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Zeolite L catalysts
Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
525
Fischer-Tropsch synthesis. Influence of the presence of intermediate iron reduction species in Fe/Zeolite L catalysts. N.G. GaUegos, M.V. Cagnoli, J.F. Bengoa, A.M. Alvarez, A.A. Yeramihn and S.G. Marchetti
CINDECA, Fac. Cs. Exactas, Fac. Ingenieria, U.N.L.P., CIC, CONICET. Calle 47 N ~ 257 (1900) La Plata, Argentina. Two catalyst to be used in the Fischer-Tropsch reaction, using zeolite-L in potassic form as support of iron species were prepared through to different methods of impregnation with iron salt. X-Ray Diffraction (XRD), Specific Surface Area (BET), M6ssbauer Spectroscopy (MS) in controlled atmosphere, between room temperature (RT) and 15 K, H2 chemisorption and Volumetric Oxidation (VO) were used to characterise the solids. The impregnation of the zeolite L under inert gas allowed to obtain a fraction ofFe ~ in contact with Fe 2+ ions that enhanced the activity of the sites. The two catalysts presented similar selectivity towards hydrocarbons and low chain growth. 1. INTRODUCTION It is well known that in iron catalysts supported on different solids such as A1203, SiO2, MgO and zeolites, it is not possible to obtain complete reduction to Fe ~ when the iron concentration is low (approximately < 10%w/w) [1, 2]. Therefore, most of the iron catalysts used in the Fischer-Tropsch synthesis have, in addition to Fe ~ intermediate iron reduction species like Fe 2+. However, the role of Fe 2§ on the activity and selectivity in the CO hydrogenation has not been studied yet. In this paper, a commercial Zeolite L in potassic form (ZLK) was used as metal support for CO hydrogenation. The choice of this support was carried out since this reaction is sensitive to the structure. This means that the activity and selectivity of the catalyst depend on its metallic crystal size [3]. Catalysts with a narrow size distribution lead to good selectivity towards to a desirable product. Making use of the structure of channels and cages ofzeolites it is possible to reach this purpose. In order to determine the influence of the intermediate iron reduction species on the activity and selectivity of the Fischer-Tropsch reaction, two Fe/ZLK catalysts were prepared by two different methods.
2. EXPERIMENTAL SECTION Two precursors were prepared using the commercial form of the ZLK (Tosoh Corp.), with the ideal unit cell composition of dehydrated form of K9A195i27072 and 290 m~/g of specific surface area. One of them, was obtained by dry impregnation in air of the
526 zeolite with aqueous solution (pH=0.5) of a concentration to yield a solid with 5.84% w/w ofFe. Then it was calcined following the programme described in [4]. This sample was called p-Fe/ZLK(a). The other precursor was obtained outgassing the support at 773K and 0.05 torr for 1 h to eliminate the water present inside the channels and cages of the zeolite. After this time the system was filled with ultra high purity He up to 500 torr. Then, the Fe(NO3)3.9HzO aqueous volume solution equal to the pore volume of the zeolite was added to yield a solid with 4.56% w/w of iron that was calcined in the same way that p-Fe/ZLK(a).This solid was called p-Fe/ZLK(v). Both precursors were reduced in H2 stream (60 cm3/min) from 298 to 698 K at 2.66 ~ and were kept at 698 K during 26 h. The resulting solids were named c-Fe/ZLK(a) and c-Fe/ZLK(v), and characterised by XRay Diffraction (XRD), Specific Surface Area (BET), M6ssbauer Spectroscopy (MS) at 298 and 15 K, 1-12chemisorption and volumetric oxidation (VO). These last two techniques were performed in a conventional static volumetric equipment with grease-free vacuum valves. The Hz uptakes at the same initial pressure, but at different temperatures between RT and 673 K were measured to determine the temperature in which the adsorption capacity is maximum. Volumetric oxidation experiments are based on the conversion of all iron species in the sample to Fe203 when it was heated in an O~ atmosphere at temperatures higher than 620 K [5]. The M6ssbauer spectra were obtained in transmission geometry with a 512-channel constant acceleration spectrometer. A source of 57Co in Rh matrix of nominally 100mCi was used. Velocity calibration was performed against a 6-1am-thick ~-Fe foil. All isomer shifts (6) mentioned in this paper are referred to this standard. The temperature between 15 and 298 K was varied using a Displex DE-202 Closed Cycle Cryogenic System. All MSssbauer spectra of the catalyst were obtained in controlled atmosphere using a cell specially built for this purpose to be used inside the cryogen [6]. The spectra were evaluated using a least-squares nonlinear computer fitting program with constraint. Lorentzian lines were considered for each spectra components. The catalytic tests were carried out in a fixed bed reactor with a H~:CO ratio of 3:1, 543 K, 1 atm of total pressure, 20 cm3/min of total volumetric flow and a space rate of 0.25 s"l. The reaction products were analyzed by gas chromatography using FID and TCD as detectors, and a GS-Alumina capilar column and Chromosorb 102 packed column respectively. 3. RESULTS AND DISCUSSION
The preservation of the crystalline structure of the samples after impregnation and calcination process was checked analysing its X-Ray difraction patterns (not shown). The same peaks as those for ZLK were obtained, although the relative intensities of these varied slightly. These results may be due to a decrease of crystallinity after impregnation and calcination process and/or a lower crystallographic planes periodicity due to the presence of Fe oxides inside the zeolite channels. The specific surface area of the precursors is of 33 m2/g for p-Fe/ZLK(a) and 45m2/g for p-Fe/ZLK(v). The very important decrease in specific surface area in comparison with ZLK would indicate that a great fraction of iron species are located inside the zeolite channels reducing the pore mouth sizes. The M6ssbauer spectra ofp-Fe/ZLK(a) and p-Fe/ZLK(v) at 298 and 15 K are shown in Figure 1.
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Figure 1" M6ssbauer spectra of the precursors at 298 and 15 K.
At 298 K a paramagnetic doublet is observed in both precursors, while an additional magnetic sextet is also displayed by p-Fe/ZLK(a). When the temperature is lowered to 15 K, a second magnetic signal is observed in p-Fe/ZLK(v). Instead, in p-Fe/ZLK(a) only two resolved signals are still noticed, but the background is significantly curved. The spectrum at 298 K of p-Fe/ZLK(v) have hyperfme parameters (Table 1) that can be assigned to two Fe 3+ species: small particles of (x-Fe203 and/or Fe 3+ ions exchanged with the support. When the temperature decreased to 15 K, it was possible to determine the existence of two magnetic signals assignable to the "core" (sextet with higher magnetic field) and "shell" (sextet with smaller magnetic field) of (~-Fe203 "clusters" [7]. Assuming homogeneous semi-spherical particles and using the ratio of the areas of the two sextuplets, it was possible to estimate an average "cluster" diameter of 1.1 nm. Therefore, these "clusters" could be located inside the channels of the zeolite L. This result, analyzed in connection with the DRX andBET results, mentioned above, confima the existence of a great quantity of iron oxide microcrystals situated inside the zeolite structure.
528 Table 1" M/3ssbauer hyperfine parameters of the precursors. Temp. 298 K
Specie s c~-Fe203
Parameters .... p-Fe/ZLK(v) p-Fe/ZLK(a) H(T) 51.2 + 0.1 8(mm/s) 0.38 + 0.02 2e(mm/s) ~ -0.24 + 0.03 Fe 3+'' 8(mm/s) 0.34~:0.01 0'.'32 _+0.0i ....... A(mm/s). . . . . . . . . . 0.90-a:0.0! 0.87 + 0.01 15 K a-Fe203 H(T) 49.3+0.1 53.7 + 0.1 ~5(mm/s) 0.50• 0.45 + 0.02 2e(mm/s) . . . . -0.06• .... 0.36 + 0,,.03 ot-Fe203 H(T) 46.3+0.1 46.3" 5(mm/s) 0.46• 0.47* 2e (mm/s) ...... -0.03+0.02 -0.01 * Fe 3§ ~5(mm/s) 0.46• 014i~ 0.01 ................... A(mm/s) 1.03• !.0! + 0.01 *Constant used for the fit. The remaining doublet was assigned to Fe 3+ exchanged with the support, and/or superparamagnetie ct-Fe203 particles. The hyperfme parameters (Table 1) at 298 K of p-Fe/ZLK(a) can be assigned to the same iron species than in p-Fe/ZLK(v). When the temperature decreased to 15 K, the spectrum displays a curved background probably originated in a fraction of small particles undergoing an incomplete magnetic splitting. The fitting was simulated with one sextet, one doublet and a second sextet of very broad lines [3]. The relative area of the sharper sextet corresponding to ct-Fe203 (11+_2%) is the same (within experimental errors) at RT and at 15 K. To estimate roughly the average size for this fraction, we applied the Collective Magnetic Excitation Model (CMEM) [8]. A diameter of 20 nm is obtained. This value indicates that these particles must be located out of the channels of the zeolite. Although the fitting procedure is a rough approximation to the physical process actually taking place, the method yields an estimate of the fraction of the particles in the relaxing magnetic regime (55+7 %). Since these particles at 15 K have not reached the degree of magnetic order of the p-Fe/ZLK(v) particles, their size must be even smaller than 1.1 nm. Figure 2 shows the M6ssbauer spectra in controlled H2 atmosphere of both catalysts c-Fe/ZLK(v) and c-Fe/ZLK(a) at 298 and 15 K. At RT both display a magnetic sextet and several intense and highly overlapped central signals. The spectra were interpreted in terms of a superposition of one magnetic sextet, one paramagnetir doublet and one singlet. In addition to the above mentioned signals, other doublet appears in c-Fe/ZLK(v). When the temperature decreases to 15 K the spectra show the presence of two magnetic sextets, a paramagnetic doublet and a superparamagnetic singlet for both solids, and c-Fe/ZLK(v) displays an additional magnetic sextet. The values of the hyperfme parameters at 298 and 15 K are shown in Table 2. At 15 K, the values are characteristic of magnetic Fe ~ (Fe~ Fe304, Fe 2+ exchanged with the support and superparamagnetir Fe ~ (Fe~ [9, 10]. The additional sextet in c-Fe/ZLK(v) is assigned to Fe 2+ ions considering its isomer shift value. Since, we found the Fe 2+ signal magnetically splitted and its hyperfine magnetic field is very similar to the ct-Fe ~ value, we
529 think that this species is magnetically coupled with Fe~ Therefore, the Fe 2+ would be decorating the F e ~ [9]. The weak magnetic signal, assignable to Fe304, that can be seen in the MS at 15 K, can be attributed to the incomplete reduction of the oxides. Its quantity is too small to sort out one more interaction in the spectrum fitted at RT from the statistical noise. The Fe~ existence at so low temperature such as 15 K indicates the presence of very small particles of Fe*, at least smaller than ~2.9nm [11] in both catalysts. The presence of the Fe~ fraction aRer a reduction treatment suggests that these microcrystals are located inside the channels of the zeolite since this situation would avoid the sintering process. The same amount of Fe ~ inside the support (Table 3) is achieved in both catalysts, although in p-Fe/ZLK(v) all the iron oxide is inside the channels and in p-Fe/ZLK(a) there is a fraction of iron oxide out of the zeolite structure. Therefore, in the former sample a percentage of the iron crystallites have migrated to the external surface, during the reduction step.
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Figure 2: M6ssbauer spectra of the catalysts at 298 and 15 K.
530 Table 2: Hyperfine M6ssbauer parameters of c-Fe/ZLK(v) and c-Fe/ZLK(a) ~
Species Fe ~ (magnetic)
Parameters
...... c-Fe/ZLK(v) ....... c,Fe/ZLK(a ) 298 K 15 K 298 K 15 K H (T) 33.2-~0.1 34.2+0.1 33.0+0.1 34.1_+0.1 8(Fe) (mm/s) 0.00+0.01 0.12+0.01 0.01+0.01 0.11+0.01 2e ( ~ s ) -0.01+0.01 -0.01a:0.01 0.00" 0.00" H (T) 49.5+0.6 48.6+0.7 8(Fe) (mm/s) 0.58+0.08 0.77+0.09 .
FesO4 (magnetic)
(mnVs) Fe 2+ (m) (Coupled with Fe~ (m)) Fe 2+ (exchanged) Fe 2+
H (T) 8(Fe) (mm/s) 2e (mm/s) A (mm/s) 8(Fe) (mm/s) A (mm/s) 8(Fe) (mm/s) LVe0sp 8(Fe) (mm/s) *Constant used for the fit
o.oo*
1.59+0.03 1.18+0.01 0.'44+0.04 1.15+0.02 0.04+0.01
35.1~0.2 1.35+0.03 0.04+0.06 2.06+0.03 1.26+0.02 ....... 0112•
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There is a good agreement between the experimental 02 uptake for the complete reoxidation of the reduced catalysts and the consumption of 02 calculated from the percentage of each species obtained fi'om the M6ssbauer spectra at 15 K (Table 4). The cross-checking of volumetric oxidation results with the complex MOssbauer spectra of these catalysts is the only reliable method that should be used in spectra of such complexity if one has not the capability to take "in situ" spectra of samples with an external magnetic field. Other choices for assignments of the iron species, different from those of Table 2, lead to unacceptable differences between both techniques. From these results, it can be deduced that although we obtained two precursors with the same iron oxide species, the catalysts have different iron species aRer reduction. The number of Fe ~ surface atoms, was determined by 1-12chemisorption at 673 K in 10-80 Torr pressure range. The high H2 consumption observed allowed us to verify the existence of very small metallic Fe crystals inside the channels [10]. Assuming that hydrogen atoms are chemisorbed only on Fe ~ surface atoms and considering that there are two fractions of Fe ~ crystals [12], it is possible to estimate their average diameter values. Assuming a semi-spherical shape, the fraction of microcrystals located inside the zeolite structure cannot exceed 2.6 nm diameter considering the dimension of the channels. This fraction is superparamagnetic (Fe~ in the M6ssbauer spectra of both Table 3" Percentages of iron species obtained by MSssbaucr spectroscopy at 15K
c-Fe/ZLK(v) c-Fe/ZLK(a)
Fe ~ FesO4 ,,(magnetic) (magnetic) 30-A:2 5q-1 45+3 13+5
Species (%) Fe 2+(Coupled with Fe~ 13+1 ---
Fe 2+ (exchanged) 38• 27+6
Fe~ 144-1 15+4
531 Table 4: Values of O2 uptake, H2 chernisorption and Fe~ crystal diameter of catalyts.
c-Fe/ZLK(v) e-Fe/ZLK(a)
Experimental 02 uptake (~tmol O4g) 496:525 521:556
Theoretical O~ uptake (txmol O2/g) 442:514 553:542
Experimental Fe~ Diameter H2 ehemisorption (nm)
(/.tmol H2/g) 31 62
>_14.3 6.0-13.0
samples, and their percentages were obtained from these spectra. The more important structural difference between both catalysts is the presence of Fe 2§ decorating the extemal Fe ~ surface crystals in c-Fe/ZLK(v). The existence of this fraction of Fe 2§ leads to a decrease of the Fe~ and Fe304 amounts. On the other hand, the quantity of Fe ~ internal crystals (that represents the 80% of the total active sites) is the same in both catalysts. The activity and selectivity results are shown in Table 5. The tumover frequencies to total hydrocarbons of both catalysts were obtained assuming one active site per Fe~ surface atom. In the pseudo-steady state, the activity per site is about three times higher in cFe/ZLK(v) than in c-Fe/ZLK(a), and the activity per gram is twice higher in e-Fe/ZLK(v) than in c-Fe/ZLK(a). Since the inner Fe~ fraction is equal in both catalysts, the activity difference between them cannot be assigned to this fraction. In consequence, the different behaviour of the catalysts may be attributed to the presence of Fe 2+ decorating the Fe~ surface of crystals located outside the channels in fresh c-Fe/ZLK(v). Theoretical models have demonstrated that the main effect of cations in contact with a metal is an electrostatic one [ 13], which is essentially of short range. However, a long range effect is possible as a result of a cumulative electrostatic field, generating zones of minimum potential energy at the surface. Consequently, the bond between the Fe ~ and the CO adsorbed becomes stronger, while at the same time, the intra-moleeular CO bond is weakened, increasing the catalyst activity. After 48hs of reaction the Fe~ is carburized in both catalyst and in c-Fe/ZLK(v) the sextet of Fe2+ appears magnetically coupled with z-FesC2 maintaining the promoter effect of this species. This behaviour was demonstrated by MS in controlled atmosphere (not shown spectra). The olefirgparaffm ratio is similar for both catalysts. These results can be justified taking into account that the conversion values and the support basicity are practically equal in both samples Finally, similar methane production and Schulz-Flory coefficients ((z) are observed in both catalysts. Therefore, it can be deduced that Fe 2+ ions magnetically coupled with Fe~ Table 5" Activity and selectivity results
Total hydrocarbon molecules/site.see xl 04 Total hydrocarbon moleeules/g.sec xl 0"~6 CO conversion (%) Olefin/paraffln ratio(without CI-I4) cn4 (%) Schulz-Flory coefficient (a)
c-Fe/ZLK(v) 8.90 3.32 1.7 1.40 38 0.22
c-Fe/ZLK(a) 2.60 1.94 1.5 2.34 42 0.30
532 do not influence on the catalysts selectivity and chain growing. Bearing in mind that the 80% of the total active sites correspond to Fe~~, the low ot coefficient values can be justified taking into account that on very small metallic particles, the chain propagation finishes at low molecular weight hydrocarbon (up to C4) [14]. The small metallic crystal size avoids the presence of enough CHx neighbour groups to produce the propagation chain, although steric impediments inside the pores of the zeolite do not be ruled out. 4. CONCLUSIONS Through two different impregnation methods we obtained two catalysts with only one structural difference: the Fe2§ ions magnetically coupled with Fe located outside the channels of the zeolite ZLK. These ions enhanced the activity of the Fe~ sites for the total hydrocarbon production by an electrostatic effect favouring the CO dissociation. Instead, the selectivity and chain growth is not modified by the presence of these ions. REFERENCES
1. G.B. Raupp and W.N. Delgass, J.Catal., 58 (1979) 337. 2. M.V. Cagnoli, S.G. Marchetti, N.G. Gallegos, A.M. Alvarez, R.C. Mercader and A.A. Yerami~, J. Catal, 123 (1990) 21. 3. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramihn and 1LC. Mercader, Hyperfme Interactions (C), 4 (1999) 61. 4. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramiatt and M Schmal, Actas do XVII Simp6sio Ibero-americano do Cat~ilise (J.M.0rfiio, J.L. Faria, J.L. Figueiredo, Eds.), p.97, Porto, Portugal (2000). 5. M. Boudart, A. Delbouille, J.A. Dumesic, S. Khamrnouma and H. Topsoe, J.Catal. 37 (1975) 486. 6. S. G. Marchetti, J. F. Bengoa, M. V. Cagnoli, A. M. Alvarez, N. G. Gallegos, A. A. Yerami~n and R. C. Mercader, Meas. Sci. Tech. 7 (1996) 758. 7. M.Vasquez-Mansilla, R.D. Zysler, C. Arciprete, M.I. Dimitrijewits, C. Saragovi, J.M. Greneche, J. of Magnetism and Magnetic Materials, 204 (1999) 29. 8. S. Morup and H. Topsae, Appl. Phys. 11 (1976) 63. 9. M.V. Cagnoli, N.G. Gallegos, A.M. Alvarez, J.F. Bengoa, A.A. Yeramihn and S.G. Marchetti, Studies in Surface Science and Catalysis, 135 (2001) 272. 10. A.M. Alvarez, S.G. Marchetti, M.V. Cagnoli, J.F. Bengoa, R.C. Mercader and A.A. Yerami~a., Applied Surface Science, 165 (2000) 100. 11. F. Bodker, S. Morup, M.S. Pedersen, P. Svedlindh, G.T. Jonsson, J.L. Garcia-Palacios and F.J. Lazaro, J. Magn. Magn. Mater., 925 (1998) 177. 12. S.G. Marchetti, M.V. Cagnoli, A.M. Alvarez, J.F. Bengoa, R.C. Mercader and A.A. Y~eramihrg Appl. Surf. Sei., 165 (2000) 91. 13. J.W. Niemantsverdriet, "Spectroscopy in Catalysis" VCH, Weinheim, (1995), p. 219. 14. L. Guczi, in "New Trends in CO Activation", Studies in Surface Science and Catalysis, Ed. L. Guezi, Elsevier, 64 (1991) 350.
525
Fischer-Tropsch synthesis. Influence of the presence of intermediate iron reduction species in Fe/Zeolite L catalysts. N.G. GaUegos, M.V. Cagnoli, J.F. Bengoa, A.M. Alvarez, A.A. Yeramihn and S.G. Marchetti
CINDECA, Fac. Cs. Exactas, Fac. Ingenieria, U.N.L.P., CIC, CONICET. Calle 47 N ~ 257 (1900) La Plata, Argentina. Two catalyst to be used in the Fischer-Tropsch reaction, using zeolite-L in potassic form as support of iron species were prepared through to different methods of impregnation with iron salt. X-Ray Diffraction (XRD), Specific Surface Area (BET), M6ssbauer Spectroscopy (MS) in controlled atmosphere, between room temperature (RT) and 15 K, H2 chemisorption and Volumetric Oxidation (VO) were used to characterise the solids. The impregnation of the zeolite L under inert gas allowed to obtain a fraction ofFe ~ in contact with Fe 2+ ions that enhanced the activity of the sites. The two catalysts presented similar selectivity towards hydrocarbons and low chain growth. 1. INTRODUCTION It is well known that in iron catalysts supported on different solids such as A1203, SiO2, MgO and zeolites, it is not possible to obtain complete reduction to Fe ~ when the iron concentration is low (approximately < 10%w/w) [1, 2]. Therefore, most of the iron catalysts used in the Fischer-Tropsch synthesis have, in addition to Fe ~ intermediate iron reduction species like Fe 2+. However, the role of Fe 2§ on the activity and selectivity in the CO hydrogenation has not been studied yet. In this paper, a commercial Zeolite L in potassic form (ZLK) was used as metal support for CO hydrogenation. The choice of this support was carried out since this reaction is sensitive to the structure. This means that the activity and selectivity of the catalyst depend on its metallic crystal size [3]. Catalysts with a narrow size distribution lead to good selectivity towards to a desirable product. Making use of the structure of channels and cages ofzeolites it is possible to reach this purpose. In order to determine the influence of the intermediate iron reduction species on the activity and selectivity of the Fischer-Tropsch reaction, two Fe/ZLK catalysts were prepared by two different methods.
2. EXPERIMENTAL SECTION Two precursors were prepared using the commercial form of the ZLK (Tosoh Corp.), with the ideal unit cell composition of dehydrated form of K9A195i27072 and 290 m~/g of specific surface area. One of them, was obtained by dry impregnation in air of the
526 zeolite with aqueous solution (pH=0.5) of a concentration to yield a solid with 5.84% w/w ofFe. Then it was calcined following the programme described in [4]. This sample was called p-Fe/ZLK(a). The other precursor was obtained outgassing the support at 773K and 0.05 torr for 1 h to eliminate the water present inside the channels and cages of the zeolite. After this time the system was filled with ultra high purity He up to 500 torr. Then, the Fe(NO3)3.9HzO aqueous volume solution equal to the pore volume of the zeolite was added to yield a solid with 4.56% w/w of iron that was calcined in the same way that p-Fe/ZLK(a).This solid was called p-Fe/ZLK(v). Both precursors were reduced in H2 stream (60 cm3/min) from 298 to 698 K at 2.66 ~ and were kept at 698 K during 26 h. The resulting solids were named c-Fe/ZLK(a) and c-Fe/ZLK(v), and characterised by XRay Diffraction (XRD), Specific Surface Area (BET), M6ssbauer Spectroscopy (MS) at 298 and 15 K, 1-12chemisorption and volumetric oxidation (VO). These last two techniques were performed in a conventional static volumetric equipment with grease-free vacuum valves. The Hz uptakes at the same initial pressure, but at different temperatures between RT and 673 K were measured to determine the temperature in which the adsorption capacity is maximum. Volumetric oxidation experiments are based on the conversion of all iron species in the sample to Fe203 when it was heated in an O~ atmosphere at temperatures higher than 620 K [5]. The M6ssbauer spectra were obtained in transmission geometry with a 512-channel constant acceleration spectrometer. A source of 57Co in Rh matrix of nominally 100mCi was used. Velocity calibration was performed against a 6-1am-thick ~-Fe foil. All isomer shifts (6) mentioned in this paper are referred to this standard. The temperature between 15 and 298 K was varied using a Displex DE-202 Closed Cycle Cryogenic System. All MSssbauer spectra of the catalyst were obtained in controlled atmosphere using a cell specially built for this purpose to be used inside the cryogen [6]. The spectra were evaluated using a least-squares nonlinear computer fitting program with constraint. Lorentzian lines were considered for each spectra components. The catalytic tests were carried out in a fixed bed reactor with a H~:CO ratio of 3:1, 543 K, 1 atm of total pressure, 20 cm3/min of total volumetric flow and a space rate of 0.25 s"l. The reaction products were analyzed by gas chromatography using FID and TCD as detectors, and a GS-Alumina capilar column and Chromosorb 102 packed column respectively. 3. RESULTS AND DISCUSSION
The preservation of the crystalline structure of the samples after impregnation and calcination process was checked analysing its X-Ray difraction patterns (not shown). The same peaks as those for ZLK were obtained, although the relative intensities of these varied slightly. These results may be due to a decrease of crystallinity after impregnation and calcination process and/or a lower crystallographic planes periodicity due to the presence of Fe oxides inside the zeolite channels. The specific surface area of the precursors is of 33 m2/g for p-Fe/ZLK(a) and 45m2/g for p-Fe/ZLK(v). The very important decrease in specific surface area in comparison with ZLK would indicate that a great fraction of iron species are located inside the zeolite channels reducing the pore mouth sizes. The M6ssbauer spectra ofp-Fe/ZLK(a) and p-Fe/ZLK(v) at 298 and 15 K are shown in Figure 1.
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Velocity (mm/s)
Figure 1" M6ssbauer spectra of the precursors at 298 and 15 K.
At 298 K a paramagnetic doublet is observed in both precursors, while an additional magnetic sextet is also displayed by p-Fe/ZLK(a). When the temperature is lowered to 15 K, a second magnetic signal is observed in p-Fe/ZLK(v). Instead, in p-Fe/ZLK(a) only two resolved signals are still noticed, but the background is significantly curved. The spectrum at 298 K of p-Fe/ZLK(v) have hyperfme parameters (Table 1) that can be assigned to two Fe 3+ species: small particles of (x-Fe203 and/or Fe 3+ ions exchanged with the support. When the temperature decreased to 15 K, it was possible to determine the existence of two magnetic signals assignable to the "core" (sextet with higher magnetic field) and "shell" (sextet with smaller magnetic field) of (~-Fe203 "clusters" [7]. Assuming homogeneous semi-spherical particles and using the ratio of the areas of the two sextuplets, it was possible to estimate an average "cluster" diameter of 1.1 nm. Therefore, these "clusters" could be located inside the channels of the zeolite L. This result, analyzed in connection with the DRX andBET results, mentioned above, confima the existence of a great quantity of iron oxide microcrystals situated inside the zeolite structure.
528 Table 1" M/3ssbauer hyperfine parameters of the precursors. Temp. 298 K
Specie s c~-Fe203
Parameters .... p-Fe/ZLK(v) p-Fe/ZLK(a) H(T) 51.2 + 0.1 8(mm/s) 0.38 + 0.02 2e(mm/s) ~ -0.24 + 0.03 Fe 3+'' 8(mm/s) 0.34~:0.01 0'.'32 _+0.0i ....... A(mm/s). . . . . . . . . . 0.90-a:0.0! 0.87 + 0.01 15 K a-Fe203 H(T) 49.3+0.1 53.7 + 0.1 ~5(mm/s) 0.50• 0.45 + 0.02 2e(mm/s) . . . . -0.06• .... 0.36 + 0,,.03 ot-Fe203 H(T) 46.3+0.1 46.3" 5(mm/s) 0.46• 0.47* 2e (mm/s) ...... -0.03+0.02 -0.01 * Fe 3§ ~5(mm/s) 0.46• 014i~ 0.01 ................... A(mm/s) 1.03• !.0! + 0.01 *Constant used for the fit. The remaining doublet was assigned to Fe 3+ exchanged with the support, and/or superparamagnetie ct-Fe203 particles. The hyperfme parameters (Table 1) at 298 K of p-Fe/ZLK(a) can be assigned to the same iron species than in p-Fe/ZLK(v). When the temperature decreased to 15 K, the spectrum displays a curved background probably originated in a fraction of small particles undergoing an incomplete magnetic splitting. The fitting was simulated with one sextet, one doublet and a second sextet of very broad lines [3]. The relative area of the sharper sextet corresponding to ct-Fe203 (11+_2%) is the same (within experimental errors) at RT and at 15 K. To estimate roughly the average size for this fraction, we applied the Collective Magnetic Excitation Model (CMEM) [8]. A diameter of 20 nm is obtained. This value indicates that these particles must be located out of the channels of the zeolite. Although the fitting procedure is a rough approximation to the physical process actually taking place, the method yields an estimate of the fraction of the particles in the relaxing magnetic regime (55+7 %). Since these particles at 15 K have not reached the degree of magnetic order of the p-Fe/ZLK(v) particles, their size must be even smaller than 1.1 nm. Figure 2 shows the M6ssbauer spectra in controlled H2 atmosphere of both catalysts c-Fe/ZLK(v) and c-Fe/ZLK(a) at 298 and 15 K. At RT both display a magnetic sextet and several intense and highly overlapped central signals. The spectra were interpreted in terms of a superposition of one magnetic sextet, one paramagnetir doublet and one singlet. In addition to the above mentioned signals, other doublet appears in c-Fe/ZLK(v). When the temperature decreases to 15 K the spectra show the presence of two magnetic sextets, a paramagnetic doublet and a superparamagnetic singlet for both solids, and c-Fe/ZLK(v) displays an additional magnetic sextet. The values of the hyperfme parameters at 298 and 15 K are shown in Table 2. At 15 K, the values are characteristic of magnetic Fe ~ (Fe~ Fe304, Fe 2+ exchanged with the support and superparamagnetir Fe ~ (Fe~ [9, 10]. The additional sextet in c-Fe/ZLK(v) is assigned to Fe 2+ ions considering its isomer shift value. Since, we found the Fe 2+ signal magnetically splitted and its hyperfine magnetic field is very similar to the ct-Fe ~ value, we
529 think that this species is magnetically coupled with Fe~ Therefore, the Fe 2+ would be decorating the F e ~ [9]. The weak magnetic signal, assignable to Fe304, that can be seen in the MS at 15 K, can be attributed to the incomplete reduction of the oxides. Its quantity is too small to sort out one more interaction in the spectrum fitted at RT from the statistical noise. The Fe~ existence at so low temperature such as 15 K indicates the presence of very small particles of Fe*, at least smaller than ~2.9nm [11] in both catalysts. The presence of the Fe~ fraction aRer a reduction treatment suggests that these microcrystals are located inside the channels of the zeolite since this situation would avoid the sintering process. The same amount of Fe ~ inside the support (Table 3) is achieved in both catalysts, although in p-Fe/ZLK(v) all the iron oxide is inside the channels and in p-Fe/ZLK(a) there is a fraction of iron oxide out of the zeolite structure. Therefore, in the former sample a percentage of the iron crystallites have migrated to the external surface, during the reduction step.
'"i
'~
'
I
9 .,:.
.......
I
I
I
I
I
..': :~,. ,%
=. (O cO u)
E r r
0~ L
p.
c-FelZLK(v) 298 K
c-FelZLK(a) 298 K
"""
~'~
~. v
.,,"
; ; z ", ~ -
~ -. . .
-
c-FelZLKla) 15K
c-FelZLK(v) 15K
II I
t
I
I
I
I
!
-12
-8
-4
0
4
8
Velocity (mmls)
I
.I
12 -12
!
I
!
L.
1
!
-8
-4
0
4
8
12
Velocity (mmls)
Figure 2: M6ssbauer spectra of the catalysts at 298 and 15 K.
530 Table 2: Hyperfine M6ssbauer parameters of c-Fe/ZLK(v) and c-Fe/ZLK(a) ~
Species Fe ~ (magnetic)
Parameters
...... c-Fe/ZLK(v) ....... c,Fe/ZLK(a ) 298 K 15 K 298 K 15 K H (T) 33.2-~0.1 34.2+0.1 33.0+0.1 34.1_+0.1 8(Fe) (mm/s) 0.00+0.01 0.12+0.01 0.01+0.01 0.11+0.01 2e ( ~ s ) -0.01+0.01 -0.01a:0.01 0.00" 0.00" H (T) 49.5+0.6 48.6+0.7 8(Fe) (mm/s) 0.58+0.08 0.77+0.09 .
FesO4 (magnetic)
(mnVs) Fe 2+ (m) (Coupled with Fe~ (m)) Fe 2+ (exchanged) Fe 2+
H (T) 8(Fe) (mm/s) 2e (mm/s) A (mm/s) 8(Fe) (mm/s) A (mm/s) 8(Fe) (mm/s) LVe0sp 8(Fe) (mm/s) *Constant used for the fit
o.oo*
1.59+0.03 1.18+0.01 0.'44+0.04 1.15+0.02 0.04+0.01
35.1~0.2 1.35+0.03 0.04+0.06 2.06+0.03 1.26+0.02 ....... 0112•
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
1.36~0.07 1.23+0.04
..... '0.03•
.
.
.
o.oo* .
.
1.82+0.08 1.39_+0.04
0.02+0.05
There is a good agreement between the experimental 02 uptake for the complete reoxidation of the reduced catalysts and the consumption of 02 calculated from the percentage of each species obtained fi'om the M6ssbauer spectra at 15 K (Table 4). The cross-checking of volumetric oxidation results with the complex MOssbauer spectra of these catalysts is the only reliable method that should be used in spectra of such complexity if one has not the capability to take "in situ" spectra of samples with an external magnetic field. Other choices for assignments of the iron species, different from those of Table 2, lead to unacceptable differences between both techniques. From these results, it can be deduced that although we obtained two precursors with the same iron oxide species, the catalysts have different iron species aRer reduction. The number of Fe ~ surface atoms, was determined by 1-12chemisorption at 673 K in 10-80 Torr pressure range. The high H2 consumption observed allowed us to verify the existence of very small metallic Fe crystals inside the channels [10]. Assuming that hydrogen atoms are chemisorbed only on Fe ~ surface atoms and considering that there are two fractions of Fe ~ crystals [12], it is possible to estimate their average diameter values. Assuming a semi-spherical shape, the fraction of microcrystals located inside the zeolite structure cannot exceed 2.6 nm diameter considering the dimension of the channels. This fraction is superparamagnetic (Fe~ in the M6ssbauer spectra of both Table 3" Percentages of iron species obtained by MSssbaucr spectroscopy at 15K
c-Fe/ZLK(v) c-Fe/ZLK(a)
Fe ~ FesO4 ,,(magnetic) (magnetic) 30-A:2 5q-1 45+3 13+5
Species (%) Fe 2+(Coupled with Fe~ 13+1 ---
Fe 2+ (exchanged) 38• 27+6
Fe~ 144-1 15+4
531 Table 4: Values of O2 uptake, H2 chernisorption and Fe~ crystal diameter of catalyts.
c-Fe/ZLK(v) e-Fe/ZLK(a)
Experimental 02 uptake (~tmol O4g) 496:525 521:556
Theoretical O~ uptake (txmol O2/g) 442:514 553:542
Experimental Fe~ Diameter H2 ehemisorption (nm)
(/.tmol H2/g) 31 62
>_14.3 6.0-13.0
samples, and their percentages were obtained from these spectra. The more important structural difference between both catalysts is the presence of Fe 2§ decorating the extemal Fe ~ surface crystals in c-Fe/ZLK(v). The existence of this fraction of Fe 2§ leads to a decrease of the Fe~ and Fe304 amounts. On the other hand, the quantity of Fe ~ internal crystals (that represents the 80% of the total active sites) is the same in both catalysts. The activity and selectivity results are shown in Table 5. The tumover frequencies to total hydrocarbons of both catalysts were obtained assuming one active site per Fe~ surface atom. In the pseudo-steady state, the activity per site is about three times higher in cFe/ZLK(v) than in c-Fe/ZLK(a), and the activity per gram is twice higher in e-Fe/ZLK(v) than in c-Fe/ZLK(a). Since the inner Fe~ fraction is equal in both catalysts, the activity difference between them cannot be assigned to this fraction. In consequence, the different behaviour of the catalysts may be attributed to the presence of Fe 2+ decorating the Fe~ surface of crystals located outside the channels in fresh c-Fe/ZLK(v). Theoretical models have demonstrated that the main effect of cations in contact with a metal is an electrostatic one [ 13], which is essentially of short range. However, a long range effect is possible as a result of a cumulative electrostatic field, generating zones of minimum potential energy at the surface. Consequently, the bond between the Fe ~ and the CO adsorbed becomes stronger, while at the same time, the intra-moleeular CO bond is weakened, increasing the catalyst activity. After 48hs of reaction the Fe~ is carburized in both catalyst and in c-Fe/ZLK(v) the sextet of Fe2+ appears magnetically coupled with z-FesC2 maintaining the promoter effect of this species. This behaviour was demonstrated by MS in controlled atmosphere (not shown spectra). The olefirgparaffm ratio is similar for both catalysts. These results can be justified taking into account that the conversion values and the support basicity are practically equal in both samples Finally, similar methane production and Schulz-Flory coefficients ((z) are observed in both catalysts. Therefore, it can be deduced that Fe 2+ ions magnetically coupled with Fe~ Table 5" Activity and selectivity results
Total hydrocarbon molecules/site.see xl 04 Total hydrocarbon moleeules/g.sec xl 0"~6 CO conversion (%) Olefin/paraffln ratio(without CI-I4) cn4 (%) Schulz-Flory coefficient (a)
c-Fe/ZLK(v) 8.90 3.32 1.7 1.40 38 0.22
c-Fe/ZLK(a) 2.60 1.94 1.5 2.34 42 0.30
532 do not influence on the catalysts selectivity and chain growing. Bearing in mind that the 80% of the total active sites correspond to Fe~~, the low ot coefficient values can be justified taking into account that on very small metallic particles, the chain propagation finishes at low molecular weight hydrocarbon (up to C4) [14]. The small metallic crystal size avoids the presence of enough CHx neighbour groups to produce the propagation chain, although steric impediments inside the pores of the zeolite do not be ruled out. 4. CONCLUSIONS Through two different impregnation methods we obtained two catalysts with only one structural difference: the Fe2§ ions magnetically coupled with Fe located outside the channels of the zeolite ZLK. These ions enhanced the activity of the Fe~ sites for the total hydrocarbon production by an electrostatic effect favouring the CO dissociation. Instead, the selectivity and chain growth is not modified by the presence of these ions. REFERENCES
1. G.B. Raupp and W.N. Delgass, J.Catal., 58 (1979) 337. 2. M.V. Cagnoli, S.G. Marchetti, N.G. Gallegos, A.M. Alvarez, R.C. Mercader and A.A. Yerami~, J. Catal, 123 (1990) 21. 3. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramihn and 1LC. Mercader, Hyperfme Interactions (C), 4 (1999) 61. 4. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramiatt and M Schmal, Actas do XVII Simp6sio Ibero-americano do Cat~ilise (J.M.0rfiio, J.L. Faria, J.L. Figueiredo, Eds.), p.97, Porto, Portugal (2000). 5. M. Boudart, A. Delbouille, J.A. Dumesic, S. Khamrnouma and H. Topsoe, J.Catal. 37 (1975) 486. 6. S. G. Marchetti, J. F. Bengoa, M. V. Cagnoli, A. M. Alvarez, N. G. Gallegos, A. A. Yerami~n and R. C. Mercader, Meas. Sci. Tech. 7 (1996) 758. 7. M.Vasquez-Mansilla, R.D. Zysler, C. Arciprete, M.I. Dimitrijewits, C. Saragovi, J.M. Greneche, J. of Magnetism and Magnetic Materials, 204 (1999) 29. 8. S. Morup and H. Topsae, Appl. Phys. 11 (1976) 63. 9. M.V. Cagnoli, N.G. Gallegos, A.M. Alvarez, J.F. Bengoa, A.A. Yeramihn and S.G. Marchetti, Studies in Surface Science and Catalysis, 135 (2001) 272. 10. A.M. Alvarez, S.G. Marchetti, M.V. Cagnoli, J.F. Bengoa, R.C. Mercader and A.A. Yerami~a., Applied Surface Science, 165 (2000) 100. 11. F. Bodker, S. Morup, M.S. Pedersen, P. Svedlindh, G.T. Jonsson, J.L. Garcia-Palacios and F.J. Lazaro, J. Magn. Magn. Mater., 925 (1998) 177. 12. S.G. Marchetti, M.V. Cagnoli, A.M. Alvarez, J.F. Bengoa, R.C. Mercader and A.A. Y~eramihrg Appl. Surf. Sei., 165 (2000) 91. 13. J.W. Niemantsverdriet, "Spectroscopy in Catalysis" VCH, Weinheim, (1995), p. 219. 14. L. Guczi, in "New Trends in CO Activation", Studies in Surface Science and Catalysis, Ed. L. Guezi, Elsevier, 64 (1991) 350.