Ind. Eng. Chem. Process Des. Dev. 1903, 22, 97-103
97
Effects of Potassium Promotion on the Activity and Selectivity of Iron Fischer-Tropsch Catalysts Hlronorl Arakawa and Alexis T. Bell' Miteriais and Molecuier Research Divisbn, Lawrence Berkeby Laboratoty, and Depa-nt University of Callfmia, M e b y , Callfornle 94720
of Chembi Engineering,
A systematic study has been carried out of the effects of potassium promotion on the performance of alumina-
supported iron catalysts for Fischer-Tropsch synthesis. The results show that potassium promotion causes a decrease in Fe dispersion, an increase in the strength of CO chemisorption on reduced Fe, a decrease 10 the turnover frequency for total CO consumptlon, an Increase in the average mol6cular weight and olefin to paraffin ratio of the products, and an Increase in the water-gas-shift activity. The addition of potassium is also found to increase the rate of catalyst carburization. This process occurs more rapWty in a mixture of CO and H, than in CO alone.
Introduction Numerous studies have shown that the addition of small amounts of potassum to iron catalysts used for FischerTropech synthesis affecta the performance of such catalysts in a variety of ways (Storch et al., 1951; Anderson et al., 1952; Anderson, 1956; Dry et al., 1966, 1969; Dry and Ooethuizen, 1968; Shah and Perotta, 1976; Dry, 1976,1981; McVicker and Vannice, 1980; Benzinger and Madix, 1980; Bonzel and Krebs, 1981; Amelse et al., 1981). Anderson et al. (1952) have reported that the rate of synthesis gas consumption per volume of catalyst increases up to a maximum at about 0.5 parts of K20 per 100 parts by weight of Fe, but thereafter it declines with further addition of the promoter. The addition of potassium also causes an increase in the average molecular weight of the product, the olefin to paraffin ratio, and the content of oxygenated products, and it causes a decrease in the H2 to CO usage ratio due to an increased water-gas-shift activity. In a more recent study, Dry and Oosthuizen (1968) have reported that the addition of potassium causes a decrease in the surface area of fused magnetite catalysts as well as a decrease in methane selectivity. Subsequent work by Dry et aL (1969) showed that potassium promotion of fused magnetite increases the heat of CO adsorption on the reduced catalyst at low coverages and at the same time increases the initial heat of H2 adsorption. On the other hand, the heat of adsorption of C02 at all coverages increases in the presence of potassium. The effects of metallic potassium deposited on an Fe(100) surface have been examined by Benziger and Madix (1980). These authors found that potassium enhances the CO and H2 binding strengths and increases the amount of CO dissociation relative to the clean surface. The effects of KzCO3 coverage of an Fe foil on Fischer-Tropsch synthesis has been studied by Bonzel and Krebs (1981). Their results show that potassium causes a decrease in the rate of methane formation, an increase in the rate of carbon deposition, and a shift in selectivity toward the formation of longer chain molecules. Furthermore, XPS measurements on the K 2p levels showed that the potassium, associated in some form with oxygen, appears not to be covered by theldepositedcarbon but rather sits on top of the carbon layer. The present work was undertaken to develop a systematic understanding of the effects of potassium promotion of iron catalysts prepared in a common fashion. The range of potassium to iron ratios explored in this work was much
broader than that investigated previously. Particular attention was given to the effects of potassium on catalyst activity, methane selectivity, olefin to paraffin selectivity, water-gas-shift activity, and carbon deposition kinetics. Experimental Section Alumina-supported iron catalysts promoted with potassium were prepared by the addition of Alon C (Cabot Corp.) to an aqueous solution of Fe(N03)3.9H20and KN03. The resulting slurry was mixed and then heated gradually to 393 K for 12 h to drive off all water. After drying, the catalyst was sieved to sub65 mesh and calcined in flowing air. The calcining temperature was raised slowly to 623 K and then maintained at this level for 12 h. Finally, the calcined material was reduced in flowing Hz. The reduction temperature was raised slowly to 673 K and then maintained at this level for 48 h. The procedure for preparing an unpromoted iron catalyst was identical with that described above, with the exception that KNOBwas left out of the initial impregnation solution.,The final catalysts contained 20% by weight of iron in every case and had K/Fe ratios of 0.0, 0.022, 0.065, or 0.20. Measurements of H2and CO chemisorption were carried out in a static adsorption system connected to a vacuum manifold. One gram of the catalyst was placed in a small glass adsorption cell and reduced at 673 K under H2 for 12 h. The sample was then evacuated to lo4 torr at 673 K and cooled to the desired adsorption temperature. Hydrogen adsorption was carried out at 373 K and CO adsorption was carried out at room temperature. For each sample, an adsorption isotherm was determined between 30 and 300 torr, allowing 1to 3 h for equilibration of the gas with the sample at each measurement point. Extrapolation of each isotherm to zero pressure provided a measure of the total amount of gas adsorbed. A determination of the amount of weakly pdsorbed gas was made by evacuating the sample for 2 min and then carrying out a second adsorption. Catalyst performance was tested using a glass microreactor heated in a fluidized sand bath. Hydrogen and CO were supplied to the reactor at 1atm from a gas manifold. The H2 was purified of oxygen by passage through a DEOXO unit (Engelhard), followed by a molecular sieve trap, and the CO was purified by passage through a dry iceacetone trap. The reaction products were analyzed by on-line gas chromatography, using a Varian Model 1420 chromatograph equipped with thermal conductivity detectors. Product separation was achieved with a 2 mm x 0 1982 American Chemical Society
Q8 Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983 Table I. Chemisorption of H, and CO (Catalyst: 20% Fe/Al,O,) adsorption: K/Fe
H, a t 383 K
0.000 0.022 0.065 0.200
45.6 35.2 26.3 20.5
p mol/g CO a t 296K
D, % b 2.4 2.0 1.4 1.2 a Measured o n used catalysts following reduction in H, Determined from H, chemisorption. for 1 2 h a t 673 K. HJCO 1.05 1.01 1.06 1.02
43.3 34.8 24.8 20.1
Table 11. Retention of Adsorbed (Catalyst: 20% Fe/Al,O,)
H, and CO
adsorbate loss, %" K/Fe
evac. time, min
H, at 383K
CO at 296K
0 2 16 80
0 9 27 40
0 25 32 43
0 2 16 80
0 16 32 45
0 3 12 24
0.00
0.20
K/Fe
Figure 1. Effects of K/Fe ratio on specific activity.
" Determined from the amount of adsorbate taken u p following evacuation. 3.6 m column packed with Chromsorb 106. The column oven was temperature programmed from 313 to 503 K at 10 K/min. Calibrations for C1 through C4 hydrocarbons and C02 were obtained by using mixtures containing known quantities of each component. Calibration curves for CH30H, C2H50H,and H 2 0 were produced by measuring the responses for helium saturated with each of these components at different temperatures. Each experiment with a new catalyst sample was initiated by placing 1.8 g of catalyst in the reactor and then reducing the catalyst in flowing H2 at 673 K for 12 h. The temperature was then reduced to the desired reaction temperature and the flow of pure H2 was replaced by a mixture of Hz and CO. The reaction was normally allowed to progress for 140 min. At the end of this time, the catalyst was again reduced in pure H2 a t 673 K for 12 h, in preparation for the next experiment. Following this procedure assured that each experiment was initiated with the catalyst in a fixed composition state. Results Chemisorption of H2 and CO. The quantities of H2 and CO adsorbed at saturation coverage are presented in Table I as a function of the potassium content of the catalyst. It is noted that for each K/Fe ratio nearly identical amounts of H2 and CO are adsorbed. Assuming that H2 adsorbs dissociatively, this suggests that CO adsorbs either in a bridge structure, so as to occupy two Fe sites, or dissociatively to form C and 0 atoms. Infrared observations were carried out in an effort to distinguish
between these two forms of CO adsorption. No bands could be observed when a stream of CO in helium was passed over the catalyst at room temperature, indicating that CO does not adsorb in a bridging mode, with the C-0 axis perpendicular to the metal surface. However, one cannot conclude from this that CO is dissociatively adsorbed,since it is possible that molecular adsorption occurs with the C-O axis parallel to the metal surface, in which case coupling of the CO dipole moment to the infrared radiation would be very weak. Table I also shows that the addition of potassium causes a decrease in the uptake of H2 and CO. These data indicate a steady decrease in the dispersion of Fe as the K/Fe ratio increases. The effect of potassium promotion on the strength of H2and CO chemisorption was investigated by determining the amount of gas that could be readsorbed following isothermal desorption of the adsorbate into a vacuum from a surface initially covered to saturation. The results of these experiments are preaented in Table 11. It is observed that potassium promotion slightly weakens the adsorption of H2 and significantly strengthens the adsorption of CO. Catalyst Activity and Selectivity. The effects of potassium loading on catalyst activity and selectivity are presented in Table III and Figure 1. In all cases the data were obtained 70 min after initiation of an experiment, using a freshly reduced catalyst sample. Figure 1 shows that the turnover frequency for CO consumption, Nco, increases and then passes through a maximum as the K/Fe ratio is increased. A similar pattern is also observed in the turnover frequencies for forming C1 through C5 hydrocarbons. As the K/Fe ratio increases, the proportion of methane in the products decreases and the fraction of Cz+ products increases. This trend is evident both from the relative position of the curves for N C ,shown in Figure 1, and the product distributions given in Table 111. The results in Table I11 indicate that olefin to paraffin ratio
Table 111. Effect of K/Fe Ratio o n Catalyst Activity and Selectivity (Catalyst: 20% Fe/Al,O,)' product distribution, mol %
co K/Fe
a
0.000 0.022 0.065 0.200 0.400 Reaction
conv, %
C,
C,=
C;
C,=
20.1 57.6 1.5 0.9 16.1 20.4 59.0 1.0 17.4 1.6 18.0 8.3 9.3 14.3 49.2 7.3 32.4 20.3 0.8 20.8 18.6 1.4 29.3 trace 22.5 conditions: T = 533 K; HJCO = 3 ; GHSV
C;
C,=
11.3 10.2 2.1 0.3 trace
0.3 0.4 4.0 10.6 11.7
C;
5.8 5.2 3.9 0.6 trace = 6000 cm3 (STP)/(h g).
C,=
C,'
MeOH
EtOH
C,C
0.2
2.4 2.2 2.7 0.4 trace
0.4 0.3 0.2 0.4 trace
0.1 0.1 0.4 1.6 trace
3.4 2.2 4.4 7.1 12.6
0.4
1.2 5.0 5.3
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983 99 Table IV. Effects o f Temperature and K/Fe Ratio o n Catalyst Activity and Selectivity (Catalyst: 20% Fe/A1,O,)a product distribution, mol %
T,K conv,%
0.00
533 513 493 468
20.1 12.3 5.1 1.5
57.6 55.8 53.2 46.6
0.9 1.0 1.2 3.4
648 533 513
11.4 7.3 3.3
35.5 32.3 23.7
19.5 20.3 21.4
0.20
a
co
K/Fe
C,
C,=
C; 16.1 14.1 11.5 8.0
1.1 0.8 trace
C,=
C;
C,=
1.5 1.0 4.6 9.3
11.3 9.6 6.2 3.6
0.3 0.4 1.0 2.9
20.4 20.8 21.6
1.0 0.3 trace
10.7 10.6 13.7
C,
C,=
C;
5.8 6.0 6.5 5.2
0.2 0.4 1.7
2.4 3.6 3.6 3.7
0.4 1.9 3.8 5.5
1.3 0.6 trace
4.1 5.0 5.3
0.7 0.4 trace
0.2 0.4 0.6
0.8
MeOH EtOH 0.1
C,+
1.3 2.8
3.4 5.4 6.5 7.6
0.7 1.6 3.2
4.9 7.1 10.6
0.8
Reaction Conditions: HJCO = 3; GHSV = 6000 cm3 (STP)/(h g). K/Fe = 0 T = 533 K , H,/CO 3 GHSV = 6000 STP cm3/hr * g
Water-Gos-Shift Equilibrium
j
I m
0.2. For K/Fe = 0.065, nearly identical quantities of olefins and paraffms are produced, and at higher K/Fe ratios the C2+ product is almost exclusively olefinic. The influence of potassium loading on the synthesis of alcohols is also given in Table 111. As may be seen, the formation of methanol is essentially unaffected by potassium promotion, but the proportion of ethanol increases as the K/Fe ratio increases. Figure 2 illustrates the effect of potassium loading on the ratio of C02/H20 observed in the products. As the K/Fe ratio increases, the ratio of COPto H20 increases monotonically and approaches the equilibrium value predicted for the wate-as-shift reaction. Consistent with this,Figure 1shows that the ratio of Nco/Nco approaches 2 at high K/Fe ratios, as would be expected for the following reaction stoichiometry 2nCO + ( ~ / 2 ) H 2 C,H, + nCO2 +
The performance as a function of time of the unpromoted catalyst and the catalyst characterized by a K/Fe ratio of 0.20 is shown in Figure 3 and 4. The overall activity of the unpromoted catalyst (Figure 3), as judged by the s u m of the activities for COz and HzO production, declines by about a factor of 2 over a 2-h period. Over the same time period, the rates for forming methane, butane, and pentane remain more or less constant, but the rates for forming ethane and propane decrease after the first 40 min. By contrast with the behavior of the paraffins, the rate of formation of olefins increases throughout the 2-h period. The overall activity of the promoted catalyst (Figure 4)also decreases twofold in 2 h. During this time, the rates of formation of C2+ paraffins steadily decrease. The patterns for methane and CP+ paraffins steadily de-
K / F e = 0.20 T = 533 K . H,/CO = 3 GHSV = 6000 STP cm3/hr * g
I
0
20 4 0
60 80 100 120 140 160 I (min)
Figure 4. Variation of catalyst activity with time on stream: K/Fe = 0.20.
crease. The patterns for methane and CP+ olefins are very similar. The rates for forming these products increase during the first 40 min of reaction and then remain nearly constant. The effects of temperature on product distribution are summarized in Table IV for two of the catalysts. As the temperature decreases, the average product molecular weight, the olefin to paraffin ratio, and the proportion of alcohols formed increase. The primary effect of potassium promotion is to maintain a high olefin to paraffin ratio. While the olefin to paraffin ratio observed using the promoted catalyst does decrease somewhat with increasing temperature, this change is very small compared to that observed using the unpromoted catalyst. Temperature also affects the COP/H20ratio. Figure 2 shows that as the temperature increases, the C02/H20ratio approaches the line characterizing the water-gas-shift equilibrium.
100
Id. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983
Table V. Effects of HJCO Ratio and K/Fe Ratio o n CO Conversion and Product Distribution (Catalyst: 20% Fe/Al,O,)a
co
product distribution, mol %
conv, K/Fe HJCO
%
C,
C,=
C,=
C;
C,=
C;
C;
C,=
C,'
MeOH
EtOH
0.00
1 3 5
7.7 20.0 32.2
48.3 57.6 59.3
2.7 0.9 0.3
13.8 16.1 17.7
6.3 1.5 0.5
7.8 11.3 10.9
1.8 0.3 0.1
6.5 5.8 5.4
1.2 0.2 0.4
3.9 2.4 2.2
1.0 0.4 0.5
trace 0.1 trace
0.20
1 3 5
2.4 7.3 12.1
12.2 32.3 31.4
19.8 20.3 18.2
0.5 0.8 2.2
18.0 20.8 16.9
trace
13.8 10.6 9.9
0.9 0.6 1.6
8.5 5.0 4.9
2.2 0.4 1.5
trace
2.8 1.6 1.8
0.3 1.7
0.4 0.3
C,' 6.9 3.4 2.7 21.4 7.1 9.6
Reaction conditions: T = 533 K; GHSV = 6000 cm3 (STP)/(h 9 ) . Table VI. Effects of Space Velocity o n CO Conversion and Product Distribution (Catalyst: 20% Fe/Al,O,)a
K/Fe 0.00
0.20 a
GHSV, CO cm3(STP)/ conv, (hg) %
droduct distribution, mol % C,
C,=
Cy
3000 6000 18000
37.6 20.1 5.1
53.7 57.6 58.8
0.4 0.9 2.0
16.9 16.1 14.7
3000 6000
11.2 7.1
27.3 32.5
16.4 20.3
2.4 0.8
C,= 0.8
C;
C,=
1.5 3.5
13.1 11.3 9.8
0.4 0.3 0.8
17.2 20.8
0.5 0.3
9.7 10.6
C,
C,=
C,'
6.7 5.8 4.2
0.4 0.2 0.7
2.4 2.4
1.1 0.6
6.3 5.0
1.9 0.4
MeOH EtOH 3.5 0.3 trace
C,'
0.7
0.1 0.2
3.7 3.5 2.2
0.7 0.4
1.7 1.6
14.9 7.1
0.4
Reaction conditions: T = 533 K; HJCO = 3.
Table V presents the influence of H2/C0 ratio on the distribution of reaction products. As the proportion of H2 in the feed gas increases, the rate of reaction increases and, concurrently, the average product molecular weight, the olefin to paraffin ratio, and the formation of alcohols all decrease. As before, potassium promotion results in a significantly higher product molecular weight and olefin to paraffin ratio than can be achieved in the absence of potassium promotion. The feed-gas flow rate affects the product composition in several ways, as shown in Table VI. For the unpromoted catalyst, little change in the product molecular weight occurs when the flow rate is increased, but the olefin to paraffin ratio increases, indicating that olefins are a primary synthesis product and that a substantial portion of the Cz+ paraffins are formed by hydrogenation of the olefm. The molecular weight of the product obtained over the potassium-promoted catalyst appears to be sensitive to the feed flow rate, decreasing as the flow rate increases. Here too,the olefin to paraffin ratio is observed to decrease as the flow rate decreases. The results presented in Table VI show that for a fixed space velocity the olefin to paraffm ratio for the potassium-promoted catalyst is substantially greater than that for the unpromoted catalyst. Since the sensitivity of the olefin to paraffin ratio to space velocity is not very great for either catalyst, the major part of the enhanced olefrn to paraffm ratio observed upon promotion must be ascribed to a reduction in the hydrogenation capacity of iron due to the presence of potassium. A similar conclusion can be drawn for the results presented in Table 111 and Figure 1. The dependence of the C02/H20ratio on flow rate is given in Table VII. It is seen that the C02/H20ratio decreases as the flow rate increases, for both the promoted and unpromoted catalyst. This trend indicates that H 2 0 is a primary product and that its reaction with CO to form COz occurs via a secondary process. Characterization of Used Catalysts. The unpromoted iron catalyst and the catalyst promoted to the level of 20 parts of potassium per 100 parts of iron were characterized after use. In each case the catalyst was exposed at 533 K to a 3:l H2/C0 mixture, flowing at 180 cm3 (STP)/min, for 140 min and then flushed with helium for 10 min at 533 K. Next, the catalyst was cooled to room temperature in flowing helium. The crystal structure of
Table VII. Dependence of CO,/H,O Ratio o n Flow Rate (Catalyst: 20% Fe/AI,O,)Q GHSV, cm3 (STP)/(h g)
a
K/Fe
3000
6000
18000
0.00 0.20
33.1
.o
0.7 29.1
0.3 0.7
1
Reaction conditions: T = 533 K ; H,/CO = 3.
Table VIII. Elemental Analysis of Used Catalyst (20% Fe/Al,O,) K/Fe
C,wt %
H, wt %
C/Fe
H/C
0.00 0.20
2.02 3.20
0.22 0.30
0.47 0.75
1.30 1.12
the catalyst was determined by X-ray diffraction and the content of carbon and hydrogen by high-temperature combustion analysis. The results of these analyses are summarized in Table VIII. The X-ray powder diffraction patterns showed lines which are best identified with a mixture of e and x phases of iron carbide (i.e., Fe2.2Cand Fe5C2,respectively (Niemantsverdriet et al., 1980). No evidence was found, though, for a-iron, wuestite (FeO), hematite (Fe203),magnetite (cu-Fe20d,or cementite (Fe,C). With the exception that the diffraction pattern of the promoted catalyst was somewhat more intense than that of the unpromoted catalyst, no differences were observed in the results obtained for the two catalysts. Elemental analysis of the unpromoted catalyst reveals a C/Fe ratio of about 0.47 in rough agreement with the stoichiometry of Fe2.2C. The promoted catalyst contains considerably more carbon, and C/Fe = 0.75 in this case, indicating a carbon content in excess of that required for Fe2.2C. In addition to carbon, both catalysts retain large amounts of hydrogen following reaction. For the unpromoted catalyst the average H/C ratio drops to 1.12. The extent to which the observed hydrogen is present as high molecular weight hydrocarbons filling the pores of the catalyst is unknown, but this mode of hydrogen storage is probably small. The catalyst as removed from the reactor appeared dry and did not have an oily consistency. This suggests that the helium flushing period removes any hydrocarbons accumulated on the catalyst surface or in the pores. Reactivity of the Used Catalysts with Hydrogen. Analysis of the products formed during hydrogenation of
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983 101
1
K/Fe :0, H2/C0 :I GHSV :1000 STP ~ m ~ 1 h r . q
573K
1
623K
4
104~
,
I
1
I
K/Fe = 0 T=533K Hp/He :I GHSV : 3300
GHSV = 3000 STP cm3/hr. g
I (hi1
Figure 5. Rates of product formation during the reduction of a catalyst following ita use under synthesis conditions: K/Fe = 0.0.
I
0
1
2
3
4
5
6
t (hr)
Figure 7. Rates of product formation during the reduction of a catalyst following its carburization in C O K/Fe = 0.0.
t
(hrl
Figure 6. Rates of product formation during the reduction of a catalyst following ita use under synthesis conditions: K/Fe = 0.20.
the spent catalyst was also used to identify the influence of potassium promotion. These experiments were carried out by passing a 1:l Hz/CO mixture over the catalyst for 2 h after which the flow of CO was replaced by a flow of Hz to begin the reduction of the catalyst. Figures 5 and 6 illustrate the composition of the produds observed both at the end of the reaction period and during reduction at different temperature levels. During hydrogenation of the unpromoted catalyst (Figure 5), only paraffinic hydrocarbons are produced and virtually no CO or COPis detected after the initial sample. Reduction of the promoted catalyst (Figure 6) proceeds somewhat more slowly. In this case, olefins are formed in nearly equivalent concentration to paraffins, and CO and C02 are released by the catalyst during the first 2 h of reduction. Effects of Potassium on the Carburization of Iron and the Reactivity of the Carbided Catalysts with Hydrogen. An investigation was made to determine the effect of potassium promotion on the rate of catalyst carburization in CO alone and the subsequent reactivity of the catalyst with hydrogen. These experiments were conducted with both the unpromoted catalyst and the promoted catalyst for which K/Fe = 0.20. The results are shown in Figures 7 and 8. Carbiding of both catalysts was carried out at 533 K in a helium stream containing 20% CO. Formation of C was determined by the observation of COPformation and the assumption that carbon deposition proceeds with the stoichiometry 2CO C + COz. Figure 7 shows that over the unpromoted catalyst the formation of carbon accelerates somewhat during the f i s t half hour or so of reaction but thereafter progressively decelerates. Integration under the curve of C02 formation indicates that after 3 h the
-
r
I I ci
t
I 310
I
1'
\*=
-I
I
1
2
3
4
5
6
t (hrl
Figure 8. Rates of product formation during the reduction of a catalyst following ita carburization in CO: K/Fe = 0.20.
accumulated carbon corresponds to C/Fe = 0.30. The addition of potassium to the catalyst increases the initial rate of carbon deposition by an order of magnitude, as is seen in Figure 8. However, with time the deposition rate rapidly decreases and actually becomes slower than the rate observed with the unpromoted sample. Integration under the curve of COPformation in this case indicates that after 3 h the accumulated carbon corresonds to C/Fe = 0.37. Hydrogenation of the carbided catalyst was carried out in a 1:l H2/He stream at 533 K. Figure 7 shows that methane, ethane, and propane are the only products detected over the unpromoted catalyst. Over the promoted catalyst Figure 8 shows that ethylene and propylene are formed in addition to the paraffinic products, and that CO and COz are released from the catalyst throughout the period of reduction. Comparison of the results shown in Figures 7 and 8 with those presented in Figures 5 and 6
102
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983
T=533K H,/CO = 3
0
02
01
03
K/Fe
Figure 9. Effect of K/Fe rato on the total rate of synthesis gas consumption.
shows that higher rates of product formation are observed when the catalyst is carbided in an Hz/CO mixture than in CO alone. Discussion The data presented in Figure 1 and Table I11 show that the turnover frequencies for CO consumption and hydrocarbon synthesis pass through a maximum as the level of potassium promotion is increased. It is significant to note that the turnover frequency for hydrocarbon synthesis reaches a maximum near K/Fe = 0.02, while the maximum in the turnover frequency for CO consumption is reached near K/Fe = 0.07. This difference reflects the fact that the proportion of CO converted to COz increases monotonically from 0.3 to 0.5, as K/Fe increases from 0 to 0.2, and is a direct consequence of the enhanced water-gasshift activity of the catalyst with increasing potassium loading. The effect of potassium on catalyst activity observed in the present study is similar in some respects to that reported earlier by Anderson et al. (1952) for a variety of iron catalysts. In those studies the catalyst activity, defined as the volumes of H2and CO converted per gram or volume of catalyst, was found to increase up to a maximum near a K/Fe ratio of 5 X to and then to decrease progressively with further increase in the K/Fe ratio. The maximum enhancement in the catalyst activity over that for an unpromoted catalyst and the K/Fe ratio a t which maximum activity was achieved depended on the specifics of catalyst preparation. Figure 9 shows that when the definition of activity of Anderson et al. is used, the catalysts studied in the present investigation exhibit no influence of promotion up to K/Fe = 0.02 and then a monotonic decrease in activity for higher levels of promotion. The absence of an effect of promotion on activity for K/Fe ratios below N 0.02 probably results from the fact that at low potassium loadings a significant proportion of the potassium is consumed by acid sites on the alumina support, leaving little available for interaction with the iron. In the studies conducted by Anderson et al. (1952), the catalysts were prepared without alumina or contained about 5 wt % A1203as a structural promoter. Figure 1 and Table I11 suggest that for K/Fe ratios below about 0.02, the distribution of hydrocarbons formed is insensitive to the presence of potassium. A t higher promoter levels, the methane content of the product decr-, and, correspondingly,the average molecular weight of the product increases. It is also observed that for K/Fe > 0.02 the olefin to paraffin ratio increases rapidly with an increase in the K/Fe ratio. The absence of an effect of potassium on product composition for K/Fe less than N 0.02 could be explained by the preferential adsorption of the promoter on the support, as discussed above.
The influence of potassium on the distribution of products is consistent with the effects of this promoter on the strengths of H2and CO chemisorption on the reduced catalyst. The results presented in Table I show that potaasium promotion significantly strengthens the adsorption of CO but weakens the adsorption of Hz. These observations are in general agreement with several earlier studies (Dry et al., 1969; Benziger and Madix, 1980). If it is assumed that potassium has a similar effect on the chemisorptive properties of the working catalyst, then the observed reduction in methane formation and the increase in olefin to paraffin ratio with increasing potassium promotion can be attributed to a suppression in the relative concentrations of Hz and CO by the promoter. This interpretation would also explain the decreased dependence of the olefin to paraffin ratio on space velocity, noted in Table VI. In agreement with previous studies of Fischer-Tropsch synthesis over iron catalysts (Anderson et al., 1952; Amelse et al., 1981), it is concluded that H20 is a primary synthesis product and that CO, is produced in a secondary step. The response of the C02/H20 ratio to increases in the space velocity and reaction temperature clearly indicate that the formation of CO, is limited by the kinetics of the watergas-shift reaction. The addition of potassium greatly enhances the water-gas-shift activity, and, as was shown in Figure 2, at high temperatures and low space velocities the CO2/H20ratio in the product gases approaches that expected when the water-gas-shift reaction is at equilibrium. The effect of temperature on the conversion of CO to COz observed here is quite different from that reported recently by Jacobs and Ollis (1981) for a potassium-promoted Fe catalyst and by Madon and Taylor (1981) for an unpromoted Fe catalyst. These authors reported that upon increasing the temperature from about 503 to 553 K, the CO conversion to COz decreased significantly, and at the same time the CO conversion to CH4 underwent a sudden increase. This pattern was attributed to a change in the bulk composition of the catalyst. X-ray diffraction studies conducted by Jacobs and Ollis (1981) revealed that at 513 and 523 K the catalyst exhibited a pattern characteristic of Fe304and an iron-carbon solution, but no evidence for iron carbide. At 553 K, however, the X-ray pattern corresponded predominantly to x and t iron crbides. Since magnetite is a very active catalyst for the water-gas-shift reaction, Madon and Taylor (1981) proposed that the presence of Fe304 at low temperatures during Fischer-Tropsch synthesis was reasonable for the high conversion of CO to CO,. Iron carbide is known to be a less active catalyst for the water-gas-shift reaction and, hence, the appearance of this phase at higher synthesis temperatures would explain the observed decrease in the formation of COP The present results are completely consistent with these observation if one assumes that the iron carbide composition, observed by X-ray diffraction following synthesis at 523 K, is retained at lower temperatures as well. This assumption is not unreasonable given the fact that at 1 atm and the CO conversion levels used in this study (generally
