Reactions of Hydrogen and Ca:rbonMonoxide In a Tubular Reactor with Iron-Copper Catalyst W. M. Campbell1and H. F. Johnstone UNIVERSITY OF ILLINOIS, URBANA, ILL.
In order to throw some light on their complicated sequence, the Fischer-Tropsch reactions were studied under highly simplified conditions using a porous tube impregnated with an ironcopper catalyst. The close control of temperature and simplicity of sampling aIong the reactor provide an opportunity to study the kinetics from the abrupt changes when the composition of the gas is altered or an inhibitor i s added. Most of the catalysts used produced carbon dioxide and were active for the water gas shift reaction. Heating the catalysts to 3 10"C. in the presence of hydrogen and carbon monoxide destroyed
THE
reactions of the Fischer-Tropsch synthesis are complex and affected considerably by the nature of the catalyst and other conditions. The mechanisms are not fully understood. I n this work several factors have been studied, using a porous tube impregnated with the catalyst instead of a conventional granular bed. The tubular reactor, which has been described in a recent paper (31, has many advantages in the study of reaction kinetics. In particular, it provides a closer control of the catalyst temperature than is possible with a packed bed, and permits sampling a t points along the reaction zone. In this way the advantages of a differential reactor can be obtained R ithout excessive experimentation. The tubular reactor is especially useful for foIIowing abrupt changes in catalyst perforniance as the composition of the gas is changed or an inhibitor is added. Thus, experiments on the effects of steam and of hydrogen sulfide on the iron-copper catalysts indicate the existence of two types of active centers which c-ontrol simultaneous reactions.
Previous Work Several reviews of the Fischer-Tropsch reactions have been published recently (10, 22-25, 27). The following are some of the conclusions that are applicable to this work. Xlthougli cobalt catalysts produce hydrocarbon fractions containing olefins with internal double bonds ( 2 , 1 1 , l 4 , 1 6 ) , iron catalysts produce larger proportions of olefins, and most of these are unsaturated in the alpha position. The higher boiling fractions, in general, contain smaller percentages of olefins. Anderson and coworkers ( 1 , 3) found that, as the space velocity increases, the proportion of olefins increases, They also observed that the olefin content of samples taken along the axis of a catalyst bed deereases as the distance from the gas inlet increases. This suggests that olefins are the primary hydrocarbon product. Wiiile the existence of metallic carbides in iron catalysts ~ R Sbeen ascertained, the exact role of these compounds in the reaction mechanism is uncertain. The hypothesis that the synthesis proceeds through formation oi an intermediate carbide was first suggested by Fischer (9, l g , I S ) . The carbide is assumed to be hydrogenated to form CH2radicals on adjacent sites and these are polymerized to form straight- or branched-chain hydrocarbon molecules which are finally desorbed. Evidence for t,he c:trbi..ie mechanism was found by Craxford and Rideal in studies on thcl conversion of para- t o ortho-hydrogen in the presence of carbon monoxide and cobalt catalysts (8). Herington I Present address, National Research Council, Chalk River, Ontario, Canada.
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the synthesis reaction activity, but did not affect the shift reaction activity. Hydrogen sulfide destroyed the activity for both reactions. The shift reaction activity of a water-producing catalyst was increased by increasing the hydrogen concentration in the feed gas. These results indicate that two types of active centers exist, one active for the shift reaction and the other active for the synthesis reaction, A dual reaction mechanism is proposed by which paraffins are formed by the termination of a chain b y hydrogenation, and olefins are formed b y splitting the (CH?), chain off the surface.
considers that the hydrocarbon chains are attached to the surface only through the two carbon atoms a t the terminal and penultimate positions, and that they can be removed from the surface without picking up a hydrogen atom (17). This suggests that the primary product is an alpha-olefin. On the other hand, Craxford, accepting Fischer's carbide theory, considers that the methylene chain growth is terminated when the methylene radical a t the end picks up a chemisorbed hydrogen from a metal-active center, forms a methyl radical, and completes the paraffin molecule (6-8). The para& molecule so formed is desorbed and, therefore, paraffins are the primary hydrocarbon product. More recently, Emmett and coworkers studied the mechanism of the synthesis reactions by means of radiocarbon tracers ( $ I ) , and examined the thermodynamics of hydrogenation oi carbides (6, $0). They conclude that bulk phase carbide participates in t h e synthesis only to a negligible extent, while surface carbide may account for a portion but not all of the hydrocarbon product. It is agreed that the mechanism may include a step in which carbon atoms are momentarily formed at the surface, perhaps in chemisorbed form, if not as a true carbide (24). Storch and coworkers propose a mechanism involving the chemisorption of carbon monoxide on surface metal atoms followed by partial hydrogenation by chemisorbed hydrogen on adjacent sites (24). Chain building occurs in two ways: by addition to end carbon atoms, and by addition to adjacent-to-end carbons. Progressive poisoning of a cobalt catalyst with hydrogen sulfide was studied by Herington and Woodward (18). At first the yield of oil and total hydrocarbons increases slightly, and then decreases rapidly. The hydrogen sulfide apparently destroys the chain-ending active centers. Fujimura found that from 0.05 to 0.25 atom of sulfur per atom of catalyst is required to decrease the activity of a nickel catalyst by 50% (16). Water is considered to be the product of the primary hydrocarbon-forming reaction, and any carbon dioxide iormed is the result of the secondary water gas shift reaction, Bashkirov and coworkers, using an iron catalyst, added water to the normal hydrogen-carbon monoxide feed, and found that the shift reaction takes place rapidly (4). Kolbel and Engelhardt studied the reaction of carbon monoxide and steam over cobalt and iron catalysts prepared in several ways and concluded that the relative amounts of water and carbon dioxide depend only on the temperature ( 1 9 ) . The conditions of their experiments, however, were different from those of the normal synthesis reactions. Watanabe and coworkers studied the effect of varying the hydrogen-carbon monoxide ratio, and found that the changes produced in the watercarbon dioxide ratio were in the opposite direction to that expected from a consideration of the water gas equilibrium alone ($6).
Materials Electrolytic hydrogen, 99.8% pure, was used from Cylinders. The carbon monoxide, also supplied from cylinders, contained 0.3% carbon dioxide, 1.2% hydrogen, 1.0% nitrogen and paraffin
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Fuel Gasification hydrocarbons. Before use, each gas was urified separately by passage over copper gauze a t 500" C. a n 8 through towers containing caustic soda and activated carbon. All other chemicals were C.P. grade. The Alundum catalyst support tubes were refractory grade RA98, 24 inches long and 0.5 inch in internal diameter, with 0.125-inch wall, and were obtained from the Norton Co., Worcester, Mass.
Catalyst Preparation The catalysts used contained iron, copper, and potassium carbonate in the ratio of 100: 16.7:0.175. Ferric and copper nitrates were mixed in the correct proportion and fused before the potassium carbonate was added. The Alundum tubes were impregnated under vacuum with the molten mixture a t about 80" C. When the excess melt had drained, the residue was converted to the oxides by slowly heating the tubes from 120' to 250" C. over a period of about 20 hours, and then heating rapidly to 380" C. During the latter part of the cycle the oxides of nitrogen nere purged with a stream of nitrogen. The final step in the preparation was the reduction of the oxides and carburization of the iron. This was done by treating the catalyst with carbon monoxide a t a constant rate for about 40 hours, while the temperature was raised from 180" to 250" C. The amount of carbon monoxide reacting was determined from the rate of flow and the amount of carbon dioxide formed. In all cases it was sufficient to reduce the oxides to the metals and to deposit carbon in the form of carbide, or free carbon, or both, in the ratio of 1 carbon to 3 iron. After the carburizing period, hydrogen was gradually added to the gas until the desired ratio had been reached. Fr,om this point, the ratio was held constant and the gas flow over the catalyst was maintained throughout the life of the catalyst.
Apparatus and Procedure The apparatus used is shown in Figure 1.
An example of the data for a typical run is shown in Table I. Contraction is reported on the dry carbon dioxide-free basis.
Catalyst Activity Several ways of pretreating the oxide catalysts were tried before the carburizing method described above was adopted. Carburization with carbon monoxide a t 0.1 atmosphere pressure and a t 325" C. was usually found to give inactive catalysts. Later work indicated that the active catalyst could not exist at this temperature. Pretreating the oxides with hydrogen or carbon monoxide, or a mixture of the two gases, a t temperatures gradually increasing from 170' t o 270' C., or with hydrogen and steam a t 400" C. followed by carbon monoxide a t 170' t o 270" C., gave catalysts of the same general properties. While some of the catalysts were more active than others, they all gave essentially the same product distribution. This indicates that all the pretreatments converted the catalyst to the same form, and that after an initial period a steady state is reached with respect t o the gas stream.
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General Layout of Reactor Unit
Preheater furnace 2. Salt bath 3. Thermowell 4. Electric stirrer 5. Salt drain pipe 6. 0.875-inch iron tube 7. Packed lass preheater 8. Sintere&lass disk 9. Catalyst tube
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ally located, as shown. A uniform temperature throughout the bath was ensured by providing a stirrer and baffles. i f ith this system, the temperatures in the salt bath and in the gas stream just ahead of the sintered disk were essentially the same. The preheater section was heated with a separate furnace. The feed gas mixtures were made up in large glass bottles, analyzed, and fed a t accurately measured rates of 0.75 to 0.80 liter per hour into the reactor unit. The rates were controlled by displacing the gases with water from a constant head siphon. The product gases, after the condensable products nere removed in the cool end of the sample tube, passed either to exhaust, or through magnesium perchlorate and Ascarite tubes to a calibrated constant pressure gasometer. Gas samples were collected from points 0, 6, 12, 18, and 24 inches from the inlet end of the catalyst tube. Each sample consisted of all the gases reaching the sample point. There was evidence of some back-diffusion of the gases during the sampling, as some reaction products were always detected a t the entrance to the catalyst tube. However because all the samples were taken in the same manner, the resuits a t the various positions should be comparable with each other. The product gases were analyzed for carbon dioxide gravimetrically, and for olefins, carbon monoxide, h drogen, paraffins, and nitrogen with a Fisher gas analyzer. A cfouble combustion procedure was used by which the average values of n in the paraffin CnH2n+2and of m in the ,olefin CmH2mwere estim:ited ( 2 7 ) . Water was not determined directly, but, was calculated from an oxygen balance.
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The reactor unit consisted of a thin-walled iron tube, 0.875 inch in diameter and 60 inches long, into which the glass preheater and catalyst tube were tightly packed with Alundum powder. The preheater tube, which was filled with glass beads, contained a sintered-glass disk to smooth out the gas flow just ahead of the catalyst tube. The reactor unit was placed in a molten bath of sodium nitrite and potassium nitrate, with the catalyst tube axi-
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IO. Sample tube 11. Manometer 12. Li uid tra 13. Ru%bertuxing joints 14. Stufng gland 15. Immersion heater 16. Magnesium perchlorate tube 17. Ascarite tube
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Table I. Performance of Catalyst CC (Basis, 100 moles dry feed gas) Temp. 245' C. Feed gas composition. Hz = 39.0%, CO = 59.6%, Nz = 1.4% Catalyst age 70 hours 12 6 0 Sample point inches frominlet 24 18 58.7 51.5 40.2 23.0 11.0 Contraction(COzandHzOfree), % Dry gas leaving reactor, moles 17.3 14.9 10.5 6.3 2.1 cog 2.2 2.5 2.0 1.4 0.6 Olefins 15.9 18.7 22.8 28.8 33.4 Hz 20.4 24.1 32.1 44.4 53.3 co 2.2 2.3 2.0 1.5 0.8 Paraffins 0.7 1.0 0.9 0.9 1.0 NZ 2.5 2.2 2.3 2.2 .... Olefin m (in CmHzm) 1.29 1.33 1.27 1.26 1.22 Paraffin n (in CnHzn+ z) 0.78 0.77 0.71 0.65 0.63 Ratio HI-CO Gas reacting, moles 5.6 16.2 10.2 23.1 20.3 Hz 6.3 39.2 35.5 2 7 . 5 15.2 co Ratio Hz-CO 0.59 0.57 0.59 0.67 0.89 Gaseous product, atoms 21.7 1 8 . 3 13.0 6.1 H in hydrocarbon 21.1 7.1 4.9 2.0 C in hydrocarbon 8.2 8.4 17.3 10.5 6.3 2.1 14.9 C in COz 4.2 21.0 12.6 34.6 29.8 0 in COz 3.05 2.58 2.65 Ratio H-C in hydrocarbon 2.57 2.58 Condensed product 2.6 2.1 Hz0, moles 4.0 5.7 6.5 H, atoms 15.9 7.5 1.1 2.2 0.9 9.9 4.0 2.2 13.7 12.2 C atoms 0.41 0.11 0.55 R6tio H-C in hydrocarbon 1.16 0.61 Moles of COz per C a t o m i n hydrocarbon product 0.79 0.72 0.62 0.71 0.60 Distribution of C reacting % in hydrocarbon was % in condensed hycdrocarbons yo in gas/% in condensate Distribution of Hz reacting % in hydrocarbon gas 7" incondensed hydrocarbon % in gas/% in condensate
20.9 35.0 0.6
23.6 34.4 0.7
25.8 36.0 0.7
32.2 26.3 1.2
31 8 35.0 0.9
45.7 34.4 1.3
53.5 18.5 2.9
56.5 3.4 1.7
63.7 10.8 5.9
54.5 8.0 6.8
Other pretreatments might convert the catalyst to different forms which might give very different results. In later stages of the work, it was found that treatment of a normal carbon dioxide-producing catalyst with steam would increase its activity. To investigate this, a normal operating catalyst was treated for 10 hours a t 250" C. with a 2 to 1 mixture of carbon monoxide and steam. A second treatment with the same mixture mas carried out for 5 hours a t 295" C. Figures 2 and 3 show the effect of these treatments on the amount of carbon mon-
oxide reacting and on the paraffin gas n value. The dotted lines represent the results from similar runs with the untreated catalysts at the equivalent ages. ?rIost of the increase takes place in the first few inches of the tube before the steam is converted to carbon dioxide by the water gas shift reaction. The increase in activity may be due to oxidation of the metal-active centers by the steam followed by conversion of the oxide to carbide with the carbon monoxide. Steam-carbon monoxide treatment a t 295' C. caused the activity to drop. This is probably due to the effect of temperature alone, since it was observed that the catalyst rapidly becomes inactive at 310" C.
Effect of Catalyst Age The changes which take place as the catalyst is used w x e studied at 280' and 250' C. Although different tubes r e r e used for the two experiments, earlier work had shown that reproducible results could be obtained from catalysts made by the standard procedure; the variations observed, therefore, were caused by the difference in temperature only. Using the contraction of the gas as a measure of the catalyst activity, the activity decreases with age, as shown in Figure 4. At 280" C., the decrease is rapid at first, but, after about 80 hours, the activity becomes practically constant. The olefin m and paraffin n values, shon n in Figures 5 and 6, vary in much the same way as the contiaction, and reach constant values of about 2.0 and 1.0, respectively. Further data show that in the constant activity period a t 280" C. the only other products of importance are carbon deposited on the catalyst, and carbon dioxide. Therefore, some catalyst-destroying reaction, such as decomposition of active carbide, must take place slowly a t 250' C. and rapidly at 280" C. Furthermore, the only hydrocarbons which can be formed by a relatively inactive catalyst are methane and ethylene.
Poisoning with Hydrogen Sulfide The effect of hydrogen sulfide on an active catalyst was studied at 250' C. Three charges of hydrogen sulfide, equivalent to 0.039, 0.086, and 0.354 atom of sulfur per atom of catalyst metal, were added to the feed gas over short periods of time. All of the sulfide was adsorbed by the catalyst. Figure 7 show8 the effect of the cumulated amount of the inhibitor on the conversion of carbon monoxide. For comparison, the dotted line sho>%sthe
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Literature Cited (1) Anderson, R. B., Krieg, A., and Friedel, R. A,, IND. ENG.CHEM.,
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Figure 9. Water Converted by Different Catalysts Based on Carbon Dioxide Formed Tern erature 250° C. HC? 20 ratio 1.0
hydrogenation center and being split off as a paraffin. This explains why the gaseous hydrocarbon fractions are more olefinic than the condensed fractions. This dual reaction mechanism, combined with the concept of clusters of metal-active centers in the iron catalysts, seems to explain the observed results better than any mechanism previously suggested.
Conclusions Stable and reproducible iron-copper catalysts on Alundum tubes, which produce carbon dioxide and hydrocarbons from synthesis gas a t atmospheric pressure, were developed. When treated with steam and carbon monoxide at 250’ C. the activity of these catalysts increases. Above 280” C., the activity drops rapidly and eventually the only products are methane, ethylene, carbon, and carbon dioxide. The loss in activity due t o hydrogen sulfide poisoning is a linear function of the amount of the sulfide adsorbed; when the ratio is about one half atom of sulfur per atom of metal the activity reaches zero. Most of the catalysts produced carbon dioxide and were active for the water gas shift reaction. Heating the catalyst to 310’ C. in the presence of hydrogen and carbon monoxide destroyed the synthesis reaction activity, but did not affect the shift reaction activity. Hydrogen sulfide destroyed the activity for both reactions. The shift reaction activity of a water-producing cata-
41,2189 (1949). (2) Anderson, R. B.; Krieg, A., Seligman, B., and O’Xeill, W. E., Ibid., 39, 1548 (1947). (3) Baron, T., Manning, W. R., and Johnstone, H. F., Chem. Ena. Progress, 48, 125 (1952). (4) Bashkirov, A. N., Kryukov, Y . B., and Kagan, Y. B., Doklady Akad. N a u k S.S.S.R., 67, 1029 (1949). (5) Browning, L. C., DeWitt, T. W., and Emmett, P. H., J . Am. Chem. Soc., 72,4211 (1950). (6) Craxford, S. R., Fuel, 26, No. 6, 119 (1947). (7) Craxford, S. R., Trans. Faraday SOC.,35,946 (1939). (8) Craxford, S. R., and Rideal, E. K., J . Chem. SOC.,1939, 1604. (9) Fisoher, F., Brennst0.f-Chem., 8 , l (1927). (10) Fisoher, F., O e l u . Kohle, 39,517 (1943). (11) Fischer, F., Petroleum Refiner, 23, No. 2, 112 (1944). (12) Fischer, F., and Tropsch, H., Brennst0.f-Chem., 7, 299 (1926). (13) Fischer, F., Tropsch, H., and Ter-Nedden, W., BeT., 60B, 1330 (1927). (14) Friedel, R. A., and Anderson, R. B., J. Am. Chem. Soc., 72, 1212 (1950). (15) Fujimura, K., and Tsuneoka, S., Sci. Papers Inst. Phys. Chem. Research ( T o k y o ) ,24,79 (1934). (16) Fujimura, K., Tsuneoka, S., and Kawamichi, K., Ibid., 24, 93 (1934). (17) Herington, E. F. G., Trans. Faraday SOC.,37, 361 (1941). (18) Herington, E. F. G., and Woodward, L. A., Ibid., 35, 958 (1939). (19) Kblbel, H., and Engelhardt, F., Erdol u. Kohle, 2,52 (1949). (20) Kummer, J. T.. Browning, L. C., and Emmett, P. H., J . Chem. Phys., 16,739 (1948). (21) Kummer, J. T., DeWitt, T. W., and Emmett, P. H., J . Am. Chem. SOC.,70, 3632 (1948). (22) Storch, H. H., Chem. Rng. Progress, 44,469 (1948). (23) Storch, H. H., Anderson, R. B., Hofer, L. J. E., Hawk, C. O., Anderson, H. C., and Golumbic, N., U. S. Bur. Mines, Tech. Paper 709 (1948). (24) Storch, H. H.. Golumbic, N., and Anderson, R. B., “The Fischer-Tropsch and Related Syntheses,” New York, John Wiley & Sons, 1951. (25) Underwood, A. J. V., IND. ENCT. CHEM.,32,449 (1940). (26) Watanabe, S., Morikawa, K., and Igawa, S., J. SOC.Chem. I d . J a p a n , 38, Suppl. binding, 328 (1935). (27) Weil, B. H., and Lane, J. C., “Synthetic Petroleum from the Synthine Process,” New York, Chemical Publishing Co., 1948. RECEIVED for review July 31, 1951. ACCEPTEDMay 13, 1952.
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