3298
A Concerted Reaction Mechanism for the Hydrogenation of Olefins on Metalel' by Nelson C. G a r d n e P and Robert S. Hansen XnsfilulcJm Atomic Rumrch and Dcporlnnl OJ Chmialru. ISWc L'niowdfu. Am% l o w 6WlO (Recn'cd May 8.1870)
I n an effort to reconcile their findings* for the decomposition of ethylene on iridium following mom temperature dose (in which hydrogen was the only gaseous product) with those of (in which methane and ethane, but no hydrogen, appeared in the gas phase), Hansen, et al., suggested a mechanism for ethane formation based on the direct transfer of hydrogen atoms from chemisorbed acetylene or ethylene to an ethylene molecule impacting it from the gas phase. Because the ethylene partial pressure was greater by a factor 106 in Roberts' experiment, this mechanism might be operative in his case and not in theirs. They cited the diimine hydrogenation of olefins' as an analog for the transfer of hydrogen atoms from chemisorbed acetylene to impacting ethylene. Recent work in this suggests strongly a modification and extension of this mechanism, and the modified mechanism appears to offer a reasonable interpretation of a number of hitherto puzzling facts. The possibility of trans diadsorption of olefins appears to have been first suggested by Winfields incident to an interpretation of Beeck's findingt0of polymerization of ethylene on nickel. Work in this laboratory has indicated that tungsten planes with a high density of 4.47-i( spacings are also planes of high site density for ethylene adsorption; these distances are readily spanned by tram diadsorption. A number of features of ethylene decomposition on body centered and face centered cubic metals have been rationalized through use of the transdiadsorbed model.'.a In fully extended transdiadsorbed ethylene, two hydrogen atoms and two carbon atoms are coplanar in a plane normal to the surface. The former two hydrogen atoms are so located as to form readily a transition state for concerted transfer to another ethylene molecule above that chemisorbed, and the transfer is permitted thermally by the Woodward-Hoffman rules." The second ethylene molecule might be adsorbed on top of or otherwise "complexed" to the first; it is not necessary to limit consideration to impacting ethylene molecules as in the suggestion of Hansen, et al. Removal of a pair of hydrogen atoms from transdiadsorbed ethylene leaves trans-diadsorbed acetylene as a residue if the transfer occurs with minimum rearrangement. Addition of hydrogen to trans-diadsorbed acetylene is proposed as the rate-limiting step in hydrogenation at other than extremely low T h Jmrrnol OJ Phuaiml Chonialry. Vol. 74. No. 17, 1970
pressures. The proposed mechanism is therefore the following CtHdg) * r C H r C H t *
*CHzCHt*
+ C2Hdg)
---t
tCH=CH*
+ CzH&
Ht +2H
2H 1
(1)
+ *CH=CH*
(11) (111)
+*CHZ-CHz*
(IV)
rCH=CH' +2CH(ads) (VI Step IV is assumed rate determining and step V o r equivalent leads to poisoning. Whether step I1 occurs directly or through an intermediate weak complex is immaterial to the kinetics analysis. Figure 1 is a photograph of cork ball models of eth lene and acetylene trans diadsorbed on tungsten 4.47- spacings. The transition state for reaction I1 is shown in Figure 2 with hydrogen atoms in the top ethylene shown eclipsed in a configuration making abstraction of two hydrogens from the lower ethylene plausible; a similar structure but with hydrogen atoms on the top ethylene molecule
x
( I ) (a) Work was performed in the Ames Laboratory of the Atomic Energy Commission. Contribution No. 2736. Based in part on B dissertation submitted by Nelson C. Gardner to the Graduate College of Iowa State University in partial fulfillment of the w u i menta for the degree of Doctor of Philoeophy. 1866. (b) Addinquiries to the Chemieal Enairteering Science Division. Case Western
Reserve University. Cleveland. Ohio. (2) H. 8. Hansen. J. II. Arthur. Jr.. V. J . Mimeault. and R. R. Rye. J . Phua. Chm.. 70.2787 (1860). !3! (a) R. W. Roberts. ibid.. 67. 2035 (1863); (b) R. W. Roberts. rbd.. MI. 2718 (1884). . . (4) S. Hanig. H. H. Maller. and W. Thier. AWM. C h o . . 4. 271 ( 1805). (5) N. C. Gardner. Thesis. Iowa State University. Nov 1866. ( 0 ) R. R. Rye and R. S. Hansen. J . Phus. C h m . . 73, 1667 (1868). (7) R. R. Rye and R. S. Hansen. J . C h m . Phpn.. 50,3585 (1968). (8)R. S. Hansen and N. C. Gardner. J . Phur. Chon.. in p m . (8) M. E. Winfield. A u d . J . Sci. Rea.. Scr. A . 4, 385 (1851). (IO) 0.Beck. Diactcra. Faraday SN., 8, 118 (1850). (11) R. Hoffman" and R. B. Wwdwuard, Scicncc. 167, 825 (IWO).
NOTES
3299
staggered would make plausible the exchange of hydrogen atoms between upper and lower ethylene molecules. Most commonly, the rate of hydrogenation satisfies'* dPczHe -- JCPHa dt
(1)
The zeroth-order dependence on ethylene pressure follows if sites suitable for trans-diadsorbed ethylene or acetylene are almost completely filled with the latter (the former reacting as fast as formed). The firstorder dependence on hydrogen pressure could result from a concerted addition Of hydrogen to acetylene (either by impact or from a weak complex, e*g*,a hydrogen phYsical1y adsorbed On top Of the chemisorbed acetylene) or by addition Of hydrogen atoms chemisorbed on the metal provided the additional sites for hydrogen adsorption are sparsely occupied. At least according to the simplest view, the concerted thermal addition from impact or weak complex is not in accord with the Woodward-Hoffmann rules so the chemisorbed hydrogen intermediate appears preferable. H
\.
I
'\c .......... . . . .. . . . . . . . . . . . . c' H
/
a small fraction of the ethylene-14C was ever removed from the surface. The authors concluded that only a small fraction of the sites were operative in the reaction, whereas we believe the ethylene-14C molecules served repeatedly as hydrogen donors without ever leaving the surface. We do not propose that this is the only mechanism operative in catalytic hydrogenation. In fact, unless an olefin has a hydrogen atom on each side of the double bond it appears sterically difficult or impossible to diadsorb it in the full trans configuration. Further, it appears that only for ethylene will trans diadsorption place two hydrogen atoms exposed for ready abstraction. This suggests the possibility of using an ethylene predose as catalyst promotor in the hydrogenation of substituted olefins; this possibility is being explored. Acknowledgment. We are indebted to Professor 0. L. Chapman for extremely helpful and stimulating discussion. (12) G. C. Bond, "Catalysis by Metals," Academic Press, New York, N. Y., 1962, pp 239-242. (13) N. C. Gardner and W. Powell, unpublished work. (14) R. Gomer, R. Wortman, and R. Lundy, J . Chem. Phys., 26, 1147 (1957). (15) C. Kemball, J . Chem. Hoc., 735 (1956). 58, (16) S. J. Thomson and J. L. Wishlode, Trans. Faraday SOC., 1170 (1962).
.
H
H
Investigations on Single Crystals of Alkali+ Biphenyl- Radical Salts Figure 2. Transition state for transfer of hydrogen atoms from trans-diadsorbed ethylene to ethylene. M denotes a surface metal atom, dotted lines are bonds in the process of forming or breaking, and the atoms connected b y dotted lines are considered coplanar. A plausible transition state for exchange of hydrogen atoms between the two ethylene molecules results (approximately) from rotating the top ethylene molecule 60' about a n axis connecting the two central hydrogen atoms.
The following observations are simply interpreted in terms of this mechanism and are otherwise rather puzzling. (1) Ethylene readily self-hydrogenates on tungsten a t temperatures as low as 160"K,13 yet hydrogen adatoms are negligibly mobile on tungsten below 200°K.14 (2) When ethylene reacts with deuterium on tungsten, nickel, rhodium, or iron the first ethane formed is Ci". Subsequently all possible ethanes are formed, CzDs being the last to appear; this can be accounted for by exchange of hydrogens between chemisorbed and complexed ethylene as previously mentioned. (3) Thomson and Wishlode'6 performed an experiment in which hydrogen and ethylene were admitted to a nickel film containing preadsorbed ethylene-14C. Only
by G. W. Canters, A. A. K. Klaassen, and E. de Boer* Department of Physical Chemistry, University of iVijmegen, Nijmegen, T h e Netherlands (Received February 12, 1970)
It is known that organoalkali salts, such as Li fluorenyl, can be obtained in a crystalline form.' While these crystals are probably ionic, they are not expected to show electronic paramagnetism as the constituent compounds are diamagnetic themselves. On the other hand, it is known that neutral free radicals such as diphenylpicrylhydraxil (DPPH) can form single crystals which are paramagnetic but not ionic.2 Therefore, it was of interest to investigate the feasibility of preparing single crystals of paramagnetic alkali radical salts. Moreover, an investigation
* To whom correspondence should be addressed. (1) J. A. Dixon, P. A. Gwinner, and D. C. Lini, J. A m e r . Chem. Soc., 87, 1379 (1965); A. K. Banerjee, A. J. Layton, R. S. Nyholm, and M. R. Truter, Nature, 217, 1147 (1968).
(2) G. E. Pake, "Paramagnetic Resonance," W. A . Benjamin, New York, N. Y., 1962, Chapter 4. The Journal of Physical Chemistry, Vol. 74, N o . 17, 1970
