August, 1962
ELECTRONIC SPECTRA OF OLEFINSADSORBED ON SILICA-ALUMINA CATALYSTS 1457
ELECTRONIC SPECTRA OF OLEFIXS ADSORBED ON SILICA-ALUMINA CATALYSTS BY HARRY P. LEFTINAND W. KEITHHALL Mellon Institute, Pittsburgh, Penna. Received March 3, 1969
The spectra of Ill-diphenylethylene and of a-methylstyrene adsorbed on silica--alumina and of acidic solutions of these olefins have been obtained. In addition to bands in the 400-mp region, assigned to the classical carbonium ions, bands of uncertain origin were observed a t longer wave lengths. The kinetic behavior of these systems was studied spectroscopically. These results, coupled with observations of the effects of oxidizing agents on the rates, indicate that the long wave length bands stem from intermediates formed during the oxidation of the olefin and that radical ions may be involved. Consideration of the available data leads to the conclusion that the long wave length bands arise from intramolecular, rather than charge-transfer transitions, as was previously supposed.
Introduction The visible spectrum of 1,I-diphcnylethylene (DPE), adsorbed on the surface of a silica-alumina catalyst, has been f ~ u n d l - to ~ consist of two principal bands a t 423 and 607 mp. The first of these has been attributed to the methyldiphenylcarbonium ion1-5 formed by addition of a proton or of a Lewis acid to the double bond. The other band is of uncertain origin. It may be supposed, however, that it is characteristic of a definite unstable intermediate formed by reaction of the olefin with the catalyst surface. Evans, Jones, and Thomas4 and Lavrushin5 observed similar behavior with DPE in acidic solutions. They found that while DPE in concentrated l[-IzS04yielded only the classical carbonium ion band, in sulfuric acid-acetic acid mixtures5 and in benzene solution with trichloroacetic both the 423 and 607 mp bands appeared. This work recently has been confirmed and extended by Grace and Symons.'j Since most reactions of olefins over silicaalumina catalysts have been explained by mechanisms involving only classical carboiiiurii ion intermedial es, knowledge of the surface species responsible for the long wave length band may prove to be of considerable theoretical interest to catalytic chemists. We have noted earlier1,'&that it has most of the attributes of an ion radical. It is the purpose of the present paper to present the detailed results of our investigations into the nature of this species. Experimental Equipment and Procedures.-The optical cells and techniqucs employed for measurement of the electronic spertra of chemisorbed species have been described elsewhere.3.7b Spectra were obtained by transmission with a Beckman Model DK-1 recording spectrophotometer using quartz optical cells in conjunction with a high vacuum system of conventional design. Where catalysts wcre ubed, they were mounted as thin, fairly transparent platelets on quartz racks which positioned them insidc the optical cells. The catalyst samples were calcined at 500" in a stream of dried oxygen for 16 Lo 24 hr. before being placed into the (1) H. P. Leftin and W. K. Hall, J . Phys. Chem., 6 4 , 382 (1960). (2) A. N. Webb, Actea Congr. Intern. Cataluse, Be, Parrs. 1, 1289 (1961). (3) H. P.Leftin, J . Phvs. Chem., 6 4 , 1714 (1960). (4) A. G. Evans, P. M. S. Jones, and J. H. Thomas, (a) J . Chem. SOC.,1824 (19541; (b) 104 (1957). (5) V. F. Lavrushin, Zh. Obshch. Khzm., 2 6 , 2697 (1954). (6) J. A. Grace and M. C. R. Symons, J . Chem. Soc., 958 (1959). (7) (a) H.P. )Leftin and W. K. Hall, Actes Conor. Intern. Catalyse, O c , Paras, 1, 1307 (1961); (b) zbrd., 1, I353 (1nGl).
optical cells. After sealing the cells directly to the vacuum line, the catalyst samples were evacuated for 1 hr., calcined in oxygen for 24 hr., evacuated for 24 hr., all a t 50Qo,and then sealed off undcr Vacuum. The organic reagent to be adsorbed was separately purified and sealed undcr vacuum in a separate compartment attached to the optical cell through a break-nff seal. It was transferred to the catalyst through the vapor phase. Gencrally, the total amount of substra,te used was only siifficient to provide for coverage of about 5 X 1012sites/cm.2 of catalyst surface; this was thc number of Lewis acid sitrs measured earlier.7b When, after the band intensities had become ncarly time-independent, it was desired to treat the adsorbed species with another reagent such at! € 1 2 0 , NH3, RFs, or HF, a stoprocl: way sealed to the cell through a second break-off seal previously provided for that purpose. Comparative experiments were made using scvcral solvent systems. For the purpose of the kinetic nieasurements, the reaction vessel consisted of two compartments separated by a break-off seal. Solutions containing olefin and solutions containing the active acid reagent were separately degassed a t -195" and sealed off. After equilibrating a t 25 EIZ 0.5", these solutions were vigorously mixed to initiate the reaction and the rates wcre monitored spectrophotometrically. Spectra of the produrts of the reaction of alkali metals with olefins were measured using evacuated cells of similsr design. Catalysts.-The dame transp:ircnt highly active8 highDuritv silica-alumina (DSA-1) and d i c a eel IDGS-4) &dxi&ts used in earlicr work37 were crnployd; they had surface areas of 278 and 550 m.2/g.,respectively. Reagents.-All reagents were commercial samplrs which were further purified until their spectra showed no bands due to impurities. Wherever comparison was possible, these and other physical properties were in good agreement with reportrd data. Aldrich Chemical Co. I ,1-diphenylcthylcne was redistilled under reduced prcssure and dried over calcium hydride (%'OD 1.6008). The 1,Ldiphenylethane (Beacon Chemical Co.) was vaEuum distilled and the distillate percolated through columns of ~ilicagel and activated alumina. Eastman White Label a-methylstyrene (wMS) was redistilled and dried over calcium hydride. The cumene was an API standard sample which was further purified by the method of Plank and n'ace.Q Triphenylethylene and tetraphenylethylene (Aldrich Chemical Co.) were recrystallized from suitable solvents; the purified materials melted a t 67.5 to 68.9' and 224.6 to 226.9", respectively. Perylene and anthracene were fluorescent grade, obtained from the same source, and used without further purification. Samples of the linear dimer (1 ,I ,3,3-tetraphenylbutene-l) and of the cyclic dimer ( I ,1,3-triphenyl-3-methylindan) of 1,l-diphenylethvlene were prepared by the method of SchoepAe and Rvan.10 The former melted a t 111 to 112' and the latter a,t 143.6 to 144.8'. The oxidation dimer (1,1,4,4tetraphenylbutadiene) from the Aldrich Chemical Co. was used without further purification. (8) W. K. Hall, D.
S. MacIver, and H. P. Weber, Ind. Cne. Chem..
52, 421 (1960). (9) C. J. Plank a n d D. M. Naoe, zbzd., 47,2374 (1955). (10) C. S. Sohoepfle and J. D. Ryan, J. Am. Chem. Soc., 52, 4021
(1930).
1458
HARRY P, LEFTIN-4ND
Fig. 1.-Spectra of hydrocarbons adsorbed on silicaalumina and dissolved in HpSQ and in (CH,O),SOp with added HsS04.
Solvents and Solutions.-Unless otherwise noted, solvents of C.P. or A.R. quality were used a2 received. Dimethyl sulfate, freshly distilled over K&OJ, afforded solutions of the olefins which were transparent in the visible region. These solutions remained rolorless for periods of weeks if stored tn vacuo. Color developed s l o d y upon exposure to the atmosphere and quite rapidly upon addition of as little as O.Olyoof concentrated H2SQ4. Acetic acid, monochloroacetic acid, dichloromethane, benzene, and tetrahydrofuran all afforded colorless solutions of the olefins. The tetrahydrofuran was distilled under vacwm into a mixture of benzophenone and potassium met:L It then was redistilled from the potassium ketyl solution into an evacuated storage bulb containing Na-K alloy.
Results Spectra of Adsorbed Species.-When the catalyst platelets were exposed to limited amounts of the phenylated hydrocarbons using high vacuum techniques, absorption bands developed more and more slowly until the spectra became nearly invariant with time With the olefins, the bands formed readily at room temperature, but with 1,l-diphenylethane ant1 cumene, tempsratures of 100 and 320°, respectively, were required. In Fig. 1, the spectra of cumene (curve B) and of a-methylstyrene (curve C) adsorbed on silicaalumina are compared with those of a-methylstyrene solutions in concentrated sulfuric acid (curve A) and in (CH~O)zSOz-H2SO~ (curve D). As previously dem~nstrated,~b silica-alumina abstracts hydride ions from tertiary aliphatic carbon atoms, to form the corresponding carbonium ions. Thus, the band at 395 mp can readily be assigned to the dimethylphenylcarbonium ion. Analogous results were obtained with 1,l-diphenylethane and 1,I-diphenylethylene, respectively. On the silica-alumina surface, the methyldiphenylcarbonium ion was formed from the f ~ r m e r , ~while b a corresponding band appeared (at 423 mp) in the spectrum of the latter.3 It cannot be decided from the present data whether the species responsible for this band (in the case of
w.KEITHHALL
Vol. G6
the olefin) was the proton or, as suggested by TYebb,z the Lewis acid adduct. When the olefins were dissolved in concentrated H2S04, these same carbonium ions were formed by proton addition.‘l-14 Provided care was taken to avoid reaction between these ions and excess olefin during the solution process,6 only the bands for the classical carbonium ions were obtained. Whe11, however, these olefins were adsorbed on a silica-alumina catalyst or dissolved in weakly acidic media, additional bands apppared in the spectra at about 607 mp with D P E (Fig. 2) and a t 470 and 570 mp with a-MS. The absence of these long wave length bands in the spectra obtained with the related saturated hydrocarbons indicates that the unknown entities cannot be formed by a simple equilibrium rearrangement of the classical carbonium ions, as has been suggested previously.4 B simple interpretation is that excess olefin is required. Evidence in support of this was provided by the observation ihat only the 423-mp carbonium ion band was formed on silica-alumina a t very low D P E coverages. Contrary to the finding of 1Vebbl2both bands always were formed at higher coverages, whether or not the D P E mas distilled from PzOs (or CaH2). A catalyst platelet as prepared by pressing finely ground Houdry S-65 catalyst to 18,000 p.s.i. In order to obtain sufficient transmission, the sample was immersed in a solution of DPE in CCL. The most intense band formed LTas at 423 mp. Additional bands appeared at 570 and 607 mp. The former had the greater intensity and may be due to an additional species formed from DPE. Alternatively, it. may arise through some reaction with the solyent. The spectrum from DPE adsorbed on silica gel (DGS-4) was identical with that of the parent hydrocarbon in non-acidic solvents, indicating that the olefin was physically adsorbed. On ihe admission of about 1 cc. (KTP) of anhydrous HP, bands at 426 and 595 mp developed rapidly, the latter being considerably more intense. Similar results were obtained when the olefin was adsorbed on silica gel and treated with BF3. The molar extinction coefficient for the melhyldipheiiylcarbonium ion was determined in sulfuric acid; in agreement with Grace and Symoiq6 a value of 3.1 X lo* I. mole-1 cm.-l was obtained. For the 607-my absorption, Beer’s law was not obeyed, so that only minimum values of the extinction coefficient could be obtained. In our work, these fell between 5 x lo3 and 2.2 x lo4 1. mole-’ ern.-’. We therefore concur with the opinion of Grace and Symonsc that, the value for the lorig wave length band is a t least as great., and possibly greater, than that for the carbonium ion. The effect of water vapor and ammonia on the spectrum of the DPE-silica-alumina complex is shown in Fig. 2 . On exposure to a small amount of (11) N. C. Deno, J. J. Jaiuzclski, and.4. Schriesheim, J . Org. Chem., 19, 155 (1954). (12) V. Gold, B. W.V. Hawes, and F.L. Tye, J. Chem. Soc , 2167
(1952).
(13) A. G. Evans, J. A w l . Chem. (London), 1, 240 (1951) (14) D. E. O’Reilly and H. P. Leftin, J . P h p . Chem., 64, 1556
(1960).
August, 1962
ELECTRONIC SPECTRA OF OLEFINSADSORBED ON SILICA-ALUXIIINA CATALYSTS 1469
water vapor at room temperature, the classical carbonium ion band was virtually eliminated, while the long wave length absorption band was unaffected (compare curves 1 and 2). On subsequent evacuation for 5 min. at 2 5 O , the band due to the carbonium ion was partially restored (curve 3). The reversibility of this process was demonstrated (curves 4 and 5) where it also is shown that the band a t 607 mp is not affected by a large excess of HZO. Both bands were completely eliminated by exposure to excess ammonia. These data emphasize 1hat the entity responsible for the 607-mp absorption is independent of the carbonium ion, as the concentration of the latter can be varied at will without affecting the concentration of the former. The behavior of the carbonium ion band closely resembled that presented earlier'b for the triiphenylcarbonium ion formed from triphenylmethane. A few measurements were made of the rates of development of the 423 and 607 mp bands on catalyst surfaces. These were sufficient to demonstrate: (a) that the 423-mp band always was detected first and (b) that when the amount of olefin exposed to the catalyst was severely limited, only the 423-mp peak appeared. Kinetic Studies in Oxidizing Media.-Evans and eo-workers4 l3 have shown that both bands appear in DPE-sulfuric acid-acetic acid solutions. Unfortunately, the properties of this solvent are such that rapid dimerization occurs and the total concentration of ionic species remains low. A mixture of monochloroacetic acid, acetic acid, and sulfuric acid gave better results. Data obtained from a typical experiment using this ternary system are presented in Fig. 3. Initially, the concentrations of the classical carbonium ion and of the 607-mp species increased simultaneously. After about 30 hr., both approached their maximum intensity, but while the former remained constant 01 increased extremely slowly, the latter decreased in concentration until after about 240 hr. the system reached a metastable state that remained invariant for at least 1000 hr. When only the data for the first 40 min. were plotted (see insert on Fig. 3), it became evident that while the intensity of the 423-mp band increased linearly with time, the time dependence of the 607-mp band was approximately parabolic. This behavior is consistent with the view4b that the initial rate of formation of the 607-mp species depends upon the concentration of the classical carbonium ion. The fall-off in concentration beyond the maximum may correspond to any of a variety of reactions including the formation of neutral dimeric species. It further was observed that the initial rate of formation of both species increased strongly with increasing mlfuric acid and with olefin concentration. The eff'ect of trace amounts of oxidizing agents on the kinetic behavior was investigated. Selenic acid, peroxydisulfuric acid, and potassium ferricyanide were selected for the following reasons: (a) selenic acid has only twice the acid strength of sulfuric acid, yet its oxidation potential is greater by a factor of five1;; (b) peroxydisulfuric acid is
1. Initial Spectrum 2. & 4. H9O added 3. & 5. Evacuated 6. SHa added
of H?O and SHp on spectrum of (CeHa)zC= cc. NTP) adsorbed on silica-alumina. The curves are identified as follows: (1) initial s ectrum; (2) spectrum after adsorbing 0.16 cc. KTP of Hz8 on dry plate (area -10 m.2); (3) evacuated for 5 min. a t 30"; (4) cell filled with 20 mm. of H,O vapor; ( 5 ) cell evacuated for 10 min. a t 30'; (6) cell filled with 200 mm. of SH8. Fig, 2.-Effect
CH2 (1.3 X
IO,
oE-----' 0
20
40
60
Fig. 3.-Kinetics
BO
'
1
1
I60 120 140 Time, Hours,
1 160
1 180
1 200
1 220
1
240
1
of formation of colored species from
(CeH&C=CH, in a ternary solvent system, vw., 2.96 Jf ClCH&OOH in CHJCOOH plus 15% €I*S04. The hydro-
carbon concentration was 1.02 X solvent,
A{ in the combined
similar in these respectslGa;and, finally, (e) potassium ferricyanide is a good oxidizing agent which usually involves the transfer of only a single electron.16b These data, presented in Fig. 4, show that with the addition of as little as 0.15% H2SeO4, the same over-all kinetic pattern was obtained but the entire process required only 30 min. as compared to over 250 hr. for the experiment of Fig. 3 . For direct comparison, the uncatalyzed process for the same acid concentration (15% H2S04) and for a higher acid concentration (22.5y0H2S04) arc shown (Fig. 4). It can be seen that the vastly increased rate cannot result from the small increase in acid strength of the medium attending (15) N, V. Sidgwick, "The Chemical Elements and Their Compounds, Val. 11. Clarendon Press, Oxford, 1950, p. 975 f . (16) W. M. Latimer and J. H. Hildebrand, "Reference Book on Inorganic Chemistry," The Macmillan Co , New York, N. Y , 1940 (a) pp. 243-251; (b) p 395.
HARRYP. LEFTINAND W. KEITHHALL
1460
Time,
minules.
Fig. 4.-Comparison of kinetics of formation of 607 mp species when catalyzed by small amounts of oxidizing agents to uncatalyzed reaction. In all cases, the same ternary solvent system shown in Fig. 3 was used except for one instance where the concentration of H 8 0 4 was increased to 22.57,. The olefin concentration was 4.62 X M.
the addition of (he HaSe04. Similar results u-ere obtained with peroxydisulfuric acid, More striking, however, was the fact that the addition of as mole/l. of K8Fe(CiY)6also strongly little as 1 X catalyzed the process. These results indicate that the species exhibiting the 607-mp band is an intermediate in an oxidat ion process involving the olefin. The facts that the band develops on the catalyst surface and that its formalion in solution is catalyzed by an electronic oxidizing agent (e.g., K3Fe(CN),) restrict the types of oxidation possible. These considerations led to the idea that the entity in question might be a cation radical (positive radical ion).1 Repeated experiments were made with trifluoroacetic acid, vacuum distilled from anthrone into a solution of the olefin dissolved in CaH2-dried, spectroscopically pure benzene. I n agreement with the work of Evans,4 both bands appeared, demonstrating that the species responsible may form in the absence of any kuown oxidizing agent other than the methyldiphenylcarbonium ion. This is in accord with the result obtained when the olefin was adsorbed on silica gel and exposed to gaseous anhydrous HF. As with the catalystolefin system, no e.p.r. absorption could be found for these solutions. Both the 4'23-mp and the long wave length (at li95 mp) bands appeared when SbCls was added to CHzClz solutions of DPE. Thesc solutions were quite unstable, but the kinetic behavior appeared to follow the same pattern as in the mixed acid solutions, i.e., the blue color developed rapidly and then faded. A single strong sharp e.p.r. line, having a width of less than 10 gauss, centered a t g = 2.00 was obtaiwd from this system, but the spin concentration did not always appear to be related to the blue color. When CClh was substituted for CHzC12, a deep blue paramagnetic complex precipitated. A sample of this precipitate, dissolved in CH2C12,also exhibited a strong e.p.r. signal. A blank consisting of 10% (by vol.) SbCls in CH2C12showed no resonance. Reaction of DPE with Alkali Metals.-In work reported earlier,l it was found that when DPE was treated with lithium, sodium, potassium, or even calcium in tetrahydrofuran, the solutions first became blue and then turned to a deep red. These were found to be paramagnetic and the
VOl. 66
paramagnetism correlated roughly with thc blue color. The red and the blue solutions appeared to be stable and interconvertible, the color depending upon whether excess metal or excess DPE was present. Thus, if additional potassium mas added to a blue solution, it became red (475 mp); the blue color (615 mp) could be restored by the addition of more 1,l-diphenylethylene. As similar behavior had been reported previously17~ l8 for stilbene (1,2-diphenylethylene) , and since, in this case, the dark green color was attributed to the paramagnetic species (C6H6CH=CHC6H6)- and the red color to the diamagnetic ion (C6H6CI-I= CHCOHtJ-, it was suggested that corresponding species were responsible for our results with 1,ldiphenylethylene. More recent experiments havc revealed that the 615-mp band appears in these solutions when some samples of DPE, but not when others, are used. Freshly distilled samples produced only the 475-mp band. As it was determined that these deteriorate noticeably within a few days, it must be concluded that the band at 615 mp may be due to an ion formed from a decomposition product of DPE. If it is due to the monomeric radical ion, as previously suggested,l then it is not clear why the dinegative (475-mp) ion forms exclusively in some cases, but not in others. We so far have not been able to resolve this question satisfactorily. Spectra of Possible Reaction Products.-Dimethyl sulfate solutions of the linear and cyclic dimers of 1,l-diphenylethylene initially showed no absorption in the visible region. Color developed rapidly, however, folIowing the addition of trace amounts of HzS04. The cyclic dimer solution deseloped a single broad band a t about 495 mp in contradistinction to the work of Evans and co~ v o r k e r swho , ~ ~ found no bands in the visible region with benzene-trichloroacetic acid solutions of this compound. As might have been predicted, solutions of the linear dimer (1,1,3,3-tetraphenylbutene1) rapidly developed the same two spectral peaks obtained from the monomeric olefin. It cannot be decided from this observation whether this result indicates a rapid depolymcrization of the dimer in the acidic medium, or whether the absorption bands can be obtained directly from the dimer. The oxidation dimer 1,1,4,4-tetraphenylbutadiene reacted rapidly with H2S04in the ternary solvent mixture to give an intensely blue solution. The spectrum from this solution, however, was composed of a broad band a t 715 and a stronger band at 560 mp. The spectrum of its potassium complex in tetrahydrofuran solution was very similar, having bands at 583 and 716 mp. Although the behavior of this dimer is similar to that of DPE, it can be seen that they are otherwise unrelated. Benzophenone is one of the more likely impurities that might be present in DPE, as it is an oxidation product of the olefin. Benzophenone, in the ternary solvent mixture, does not give a (17) D. E. Paul, D. Lipkin, and S. I. Weifisman, J. Am. Chem. Soc., 78, 116 (1956).
(18) G. J. Hoijtink and P. H. van der N o i j , Z. phystlc. Chem., 20, 1 (1959).
August, 1982
ELECTROXC SPECTRA OF OLEFINSADSORBED o s SILICA-ALUXINA CATALYSTS 1461
blue solution, even with added selenic acid. However, the potassium ketyl in tetrahydrofuran exhibited a single broad absorption band at 710
+
(C~HE,)CCH~ 4HzS04 + [(CsHrj)zC=CHz]t 2H30+
+
+ SO2 + 3HSO4(4)
mp.
Evans and ~ o - w o r k e r s ~have ~ ~ 3 considered some of these and other possibilities and have concluded that the 607-mp band is not related to any expected reaction product. Our results tend to corroborate this view.
where it is understood that both the carbonium ion and the cation radical may be removed by additional reactions to form stable products such as the cyclic dimer of DPE. These considerations led to the suggestion1 that the 607-mp band corresponded to an ion radical. Discussion (4) The ability of the silica-alumina surface The results presented herein can be qualitatively to act as an oxidizing agent already is well estabrxplaincd if‘ it is assumed that the species respon- lished; it can abstract hydride ions from phenylsible for the 607-mp absorption band, obtained ated alkanes” and it can oxidize polynuclear from adsorbed DPE, is formed during the oxida- aromatic hydrocarbons to their corresponding tion of the olefin. In a number of ways our data radical i0ns.21-2~ Moreover, it is known that the are inconsistent with the assignmentzr4of this band oxidation state of the surface is in some way into a charge-transfer complex, involving the inter- volved in these processes.21 In agreement with action of the proton of a weak acid with the ole- eq. 4,it is known that SO2 is evolved when olefins finic double bond. Evidence in support of these are treated with sulfuric Hence, the oxistatements is listed below. dation involves transfer of oxygen. The posOxidation Hypothesis.-(1) The fact that ex- sibility exists that silica-alumina also supplies oxycess olefin is necessary to obtain the long wave gen atoms. Shortcomings of the Charge-Transfer Complex length band may be taken as evidence that the It offers no explanation for the 607-mp species can be formed through reaction of Hypothesis.-(1) kinetic effects nor for the effects of oxidizing a carbonium ion with an olefin molecule, above. (2) The kinetic behavior shown in Fig. 3 sup- agents noted (2) The data strongly support the view that ports this view and, in addition, is typical of a free radical oxidation process. Although insufficient the species responsible for the 607-mp band is kinetic data were obtained for detailed analysis, formed by reaction of a D P E molecule with its certain characteristics of the mechanism are evi- carbonium ion. They appear to be inconsistent dent, uix., (a) the initial rate of formation of the 607- with the suggestion of Evans, et u Z . , ~ that the mp species is proportional to the carbonium ion carbonium ion rearranges into the new species. concentration, and (b) the former is destroyed by It should be noted that Evans’ assignment4 was subsequent reactions. The first of these items is derived by a process of elimination. (3) The effects of water, shown in Fig. 2, demonin agreement with the findings of Evans4b and the first and second with those of Aalbersberg’g for strate that the 423 and 607 mp bands are due to D P E in the Cd&-CF3COOH and polynuclear two independent species. Webb,2 in accord with Evans’ hypothesis, assumed that the long wave aromatic hydrocarbons in the BFrCF3COOHHzO-02 systems, respectively. Aalbersberg writes length band originated from a charge-transfer complex involving the olefin and an OH group of (for 9-bromoanthracene) the catalyst surface. Since the intensity of the 607-mp band does not increase as the 423-mp band d(cation radical) ~ -= kl(car.bonium decreases, we believe that the hydrogen associated ion) dt with hydrated Lewis acid sites is not responsible for its formation. Moreover, since the spectrum ks(cation radical) (1) of DPE on high area silica gel is identical with an equation which describes the principal features that of the parent olefin, Webb’s suggestion that of our results if it is supposed that the 607 mp band surface hydroxyl groups are the source of the pi bonding protons also seems very unlikely; were stems from the species: [ (C6H&C=CH2]+. (3) Trace amounts of oxidizing agents strongly this the case, a decided difference in hydrogen accelerate these processes, as shown in Fig. 4. bonding ability between the hydrogen atoms of Although these effects are not entirely understood, silica and silica-alumina mould be required. they may be rationalized in terms of a reaction The available evidence does not support this sequence anstlogous to one suggested by Symons,20 view.26p27Webb’s interpretation was backed by the observation that the formation of the long i.e. wave length band on the catalyst surface appeared @
(CJZi)&=CH,
f I-!+ JJ (CJ&)&-CHS
@
Jr
(c6&)2cC& f (CsHs)zC=CH, (C&)&C& [(CeH&C=CH,]t
+
(2)
(3)
(19) m’. I. Aalbersberg, ‘Thesis, Free University of Amstrrdam, 1960. (20) A. Carrington, F. Drainieks, and hi. C. R. Synions, J . Chem. S O L . , 947 (1959).
( 2 1 ) W. K. Hall, J . Cut., 1, 53 (1962). 70, 142 (1961). (22) J. J. Rooney and R. C Pink, Proc. Chem. SOC., (23) D. M. Brouwer, Chem. Ind. (London), 177 (1961).
(24) V F. Lavrushin, D. N. Kursanov, and V N. Setkina, Dakl. Akad. Xauk SSSR, 97, 265 (1954). (25) J. Gonzales-Vidal, E. Kohn, and F. A. RIatsen, J . Ciiem. Pkgs.. 26, 181 (1956). ( 2 6 ) M. R. Basila, Abstracts of Papers presented a t the 141st National Meeting, Bmerican Chemical Society, Washington, D. C., March. 1962. (27) W. K. Hall, H. P. Leftin, and D. E. O’Reilly, to be published
1462
ITARRY P. LEFTIKA U D N.KEITHIIALL
to be favored by “added-back” water. Our results show that when water is added to a system where both species are present, carbonium ions are preferentially desorbed. Hence, if H 2 0were added first, one might expect to obtain only the long wave length band. (4) The observation of the species in some solutions of DPE, when treated with alkali metals, appears to have no explanation in terms of pi complex theory. (5) Grace and Symons6 noted a similarity between the behavior of 1,l-diphenylethylene and diphenylmethanol in strong and weakly acidic media and pointed out that there is no way of deriving a complex of the type suggested by Evans from the latter compound. On the other hand, such a similarity might be expected to result from an oxidation process. (6) The band appeared in systems which are substantially free of hydrogen, e.y., anhydfous A1C132 and in the present work when D P E was adsorbed on silica gel and treated with BF3. In these instances, as with a number of other systems,lgj2sthe formation of a proton-containing complex such as postulated by JTTebb2and by Evans*is very unlikely. (7) On more general grounds, a wave length of 607 mp is too long to correspond to a chargetransfer complex of the benzene-iodine type. Buckles and ~ o - w o r k e r sshowed ~~ that tetra-pmethoxyphenylethylene interacts with a number of electron acceptors to form blue solutions (A, 575 mp) which slowly turn yellow on standing. This; band had the high malar extinction coefficient (-lo4) typical of those observed in the present work. The systems forming this band with the olefin included : H2S04in CH&UOH, CF&OOH in CC14, BF3 in CH2C12, PCls in C C 4 , and finally, of particular interest here, the halogens including T2 in C1CKaCE2Cl. The wave length of the maximum was quite insensitive to the acceptor molecule used, as observed for DPE in the present and related mork.4r6 It also is knownGthat styrene forms bands a t 435 and 615 mp in H2804 and in H2SO~-CH3COOHmixtures, respectively. It may be inferred, therefore, that these three olefins all form similar complexes from svhich the long wave length bands arise. McConnell, Ham, and Platt30 showed that the frequency of transitions of CT complexes with I, form a linear correlation with the ionization potential of the donor molecule. Recent ionization potential rneasureme~lts~~ yielded values of 9.0 and 8 3 for styrene and DPE, respectively. Accordingly, their CT complexes (with 12) would be expected30 to provide bands in the 310 to 330 mp region. Moreox-er, the work of Hastings, Franklin, Schiller, and M a t ~ e nsuggests ~~ that bands in the 600-mp region cannot be expected ( Z A ) XI7. I. Aalbersberg. G. J. Hoijtink, E. 12. Mackor, and W. P. Wciiland, J . Chem. SOC.,3049, 3055 (1959). (XI) R. E. Bucklrs, R. E. Erickson, J. D. Snyder, and W. B. Person, J . Am. Chcm. SOC.,82, 2444 (1960). (30) H. L. McConnell, J. S. Ham. and J. R. Platt, J . Chem. Phys., 21, 66 (1953) (31) G. F Crable, Gulf Research z! Development Company, private communication. ( 3 2 ) S €3. I-lantings,J . L. Franklin, J. C . Schillev, and F. .4. hihtsen, J . Ana. Chem. Soc., 76, 2Qc)c)(1963).
Vol. 66
from this system; this would require z\, much stronger acceptor molecule, e.g., tetracyanoethyl-
(8) Charge-transfer theory offers no explanation for the appearance of a band a t nearly the same wave length in a wide variety of systems, i.p., in dimethyl sulfate, in sulfuric acid-acetic acid mixtures, in benzene-CF3COOH solutions, in the CH&l&bC15, and in tetrahydrofuran-alkali metal systems, as well as adsorbed on silica-alumina or on silica gel with BF3 or HF. It is a well known consequence of charge-transfer theory that the frequency of the transition varies appreciably This obserwith both the donor aiid vation indicates that, regardless of the nature of the species, the band arises from an intramolecular transition, rather than one of the charge-transfer type. Concluding Remarks.-The nature of the species responsible for the 607-mp absorption band formed from adsorbed D P E is of considerable interest as it may correspond to an ionic intermediate not previously considered in the catalytic literature. Evidence that such intermediates may exist is to be found in ssveral places. Taft and ~ o - w o r k e r s ~ ~ have studied the mechanism of the hydration of isobutene and the reverse reaction in dilute acid solutions and have been forced to the conclusion that all of their kinetic results cannot be rationalized by the assumption of a single carbonium ion intermediate. They advanced the idea that the missing entity is a pi compIex or possibly a hydrated carbonium ion. Haag and Pines36 and LucchesF also have used the pi complex concept to explain the excess (above equilibrium) production of cis-butene-2 over the trans-isomer in the isomerization of butene-1. These studies are of particular interest in view of the fact that Webb2 has found spectral evidence of complexes formed from butene2 adsorbed on silica-alumina analogous to those reported here for 1,1-diphenylethylene and amethylstyrene. The kinetic evidence (Fig. 3 and 4) is readily understood in terms of eq. 1-4, suggesting that the 607-mp band is due to a cation radical. The free radical formed in (3) is a pon-erful electron and/or hydrogen atom donor. Step (4) is one of several possible reactions. It can be seen, therefore, that cation radicals may be formed according to (3) even in the absence of strong oxidizing agents so long as excess olefin is present in the system. Hence, they could form on the surface of silica gel when treated with anhydrous HF yet not farm when the same olefin is dissolved in liquid HF (for the same reason that only the carbonium ion forms in concentrated H2S04). On the other hand, as shown by (4), the formation will be favored by the presence of an oxidizing agent, affording a qualitative explanation for our results. Repeated attempts to prevert the formation of the (331 1% Kuroda, M. Xobayashi, M. Kinoshlta, and 8.Takemoto, J . Chem Phyr., 36, 457 (1962). (34) R. Foster, Tetrnhedion, 10,96 (1960) (35) R. ’pv. Taft, Preprints, Petroleum Division ACS, General Papers, Vol. 5 , 133, Maroh, 1960, J. Am Chem Soc., 82, 4729 (1960). (36) 0. Haag and E. Pines, ahd., 82, 2488 (1960). (37) P. J. Lucchesi, b, L Dmeder, and d . F. Longwell, z b d , 81, $286 (1959),
w.
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ELECTROKIC SPECTRA O F OLEF’IIJS ADSORBED
ON S I L I c i l - A L u M I K i i
CATALYSTS
1463
blue (607 mp) species in CsH6-CF3COOH mixtures admittedly serious shortcoming, particularly as failed, even though great care was taken t o remove the radical ion of anthracene can be readily identiall possible oxidizing agents. Yet, Aalbersberg’g 2* fied in the same systems. However, it is generally produced the spectra of the cation radicals of the recognized that the inability to observe parapolynuclear aromatic hydrocarbons in liquid HF magnetism by the e.p.r. technique does not prove and in aqueous CF3COOH saturated with BF3 its absence. As the observation of an e.p.r. signal when these were contacted with air; only the from adsorbed DPE (on a different silica-alumina proton adducts were formed in the absence of catalyst) has recently been reported by another ~ ~ interpretation must remain a oxygen. All these results are consistent with eq. L a b o r a t ~ r y ,this 1-4 and with the requirement that the 607-mp definite possibility. It now is known that the oxidation state of the species be an intermediate formed by the oxidation silica-alumina surface influences its ability to of the olefin. Recently, it has been unambiguously demon- generate ion radicals from the polynuclear aromatic strated that cation radicals do form on the surface hydrocarbon^*^.^^ and it was pointed out21 that of silica-alumina catalysts when the substrate is oxygen atoms may be involved in the process. one of the closely related polynuclear aromatic I n the interpretation of our data, the possibility hydrocarbons, such as perylene or a n t h r a ~ e n e . ~ l -that ~ ~ the 607-mp species includes an atom of From the quantum mechanical point of view, oxygen cannot be overlooked. Buckles, et U Z . , ~ ~ anthracene and 1,1-diphenylethylene are closely found that, in the presence of traces of HzO, related molecules. Their electronic spectra should the corresponding pinacolone was formed in 22% be closely similar and this also should apply to yield when tetra-p.methoxyethylene was treated their carbonium ions and to their radical ions. with a slight excess of C12 in dilute solution in These relationships have been noted elsetvhere.6112v21 dichloroethane. On the other hand, the olefin The strong band a t 720 mp of the radical ion of could be quantitatively recovered from the same anthracene has been justified by quantum me- blue (A, = 575 mp) solutions by the addition of chanical ~alculations.~8-~~ Both anthracene and Cu powder. Moreover, Webb2 contended that 1,I-diphenylethylene are alternant polynuclear traces of H20 were essential for the formation of aromatic hydrocarbons. Hence, the strong theo- the 607-mp band from DPE. Thus, although it is retical support for the existence of the polynuclear not yet possible to assign the long wave length aromatic radical ions also favors the view that the bands observed with aryl olefins to a particular aryl olefins should behave similarly, and in many species, the rough outlines of some previously mays they do. unsuspected chemistry of surfaces have been Repeated attempts to observe the paramag- made apparent. netism expected for the radical ion of DPE by the Acknowledgment.-This work was sponsored by e.p.r. technique, both in acidic solutions6,20j21 the Gulf Research &. Development Co. as part and with catalysts,21322 have failed. This is an of the research program of the Multiple Fellowship on Petroleum. Thanks are due to Dr. A. J. (38) G. J. Hoijtink and W. P. Weijland, Rec. traz. chwn., 76, 836 (1957). Saracen0 at Gulf Research and Development Co. (39) A. A. Verrijn-Stuart and E. L. Mackor, J . Chem Phya., 27, for making the e.pr. measurements. 826 (1957). (40) D E Paul, D. Lipkin, and S I. Weissman, J . A m . Chem. Soc., 78, 116 (1946). (41) T. L. Chu, G. E. Pake, D. E. Paul, J. Townsend, and 9. I. Weissman, J . Phys. Chem , 67, 504 (1953).
(42) J. Turkevich, 140th National Meeting of the American Chemical Society, Division of Colloid and Surface Chemistry, Chicago, Illinois, September 3-8, 1961. (43) J. K. Fogo, J . Phys. Chem., 65, 1919 (1961).
