Potassium Effects on Activated-Carbon-Supported Iron Catalysts for

1832

Energy & Fuels 2007, 21, 1832-1842

Potassium Effects on Activated-Carbon-Supported Iron Catalysts for Fischer-Tropsch Synthesis Wenping Ma, Edwin L. Kugler, and Dady B. Dadyburjor* Department of Chemical Engineering, West Virginia UniVersity, Morgantown, West Virginia 26506 ReceiVed December 27, 2006. ReVised Manuscript ReceiVed April 26, 2007

The effect of potassium on the activity, selectivity, and distribution of products (hydrocarbons and oxygenates) was studied over iron catalysts supported on activated carbon (AC) for Fischer-Tropsch synthesis (FTS). This is part of a wider study on the incremental effects of components (including the support) of a multicomponent (Fe-Cu-Mo-K/AC) FTS catalyst. The range of potassium loading used was 0-2 wt %. A fixed-bed reactor was used under the conditions of 260-300 °C, 300 psig, and 3 Nl/g cat/h, using syngas with a H2/CO molar feed ratio of 0.9. Both FTS and water-gas shift activities increase after the addition of 0.9 wt % potassium, whereas an opposite trend is observed with the addition of 2 wt % potassium. This is shown to be the result of interaction between the decrease of both the activation energy (Ea) and the pre-exponental factor (k0) with the amount of potassium promoter added. Detectable hydrocarbons up to C34 and oxygenates up to C5 are formed on the Fe/AC catalysts with or without potassium. The potassium promoter significantly suppresses formation of methane and methanol and shifts selectivities to higher-molecular-weight hydrocarbons (C5+) and alcohols (C2-C5). Meanwhile, the potassium promoter changes paraffin and olefin distributions. At least for carbon numbers of 25 or less, increasing the K level to 0.9 wt % greatly decreases the amount of n-paraffins and internal olefins (i.e., those with the double bond in other than the terminal positions) and dramatically increases branched paraffins and 1-olefins, but a further increase in the K level shows little additional improvement. The addition of potassium changes the effect of temperature on the selectivity to oxygenates. In the absence of K, oxygenate selectivity decreases with temperature. However, when K is present, the selectivity is almost independent of the temperature.

Introduction Potassium is an essential promoter for precipitated-iron catalysts in the Fischer-Tropsch synthesis (FTS) reaction. This promoter has been found to have a greater impact on catalytic performance than others such as Ca, Co, Mn, Zn, Ru, and Zr.1-3 Potassium is generally believed to function as an electronic promoter in the iron-based catalysts.2-5 The interaction between iron and potassium leads to iron particles being more difficult to reduce but decreases the surface area of iron catalysts.4-7 Some studies report that potassium changes the adsorption of CO and H2 molecules on the iron-based catalyst due to different electronic features of the dissociation of CO and H2; this eventually causes potassium to enhance the strength of the Fe-C bond and to weaken the Fe-H bond.3,8-13 * Corresponding author. Phone: (304) 293-2111 ex 2411. Fax (304) 2934139. E-mail: [email protected]. (1) Anderson, R. B. Catalysis for the Fischer-Tropsch Synthesis. In Catalysis; Emmett, P.H., Ed.; Van Nostrand-Reinhold: New York, 1956; pp 119-236. (2) Dry, M. E. The Fischer-Tropsch Synthesis. In Catalysis Science and Technology 1; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1981; pp 175-183. (3) Bukur, D. B.; Mukesh, D.; Patel, S. A. Ind. Eng. Chem. Res. 1990, 29, 194-204. (4) Li, C. Effect of Potassium and Copper Promoters on Reduction Behavior of Precipitated Iron Catalysts. Ph.D. Dissertation, Texas A&M University, College Station, TX, 1998. (5) Yang, Y.; Xiang, H. W.; Xu, Y. Y.; Bai, L.; Li, Y. W. Appl. Catal. 2004, 266, 181-194. (6) Li, S. Z.; Li, A. W.; Krishnamoorthy, S.; Iglesia, E. Catal. Lett. 2001, 77, 197-205. (7) Dry, M. E.; Oosthuizen, G. J. J. Catal. 1968, 11, 18-24. (8) Dry, M. E.; Shingles, T.; Boshoff, L.; Oosthuizen, G. J. J. Catal. 1969, 15, 190-199.

Several studies of precipitated-iron catalysts report that potassium improves water-gas shift (WGS) activity,1-3,5,13-16 but modifying FTS activity depends on the potassium content: low potassium loadings improve the FTS rate, while high potassium loadings decrease the FTS rate.2,5,17,18 Extra potassium is believed2,4,5,9 to lead to a smaller surface area of the catalyst and a smaller extent of iron reduction. Potassium was also found to modify the hydrocarbon (HC) distribution of precipitatediron catalysts.1-18 Oxygenates are produced during FTS, in particular, for precipitated-iron-based catalysts.2,3,5,13,15,19 Dry2 indicated that oxygenate formation increases with potassium content, while Li and coworkers5,19 found that there is an optimized potassium content for oxygenate yield over the precipitated-iron catalyst. (9) Kolbel, H. Kalium als Strucktureller und Energetischer Promoter in Eisenkatalysatoren. In Actes du Deuxieme Congres International de Catalyse; Technip: Paris, 1960; pp 2075-2099. (10) Storch, H. H.; Golumbic, N.; Anderson, R. B. The Fischer-Tropsch and Related Syntheses; Wiley: New York, 1951; pp 57-246 (11) Anderson, R. B. The Fischer-Tropsch Synthesis; Wiley: New York, 1984; pp 140-159. (12) Soled, S.; Iglesia, E.; Miseo, S.; DeRites, B. A.; Fiato, R. A. Top. Catal. 1995, 2, 193-205. (13) Arakawa, H.; Bell, A. T. Ind. Eng. Chem. Process. Des. DeV. 1983, 22, 97-103. (14) Raje, A. P.; O’Brien, R. J.; Davis, B. H. J. Catal. 1998, 180, 3643. (15) Dictor, R. A.; Bell, A. T. J. Catal. 1986, 97, 121-136. (16) Poznan, P. Catal. Lett. 1997, 43, 59-61. (17) Miller, D. G.; Moskovits, M. J. J. Phys. Chem. 1988, 92, 60816085. (18) Anderson, R. B.; Seligman, B.; Schulz, J. F.; Kelly, R.; Elliott, M. A. Ind. Eng. Chem. 1952, 44, 391-397. (19) Teng, B. T.; Zhang, C. H.; Yang, J.; Cao, D. B.; Chang, J.; Xiang, H. W.; Li, Y. W. Fuel 2005, 84, 791-800.

10.1021/ef060654e CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007

Potassium Effects on Iron Catalysts

and Bell.15 and Bukur et al.3 found that potassium suppresses the formation of methanol and increases higher-molecularweight alcohols. A common feature of all precipitated FTS catalysts and the majority of supported FTS catalysts is their wide distribution of HCs. The chain length of HCs needs to be adjusted so that primary gasoline- and diesel-range HCs can be selectively produced from syngas.20-23 Activated carbon (AC) has a large surface area and pores ranging from the micro- to macrolevel. AC was initially used as the support for an iron FTS catalyst by Vannice and co-workers.24,25 The catalysts were reported to produce HCs only up to C6 under atmospheric pressure and at 300-350 °C. AC has been used as a promising support for the selective production of primary gasoline and diesel HCs via FTS.26 The Fe/AC catalyst produces mainly gasoline- and dieselrange HCs, and Mo is a very effective additive in the maintenance of catalyst stability and the high selectivity of internal olefins (int-olefins, i.e., where the double bond is not in the terminal position). Due to the significance of the AC catalyst system for the FTS reaction, we have conducted a systematic study of the effects of components of the Fe-Cu-Mo-K/AC catalyst for FTS. The current work reports on the effect of K loading on catalytic performance (FTS and WGS activities, HC selectivity, stability, and product distribution) over AC-supported Fe catalysts. Potassium loadings of 0-2 wt % and a broad range of reaction temperatures of 260-300 °C have been used in a fixed-bed reactor. Experimental Section Catalyst Synthesis. AC from peat (Sigma-Aldrich) was used as a catalyst support. The AC was washed in hot distilled water, calcined at 500 °C for 2 h in flowing N2, and sieved to 20-40 mesh as the final support for catalyst preparation. AC-supported unpromoted and potassium-promoted iron catalysts were prepared using incipient-wetness impregnation. Details of preparation procedures can be found elsewhere.26,27 In brief, an aqueous solution containing ferric nitrate corresponding to 15.7 wt % iron on the catalyst was first impregnated on the AC support. The resultant materials were dried in the air at 90-100 °C overnight. An aqueous solution containing potassium nitrate corresponding to 0.9 wt % potassium on the catalyst was then impregnated onto the prepared Fe/AC, again followed by drying in the air at 90100 °C overnight. This produces the 15.7% Fe/0.9% K/AC catalyst. The 15.7% Fe/2% K/AC catalyst is obtained by using a different concentration of the KNO3 solution. For convenience of discussion, the abbreviations of 0K, 1K, and 2K represent the 15.7% Fe/AC catalyst, the 15.7% Fe/0.9 K/AC catalyst, and the 15.7% Fe/2% K/AC catalyst, respectively. Reactor System. Catalyst performance was examined in a computer-controlled down-flow fixed-bed reactor system. A detailed description of the reactor system has been provided previously.28,2928 (20) Ma, W. P.; Ding, Y. J.; Lin, L. W. Ind. Eng. Chem. Res. 2004, 43, 2391-2398. (21) Ding, Y. J.; Ma, W. P.; Lu, Y.; Lin, L. W. Process for direct synthesis of diesel distillates with high quality from synthesis gas through Fischer-Tropsch synthesis; July 20, 2004. U.S. Patent 6,765,025. (22) Ding, Y. J.; Ma, W. P.; Lin, L. W. Activated carbon supported cobalt based catalyst for direct conversion of synthesis gas to diesel fuels. U.S. Patent 6,720,283; April 13, 2004. (23) Dry, M. E. Appl. Catal. 2004, 276, 1-3. (24) Jung, H. J.; Vannice, M. A.; Mulay, L. N.; Stanfield, R. M.; Delgass, W. N. J. Catal. 1982, 76, 208-224. (25) Venter, J. J.; Kaminsky, M.; Geoffroy, G. L.; Vannice, M. A. J. Catal. 1987, 103, 450-465. (26) Ma, W. P.; Kugler, E. L.; Wright, J.; Dadyburjor, D. B. Energy Fuels 2007, 20, 2299-2307. (27) Ma, W. P.; Kugler, E. L.; Wright, J.; Dadyburjor, D. B. Prepr.s Am. Chem. Soc., DiV. Pet. Chem. 2005, 50, 161-163. (28) Li, X. G.; Feng, L. J.; Liu, Z. Y.; Zhong, B.; Dadyburjor, D. B.; Kugler, E. L. Ind. Eng. Chem. Res. 1998, 37, 3853-3863.

Energy & Fuels, Vol. 21, No. 4, 2007 1833 Briefly, the flow rate of syngas (H2/CO ) 0.9) containing an internal standard of 5% He was adjusted by a mass-flow controller and directed to a stainless-steel reactor (8 mm × 630 mm). Typically, about 1.0 g of catalyst (20-40 mesh) was used in the reaction study. The catalyst was diluted 1:4 (v/v) with quartz chips of the same size before being loaded into the reactor. The exit gas stream (unreacted syngas, CO2, hydrocarbons, water, and alcohols) passed through a product trap to condense liquid products. A liquid sample was generally withdrawn every 24 h. Incondensable gases passed through a back-pressure regulator and then went either to a bubble flow meter or to a gas chromatograph (GC) for online product analysis. The catalyst was reduced in situ by H2 at 400 °C, 0.5 MPa, and 3 Nl/g cat/h for 12 h. Following the pretreatment, the reactor was cooled to 250 °C in flowing He. The system was then pressurized to 300 psig; the He flow was cut off, and syngas was introduced. Finally, the reactor temperature was gradually increased to the desired value, and the run was started. Runs at each reaction temperature generally lasted for approximately 24 h. Product Analysis. Inlet and outlet gases were analyzed online every 2 h by a HP-5890 GC using a HayeSep packed column and a thermal conductivity detector and using a DB-1 capillary column and a flame-ionization detector (FID), respectively. The organic phase of the liquid sample (C4-C34 HCs) was analyzed in a Varian 3400 GC by a MXT-5 capillary column and FID. Branched paraffins were analyzed by comparison with calibrations using standard samples of C8 HCs. The water phase of the liquid sample was analyzed in the same Varian 3400 GC by a Porapak-Q packed column and FID. The alcohol components in the water phase were identified by matching retention times of known standards. The water-phase composition was quantified using tert-amyl alcohol as an internal standard. Mass balance closures generally were 100 ( 3%. In this paper, we use the following definitions to calculate syngas conversion, usage ratio (of H2 to CO), partial pressure coefficient (Jp), hydrocarbon productivity, the ratio of olefins to paraffins, and product selectivities. CO conversion (%) ) 100 × mole rate of CO consumption/mole rate of CO inlet (1a)

H2 conversion (%) ) 100 × mole rate of H2 consumption/mole rate of H2 inlet (1b)

syngas conversion (%) ) 100 × mole rate of syngas consumption/mole rate of syngas inlet (1c) usage ratio of H2 to CO ) mole rate of H2 consumption/mole rate of CO consumption (2)

partial pressure coefficient, Jp ) (PCO2PH2)/(PCOPH2O)

(3)

hydrocarbon productivity (g/kg cat/h) ) hydrocarbon production rate (g/h)/catalyst mass (kg) (4)

selectivity of hydrocarbons with carbon number of n (wt%) ) 100 × rate of hydrocarbon with carbon number of n/rate of total hydrocarbon produced (5) (29) Kugler, E. L.; Feng, L.; Li, X.; Dadyburjor, D. B. Stud. Surf. Sci. Catal. 2000, 130A, 299-304.

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Ma et al.

CO2 selectivity (%) ) 100 × mole rate of CO2 produced/mole rate of CO consumption (6) int-olefin selectivity (wt %) ) 100 × rate of int-olefins/ (rate of 1-olefins + rate of int-olefins) (7) oxygenate selectivity (wt%) ) 100 × production rate of oxygenates/(production rate of oxygenates + production rate of hydrocarbons) (8) ratio of olefin to paraffin ) production rate of olefin/production rate of paraffin (9) Note that in the calculation of Jp, eq 3, the partial pressure of water, PH2O, is obtained by difference. The value of Jp is basically less than or equal to the thermodynamic equilibrium constant (Kp) of the WGS reaction and, so, can be used to determine the approach to equilibrium of the WGS reaction.

Results and Discussion Effect of Potassium on Activity and Stability. The effects of potassium loading on catalyst activity and stability are shown in Figure 1 and are summarized in Table 1. For the 1K catalyst, the initial syngas conversion at the lowest temperature (260 °C) is higher than that for the 0K catalyst, but the 1K catalyst deactivates very quickly. This superior activity for the 1K catalyst is maintained at 270 °C and at 280 °C. For the 2K catalyst at the lowest temperature, the initial syngas conversion is again higher than that for the 0K catalyst, but less than that for the 1K catalyst. However, for the 2K catalyst, deactivation is even more rapid than that for the 1K catalyst; after only about 12 h on stream, the conversion of the 2K catalyst is smaller than that of the 0K catalyst. During testing at higher temperatures, the activity of the 2K catalyst is the lowest of the three catalysts, and the catalyst is deactivated. Even at the higher temperatures, the 0K catalyst is relatively stable and little deactivation occurs. Hence, a small amount of potassium increases FTS activity, while a larger amount decreases FTS activity. Further, the potassium promoter generally reduces catalyst stability. These changes may stem from the potassium promotion of carbon deposition, since potassium facilitates CO dissociation on iron catalysts.9,13,18 Moreover, the low H2/CO ratio of the synthesis gas used in the current study makes the catalyst more amenable to carbon deposition by potassium. Several groups have studied the effect of potassium on precipitated Fe-Cu/SiO2 catalysts in both fixed-bed reactors and slurry-bed reactors. For example, Bukur et al.3 studied the effect of potassium promotion on the activity and selectivity of a precipitated-iron catalyst. They observed an increase in activity with increasing potassium content, up to 1 part in 100 parts of Fe. Recently, Yang et al.5 reported results of the effect of a K promoter on the catalytic performance of a precipitated-iron catalyst by changing the potassium loading up to 3 wt %. They found that the catalyst activity passes through a peak at a potassium loading of 0.7 wt % (1.14 parts K in 100 parts of Fe). However, when the K content is over 1.5 wt %, the catalyst activity is even lower than that for the precipitated-iron catalyst without any promoter. Finally, Raje et al.14 used potassium loadings of 0.36 wt % (0.36 parts K in 100 parts Fe), 1.4 wt % (1.43 parts K in 100 parts of Fe), and 2.2 wt % (2.21 parts K in 100 parts Fe) to investigate the effect of potassium on the catalytic performance of a precipitated-iron catalyst. Their results show that the catalyst with 1.43 parts K in 100 parts Fe is the most active, while the catalyst with 2.21 parts K in 100 parts Fe is the least active.

Figure 1. Change of syngas conversion with time on stream over Fe/ AC (0-2K) catalysts. Other reaction conditions: 300 psig, 3 Nl/g cat/ h, H2/CO ) 0.9.

The trend of catalyst activity with potassium content in the current study over an AC support is not in quantitative agreement with the results over precipitated-iron catalysts reported above. Our current studies show that the 1K catalyst is more active than the 0K catalyst and the 2K catalyst is the least active. However the 1K and 2K catalysts in this study have 6 parts K and 12.8 parts K, respectively, per 100 parts Fe, much higher than the values used in the literature. AC-supported iron catalysts need more of the potassium promoter to increase catalyst activity, compared to the precipitated-iron catalysts. This may be due to the higher surface area of the supported catalyst. Both micropores and macropores are present in this material. Also, some K may be interacting with the AC support during the last impregnation of K. Effect of Potassium on the Extents of FTS and WGS. The overall CO consumption rate, rCO, FTS reaction rate, rFTS, CO2 formation rate, rCO2, and WGS rate, rWGS, in the FTS system can be correlated by following equations:

-rCO ) NCO,in - NCO,out ) rFTS + rWGS

(10)

rWGS ) rCO2

(11)

and

In eq 10, NCO,in and NCO,out are CO mole flow rates of the inlet and outlet of the reactor. The ratio of WGS rate to FTS rate, RWF, (≡ rWGS/rFTS) is a reliable parameter to measure the extent of the WGS reaction. Figure 2 illustrates the variation of RWF values for the three catalysts with temperature. For all three Fe/AC catalysts, the extents of WGS increase with the temperature. At low temperatures, RWF values of the 1K and 2K catalysts are much higher than that of the 0K catalyst, suggesting that the potassium promoter increases the extent of WGS significantly more than FTS. The RWF value of the 1K catalyst is slightly greater than that of the 2K catalyst, indicating that more than a small amount of potassium promoter increases WGS less than FTS. At temperatures above 290 °C, RWF values for the 0K catalyst increase rapidly and become greater than those of the 2K catalyst. This demonstrates that potassium suppresses the effect of temperature on the WGS reaction. Average values of CO2 selectivity, the partial pressure coefficient, Jp () PCO2PH2/PCOPH2O), and the H2/CO usage ratio listed in Table 1 are also parameters to measure the extent of WGS activity in the FTS system.3,5,30-33 The values of Jp (30) Newsome, D. S. Catal. ReV. Sci. Eng. 1980, 21, 275-318.


Energy & Fuels, Vol. 21, No. 4, 2007 1835

Table 1. Activity Summary of Fe/AC Catalysts with or without Potassiuma catalyst temperature,

15.7 Fe/AC (0 K) °Cb

TOS, h CO conversion, % H2 conversion, % CO + H2 conversion, % productivity, g HC/kg cat/h (H2/CO) usage ratio CO2 selectivity, mol %c JP ) PCO2PH2/PCOPH2Od KP,WGSe

15.7 Fe/0.9 K/AC (1 K)

15.7 Fe/2 K/AC (2 K)

260

270

280

290

300

260

270

280

260

270

280

290

300

8-30 29.4 40.7 34.9 201.6

30-53 50.5 54.5 52.4 303

53-77 71.2 66.9 69.1 398.3

77-101 93.1 82 87.7 487.4

101-126 96.9 88.5 92.9 510.2

5-22 50.7 40 45.5 275.6

22-46 59.9 44.9 52.7 332.9

46-70 85.7 63 74.8 449.9

3-22 35.5 27.6 31.7 192.7

22-46 41.1 31.1 36.3 212.4

46-70 49.7 36.6 43.4 266.1

70-95 56.3 40.8 48.9 300.8

95-117 70.2 52.2 61.5 372.8

1.28 30.1 0.7 73.2

1 39 1.9 62.1

0.87 44 5.7 53

0.82 48.4 29.0 45.5

0.85 47.6 35.7 39.3

0.73 45.5 22.2 73.2

0.7 47 27.9 62.1

0.68 47.5 39.8 53

0.72 44.7 10.5 73.2

0.7 45.8 14.4 62.1

0.68 46.8 20.6 53

0.67 46 17.8 45.5

0.69 47.1 28.1 39.3

a Mean values. b Other process conditions: 3 N1/g cat/h, 300 psig, H /CO ) 0.9. c Obtained from eq 6. d Obtained from eq 3. e K ) exp[(4577.8/T) 2 eq 4.33] in ref 30.

Figure 2. Variation of the ratio of WGS rate to FTS rate with temperature and potassium loading. Other reaction conditions: 300 psig, 3 Nl/g cat/h, H2/CO ) 0.9.

corresponding to thermodynamic equilibrium (≡ Kp) are also given in Table 1. CO2 selectivity on all three catalysts is high and increases with the temperature. Moreover, the 0K catalyst shows the largest increase of CO2 selectivity with temperature, consistent with Figure 2. The same trend can also be observed with the values of Jp over the three catalysts. Jp values of the 0K catalyst are much smaller than Kp at lower temperatures but approach Kp at higher temperatures. Hence, the WGS for the unpromoted Fe/AC catalyst is far from equilibrium at the low temperatures but approaches equilibrium at high temperatures. The 1K catalyst shows the highest Jp values of the three catalysts, consistent with higher RWF values observed on the 1K catalyst discussed above. Finally, in conformity with CO2 selectivity and Jp, decreased trends of the H2/CO usage ratio are observed in Table 1. These results are qualitatively consistent with previous studies over precipitated-iron catalysts.3,5,13,66 However, there are some quantitative differences. Bukur et al.3 report that the “WGS quotient” (Jp in this work) for a precipitated catalyst containing 100 Fe/1K (their most active catalyst) at 250 °C is 52, while our value of Jp is much lower, about 22, at 260 °C over the 1K catalyst (our most active Fe/AC catalyst). Similarly, Yang et al.5 report that the “WGS quotient” over the 100 Fe/1.14K (0.7 wt %) precipitated catalyst (again, their most active catalyst) at (31) O’Brien, R. J.; Xu, L.; Bao, S.; Raje, A.; Davis, B. H. Appl. Catal. 2000, 196, 173-178. (32) Bukur, D. B.; Koranne, M.; Lang, X. S.; Rao, K. R. P. M.; Huffman, G. P. Appl. Catal. 1995, 126, 85-113. (33) Rao, K. R. P. M.; Huggins, F. E.; Huffman, G. F.; Gormley, R. J.; O’Brien, R. J.; Davis, B. H. Energy Fuels 1996, 10, 546-551.

280 °C is 51, while for our case, the value of Jp is around 40 at 280 °C. Considering that the WGS quotient (or Jp) increases with the temperature, it is obvious that the extent of WGS over K-promoted precipitated-iron catalysts is greater than that over the current K-promoted Fe/AC catalysts. The AC support itself probably has some influence on the extent of WGS over the current catalysts. Effect of Potassium on Selectivities of Hydrocarbons. Effects on hydrocarbon selectivity are illustrated in Table 2. At the lowest temperature considered, the selectivity of C1-C4 hydrocarbons decreases with the addition of a small amount of K and decreases further (but to a smaller extent) with the addition of more K. Correspondingly, the selectivity of C5+ hydrocarbons increases with the addition of K. As the temperature increases, the C1-C4 selectivity increases for the K-free catalyst but decreases for the K-containing catalyst. Correspondingly, the C5+ selectivity decreases with the temperature for the K-free catalyst but increases for the K-containing catalyst. Considering all temperatures and K loadings, the largest values of C5+ selectivity occur at the highest loading of K at intermediate temperatures. These results are consistent with previous studies of potassium on iron-based catalysts.2,3,5,11,14,18,34,35 Potassium has been reported to enhance CO adsorption but to restrict H2 adsorption on the iron surface.3,8-13 Hence, changes of surface concentrations of CO, H2, and chain-growth intermediates with potassium level improve the probability of continued chain growth and the formation of more highmolecular-weight hydrocarbons.3,7 Meanwhile, the effects of temperature on the K-containing Fe/AC catalysts can be smaller, resulting in a lesser change of hydrocarbon selectivity with the temperature on the K-containing Fe/AC catalysts. There are two possible reasons for the effect of temperature on the K-containing catalysts. One is that the temperature of 260 °C is not high enough to form as many iron carbides over the K-promoted Fe/AC catalyst due to strong interactions created between potassium and iron during catalyst reduction.5,36 As temperatures increase to 270 °C, more carbides are formed, and these lead to the formation of less CH4 and more C5+ hydrocarbons.32 Another reason could be that more CO is dissociated at the initial reaction stage (260 °C) on the K-promoted catalysts since potassium enhances CO adsorption.8-13 This causes higher CH4 selectivity and smaller C5+ selectivity at 260 °C over the K-containing catalysts. Raje et al.14 report that hydrocarbon selectivity is related to catalyst activity to some extent. They found that the methane (34) Yeh, E. B.; Schwartz, L. H.; Butt, J. B. J. Catal. 1985, 91, 241253. (35) Luo, M. S.; Davis, B. H. Appl. Catal. 2003, 246, 171-181. (36) Feng, L. J.; Li, X. G.; Dadyburjor, D. B.; Kugler, E. L. J. Catal. 2000, 190, 1-13.


Ma et al.

Table 2. Selectivity Summary of Fe/AC Catalysts with or without Potassiuma catalyst temperature,

15.7 Fe/AC (0 K) °Cb

TOS, h CH4 C2-C4 C5+ C2)-C4)/C2O-C4O C5)-C11)/C5O-C11O C4 C5-C11 a

15.7 Fe/0.9 K/AC (1 K)

15.7 Fe/2 K/AC (2 K)

260

270

280

290

300

260

270

280

260

270

280

290

300

8-30

30-53

53-77

77-101

101-126

5-22

22-46

46-70

3-22

22-46

46-70

70-95

95-117

21.7 51.9 26.4

24.6 52.7 22.7

28.2 51.2 20.6

5.7 33.2 61.1

6.2 35.0 58.8

6.8 35.5 57.7

7.6 36.2 56.3

18.4 51.1 30.6

Hydrocarbon Selectivity, wt % 34.4 7.8 6.6 49.1 41.7 34.4 16.5 50.5 59.0

8.6 34.9 56.5

7.2 44.0 48.8

0.79 0.75

0.58 0.85

0.41 0.58

0.16 0.30

Olefin/Paraffin Ratio 0.10 5.67 4.80 0.17 1.15 1.03

4.64 0.98

6.86 1.29

5.84 1.23

5.92 1.21

5.76 0.85

5.10 0.79

71.49 78.22

73.41 70.87

74.80 78.47

72.62 75.20

Internal-Olefin Contentc, wt % 70.83 8.53 5.96 65.75 18.37 13.67

6.45 14.84

0.00 3.32

6.54 8.87

5.34 9.52

5.45 10.13

6.40 11.17

Mean values. b Other process conditions: 3 Nl/g cat/h, 300 psig, H2/CO ) 0.9. c Obtained from eq 7.

selectivity increases slightly with increasing conversion over precipitated-iron catalysts containing 0.36-2.21 parts K per 100 parts Fe. Quantitatively, the net increase of CH4 selectivity is less than 1.0% when CO conversion changes from 30 to 80%. In the present work, however, increasing K to 12.6 parts per 100 parts of Fe increases CO conversion less than 20% (see Table 1), while CH4 selectivity decreases up to 13%. Apparently, the significant change of hydrocarbon selectivity over the K-promoted Fe/AC catalysts is primarily due to the potassium promoter, but not to the change of conversion on the catalyst. Ratios of olefins to paraffins for the three catalysts at different temperatures are also presented in Table 2. These parameters are given separately for C2-C4 and C5-C11 products. At all temperatures, these values show a large increase for the 1K catalysts relative to the 0K catalyst, but only a small increase when the 2K catalyst is used relative to the 1K catalysts. Clearly, a small amount of potassium greatly improves the formation of primary olefins (1-olefins), while a larger amount of K has a proportionately smaller effect. This observation is consistent with several studies of the potassium effect3,5,6,13,15,20,37-39 and has been explained by potassium having a low hydrogenation rate of 1-olefins.1,3,9,15 Values of the internal-olefin (int-olefin) content for the three catalysts are also shown in Table 2. The 0K catalyst shows much higher int-olefin selectivity relative to that of the 1K catalysts, and the values for the 2K catalyst are somewhat smaller than those for the 1K catalyst. This is true for both HC ranges of C2-C4 and C5-C11. This again indicates that potassium suppresses the rate of isomerization of 1-olefins to int-olefins. The int-olefin selectivity does not appear to change consistently with temperature. In summary, increases in potassium loading for the Fe/AC catalysts enhance 1-olefin formation but suppress the isomerization of 1-olefins to int-olefins. Overall modification of the distributions of olefins by temperature is much less pronounced than that by potassium loading, as shown in Table 1. For the 0K catalyst, the ratio of olefins to paraffins for C2C4 HCs is approximately equal to that for C5-C11, as shown in Table 2. For the 1K and 2K catalysts, the ratios for C2-C4 are larger than those for C5-C11 by a factor of 4-7. This shows that potassium improves olefin formation for lower carbon numbers. Meanwhile, the selectivity of int-olefins is greater at higher carbon numbers, indicating the extent of secondary reactions of olefins. This trend is in good agreement with our (37) Duvenhage, D. J.; Coville, N. J. Catal. Lett. 2005, 104, 129-133. (38) Rankin, J. L.; Bartholomew, C. H. J. Catal. 1986, 100, 526-532. (39) Venter, J. J.; Vannice, M. A. Catal. Lett. 1990, 7, 219-240.

earlier study with Fe-Mo catalysts26 and is consistent with reports over precipitated-iron catalysts.3,14,40-44 It was reported3,5,15,45 that increases in temperature lead to some decrease in 1-olefin selectivity for low potassium loadings, that is, 0.5-0.7 wt %. The opposite trend was reported for high potassium loadings, because high potassium loadings decrease rates of secondary reactions, such as hydrogenation and isomerization.3,5 Alternatively, the higher loadings of potassium result in an enhanced desorption rate of 1-olefins.3 Our results are in good agreement with the literature for low levels of K promotion of precipitated-iron catalysts (K/Fe < 0.016) but not for precipitated-iron catalysts with high K loadings (K/Fe > 0.016), although the potassium loadings in the current Fe/AC catalysts are higher (K/Fe ) 0.09-0.18). It is also possible that the AC support decreases the effectiveness of potassium for the formation of olefins. Effect of Potassium on Carbon Number Distribution. The distribution of hydrocarbons formed over the Fe/AC catalyst covers a shorter carbon range (C1-C34) compared to the broad range of hydrocarbon distribution over the precipitated-iron catalyst. Carbon number distributions of n-paraffins and isoparaffins produced over catalysts 0K, 1K, and 2K are shown in Figures 3-5, respectively. Analogous distributions of olefins are shown in Figures 6-8. Paraffinic products of the 0K catalyst (Figure 3) consist mainly of n-paraffins with a small amount of branched paraffins. Adding potassium (Figures 4 and 5) significantly decreases the n-paraffin fraction but greatly increases the fraction of the branched paraffins. This indicates that the addition of potassium improves the fuel quality of the liquid. In all cases, n-paraffins can be detected up to C34, regardless of the amount of potassium present. However, branched paraffins can be detected only up to C16 for the 0K catalyst and only up to C25 for the 1K and 2K catalysts. Hence, the potassium appears to increase the branching of short- and medium-chain paraffins (up to about C25). The decrease of n-paraffins and the concomitant increase of branched paraffins by potassium have been little reported so far, and the formation mechanism of the branched paraffins remains unclear. Three possible reaction pathways have been (40) Raje, A. P.; Davis, B. H. Catal. Today 1997, 36, 335-345. (41) Pennline, H. W.; Zarochak, M. F.; Stencel, J. M.; Diehl, J. R. Ind. Eng. Chem. Res. 1987, 26, 595-601. (42) Lou, M. S.; Davis, B. H. Fuel Process. Technol. 2003, 83, 49-65. (43) Bukur, D. B.; Nowicki, Z.; Manne, R. K.; Lang, X. S. J. Catal. 1995, 155, 366-375. (44) Bukur, D. B.; Lang, X.; Ding, Y. Appl. Catal. 1999, 186, 255275. (45) Donnelly, T. J.; Satterfield, C. N. Appl. Catal. 1989, 52, 93-114.



Figure 3. Paraffin distributions over 0K catalyst at (a) 270 °C and (b) 280 °C. Other reaction conditions: 300 psig, 3 Nl/g cat/h, H2/CO ) 0.9.


proposed. In the first,46,47 alkene and/or alkyl intermediates formed during FTS undergo skeletal isomerization and then hydrogenation to form the isomers of iso-paraffins. In the second,48,49 n-paraffins are isomerized to form branched paraffins. The third pathway postulates hydrocracking/decomposition of long-chain hydrocarbons or unsaturated branched alkenes and their subsequent hydrogenation.50,51 It is known that the secondary reactions (isomerization, hydrocracking/decomposition) particularly take place on acid sites, either Lewis acid sites (L sites) or Bronsted acid sites (B sites).50,52,53 As indicated by our previous work,26 Fe/AC catalysts contain iron carbides and some unreduced iron oxide phases, and/or unreduced Fe-K oxides if a potassium promoter is present, since potassium is not reduced during H2 reduction at 400 °C.36 Iron carbides are active for FTS, while metal oxides are generally inactive.11,32,33 Therefore, it is possible that L sites could be created on the

Fe/AC catalyst after adsorbed CO molecules interact with unreduced oxides of iron or iron-potassium. However, even if L sites could be created on the Fe/AC surface, it is unlikely that all three pathways mentioned above take place to form branched alkanes under FTS conditions. First, alkanes formed during FTS are normally inert and do not participate in any secondary reactions.54 Moreover, the hydrocracking/decomposition of hydrocarbons is much more difficult than the isomerization of alkenes and needs critical conditions and special acid catalysts.50 However, alkenes formed during FTS are active and can be readily readsorbed on the catalyst surface to participate in secondary reactions.11,54-61 Therefore, the second and third reaction pathways above may not be the source of branched paraffins. The predominant method of formation of branched paraffins under FTS conditions over an Fe/AC catalyst is probably by skeletal isomerization of the readsorbed alkenes or intermediates on some Fe-K oxide acid sites followed by hydrogenation on the Fe-K/AC catalysts.

(46) Lappidus, A. L.; Krylova, A. Y. R. Russ. Chem. ReV. 1998, 67, 941-950. (47) Song, X. Sayari, A. Appl. Catal. 1994, 110, 121-136. (48) Liu, Z. W.; Li, X. H.; Asami, K.; Fujimoto, K. Appl. Catal. 2006, 300, 162-169. (49) Orlov, Kh. Ya.; Martynov, A. A.; Bulychev, V. P. IzV. Akad. Nauk SSSR Ser. Khim. 1963, 9, 1636-1641. (50) Van Bekkum, H.; Flanigen, E. M.; Jansen, J. C. Stud. Surf. Sci. Catal. 1991, 58, 455-473. (51) Engelen, C. W. R.; Wolthuizen, J. P.; van Hoff, J. H. C. Appl. Catal. 1985, 19, 153-163. (52) Venuto, P. B.; Hamilton, L. A.; Landis P. S. J. Catal. 1966, 5, 484493. (53) Rao, V. U. S.; Gormley, R. J. Catal. Today 1990, 6, 207-234.

(54) Kuipers, E. W.; Vinkenburg, I. H.; Oosterbeek, H. J. Catal 1995, 152, 137-146. (55) Iglesia, E. Appl. Catal. 1997, 161, 59-78. (56) Iglesia, E.; Reyes. S. C.; Madon. R. J. J. Catal. 1991, 129, 238256. (57) Jordan, D. S.; Bell A. T. J. Phys. Chem. 1986, 90, 4797-4805. (58) Wang, Y. N.; Ma, W. P.; Lu, Y. J.; Yang, J.; Xu, Y. Y.; Xiang, H. W.; Li, Y. W.; Zhao, Y. L.; Zhang, B. J. Fuel 2003, 82, 195-213. (59) Nowicki, L.; Ledakowicz, S.; Bukur, D. B. Chem. Eng. Sci. 2001, 56, 1175-1180. (60) Schulz, H.; Claeys, M. Appl. Catal. 1999, 186, 91-107. (61) Ji, Y. Y.; Xiang, H. W.; Xu, Y. Y.; Li, Y. W. Appl. Catal. 2001, 214, 77-86.



The increase in secondary reactions of intermediate-sized alkenes with an increase in the number of carbon atoms has been ascribed1,43,55,62 to larger solubility, and/or the slower diffusion ability of high-molecular-weight hydrocarbons in catalyst pores. This is evident experimentally by larger values of int-olefin selectivity at C5-C11, as shown in Table 2. The fact that potassium enhances the formation of branched paraffins and decreases the formation of n-paraffins suggests that potassium increases the isomerization of alkenes and/or alkyl intermediates to iso-paraffins but suppresses chain termination by the hydrogenation of alkyl intermediates. From Figures 3-5, the 0K catalyst and the K-containing catalysts show different distribution trends. Over the 0K catalysts, both n-paraffins and branched paraffins are formed, and both distributions decrease monotonically with increasing carbon number. Over the 1K and 2K catalysts, both n-paraffins and branched paraffins (but mainly branched paraffins) show a minimum around C2-C5, then a secondary maximum around C8-C10, followed by a monotonic decrease with the carbon number. Compared to the 0K catalyst, the paraffin content at C1 is much lower for the 1K and 2K catalysts, and individual hydrocarbons after C5 are much higher, also indicating that potassium shifts hydrocarbon distribution to high-molecularweight hydrocarbons. Olefin distributions over the 0K, 1K, and 2K catalysts are shown in Figures 6-8, respectively. Linear olefins up to C18 (62) Tau, L. M.; Dabbadh, H. A.; Davis, B. H. Energy Fuels 1990, 4, 94-99.

Ma et al.

Figure 6. Olefin distributions over 0K catalyst at (a) 270 °C and (b) 280 °C. Other reaction conditions: 300 psig, 3 Nl/g cat/h, H2/CO ) 0.9.

are detected for all three catalysts. For int-olefins, the addition of potassium increases the chain maximum length of the intolefins detected from C10 to C16-C18. 1-Alkenes are considered40,43 to be primary products of FTS; they can subsequently hydrogenate to alkanes or be readsorbed on the catalyst surface to isomerize. Therefore, the production of int-olefins and branched paraffins with larger carbon numbers may be explained by postulating that potassium decreases the desorption rate of lower-molecular-weight intermediates, since lower C2-C5 products over the K-containing catalysts are observed in Figures 3-5. The decrease in desorption rate leads to higher concentrations of higher-molecular-weight alkyl intermediates and, consequently, the production of longer branch alkanes or isoalkenes by the isomerization of readsorbed alkenes or alkyl intermediates. The distribution of overall olefin products with carbon numbers is qualitatively the same over both the potassiumpromoted and unpromoted catalysts (Figure 6 compared to Figures 7 and 8). However, the olefin distributions are different from the paraffin distributions (Figures 3-5): C2 olefins are low, and the olefin distribution has a single maximum around C3-C4. In addition, changes with temperature are minor, consistent with the results shown in Table 2. When Figure 6 is compared to Figures 7 and 8, potassium increases the selectivity to linear olefins and decreases the selectivity to int-olefins. The largest changes of int-olefins by potassium can be found at C4C8.





with and without a potassium promoter. The FTS kinetic model, which is first-order in hydrogen partial pressure,1,11,63-65 Overall hydrocarbon distributions over the three catalysts are plotted in Figure 9 over the entire hydrocarbon range of C1-C34. The data are described by Anderson-ShulzFlory (ASF) coordinates, each with a single value of the chaingrowth probability (R). The unpromoted 0K catalyst shows a low value of R, about 0.64 at 270-280 °C, whereas the two K-promoted Fe/AC catalysts show slightly higher R values, close to 0.75. To sum up, the Fe/AC catalyst is characterized by the distribution of hydrocarbons efficiently ending at C34, substantially different from the very wide hydrocarbon distribution over the traditional precipitated-iron catalysts. The “limited” hydrocarbon distribution on the AC-supported iron catalysts is presumed to be accounted for by special textural properties of the AC support.26 Potassium enhances the formation of 1-alkenes and branch paraffins but suppresses formation of the int-olefins and n-paraffins. Effect of Potassium on Kinetic Parameters. BrunauerEmmett-Teller, X-ray diffraction analysis, and Mossbauer have been used5,11 to interpret qualitatively the effect of potassium on catalyst performance. In order to explain the effect of potassium quantitatively, we calculated the pre-exponential factor, k0, and the activation energy, Ea, of the Fe/AC catalysts

rFTS ) kPH2

(12)

has been considered to be valid for syngas conversion up to about 70%. Here, k is the apparent kinetic rate constant, related to temperature by

k ) k0 exp[-Ea/(RgT)]

(13)

Values of rFTS are obtained using eq 10. Values of k for the catalysts 0K, 1K, and 2K are plotted in Figure 10a-c. Note that data at 290 and 300 °C for the 0K catalyst are not shown, as syngas conversions are greater than 70% in Table 1 and Figure 1. Reasonably good correlations of the rate constant with temperature (R2 ) 0.97-0.99) for all three catalysts imply that the assumption of first order in H2 is valid at least for temperatures below 280 °C. The rate constants shown in Figure 10 are apparent, or macrokinetic, values that include transport effects of reactant products in the AC pores. Values of the apparent activation energy and apparent pre-exponential factors for the three (63) Zimmerman, W. H.; Bukur, D. B. Can. J. Chem. Eng. 1990, 68, 292-301. (64) Dry, M. E.; Sgingles, T.; Boshof, L. J. J. Catal. 1972, 25, 99-104. (65) Shen, W. J.; Zhou, J. L.; Zhang, B. J. J. Nat. Gas Chem. 1994, 4, 385-400.


Figure 9. Overall hydrocarbon distributions over unpromoted and potassium-promoted Fe/AC catalysts at (a) 270 °C and (b) 280 °C. Other reaction conditions: 300 psig, 3 Nl/g cat/h, H2/CO ) 0.9.

Figure 10. Arrhenius plot for (a) 0K, (b) 1K, and (c) 2K catalysts. Other reaction conditions: 300 psig, 3 Nl/g cat/h, H2/CO ) 0.9.

catalysts are shown in Table 3. Clearly, the potassium promoter significantly decreases the apparent activation energy of the catalysts. The values are essentially consistent with the values

Ma et al.

of 80-125 kJ/mol reported on unpromoted iron catalysts15,66,67 and values of 55-95 kJ/mol for K-containing iron catalysts.58,63-65,68,69 Further, the apparent pre-exponential factor of the 0K catalyst is larger than those of the 1K and 2K catalysts by 3 and 7 orders of magnitude, respectively. This suggests that potassium loading significantly reduces the value of the preexponential factor as well. Several studies3,8-13 have pointed out that potassium causes an increase in the adsorption equilibrium constant for carbon monoxide on the iron surface but a decrease in H2 chemisorption. The decrease in both Ea and k0 with the addition of potassium is consistent with these studies, since potassium-facilitated CO adsorption can lead to increased formation of the C* intermediates, while potassium-weakened H2 chemisorption can slow down the frequency of the effective collisions between the molecules of CO and H2. The decrease in Ea with increasing potassium content in the Fe/AC catalysts implies enhancement of FTS by the potassium promoter, while the decrease in k0 suggests inhibition. The net result of these two opposing effects of potassium may be an increase or a decrease in the apparent kinetic rate constant, k. From Table 3, for the 1K catalyst, the increase of the FTS rate by lowering Ea is greater than the decrease of the rate by lowering k0, except at the highest temperature considered (280 °C). However, for the 2K catalyst, increasing the potassium level results in an increase of the FTS rate by lowering Ea, which is always less than the decrease of the rate by decreasing k0. Therefore, the higher FTS activity of the 1K catalyst relative to the 0K catalyst is primarily due to the smaller amount of potassium contributing more to the decrease in the activation energy (Ea), while the 2K catalyst is the least active, because the high potassium loading has a greater contribution to the decrease in the pre-exponential factor (k0). In addition to the change in rFTS, the higher WGS activity on the 1K catalyst gives rise to higher hydrogen partial pressures. According to eq 12, this would be also responsible for the higher activity of the 1K catalyst. Effect of Potassium on Yield and Selectivity of Alcohols. Oxygenates are formed over all three catalysts. The yields of oxygenates for various potassium loadings and temperatures are shown in Figure 11. Over the 0K catalyst, the yield is high at lower temperatures but decreases markedly with increasing temperature. Over the 1K and 2K catalysts, yields are low at lower temperatures but increase slightly with increasing temperature. This result is consistent with some previous studies over precipitated-iron catalysts.3,5,34,70 These trends are mirrored by the oxygenate selectivity shown in Figure 12. Basically, the values of oxygenate selectivity are low, less than 12% and 3% on the unpromoted and K-containing catalysts, respectively. The effects of potassium on the variation of alcohol yield and selectivity with the temperature suggest that oxygenate formation and hydrocarbon formation are in competition. At lower temperatures, more CO is adsorbed, due to potassium increasing CO adsorption on the catalyst surface.8-13 The CO can be dissociated to form hydrocarbons on the K-containing Fe/AC catalysts. With increasing temperature, these adsorbed CO molecules might participate directly in alcohol formation (66) Huff, G. A., Jr.; Satterfield, C. N. Ind. Eng. Chem. Process. Des. DeV. 1984, 23, 696-705. (67) Atwood, H. E.; Bennett, C. O. Ind. Eng. Chem. Process. Des. DeV. 1979, 18, 163-170. (68) Lox, E. S.; Froment, G. F. Ind. Eng. Chem. Res. 1993, 32, 71-82. (69) Ledakowicz, S.; Nettelhoff, H.; Kokuun, R.; Deckwer, W. D. Ind. Eng. Chem. Process. Des. DeV. 1985, 24, 1043-1049. (70) Claeys, M.; Schulz, H. Prepr.sAm. Chem. Soc., DiV. Pet. Chem. 2004, 49, 195-199.



Table 3. Summary of Macrokinetic Parameters Estimated over Fe/AC Catalysts with or without Potassiuma catalyst

Ea, kJ/mol

k0, mmol/g Fe/h/MPa

decreased amount of k by decreasing k0

0K 1K 2K

121.00 88.00 51.00

9.0E + 13 6.7E + 10 9.5E + 06

1.3E+03 9.4E+06

a

increased amout of k by decreasing Ea 260 °C 270 °C 280 °C 1.6E + 03 7.0E + 06

1.4E + 03 5.2E + 06

1.2E + 03 4.0E + 06

k ) k0 exp[-Ea/(RgT)].

through a CO-insertion mechanism.71,72 This would result in relatively higher oxygenate selectivity at high temperatures on the K-promoted catalyst, compared to the unpromoted Fe/AC catalyst. Effect of Potassium on Alcohol Distribution. Oxygenates produced on all three catalysts consist primarily of C1-C5 straight-chain alcohols. Other oxygenates, such as acetaldehyde, acetone, and C3-C5 branched alcohols, were also detected and quantified, but the amounts are small and are not shown here. Figure 13 illustrates the change of the alcohol distributions with the temperature over the three catalysts. Ethanol is the dominant oxygenate in all cases, followed by C3-OH, C1-OH, C4-OH, and C5-OH in turn. This order of alcohol selectivity is also found on the Mo-Fe/AC catalysts reported elsewhere26 and is consistent with the result reported by this laboratory29 over MoNi-K/AC alcohol catalysts.

When the distributions of C1-C5 alcohols for the 0K and the K-containing catalysts are compared, the oxygenates formed over the unpromoted Fe/AC contain approximately 35% methanol, 45% ethanol, 10% propanol, and a balance of butanol and pentanol. After K is added, methanol in the oxygenate decreases to less than 10%, while ethanol increases to approximately 60%, propanol to approximately 15%, and the balance is butanol and pentanol. This result is in good agreement with previous studies on precipitated-iron catalysts,3,15 which reported that potassium suppresses the formation of methanol and increases the selectivity of higher-molecular-weight alcohols. Finally, Figure 13 shows that an increase in temperature enhances the formation of methanol and suppresses the formation of ethanol and propanol on the unpromoted Fe/AC catalyst, but this trend is indiscernible for the K-containing Fe/AC catalysts. Conclusions The change of catalytic activity by potassium depends greatly on the K content of the catalyst. The addition of 0.9 wt % potassium into the Fe/AC catalyst increases the activity of the catalyst, more so than the addition of 2 wt % K. Increasing K

Figure 11. Change of oxygenate yield with temperature on the unpromoted and potassium-promoted Fe/AC catalysts. Other reaction conditions: 300 psig, 3 Nl/g cat/h, H2/CO ) 0.9.

Figure 12. Change of oxygenate selectivity with temperature on the unpromoted and potassium-promoted Fe/AC catalysts. Other reaction conditions: 300 psig, 3 Nl/g cat/h, H2/CO ) 0.9.

Figure 13. Alcohol distribution over (a) 0K, (b) 1K, and (c) 2K catalysts. Other reaction conditions: 300 psig, 3 Nl/g cat/h, H2/CO ) 0.9.


levels increases the deactivation rate. The activity and stability characteristics of this catalyst are quantitatively different from those of the unsupported precipitated-iron catalyst. The ratio of water-gas shift to Fischer-Tropsch reactions, RWF, is also greatest for the 0.9 wt % K catalyst. These results are consistent with values of CO2 selectivity, the partial pressure coefficient, and the H2/CO usage ratio. For the selectivity of C5+ (liquid phase) hydrocarbons, the 0.9 wt % K catalyst is far superior to the unpromoted catalyst, and a small improvement is obtained for the 2 wt % K catalyst. The effect of K addition on C1-4 (vapor phase) selectivity is opposite. This behavior can be attributed to the formation of more carbide phases by potassium. For the K-containing catalyst, as the temperature increases, the C5+ selectivity decreases. The olefin/paraffin ratios are much greater for the 0.9 wt % K catalyst than for the unpromoted catalyst, with again a slightly higher value for the 2 wt % K catalyst. This holds for hydrocarbons in the C2-C4 range and in the C5-C11 range. Finally, the ratios of int-olefin/1-olefin are much larger for the unpromoted catalyst and least for the 2 wt % K catalyst. This indicates that the promoter is less active for secondary reactions. (71) Pichler, H.; Schulz, H. Chem. Ing. Tech. 1970, 42, 1162-1174. (72) Chuang, S. S. C.; Stevens, R. W.; Khatri R. Top. Catal. 2005, 32, 225-231.

Ma et al.

At least for carbon numbers of 25 or less, increasing the K level to 0.9 wt % greatly decreases the amount of n-paraffins and int-olefins and dramatically increases branched paraffins and 1-olefins. These effects of K are smaller for the further increase of the K level to 2 wt %. In all cases, ASF distributions are obtained, with R values higher for the K-containing catalysts. The kinetics of FTS were analyzed for all three catalysts using a simple first-order model for conversions of less than 80%. For the 0.9 wt % K catalyst compared with the unpromoted catalyst, the decrease of activation energy increases the rate more than the decrease of the pre-exponential decreases the rate. For the 2 wt % K catalyst relative to the 0.9 wt % K catalyst, the opposite situation occurs. Alcohol products from C1 through C5 can be easily detected. In all cases, the C2-alcohol is the highest, followed by the propanol, methanol, butanol, and pentanol in turn. As the K level of the catalyst is increased, methanol is decreased and the higher alcohols are increased. Alcohol yields are lower at low temperatures, and analogous results are obtained for alcohol selectivity. Acknowledgment. This study was supported by the U.S. Department of Energy under Cooperative Agreement DE-AC2299FT40540 with the Consortium for Fossil Fuel Science (CFFS). EF060654E

Potassium Effects on Activated-Carbon-Supported Iron Catalysts for

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