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Quantitative Comparison of Supported Cobalt and Iron Fischer Tropsch Synthesis Catalysts
Studies in Surface Science and Catalysis J.J. Spivey, E. Iglesiaand T.H. Fleisch (Editors) 9 2001 Elsevier ScienceB.V. All rights reserved.
513
Quantitative Comparison of Supported Cobalt and Iron Fischer Tropsch Synthesis Catalysts Roberto Zennaro a, Gianni Pederzani a, Sonia Morselli a , Shiyong Cheng b and Calvin H. Bartholomew b aEniTecnologie S.p.A., Via F. Maritano 26, 20097 San Donato Milanese (Milano) Italy bBYU Catalysis Lab, Department of Chemical Engineering, Brigham Young University, Provo, UT 84602, USA
Abstract A collaborative study between two laboratories was undertaken to determine FischerTropsch activity and selectivity properties of representative cobalt and iron catalysts under conditions relevant to natural-gas to liquids conversion. In FTS activity/selectivity tests of Co/A1203 and Fe/K/Cu supported on silica, conducted under comparable operating conditions, i.e., H2/CO = 2, Ptot = 20 bar, GHSV = 2.1 NL/gcat/h and 50-70% CO conversion, 20-25% higher catalyst-mass-based CO conversion activity is observed for Co/A1203 relative to Fe/K/Cu/Si. Site-based rates (i.e., turnover frequencies based on H2 chemisorption data) are two times higher for Co/A1203 relative to Fe/K/Cu/Si. C5+ selectivity for Co/A1203 is 50-60% higher due to its higher propagation probability and its negligible CO2 selectivity; its C2+ productivity is also 50-70% higher because of its higher activity and higher selectivity to C2+ hydrocarbons. Activity and selectivity data obtained in the laboratories of Brigham Young University and EniTecnologie using micro and bench scale fixed bed reactors are in the case of Co/A1203 in excellent agreement and in the case of Fe/K/Cu/Si in fairly good agreement. 1. INTRODUCTION Wax-crack Fischer-Tropsch synthesis (FTS) is an economically promising, developing technology for conversion of natural gas to middle distillate hydrocarbons. Both cobalt [ 1-8] and iron [6,7,9] catalysts are active and selective for FTS and have found previous large-scale application: supported cobalt in the production of middle-distillates from natural gas by Shell in Malaysia during the last decade and precipitated iron in the conversion of coal synthesis gas to gasoline and chemicals for the past five decades at Sasol, South Africa. Because of its higher CO conversion activity and hydrocarbon selectivity, high activity maintenance, and potential regenerability, supported cobalt is presently the leading FTS catalyst technology [5]; nevertheless, iron catalysts can reportedly achieve higher productivities at higher space velocities (lower conversions) relative to cobalt catalysts [6,7] and might find application in multi-reactor process designs or as a throw-away catalyst because of their significantly lower cost. An important factor driving the choice of cobalt or iron catalyst for natural gas-to-liquid (GTL) conversion is the technology of the syngas production process which in turn determines H2/CO ratio. Commercially available technologies produce syngas from methane with a H2/CO ratio of 1.7 to 3.8; however, Fe-catalysts, because of their WGS activity, perform best at H2/CO ratios of 0.6-1.5, while Co-based catalysts have optimum performance at higher HdCO ratios (1.5-2.2). Nevertheless it may be possible to operate economically with an iron catalyst or a combination of iron and cobalt catalysts at HdCO ratios of 1.7-2.0.
514 In previous comparisons of cobalt and iron catalysts [6,7], CO conversion and hydrocarbon selectivity data were obtained at a different H2/CO ratio for each catalyst type; moreover, in none of the previous literature are comparisons of CO conversion rate per gram catalyst or turnover frequency made under comparable high conversion conditions. Nor are comparisons published for well-characterized, supported cobalt and iron catalysts under conditions relevant to natural gas-to-liquids (GTL) conversion. The aim of this study was to compare activities and selectivities of commercially representative iron and cobalt catalysts under similar operating condition, i.e. at 20 bar, H2:CO = 2:1, and CO conversions of 50-70%. This study also included comparison of data obtained at Brigham Young University (BYU) and EniTecnologie (ET) laboratories in micro and bench scale fixed-bed reactor units. 2. EXPERIMENTAL
2.1. Catalyst Preparation A 14% Co catalyst supported on alumina (Co/AI) and a silica supported Fe/I~Cu catalyst (52% wt% Fe, 1.9% K, 2.6% Cu) were prepared at EniTecnologie using incipient wetness and precipitation techniques respectively.
2.2. Catalyst Characterization X-ray diffraction of calcined Co/AI showed the crystalline phase to contain 20.3 wt% of spinel phase with an average formula C02.8A10.204, while the alumina support (Condea) was found to be a mixture of gamma and delta phases. The calcined Fe/K/Cu/Si consisted mainly of well-crystallized hematite (t~-Fe203). BET surface areas and pore volumes of the fresh catalyst are reported in Table 1. M6ssbauer spectra of the Fe/K/Cu/Si, in calcined and actTable 1 Surface area andpore volumes for fresh catalysts . ivity-tested, passivated forms, were collected as previously desSample Surface area Pore volume (m2g"l) (cm3g"l) cribed [10]. Spectra of used catalysts were collected at both Alumina 174 0.52 295 K and 77 K to better resolve Co/AI 152 0.35 the more complex spectral patFe/K/Cu/Si 229 0.47 terns of the four or five iron phases present. Based on a match of M6ssbauer parameters for compounds reported in the literature, peaks from the best fit of each data set were assigned to corresponding iron species. Selective H2 chemisorption uptakes were measured by a flow desorption method using a custom flow system with a thermal conductivity detector (TCD) similar to that described by Jones and Bartholomew [11 ].
2.3 Activity tests FTS activity tests were conducted in two different fixed-bed reactor systems: (1) a bench scale reactor (ET) charged with 20 cm 3 catalyst, shaped into particles of 20-40 mesh, and diluted with quartz chips (1:2 by volume), the latter of which served to minimize the axial temperature gradient in the catalyst bed and control reactor temperature and (2) a micro scale reactor (BYU) charged with 2 g of iron catalyst (about 60-100 mesh) diluted with 6 g of quartz chips or with 1 g of cobalt catalyst diluted with 8 g of quartz chips. Co/A1 was activated in situ in H2 at 400~ and 2.0 NL/gcat/h for 16 h. The Fe/KJCu/Si catalyst was activated in situ in CO at 300~ and 1.0 NL/gcat/h for 16 h. The use of CO instead of H2 for precipitated Fe has been demonstrated [ 12] to improve the catalyst activity. Catalysts were tested for FTS activity and selectivity at 20 bar, 200-280~ H2/CO = 2, and GHSV = 3.75-10 NL/gcat/h (BYU) or GHSV = 2 NL/gcat/h (EniTecnologie).
515 3. RESULTS AND DISCUSSION Co supported on alumina is one of the most common FT catalyst formulations cited in the literature [1-5], while impregnation to incipient wetness is a common synthesis route for cobalt catalysts [1,3]. The alumina (Condea) used in this study had a particle size and mechanical characteristics suitable for slurry phase application [13]. Fe/K/Cu/Si was prepared according to ref. [9]. Potassium is an important promoter for iron catalysts, increasing both activity and the average molecular weight of products, while decreasing the formation of graphite on the catalyst surface [14,15]. The addition of copper facilitates iron reduction at lower temperatures and improves thermal stability, thereby maintaining higher active surface area. Silica is used as a binder to stabilize iron crystallite size and improve the mechanical strength of the catalyst. M6ssbauer parameters and iron phase distributions of Fe/K/Cu/Si (52 wt% Fe) in its calcined and passivated forms after activity testing are listed in Table 2. The spectrum of the calcined catalyst consists of a doublet of superparamagnetic o~-Fe203 (sp-tx-Fe203). After a 16 h pretreatment in CO at 300~ and subsequent FTS reaction conducted at ET, Fe203 was converted to oxides and carbides of lower oxidation state. The working catalyst (see Table 2) consists of a mixture of superparamagnetic Fe304 (30.1%), ferromagnetic Fe304 (13.9%; 4.9% in A-sites and 9.0% in B-sites), and Z-carbide or Fe2.sC (49.2%). The peak of low intensity at about 2.4 mm/s in the spectrum has an isomer shift and quadruple splitting parameters consistent with those of Fe2§ this peak accounts for 6.8% of iron. The estimated percentages of Fe304, Fe2.sC, and Fe E+ at 295 K of 44, 49, and 6.8% are in very good agreement with those at 77 K of 44, 47, and 7.8% respectively. Thus, these data provide strong evidence that Fe2.sC and Fe304 are the predominant phases in the working catalyst. HE chemisorption capacities for the fresh Co/Al and Fe/K/Cu/Si precursors after reduction for 16 h at 400 and 300~ were 52.7+6.6 and 81.4_+10.5 txmoles/g respectively; dispersions were 10.1_+1.3 and 3.5_+0.4. These data are based on extents of reduction of 43.3 and 50% obtained by temperature programmed reduction (TPR) and M6ssbauer spectroscopy. In this study activity/selectivity data were obtained on the same cobalt and iron catalysts under comparable conditions in two different laboratories fixed bed reactors. Activity and selectivity data obtained in a bench-scale, fixed-bed reactor (ET) are compared in Table 3 with those obtained in a fixed-bed microreactor (BYU); excellent agreement between the results obtained in the two laboratories in the fixed bed units of different size (by a factor of 10) is evident. The steady-state TOF from both labs for the Co/A1 catalyst at 200~ of 2.2 x 102 s"1 (extrapolated using an activation energy of 100 kJ/mol [ 16]) is the same within experimental error as those reported earlier [5,14,16] for Co catalysts under the same conditions. This validates the comparison of rates between the micro and bench scale reactors for cobalt catalysts. For the Fe catalyst rates/g obtained in the micro reactor were 25-60% higher, while TOF value were only 10-25% higher, i.e., almost the same within experimental error. While comparison of the TOF values between Fe catalysts of this study may provide a useful relative measure of site activity, a similar comparison between Fe and Co catalysts is questionable, since the iron metal sites were measured by HE chemisorption on iron catalysts reduced in H2, while the active catalytic phase is presumably an iron carbide. On the other hand, comparisons of the TOF values based on H2 chemisorption among cobalt and ruthenium catalysts reduced in H2 is warranted, since (1) the reduced metal is known to be the active phase, (2) site density- activity correlations have been shown to be linear [4,5,14,16], and (3) excellent agreement has been observed for TOF values obtained in a number of laboratories for cobalt on different supports at different metal loadings [ 1,4,5,14,16].
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Table 2 . M 6 s s b a u e r p a r a m e t e r s o f precipitated F e / C u / K / S i Species Iron IS a AEQb HFS r Site (mm/s) (mm/s) (KOe) Fe/K/Cu/Si calcined (M6ssbauer 36 h at 295 K) F e 2 0 3 (SPi d 0.34 0.68 -Fe/K/Cu/Si after ~ n (M6ssbauer 48 h at 295 K) Fe304 (SP) 0.41 1.00 Fe304 (FiM) e A 0.34 -477 B 0.72 -447 I 0.27 -176 x-Fe2.sC II 0.36 -214 III 0.52 -114 Fe 2+ 1.04 2.03 -Fe/K/Cu/Si after run (MOssbauer 9 h at 77 K) Fe304 (SPi d 0.44 1.08 -Fe304 (FiM) e A 0.42 -493 B 0.78 -466 I 0.33 -196 z-Fe25C II 0.44 -240 III 0.32 -109 Fe 2§ 1.32 2.30 -. . . . 342 o~-Fe alsomer shift relative to ct-Fe. b Quadrupole splitting. r Hyperfine (magnetic) field splitting. d Superparamagnetic species. Ferrimagnetic species.
% Area ( 295 K)
% Area (77 K)
100 30.1 4.9 9.0 17.3 17.2 14.7 6.8 33.4 5.3 5.7 17.5 15.7 13.4 7.8 1.2
Table 3 C o m p a r i s o n o f activity test results for Co/A1 f r o m EniTecn91ogie and B Y U . a Activity Test Tar b XC O -rco TOF x 10 2 SCH4 SCO2 (~ (%) (Bmol/g's) (s l ) (%) (%) EniTecnologie d
202 29.2 2.7 210 54.0 5.3 BYU e 202 7.6 2.8 212 13.4 5.0 a Reaction conditions: 20 bar; H2/CO = 2. b For bench scale reactor temperature was a geometric average. c CO2 selectivity was below the detection limit. d GHSV = 2.1 NL/gcat/h; H2:CO = 2, no He; 20 cm 3 catalyst. e GHSV = 10 NL/gcat/h; H2:CO:He = 2:1:0.375" 2 cm 3 catalyst.
1.1 2.1 1.2 2.1
7.3 7.6 8.6 8.5
0.4 0.4 n.d. r n.d. r
In a fixed-bed m i c r o r e a c t o r test at 2 6 0 ~ ( B Y U ) , the C O c o n v e r s i o n increased sharply to m o r e than 9 5 % during the first 5 h (Fig. 1) p r o b a b l y due to formation o f active surface sites on the carbide. D u r i n g the next 37 h, C O c o n v e r s i o n d e c r e a s e d steadily f r o m 95 to 62%. T h e o b s e r v e d steady d e c r e a s e in c a t a l y s t a c t i v i t y at 2 6 0 ~ d u r i n g the p e r i o d o f 5-42 h m a y be a t t r i b u t e d to: (1) the f o r m a t i o n o f i n a c t i v e c a r b o n a c e o u s d e p o s i t s on the c a t a l y s t surface and/or (2) oxidation o f the active carbide p h a s e to inactive magnetite.
517 After 42 h of reaction the temperature was decreased to 80 230~ and gradually raised to 280 ,_, o 245~ over the next 10 h and g 60 260 ~ then held at 245~ CO conversion first decreased and then ~ 40 240 E increased, reaching a steady state [9 ... ~ CO conversion .... ~ .................. value of 55% after a total of 70 h ~ 20 220 Temperature of reaction. By contrast, if the 0 ,, '. . . . , ' .... : ........... ~ ..... ,,, temperature was increased grad200 0 10 20 30 40 50 60 70 ually from about 200 to 240~ ReactionTime(h) the deactivation rate was significantly lower and steady-state Fig. 1. CO conversion for Fe/K/Cu/Si tested conversion could be reached on FB-micro-reactor (BYU). after 60-80 h. Thus, in comparing the two catalysts (Fe/K/Cu/Si and Co/AI) data were collected at steady state following a gradual increase of the reaction temperature during the initial 20-40 h of reaction and usually after 100 h from the start of the reaction to ensure stable conditions. Results of selected FTS activity and selectivity tests of Co/A1 and Fe/K/Cu/Si catalysts are summarized in Tables 4 and 5. The data in both tables were obtained at high temperatures and conversions typical of commercial operation. From these data it is evident that the cobalt is significantly (20-25%) more active on a catalyst weight basis and significantly (two times) more active on a turnover frequency basis; its C2+ productivity is 50-70% higher because of its higher activity, negligible CO2 selectivity (relative to a high CO2 selectivity for Fe/K/Cu/Si because of its high water-gas-shift activity), and hence higher selectivity to C2+ hydrocarbons. On the other hand the cobalt catalyst has a higher selectivity S (based on at.% C) to methane than the iron catalyst, and its methane selectivity is more temperature dependent (see Table 4). Nevertheless, the selectivity S' of cobalt for C2-C4 hydrocarbons (CO2-free hydrocarbon wt. basis) is lower and its C5+ selectivity higher than for Fe, consistent with its higher propagation probability (see Table 5). It should be emphasized that these activity/selectivity data (Tables 4 and 5) were obtained under conditions of low pore diffusional resistance based on common criteria [5,16]; this is especially true of the data obtained at 50% conversion or less. 100
300
l
Table 4 Fixed bed bench scale reactor (EniTecnologie) Steady-state activity and selectivity data for Fe/K/Cu/Si and Co/AI catalysts, a Catalyst Tar Xco -rco 106 TOF ScH4b S C O 2 b 9c5+ b (~ (%) (mol/g/s) ( 102s"l) (%) (%) (%)
Or, r
C10+
Fe/K/Cu/Si 247 68.5 5.3 1.7 5.3 25.5 47.9 0.90 Fe/K/Cu/Si 221 49.9 3.9 1.3 5.6 18.4 52.2 0.91 Co/AI 219 64.6 6.6 2.7 10.6 0.6 75.7 0.91 Co/AI 210 54.0 5.3 2.1 6.9 0.4 84.4 0.92 a Reaction conditions: 20 bar; H2/CO=2:1;GHSV = 1.82 NL/gcat/hfor Fe/K/Cu, 1.82 NL/gcat/hfor Co/AI. b Selectivity in atomic C % based on CO conversion. Propagation probability based on Anderson-Schulz-Florykinetics. dProductivity in grams C2+hydrocarbonper gram catalyst per hour.
rc2+d
(go2+/goat/h) 0.15 0.12 0.23 0.20
518 Table 5 Hydrocarbon distribution a Catalyst T~v Xco
(~
(%)
S'cH4
S'C2-C4
S'C5-C9
S'C 10"C21
S'c22+
(%)
(%)
(%)
(%)
(%)
S'olefin S'alcoholsb
(%)
(%)
l~e/K]Cu/Si 247 68.5 8.1 21.3 19.4 24.7 20.7 26.7 5.8 Fe/K/Cu/Si 221 49.9 7.8 19.3 14.7 27.4 22.9 25.5 7.9 .... Co/~,l 219 64.'6 12.1 9.2 23.1 32.4 19.9 13.7 3.3 Co/AI 210 54.0 8.0 7.2 13.2 35.8 33.1 11.6 2.6 ;' S' denotes selectivityin wt %, based o n overall hydrocarbonproduction (CO2not included): 100-~S'hy-S'olefin. b Water soluble alcohols not included. CONCLUSIONS 1. In FTS activity/selectivity tests of C o / A 1 2 0 3 and Fe/K/Cu supported on silica, conducted under comparable operating conditions, i.e., HE/CO = 2, Ptot = 20 bar, GHSV = 2.1 NL/gcat/h and 50-70% CO conversion, 20-25% higher catalyst-mass-based CO conversion activity is observed for Co/AI203 relative to Fe/K/Cu/Si. Site-based rates (i.e., turnover frequencies based on HE chemisorption data) are two times higher for Co/A1203 relative to Fe/K/Cu/Si. 2. C5+ selectivity S (based on C at.% from CO conversion) of Co/AI203 is 50-60% higher due to its higher propagation probability and its negligible CO2 selectivity; its C2+ productivity is also 50-70% higher because of its higher activity and higher selectivity to C2+ hydrocarbons. On the other hand the cobalt catalyst has a higher selectivity to methane than the iron catalyst, and its methane selectivity is more temperature dependent. Nevertheless, the selectivity S' of cobalt (on a CO2-free wt. basis) for C2-C4 hydrocarbons is lower and its C5+ selectivity higher than for Fe, consistent with its higher propagation probability. 3. Activity and selectivity data obtained in the laboratories of Brigham Young University and EniTecnologie using micro and bench scale fixed bed reactors are in the case of C0/A1203 in excellent agreement and in the case of Fe/KJCu/Si in fairly good agreement. REFERENCES 1. C.H. Bartholomew and R.C. Reuel, Ind. Eng. Chem. Prod. Res. Dev., 24 (1985) 56; J. Catal., 85 (1984) 78. 2. J.H.E. Glezer, K. P. De Jong, and M. F. M. Post, European Patent EP 0,221,598, (1989). 3. E.S. Goodwin, J.G. Marcelin, T. Riis, US Patent 4,801,573 (1989); US Patent 4,857,559. (1989); US Patent 5,102,851 (1992). 4. E. Iglesia, S.L. Soled, R. A. Fiato, J. Catal., 137 (1992) 212-224. 5. E. Iglesia, Applied Catalysis A: General 161 (1997) 59-78. 6. P.J. van Berge and R. C. Everson, Stud. Surf. Sci. and Catal.,107 (1997) 207. 7. A Raje, J. Inga and B. H. Davis, Fuel 76 (1997) 273. 8. J.J.C. Geerlings, J.H. Wilson, G.J. Kramer, H.P.C.E. Kuipers, A. Hoek and H.M. Huisman, Appl. Catal. A 186 (1999) 27. 9. M.E. Dry, Catalysis- Science and Technology, Vol. 1, Springer Verlag, NY (1981) 175. 10. M.W.Stoker, BYU thesis, 1999. 11. R. D. Jones, C.H. Bartholomew, Applied Catalysis 39 (1988) 77. 12. A.K. Datye, M.D. Shroff, M.S. Harrington, A.G. Sault, N.B. Jackson, Stud. Surf. Sci. and Catal., 107 (1997) 169. 13. R.L. Espinoza, EP Patent No.736 326 (1996). 14. C.H. Bartholomew, Stud. Surf. Sci. and Catal.,64 (1991) 158. 15. A.Datye, N.B. Jackson, L. Evans, Stud. Surf. Sci. and Catal.,119 (1998) 137. 16. R. Zennaro, M. Tagliabue, C.H. Bartholomew, Catalysis Today 58 (2000) 309-319.
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Quantitative Comparison of Supported Cobalt and Iron Fischer Tropsch Synthesis Catalysts Roberto Zennaro a, Gianni Pederzani a, Sonia Morselli a , Shiyong Cheng b and Calvin H. Bartholomew b aEniTecnologie S.p.A., Via F. Maritano 26, 20097 San Donato Milanese (Milano) Italy bBYU Catalysis Lab, Department of Chemical Engineering, Brigham Young University, Provo, UT 84602, USA
Abstract A collaborative study between two laboratories was undertaken to determine FischerTropsch activity and selectivity properties of representative cobalt and iron catalysts under conditions relevant to natural-gas to liquids conversion. In FTS activity/selectivity tests of Co/A1203 and Fe/K/Cu supported on silica, conducted under comparable operating conditions, i.e., H2/CO = 2, Ptot = 20 bar, GHSV = 2.1 NL/gcat/h and 50-70% CO conversion, 20-25% higher catalyst-mass-based CO conversion activity is observed for Co/A1203 relative to Fe/K/Cu/Si. Site-based rates (i.e., turnover frequencies based on H2 chemisorption data) are two times higher for Co/A1203 relative to Fe/K/Cu/Si. C5+ selectivity for Co/A1203 is 50-60% higher due to its higher propagation probability and its negligible CO2 selectivity; its C2+ productivity is also 50-70% higher because of its higher activity and higher selectivity to C2+ hydrocarbons. Activity and selectivity data obtained in the laboratories of Brigham Young University and EniTecnologie using micro and bench scale fixed bed reactors are in the case of Co/A1203 in excellent agreement and in the case of Fe/K/Cu/Si in fairly good agreement. 1. INTRODUCTION Wax-crack Fischer-Tropsch synthesis (FTS) is an economically promising, developing technology for conversion of natural gas to middle distillate hydrocarbons. Both cobalt [ 1-8] and iron [6,7,9] catalysts are active and selective for FTS and have found previous large-scale application: supported cobalt in the production of middle-distillates from natural gas by Shell in Malaysia during the last decade and precipitated iron in the conversion of coal synthesis gas to gasoline and chemicals for the past five decades at Sasol, South Africa. Because of its higher CO conversion activity and hydrocarbon selectivity, high activity maintenance, and potential regenerability, supported cobalt is presently the leading FTS catalyst technology [5]; nevertheless, iron catalysts can reportedly achieve higher productivities at higher space velocities (lower conversions) relative to cobalt catalysts [6,7] and might find application in multi-reactor process designs or as a throw-away catalyst because of their significantly lower cost. An important factor driving the choice of cobalt or iron catalyst for natural gas-to-liquid (GTL) conversion is the technology of the syngas production process which in turn determines H2/CO ratio. Commercially available technologies produce syngas from methane with a H2/CO ratio of 1.7 to 3.8; however, Fe-catalysts, because of their WGS activity, perform best at H2/CO ratios of 0.6-1.5, while Co-based catalysts have optimum performance at higher HdCO ratios (1.5-2.2). Nevertheless it may be possible to operate economically with an iron catalyst or a combination of iron and cobalt catalysts at HdCO ratios of 1.7-2.0.
514 In previous comparisons of cobalt and iron catalysts [6,7], CO conversion and hydrocarbon selectivity data were obtained at a different H2/CO ratio for each catalyst type; moreover, in none of the previous literature are comparisons of CO conversion rate per gram catalyst or turnover frequency made under comparable high conversion conditions. Nor are comparisons published for well-characterized, supported cobalt and iron catalysts under conditions relevant to natural gas-to-liquids (GTL) conversion. The aim of this study was to compare activities and selectivities of commercially representative iron and cobalt catalysts under similar operating condition, i.e. at 20 bar, H2:CO = 2:1, and CO conversions of 50-70%. This study also included comparison of data obtained at Brigham Young University (BYU) and EniTecnologie (ET) laboratories in micro and bench scale fixed-bed reactor units. 2. EXPERIMENTAL
2.1. Catalyst Preparation A 14% Co catalyst supported on alumina (Co/AI) and a silica supported Fe/I~Cu catalyst (52% wt% Fe, 1.9% K, 2.6% Cu) were prepared at EniTecnologie using incipient wetness and precipitation techniques respectively.
2.2. Catalyst Characterization X-ray diffraction of calcined Co/AI showed the crystalline phase to contain 20.3 wt% of spinel phase with an average formula C02.8A10.204, while the alumina support (Condea) was found to be a mixture of gamma and delta phases. The calcined Fe/K/Cu/Si consisted mainly of well-crystallized hematite (t~-Fe203). BET surface areas and pore volumes of the fresh catalyst are reported in Table 1. M6ssbauer spectra of the Fe/K/Cu/Si, in calcined and actTable 1 Surface area andpore volumes for fresh catalysts . ivity-tested, passivated forms, were collected as previously desSample Surface area Pore volume (m2g"l) (cm3g"l) cribed [10]. Spectra of used catalysts were collected at both Alumina 174 0.52 295 K and 77 K to better resolve Co/AI 152 0.35 the more complex spectral patFe/K/Cu/Si 229 0.47 terns of the four or five iron phases present. Based on a match of M6ssbauer parameters for compounds reported in the literature, peaks from the best fit of each data set were assigned to corresponding iron species. Selective H2 chemisorption uptakes were measured by a flow desorption method using a custom flow system with a thermal conductivity detector (TCD) similar to that described by Jones and Bartholomew [11 ].
2.3 Activity tests FTS activity tests were conducted in two different fixed-bed reactor systems: (1) a bench scale reactor (ET) charged with 20 cm 3 catalyst, shaped into particles of 20-40 mesh, and diluted with quartz chips (1:2 by volume), the latter of which served to minimize the axial temperature gradient in the catalyst bed and control reactor temperature and (2) a micro scale reactor (BYU) charged with 2 g of iron catalyst (about 60-100 mesh) diluted with 6 g of quartz chips or with 1 g of cobalt catalyst diluted with 8 g of quartz chips. Co/A1 was activated in situ in H2 at 400~ and 2.0 NL/gcat/h for 16 h. The Fe/KJCu/Si catalyst was activated in situ in CO at 300~ and 1.0 NL/gcat/h for 16 h. The use of CO instead of H2 for precipitated Fe has been demonstrated [ 12] to improve the catalyst activity. Catalysts were tested for FTS activity and selectivity at 20 bar, 200-280~ H2/CO = 2, and GHSV = 3.75-10 NL/gcat/h (BYU) or GHSV = 2 NL/gcat/h (EniTecnologie).
515 3. RESULTS AND DISCUSSION Co supported on alumina is one of the most common FT catalyst formulations cited in the literature [1-5], while impregnation to incipient wetness is a common synthesis route for cobalt catalysts [1,3]. The alumina (Condea) used in this study had a particle size and mechanical characteristics suitable for slurry phase application [13]. Fe/K/Cu/Si was prepared according to ref. [9]. Potassium is an important promoter for iron catalysts, increasing both activity and the average molecular weight of products, while decreasing the formation of graphite on the catalyst surface [14,15]. The addition of copper facilitates iron reduction at lower temperatures and improves thermal stability, thereby maintaining higher active surface area. Silica is used as a binder to stabilize iron crystallite size and improve the mechanical strength of the catalyst. M6ssbauer parameters and iron phase distributions of Fe/K/Cu/Si (52 wt% Fe) in its calcined and passivated forms after activity testing are listed in Table 2. The spectrum of the calcined catalyst consists of a doublet of superparamagnetic o~-Fe203 (sp-tx-Fe203). After a 16 h pretreatment in CO at 300~ and subsequent FTS reaction conducted at ET, Fe203 was converted to oxides and carbides of lower oxidation state. The working catalyst (see Table 2) consists of a mixture of superparamagnetic Fe304 (30.1%), ferromagnetic Fe304 (13.9%; 4.9% in A-sites and 9.0% in B-sites), and Z-carbide or Fe2.sC (49.2%). The peak of low intensity at about 2.4 mm/s in the spectrum has an isomer shift and quadruple splitting parameters consistent with those of Fe2§ this peak accounts for 6.8% of iron. The estimated percentages of Fe304, Fe2.sC, and Fe E+ at 295 K of 44, 49, and 6.8% are in very good agreement with those at 77 K of 44, 47, and 7.8% respectively. Thus, these data provide strong evidence that Fe2.sC and Fe304 are the predominant phases in the working catalyst. HE chemisorption capacities for the fresh Co/Al and Fe/K/Cu/Si precursors after reduction for 16 h at 400 and 300~ were 52.7+6.6 and 81.4_+10.5 txmoles/g respectively; dispersions were 10.1_+1.3 and 3.5_+0.4. These data are based on extents of reduction of 43.3 and 50% obtained by temperature programmed reduction (TPR) and M6ssbauer spectroscopy. In this study activity/selectivity data were obtained on the same cobalt and iron catalysts under comparable conditions in two different laboratories fixed bed reactors. Activity and selectivity data obtained in a bench-scale, fixed-bed reactor (ET) are compared in Table 3 with those obtained in a fixed-bed microreactor (BYU); excellent agreement between the results obtained in the two laboratories in the fixed bed units of different size (by a factor of 10) is evident. The steady-state TOF from both labs for the Co/A1 catalyst at 200~ of 2.2 x 102 s"1 (extrapolated using an activation energy of 100 kJ/mol [ 16]) is the same within experimental error as those reported earlier [5,14,16] for Co catalysts under the same conditions. This validates the comparison of rates between the micro and bench scale reactors for cobalt catalysts. For the Fe catalyst rates/g obtained in the micro reactor were 25-60% higher, while TOF value were only 10-25% higher, i.e., almost the same within experimental error. While comparison of the TOF values between Fe catalysts of this study may provide a useful relative measure of site activity, a similar comparison between Fe and Co catalysts is questionable, since the iron metal sites were measured by HE chemisorption on iron catalysts reduced in H2, while the active catalytic phase is presumably an iron carbide. On the other hand, comparisons of the TOF values based on H2 chemisorption among cobalt and ruthenium catalysts reduced in H2 is warranted, since (1) the reduced metal is known to be the active phase, (2) site density- activity correlations have been shown to be linear [4,5,14,16], and (3) excellent agreement has been observed for TOF values obtained in a number of laboratories for cobalt on different supports at different metal loadings [ 1,4,5,14,16].
516
Table 2 . M 6 s s b a u e r p a r a m e t e r s o f precipitated F e / C u / K / S i Species Iron IS a AEQb HFS r Site (mm/s) (mm/s) (KOe) Fe/K/Cu/Si calcined (M6ssbauer 36 h at 295 K) F e 2 0 3 (SPi d 0.34 0.68 -Fe/K/Cu/Si after ~ n (M6ssbauer 48 h at 295 K) Fe304 (SP) 0.41 1.00 Fe304 (FiM) e A 0.34 -477 B 0.72 -447 I 0.27 -176 x-Fe2.sC II 0.36 -214 III 0.52 -114 Fe 2+ 1.04 2.03 -Fe/K/Cu/Si after run (MOssbauer 9 h at 77 K) Fe304 (SPi d 0.44 1.08 -Fe304 (FiM) e A 0.42 -493 B 0.78 -466 I 0.33 -196 z-Fe25C II 0.44 -240 III 0.32 -109 Fe 2§ 1.32 2.30 -. . . . 342 o~-Fe alsomer shift relative to ct-Fe. b Quadrupole splitting. r Hyperfine (magnetic) field splitting. d Superparamagnetic species. Ferrimagnetic species.
% Area ( 295 K)
% Area (77 K)
100 30.1 4.9 9.0 17.3 17.2 14.7 6.8 33.4 5.3 5.7 17.5 15.7 13.4 7.8 1.2
Table 3 C o m p a r i s o n o f activity test results for Co/A1 f r o m EniTecn91ogie and B Y U . a Activity Test Tar b XC O -rco TOF x 10 2 SCH4 SCO2 (~ (%) (Bmol/g's) (s l ) (%) (%) EniTecnologie d
202 29.2 2.7 210 54.0 5.3 BYU e 202 7.6 2.8 212 13.4 5.0 a Reaction conditions: 20 bar; H2/CO = 2. b For bench scale reactor temperature was a geometric average. c CO2 selectivity was below the detection limit. d GHSV = 2.1 NL/gcat/h; H2:CO = 2, no He; 20 cm 3 catalyst. e GHSV = 10 NL/gcat/h; H2:CO:He = 2:1:0.375" 2 cm 3 catalyst.
1.1 2.1 1.2 2.1
7.3 7.6 8.6 8.5
0.4 0.4 n.d. r n.d. r
In a fixed-bed m i c r o r e a c t o r test at 2 6 0 ~ ( B Y U ) , the C O c o n v e r s i o n increased sharply to m o r e than 9 5 % during the first 5 h (Fig. 1) p r o b a b l y due to formation o f active surface sites on the carbide. D u r i n g the next 37 h, C O c o n v e r s i o n d e c r e a s e d steadily f r o m 95 to 62%. T h e o b s e r v e d steady d e c r e a s e in c a t a l y s t a c t i v i t y at 2 6 0 ~ d u r i n g the p e r i o d o f 5-42 h m a y be a t t r i b u t e d to: (1) the f o r m a t i o n o f i n a c t i v e c a r b o n a c e o u s d e p o s i t s on the c a t a l y s t surface and/or (2) oxidation o f the active carbide p h a s e to inactive magnetite.
517 After 42 h of reaction the temperature was decreased to 80 230~ and gradually raised to 280 ,_, o 245~ over the next 10 h and g 60 260 ~ then held at 245~ CO conversion first decreased and then ~ 40 240 E increased, reaching a steady state [9 ... ~ CO conversion .... ~ .................. value of 55% after a total of 70 h ~ 20 220 Temperature of reaction. By contrast, if the 0 ,, '. . . . , ' .... : ........... ~ ..... ,,, temperature was increased grad200 0 10 20 30 40 50 60 70 ually from about 200 to 240~ ReactionTime(h) the deactivation rate was significantly lower and steady-state Fig. 1. CO conversion for Fe/K/Cu/Si tested conversion could be reached on FB-micro-reactor (BYU). after 60-80 h. Thus, in comparing the two catalysts (Fe/K/Cu/Si and Co/AI) data were collected at steady state following a gradual increase of the reaction temperature during the initial 20-40 h of reaction and usually after 100 h from the start of the reaction to ensure stable conditions. Results of selected FTS activity and selectivity tests of Co/A1 and Fe/K/Cu/Si catalysts are summarized in Tables 4 and 5. The data in both tables were obtained at high temperatures and conversions typical of commercial operation. From these data it is evident that the cobalt is significantly (20-25%) more active on a catalyst weight basis and significantly (two times) more active on a turnover frequency basis; its C2+ productivity is 50-70% higher because of its higher activity, negligible CO2 selectivity (relative to a high CO2 selectivity for Fe/K/Cu/Si because of its high water-gas-shift activity), and hence higher selectivity to C2+ hydrocarbons. On the other hand the cobalt catalyst has a higher selectivity S (based on at.% C) to methane than the iron catalyst, and its methane selectivity is more temperature dependent (see Table 4). Nevertheless, the selectivity S' of cobalt for C2-C4 hydrocarbons (CO2-free hydrocarbon wt. basis) is lower and its C5+ selectivity higher than for Fe, consistent with its higher propagation probability (see Table 5). It should be emphasized that these activity/selectivity data (Tables 4 and 5) were obtained under conditions of low pore diffusional resistance based on common criteria [5,16]; this is especially true of the data obtained at 50% conversion or less. 100
300
l
Table 4 Fixed bed bench scale reactor (EniTecnologie) Steady-state activity and selectivity data for Fe/K/Cu/Si and Co/AI catalysts, a Catalyst Tar Xco -rco 106 TOF ScH4b S C O 2 b 9c5+ b (~ (%) (mol/g/s) ( 102s"l) (%) (%) (%)
Or, r
C10+
Fe/K/Cu/Si 247 68.5 5.3 1.7 5.3 25.5 47.9 0.90 Fe/K/Cu/Si 221 49.9 3.9 1.3 5.6 18.4 52.2 0.91 Co/AI 219 64.6 6.6 2.7 10.6 0.6 75.7 0.91 Co/AI 210 54.0 5.3 2.1 6.9 0.4 84.4 0.92 a Reaction conditions: 20 bar; H2/CO=2:1;GHSV = 1.82 NL/gcat/hfor Fe/K/Cu, 1.82 NL/gcat/hfor Co/AI. b Selectivity in atomic C % based on CO conversion. Propagation probability based on Anderson-Schulz-Florykinetics. dProductivity in grams C2+hydrocarbonper gram catalyst per hour.
rc2+d
(go2+/goat/h) 0.15 0.12 0.23 0.20
518 Table 5 Hydrocarbon distribution a Catalyst T~v Xco
(~
(%)
S'cH4
S'C2-C4
S'C5-C9
S'C 10"C21
S'c22+
(%)
(%)
(%)
(%)
(%)
S'olefin S'alcoholsb
(%)
(%)
l~e/K]Cu/Si 247 68.5 8.1 21.3 19.4 24.7 20.7 26.7 5.8 Fe/K/Cu/Si 221 49.9 7.8 19.3 14.7 27.4 22.9 25.5 7.9 .... Co/~,l 219 64.'6 12.1 9.2 23.1 32.4 19.9 13.7 3.3 Co/AI 210 54.0 8.0 7.2 13.2 35.8 33.1 11.6 2.6 ;' S' denotes selectivityin wt %, based o n overall hydrocarbonproduction (CO2not included): 100-~S'hy-S'olefin. b Water soluble alcohols not included. CONCLUSIONS 1. In FTS activity/selectivity tests of C o / A 1 2 0 3 and Fe/K/Cu supported on silica, conducted under comparable operating conditions, i.e., HE/CO = 2, Ptot = 20 bar, GHSV = 2.1 NL/gcat/h and 50-70% CO conversion, 20-25% higher catalyst-mass-based CO conversion activity is observed for Co/AI203 relative to Fe/K/Cu/Si. Site-based rates (i.e., turnover frequencies based on HE chemisorption data) are two times higher for Co/A1203 relative to Fe/K/Cu/Si. 2. C5+ selectivity S (based on C at.% from CO conversion) of Co/AI203 is 50-60% higher due to its higher propagation probability and its negligible CO2 selectivity; its C2+ productivity is also 50-70% higher because of its higher activity and higher selectivity to C2+ hydrocarbons. On the other hand the cobalt catalyst has a higher selectivity to methane than the iron catalyst, and its methane selectivity is more temperature dependent. Nevertheless, the selectivity S' of cobalt (on a CO2-free wt. basis) for C2-C4 hydrocarbons is lower and its C5+ selectivity higher than for Fe, consistent with its higher propagation probability. 3. Activity and selectivity data obtained in the laboratories of Brigham Young University and EniTecnologie using micro and bench scale fixed bed reactors are in the case of C0/A1203 in excellent agreement and in the case of Fe/KJCu/Si in fairly good agreement. REFERENCES 1. C.H. Bartholomew and R.C. Reuel, Ind. Eng. Chem. Prod. Res. Dev., 24 (1985) 56; J. Catal., 85 (1984) 78. 2. J.H.E. Glezer, K. P. De Jong, and M. F. M. Post, European Patent EP 0,221,598, (1989). 3. E.S. Goodwin, J.G. Marcelin, T. Riis, US Patent 4,801,573 (1989); US Patent 4,857,559. (1989); US Patent 5,102,851 (1992). 4. E. Iglesia, S.L. Soled, R. A. Fiato, J. Catal., 137 (1992) 212-224. 5. E. Iglesia, Applied Catalysis A: General 161 (1997) 59-78. 6. P.J. van Berge and R. C. Everson, Stud. Surf. Sci. and Catal.,107 (1997) 207. 7. A Raje, J. Inga and B. H. Davis, Fuel 76 (1997) 273. 8. J.J.C. Geerlings, J.H. Wilson, G.J. Kramer, H.P.C.E. Kuipers, A. Hoek and H.M. Huisman, Appl. Catal. A 186 (1999) 27. 9. M.E. Dry, Catalysis- Science and Technology, Vol. 1, Springer Verlag, NY (1981) 175. 10. M.W.Stoker, BYU thesis, 1999. 11. R. D. Jones, C.H. Bartholomew, Applied Catalysis 39 (1988) 77. 12. A.K. Datye, M.D. Shroff, M.S. Harrington, A.G. Sault, N.B. Jackson, Stud. Surf. Sci. and Catal., 107 (1997) 169. 13. R.L. Espinoza, EP Patent No.736 326 (1996). 14. C.H. Bartholomew, Stud. Surf. Sci. and Catal.,64 (1991) 158. 15. A.Datye, N.B. Jackson, L. Evans, Stud. Surf. Sci. and Catal.,119 (1998) 137. 16. R. Zennaro, M. Tagliabue, C.H. Bartholomew, Catalysis Today 58 (2000) 309-319.