1072
Energy & Fuels 2000, 14, 1072-1082
Production of C4 Hydrocarbons from Modified Fischer-Tropsch Synthesis over Co-Ni-ZrO2/ Sulfated-ZrO2 Hybrid Catalysts Raphael O. Idem* Industrial/Petroleum Systems Engineering, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada S4S 0A2
Sai P. R. Katikaneni Advanced Technology Group, Fuel Cell Energy Inc., 3 Great Pasture Road, Danbury, Connecticut 06813
Ramakrishnan Sethuraman and Narendra N. Bakhshi Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, 110 Science Place, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5C9 Received March 15, 2000. Revised Manuscript Received June 8, 2000
Fischer-Tropsch synthesis was carried out at atmospheric pressure in a fixed-bed microreactor at temperatures and weight hourly space velocities (WHSV) ranging from 513 to 533 K and 5 to 25 h-1, respectively, over hybrid catalysts (physical mixtures) containing Co-Ni-ZrO2 and sulfated-ZrO2 catalysts. The sulfated-ZrO2/Co-Ni-ZrO2 catalyst weight ratios (SZ/CN) ranged from 0 to 3, whereas sulfate concentrations in sulfated-ZrO2 catalyst (sulfate loading) ranged from 5 to 15 wt %. Fischer-Tropsch synthesis over Co-Ni-ZrO2 catalyst alone produced a maximum C4 hydrocarbon selectivity of 14.6 wt % at a temperature of 523 K and WHSV of 15 h-1. There was an impressive increase in C4 hydrocarbons selectivity to a maximum of 32.4 wt % when catalyst HB5,1 (SZ/CN of 1 and sulfate loading of 5 wt %) was used. This catalyst also gave an extremely high selectivity for isobutane (maximum of 10.6 wt % of total hydrocarbon products) as compared to 0.1 wt % obtained with Co-Ni-ZrO2 catalyst. A time-on-stream study on catalyst HB5,1 showed a decrease in activity of this catalyst with reaction time. In contrast, the use of hybrid catalyst HB5,0.5 (SZ/CN of 0.5 and sulfur loading of 5) where the overall sulfur content was low resulted in almost no deactivation. However, the activity obtained in the case of catalyst HB5,0.5 was lower than that obtained for catalyst HB5,1 but was much higher than that for Co-Ni-ZrO2 catalyst. On the other hand, for hybrid catalysts HB5,2 and HB15,1,which had high overall concentrations of sulfur, there was no activity at all. The results show that interactions brought about by close proximity of Fischer-Tropsch catalyst active sites and acid sites produce favorable effects when the overall sulfur content in the hybrid catalyst is low.
Introduction Isobutane, isobutylene, and n-butane are highly desirable hydrocarbons. Isobutylene is used in the production of methyl tert-butyl ether (MTBE) and ethyl tertbutyl ether (ETBE) which are used as oxygenate additives in producing reformulated gasoline. This type of gasoline is regarded as the fuel most likely to meet the stringent requirements of the U.S.A. Clean Air Act of 1990.1,2 On the other hand, n-butane is widely used for the production of acetic acid, maleic anhydride, and butanediol which are important feedstocks for the manufacture of resins and other fine chemicals. All the * Author to whom all correspondence should be addressed. Fax: (306) 585-4855. E-mail:
[email protected]. (1) Parkinson, G. Chem. Eng. 1992, April, 35. (2) Unzelman, G. H. Oil Gas J. 1990, April, 91.
C4 hydrocarbons (i.e., i-butane, n-butane, i-butylene, 1and 2-butenes) are used for alkylation processes to produce alkylate gasoline which has a high octane rating. Traditionally, C4 hydrocarbons are obtained from petroleum sources such as natural gas and steam cracking of naphtha and gas oil.3 However, considering the large need for C4 hydrocarbons, it is worthwhile to investigate alternative sources of these hydrocarbons other than the conventional petroleum sources. As is well-known, a variety of hydrocarbons can be produced from synthesis gas using the Fischer-Tropsch (FT) chemistry.4-15 Synthesis gas (syngas) is generally pro(3) Gary, J. H.; Handwerk, G. E. Petroleum Refining Technology and Economics; Marcel Dekker: New York, 1994. (4) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic Press: Orlando, 1984.
10.1021/ef000052+ CCC: $19.00 © 2000 American Chemical Society Published on Web 07/22/2000
C4 Hydrocarbons from Modified Fischer-Tropsch Synthesis
duced from steam reforming of methane or naphtha. In addition to these feedstocks, the potential also exists of producing syngas from renewable biomass materials such as crop residues and biomass derived chars. Thus, by utilization of biomass or waste biomass-type materials for syngas production, Fischer-Tropsch processes may have an environmental impact as well. Generally, catalysts for Fischer-Tropsch synthesis consist of metals such as cobalt, nickel, iron, and ruthenium impregnated on supports ranging from metal oxides to zeolites.16,17 As is well-known,18 FT synthesis lacks specificity for a particular hydrocarbon type or chain length. However, literature16-28 indicates that specificity may be improved by using a second catalyst in conjunction with the FT catalyst. In our earlier work,29 we employed such an approach to improve C4 hydrocarbon specificity (or selectivity) by using a follow bed reactor setup. The Fischer-Tropsch synthesis catalyst used in the study (placed in the first catalyst bed) was Co-Ni-ZrO2 whereas the modifier catalysts (placed in the second catalyst bed) was sulfatedZrO2 solid acid catalyst. The setup was such that the product from the first catalyst bed flowed over the second catalyst bed. It was observed that this setup resulted in a fairly high selectivity for C4 hydrocarbons. However, in addition to the follow bed reactor scheme, it appears30 there is the possibility of improving C4 hydrocarbon selectivity from modified FT process through the use of a mixed (FT + solid acid) catalysts bed. The major reason is that both the FT and acid sites are available side by side. Thus, the scheme provides the possibility of very close contact of products of the FT reactions to immediately react over the acid sites. It is therefore possible to effect reactions of hydrocarbons (5) Pichler, H.; Schulz, H.; Kuhne, D. Brennst-Chem. 1968, 49, 344. (6) King, D. L.; Cusumano, J. A.; Garten, R. L. Catal. Rev.sSci. Eng. 1981, 23, 233. (7) Dry, M. E. In CatalysissScience and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 1, Chapter 4, pp 159-255. (8) Anderson, R. B. In Catalysis; Emmett, P. H., Ed.; Reinhold: New York, 1956; Vol. 4. (9) Shaw, Y. T.; Perotta, A. J. Ind. Eng. Chem. Prod. Res. Div. 1976, 15, 123. (10) Vannice, M. A. Catal. Res.sSci. Eng. 1976, 14, 153. (11) Denny, P. J.; Whan, D. A. In Catalysis; The Chemical Society: London, 1978; Vol. 2. (12) Bartholomew, C. H.; Pannel, R. B. J. Catal. 1980, 65, 390. (13) Holm, V. C. F.; Bacles, G. C. U.S. Patent 3,032,599, 1962. (14) Hini, M.; Kobayashi, S.; Arata, K. AIChE J. 1979, 101, 6439. (15) Rostrup-Nielson, J. R. Catal. Today 1994, 21, 305-324. (16) Bruce, L. A.; Hope, J. G.; Mathews, J. F. Appl. Catal. 1983, 8, 349. (17) Adesina, A. A. Appl. Catal. A: General 1996, 138, 345-367. (18) Bruce, L. A.; Mathews, J. F. Appl. Catal. 1982, 4, 353-370. (19) Chang, C. D.; Lang, W. H.; Silvestri, A. J. J. Catal. 1979, 56, 268. (20) Snell, R. Catal. Rev.sSci. Eng. 1987, 29, 361. (21) Bruce, L. A.; Hope, G. J.; Mathews, J. F. Appl. Catal. 1984, 9, 351. (22) Varma, R. L.; Bakhshi, N. N.; Mathews, J. F.; Ng, S. H. Can. J. Chem. Eng. 1985, 63, 612. (23) Haag, W. O.; Huang, T. J. U.S. Patent 4,279,830, 1981. (24) Guo, C.; Liao, S.; Qian, Z.; Tanabe, K. Appl. Catal. A: General 1994, 107, 239-248. (25) Guo, C.; Yao, S.; Cao, J.; Qian, Z. Appl. Catal. A: General 1994, 107, 229-238. (26) Comelli, R. A.; Canavese, S. A.; Vaudagna, S. R.; Figoli, N. S. J. Catal. 1996, 135, 287-299. (27) Song, X.; Sayari, A. Appl. Catal. A: General 1994, 110, 121136. (28) Song, X.; Sayari, A. Catal. Rev.sSci. Eng. 1996, 38, 329. (29) Sethuraman, R; Katikaneni, S. P. R.; Idem, R. O.; Bakhshi, N. N. Fuel. Proc. Technol., submitted. (30) Song, X; Sayari, A. Energy Fuels 1996, 10, 561-565.
Energy & Fuels, Vol. 14, No. 5, 2000 1073 Table 1. BET Surface Areas of Sulfated-ZrO2, Co-Ni-ZrO2, and Hybrid Catalystsa catalyst identity
sulfated-ZrO2/ Co-Ni-ZrO2 wt ratio
sulfate loading on sulfated-ZrO2 catalyst, wt %
BET surface area, m2/g
SZR5 SZR7 SZR10 SZR12 SZR15 CN HB5,1 HB5,0.5 HB5,2 HB7,1 HB10,1 HB12,1 HB15,1
N/A N/A N/A N/A N/A N/A 1 0.5 2 1 1 1 1
5 7 10 12 15 N/A 5 5 5 7 10 12 15
64 63 55 58 89 188 119 135 105 116 120 122 136
a
CN ) unmodified Co-Ni-ZrO2 catalyst; N/A ) not applicable.
produced from FT synthesis thus leading to improvement in C4 hydrocarbon selectivity. In this work, we studied improvement in selectivity for C4 hydrocarbons using a physical mixture consisting of Co-Ni-ZrO2 catalyst and sulfated-ZrO2 solid acid catalyst (hybrid catalyst) for modified FT synthesis at atmospheric pressure at temperatures and space velocities in the range 513-533 K and 5-25 h-1, respectively, as a function of sulfated-ZrO2/Co-Ni-ZrO2 catalyst weight ratio (SZ/CN) and sulfate loading. All the catalysts were characterized thoroughly in order to obtain an understanding of the relationship between catalyst characteristics and catalyst performance. Experimental Section Preparation of Catalysts. Co-Ni-ZrO2 Catalyst. The Co-Ni-ZrO2 catalyst was prepared by first preparing a dried Ni-Zr coprecipitate and then incorporating Co in the coprecipitate. Ni-Zr coprecipitate was prepared by coprecipitation techniques involving dropwise addition of a 400 mL aqueous solution containing zirconium dinitrate and nickel nitrate (92 g of zirconium dinitrate oxide (ZrO(NO3)2.xH2O, obtained from Alpha Products, Denver, CO) and 12.5 g of nickel nitrate (Ni(NO3)2‚6H2O, obtained from BDH Chemicals, Poole, England) in 1000 mL of distilled water) to a continuously stirred 300 mL of 2.78 mol/L aqueous sodium hydroxide solution. An airtight container was used for coprecipitation to prevent diffusion of atmospheric carbon dioxide into the system during coprecipitation. We found that if carbon dioxide were present, it would react with sodium hydroxide to form a carbonate which we also found to lead to production of a poor quality catalyst. Thus, prior to and during coprecipitation, the container was purged with nitrogen gas to ensure the system was carbon dioxide free. After the precipitation step, the Ni-Zr coprecipitate was filtered and washed several times with distilled water until the filtrate was neutral, then twice with acetone, and finally dried in an oven at 333 K for 24 h. The dried coprecipitate was ground and sieved into particle sizes ranging from 15 to 74 µm before impregnating cobalt by adding 15 mL of 1.14 mol/L aqueous cobalt nitrate (Co(NO3)2‚ 6H2O, obtained from BDH, Poole, England) solution to all of the dried and sized Ni-Zr coprecipitate obtained from the previous coprecipitation step. The impregnated catalyst was dried at 333 K for 24 h and then calcined at 773 K for 16 h. The Co-Ni-ZrO2 catalyst was designated as CN. Earlier characterization29 showed that this catalyst had a composition of 5.6 and 3.8 wt % Co and Ni, respectively. Also, the BET surface area was 188 m2/g (see Table 1), while the pore volume and average pore size were 0.35 mL/g and 7 nm,
1074
Energy & Fuels, Vol. 14, No. 5, 2000
respectively. Furthermore, hydrogen and carbon monoxide chemisorption tests, and X-ray line broadening measurements for Co-Ni-ZrO2 catalyst reduced with hydrogen at 673 K29 showed that the average crystallites size of Ni species was very small (
