Vol. 64
HARRY P. LEFTIN
1714
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chloride precipitation had become appreciable and decreased the turbulence in the partially solidified melt. The thermal decomposition of potassium perchlorate becomes rapid only after fusion of the material.1.18 As in the case of lithium perchlorate, two decomposition reactions are applicable, wiz., (a) KC104 (solution) KC1 (solution) 2 0 2 (gas), and (b) KC104 (solution) KC1 (solid) 202 (gas). The nature of the thermal effect accompanying reaction (a) is very difficult to determine by DTA. This is probably due to the relatively small heat effect involved in the decomposition which has been computed to be about 0.8 to 1.7 kcal./mole end0thermic.l Reaction (a) may be masked by the concomitant occurrence of the exothermic reaction (b) for which an enthalpy change of - (4.4to 5.3) kcal./mole was calculated. The net result of the DTA run is a single, large exothermic break attributable to over-all decomposition. As a result of this study, it appears that the following points are worthy of consideration: (a) the use of an open furnace in DTA studies of highly turbulent condensed phase-gas reactions gives spurious results because of sample recession from the thermocouple sensor leading to thermocoupleatmosphere interactions, (b) use of an open furnace is permissible for the study of completely condensed phase reactions, e.g., crystalline transitions, melting phenomena, solid phase reactions, etc., (c) quantitative information may not be obtainable from highly turbulent condensed phase-gas reactions using the open furnace technique, and probably in the closed furnace as well, because of the heat transients due to sample-thermocoupleatmosphere interactions, and (d) open furnace decomposition studies reported in the literature should be reviewed in the light of the present investigation.
-
01
0 Endothermal t AT + Exothermal. Fig. 4.-Constant temperature thermogram (480’) of anhydrous lithium perchlorate in the open furnace
autocatalyzed by the lithium chloride product. Thus, it was possible to secure a complete rapid decomposition of the perchlorate salt once the initially slow decomposition produced sufficient lithium chloride. The resulting trace is shown in Fig. 4. All the features of the dynamic DTA curve in the decomposition region (see Fig. 1, A) obtained in the open furnace are seen except that the endotherm in Fig. 4, whlch would correspond to the area IVA in Fig. 1, A has a saw-tooth appearance. Visual observations showed that as the bubbling melt rose in the tube, the initial exotherm (Fig. 4) reversed in direction and turned into an endotherm. The periodic rise and fall of the decomposing melt coincided with the periodic oscillations of the pen. The large exotherm occurred after the lithium
+
+
(18) A. E. Harvey, Jr., M. T. Edmison, E. D. Jones, R. A. Seybert and K. A. Catto. J . Am. Chem. Sac., 76, 3270 (1954).
ELECTRONIC SPECTRA OF ADSORBED MOLECULES : STABLE CARBONIUM IONS ON SILICA ALUMINA BY HARRY P. LEFTIN’ Mellon Institute, Pittsburgh 13,Pennsylvania Received M a y 13, 1980
A novel method for the measurement of the electronic absorption spectra of adsorbed molecules is described. The electronic spectra of triphenylmethane and of 1,l-diphenylethylene adsorbed on silica gel and on a silica-alumina catalyst are presented. While on silica gel the spectra are identical to those of the parent hydrocarbons (indicating physical adsorption), on silica-alumina the spectra are those of the corresponding carbonium ions formed by reaction of the hydrocarbons with the active (acidic) centers of the catalyst, (chemisorption). It is shown that on these acid sites carbonium ions can be formed either by addition to an olefinic bond or by the rupture of a tertiary aliphatic carbon hydrogen bond.
Introduction and isotope exchange studies4 has led workersas2&,6 The demonstration of surface acidity of cracking to suggest reaction mechanisms which differ in detail catalysts2 in conjunction with product distribution3 but most of which assume an initial chemisorption (1) The M. W. Kellogg Company, Jersey City, New Jersey. (3) B. S. Greensfelder, H. H. Voge and G. M.Good, Ind. Eno. Chem.. (2) (a) C . L. Thomas, Ind. Bng. Chem., 41, 2564 (1949); (b) RI. W. Tamele, Disc. Faraday Soc., 8, 2701 (1950); (c) A. G. Oblad, T. H. Milliken, Jr., and G. A. Mills, in “Advances in Catalysis,” W. G. Frankenburg, Ed., Academic Press, Ino., New York, N. Y.. 1951, Vol. 3, pp. 199-247.
41, 2573 (1949). (4) (a) G. Parrrtvano, E. F. Hammel and H. S. Taylor. J . Am. Chem. Soc., 1 0 , 2269 (1948); (b) For a recent review, see H. H. Voge, in “Catalysis,” P. H. Emmett, Ed., Reinhold Publ. Corp.. NewYork, N. Y., 1958, Vol. 6, pp. 435-439.
Nov., 1960
ELECTRONIC SPECTRA OF ADSORBED MOLECULES
process leading to the formation of adsorbed carbonium ions. Although, carbonium ion formation has been demonstrated for the chemisorption of an olefin,B there is no direct evidence for this process in the case of paraffin chemisorption. Moreover, while the uncertainty concerning the chemisorption of paraffins on silica-alumina' has very recently been resolved in favor of a small amount of chemisorptions in the case of isobutane, the identity of the chemisorbed species remains unknown. Since chemisorption is generally considered to be a necessary prerequisite for heterogeneous catalysis, it is of interest to inquire into the structure and properties of chemisorbed molecules. Detailed knowledge within this area would be of prime importance to any discussion of catalytic reaction mechanisms. Studies of the infrared spectra of adsorbed molecules have already provided insight into the molecular transformations associated with adsorption. lo In the visible and ultraviolet regions, however, owing to the difficulty in preparing suitably transparent samples, few results of catalytic significance have been reported. Early work in this field, which employed either slurry, reflection or evaporated film techniques, has been summarized.'l The purpose of the present paper is to describe a simple and practical technique for the measurement of the electronic spectra of adsorbed molecules and to show from the data obtained, that carbonium ions are formed by the chemisorption of paraffinic and olefinic hydrocarbons on an active silica-alumina catalyst. Experimental Catalysts.-Silica-alumina catalyst (DSA-1) was prepared by the neutral hydrolysis of an alcoholic solution of aluminum isopropoxide (Eastman, Technical grade, twice redistilled) and ethylorthosilicate (Carbide and Carbon Co., pure grade). The hydrolysis was carried out in a sealed container at 100". Large pieces of the resultant gel were heated in air to 600" over an 8 hour period and then calcined a t this temperature for an additional 22 hours. The finished catalyst (8870Si02 and 12% Al2O3)which was in the form of large semi-transparent chunks had a B.E.T. surface area of 278 m.*/g. Emission analysis indicated only the following impurities, in p.p.m. Ba, < l ; Fe, 10-20; Ca, 4; Mg, 3; Ka, 4; Ti, 4;andV, 9 The catalytic activity per unit surface area of DSA-1 was found to be 30% greater than that of a commercial silicaalumina catalyst for the cracking of 2,3-dimethylbutane a t 525".'2 A pure silica gel sample was prepared by the hydrolysis of ethylorthosilicate. After calcination a t 600" the adsorbent was in the form of glassy chunks and had a B.E.T. surface area of 550 m2/g. (5) (a) T. H. Milliken, Jr., G. A. Mills and A. G. Oblad, Disc. Farad a y Soc., 8, 279 (1950); (b) J. D. Danforth, Preprints of General Papers, Dw. of Pet. Chem., 9 C S September 1956, Vol. 1, p. 15: ( e ) For a recent review of this subject, see ref. 4b. (6) A. G. Evans, Disc. Faraday S O C .8, , 302 (1950). ( 7 ) R. C. Zabor and P. H. Emmett, J . A m . Chem. SOC.,73, 5639, (1951). (8) D.9. MacIver, P. H. Emmett and H. S. Frank, THISJOURNAL, 62,935 (1958). (9) K. J. Laidler, in "Catalysis," P. H. Emmett, Ed., Reinhold Publ. Corp., New York, 1954, Vol. 1, Ch. 3. (10) R. P. Eischens and W. A. Pliskin, in "Advances in Catalysis," W. G. Frankenburg, ed., -4cademic Press, Inc., New York, N. Y., Vol. 10,Pp. 1-56. (11) M. Robin and K. N. Trueblood, J . Am. Chem. Soc., 79, 5138 (1957). (12) W. K. Hall, IT. P. V-eber and D. S. RIacIver, Ind. Eng. Chem., 62, 121 (1960).
/o"orlz
10
-Absorption
1715
Pyre* Cell
b
Fig. 1.-Apparatus for spectral measurements of adsorbed molecules: a, sample holder; b, vacuum cell. Reagents.-The reagents used in this work were commercial samples which were further purified by conventional methods until constant infrared, ultraviolet and proton n.m.r. spectra were obtained. These spectra showed no bands due to impurities and, wherever comparison was possible, were in good agreement with reported data. Eastman White Label triphenylmethane after 4 reczystallizations from 95y0 ethanol had m.p. 93.2-94.2 , lit. 91-93' .13 Aldrich Chemical Co., 1,l-diphenylethylene was redistilled under reduced pressure and dried over calcium hydride; WD1.6088, lit.14n20~1.6087. Procedure.-All spectra were measured with a Beckmann recording spectrophotometer, model DK-1. A light tight box was fitted over the detector and cell compartment in order to accommodate the cells used in this work. Catalyst samples for optical measurements were prepared in the form of thin platelets (0.5 f 0.2 mm. thick) by grinding on emery cloth. These platelets were mounted on the vertical uprights of rectangular quartz cages with 6 mil platinum wire as shown in Fig. l a . The cages fit snugly into commercial fused silica absorption cells having a 1 cm. path length. Mounted samples were cleaned and regenerated between runs by calcination in a stream of dried oxygen a t 500" for 16 to 24 hours. They were then placed in the optical cells, (Fig. lb), sealed directly to a vacuum system, evacuated for 1 hour at 500", and further calcined in situ with oxygen at 500" for 24 hours. After final evacuation for 24 hours a t 500" (to ~ 1 0 - 6mm.) the cells were sealedoff under vacuum. For each catalyst sample studied, a closely matched sample was identically pretreated in a separate cell. This was used as a blank in the reference beam in order to compensate for the light scattering of the catalyst samples so that the full absorbance range of the spectrophotometer could be utilized. By selecting pairs of closely matched samples, it was possible to make adequate corrections for background absorbance. Since, however, exact compensation would require tedious matching of fragile samples, it was more convenient to measure background curves prior to chemisorption. These curves were used to correct the observed spectra so that the resulting curves represent the absorbance due only to the adsorbed species. Weighed samples of the adsorbates were introduced into the reagent compartment of the optical cell which was then sealed to the vacuum system, evacuated for at least 4 hours, and sealed under vacuum. After recording the background curve, the catalyst sample was exposed to the hydrocarbon vapors by rupturing a break-off membrane. Due to the low vapor pressures of these hydrocarbons, it was often advantageous to thermostat the entire cell at 100". Therefore, in order to avoid contamination of the catalyst by constituents of the stopcock lubricant, the stopcock shown in Fig. l a was not joined to the second side arm of the cell until the chemisorption process was completed and an initial spectrum measured. This stopcock could be used to admit more volatile gases such as HzO and NH3 into the system. As a precaution against contamination of the catalyst samples by residual hydrocarbons from previous experiments the o tical cells were cleaned between runs in a heated sulfuric aci8nitric acid bath. (13) A. G. Brook and €1. Gilman, J . Org. Chem., 18, 447 (1953). (14) K. T. Serijan and P. H. Wise, J . A m . Chem. SDC., 74, 4766 (1951).
HARRY P. LEFTIN
1716
A -Adsorbed on S I c o Ptunim 0-1" Oimcthylrullate + H,S0. C-In 98% HzSO.
L' / ir
)
/
450
i
\
600
ms. I'ig. 4.--Al)sorption spectra of 1,l-diphenylethylene.
Results Chemisorption of Para5ns.-Alkyl carbonium ions formed on the surface during a catalytic reaction rapidly decompose according to the usual Whitmorels type transformations which are permitted by their structural features. Once these transformations have occurred, the products are desorbed and, hence, it is not surprising that alkyl carbonium ions have never been isolated or identified on catalyst surfaces. Since the structural features of the triphenylcarbonium ion prohibit rapid decomposition, it is sufficiently stable to permit its identification under suitable circumstances. Thus, the triphenylcarbonium ion in solution is (15) (a) F. C. Whitmore, J . Am. Chem. Soc., 54, 3274 (1932); (b) Chem. Eng. hiews, 26, 668 (1948).
Vol. 64
well known. Its existence as a stable species has been amply demonstrated by cryoscopic measurements in sulfuric acid solutions of triphenylcarbinol16and by conductivity measurements in liquid sulfur dioxide solutions of the corresponding hali d e ~ . The ~ ~ spectrum of the ion is also well established18 and is characterized by a double peaked absorption band in the 404 to 432 mp region. Curve B of Fig. 2 is a redetermination of this spectrum obtained from a solution of triphenylcarbinol in concentrated sulfuric acid. Curve A is the spectrum obtained when triphenylmethane is adsorbed on DSA-1 at 50". Comparison of these two curves provides convincing evidence for the existence of the triphenylcarbonium ion on the surface. Also shown in Fig. 2 are the curves for the triphenylmethyl radical (curve C) as observed by Andersonlg in an ether solution of hexaphenylethane and for the triphenylmethide (curve D). Comparison with these curves demonstrates that neither the carbanion nor the free radical are formed in the process of chemisorption. That the observed spectrum was the result of a chemical reaction between the hydrocarbon and the catalytically active acidic centers of the silicaalumina surface (chemisorption), and not due to a spectral shift caused by perturbations of the electronic states of the parent hydrocarbon molecule upon adsorption, can be demonstrated from the data given in Fig. 3. Here it is evident that the spectrum of silica gel exposed to triphenylmethane vapor for 1000 hours at 100" (curve B) is identical with the spectrum (curve A) of an alcoholic solution of this hydrocarbon. Thus, on the non-acidic or weakly acidic surface of silica gel, the triphenylcarbonium ion is not formed. The close agreement of these spectra suggests that on silica gel the triphenylmethane is physically adsorbed. This is further evidenced by the marked loss of spectral intensity after evacuation at 100" for 4 hours (curve C). In contrast, in the case of silica alumina where the hydrocarbon is chemisorbed as the carbonium ion, no decrease in absorbance was noted even after evacuation at 275" for 48 hours. These data are believed to constitute the first direct demonstration of the formation of carbonium ions as a consequence of the chemisorption of a "tertiary" hydrocarbon on the surface of a silica-alumina catalyst by a reaction involving the rupture of an aliphatic C-H bond. Chemisorption of Olefins.-Carbonium ion formation by the addition of a proton or Lewis acid to an olefinic bond is well known. It is expected, therefore, that carbonium ions should be formed as a result of the chemisorption of olefins on acid catalysts such as silica-alumina, and if the olefin employed is conjugated with one or more phenyl substituents, the surface carbonium ion should easily be detected from its characteristic spectrum. (16) L. P. IIammett and A. Deyrup, J. Am. Chem. Soc., 55, 1900
(1933).
(17) (a) P. Walden, Ber.. 36, 2018 (1902); (h) h l . Gomberg, ibid., 35, 2403 (1902); (0) N. N. Lichtin and H. P. Leftin, THIS JOURNAL, 60, 164 (1956). (18) (a) A. Hantzsch, 2. physzk. Cham., 61, 257 (1908); (b) 0. Branch and H. Walba. J. Am. Cham. Soc.. 76. 1564 (1954). . . (19) L. C. Anderson, ibid., 67, 1673 (1935):
Nov., 1960
ELECTRONIC SPECTRA OF ADSORBED MOLECULES
1717
When 1,ldiphenylethylene is chemisorbed on the surface of a silica-alumina catalyst, an absorption spectrum (Fig. 4) composed of two principal bands in the visible region is obtained. One of these, the band at 423 mp (e = 3 X lo4),can be assigned to the methyldiphenylcarbonium ion, formed by proton addition to the double bond. The existence of this species has been adequately demonstrated by cryoscopic measurements on solutions of the olefin and the corresponding carbinol in concentrated sulfuric acid.20 The spectrum of this ion has been determinedzl and its structure has recently been verified from the proton n.m.r. spectrum of a solution of the olefin in concentrated sulfuric acid.22 The other absorption band occurs at 600 mp and is of uncertain origin. In Fig. 4, it is demonstrated that these same two bands can be developed in solutions of this olefin in dimethyl sulfate +5% concentrated H2S04, but that only the band corresponding to the carbonium ion (423 mp) appears in concentrated H2S04 solutions. It has previously been notedzasz4 that both bands appear in sulfuric acid-acetic acid mixtures. Discussion On the basis of these results, the over-all course of the reaction involved in the chemisorption of triphenylmethane must be
t e free to rotate and may also be free to migrate over limited regions of the surface. Recent n.m.r data,z6tend to support this view. I n the case of olefin chemisorption, the mode of carbonium ion formation cannot be defined on the basis of the present spectral data. This is due to the fact that closely similar spectra would be expected from the addition of either a Lewis acid or a proton to the double bond of 1,l-diphenylethylene, since the spectrum of the resulting carbonium ion,
(21) A. G. Evans, J . A p p l . Chem., 1, 240 (1951). (22) D. E. O’Reilly and H. P. Leftin, unpublished results. (23) A. G. Evans. P. AI. S. Jones and J. H. Thomas, J . Chem. Soc., 104 (1957). (24) Lavrushin, Zhur. Obehchei Rhim., 26, 2697 (1954). (25) H. P. Leftin and W. X. Hall, Proceedings of 2nd International Congress on Catalysis, Paris, France, July 1960.
(26) D. E. O’Reilly, H. P. Leftin and W. K. Hall, to be published. (27) V. Gold and F. L. Tye, J . Chem. Soc., 2172 (1952). (28) H. P. Leftin and W. K. Hall, Abstracts of Papers, 134th National Meeting, ACS, Chicago, Illinois, September 1958, p. 94-P. (29) A. N. Webb, 135th National Meeting B C S , Boston, Massachusetts, April 1959, Petroleum Preprints, p. C-171. (30) €1. P. Leftin and W. K. Hall, THISJ O u n s . 4 ~ .64, 382 (1960).
+
namely, (CeHa)&-CH2Y, would be only slightly influenced by the nature of the substituent on the ,8 carbon atom. In accord with this reasoning is the observationz7that the spectra of the protonated forms of 1,l-diphenylethylene, triphenylethylene and anthracene are closely similar. It is of interest to note, however, that while a carbonium ion formed by proton transfer from a Bronsted acid site would be free of the surface (as in the case of the triphenylcarbonium ion), an ion formed by interaction with a catalyst Lewis acid would be bonded to the surface. Evans6 reported that a broad absorption band in the 430 mp region appeared when a solution of 1,l-diphenylethylene was added to a suspension of activated floridin clay. This result is in agreement with the present work insofar as formation of the methyldiphenylcarbonium ion is concerned. The long wave length absorptionz8 (A 600 mp) was not observed by Evans; however, it silica-alumina (CeHb),CH (Ct.H6)&+ + Hhas been independently reported by Webb.2g where clearly the hydrocarbon molecule has lost On the basis of a suggestion of Evans, et U Z . , ~ ~ a hydride ion to form the observed tertiary car- Webb has attributed this absorption band to a bonium ion. The fate of the hydride ion and the ?r-complex formed between the olefin and a hyrole of the catalyst in this process is discussed drated Lewis acid center on the catalyst surface. elsewhereeZ5 That the carbonium ion does not The nature of the species responsible for the form a covalent carbon-to-surface bond, as had 600 mp band will be discussed elsewhere; however, been suggested p r e v i o ~ s l y ,is~ ~indicated by the it may be stated, our recent dataaa suggest a fact that such bonding would destroy the resonance quite different interpretation. The results presented in this paper are in agreesystem responsible for the stability and characteristic spectrum of this ion. The carbonium ions ment with the hypothesis that a carbonium ion must, therefore, be held in the vicinity of the sur- mechanism is involved in catalytic cracking and face by coulombic forces and may be considered related processes. Acknowledgment.-This work was sponsored by to constitute the positively charged half of an electrical double layer. In this state, they may the Gulf Research & Development Company as a part of the research program of the Multiple Fel(20) V. Gold, B. W. V. Hawes and F. L. Tye, J . Chem. Soc., 2167 lowship on F’etroleum. (1952).
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