23.6 SubC
89.1 1.7 9.2 hvCb
67.0 9.0 19.0 mvb
72.0 5.0 23.0 hvCb
1.22
A-A Andorra-Arifio. M.M.: mineral matter.
liquefaction catalysts, such as pyrite15or iron oxide,16are insoluble in common solvents and have to be physically mixed as powders or as a gas. Nevertheless, the most important catalytic aspects are the particle size, the dispersion degree, and the fact that the active species must be generated in situ. Mossbauer spe~troscopy~~-19 has since led to greater understanding of the behavior of Fe/S species under liquefaction conditions. SEM and TEM coupled with EDX have been useful complementary tools to gauge the dispersion degree of the catalyst and in following the agglomeration and synterization phenomena throughout the catalytic In this paper, the catalytic activities of three iron-based precursors in different chemicd stages and degrees of dispersion have been studied in dry hydroconversion of a set of low to high rank coals with different sulfur contents. Results of conversion and selectivity to derived products are discussed in relation to these parameters. Proximate and ultimate analysis, NMR, infrared and Mossbauer spectroscopies, FM, SEMEDX, and XRD are the analytical techniques applied to this study.
Experimental Section Coals. Seven coals denoted S9 (Mequinenza); S13, S16, and S18 (Andorra-Arifio);B19 (Illinois No. 6); B22 (Zollverein);and B25 (Bagworth)were studied. Table 1showstheir characteristics. Coals were ground to pass though a 0.25." sieve (-60 mesh, Tyler scale) and stored in an argon atmosphere until use. Catalysts. Three catalytic precursors were studied iron(I1) sulfate heptahydrate (Merck, with purity higher than 99.5%), iron(II1)oxide (asred mud composed of 36.5% FezO3,5.3% CaO, 8.5% Si02,13.5% TiO2, and 23.8% A1203, courtesy of Deustche Montan Technologie),and iron pentacarbonyl (Strem Chemicals, with purity higher than 99%). The three catalytic precursors were dispersed onto the coal using different methods: For the iron(I1) sulfate, the intermediate oxo-thio salt was prepared by bubbling HzS for 15 min through an aqueous
F e S 0 ~ 7 H z 0solution in the presence of air. The neutrality of the solution is maintained by adding several drops of NaOH concentrated solution when necessary while bubbling. The conversion of the thio salt takes place quickly and is accompanied by a color change. Then, after filtration and washing with distilled water, the precipitate was added to an aqueous slurry of a quantitative amount of coal to obtain a 5 % weight iron load. The mixture was magnetically stirred for 30 min. After that, the slurry was freeze-dried in order to avoid altering the original catalyst dispersion. This method of dispersion, in which the coal is the catalytic support, has been widely used previously.12J1 The iron amount loaded was verified by atomic absorption. For the iron(II1) oxide, the red mud was directly mixed with coal as powder by hand stirring. With iron pentacarbonyl as catalytic precursor, this was directly embedded on coal in inert atmosphere in the interior of a drybox. For the seven coals and the three precursors, the catalyst loading was 5 wt % Fe on daf basis. Hydrogenation Procedure. About 10 g (dmmf) of catalyst containing coal was loaded into the reactor-type tubing bomb of 160-cm3capacity, and this was pressurized with nitrogen to 15 MPa in a check for leaks. Afterward the reactor was purged 3 times with hydrogen before setting up the initial pressure of operation (10 MPa). The reactor placed at the holder was hung from the oscillation system and immersed in a preheated sand bath at the temperature reaction. The oscillation device was provided with a frequency range of 0-200 vibrationelmin and a variable amplitude up to 15 cm, so the reactor was agitated at 100vibration/min with an amplitude of 2.5 cm for 30 min which was the fixed reaction time for all the reactions. At the end of the process, the reactor was cooled by quenching in water. Product Workup. The gases were vented from the cold reaction vessels into a gas sampling bag. The reactor content was transferred into a Soxhlet extraction thimble for extraction with THF for 24 h. The THF was removed from the extract in a rotary evaporator. Afterward, both residue and the THFsolubles were dried at 50 "C under vacuum overnight. Then, the THF-solubles were fractionated with n-hexane into oils and asphaltenes. Yields. The obtained yields are calculated as follows:
- solid residue w t (loaded coal wt)& conversion % = x 100 Suzuki,T.Prep. Pap.-ACS Diu. Fuel Chem. 1993,38(11,137-141. (loaded coal wt)daf (15) Montano, P. A.; Granoff, B. Fuel 1980,59, 214-216. (1) (16) Herrick,D.E.;Tierney,J. W.;Wender,L;Huffman,G.P.;Huggins, F. E. Energy Fuels 1991,4, 231-236. where the red mud was the catalyst precursor, the weight of the (17) Pradhan,V.R.;Tierney,J. W.; Wender,I.;Huffman,G.P.Energy Fuel3 1991, 5, 497-507. initially added red mud is subtracted from the solid residue (18)Gangulu, B.; Huggin, F. E.; Rao, K. R. P. M.; Huffman, G. P. weight. Prep. Pap.-ACS Diu. Fuel Chem. 1 9 9 3 , s (l),190-195. (19)Srinivasan, R.; Keoghand, R. A.; Davis, B. Prep. Pap.-ACS Diu. (21) Davis, A.; Derbyshire, F. J.; Mitchel, G. D.; Schobert, H. H. Final Fuel Chem. 1 9 9 3 , s (l),203-210. Report, 1987. DOE-PC-90910-1. The PennsylvaniaState University,PA, (20) Mastral, A. M. Coal valoration by swelling measuring. Final 1987. Report. CSIC to EC SC, Contract 722O/ECl755, 1993. (14)
Mastral et al.
96 Energy & Fuels, Vol. 8, No. 1,1994
The identification of chemical species in the mineral matter and the characterizationof catalystsand their state of dispersion were carried out by powder X-ray diffraction (XRD) in a Seifert 3000 using Cu Ka radiation and by scanningelectron microscopy (SEM) with energy dispersive X-ray (EDX) in an electron microscope IS1DS-130with a Si/Li detector and processor 8OOOI1 Kevex. The Mhsbauer spectra were obtainedby transmission at rmm temperature and constant acceleration using ~'COin a Rh matrix.
n
!!i
f
Results and Discussion This work is part of a broader study20 on dry catalytic hydrogenation of a set of 25 coals from diverse mining areas around the world. Sevenof them were hydrogenated with iron catalysts, and results obtained are reported in this paper. Many variables influence this work there are low (S9,S13,S16,and S18)and high (B19,B22,and B25) rank coals; low (B22and B25)and high (S9, S13,S16,S18, and S19) sulfur content coals; and well-dispersed (iron sulfate and iron pentacarbonyl) and just-added (red mud) catalytic precursors. Furthermore, the iron oxidation degree in the precursors was 0, 11, and 111. Data obtained have to be analyzed as a function of the coal rank, temperature hydrogenation, sulfur content in the reactor, and catalyst precursor. The influence of the process variables, according to Figures 1 and 2, confirms the different behavior between low and high rank coals. Blank Tests. Results obtained when no catalyst was added show that with high sulfur content coals important conversionswere reached even a t 350"C, the THF-solubles ranging from 35.5% (subbituminous coal with 9.7% of total sulfur) to 20% (bituminouswith 4.0% of total sulfur). The concentration and dispersion of the S13 raw coal and its solid residues after hydrotreatment were characterized by SEM-EDX and XRD techniques.22 SEM-EDX allows the identification of the Fe compounds as pyrite more or less oxidized to iron sulfate, presumably in a noncrystalline state and consequently not detected by XRD. The pyrite appears mainly as very small particles (50.5pm) embedded in the coal matrix and also eventually in large particles. During the reaction, the natural iron-based catalyst in the mineral matter is chemically transformed. The fluorescence microscopy shows the superficial transformation of the pyrite nodules from the inherent mineral matter into pyrrhotite, and M6ssbauer analysis corroborates 39% of this transformation at 350 "C. This transformation is nearly complete at 400 O C , which is clearly evidenced by the steep decrease in sulfur concentration in solid residue and the high emission of H2S in the gas fraction. These chemical changes cause a decrease in dispersion of the iron-based compounds: the small particles aggregate and the larger particles break into smaller ones. The overall effect is more uniform distribution with an increase in the particle size. The sulfur liberated in the pyrite to pyrrhotite conversion, together with the organic sulfur released at 350 "C (mostly from the thio ether bridges and thiols) at the working conditions will be HzS. Thus, the high conversions achieved even at these mild conditions can be explained by a homogeneouscatalytic mechanism involving catalysis of hydrogen transfer from this H2S to the radicals from thermal breaking of the low rank high sulfur content coals. Sulfur Influence. While 300 "C is a quite low temperature for coal hydrogenation, the S9 coal with the (22) Prado, J. G.,private communication.
Qtht
NOM
CltPrac:
None
Red Mud
Red Mud
Iron Sulphate
Iron Pantrarbonyl
Iron Sulphite
Iron Pcntaarbonyl
I
IwTl
CltPlsc:
I
None
% Aaphiltena
Rad Mud % Oilr
Iron Sulphate
Iron Pcnuarbonyl
0Conv.(%)-7HFwl.(%)I
Figure 1. Obtained yields with the low rank coals with the three catalyst precursors. Reaction time: 30 min. Pressure: 10 MPa Hz cold.
highest organic sulfur content shows a relevant difference in conversion yields. This difference is also observed at 350 "C. A t this temperature the catalytic hydrogenation of low rank coal takes place, S9 again showing the best conversion yields. This behavior could be due to the higher H2S pressure during the process. The highest percentages in THF-solubles with low rank coals are achieved at 400 "C. Higher temperatures with these low rank coals potentiate the retrogressive reactions. A t 350 "C the bituminous coals show low conversion percentages. For them, and with CS2 addition, the catalyst role is observed at 400and 425 "C. A t 450"C,the bulk of the retrogressive reactions have a negative effect on conversionyields, due to an uncontrolled thermal breaking. The influence of the CS2 addition depends on the coal sulfur content and is determinant in the hydrogenation of the low sulfur content coals. For instance, the total sulfur/ iron from catalyst ratio affects the B25 hydrogenation coal with red mud improving conversions from 38% in THFsolubles when this ratio is 0.26 to 60% when the ratio is 0.78. On the other side, the CS2 addition when RM is the catalytic precursor enhances the pyrite to pyrrhotite transformation from 59% to 68% when S13 coal is
Fe Catalytic Precursors
Energy & Fuels, Vol. 8, No. 1, 1994 97
1
1
Red Mud
CaLPnr:
None
% Asphrltcncr
Red M u d
@ % OiL
Iron Sulphate
Iran Pentlorbony1
0Conv.(%)-THFwl.(%)
Figure 2. Obtained yields with the high rank coals with the three catalyst precursor CS2 added. Reaction time: 30 min. Pressure: 10 MPa H2 cold.
hydrogenated at 350 "C, 10 MPa, and 30 min, according to the Mossbauer analysis. CatalyzedHydrogenation. The analysis of the results obtained in the catalyzed reactions is very complexbecause many different variables operate simultaneously: (a) the sulfur inherent to coal, (b) the total amount of sulfur in the reactor, (c) the iron oxidation degree in the precursor, and (d) the catalyst dispersion. As the analytical techniques applied show, and due to the dispersion procedure for each precursor, the degree of dispersion in decreasing order was iron sulfate, iron pentacarbonyl and iron oxide. (i) Iron Sulfate Precursor. In the case of the iron sulfate precursor, the iron compound in the fresh material is not crystalline and could not be identified by XRD; however, SEM-EDX shows that it is iron(I1) sulfate. Because of the method of preparation used for the addition to coal, the catalyst is expected to be found in a high degree of dispersion. The morphological appearance and the correspondingline profile confirm this general expectation, but some heterogeneities in the catalyst dispersion are observed. They can be due to the iron naturally present in the mineral matter that eventually appears in large particles of pyrite, as commented above, and/or perhaps to the mesoporous nature of the coal matrix, which does not completely prevent the accumulation of catalyst in some large pores. Consequently, a good performance of this catalyst is expected. Pyrrhotite was always the major component, and its transformation seems to depend on the degree of dispersion. Temperature acts as a highly influencing parameter on the agglomeration of small particles and on the segregationtoward the external surface of the coal particles as evidenced by SEM-EDX. (ii) Iron Pentacarbonyl. Iron pentacarbonyl precursor is expected to be formed in a fine degree of dispersion. However, the catalyst performance a t 350 "C indicates
that conversion is similar to that obtained in the absence of catalyst due only to the contribution of the mineral matter. SEM-EDX studies indicate that the degree of dispersion is not as good as expected because of the formation of molecular crystals. Even when the degree of dispersion is fine, it is the chemical state of iron which determines the bad performance in coal hydrogenation at 350 "C. At 400 "C the formation of pyrrhotite as a convenient active phase for the coal hydrogenation is demonstrated by XRD, and the behavior is similar to that of iron sulfate at the same temperature. These results emphasize the importance of the presence of iron in the suitable chemical state. Previous resultszs demonstrated that sulfiding elemental iron is more difficult than when the iron is oxidated. When elemental iron is first reduced and then sulfided, the corresponding species is active, while when elemental iron is first sulfided and then reduced, the obtained species is less active. Due to the high H2S partial pressure at the working conditions, that could be the explanation to the low performance of iron pentacarbonyl. At 350 "C, iron from iron pentacarbonyl has not yet been converted to pyrrhotite, but in the THF-insolubles from 400 "C there is a morphological change in the particles, and pyrrhotite is identified. With increasing temperatures, the iron dispersion was not affected according to the line profile of the K, line. (iii) Red Mud. Precursor RM was added to coal by physical mixture. In these circumstances it is expected that the catalyst will be in a poor degree of dispersion determined mostly by the particle size and located on the external surface of the coal particles. Actually, the SEMEDX shows that the iron dispersion degree is comparatively the lowest of the three studied. Under the reaction conditions XRD presents the iron oxides are almost completely converted into pyrrhotite, even at 350 "C,and the degree of dispersion is not modified. Nevertheless, the Miissbauer spectroscopy only corroborates 59 % of the iron transformation into pyrrhotite. Another component found in RM is Ti02 as an impurity, but it does not seem to play any role in the hydrogenation of coal. It is worth noting the high conversions achieved when red mud is the catalyst precursor. At 400 and 425 "C, conversionswere higher than those obtained with the other precursors. Previous workers24showed that iron oxide could react with sulfur at 450 "C and 10 MPa when a very small amount of moisture was present. Later,26 working with sulfated metal oxides it was found that red mud was very reactive at 400 "C due to the SO3 chemisorption by iron oxides. At the conditions studied in this work, the sulfur source would be the pyrite to pyrrhotite conversion and would release the heteroatoms. The water would be supplied by the coal moisture, but in this work the reaction would take place at 400 "C. These facts could explain that, in spite of the red mud not being well dispersed, due to an acid catalysis it was the most active precursor of the three studied a t 400 and 425 "C. This acid catalysis overlaps pyrrhotite heterogeneous catalysis and the HzS homogeneous catalysis. The acid catalysis produced by the sulfated iron oxide formation is corroborated by the fact (23) Zimmer, H.; Andrb, M.; Charcosset, H.; Djega-Mariadassou, G. Appl. Cotal. 1983, 7,295-302. (24) Kotaniaawa, T.; Takahaahi,H.; Yokovama, S.:Maekawa,Y. Fuel 1988,67,927431. (25) Pradhan, V. R.; Herrik, D. E.; Tierney, J. W.; Wender, I. Energy Fuels 1991,5,712-720.
98 Energy & Fuels, Vol. 8,No. 1, 1994
that at 450 "C the conversions achieved with red mud underwent a decrease, in comparison to iron sulfate precursor yields. This would be due to the potentiation of the retrogressive reactions by the acid
Conclusions From the characterization of iron-based catalysts in the coal hydrogenation process, it can be deduced that their catalytic performance is determined not only by the chemical state and the degree of dispersion but also by acid catalysis. During reaction the precursor is usually converted into pyrrhotite, and in the case that this change does not happen due to the low temperature, the ironbased compounds naturally present in the mineralmatter, mainly pyrite more or less altered, influence the process strongly. The sulfur from this reduced pyrite together with part of the organic sulfur from the coal matrix is involved as HzS in a parallel homogeneous catalytic mechanism that would explain the high conversions obtained with the high sulfur content coals in the blank test. The higher the organic sulfur content, the higher the conversions. (26) Olah, G.A.; Prakash, G. K.; Sommer, J. Superacids; Wiley: New York, 1985; pp 15-42.
Mastral et al.
The degree of catalyst dispersion is another important parameter to be considered that is entirely determined by the preparation method applied. Nevertheless, the good results achieved when red mud was the catalytic precursor, with the lowest dispersion degree, might be due to the formation of sulfated iron oxides under the more severe hydrogenation conditions which are very active with an acid catalyst. Iron pentacarbonyl is the precursor with the highest stability and does not generate pyrrhotite at temperatures lower than 400 OC.
Acknowledgment. We thank Dr. R. Gancedo and Dr. M. Gracia for their help with the Mbsbauer spectroscopy, Dr. J. G. Prado for his FM comments, and the European Communities and the Spanish CICYT for their financial support. Supplementary Material Available: Scanning electron microscope/energy-dispersiveX-rayphotographeof 5-13coal and THF-insoluble6 from S-13 coal hydrogenation and Mtkiesbauer spectra of 5-13coal (12pages). Ordering informationis available on any current masthead page.
