March, 1961
HYDROCARBON SYNTHESIS ON PURE IRON
417
the concentration of hydrogen in the burned gas activation energy obtained would be different dcdepends on many factors, including the temperature pending on whether the assumed hydrogen order of the downstream zone, one would expect that the was 1or 3//2.
HYDROCARBON SYNTHESIS ON PURE IRON BY P. B. WEISZAND W. P. KERN Socoray Alobil Oil Company, Inc., Research Department, Paulsboro Laboratory, Paulsboro, A'. J . Received August 8. 1060
Pure iron was observed gradually to develop activity for the production of paraffin hydrorarbons, from carbon monoxide and hydrogen, over a period of 350 hours of contact. The fraction of olefin hydrocarbons produced, however, was noted to decay to zero within about 150 hours of contact. Photoelectric measurements of the iron surface indicated only a small shift of work-function, photoelectric emission shifting toward longer wave lengths. The gradual shift in photoelectric properties parallels the slow transition of the catalytic properties.
Introduction Over the many years of investigation of iron as a synthesis catalyst for the conversion of carbon monoxide and hydrogen little attention has been given the behavior of pure iron. An important role of "promoter" additions, especially of alkali, has been indicated starting with the original work of Fischer and Tropsch,' for achieving practical synthesis catalysis. Moreover, Fischer and Tropsch suggested that alkali is an essential mechanistic factor for achieving the synthesis of molecules with more than one carbon number, the iron acting as hydrogenation catalyst for carbon monoxide, and alkali catalyzing condensation reaction steps. The present report deals with a study employing iron introduced into the experiment as 99.99 (plus) % purity iron wire, without the aid of preparation techniques which may introduce other components. The gaseous products from repeated 20 hour contacts with a mixture of 1: 1 CO-H2 a t 40 cm. pressure and 320" were studied over a period of several hundred hours. At the same time intervals the photoelectric properties of the surface were examined as a non-destructive tool which would indicate compositional changes occurring at the surface during the period of activity study. Experimental The reactor vessel was designed to allow its use for catalytic contact with the reacting gases, as well as to offer part of the iron surface contained therein as the cathode of a photo-responsive tube geometry, which could be irradiated with the light from a Beckman monochromator, and the photoelectric response measured as a function of wave length. The low photoeliectric yield attainable from iron, especially in the presence of gas,zss and the relatively low light intensity available after passage through a monochromator, and through a sighting passage of a surrounding furnace, made the conventional photo-current measurement a relatively unattractive prospect. Instead, a Geiger counter tube geometry, consisting of a pure iron cathode cylinder and coaxial pure iron anode wire was created as part of the reactor's total iron surface. The monochromator light beam could be directed onto the interior cathode surface, and with thc gas fillings of the chemical experiment itself, the tube was operated as a photon counting Geiger counter. In Fig. 1 is pictured the construction of the reactor tube constructed from Konex glass, containing the bulk of the (1) F. Fischer and N. Tropsch, Her. deut. Chem. Ges., 56, 2428 (1923). (2) A . K. Brewer, J . Am. Chem. SOC.,53, 74 (1931). (3) A. K . Brewer, i b d , 54, 1888 (1932).
catalytic iron in the form of a multiple honeycomb coil of 68 g. of 0.007" diameter iron wire (99.99 plus yo highest purity iron from Swedish Iron and Steel Corp., New York City), presenting a geometric surface area of 2000 cm.'. This was wound around a 2 cm. diameter cylinder of the same iron, serving as a support as well as the photoelectric Geiger tube cathode. The axially suspended (0.007" iron) wire serves as anode. The ultraviolet transmitting window was blown t o approximately 0.005'' thickness from Corning 9741 glase. This reactor tube was contained in a tubular furnace thermostatically controlled to a temperature of 320' during reaction periods. The furnace cylinder was provided with hole and side tube t o allow proper injection of the monochromator beam. The two gas leads t o the reactor tube were attached to a glass circulating system comprising a glass bulb reservoir, a magnetically rotatable glass vane (with sealed-in iron bar) to provide continuous circulation of the gaseous content through the entire system, a mercury manometer and valving to allow evacuation to lo-' mm. pressure. The system, including the reactor itself, contained a volume of 2510 cm.a. The Geiger tube electrodes were connected to a conventional (Instrument Development Co.-now Nuclear-Chicago Corporation) electronic quenching circuit and 64: 1 scaler. Hydrogen (Mstheson Company, East Rutherford, N. J.2 was used after passing i t over reduced copper filings a t 700 and liquid nitrogen cooled glass trap. Carbon monoxide was passed over the same purification train. Preparation of Iron Surface.-The iron containing reactor tube was washed with large quantities of acetone and dried before sealing to the system. It was pumped t o about lo-' mm. pressure over a period of 2 days; the iron was then oxidized in oxygen with 1 atm. oxygen for 20 minutes at 300'. The photoelectric response of the oxidized surface2 in helium as the counting gas-is seen in taken with 5y0 0 Fig. 2A. Then the tube was pumped back to a vacuum, and reduced by filling the system to 1 atm. of hydrogen circulating over the iron at 420' for one hour and through a liquid nitrogen cooled trap before re-evacuation for several reduction cycles. This reduction was repeated beyond the state of no measurable hydrogen pressure drop, until no more change in the photoelectric spectral response of the surface occurred. The latter response was measured after each reduction cycle with the tube at room temperature and containing 10 mm. hydrogen pressure; Fig. 2B shows the response of the final surface. As previously pointed out by B r e ~ e rthe , ~ photoelectric method is very sensitive to oxide (or oxygen) content and reveals changes toward the ultimate reduced state after pressure changes are no longer observable. Figure 2C shows the spectral response of the surface after room temperature exposure to 5% 02 in 1 atm. helium for 15 minutes obtained in a previous preliminary experiment. Catalytic Reaction.-After evacuation a t room temperature, a 50/50 mixture of hydrogen and carbon monoxide were admitted to the system, to a total pressure of 3 cm. A (4) J.
T. Kummer and P. H. Emmett. ibid., 75, 5177 (1953).
P. B. WEISZAND W. P. KERN
418 *
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Vol. 65
reaction tube cooled t o room temperature, and pressure pumped back to 3 cm., a photoelectric response taken ( L e . in the presence now of any product species made), the syst.em evacuated, and the same cycle of operations repeated. The set of measurements was carried out over a total catalytic contact time of 380 hours. Results of Catalytic Contact.-Figure 3 summarizes the following results of the catalyst runs for each reaction interval: (a) the average rate of pressure change; (b) the eoncentration of COZ, (c) of total paraffin hydrocarbons, (d) of total olefin hydrocarbons in the product gas, (e) the wave length "intercept" and (f) of the wave length of the "maximum" of the photoelectric response spectrum of the iron surface, both measured in the absence as well as presence of reaction products (pumped, and unpumped). Furthermore Table I shows the composition of paraffin and olefin components for the same reaction intervals.
300
TABLE I
{"MAXIMUM"
COMPOSITION OF HYDROCARBON COMPONENTS IN PROD V
Total contact time, hr.
W rT)
\
2
200
2
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TIME,
GASES Mole 3'0' in product gas C 01efi11s-Ca Cs Cn C4
--
Cs
19.5 0.2 T T 0.1 0.1 0.1 0 . 1 35 .5 0.1 T .3 .2 .I .I 53.5 .7 .2 0 . 1 .4 .3 .1 .1 66.5 .9 .2 T .6 .4 .1 .1 86 3.6 .Y 0.2 1.0 .7 .2 .1 105.5 4.2 .9 .3 1.0 .6 .2 .2 125 4.4 1.0 .1 0.8 .G .2 .I 145.5 9 . 1 1.1 T .2 .2 T T 166 8 . 3 1.2 T .2 .I T 0 .2 .2 0 0 185.1 10.3 1 . 6 0 . 3 205 9.8 l . G .3 .2 .2 0 0 226 11.5 1 . 5 .3 .1 .1 0 0 247 13.3 1 . 3 .3 T .1 0 0 266 11.2 1 . 5 .3 T .I 0 0 287 12.0 1 . 7 .4 T .1 0 0 310 13.6 1 . 5 .3 0.1 T 0 0 329 11.3 1 . 5 .4 . I 0.1 0 0 351 12.5 1 . 6 .4 .I 0 0 0 a Only trace quantities of C4-paraffins were detected, and no reliably detectable amounts of Ce-paraffin were observed.
Z
W
-
----Paraffinsa-CI Cz
30C
tOJXS
Fig. 3. photoelectric response was taken, and the reactor heated to 320". Additional gas mixture was added t o a total pressure of 40 cm. Reaction was allowed to proceed for 20 hours with the circulating vane operating. The pressure drop was followed for that period. Thereafter, a gas sample was drawn from the system for mass spectrometer analysis, the
The Surface after Reaction Contact.-After the catalytic contact, the surface was subjected to contact with hydrogen alone, to observe any continued production of hydrocarbon products due to interaction with the surface composition, and to test for the presence of iron carbide by examination of the product,s of hydrogen reduction. Methane production alone did cont'inue. Figure 4 shows the m a s spectrometer analysis of the hydrogen gas after successive contacts for 20 hour intervals a t 270,320 and 420", as indicated along the abscipsa. Water was the only other observed product (no quantitative amounts recorded). The reduction, as indicated by Fig. 4, generated methane equivalent in carbon cont'ent to 52 cm. pressure of CO in the volume of the reaction system. For the sum of all reaction runs, the calculated carbon balance shows a deficiency of 59 + 4 cm. pressure equivalent of CO. Thus most of this deficiency is accounted for by carbon remaining on the iron, and which was successfully reduced to methane with perhaps n small additional unreduced or irreducible carbonaceous material remaining on the catalyst. The photoelectric response of this surface remained identical to that after reaction contact; moreover, the addition of oxygen (air) a t room temperature did not cause a distinct change in photoelectric response of this surface. In fact, even a contact with 0 2 a t 300°, for one hour, produced a spectral shift of only about, - 5 mp of the maximum. These results are consistent with the conclusion that a clean iron surface had not been attained by the above hydrogcsn contacts.
Discussion The gas analyses reveal progressi7-e development of activity of an initially unreactive surface. The
March, 1961
HYDROCARBON SYNTHESIS ON PURE IRON
rate of development is fastest within the first 100 hours, and tapers off thereafter to possibly a constant activity. Hydrocarbons of larger carbon numbers begin to be detectable as the activity rises. After about 200 hours a nearly constant level of catalytic behavior is approached. h qualitative change in the nature of the products, and therefore presumably in the nature of the catalytically active solid, is observed between 50 and 150 hours, during which time olefin product concentration reaches a peak but subsequently decays to essentially zero. If we accept the hypothesis that olefinic hydrocarbons are generated prior to their saturated then the product observations indicate gradual d e velopment of increasing hydrogenative activity as a catalytic characteristic of the surface change. During the course of the qualitative surface changes, the course and concentrations of total C3,C4,Cshydrocarbon concentrations, individually, as shown in Talde I, indicate that the ability to synthesize increasingly larger chains is arrested during the development of increased hydrogenation activity. Furthermore, the active disappearance of C4and Cg olefins past the 100 hour contact time, in the absence of detectable Cq and Csparaffins indicates an actual increased inhibition of chain growth. The photoelectric behavior of the pumped surface parallels thiis gradual change of surface properties for the 6rst 200 hours. The long wave length intercept changes, by a relatively small absolute amount, toward larger wave length, indicating a somewhat lower work-function. The location of the observed maximum also changes by a small amount (toward shorter wave length) during this contact period. The qualitative progression of the nature of the surface parallels the course of catalytic behavior. Also the relative effect of product molecules on the photoelectric nature of the surface (unpumped vs. pumped response) also reflects a qualitative change of surface property during the initial phase. Beyond this, a quantitative interpretation of the response patterns, measured in absence of high vacuum techniques or of gases of highly controlled compositio 11, is probably not justified. However, no major :Imount of surface appears to be in the oxide form, since a very rapid drop in photoelectron intensity and sharp increase of work-function would
-" I
419
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120
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220 H O U R S
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result. The opposite behavior in the shifts (pumped surface) of long wave length intercept and of the location of the observed maximum would, however, be consistent with development of a composite or heterogeneous structure. The observations do not exclude the possibility of some surface oxygen-iron bonding. However, considering the small changes in the photoelectric spectrum no more than an estimated 10% (and probably much less) of the surface atoms could be involved. The presence of large amounts of iron carbide is indicated by the hydrogenation subsequent to the synthesis use of the iron. However, the production of such carbide does not indicate its participation as an intermediate in the synthesis. We prefer not t o seek to identify the chemical nature of the catalyzing surface in terms of bulk chemical terminology. One may conclude that "pure" iron, after conis capable of forming a surface tact with CO and HP, composition which will produce addition of carboil units to form hydrocarbon beyond methane, a i d that promoters may act to favor production and maintenance of a state of surface which with pure iron is approached somewhat during the transitory period around 100 hours contact time in these experiment s, We are indebted to Prof. P. H. Emmett for helpful and stimulating discussions of the results and findings in this work.
