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C O M M U N I C A T I O N S TO THE EDITOR
Identification of Reaction Intermediates by Microwave Spectroscopy. The Catalytic Reactions between Propylene and Deuterium over Zinc Oxide Publication costs assisted by Sagami Chemical Research Center
Xiv: It has been generally considered that the hydrogenation and isomerization of unsaturated hydrocarbons over various metals proceeds through the same reaction intermediate,’ the half-hydrogenated state. The authors previously studied2 the mechanism of the catalytic reaction between propylene and deuterium over ZnO and suggested that a r-allyl species was the reaction intermediate of the exchange reaction, on the basis of the deuterium distribution determined by microwave spectroscopy. The deuterium addition to propylene, on the other hand, seemed to proceed via interaction between the chemisorbed hydrocarbons and deuterium molecules in the ambient gas. It is very interesting to note that the formation of a r-allyl species of zinc oxide has also been observed by Dent and Kokes3 by means of an infrared spectroscopic technique. Infrared spectroscopy is one of the most powerful techniques for studying the nature of surface species, but even if a certain chemical species is detected on the catalyst surface, it does not follow that it is necessarily a reaction intermediate. I n this communication, the identification of the reaction intermediate in the propylene-deuterium exchange reaction over zinc oxide will be reported and the mechanism of the deuterium addition t o propylene will also be discussed. As described in the previous report,2 the deuterium distribution of the product propylene-& suggested that the reaction intermediate of the propylene-deuterium exchange reaction was a r-allyl species. I n the initial stage of the exchange reaction, only propylene-341 (H3DC.CH=CHz) was formed, but in the latter stages three kinds of monodeuteriopropylene, 3 4 , cis-1-dl, and t r ~ n s - 1 - d(H,C.CH=CHD), ~ were formed in the approximate ratio of 3: 1: 1, while, propylene 2 4 (HzC .CD=CH2) u’as not observed during the reaction. These results exclude the associative adsorption mechanism of propylene which u-ould produce the halfhydrogenated state of the normal propyl or isopropyl radical, which would give propylene-241 (100%) or propylene-3-dl :cis-l-dl :truns-l-dl = 3 :1: 1, respectively. They also exclude the u-allyl adsorption The Journal of Physical Chemistry, Vol. 76, No. 16, 1079
mechanism because of the deuterium distribution of the dl species (and dz species) but the possibility of a concerted (push-pull) exchange mechanism remains. To distinguish this mechanism from that of a .Ir-allyl intermediate, the deuterium distribution in the propylene-& was observed as shown in Figure 1. It is obvious from the figure that at the beginning of the reaction, propylene-l,3-dz (HzDC.CH=CHD) and propylene-3,3-dz (HDZC .CH=CHz) are formed in equal amounts (50:50), which indicates that the 1 and 3 carbons of the propylene become equivalent in the reaction intermediate. These results exclude a concerted mechanism which would give only the 1,3-dz species of dz-propylene at the beginning of the reaction. These experimental results lead to the conclusion that the r-allyl species, which mas detected on the catalyst surface by Dent and Kokes, is the real intermediate of the propylene-deuterium exchange reaction over ZnO. On the other hand, in the deuteration of propylene over ZnO, propane-& was predominant from the beginning of the reaction, which suggests that the r-allyl species is not the reaction intermediate in this process as discussed previously.2*3 To confirm this, the deuterium distribution of propane-& was studied and is Conversion.
o *
f --.-..
__o
40
--D--o
Q, (mean deuterium content).
Figure 1. The deuterium distribution in propylene-& and propane-& molecules; Pcaae= PO, = 12.5 cm, Zno (10 g), room temp: 0,HDzC.CH=CHZ (3,3-&); A, HzDC.CH=CHD (cis) (1,3-dz); x, HzDC.CH=CHD (trans) (1,3-&); 0, H& CH=CDz (1,l-dz); 0, H& CHD CHzD ( 1,2-&-propane).
-
3
R
(mean deuterium content) conversion = propane/(propylene
fi
+ propane).
(1) G. C. Bond, “Catalysis by Metals,” Academic Press, London, 1962. (2) S. Naito, Y. Sakurai, H. Shimizu, T. Onishi, and K. Tamaru, Trans. Faraday Soe., 67, 1529 (1971). (3) A. L.Dent and R. J. Kokes, J . Amer. Chem. SOC.,92,6709,6718 (1970).
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COMMUNICATIONS TO THE EDITOR shown in Figure 1. This demonstrates that propane 1,2-dz (CH2D'CHD .CHa) is predominantly formed at the beginning of the reaction. These results indicate that propme-ds is formed by the simple addition of deuterium to the double bond of normal propylene or to the propyl group in the surface. Consequently, it is concluded that the hydrogenation and isomerization proceed through different reaction intermediates over zinc oxide.
Acknowledgment. The authors wish to thank Dr. Shuji Saito for his continuous help during the measurement of microwave spectra. SAQAMI CHEMICAL RESEARCH CENTER SHUICHINAITO* OHNUMA, SAQAMIHARA, KANAQAWA-KEN TOSHIHIKO KONDO JAPAN MASARU ICHIKAWA DEPARTMENT OF CHEMISTRY THEUNIVERSITY OF TOKYO HONQO, BUNKYO-KU, TOKYO, JAPAN RECEIVED MARCH16, 1972
KENZITAMARU 4
TIME
(MINI
Figure 1. Transmission changes in a solution containing 2,4-pentanedione (0.05 M), MnS04 (0.017 M ) , and KBrOa (0.07 M ) in 1 M HzS04at 25", 8-om light path. Magnetic stirring was stopped a t A and restarted at B.
Kinetic Oscillations in the Oxidation of 2,4-Pentanedione by Bromate Ion, Catalyzed by Manganese(I1) Publication costs borne completely by The Journal of Physical Chemistry
Sir: The increasing interest which is currently being shown in the oscillatory kinetics of Belousov's reaction (bromate oxidation of malonic acid)'-& prompts us to report our observations on a related system in which the organic acid is replaced by 2,4-pentanedionea Several novel features make the diketone oxidation worthy of attention, particularly since in constrast to both the malonate-bromate reaction and the well-known hydrogen peroxide-iodate reaction, no permanent gas is evolved, so that experiments can be conducted in a materially (if not thermally) "closed" situation. Our chief findings may be most succinctly summarized as follows. (i) I n acid solution, with Mn(I1) present, the bromate ion oxidation of 2,4-pentanedione proceeds in a periodic fashion. Fluctuations in the absorbance of the solution begin to build up immediately on mixing the reagents, as Figure &shawa+with only a very brief induction period. (ii) The reaction is affected by stirring in the sense that the oscillations damp away more rapidly if stirring is discontinued. We originally thought it possible that dissolved oxygen could be responsible for this effect in a system which was open to the atmosphere. However, a stirring effect exactly similar to that shown in Figure 1 was observed in a sealed system where the components had been rigorously degassed by standard
X
(nml
Figure 2. Spectra of the transient intermediates, obtained by the flow method. The initial concentrations after mixing for KBr08, MnSOd, and 2,4-pentanedione were (A) 0.052, 0.037, 0.022 M and (B) 0.30, 0.42, and 0.025 M, respectively, in 1.5 M HZSO4.
freeze-thaw-pump methods under high-vacuum conditions before mixing. We therefore conclude that the mechanism of this reaction includes at least one heterogeneous step which involves neither gas evolution nor the presence of atmospheric oxygen. The origin of the heterogeneity may be similar to that suggested by Kasparek and Bruicel for the cerium-catalyzed malonate-bromate reaction, namely transient formation (1) G.J. Kasparek and T. C. Bruice, Znorg. Chem., 10, 382 (1971). (2) A. N. Zaikin and A. M. Zhabortinski, Nature (London) 225, 535 (1970). (3) R. M.Noyes, R. J. Field, and E. Koros, J. Amer. Chem. Soc., 94,1394(1972). The Journal of Phu,aical Chemistry, Vol. 76,No. 16, 197.9
