NOTES
198
Vol. 67
NOTES RADIATION DOSE AND IODINE SCAVENGER EFFECTS ON RECOIL C1l REACTIONS I N CYCLOHEXANE’ BY
EDWaRD
P. RACKA S D
.kDOLF
F.
S’OIGT
Contribution hTo. 1166 from The Institute j o y Atonzc Research and Department of Chendstry, Iowa State University, Ames, Iowa Received March 16, f 068
The chemistry of recoil carbon atoms has received considerable attention in recent years. The experiments of Wolfgang and his c o - ~ o r k e r s ~on - ~the reactions of CI1 with gaseous hydrocarbons have been interesting, in particular their discussion of the formation of products by Cll or Cll H insertions into a C-H bond in the target molecules. This paper presents the results of a study of the C*I+ liquid cyclohexane system in which the (y, n) reaction was used as the method of activation. The relative yields of gaseous products were determined as a function of the radiation dose and of the concentration of iodine dissolved in the cyclohexane. During the irradiation, the samples were subjected to rather large radiation doses and it was important to establish the contribution of radiation damage to the observed yields. Experiment a1 Materials.-Phillips research grade cyclohexane (99.94 mole 70 purity) was used after drying over sodium wire. “Baker Analyzed” iodine was used without further purificition. The tantalum foil used as a beam monitor was from the Ethicon Suture Laboratory. The irradiated tantalum foil was shown to be free of radioactive impurities. Helium which was used as a carrier gas in the chromatograph had a stated purity in excess of 99.99%. Sample Preparation.-The systems studied were cyclohexane, either pure or with dissolved iodine, prepared by vacuum-line techniques and sealed in Pyrex glass bulblets with a volume of about 0.5 ml. Iodine concentrations were determined by a Hellige Duboscq colorimeter. Synchrotron Irradiations .-The samples were irradiated in a thin-walled thimble which projected into the cavity of the doughnut-shaped acceleration chamber of the Iowa State University electron synchrotron, operated at 47 MeV. Irradiations were for periods of 5 to 60 min. and all runs were made at room or ambient temperature. Tantalum foil strips used as X-ray dose monitors were placed around the bulblets at reproducible positions. A description of the irradiating position can be found elsewhere.5 In addition to the bremsstrahlung which cause the nuclear transformation in carbon, samples were exposed to a high electron flux. These fluxes combined with the recoiling Cii atoms, with a kinetic energy of -0.5 Mev., to produce appreciable radiation decomposition in the samples. The only activity observed in the samples was that of C11. I n previous work6 using this synchrotron irradiations were made on samples outside the doughnut. I n that position the flux responsible for activating the Ci1 and producing decomposition was much lower. Gaseous Product Analysis.-A radio-gas chromatograph designed and built by R. Clark and K. Stensland of this Laboratory was used. The only column used was a diisodecylphthalate-di(1) Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission. ( 2 ) C. MaoKay and R. Wolfgang, J. Am. Chem. Soc., 83, 2399 (1961). (3) C. MaoKay, M. Pandom, P. Polak, and R. Wolfgang, “Chemical Effects of Nuclear Transformations,” Val. 11, International Atomic Energy Agency, Vienna, 1961, pp. 17-26. (4) C . MaoKay, P. Polak, €1. E. Rosenberg, and R. Kolfgang, J . Am. Chem. SOC.,84, 306 (1962). ( 5 ) A. J. Bureau and C. L. Hammer, Rev. S e i . Instr , 32, 93 (1961) (6) C. E. Lang and A. 17. T’oigt, J. Phys. Chem., 66, 1542 (1961).
methylsulfolane (DIDP-DMS) mixed column consisting of a 6ft. section of D I D P and a 16-ft. section of DMS, both 40% by weight, on celite-22 (48-65 mesh). All the gaseous products appeared on a DIDP-DMS column, except that C2H4and C2Hoappeared as one product peak because of similar retention times. KO attempt was made to resoIve these activities since earIier work has shown that about four times as much ethylene as ethane is produced in the C11 cyclohexane reaction. Activity Calculations.-The counting system was described by Lang and Voigta; the gas stream was passed through a vial placed in the well of a KaI( T1) crystal. For each radioactive product the count rate 11s. time was recorded on a strip chart recorder or summed by a scaler, or both. The scaler sums and the areas on the recorder charts were in quite good agreement. These data were corrected for background and for decay to the end of the irradiation (using 20.4 min. for the half-life of GI1) and standardized to a 1-g. sample. The corrected areas were multiplied by the flow rate t o obtain values for the activities of the products.
-+
Results . Sixteen experiments were done with pure cyclohexane
and seventeen with iodine at various initial concentrations, in mole fraction: 1.6 X (6), 3.2 X (3, and 2.1 X (6). The labeled products observed were methane, ethane 3. ethylene, propylene, acetylene, and 4-carbon products. Tantalum foil monitors, in which the 8.15 hr. was produced, were used to correct for variations in irradiation time and beam intensity. However, there was considerable scatter in the results when this external monitor was used. It was observed that the acetylene activity compared to tantalum and corrected for irradiation time with (1 - e--ht) terms mas remarkably constant. In 12 experiments the acetylene/tantalum activity ratio was found not to depend on irradiation time (from 5 to 40 min.), on dose (over a 9-fold range), on dose rate (over a 2-fold range), or on iodine concentration. Hence, the acetylene yield could be and was used as an internal monitor, and the yields of other products are presented as ratios to acetylene. I n this may agreement between experiments was greatly improved. Also, if the absolute yield of acetylene is known (Lang and Voigt estimate it as 140J,)6the absolute yields of other products can be obtained by multiplying the appropriate ratio by the absolute acetylene yield. It is legitimate to use the acetylene activity as a measure of the total radiation dose as well. Although the total dose includes the contribution of electrons and bremsstrahlung of all energies, while the yield of acetylene measures only those X-rays of energy sufficient to induce the C12(y, n) reaction, the method of producing the bremsstrahlung is the same in all experiments. Hence, the relative total doses in different experiments will be related to the acetylene activities, if these are corrected for Cll decay during irradiation with the factor t / ( l - e-Xt). The total radiation intensity in the system was estimated in two ways. The irradiation of cyclohexane containing low concentrations of iodine led to the disappearance of the iodine color after about 5 min. from mole fraction. an original concentration of 3.2 X With the assumption that G(-Iz) = 3.2,’ this corresponds to an intensity of -1.5 x 1OI8 e.v./g. min. Ex( 7 ) E. H, Weber, P . F. Forsyth, and R. 11.Schuler, Radiatzon Res., .?, 66 (1955).
XOTES
Jan., 1963 periments with the Fricke dosimeter indicated the oxidation of -6 X mole/l. min. of ferrous ion, corresponding to an intensity of -2 X 1OL9e.v./g. min. The actual dose rate probably lies between these limits. Results on the effects of changing the radiation dose and the iodine concentration on the yields of CH4 and CzH4 4- CZH6 are shown in Fig. 1 and 2. I n all of the mole fraction iodine and in experiments at 1.6 X those for longer than 5 min. at 3.2 X m.f., the iodine color disappeared. In these experiments yields lay between the upper and lower curves of Fig. 1 and 2. Since iodine was not present during the complete irradiation, the results are not included in the figures. The yield of methane relative to acetylene can be extrapolated to 0.29 f 0.05 at zero radiation dose, independent of the iodine concentration. Similarly, the yield of ethylene plus ethane extrapolates to 0.13 d= 0.03. Experinients with a silica gel column which does separate them indicate that the ratio for ethylene is The data for the -0.10 and that for ethane -0.03. production of propylene are similar to those of Fig. 1 and 2 giving, on extrapolation, -0.08 for the propylene/ acetylene ratio. The yields of 4-carbon compounds show some variation, with an average of 0.40 i 0.06 relative to acetylene for the iodine-free systems. trans-Butene-2 has been identified tentatively as one of the 4-carbon products. The data show that in the presence of sufficient iodine the yield is reduced to zero. Experiments were conducted to determine what effect traces of oxygen might have on the shape of the yield us. dose curve. Solvent extraction experiments on pure cyclohexane, with CHC& and Iz as one phase and an aqueous solution 0.5 M in Na2S03and in NaOH as the other, showed that about 99% of the Cll activity was organic. A gas chromatographic separation on a silica gel column showed that labeled CO and COz, two products expected in the presence of oxygen, were not present. Experiments on cyclohexane saturated with oxygen at atmospheric pressure gave results similar to mole fraction I2 systems, those from the 1.6 X indicating that dissolved O2behaves similarly to Iz.
Discussion The results show that the radiation-induced portions C2H6, and C3H6 are reof the yields of CH4, C2H4 moved by iodine and hence involve species that react with iodine. While the short lives of the usual radicals exclude them from consideration as causing the build-up of yields to a maximum over a period of 20 to 30 min., other active species with lifetimes of a few minutes may be formed. In any case, additional data will be required before the nature of the radiation damage in this system is understood. That portion of the yields which is produced at zero dose or in the presence of iodine can be considered as the true hot-atom yield, resulting from the fact that the C'l fragment has energy above that of its surroundings. Although the Cll fragment may lose electrons initially and start out as an ion, the ionization potentials of the various C-H radicals and cyclohexane are such that the radicals would be neutralized before reaching the energy range for reaction. Hence ion-molecule reactions can be eliminated. Since the steady state concentration of radicals is much less than the con.. centration of cyclohexane molecules, reaction of the
+
0
199
5
I
I
I
I
I
J
IO
15
20
25
30
35
X-RAY
DOSE CC2 H2ACTIVITY x
&) At
I-e
Fig. 1-Relative yields of CH, us. radiation dose a t various iodine concentrations. Mole fraction 11:0 , 0; 0,3.2 X B, 2.1 x 10-3.
0
0
5
I
I
I
I
I
I
IO
15
20
25
30
35
X-RAY DOSE ( C z H z ACTIVITY x
+
+ I-e-At
)
Fig. 2-Relative yields of CzHa C I H ~ us. X-ray dose at various iodine concentrations. Mole fraction 1 2 : 0 , 0; 0, 3.2 x 10-5; m, 2.1 x 10-3.
Cll fragment with the latter is more probable, particularly since this reaction would he exothermic even when the C1l fragment is at thermal energies. Thus, the various products appear to be formed predominantly by reactions of the recoiling fragment with the molecules of cyclohexane. Displacement and abstraction reactions can be expected to be important. The most abundant gaseous product, acetylene, could be formed by a reaction path involving displacement of hydrogen by a C*H radical, or it could be considered as insertion of a carbon in a C-H bond. I n this mechanism sufficient energy would be contributed to the intermediate so that two C-C ring bonds would be broken and the labeled product would rearrange to acetylene. The formation of the other two-carbon compounds could be quite similar, possibly involving C*H2 and C*H, radicals, or an energetic C2*H, fragment could abstract hydrogens from the cyclohexane. Of the other products, methane probably is produced by a series of abstraction reactions, as the Cl1 fragment cools down but is still above thermal energies, since its yield at zero dose is not reduced by the addition of iodine. Three-carbon compounds could result from the rupture of a CeHll. C*H intermediate in such a way that st 3-carbon chain results, followed by stabilization of the
200
Vol. 67
NOTES
fragment by hydrogen abstraction. Since the yield of 4-carbon compounds was reduced to zero in the presence of iodine, reactions to form these are more likely radiation induced or thermal. Acknowledgment.-The authors gratefully appreciate the assistance of Dr. A. Bureau and the synchrotron staff, particularly Mr. J. R. McConnell, who operated the synchrotron for the many irradiations, and nlr. D. Clark, who performed Fricke dosimetry in the synchrotron. THE STATE OF PLATINUM IN RE-FORMING CATALYSTS BY MARVINF. L. JOHNSON AND CARLD. KEITH Sinelair Research, Inc., Harvey, Illinozs Received August 0, 1066
Alumina with a small amount of platinum is widely used as a re-forming catalyst-to catalyze reactions of dehydrocyclization, dehydrogenation, isomerization, and hydrocracking of hydrocarbons. Such catalysts normally are prepared by impregnating alumina with a soluble platinum compound, followed by calcination in air a t an elevated temperature. The chemical state and degree of dispersion of the platinum in platinum-alumina have been the subject of several recent papers. Since the catalysts are employed under reducing conditions, one presumes that platinum has been converted to the metallic state. Indeed, proposed mechanisms for the various reforming reactions1 presuppose the existence of metallic platinum as the dehydrogenation component of these dual-function catalysts. Hydrogen consumption measurements in these Laboratories and by Mills, et C L E . , ~ indicate nearly complete reduction of Pt+4to Pto in a few minutes in hydrogen, a t temperatures as low as 245’. As suggested by Kluksdahl and Houston,3 the darkening and dehydrogenation activity which appear upon reduction with hydrogen suggest the production of platinum metal. Furthermore, it has been observed in these Laboratories that hydrogen reduction a t an elevated temperature is necessary to cause platinum-alumina to catalyze HrD2 exchange; rate constants of the order of 20-30 min.-l at 195.5’ and 518 mm. are observed for various preparations containing 0.6% Pt. The above constitutes evidence for the existence of platinum metal upon treatment of platinum-alumina with hydrogen at elevated temperatures. Several workers have employed adsorption techniques to show that this platinum exists in a high degree of dispersion. Hughes, et ~ 1 . ~ used 4 carbon monoxide adsorption, while 0thers6-~employed hydrogen adsorption to draw this conclusion. Hughes, et u L , ~showed, in addition, that hydrogen reduction must be carried out above a t least 200° to obtain adsorption by platinum metal; they showed further that a minimum extent of plati-
n u n area, as measured by carbon monoxide adsorption, is necessary to obtain activity for re-forming methylcyclopentane. Similarly, Mills, et u L , ~observed that a loss of catalytic effectiveness paralleled the growth of platinum metal particles as observed by X-ray diffraction. Platinum surface area thus is an important factor in re-forming, A different point of view has been proposed by McHenry, et aL18who postulated the active component of these catalysts to consist of a platinum-alumina complex. This conclusion was based on their finding that dehydrocyclization activity was dependent on the amount of complex present, defined as that portion of the platinum which was soluble with the alumina in aqueous hydrofluoric acid or acetylacetone. The present paper presents evidence to show that this apparent discrepancy can be resolved if one considers the “complex” to exist in the oxidized state and to be responsible for high platinum metal dispersion upon subsequent reduction. Experimental Platinum solubilities of Table I were measured by the method described by McHenry, et aZ.8 Table I1 results were obtained by digestion in 1:4 H&Ok a t 60’ for several days; the solutions and the residues each were analyzed. Chemisorptions of carbon monoxide were performed after reduction for 12 hr. at 482” in flowing purified hydrogen, and evacuation for 9 hr. at 300°, at about 5 X 10-8 mm.; this temperature of evacuation was chosen to minimize adsorption by alumina t o about 0.05 cc. (STP)/g. After cooling to room temperature in a few mm. of helium and evacuation of the helium, a measured quantity of carbon monoxide is admitted to the catalyst. One hour later, automatic Toepler pumping is started to return unadsorbed gas to the calibrated system. This pump-
TABLE I EFFECT OF OXIDATION ON SOLUBLE PLATINUM yo soluble Sample
A Used catalyst B A 3 hr., 440°, one atm. C A 3 hr., 480°, one atm. D A 3 hr., 505’) one atm. E C 20 hr., 505’, 10 atm. Solubilityin HF; 0.6% totalPt.
+
air air air air
+ + +
-
(1) F. G. Ciapetta, R. M. Dobres, and R. W. Baker, “Catalysis,” Vol. 6, ed. by P. H. Emmett, Reinhold Publ. Corp., New York, N. Y.,1958. (2) G. A . Mills, S. Weller, and E. B. Cornelius, “Second International Congress on Catalysis,” Vol. TI, Paris. 1960, Paper 113. (3) H. E. Kluksdahl and R. J. Houston, J . Phys. Chsm., 66, 1469 (1961). (4) T. R. Hughes, R. J. Houston, and R. P. Sieg, Preprints Pet. Div., ACS, April, 1959. ( 5 ) L. Spenadel and M. Boudart, J. P h w Chem., 64, 204 (1960). ( 6 ) S. F. Adler and J. J. Keavney, ibid., 64, 208 (1960). (7) €1. L. Gruber, &id., 66, 48 (1962).
Pt”
History
CO ohemisorption (00.
0.23 .27 .32 .34 -48
STP/g.)
0.14 .20 * 22 .24 .33
TABLE I1 EFFECTOF OXIDATIVECONDITIONS ON SOLUBLE PLATINUM Temp., OC.
Partial pressure of Oe, atm.
% solubfe platinum”
HE-Dn exchange rateb
..
427 0.015 0.27 427 0.11 .427“ 22 427 1.o .452 30 482 0.21 .493 30 1.0 .509 46 482 510 0.015 .433 39 510 0.21 .508 50 510 1.o .52 45 538 0.21 .504 57 1.o .53 47 538 582 0.015 .367 18 582 0.21 .378 43 582 1 .o .533 44 650 1.o .374 33 “Solubility in HzS04; 0.55% total Pt. bSee text. OThis sample wm used for the other treatments. (8) K. W. McHenry, R. J. Bertolacini, H. M. Brennan, J. L. Wilson, and H. 8. Seelig, “Second International Congress on Catalysis,” Vol. 11, Paris, 1960, Paper 117.
