487
BUTENE ~SOMIBRPZA'J'IONOVER ZINC OXIDE
utene Isomerization over Zinc Oxide
y A. L. Dent and R. J. Kokes" Depariment of Chemistry, The Johns Hopkins University, Baltimore, Maryland
21$18
(Received August 14, 1970)
Publieztion costs assisted by the Petrolsum Research Fund
Infrared and traditional techniques have been used to examine butene-1 and cis-butene isomerization over zinc oxide. On a per unit area basis it is found that zinc oxide is comparable to alumina as an isomerization catalyst. cis-Butene isomerization occurs readily between room temperature and 100' with a n activakon energy of about 18 kcal. Infrared studies of adsorbed butene show that adsorption occurs with dissociation; :t is concluded that adsorbed butene forms a .ir-bonded allylic species. This species, which can oceur in two forms, favors the species which is the precursor to cis-butene. The isomerization of cis-butene (and presumably butene-1) appears to occur by simultaneous isomerization to both isomeric forms, Le., trans- and 1butene. It IS suggested that the n-allyl form of butene is likely to be the intermediate 111 isomerization.
Introduction Details of double-bond isomerization of olefins over metals* and many transition metal complexes2 are consistent u itl- a sequence involving reversible addition of a hydrogen atom 1,o form a cr-bonded alkyl from complexed or adaorbed olefins. Over many metal oxi d e ~ , however, ~-~ alkyl formation from olefins is irreversible; hence, alkyl reversal cannot be the mechanism of isomeriza tion. Kinetic evidence suggests5f6that allyl species may be formed as intermediates over these oxides. To date, homever, the best evidence that these species are in fact intermediates in the isoinerization of butene and the higher olefins remains mechanistic inference. R e ~ e n t l y ,we ~ ,presented ~ evidence based on ir studies that propylene adsorbs dissociatively on zinc oxide to form a symnietric allyl species analogous to those formed ah ligands in transition metal c o m p l e ~ e s . ~The proposed site l o r adeorption is a zinc ion surrounded by oxide ions. In this picture adsorption of CH8CHCD2 is presumed l o involve the following steps B
lated over zinc oxide support this picture inasmuch as they show that I1 was the initial product and the central CH bond is undisturbed. Similarly, related reactions of propylene are consistent with this picture, Isomerization of butene via a n-allyl species introduces some new stereochemical features. The n-allyl species formed from propylene is presumed to be planar with its plane approximately parallel to the surface, Since it is attached to the electropositive zinc, it may have considerable carbanion character. ih corresponding structure for adsorbed butene would lead to two isomeric forms, vix. H
"s yn"
"anti"
cis-Butene should lead initially to the anti form; transbutene should lea,d initially to the syri form, and 1butene should give rise initially to both. The equilibrium distribution of syn and anti forms usually differs
I
Q--Zn--13' C&D-
CII==CDH I11
wherein the cpecies shown without zinc oxide are a loosely bound "'.lr-eomplex" nearly in equilibrium with the gas phase. Kinetic studies'O in which I was circu-
(1) G. C. Bond, "Catalysis by Metals," Academic Press, Kew York, N. Y., 1962. (2) R. Cramer, Accounts Chem. Res., 1, 186 (1068). (3) W. C. Canner, R. A. Innes, and R. J. Kokes, J . Amer. Chem. Soc., 90, 6858 (1968). ( 4 ) A. L. Dent and R. J. Kokes, J . Phys. Chem., '63, 3772, 3781 (1969). (5) R. L. Burwell, Jr., G. L. Haller, K. C. Taylor, and J. F. Bead, Advan. Cutal., 20, 1 (1969). (6) H. H. Voge and C. R. Adams, ibid., 17, 161 (1967). (7) A. L. Dent and R.J. Kokes, J. Amer Chem. Soc., 92, 1092 (1970). (8) A. L. Dent and R. J. Kokes, ibid., 9 2 , 6100 (1970). (9) (a) W. R. McClellan, H. H. Hoen, ]-I. N. Cripps, E. L. Muetterties, and B. W. Howk, ibid., 83, 1601 (1961); (b) G. Wilke, B. Bogdanovic, P. Hardt, P. Heimbach, ITr. Keim, M. Kroner, $57. Oberltirch, K. Tanaka, E. Steinrilcke, D. Walter, and H. Zimmerman, Angew. Chem., Int. Ed. Engl., 5 , 151 (1966). (10) A. 1,. Dent and R. J . Kokes, J. Amer. Chem. SOC.,92, 6718 (1970). The Journal of Physical Chemistry, Vol. 76,N o . 4, 19rf
A. L. DENTAND R. J. KOKES
488 greatly from the !equilibrium distribution of cis- and trans-butene for cobalt complexesll the syn form precursor of truns-Mrne is by far the most 'stable. By way of cont~astfor the corresponding carbanion, the cis anion seems by far the more stable.I2 In the basecatalyzed isomerieiation of more complex cis-olefins (cis-a-me t hy Istilbene) the ions corresponding to syn and anti are not interconvertible and cis-trans isomerization involx es the a-olefin as an intermediate;13 for the simpler czs-olefins (cis-butene) heterogeneous basecatalyzed cis-trans isomerization is direct and the a-olefin is n o t an intermediate. l4 Isomerization via n-allyl ligands of " x i t i o n metal complexes may be important, in some cases15 but it has not been established if the syn-anti conversion i s direct. The observation that many ,r-allyls are and undergo rapil I o to .R--allylinterconversion suggests the possibility of a mechanism for cis to trans conversion which does not involve butene-1 as an intermediate
where * represents the complexed atom. Thus, in the isomerization of cis-butene over oxide catalysts wherein surface a-allyl's are intermediates, one can expect either the sequential!pathway cis-butme
z3"1-butene
circulating system with a volume of about 100 em3. Small samples of gas were withdrawn periodically for chromatographic analysis. Runs at room temperature were made with a 10.0-g sample of zinc oxide; runs a t elevated temperature were made with a 1.0-g sample of zinc oxide. No detailed studies were made of the life of the catalyst during repeated isomerization runs. Nevertheless, the fact that the results of the room temperature run (on one catalyst sample) coupled with results on runs at 100 and 68" (in that order) yielded a good Arrhenius plot suggests that poisoning is not severe and the reported rates are reasonably reliable. Results and Discussion Injuwd Studies. Figure 1 shows the spectrum in the OH region for zinc oxide after admission of butene-1 a t a pressure of about 8 mm. Spectrum a, taken after 8 min of exposure, shows two features: first, the strong surface hydroxyl band at 3615 em-' is shifted about 5 em-1 to lower frequencies; second, a new band appears at about 3587 cm-l. This new band, clearly an OH, appears to arise from dissociation of the adsorbed butene. Spectrum b shows this region after exposure to the gas phase for 1 hr. It is clear that the OH band formed from butene grows with time; detailed studies, however, reveal that there is little change after the first 20 min. Spectrum c shows the results after 20 min evacuation. Two features are evident from this spectrum; first, we find that in the absence of the gas phase
3 6 0 0 3500cm.' 3600 3500 I -
I_T__r_
trans-butene
or the simultaneous conversion of cis-butene to 1- and trans-butene. This paper deals with three questions. (1) Does butene form s-allyl species over zinc oxide? (2) Is zinc oxide an effective isomerization catalyst for butenes? (3) 1s butene-1 an intermediate in the cistrans isomerization? In view of the results with propylene' s question 1 may seem fatuous. This is not true. Steric restrictions 011 surface species are often severe;16 the presence of the additional methyl group in butene may inhibit formation of the a-allyl species. I n our approach to these questions we shall utilize ir criteria developed for propylene to judge if a n-allyl species is formed and shall use traditional mechanistic techniques to answer the remaining questions. Experimental Sectim The infrared techniques, materials (Kadox-25), and pretreatment have been described in detail elsewhere."' 8,10 Kinetic runs were carried out in a closed The Journal of Physical Chemistry, Vol. 76, No. 6,1971
Figure 1. Spectra of zinc oxide in the OH region in butene-] background is shown as a dotted line: a, 8 min after exposure to butene-1 a t 8 mm; b, 60 min after exposure to butene-1 at 8 mm; c, 70 min of exposure t o butene-1 followed by 20 min of degassing; d, c after 90 min degassing. (11) C. L. Aldridge, H. B. Jonassen, and E. Pulkkinen, Chem. I n d . (London), 374 (1960); D. W. Moore, H . B. Jonassen, T.B. Joyner, and A. J. Bertrand, ibid., 1304 (1960). (12) S. Bank, A. Schriesheim, and C. A. Rowe, Jr., J . Amer. C h e n . Soe., 87, 3244 (1965). (13) D. H . Hunter and D. J. Cram, ibid., 86, 5478 (1964). (14) W. 0. Haag and H. Pines, ibid., 8 2 , 387 (1960). (15) J. F. Harrod and A. J. Chalk, ibid., 88, 3491 (1966). (16) E. F. Meyer and R. L. Burwell, Jr., {bid., 85, 2881 (1963).
BUTENE ISOMERIZA’.TION OVER ZINC OXIDE
489
the hydroxyl h n of the zinc oxide has shifted back its previous position; second, the OH band formed from butene is reduced somewhat in intensity. Spectrum d shows the result after degassing for 90 min; further reduction in the band intensity due to the adsorbed species is evident. When the sample was degassed a total of 16 hr, this band intensity decreased by roughly an additional 20%. These results are similar to those with propylene insofar as they indimte dissociative adsorption of the olefin. The hydrogen m butene which yields the hydroxyl has not been identified but the tracer results with propylene make r t reasonable to suppose that the allylic hydrogen is lock. Results with butene, however; differ from fhnse with propylene in two respects: first, the dissociation ( a i evidenced by the OH band) is rapid but not instantaneous as found for propylene; second, dissociativelLyadsorbed butene is more easily removed by room temperature evacuation than dissociatively adsorbed propylene. These facts suggest that steric effccts may be present and that the driving force for foxmdion of adsorbed methyl allyl species is not as great as for i,he allyl species. Accordingly, the kinetic behavior of these two species may be quite different. Figure 2a sh,ows the CH region of the spectrum about 10 min (solid line) and 60 min (dotted line) after admission of 8 mm of butene-l to a sample of Binc oxide. These spectrar which are primarily due to physically adsorbed and gas-phase butenes, show that sizeable changes occur as a, function of time. Table I compares Table H: Dominant Rands in Spectrum of Zinc Oxide plas Rutme-1 1nitialra om -1
Final,* om-’
3082
Butene-l,e
cisButene,B
om-1
om-’
truns-
Butene,B om-’
3086 3035 3020
3030
3021 2985
2972
2976 2972
2945
2945c
2967 2941
2941 2932
2925
2924 2903 167@ 1660
I655 1630 1610 1550-15700
1645 1630
1550-L570C
a This scan was begun after 10 min of exposure to the region near I600 was scanned after 20 min of b This scan was begun after 60 min of exposure to the region near 1600 was scanned after 70 min of There may be more than one band. d Infrared e See ref 17. ____8_
butene-1; exposure. butene-1; exposure. inactive.
C
w 100 ._
3100
3000
2900cm
1700
l6OOcm? 1500
0
-r----
Figure 2. Spectra of zinc oxide in the presence of butene-1. Background is shown as a dashed line in a and as a dotted line in b and c: a, solid line, after 10 min of exposure to butene-1 (8 mm); dotted line, after 60 min of exposure to butene-1 (8 mm); b, after 20 min of exposw-e to butene-1 (8 mm); c, after 70 min of exposure to butene-l (8 mm). Arrows mark positions of peaks referred to in text.
the dominant bands for the initial and final spectrum with the corresponding region for the 11-butene isomers.’’ The region above 3000 cm--l is particularly clear cut. Initially, a band is observed only a t 3082 cm-’; this corresponds quite closely to the 3086 cm-I band for gaseous butene-1. After 1 hr the band at 3082 cm-l is gone and two new bands (a,bove 3000 cm-l) have appeared at 3035 and 3018 cm-l which correspond within the experimental uncertainty to the expected bands for cis-butene (3030 cm-l) and trans-butene (3021 cm-I). Other bands listed suggest similar bchavior. Thus, it is evident that double bond isomerization has occurred. Figure 2b and c show the spectrum in the double bond region after 20 and 70 min of exposure to the gas phase, respectively. Bands due to adsorbed species dominate this region of the spectrum. The initial spectrum (b) shows a t least four bands at about 1650, 1630, 1610, and 1550-1570 cm-l. Of these only the band a t about 1650 cmS1 can be assigned to the gas phase and the most reasonable assignment is to gaseous butene-1. In the spectrum after 70 min of exposure, the bands a t about 1650 and 1610 cm-l are no longer prominent. No bands due to gaseous species are observed. This is not unexpected if the composition of the gas phase has approached equilibrium. At equilibrium the dominant species is trans-butene (77% 18) which shows no ir active 6=@ band and the cis-butene (20% at equilibrium’s) has a much weaker C=C band than the b ~ t e n e - 1 ’(3% ~ at ~ q u i l i b r i u ~ ~ l ~ ) ~ (17) N. Sheppard and D. M. Simpson, Quart. Rea. Chem. Soc., 6, 1 (1952). (18) D. M. Golden, E(. W. Eggers, and 8. W. Benson, .J. Amer. Chem. Soc., 86, 5416 (1964). (19) L. J. Bellamy, “The Infrared Spectra of Comglex Molecules,” Wiley, New York, N.Y., 1958, p 39; R. T , Conley, “Infrared Spectroscopy,” Allyn and Bacon, Boston, Mass., 1866,p 9’7.
490
A. L. DENTAND R. J. KOKES
Firm assignments for these C=C bands require more detailed experiments but a tentative assignment can be made based on analogies with p r ~ p y l e n e . ~ , ~ 6 0 The bands at 15543-1570 cm-*, we believe, are due to a a-allyl species; the shift from the double bond region for butenea i b about 100 cm-' compared to the shift of 40 for the a-allyl formed from propylto such species for propylene, we Lia-lmmplexl'in which the shift in C=C 20 ow 30 cm-'. We believe the band at n in the initial spectrum represents a butene-I. "a-oomplex;" disappearance of this band in 0 time is conshtent with this assignment. Similarly, 0 200 400 Time minutes the band at 1630 which persists and may intensify after 70 min, may be tentatively assigned to r-comFigure 3. &Butene isomerization over zinc oxide: 0, plexed butene-:! (cis and/or trans). cis-butene (right-hand ordinate); A, trans-butene; 0, butene-1. Some suppor.l for the above assignments is offered by the behavior of the bands a t 1630, band A, and 15501570 cm-l, band 13, on degassing. Both bands are conversion shows that the initial ratio of butene-1 to still present aiter 8 brief degassing. In time, however, trans-butene formation is about unity. Above room both bands dacreaee in intensity. Band A, which is temperature the ratio is somewhat less than unity. presumably due to the more weakly held rr-complex, Thus, butene-1 is not an intermediate in the cis-trans decreases on degassing much faster than band B, which isomerization as one finds for the homogeneous base is presumahl> due to the more strongly held a-allyl catalyzed isomerization of more complex ions. l 3 Thus, species. After 90 min of degassing band A is gone but in this respect, zinc oxide is similar to heterogeneous band B persists. After 16 hr of degassing, however, the base-catalyzed isomerization. l 4 intensity of bsnd 13 is less than half that at the start of An attempt was made to see if the surface species on degassing. This &crease in band B is qualitatively the catalyst showed a strong preference for the precursor similar to that observed for the OH band formed on of cis- or trans-butene by Aash desorption. I n this adsorption; hence, il offers some support for assignment experiment at the end of the room temperature run l which gives rise to the of band B to a ~ a l l y species before gas-phase equilibrium was achieved (gas phase dissociative ad sorption. composition 31y0 cis, 65% trans, 4.0% butene-1) the C E stretching region due to adsorbed catalyst was evacuated for 1 hr to remove the bulk of tiveiy weak. No prominent bands are the a-complexed butene. Then it was stored i n oacuo evident above 3000 em-', the region diagnostic for 2 days to permit further equilibration of the surface olefins. Were the assignment based on the CH stretchspecies. After further evacuation at room temperature ing region alcne: one might conclude only saturated for 1 hr, the catalyst was rapidly heated to 150", a species were present. Nevertheless, the correlation of temperature shown to be sufficient to completely rethe decrease 0-3 degassing of bands in this region to the move the adsorbed butenes. The composition of the decrease on degassing of band B suggests that all bands evolved gas is compared in Table II to the computed in the CW stretching region stem from olefinic species. equilibrium values at room temperature and 150°.'* Isome&alaoie of cis-Butene. Figure 3 shows the It seems clear from these data that the equilibrated cotme of cis-butene isomerization as a function of strongly held butene favors the precursor of cis-butene time at room temperature. Similar data were also gas to an extent greater than that in the equ~~ibrated obtained a t 6ti arid 100". The initial rates conformed well t o an Amhenitis plot and yielded an activation energy of aboai 18 kcal,/mol. On a per unit area basis Table I1 : Composition of Butenes the rate at rocm temperature is 4 X 1O1O (molecules/ sec)/cm2. IData of ightower and Hallz0 for GA48 EquiEquiFlash libriulnls Iibrium's desorbed, alumina yield %ninitial rate of 14 X 1O1O (molecules/ 2j0, % 150°,% % sec)/em2. Minee the activation energy for alumina is Butene-1 7 3 10 less than that loiund for zinc oxide, this means that zinc cis-Butene 37 20 28 oxide is comparable (on a per unit area basis) to alumina trans-Butene 56 77 62 as an isorneriza tion catalyst between room temperature and 400". 4t G ~ lower C temperature zinc oxide is less active and ai the "nigher temperature it is more active. (20) J. W. Hightcwer and W. K. Hall, . I Amer. . Chem. Sm., 89, 778 Extrapolation of Ihe rate data in Figure 3 to zero (1967). The Journal of Phjjskal Chemistry, Vol. 76, N o . 4, 1971
CATALYTIC ]1)ECOR/IPC)STTION OF
PERCHLORIC
ACIDVAPOR
phase or even the gas phase at the end of the run two days earlier. This preference, most evident for the strongly held butene, may explain the stereochemical preference for cis-butene found in butene-1 isomerization over oxide Conclusions. It appears that butene adsorbs dissociativcly on zinc oxide to form a species analogous to In addition it the r-.allyl fornzed from forms .rr-complexe species analogous to those formed by p r ~ p y l e n e . ~The equilibrated strongly held butenes corresponding to x-allyls preferentially form cis-butenes on desorption compared to the corresponding equilibrium distribution in the gas phase. Zinc oxide is a catalyst for thc isomerization of butenes comparable in activhy on a per unit area basis to alumina. I n cisbutene i s o ~ ~ e r ~ ~both a ~ i cis-trans o~i and double bond
ON ZINC
OXIDE
491
migration take place simultaneously ; Le., butene-1 is not an intermediate in cis-trans isomerization as has been found in homogeneous base-catalyzed isomerizat i o n ~ . The ~ ~ results show that .rr-allyls form readily and that isomerization occurs readily on zinc oxide; hence, it is plausible to conclude that n-allyls are intermediates in isomerization over zinc oxide but until further data are available, it must be noted that this conclusion is tentative and as yet not proven.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. The authors also wish to express their gratitude to C. C. Chang who rechecked the spectra in Figures 1and 2.
eeomposition of Perchloric Acid Vapor on Zinc Oxide p F. Solymosi" and L. Gera Gas Kinetics Research Group of the Hungarian Academy of Sciences, Szeged, Hungary
(Received August 81, 1970)
Publication costs borne completely by T h e Journal of Physical Chemistry
The effect of zinc oxide was studied on the vapor-phase decomposition of perchloric acid. The interaction betw ecn the catalyst and the substrate was investigated by infrared spectroscopy, electric conductivity, thermal and chemical analysis. The interaction was most pronounced at 250" when a great amount of zinc perchlorate was formed. Zinc oxide catalyzed the decomposition of perchloric acid above 310'. The reaction fo!lowed first-order kinetics. The value of the activation energy was 45 kcal/mol. Expcriments were performid with magnesium and cadmium oxides too; however, both oxides were found to be inactive substances. Taking into account the stability of perchlorate salts and the activity order of the oxides, the conclusion was drawn that the formation and decomposition of surface perchlorate may play an important role in the catalytic decomposition of perchloric acid. Measurements were also carried out on the catalysis of the gas-phase deeoinposition of ammonium perchlorate; in this process zinc oxide was found to be an outstanding catalyst.
Introduetion In order to elucidate the role of catalysts used in propellents containing ammonium perchlorate (AP) it seems very important to know their influence on the thermal stability of perchloric acid. The first direct evidence showing that oxides are able to catalyze the decomposition of perchloric acid was published in 1968. Chromium oxide wzis found to be t,he most active catalyst, and its effect has recently been studied in more detaiL2 Its excellint catalytic effect was exhibited even if a small amount of it' was incorporated into the surface layer of tin dioxide, a considerably less active catalyst . 3 The present paper xi11 deal primarily with the effect of zinc oxide. ]in its presence AP was found to decompose
at a measurable rate a t a temperature as low as 200" and it,s ignition occurred at a temperature lower by 200" than in the case of the pure s u b ~ t a n c e . ~For the purpose of comparison, the behavior of cadmium oxide and of magnesium oxide was also examined. The effects of these on the stability of AP were commensurable with that of zinc
(1) F. Solymosi, 9 . Borcsijk, and E. Lazar, Combust. Flame, 12, 397 (1968). (2) F. Solymosi and S. Borcsok, J. Chem. SQC.A , 601 (1970). (3) F. Solymosi and T. BBnsLgi, Froceedings of the 2nd International Conference on Space Engineering, D. Reidel Publ. Co., Dordrecht, Holland, 1970, p 145. (4) F. Solymosi and L. RBvBsz, Nature, 192, 64 (1961); 2. Anorg. Chem., 322,86 (1963).
T h e Journal of Physical Chemistry, Vol. 76, No. 4, 1971
