NOTES
2436 preferred to absolute values where the coefficient itself is required, since they prevent error accumulation and are much less sensitive to concentration and to ion association. Acknowledgments. The authors extend their appreci-
ation to H. Lingertat, who assisted in carrying out the experimental work. We are pleased to acknowledge that this work was supported by the Office of Saline Water under Contracts OSW-14-01-0001-425 and OSW14-01-001-966.
NOTES
Addition of Hydrogen-Deuterium Mixtures to Ethylene over Chromia and Zinc Oxide by W. C. Conner and R. J. Kokes Department of Chemistry, The Johns Hopkins University, Baltimore, iMaryland 21218 (Received December 4, 1068)
Metals that are catalysts for the reaction of ethylene or acetylene with hydrogen are also catalysts for the hydrogen-deuterium exchange reaction. I n the absence of reactant hydrocarbon, the rate of the exchange reaction (or the parahydrogen conversion) is usually faster than the hydrogenation but in the presence of the reactant hydrocarbon the exchange reaction is reduced, often by more than an order of magnitude. Twigg3 has studied the reaction of ethylene with a 50:50 Hz-D2 mixture over nickel. He finds that the deuterium distribution in the product ethane is the same for the preequilibrated isotopic hydrogen (about 50% HD) as for the unequilibrated isotopic hydrogen (about 0% HD). This result cannot be a consequence of rapid equilibration of the isotopic hydrogen in the gas phase; in the presence of ethylene, the equilibration rate is much slower than hydrogenation. These observations, however, could be a consequence of the occurrence of the surface processes
+ H 1_ C2HS fast C2Hj + H C2H6 slow CzH4
--t
alkyne, addition of unequilibrated 50:50 H2-D2 to 2butyne resulted in cis-2-butene with a random distribution of deuterium. Thus, i t appears that the hydrogendeuterium reactant over metals undergoes isotopic self-mixing before it reacts even though very little H D returns to the gas phase. The following represents a plausible interpretation of these results. All empty metal sites are capable of dissociating hydrogen. Furthermore, even in the presence of reacting hydrocarbons, there are enough of these sites close together to permit isotopic mixing prior to reaction. If reaction with adsorbed hydrocarbon is much more rapid than hydrogen desorption, this could be the cause of the observed inhibition of gas-phase exchange even though the hydrogen that adds to the hydrocarbon is isotopically equilibrated. A number of oxides can function as hydrogenation catalysts.6 These are similar to metals insofar as they also catalyze the hydrogen-deuterium exchange,’ but they differ from metals insofar as (at least for chromias and zinc oxide9) addition of deuterium to ethylene results in CH2D-CH2D only. To gain further information on the differences of metals and oxides, we have repeated Twigg’s experiments over zinc oxide and chromia.
(1) D. D. Eley, “Catalysis,” Vol. 111, I?. H. Emmett, Ed., Reinhold Publishing Corp., New York, N. Y., 1955, pp 63, 66. (2) T. I. Taylor, “Catalysis,” Vol. V, P. H. Emmett, Ed., Reinhold Publishing Corp., New York, N . Y., 1957, pp 260-264. (3) G. H. Twigg, Discussions Faraday SOC.,8 , 152 (1950). (4) E. F. Meyer and R. L. Burwell, Jr., J . Amer. Chem. SOC.,85,2877
which are often invoked5 to explain the fact that addi(1963). tion of deuterium to ethylene over metals leads to a (5) G. C. Bond, “Catalysis by Metals,” Academic Press, Inc., London, 1962, pp 258-270. distribution of ethanes with the general formula, (6) D. L. Harrison, D. Nicholls, and H. Steiner, J . Catal., 7 , 359 C2Ho-zDz: The observations of RIeyer and B u r ~ e l l , ~ (1967). however, cannot be rationalized in this manner. These (7) D. A. Dowden, N . Mckensie, and B. M . W.Trapnell, Proc. Roy. Soc., A237,245 (1956). authors showed that addition of deuterium to 2-butyne (8) R. L. Burwell, Jr., A. B. Littlewood, M . Cardew, G. Pass, and over palladium yielded about 98% cis-2-butene-2,3-d~. C. T. H. Stoddard, J . Amer. Chem. Soc., 82, 6272 (1960). They also showed that, although the H2-D2 equilibra(9) W. C. Conner, R. A. Innes, and R. J. Kokes, ibid., 90, 6858 tion over palladium was poisoned by the presence of the (1968). The Journal of Physical Chemistry
NOTES Experimental Section The zinc oxide catalyst used in these studies was liadox-25, a product of the Xew Jersey Zinc Co. The particular 1-g sample used for these experiments had been pressed into tablets to improve flow characteristics for a previous series of kinetic experiments. Prior to the experiments reported herein, it was exposed alternately for several hours to oxygen and hydrogen at 300" and then degassed for 12 hr at 450". Chromia was prepared by the urea hydrolysis procedure described by Burwell, et aL8 The activation procedure recommended yielded a catalyst so active at -78" that even with 0.1 g of catalyst it was difficult to control the amount of the conversion. After exposure to oxygen a t 375" for 30 min and subsequent degassing for 12 hr at 425") the extent of reaction could be controlled. All runs were made in a circulating system with a volume of about 100 cc. Initial studies with the very active chromia catalyst showed that for the later studies the rate of mixing was quite adequate (ie., the conversion was less than 1% per pass). Reactants were premixed by circulating them in a reaction loop with the catalyst bypassed. I n all cases, the ratio of hydrogen to ethylene was 2:1, and the total initial pressure was 270-280 mm. Runs with "unequilibrated" hydrogen were carried out with an approximately equimolar mixture of hydrogen and deuterium. For runs with "equilibrated" hydrogen, the 50: 50 mixture was circulated over the catalyst for a t least 15 min at room temperature. This equilibrated hydrogen was stored while the catalyst was degassed for about 1 hr at room temperature. Then the reactant, mixture was made up as before, and the run was carried out. The reaction temperature for chromia was -78", whereas for zinc oxide, the reaction temperature was 25". Conversion was monitored manometrically, and except for the run over chromia with pure deuterium, the catalyst was bypassed before more than 10% of the ethylene reacted. Between runs the catalysts were evacuated at room temperature for 1 hr. With this procedure, the activity for the chromia catalyst decreased about 16% from one run to another whereas the zinc oxide showed no such decline. The distribution of deuterium in the ethane was estimated by mass spectroscopy. The cracking patterns for C2H6and C2H4D2(1,2) were those obtained on our instrument for authentic samples. These cracking patterns were quite similar to those reported by Bell and I
