August, 1962
HPDROGESOLYSIS O F DICYCLOPROPYLMETHASE OK
for the decomposition of azomethane follows an Arrhenius expression over approximately twelve decades. The combination of the low temperature data and our results indicate that the best value for the activation energies lies somewhere between 51.2 and 55.4 kcal., with a value of 53 kcal. preferred. Acknowledgments.-This work was supported by the AFOSR, Mechanics Division, under Contract
PLrlTINUM
C.4TALYSTS
1431
AF49(638)-716. Sincere thanks are given to Dr. R. E. Duff (LASL) for computing the thermodynamic functions of azomethane and for furnishing the IBM program for the shock temperature computations. We also thank hlr. Paul Jlarrone (CAL) for supervising the shock speed computations. Frequent consultations with Professor E. L. Resler, Jr., on the design and construction of the shock tube are acknowledged with sincere thanks.
T H E HYDROGENOLYSIS OF DICYCLOPROPYLMETHAYE ON PLATINUM CATALY STS BY JOHKSEWHAM ASD ROBERT L. BURWELL, JR.' Department of Chemistry, Yorthwestern University, Evanston, Illinois Recezted February 16, 1962
The hydrogenolysis of dicyclopropylmethane ( A ) has been studied in a flaw reactor at 52O, H2/A = 16, on coprecipitated platinum-a'lumina, two impregnated platinum-aluminas, platinum-pumice, and platinum-charcoal. Compound A hydrogenolyzes to butylcyclopropane (B) and isobutylcyclopropane (C). B further hydrogenolyzes to heptane ( D ) and 2-methylhexane ( E ) , C to E and 2,4-dimethylpentane (F). The ratios B/C in the first reaction and D/E in the second are about and E/F in the third is about I / I ~ , but there is some variation from catalyst to catalyst. Over half of the sequence A + B -+D El proceeds directly to D E by a surface reartion which intercepts the formation of vapor phase B. The surface C -+ E F. A very active impregnated platinum-alumina was inreaction is much less prominent in the sequence A vestigated i n three mesh sizes, 60--100, 40-60, and 20-40. The first behaved normally but the third gave much less C and a large initial ratio F/C. This behavior is diagnostic of large concentration gradients in the catalyst pores. The very active platinum-charcoal gave even more extreme behavior. The extreme cases of diffusional control do not agree very well with a model which assumes cylindrical pores. This matter and the effect of channelling upon selectivity are discussed.
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On transition metal catalysts, cyclopropanes undergo ring-opening hydrogenolysis at' rates intermediate between those of hydrogenation of olefins and those of isotopic exchange bet'ween alkanes and deuterium. hlkylcyclopropanes open in two ways
CHZ
CH3
/ H? + R-CH--. ------x = x R -C€I \,' \
/I
+
/'\!
(I
- x)/2
CH,
CH3 (1 - x) R-CHzCH?CI-I3 (1)
but x is much larger t'han (1 - x ) . ~ , ~ Consider now the hydrogenolysis of dicyclopropylmethane (A). It should hydrogenolyze into two products each of which would furt'her hydrogeiiolyze in two ways as shown in Fig. 1. The quantities x, y, and x express the probabilities t'hat cleavage occurs between t.he unsubstituted positions. The coneent'rations of B and C must be greater in the catalyst pores t'han in the bulk vapor phase if there is to be net conversion of A into B and C, i.e., if B and C are to flow from the catalyst pores into the bulk vapor p h a ~ e . ~If' ~such gradients are a substantial. fraction of the concentrations of B and (1) To whom queries concerning this paper should be addressed.
+
C, then the concentrations of B and C will be substantially greater inside the catalyst pores than outside. Judged only from analysis of the vapor phase, some A will seem to be converted directly into D, E, and F without proceeding through vapor phase, ie., through desorbed B and C as intermediates. In an extreme case of diffusional control, the dependence of concentrations with distance from the end of a cylindrical pore is shown in Fig. 2. For simplicity, B and C are considered to behave identically. An analogous s h a t i o n has been reported receiitlya for hydrogenation of 3,3-dimethyl-1,4pentadiene. CH3 CHe=CH--C--C€I=CH2
--j
I
CH3 (A)
CH,
I
CH2-CH-C-CH2-CH3
+
I CH3
(B(+ C))
(2) For reviews dealing with t h e rather extensive literature on this
subject see (a) B. B. Corson, "The Chemistry of Petroleum Hydrocarbons," edited b y R. T. Brooks, C. E. Boord, S.S.Kurtz, Jr., and L. Schmerling, Reinhold Publ. Corp., New York, N. Y., 1955, Vol. 111, pp, 298-307; (12) A. L. Liberman, U s p . Khim., 30, 564 (1961). ( 3 ) G. C. Bond and J. Newham, Trans. Faraday Soc., 56, 1501 (1960). (4) (a) PI. Wheeler, "Catalysis," edited by P. H. E m m e t t , Vol. 11, Chapt. 2 , Reinhold Publ. Corp., New York, PI'. Y., 1955. ( 5 ) E. Wicke. 2. Elektroehem., 60, 774 (1956).
CH3
I
CH,-CH2--C-CH2-CH3
I
CHa (D(+ E F))
+
(2)
1.132
i
---
ky(l r--
I
-
2-mcthylhcxanc (E)
2)
li?( J )
CH?
I \ CITZ
C1-L
\ /
-
Fig. 2.-Schcmnt~ic rcprcscnt,ntion of conccntrntion vnri:i1,ioii with ponct,r:btion into a pore for the reaction A -L l3 I). Concci1tr:itions :m plot,t,ctl i n t,hc dircctioii of t,hc wrow.
Of that portion of h which react's, the fraction, a , is considcrcd t o form I1 dircvtly, thofimtion, 1 - 0, to form 13. Of course, the 1) which :ippcars to be formed dircct'ly is actually formed via thc desorbed iiit,ermediatc 13 i n the cat;alyst,porcs. The degree of diffrisional control is mcasurod by a or nlternativcly hy (ll,'13)initia~whcrc 1) and I? arc the hydrocarbon fractions of I> arid 13. If diffiisional control is ncgligiblc, cy = 0 and no I) appears iiiitially. Thus, the degree of dif'fusional ciontrol is diagnosed from measurement of the composition of the bulk v:ipor phase and withoiit, rc(wursc t o mcasimmciits of rate constants, diffusion cocfIicic!iits, cat:ilgst geometry, etc. The incursion of scrioiis concentration gradients into t h o whcmc of I'ig. I will change the rate constant for foim:itioii of' n to k 1 ( I - CY) ( I - 2 ) and that for C: to kl(l - p ) ( z ) . T w o different diffusional contml terms : ~ r ciicwssary if 11.2 # I;:the same value deliberate poisoning in runs B7, R8, and I39 of Pt-I1 might have led of X applies to all hydrocarbon species. to a case of poisoned pore mouths 6 This model uould somcuhat reFurther semble the model of eq 7. In fact, howeier, decline in aotlvity causes 6
The analyses ignore any cyclopropyl-
% ( A - A') - k,&S'A'
=
0 and X ( B - B') kzSB' k1SA'
+
0
the results to aprroach the behavior characteristic of low ooncentration gradients Thus, the deactivation a n d the poisoning seem t o reduce the catalytic activity iathcr uniformly thioughout the catalyst particlos.
1438
JOHN NEWHAM AND ROBERT L. BURWELL, JR.
essentially duplicates that of eq. 6 for A from 1.0 to about 0.75. Channelling.-Consider the system X + C + F under conditions such that concentration gradients in the catalyst pores are negligible. Th.: occurrence of channelling in flow through the catalyst bed will introduce a new type of concentration gradient in the plane perpendicular to the direction of flow and many of the characteristics of diffusional control will reappear. Suppose, for example, that the flow is divided into two streams which remix a t the end of the bed and let the compositions of the two streams just before mixing be A',C' and A",C". If we are employing Pt-VI, both compositions mill lie on the line C us. A in Fig. 3. The composition after mixing, A,C, will be on the h e joining A',C' and A",C" and it will lie below the line C us. A . This mill be the general result of channelling as it mas of diffusional control. However, if conversion is so low that the most converted stream still lies on the initial linear section of C vs. A , i.e., A greater than about 0.8, then the effect of channelling will be very small. That is, channelling will not ordinarily affect (CIA),,but concentration gradients in catalyst pores will. Since C closely follom eq. 5 for the less active catalysts, channelling iii our experiments was small or negligible.
Vol. 66
Conclusion.--We believe that the techniques described in this paper constitute in appropriate cases a very powerful technique for characterizing the presence or absence of various types of coiicentration gradients. Furthermore, this work reemphasizes the importance of establishing the absence of significant concentration gradients in any catalytic study aimed a t mechanism. For example, had one had only our results on platinum-charcoal, one might well haye concluded that 2,4-dimethylpentane was a major initial product in the hydrogenolysis of dicyclopropylmethane whereas it is, in fact, a very minor one. This work also emphasizes that it frequently is easier to elucidate mechanistic details on a complicated system than on a simple system. The simultaneous occurrence of the reactions A --2- B D E and A + C + E F permitted us to characterize the relative contributions of the doubleopening reaction and of concentration gradients rather clearly. I n a simpler reaction in which only one of these reactions occurred, this would have been much more difficult. However, as paper I1 will shorn, complications can become excessive. Acknowledgment.-This work was supported by the Air Force Office of Scientific Research (Direciorate of Chemical Sciences).
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--f
T H E HYDROGESOLYSIS OF DICYCLOPROPYLRIETHAKE O S NICKEL CATALYSTS B Y JOHN NEWHA4MA S D ROBERT L. BURTTELL, JR.~ Department of Chemistry, iYorthwestern Unaversity, Evanston, Illinois Receated Februarlj 16, 2962
This paper deals with the hydrogenolysis of dicyclopropylmethane ( A ) on nickel-silica, reduced nickel oxide, and nickel wire a t 55" in a flow reactor. Diffusional control was negligible. Results differ in t v o major ways from those on platinum (paper I), a much increased yield of 2-methylhexane (E) and the intrusion of demethanation. E amounts to about 70% of the final heptane product which indicates that cleavage of the first and of the second rings occurs preferentially in opposite locations and, therefore, with some difference in mechanism. The double ring opening reaction ma a surface reaction intercepts the formation of desorbed isobutylcyclopropane (C) to a much greater extent than on platinum. The large yield of E results from predominant cleavage of a bond adjacent to the side chain during the opening of the second ring. The mechanism of ring opening is discussed nith the tentative conclusion that the initial ring opens via formation of 1,3-diadsorbed 2substituted cyclopropane either by direct reaction of physically adsorbed A or from 1-monoadsorbed 2-substituted cyclopropane. The adsorbed species formed in this reaction either desorb or further react to open the second ring before desorption. About 10% of the initial ring cleavage leads to demethanation by breaking two bonds in the cyclopropyl ring. The effect of temperature upon selectivities is much smaller than on platinum.
This paper reports the results of the hydrogenolysis of dicyclopropylmethane on t'hree nickel cata-
lysts, nickel-silica, reduced nickel oxide, and nickel wire. Experimental The apparatus and techniques were the same as reported in paper 1.2 The procedures for gas chromatography were also the same except that four new product,s were present of which hexane, 2-methylpenta,ne, and pentane were assigned by comparison with authentic samples and propylcyclopropane, on the basis of intkrnal chemical evidence. Sickel I consisted of 0.1 g. of 60-100 mesh nickel-kieselguhr (Harshaw Chemical Company) diluted with about 4.5 (1) T o whom queries about this paper should be directed. (2) J. Xewham and R . L. Burwell, Jr., J . Phus. C h e m . , 66, 1131 (1962).
g. of 60-80 mesh glass beads. Sickel I1 was prepared from nickel nitrate via the carbonate and oxide3; 1.01 g. of 60100 mesh material was mixed with 3 g. of glass beads. Kickel wire of a reported purity of 99.95% (United Mineral and Chemical Corporation) and of a diameter of 0.01 in. was made into helices of about 2 mm. diameter. Thcse were packed in a tube parallel t o its axis and the gaps between the helices were largely filled with thin glass rods. Ni-111-A weighed 10.2 g.; Hi-111-B, 23 g. Xi-111-A \vas treated with oxygen for 1 hr. a t 600" and then reduced in hydrogen a t 450". The catalytic activity of the resulting material was very low. The oxidation-reduction sequence was repeated seven times more after which the nickel wire had subst,antial catalytic activity a t 50". The wire now had a matte appearance. Si-111-B was treated with hydrogen a t 450" for 8 hr. but it displayed no catalytic activity a t up (3) H. C. Rowlinsnn, 69, 226 (1965).
K.L. Burwell. J r . , and R . H. T u x w o r t h , iiiid.,
