Energy & Fuels 1994,8, 77-82
77
Evaluation of the Effect of a Nonporous Ultrafine Iron Catalyst on the Hydroliquefaction of a Highly Volatile Bituminous Coal Vicente L. Cebolla' Instituto de Carboqulmica, Consejo Superior de Investigaciones Cientlficas, Plaza de Paratso, 4, 50004 Zaragoza, Spain
Moustapha Diack and DBnise Cagniant Laboratoire de Chimie Organique, Universitg de Metz, Ile du Saulcy, 57045 Metz, France
Michele Oberson, Robert Bacaud, and Brigitte Nickel-PBpin-Donat Institut de Recherches sur la Catalyse, CNRS, 2, Avenue Albert Einstein, 69626 Villeurbanne, France Received July 12, 1993. Revised Manuscript Received October 26, 1999
The effect of a nonporous ultrafine iron oxide precursor, prepared by a flame method, was evaluated in the hydroliquefaction of a bituminous coal in tetralin (350,400,430 "C) on a laboratory scale, and compared to that of other aerosols (silica, alumina, SnO2, Moos, NiMo/A12Oa). The precursors were sulfided with S during heating, according to a previously established conditions set. Methods of evaluating catalytic activity in hydroliquefactionruns based on extraction yieldsare strongly dependent on the experimental conditions. Special emphasis has been given here in the application of two alternative parameters based on electron spin resonance (ESR) data, and on the ratio of hydrogen consumption from the gas to that from the solvent (HgJHsolv). Our results show that with increasing the temperature, the lower the differentiation between catalysts measured by extraction conversions. However, HgdH,lV and the measurements by ESR of the stable radicals of the tetrahydrofuraninsoluble fractions are more clearly affected by the nature of the catalysts and related at a given temperature. Fez03 and Moo3 are the most active catalysts at 350 "C. The order of activity is the same regardless of the method used. The spillovereffect can explain the hydroliquefaction mechanism. At 400 "C,HgJHsOlv permits the clearest differentiation between catalysts. At this temperature, effects other than spillover can contribute to hydrogen-transfer mechanisms. Analytical data of the coal-derived liquids (capillarygas chromatography,size exclusion chromatography,and extrography) show a large temperature effect, independent of the nature of the catalyst: as the temperature increases from 400 to 430 "C, the percentage, in the oils, of the four or more ringed compounds decreases, and the heaviest components of the asphaltenes are degraded. Furthermore, the oils obtained with sulfided Fez03 or Moo3 contained significantly more two-ringed aromatic compounds than the oils obtained with the other catalysts, or with no added catalyst. Possible explanations are discussed in the light of the literature research with model compounds.
Introduction
A summary of the research conducted in France between 1980 and 1990 on the development of highly dispersed, disposable iron-based catalysts has been presented in a previous paper.' Highly dispersed iron oxides, considered as precursors for the in situ generation of iron sulfides duiing coal liquefaction, were prepared using several procedures. The research demonstrated that the pertinent parameter for the activity of these precursors is not the total surface but the accessible external surface. Thus, nonporous nano-sized aerosol oxides were synthesized by the combustion of the corresponding metal chlorides in a hydrogen-oxygen flame.2 Other workers also reported a positive effect of ultrafine iron catalysts in coal liquefacti0n.w Abstract published in Advance ACS Abstracts, December 1, 1993.
(1)Bacaud, R.;Besson, M.; Djega-Mariadassou,G. Prep. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1993, 38 ( I ) , 1-7.
0887-0624/94/2508-0077$04.50/0
The sulfidation of flame iron precursors generated a pyrrhotite as an active phase at moderate temperature.' However, results showed that prior reduction of the oxide precursor in the hydroliquefaction medium had an inhibiting effect upon its hydrogenation activity? Furthermore, the procedure of sulfiding affected the textural and catalytic properties of the active phases.'~~ (2) (a) AndrBs, M.; Charcosset, H.; Chiche, P.; Djega-Mariadassou,G.; Joly, J. P.; Pregermain, S. In Preparation of Catalysts ZZ4 Poncelet, C., Grange, P., Eds.; Elsevier; Amsterdam, 1983; pp 675-682. (b) AndrBsBesson, M.; Charcosset,H.; Chiche, P.; Davignon, L.;Djega-Mariadassou, G.; Joly, J. P.; PrBgermain, S.Fuel 1983,62, 69-72. (3) Pradhan, V. R.;Tierney, J. V.; Wender, I.; Huffman, G. P. Energy -_ Fuels 1991, 5, 497. (4) Matson,D. W.;Linehan,J.C.;Darab,J.G.Prepr.Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1993, 38 ( I ) , 14-19. (5) Anderson,R.; Givens, E. N.;Derbyshire, F. Prep. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1993, 38 ( Z ) , 495-502. (6) Zimmer. H.: AndrBs, M.: Charcosset.. H.:. Dieaa-Mariadassou, G. _&pi. catai. i983; 7,295-306: (7) Bacaud, R.; Besson, M.; Brodzki, D.; BussiBre, P.; Charcosset, H.; Djega- Mariadassou, G.; Oberson, M. Catalysts Deactivation; Elsevier: Amsterdam, 1989; pp 289-301.
0 1994 American Chemical Society
Cebolla et al.
78 Energy & Fuels, Vol. 8, No. 1, 1994
Iron oxide precursors obtained by t h e flame method, a n d further sulfided with elemental sulfur during heating, showed high catalytic activity for t h e liquefaction of a highly volatile bituminous coal. Furthermore, t h e sintering of the initially dispersed iron sulfide was apparently prevented by t h e presence of carbonaceous solid subs t a n c e ~ Another . ~ ~ ~ study also makes reference t o this same textural effect.8 T h e research was extended at the laboratory scale t o include aerosol types other t h a n Fez03 (SnOz, Moos, also prepared by t h e flame method; SiOz, A1203 aerosols from Degussa, and a NiMo/AlzOa industrial supported catalyst). T h e general aim of this research was to evaluate the activity a n d selectivity of these precursors in order t o give a better understanding of t h e role of catalysts in hydroliquefaction a n d the mechanisms of hydrogen transfer. In this paper, t h e effect of a sulfided Fez03 aerosol on the hydroliquefaction of a highly volatile bituminous coal, in a hydrogen donor solvent (tetralin) is compared t o t h a t of the other above-mentionedaerosols. Their activity a n d t h e chemical structure of t h e derived coal liquids are evaluated. For this purpose, methods t o evaluate t h e catalytic effects in hydroliquefaction runs in batch reactors a n d analytical techniques which permit differentiation between t h e effects of t h e catalysts have been developed.
Experimental Section Catalysts. FezO3, MOOS,and SnO2 aerosol precursors were obtained by combustion of the corresponding chloride vapors in a Hz-02 flame. Details of the experimental device are described in detail elsewhere.2~9This technique allows nonporous oxides with mean particle sizes as small as 10 nm, to be obtained. The size and shape characteristics of the solids can be varied through control of the thermodynamic (flame temperature, flows of metallic compound vapors, and gases in the torch) and the kinetic parameters (residence time of these species). In our case, the formed oxides were nonporous, with particle sizes lower than 50 nm, and with BET surfaces (carried out using N2 at 77 K) from 20 to 60 m2 g-1. Si02 and A1203were commercial aerosols from Degussa (200 and 100 m2gl,respectively), and NiMo/AlzOswas a conventional supported catalyst from the Institute FranCais du PBtrole (200 m2 g1SBET). Hydroliquefaction Procedure. The experimentalprocedure and analysistechniques data were described in detail elsewhere.1° Briefly, 40 gof a low-sulfur (0.8% daf), highly volatilebituminous coal (from Freyming, France) was suspended in tetralin (95 g) with (or without in the blank run) a 2 w t % loading of catalyst precursor. Elemental sulfur was added in such an amount that catalyst precursor was sulfided during heating, and a partial pressure of 1% H2S was obtained, avoiding in this manner the possiblereduction of oxides to their correspondingmetallicstates. The magnetically stirred reactor (300 mL) was pressurized (14 MPa) with Hz. After the heating period (3 "C m-l), the nominal temperature (350,400, or 430 "C) was maintained for 1h before cooling. Runs were carried out once. The influence of catalyst loading was previouslystudiedlousing the iron precursor and NiMo/AlzO3 in order to choose standard conditions of hydroliquefaction. Two percent of catalyst precursor (wt % daf coal) was chosen to compare catalyst performance as a compromise between several factors: (i) this is the minimum amount required for further characterization of (8) (a) Cugini, A. V.; Krastman, D.; Martello, D. V.; Holder, G. D. Prep. Pap.-Am. Chem, Soc., Diu. Fuel Chem. 1993,38 ( I ) , 99-106. (9) Bacaud, R.; Besson, M.; Brodski, D.; Bussiere, P.; Cagniant, D.; Cebolla, V. L.; Charcosset, H.; Djega-Mariadassou, G.;Jamond, M.; Nickel, B.; Oberson, M. In Progress in Synthetic Fuels; Bemtgen, J. M., Imarisio, G., Eds.; Graham & Trotman, Ltd.: London, 1988, pp 9-18. (10) Besson, M.; Bacaud, R.; Charcosset, H.; Cebolla, V. L.; Oberson, M. Fuel Process. Technol. 1986,12, 91-109.
catalysts in the hydroliquefaction residues (THF-insolubles), (ii) a further increase of loading gives a constant percentage of oils and tetralin dehydrogenation, and (iii) the percentage of tetralin dehydrogenation is relatively low. This concentration is higher than the "critical catalyst loading" for each case.1 Blank runs were carried out in the absence of coal to evaluate the behavior of tetralin. The formation of bitetralyl dimers was studied, in the presence and absence of the NiMo/AlzO3 catalyst, under standard hydroliquefaction conditions (400 and 430 "C), and results were compared to the experiments carried out in presence of coal. Bitetralyl dimers were detected by capillary gas chromatography, using 1,l'-bitetralyl, 1,2'-bitetralyl and 2,2'bitetralyl as standards, and by capillary gas chromatographymass spectrometry (70 eV)(NERMAGR1010-C apparatus with SE 30 column (25 m length): initial temperature: 60 "C (3 min); 5 "C m-1; final temperature: 300 "C. Evaluation of catalytic activity i n the hydroliquefaction runs. Conversions were evaluated by parallel solvent extraction, using three microfiitration-under-pressure(for n-hexane,toluene, and THF, respectively, as has been described in detail.lOJ1The results were expressed in terms of the percentage of conversion into soluble products, calculated by the difference in weight of the insoluble fractions, % conversion =
wt daf coal - wt daf residue wt daf coal
assumingthat the amount of ashes present in coal and the residue are the same, and preasphaltenes % = conversion 5% in THF conversion % in toluene, asphaltenes % = conversion % in toluene - conversion % in n-hexane, and oil % = conversion % in n-hexane. The activity of the catalysts in hydrogen-transfer reactions was also evaluated by the extent of dehydrogenationof the solvent (expressed as the ratio of naphthalene to naphthalene tetralin, measured by gas chromatography) and by the total consumption of hydrogen gas determined by the change in the total pressure over a run, because the production of gases can be neglected ( Moo3 > Si02 > no catalysts (Table 2). The trends are also similar to H,,/ Hsolv,although the latter allows a clearer differentiation among catalysts. At 400 "C, active hydrogen is not the only way of radical stabilization. Catalysts operate in a very complex and highly competitive medium where several species can contribute to hydrogen-transfer mechanisms: shuttle mechanisms involving tetralin or coal liquids cannot be ruled out. The same is true for the indigenous pyrite of coal, as shown by the results of the stabilization of inertinite radicals (400"C, without catalyst).
-
-
(29)Robell, A. J.;Balldu, E. V.; Boudart, M. J. Phys. Chem. 1964,68, 2748. (30)Fujimoto, K.;Ohno, A.;Kunugi, T. In Studies in Surface Science and Catalysis 17.-Spillover of Adsorbed Species; Pajonk, G . M., Teichner, S.J., Germain, J. E., Eds.; Elsevier: Amsterdam, 1983;p 241. (31)Makabe, M.; Ohe, S.; Itoh, H.; Ouchi, K. Fuel 1986,65, 296.
~
~
~~
350 "C
no catalyst alumina SnOz Fez03 MOOS
0.24 0.24 0.24 0.16
0.20 0.45 1.51 1.40 1.25
0.08
0.44 0.69 1.75 1.56 1.33
0.83 1.87 6.29 8.75 15.62
1.06 0.81 1.30 0.23 0.17 0.57 0.17
1.41 1.34 1.96 2.01 1.62 2.38 2.39
0.33 0.65 0.51 7.74 8.53 3.17 13.05
1.36 1.28 0.86 0.55
2.10 2.50 2.50 2.40
0.54 0.95 1.91 3.36
400 o c no catalyst alumina silica SnO2 Fez03 MOOS NiMo/AlzOs
0.35 0.53 0.66 1.78 1.45 1.81 2.22
no catalyst silica SnOz NiMo/AlzOs
0.74 1.22 1.64 1.85
430 "C
At 430 "C the situation is still more complex because the HgJH,l, ratios decrease significantly as the amount of hydrogen transferred from the solvent increasesbecause of the thermal bond cleavage (Table 3). Our results agree with the idea that a major role of the catalysts at 1400 "C is to decrease the fraction of free radicals (1) by direct transfer of hydrogen, as active hydrogen H* from the gaseous phase to the coal/coal products rather than through hydrogen transfer from solvent or hydroaromatic compounds, and (2) by acting as radical scavengers which would avoid the recombination reactions and favor their further stabilization by hydrogen. This latter effect could explain the relatively high conversions (oilsand asphaltenes) obtained, at our conditions, in runs using silica and alumina with high surface and fine granulometry (Table 2). These solids were inactive in the hydrogenation of model c0mpounds.3~ The idea of the role of catalysts as inhibitors of recombination reactions was proposed by Hatswell et al.33 Considerations on the Use of Tetralin for Analytical Purposes. Tetralin is the most commonly used hydrogen donor solvent to model coal liquefaction at laboratory scale because it is a good compromise, taking into account its physicochemical and thermal properties and hydrogen donation ability. However, its reactivity when the temperature increases to above 410 "C somewhat obscures the interpretation of results at these conditions. Secondary reactions of tetralin have been extensively studied using model compounds and involve mainly isomerizationto give 1-methylindaneand further indane,34 and rupture of hydrogenated cycle to give alkylbenzenes.36 Disproportionation reactions to give naphthalene and decaline have also been described.36 The possible direct incorporation (adsorption or reaction) of tetralin to coal and/or coal products should not be neglected. However, Kabe et a1.3' concluded that chemical addition of tetralin or tetralin-derived compounds to coal or coal products (32)Curtis,C. W.;Guin,J.A.;Tarrer,A.R.;Huang,W. J.FuelProcess. Technol. 1983,7, 277-291. (33)Hatswell, M.R.; Jackson, W. R.; Larkins, F. P.; Marshall,M.; Rash, D.;Rogers, D. E. Fuel 1983,62,336-341. (34)Curran, G.P.; Struck, R. T.; Gorin, E. Ind. Eng. Chem. Process. Des. Deu. 1967,6 , 166. (35)Hooper, R. J.; Battaerd, H. A. J.; Evans, D. G. Fuel 1979,58, 132-138. ~~
(36)Yen, Y.K.;Furlani, D. E.; Weller, S. W. Ind. Eng. Chem. Prod. Res. Dev. 1976,15, 24.
Energy & Fuels, Vol. 8, No. 1,1994 81
Effect of Ultrafine Iron on Hydroliquefaction Table 4. Formation of Bitetralyl Speciee during Blank Experiments at 400 and 430 "C8 run catalyst bitetralyls (400 "C) bitetralyls (430 O C ) blank no 0.07 0.07 blank NiMo/Al203 1.7 0.7 coal no 0.23 0.35 coal NiMo/Al203 0.06 0.18 Results expreaeed as w t 95 of tetralin transformed.
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Figure 1. Results of extrography of the raw hydroliquefaction products. Influence of the iron catalyst and temperature. F1: paraffins, PAHs; F2: condensed PAHs, N-PAH, some hydroxyPAHs; F3: mainly hydroxy-PAH, other condensed PAHs; F 4 polyfunctional heterocyclic compounds; in black, residues.
hardly occurs in the hydroliquefaction of a bituminous coal carried out a t 400 "C with tritium-labeled hydrogen and alternatively 14C-and 3H-labeled tetralin. Results show (Table 4) that the formation of pure bitetralyl species from tetralin is favored by the presence of the catalyst in the blank runs. However, they are drastically inhibited by the presence of coal. Secondary reactions of tetralin under hydroliquefaction conditions are not totally understood. Tetralin is not an inert element and can mask somewhat the catalytic effect; however, its utilization allows a comparative global interpretation of analytical data of the derived products with different catalysts under conditions relatively similar to the systems used in larger scale coal liquefaction in which the solvent is also an active element. Influence of Fez03 Precursor on the Structural Composition of Coal-Derived Liquids. According to the results of extrography of the raw liquids (Figure l), the formation of PAHs (main constituents of F1 and F2 fractions) increases significantly as the temperature increases, at the expense of polar compounds (F4) and residues. However, the differentiation, for a given temperature, between runs with or without Fe2O3, is hardly distinguishable using this technique, except for a small increase of the lightest compounds (Fl) for Fen03 runs a t 400 and 430 "C. More details on the structural analyses were found when characterization of oils and asphaltenes were performed by GC on capillary columns and HPLC-GPC. (37) Kabe, T.; Nitoh, 0.; Kawakami, A.; Funatau, E. R o c . Int. Conf. Coal Sci., Sydney 1985,79.
E]1 0 0 - 2 0 0
200-300
300-400
_--.
400-500
,500
Figure 2. GC analysisof derivedoils. Classification by retention index zones.
GC data, using the classification according to ring size as previously described in the Experimental Section, revealed the higher and significant proportion of tworinged aromatic compounds in the oils when the runs were performed with nonporous sultided Fee03or MOOScatalyst rather than with SiO2, SnO2 or NiMolAlpO3 (Figure 2). This effect was found at 400 and 430 "C for the iron precursor. This fact could be attributed to different possibilities: (1)One is hydrogenolysis of ether oxide or alkyl linkages between doubled two-ringed structures, which are known to exist in the products of coal liquefaction.38 This is supported by literature data on model compound reactions using iron oxide catalyst39d1 and p y r r h ~ t i t e ~in~presence e~~ of sulfur. (2) The hypothesis of hydrocrackingPAHs cannot be eliminated. Someworks reported that pyrrhotite Fe1,S formed from our Fe2O3 aerosol does not have both the hydrogenative and acid sites needed for a hydrocracking reaction of some model a more recent work reported c o m p o u n d ~ . ~ *However, ~5 (38) Shinn, J. H. R o c . Int. Conf. Coal Sci., Sydney 1986,738. (39) Utoch, S.;Hirata,T.;Oda, H.; Yokokawa, C. Fuel Process. Technol. 1986,14,221-229. (40) Cassidy, P. J.; Jackson, W. R.; Larkins, F. P.; Hertan, P. A,; Rash, D.;Marshall, M. In Advanced Topics and Applications to Fossil Fuel Energy; Petrakis, L., Fraissard, J. P., Us.; D. Reidel: Dordrecht, 1984; p 739. (41)Yoehimoto, I.; Itoh, H.; Makabe, M.; Ouchi, K. Fuel 1984, 63, 978-983. (42) Montano,P.A.;Lee,Y.C.;Yeye-Odu,A.;Chien,C.H.ACSSymp. Ser. 1986,301,416. (43) Ogawa, T.; Stenberg, V. I.; Montano, P. A. Fuel 1984,63,16601663. (44) Lemberton, J. L.; Guianet, M. Appl. Catal. 1984,13,181. (45) Lemberton, J. L.; Guisnet, M. J . Chem. Res. Synop. 224.
02 Energy & Fuels, Vol. 8, No.1, 1994
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This activation was also found in the case of acid sulfate metal oxides.47 The formation of iron(II)sulfatesand FeS2 when Fez03 + S is heated at 450 "C in the presence of water has also been detected? (3)Finally, the possibility of hydrodesulfurization activity of some carbon-supported iron sulfide catalysts has also been reported and should also be c ~ n s i d e r e d . ~ ~ GC and HPLC-GPC data showed the important effect of temperature for oils and asphaltenes, in general, and for a given catalyst run. On one hand, the weight percent of nondosed GC compounds decrease as the temperature increases (from 400to 430 "C), the cases of Si02 and SnO2 being the most significant. On the other hand, HPLCGPC showed that the same increase of temperature involves degradation of the heaviest components of asphaltenes (0 < k < 0.6) (Figure 3). However, this technique did not permit the observation of differences among the catalysts for a given temperature neither for oils nor for asphaltenes.
Acknowledgment. H. Charcosset, presently retired, initiated and gave the impulse to this research. His role, as a group leader of the French laboratories included in this project, has been decisive. We wish, by means of this paper, to express our gratitude for his essential contribution.
0.75
Figure 3. Size exclusion chromatography of oils and asphaltenes: - -, 400 "C;-, 430 "C. (a) Asphaltenes; (b) oils.
some activityof an ultrafine Fe203 + S in the hydrocracking of 4-(1-naphthylmethyl)bibenzylat 420-430 "C.4 Furthermore, this activity was enhanced in presence of water.
~~~~~~~
(46) Farcasiu, M.; Smith, C.; Pradhan, V. R; Wender, I. Fuel h e s s . Technol. 1991,29,19!+208. (47) Wen, M . Y.; Wender, I.; Tiemey, J. W. Energy Fuels 1990,4, 372-379. (48) Kotanigawa, T.;Yokoyama,S.;Yamamoto, M.; Maekawa, Y. Fuel 1987,66,1452-53. (49) Abotsi, G. M. K.; Scaroni, A. W. f i e 1 Process. Technol. 1989,22, 107-133.
