470
GEORGEBLPHOLDEIZ AND P. H. EMMETT
effect is disregarded; at 418.8"K. a negative slope would be observed which is completely incomprehensible. Further the "corrected" results yield values for constants k3//k8"? and k3(k2/k&,) giving nice linear logarithmic plots'2 vs. 1/T (from 360 to 465 and from 385 to 590°K.); the "uncorrected" results give a curvature a t low temperatures aq expected for this effect. I11 fact thehe negative slopes and disagreements i n .irrhmius plots called our attention to this effect. To :11! oiir re.ult.: on photochlorination reactions
Yol. 64
we now apply corrections using tq.4 and cl, = 2 m i . and caleulatiiig K as described abovrh. Acknowledgments.-Thanks are due to tlic Centre National Belge de Chimie Physique Illol6culaire and the Fonds National Belge de la Recherche Scientifique for financial assistance. Scholarships of the Institut pour I'Encouragement de la Recherche Scientifique dans 1'Industrie et 1'Agriculture (1.R.S.I.A.-Brussels) have permitted 9. 31. ;\InhieuVan der Auwera and D. Van der huwera to perform this research.
FISCHER-TROPSCH SYSTHESIS XSECHSXISM STUDIES. 11. THE :4DDITIOK OF RADIOACTIVE KETESE TO THE SYNTHESIS G.iS BY
G. BLYHOLDER AND P. H. EMMETT
T h e J o h n s H o p k i n s University, Department of Chemistry, Baltimore, Maryland Received October f2Y9 1969
When carbonyl labeled radioactive ketene is added to the extent of 0.7% of Fischer-Tropsch synt,hesis gas being passed over a singly promoted iron cahlyst a t 256" and one atmosphere, the hydrocarbon pr0duct.s are found t,o have a Small activity which is directly proportional to the number of carbon atoms in the molecule. Similar results are obtained d e n 0.50% ketene is added to synthesis gas passed over a cobalt cat,alyst a t 189" and one atmosphere. These reeults are interpreted as indicating that ketene, which has previously' been demonstrated to act as a chain init,iator when added in a m a l l percentage to Fischer-Tropsch synthesis gas, dissociat,es on the surface SO that only the methylene part of the molecule funct,ions as t,he chain initiator.
For a general iiit,roduction to the use of radioactive tracers in the study of t8heFischer-Tropsch mechanism the reader is referred t,o the previous papers'--; on tdheaddition of radioactive compounds to the synthesis gas. I n t>he preceding paper!' ketene labeled with C14 in the methylene group when added in a small percentage to the synt,hesis gas was fouiid to produce radioactive products which had an approximately (.onstant radioactivity per mole. When 1 : 1 HZ: CO synthesis gas containing 2% ketene was passed over a singly promoted iron catalyst, the products had about the same molar activit,y a': the ketene. Firhen 2 : 1 1% :CO :synthesis gas cwitaining 0.257, ketene was passed over a cobalt catalyst the molar activity of tlhe pro(l.u(+ tis about,one-third that of t,he ket'ene. These rosults led to the cmiclusion t'hat t,he ket,ene IKLS strongly ndsorhed on hoth t'he iron and cobalt, catalyst:: aiid arted as a chain initiator. In order to check n-het,her the ketene molecule behaved as ti unit or split up when it acted as a chain initiator the experiments herein reported were performed. These experiments are similar to the preyious ones except that the ketene is 1xl)elcd i l l the c*:trboiiyIgroup rather than the methylene group. If the whole ketene molecule is the chaiii initiator t,he results of this experiment, should be ideiit,ic.:il with t,hr previous results. If the ket'ciie niolecule splits up before initiating a chain the results could he quite different. ( 1 ) G. F%lylir~ldt~r anit 1'. 11. E m m c t t , T i n s .JOURXAL, 6 3 , 962 (1L159). ( 2 ) J. T. K I I I ~ I I ~11. I C13. ~ , Podgurski, 7T. B. Spencer and P. H. E m i n r t t . J . :lm. C h r m . S o r . , 73,504 (1951). i:O .J. T.Kiiiiiriivr and P. IT. Enimrtt, ibid., 75,5177 (1953). ( 4 j FV. IC. Hnll. K. . J . Kokes a n d P. H. E m m e t t , ibid., 79, 2983 (1957). ( 5 ) R. .I K O ~ PW. S , X. Hall and P. H. E m m e t t , ibid., 79, 2989 (1957).
Experimental The ketene was made by decomposing acetonr over a hot wire in the previously described equipment. Acetone labeled with C11 in the 2-position m ~ obtained s from the Isotope Specialties Company. The ketene produced was purified by vacuum distillation and its purity checked by vapor phase chromatography. The ketene had an activity of 5000 counts per minute per cc. at S.T.P. The same Fischer-Tropsch synthesis apparatub m7as used for these experiments as was the earlier reported ketene work.' The ketene was introduced into the synthesis gas stream by bubbling the mixture of carbon monoxide and hydrogen through liquid ketene immersed in an ethyl alrohol slush bath. The percentage ketene in the synthesis gas was determined by measuring the volume of liquid ketene in the trap before and after the run. Synthesis products were analyzed on a vapor phase chromatography apparatus (Perkin-Elmer Model S o . 154-B) using columns A and J. The radioactivitv of the products was determined using a conventional Geiger rounter n l t h the previously described special cell.' Runs wcre made on the h T - 0 previously used catalysts. One ratalyst (No. 613) is a reduced fused iron oxide. It is a singly promoted catalyst containing 1.80% SiOJ, 1.6% ZrO? and 0.58% A1203. The furnace was charged with 50 rc. o,f 10 to 20 mesh catalyst. The catalyst Tyas reduced a t 500 overnight with H, a t a space velocity of 1000 hr.-I. 1 1 : 1 H2:CO mixture containing 0 . i % ketene as passed over the iron catalyst a t 207 cc./min. for 5 hi; T h r average contraction was 18Tcwith the furnace at 256 . The cobalt catalyst (KO. 89EE) whose preparation is described by Anderson6 and co-workers is composed of cobalt, thoria, magnesia and kieselguhr in the ratio 100:6 : 12,200. The furnace was charged v i t h 50 cc. of the pelletized catalyst. It was reduced at 400' overnight with a H velocity of 1000. A 5 hour run was made with 0.53s;b added to the 2 : l H2:C0 synthesis gas d i i c h was passed over the catalyst a t 210 cc./min. Thr contraction nveraged 1670 a t 189".
Results I n Table I is given the product distribution determined on the fractometer by directly ruiiiiiiig (6) R . B. Anderson, W. K. Hall, H. Henlctt and B. Selieman, zbzd., 69,3114 (1947).
ADDITIONOF RADIOACTIVE KETENETO SYNTHESIS GAS
April, 1960
product gas collected just after the furnace. Since one complete product analysis required three separate chromatographic runs, one each for the fractions C1 to C3, C3 to Cg and Csto Cs, the samples to be run for each fraction were collected a t 45 minute intervals. It was assumed that the product distribution did not change during the time between sample collections. The product distribution for both the cobalt and iron catalysts is seen to be shifted toward a greater number of low molecular weight products for the ketene runs relative to the control runs.
s
0
2
4
6
47 1
8
1
0
CARBON NUMBER.
TABLE I PRODVCT L)ISTRIBUTION I N
-Fe-cat.-
No
ketene
RELATIVE MOLES 0.7% ketene
9.56 1 9 . 5 7.0 3.5 2.4 1.20 1.9 Ethane 1.10 0.092 0.48 Ethylene 1.00 1.00 C:, t o h l 0.87 Propane 0.68 .32 .13 Propylene .59 .79 C4 total .26 n-C4 isc-Ca .29 .33 But ene-2 .50 .33 .31 C 5 total .17 .12 ?& ,026 .038 iso-C5 Pentene-2 pentene-3 .12 .16 n-Ce .Oi .043
COP CH4 C z total
+
+
n-Ci 72-C*
-Cobalt-oat.No 0.53% ketene ketene
0.49 1.93 11.0 33.0 0.59 1.53 1.27 0.56 0.028 0.26 1.00 1.00 0.72 1.0 .28 0.0 .91 .76 .68 .53 .23 .23 1.20 .67 0.72 .46 .28
