Fischer−Tropsch Synthesis over Cobalt Catalysts Supported on

E-mail: [email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck,. J. S. Nature 1992, 359, 710-712. (2) Beck, J. ...
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Fischer-Tropsch Synthesis over Cobalt Catalysts Supported on Mesoporous Metallo-silicates Kiyomi Okabe,* Mingdeng Wei, and Hironori Arakawa National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central #5, Higashi, Tsukuba, Ibaraki 305-8565, Japan Received October 15, 2002

Fischer-Tropsch synthesis was carried out in slurry phase over cobalt-based catalysts supported on mesoporous metallo-silicates prepared by the rapid room-temperature synthesis method. The incorporation of Al and Ti into the silica framework was confirmed by NMR, FT-IR, and UV. Although the catalyst supported on mesoporous silica (MPS) was deactivated during the reaction, the catalysts supported on mesoporous Al- and Ti-silicates (MPAS and MPTS) showed high and stable activity. The selectivity for higher hydrocarbons (R) increased with the amount of tetrahedral Al incorporated into the silica framework.

Introduction Since the discovery of a series of mesoporous silica, MCM-41,1-3 the materials have been used as catalysts and supports for various catalytic reactions,4-6 because of the uniform structure and the tunable mesopore diameters.7,8 It is expected that the wider pores are advantageous to mass transfer of the products and to the chain growth of the reaction intermediates in the Fischer-Tropsch synthesis. Several papers have been concerned with mesoporous materials as the support for the Fischer-Tropsch synthesis catalysts so far.9-11 However, most of them are supported on mesoporous silica, and the effect of metal incorporated into the silica framework has not been reported yet. In the present paper, various mesoporous metallo-silicates (MPMS) were prepared by the rapid room-temperature synthesis method, and Co-Ir catalysts supported on MPMS were used for the Fischer-Tropsch synthesis in slurry phase. * Author to whom correspondence should be addressed. Fax: +8129-861-4487. E-mail: [email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. (3) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680-682. (4) Liepold, A.; Roos, K.; Reschetilowski, W.; Esculcas, A. P.; Rocha, J.; Philippou, A.; Anderson, M. W. J. Chem. Soc., Faraday Trans. 1996, 92, 4623-4629. (5) Chen, X.; Huang, L.; Ding, G.; Li, Q. Catal. Lett. 1997, 44, 123128. (6) Zheng, S.; Gao, L.; Zhang, Q.-H.; Guo, J.-K. J. Mater. Chem. 2000, 10, 723-727. (7) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988-992. (8) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 1535-1537. (9) Iwasaki, T.; Reinikainen, M.; Onodera, Y.; Hayashi, H.; Ebina, T.; Nagase, T.; Torii, K.; Kataja, K.; Chatterjee, A. Appl. Surf. Sci. 1998, 130-132, 845-850. (10) Yin, D.; Li, W.; Yang, W.; Xiang, H.; Sun, Y.; Zhong, B.; Peng, S. Microporous Mesoporous Mater. 2001, 47, 15-24. (11) Su, C.; Zou, Y.; Pan, W.; He, D.; Zhu, Q. Ranliao Huaxue Xuebao 1998, 26, 297-302.

Table 1. X-ray Fluorescence Analysis of Mesoporous Metallo-silicates (MPMS) MPAS MPTS MPZS MPVS

M

M/Si (nominal)

M/Si (actual)

Al Ti Zr V

0.053 0.050 0.053 0.053

0.045 0.056 0.049 0.009

Experimental Section Hexadecylpyridinium chloride (C16-PyCl) was used as cationic surfactant in the preparation of mesoporous metallosilicates (MPMS), according to the method reported by Teraoka et al.12 C16-PyCl (3.0 mmol) was dissolved in 50 cm3 of water. With vigorous stirring and optional heating, HCl was added to the surfactant solution in order to adjust pH ) 0.5. The clear solutions of Na2O‚2SiO2‚nH2O, NaOH, and metal salts (such as Al(NO3)3‚9H2O, TiOSO4‚2H2O, ZrO(NO3)2‚2H2O, and NH4VO3) were added to the surfactant solution at room temperature with vigorous stirring, in the molar ratio of (Si + M)/ C16-PyCl/NaOH/HCl/H2O ) 1/0.12/1.06-1.27/0.61/141, where M was the metal. The final pH of the MPMS mixture was in the range of 10-11. After further stirring for 3 h, the precipitated product was filtered, washed thoroughly with water, dried in an oven at 373 K for 6 h, and calcined in air at 873 K for 6 h to remove the template. The composition of MPMS was determined by X-ray fluorescence analysis, as shown in Table 1. Except mesoporous vanado-silicate (MPVS), the M/Si was close to the nominal value, and the metals were effectively incorporated into the materials. Mesoporous silica (MPS) was similarly prepared without adding the metal salts. The MPS and MPMS thus prepared were co-impregnated with aqueous solutions of Co(NO3)2‚6H2O and IrCl4‚H2O, dried, and calcined at 573 K for 1 h, and then reduced in H2 flow at 673 K for 15 h to form 20 wt % Co-0.5 wt % Ir/MPS and 20 wt % Co-0.5 wt % Ir/MPMS catalysts, respectively. The catalyst powder (2 g) was slurried with 50 cm3 of n-hexadecane under an inert atmosphere. The FischerTropsch reaction was carried out with the catalyst slurry in an autoclave-type semi-batch reactor (flow type for gas phase) as illustrated in Figure 1. The feed gas (H2/CO/Ar ) 60:30:10) (12) Setoguchi, Y. M.; Teraoka, Y.; Moriguchi, I.; Kagawa, S. J. Porous Mater. 1997, 4, 129-134.

10.1021/ef020237b CCC: $25.00 © 2003 American Chemical Society Published on Web 05/15/2003

Fischer-Tropsch Synthesis over Cobalt Catalysts

Figure 1. Autoclave-type slurry reactor for the FischerTropsch synthesis. was bubbled into the slurry, and the dissolution of the gas was promoted by a specially designed stirring rod.13,14 The slurry was stirred vigorously by a T-shaped tubular stirring rod with small holes at the upper part of it. When the stirring rate was higher than 600 rpm, the reaction gas over the slurry was aspirated into the rod through the holes, and bubbled out from the ends of the tubular T-rod, probably owing to centrifugal force. Thus, the gas-phase syngas was effectively circulated in the reactor through the stirring rod, and the complete mixing system was accomplished. The reaction conditions were as follows: T ) 503 K, P ) 1 MPa, W/F ) 10 g-catal.h/mol. The effluent gas was periodically analyzed by on-line gas chromatography, and the contents of inorganic gases and C1-14 hydrocarbons were determined, with using Ar in the sample gas as the internal standard. The contents of C11+ hydrocarbons in the slurry were determined separately by gas chromatography after the reaction.

Results and Discussion Figure 2 shows the typical XRD patterns of mesoporous silica (MPS), alumino-silicate (MPAS) with Si/Al ) 19, and titano-silicate (MPTS) with Si/Ti ) 20, prepared by the rapid room-temperature synthesis method, as well as the XRD pattern of 20 wt % Co-0.5 wt % Ir/MPTS catalyst. The XRD patterns of the other catalysts were almost the same as those shown in Figure 2d. Three peaks are observed on each pattern in the region of 2θ ) 2-6°. They correspond to the (100), (110), and (200) reflections of a typical hexagonal lattice.1 Therefore, the structures of these materials are (13) Fan, L.; Han, Y.-Z.; Yokota, K.; Fujimoto, K. Sekiyu Gakkaishi 1996, 39, 111-119. (14) Okabe, K.; Li, X.; Matsuzaki, T.; Toba, M.; Arakawa, H.; Fujimoto, K. Sekiyu Gakkaishi 1999, 42, 377-382.

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Figure 2. XRD patterns of (a) MPS, (b) MPAS with Si/Al ) 19, (c) MPTS with Si/Ti ) 20, and (d) 20 wt % Co-0.5 wt % Ir/MPTS catalyst.

analogous to MCM-41. The d(100) peak position of MPAS and MPTS shifted to lower angle side than that of the purely siliceous MPS by 0.1-0.2°; i.e., the unit cell parameters of the former were larger than those of the latter, suggesting that Al and Ti were incorporated into the silica framework. The bond lengths of Si-O, Al-O, and Ti-O are reported to be 0.159 nm, 0.175 nm, and 0.176-0.181 nm, respectively.15 The low peak intensity of the catalyst (d) is ascribed to a decrease in the long distant regularity due to the impregnation of metals. By the impregnation, some pores might be filled with Co and/or Ir metals, and others were not completely filled, resulting in a decrease in the structural regularity. An inflection at about P/P0 ) 0.25 in the nitrogen adsorption isotherm and a very sharp peak at about 2.5 nm in the pore size distribution curve of the mesoporous materials prepared by the rapid room-temperature synthesis method clearly indicate mesopores, as shown in Figure 3a-c. Compared with MPMS, the adsorption amount of N2 on the catalysts (Figure 3d,e) was relatively small, because the pores were partially filled with Co and/or Ir. However, the inflection at about P/P0 ) 0.25 and the sharp peak of pore size distribution of Figure 3d,e indicate that the mesopore structure was still retained after the impregnation. It is noticed in the pore size distribution that the peak position of the catalyst shifted to the smaller-size side a little, compared with the corresponding MPMS. It indicates that Co and Ir were effectively loaded on the inner walls of the mesopores. The pore properties of the catalysts were (15) Tuel, A.; Diab, J.; Gellin, P.; Dufaux, M.; Dutel, J.-F.; Taarit, Y. B. J. Mol. Catal. 1990, 63, 95-102.

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Figure 3. N2 adsorption isotherm (A) and pore size distribution (B) of (a) MPS, (b) MPAS with Si/Al ) 19, (c) MPTS with Si/Ti ) 20, (d) 20 wt % Co-0.5 wt % Ir/MPAS catalyst, and (e) 20 wt %Co-0.5 wt % Ir/MPTS catalyst. Table 2. Pore Properties and Surface Area of 20 wt % Co-0.5 wt % Ir Catalysts Supported on Mesoporous Metallo-silicate (MPMS) support

SBET m2 g-1

pore volume cm3 g-1

pore diameter nm

Co dispersiona %

MPS MPAS MPTS MPZS MPVS

660 570 690 490 600

0.4 0.6 0.6 0.4 0.4

2.3 2.5 2.4 2.4 2.5

0.5 0.7 1.2 0.5

a

Determined by H2 chemisorption.

listed in Table 2. The properties did not depend on the support significantly, but the Co dispersion determined by H2 adsorption increased a little when Al and Ti were incorporated. The higher dispersion might be ascribed to the following reason: Co2+ may be selectively adsorbed adjacent to the atomically dispersed Al or Ti incorporated into the silica framework to form strong interaction, resulting in immobilization of Co particles, after calcination and H2 reduction. In contrast, since the plain surface of MPS does not have such an anchor effect, Co particles may aggregate to larger particles after calcination and reduction, resulting in lower dispersion. The honeycomb structure with a diameter of about 3 nm was observed in the TEM image of the mesoporous materials. The typical TEM image of MPTS with Si/Ti ) 20 is shown in Figure 4a. When 20 wt % Co and 0.5 wt % Ir were loaded on the MPTS, particles with the size similar to the pore diameter were observed, as shown in Figure 4b, suggesting that the metals were effectively loaded in the pores. These results clearly indicate that the metallosilicates prepared by the rapid room-temperature synthesis method have uniform mesoporous structure

analogous to MCM-41, and the structure was retained after the impregnation with Co and Ir to form a catalyst. Figure 5 shows the 27Al-MAS NMR spectrum of mesoporous MPAS with Si/Al ) 19. The position of the main peak is about 50 ppm, and the peak at 0 ppm is almost negligible, indicating that most of the Al atoms are in the state of tetrahedral coordination, and the octahedral Al is almost negligible. In other words, most of the Al is incorporated into the silica framework.16 Figure 6 shows FT-IR spectra of MPS, MPAS, MPTS, zirconium-silicate (MPZS) and vanado-silicate (MPVS). The broad band at about 960 cm-1 of MPTS with Si/Ti ) 20 is assigned to the Si-O-Ti stretching vibration,17-19 suggesting the incorporation of Ti into the silica framework. While the Si-O-M bands were not clear in the spectra of MPZS and MPVS with Si/M ) 19. Figure 7 shows UV spectra of TiO2 supported on SiO2, MPTS with Si/Ti ) 20, ZrO2, MPZS with Si/Zr ) 49, and MPS. The MPS does not have any absorption band in this region. The large absorption band at about 350 nm and less is the absorption of TiO2. When the Si/Ti ratio is 20, the band disappeared in the spectrum of MPTS, and the peak position shifted to 230 nm. This blue-shift17,18,20 suggests the incorporation of Ti into the silica framework. A similar shift was also observed in the UV spectrum of MPZS. When the Si/Zr ratio was 49, the shoulder band of ZrO2 was not observed, and the peak position (16) Schmidt, R.; Akporiaye, D.; Stocker, M.; Ellestad, O. H. J. Chem. Soc., Chem. Commun. 1994, 1493-1494. (17) Reddy, J. S.; Kumar, R. J. Catal. 1991, 130, 440-446. (18) Thangaraj, A.; Kumar, R.; Mirajkar, S. P.; Ratnasamy, P. J. Catal. 1991, 130, 1-8. (19) Huybrechts, D. R. C.; Vaesen, I.; Li, H. X.; Jacobs, P. A. Catal. Lett. 1991, 8, 237-244. (20) Hanley, T. L.; Luca, V.; Pickering, I.; Howe, R. F. J. Phys. Chem. B 2002, 106, 1153-1160.

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Figure 4. TEM images of (a) MPTS, and (b) 20 wt % Co-0.5 wt % Ir/MPTS catalyst (Si/Ti ) 20).

Figure 5.

Al-MAS NMR spectrum of MPAS with Si/Al ) 19.

27

shifted to 205 nm, suggesting the incorporation of Zr into the silica framework.21 However, the absorption band of ZrO2 was observed at about 230 nm in the UV

spectrum of MPZS with Si/Zr ) 19, indicating that not all of the Zr atoms were incorporated into the silica framework when the Zr amount was large. Thus, the incorporation of Al and Ti into the silica framework was confirmed by NMR, FT-IR, and UV spectra. As for MPZS, the incorporation was confirmed by UV spectra, only when the Zr content was small. As for MPVS, the incorporation was suspicious from the FT-IR spectrum. The time course of the Fischer-Tropsch synthesis over the catalysts supported on the above metallosilicates (MPMS) is illustrated in Figure 8. As mentioned above, X-ray fluorescence analysis results indicated that all these metals were effectively incorporated, except vanadium. The catalyst supported on MPS was gradually deactivated during the reaction. In contrast, the catalysts supported on MPTS and MPAS showed high and stable CO conversion of about 60%, and almost no deactivation was observed within 40 h of the reaction, while the catalysts supported on MPZS and MPVS were deactivated, just like the catalyst supported on the purely siliceous MPS. As described above, Al and Ti were incorporated into the silica framework, while Zr was incorporated only when Zr content was low, and the incorporation of V was suspicious. From these results, it is suggested that the incorporation of the metal into the silica framework is responsible for the stable activity. The fact that the impregnation of Al salts to MPS does not improve the catalyst stability nor the selectivity supports the above contribution of the metals (21) Tuel, A. Microporous Mesoporous Mater. 1999, 27, 151-169.

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Figure 8. Time course of the reaction over 20 wt % Co-0.5 wt % Ir catalysts supported on (a) MPTS with Si/Ti ) 20, (b) MPAS with Si/Al ) 19, (c) MPZS with Si/Zr ) 19, (d) MPVS with Si/V ) 19, and (e) MPS. Table 3. Reaction Results over 20 wt % Co-0.5 wt % Ir Catalysts Supported on Mesoporous Metallo-silicate (MPMS)

Figure 6. FT-IR spectra of (a) MPS, (b) MPAS with Si/Al ) 19, (c) MPTS with Si/Ti ) 20, (d) MPZS with Si/Zr ) 19, and (e) MPVS with Si/V ) 19.

Ma

surface area m2 g-1

CO conv. %

Al Ti Zr V

1216 1030 968 996 921

50.8 57.4 60.8 47.0 42.2

a

Figure 7. UV spectra of (a) TiO2/SiO2, (b) MPTS with Si/Ti ) 20, (c) ZrO2, (d) MPZS with Si/Zr ) 49, and (e) MPS.

incorporated into the silica framework, since the impregnated Al is not in the tetrahedral coordination form

selectivity/C-% CH4 C2-4 C5+ 14.2 7.8 7.9 7.0 5.5

25.2 8.9 7.6 7.3 6.3

59.6 82.3 83.0 83.8 91.2

Rb 0.76 0.89 0.83 0.85 0.86

Si/M ) 19-20. b The chain growth probability.

but in the octahedral coordination form of Al2O3 particles on MPS, as reported previously.22 The mesoporous structure was partially retained in the used catalysts supported on MPAS and MPTS, which was suggested by the pore size distribution (see Figure 9) and XRD measurements after calcination to remove most of the carbonaceous materials in the pores. In contrast, the mesoporous structures of the used catalysts supported on MPS, MPZS, and MPVS were not obvious. The Co metal peak was observed in the XRD patterns of the used catalysts supported on MPZS and on MPVS, suggesting aggregation of Co particles during the Fischer-Tropsch reaction. In contrast, no Co peaks were observed in the XRD patterns of the used catalysts supported on MPAS and MPTS, as shown in Figure 10. Therefore, the incorporation of the metals into the silica framework is responsible to retain the mesoporous structure to maintain high dispersion of Co, resulting in the high and stable activity during the reaction. The reaction results are summarized in Table 3. The average values at the reaction time of t ) 20-40 h are tabulated. The selectivity for higher hydrocarbons (R) was low over the catalyst supported on MPS, while the selectivity was high over the catalysts supported on (22) Wei, M.; Okabe, K.; Arakawa, H.; Teraoka, Y. New J. Chem. 2002, 26, 20-23.

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Figure 11. Effect of Al content in MPAS on the FischerTropsch reaction over 20 wt % Co-0.5 wt % Ir/MPAS catalysts. (a) The chain growth probability (R), (b) CO conversion, and (c) CH4 selectivity.

Figure 9. Pore size distribution of used catalysts supported on (a) MPTS with Si/Ti ) 20, and (b) MPAS with Si/Al ) 19.

Figure 10. XRD patterns of used catalysts supported on (a) MPVS, (b) MPZS, and (c) MPAS (Si/M ) 19).

MPMS. The CH4 formation was greatly suppressed over the catalysts supported on MPZS and MPVS, although these catalysts were deactivated. Since the incorporation of Zr and V into the silica framework is suspicious, the improvement in the selectivity is thought to be merely the additive effect of these metals. In contrast, the improved activity and the selectivity of the catalysts

supported on MPAS and MPTS are ascribed to the metals incorporated into the silica framework, although the selectivity for higher hydrocarbons (R) was relatively lower than that of the catalysts supported on MPZS and MPVS. Figure 11 shows the effect of the Al amount incorporated into the silica framework. The CO conversion and the selectivity for higher hydrocarbons (R) increased with the Al amount. Consequently, the selectivity for diesel fuel fraction of more than 30% was obtained over the catalyst supported on the MPAS with the Al/Si ratio greater than 0.03, without significant deactivation. It is expected that the incorporation of Al into the silica framework will give rise to an acidic site. If MPAS has acidic sites, some hydrocracking will occur at an elevated temperature, resulting in higher selectivity for light hydrocarbons and lower R-value of the products. However, Bronsted acidity was not observed significantly on the catalyst by FT-IR. And, actually, the selectivity for light hydrocarbons was low and the R-value was high on the catalysts supported on MPAS. Thus, we interpret the effect of Al on the reaction as the acidity is excluded under the reaction conditions employed. When Al and Ti were incorporated into the silica framework, the higher Co dispersion was obtained over the catalysts supported on MPAS and MPTS than over the catalyst supported on MPS, as described above. It is reported that the F-T reaction activity generally increases with Co dispersion.23 If the chemical characteristics around the active sites are identical, it is well accepted that C5+ selectivity is high at high conversion, since partial pressure of H2 is low under the steadystate conditions of the F-T reaction.24 The correlation between Co dispersion and R is also reported by Sun et al.25 Consequently, CO conversion and C5+ selectivity (23) Iglesia, E.; Reyes, S. C.; Madon, R. J. Adv. Catal. 1993, 39, 221302. (24) Iglesia, E.; Soled. S. L.; Fiato, R. A.; Via, G. H. J. Catal. 1993, 143, 345-368. (25) Sun, S.; Tsubaki, N.; Fujimoto, K. J. Chem. Eng. Jpn. 2000, 33, 232-238.

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might increase with the amount of Al incorporated into the silica framework. It is pointed out that the fcc phase of Co is predominant on the catalyst with high loading, which shows high conversion.25,26 However, phase identification of Co on the used catalysts supported on MPAS and MPTS was difficult in the present study, since the Co particles were too small to be detected by XRD. The precise mechanism of the enhancement in the activity (26) Wei, M.; Okabe, K.; Arakawa, H.; Teraoka, Y. React. Kinet. Catal. Lett. 2002, 77, 381-387.

Okabe et al.

and the selectivity is not clear now, and further investigation is needed Acknowledgment. This investigation was financially supported by Research for the Future Program of Japan Society for the Promotion of Science (JSPS) under the Project “Synthesis of Ecological High Quality Transportation Fuels” (JSPS-RFTF98P01001). EF020237B