ADSORPTION OF HEXACHLOROACETONE ON ALUMINA-CONTAINING SURFACES
These equations were applied to liquids by evaluating the measured a > i j at the equivalent binary composition, ;Pi = Xi/(Xi Xj) of the ternary mixture in question. By applying eq. l l a and l l b to the present system in this manner using the binary data as presented in Figures 1, 2, and 3, diffusion coefficient surfaces analogous to Figures 6, 7, 8, and 9 could be generated. The four diffusion coefficient surfaces predicted by eq. 11 are presented in Figures 10, 11, 12, and 13. It is noted that the equations were used to predict the ternary coefficients at the borders as well as within the ternary field. As can be seen from these representations, although the prediction of the ternary coefficients by means of eq. 11 are not quantitative, the general behavior of the four diffusion coefficient surfaces are fairly well reproduced in all four cases. Although there is no a priori reason for applying the Stefan-Maxwell equations to liquid systems, these
+
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equations represent essentially Bearman’sll and Lightfoot, Cussler, and RettigW equations written for a thermodynamically ideal system. The binary activity data which are availablela indicate that the present ternary system is far from ideal, but the lack of ternary activity data precludes any meaningful attempt to use the above equations to account for lack of ideality. For the same reason, it is not possible to carry out a meaningful check of the Onsager reciprocal relations.
Acknowledgment. The authors wish to thank the National Science Foundation for its financial support of this work, Mr. C. M. Yon for his assistance, and Professor G. J. Mains and Mr. S. Wrbican for the mass spectrometer analyses. ~
~~
~~
~~
(11) R. J. B e m a n , J . Phys. Chem.,65, 1961 (1961). (12) E. N. Lightfoot, E. L. C d e r , Jr., and R. L. Rettig, A.I.Ch.E. J., 8 , 708 (1902). (13) J. Tirnmermsns, “Phyaic*Chemical Constants of Binary Syn-
terne in Concentrated Solutions,” Vol. I, Interscience Publiaherr, Ina., New York, N. Y., 1959.
The Adsorption of Hexachloroacetone on Alumina-Containing Surfaces
by M. L. Hair and I. D. Chapman Exploratory Chemieal &search Department, Research and Developmat Laboratory, Corning Qkasa Works, Corning, New York (Received June 10, 1966)
The infrared spectra of hexachloroacetone adsorbed upon alumina-containing surfaces have been obtained. By analogy with the spectra of similar complexes with aluminum and boron halides, these spectra have allowed certain conclusions to be drawn concerning the strength and distribution of Lewis acid sites on such surfaces. The strengths of the sites on a silica-alumina gel were very similar to those on an alumina gel, and both of these surfaces contained at least three sites of differing strength.
The adsorption of nitrogenous bases on aluminacontaining surfaces has been used by many workers as a method for the determination of surface acidity. The quantitative desorption of ammonia from such surfaces has been widely regarded as a measure of surface acidity, but the recent results obtained by Peril on the infrared adsorption spectra of adsorbed ammonia species
show considerable decomposition of the ammonia and cast doubt on this technique as a method of determining surface acidity. It is generally accepted that the surfaces of catalytic aluminas and silica-aluminas contain sites which exhibit Lewis acidity, and many (1)
J. B.Peri, J . Phys. them., 69,231 (1865).
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workers believe that the silica-alumina surface contains acidic sites which are protonic in character, Parry2 has described the infrared spectrum of pyridine adsorbed on both alumina and silica-alumina surfaces, and from a comparison of the observed spectra with those of pyridine complexes with inorganic Lewis acid halides and hydrogen acids, concluded that both alumina and silica-alumina contained Lewis acid sites whereas only the silica-alumina-pyridine spectra were cognizant of a protonated molecule. Parry’s work has been repeated and extended by Basila3 and co-workers, who confirmed Parry’s observations. No work, however, has yet been able to demonstrate the presence of protonic sites on alumina surfaces. Whereas these studies of the infrared spectra of adsorbed molecules have provided valuable proof of the existence of Lewis and Br$nsted acid sites on these acidic surfaces, they have suffered from the serious disadvantage in that the use of nitrogenous bases leads to spectra which show little perturbation attributable to differences in the strengths of the acid sites or the number of sites of differing acidity. Also, as the nitrogenous bases react with both Lewis and Br$nsted sites, the spectra on some surfaces tend to be complicated and difficult to interpret. The purpose of the present work is to demonstrate that the use of a carbonyl-containing basic molecule can be used to give an indication of both the strength and distribution of the Lewis acid sites on the alumina-con t aining surfaces. Lappert4 has examined the infrared spectra of a number of complexes of ethyl acetate and inorganic Lewis acid halides and suggested that the shift in the carbonyl stretching frequency of the carbonyl group may be taken as a measure of the Lewis acidity of the coordinating halide. This work has been extended by COO^,^ who examined the spectra of a series of metal halide complexes using xanthone as a ligand. I n Figure 1 the shifts in the carbonyl stretching frequency for xanthone complexes of the boron halides are plotted as a function of the heat of formation of the pyridine complexes of these halides in nitrobenzene solution as determined by Brown and co-workers.6 It is seen from this linear correlation that, at least in this series of similar complexes, the fundamental C=O stretching frequency can be taken as a measure of the Lewis acidity of the halides. I n these carbonyl compounds, the complex formation takes place via the oxygen atom of the carbonyl group. The donation of electrons from the oxygen atom into the vacant orbital of the boron halide leads to a delocalization of the electrons in the carbonyl double bond. This perturbation results in a shift of the carbonyl stretching frequency to lower wave numbers, the greater shifts The Journal of Physical Chemistry
M. L. HAIRA N D I. D. CHAPMAN
20
1
I
I
25
30
35
A H (Kcols/mola)
Figure 1. Shift in carbonyl stretching frequency of xanthone complexes of boron halides us. heat of formation of analogous pyridine complexes in nitrobenzene solution.
being associated with increased Lewis acidity. This is clearly demonstrated in Figure 1 where the shift to lower wave numbers is in the order BIa > BBr3 > BCll > BF3, in agreement with thermal data on the acidity of the halides. Cook has specified five essential criteria which the ligand should fulfill before it can be used to estimate Lewis acidity. (1) The ligand should not be easily protonated. This becomes even more important in dealing with surfaces on which strong protons may exist, as such sites would react with the donor molecule in the same manner as the Lewis acid sites. (2) The complex formed should be stable and nonhygroscopic. This assumes importance if the technique is to be adapted to a routine estimation of surface acidity. (3) Steric hindrance should be minimized. Again, this assumes greater importance in dealing with catalytically active solids where the pores may be small, rendering the sites inaccessible to large molecules. (4)The carbonyl group should possess a large polar(2) (3) 68, (4)
E. P. Parry, J . Catalysis, 2, 371 (1963).
M. R. Basila, T. H. Kantner, and K. H. Rhee, J. Phys. Chem. 3197 (1964).
M. F. Lappert, J. Chem. Soc., 542 (1962). (6) D. Cook, Can. J . Chem., 41, 522 (1963). (6) H. C. Brown and R. R. Holmes, J . Am. Chem. SOC.,78, 2173 (1956).
ADSORPTION OF HEXACHLOROACETONE ON ALUMINA-CONTAINING SURFACES
izability-the larger the polarizability, the greater will be the shift for any given Lewis acid. ( 5 ) There should be an absence of mechanical effects on the carbonyl group. Cook suggested that the benzaldehyde molecule might fulfill all these criteria. However, the present authors' have shown that the benzaldehyde molecule is readily oxidized by alumina-containing surfaces, and application of this technique to such surfaces requires the further criterion that the donor molecule must not be easily oxidized. From experiments in this laboratory it has been shown that acetone, cyclohexanone, benzylnitrile, and other more complicated ligands are oxidized by alumina surfaces. The oxidation apparently proceeds very rapidly if the molecule is able to undergo the keto-enol transformation and takes place at lower temperatures on silica-alumina surfaces than on alumina surfaces. This might suggest that the reaction is catalyzed by the protons which have been shown to exist on the silica-alumina surfaces. In order to negate this oxidative effect, hexachloroacetone was chosen as the donor molecule. This liquid (b.p. 203.6') contains no hydrogen atoms and thus cannot undergo the tautomeric effect. Furthermore, it is oxidized only destructively.
Experimental Section The hexachloroacetone used in these experiments was supplied by K & K Laboratories. The silica-alumina cracking catalysts Aerocat 2000 and Aerocat Triple A were supplied by Cyanamid, and the sample of alumina used was prepared by the hydrolysis of aluminum isopropoxide and dried at 500". X-Ray analysis showed it to be in the y form. All the powdered solids were given a standard pretreatment which consisted of heating in oxygen at 500' for 12 hr. before being contacted with the hexachloroacetone. Initially, the solid sample was contacted with hexachloroacetone vapor at the required temperature before being cooled to room temperature, and the spectra were recorded. However, as there were no major differences in the spectra recorded using this method and those obtained by adsorption of the hexachloroacetone from benzene solution, evaporating and heating to the required temperature, this latter procedure was adopted for the spectra shown in Figures 2-5. All spectra were recorded on a Perkin-Elmer doublebeam 221G grating spectrophotometer. A mulling technique was employed, the mulling agent being a fluorolube oil supplied by Hooker Chemical Co. The reference sample was identical for any given series of experiments, and the quantity of samples used was adjusted in situ so that the absorptjvjties of all samples
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CM-I
Figure 2. Infrared spectrum of hexachloroacetone adsorbed on Aerocat Triple A a t room temperature.
c
f
1620
P
i
1620
250.C
k 500% ( d )
IC)
I
I
I
I
I
I
CY-'
Figure 3. Spectrum of hexachloroacetone adsorbed on Aerocat Triple A (a) at room temperature, (b) after heating to 150°J (c) 250°J and (d) 500".
were constant and equal to that of the reference sample at 1900 cm.-'. Use of this procedure enables the intensities for any one series to be compared, and exceptionally straight backgrounds were obtained.
Results The spectrum of hexachloroacetone dissolved in carbon tetrachloride is shown in the region of interest (7) I. D. Chapman and M. L. Hair, Actes Congr. Intern. Catdyse, Se, Amsterdam, 2 , 1091 (1964).
Volume 69, Number 11
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M. L. HAIRAND I. D. CHAPMAN
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I
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/(el
Figure 4. Hexachloroacetone adsorbed on ?-alumina at (a) room temperature, (b) 150°, and (c) 250".
I
I
1250
1%
I 1750
I zoo0
CM'
Figure 6. Infrared spectrum of hexachloroacetone in CCl, solution. ieie I
iCl
Figure 5. Hexachloroacetone adsorbed on Aerocat 2000 a t (a) room temperature, (b) 150", and (c) 200".
(2000 to 1250 cm.-l) in Figure 6. The peak of interest occurs at 1780 cm.-' and is identified &s the fundamental stretching frequency of the carbonyl group. A second peak is observed at 1750 cm.-l, An examination of the literature reveals that the hexachloroacetone molecule has not been subjected to vibrational analysis, and it is difficult at this stage to explain the presence of the second band. This band disappears on complex formation and thus cannot be attributed to a C-C1 overtone. Moreover, the high wave number renders this unlikely. Since the intensity of the 1750-cm.-l band relative to the 1780-cm.-l band decreases on raising the temperature to loo", it is clear that the 1750-cm.-l absorption is not due to a hot band. The doublet formation is not observed in halogenated ketones which contak two or more fluorine atoms,8 and it is possible that the second band arises from splitting due to the steric hindrance of the bulky chlorine atoms and consequent interaction with the a carbonyl group. The Jourraal of Physical Chemistry
The spectrum obtained with hexachloroacetone adsorbed on Aerocat Triple A at room temperature is shown in Figure 2. The large peak at 1780 cm.-l shows that there is a considerable quantity of physically adsorbed hexachloroacetone. However, comparisons with the hexachloroacetone spectrum of Figure 6 shows that a broad band has appeared which extends from approximately 1720 to 1550 cm.-l. This band is attributed to perturbation of the fundamental carbonyl stretching frequency due to complex formation with the surface Lewis acid sites. This portion of the spectrum is shown on an expanded scale in Figure 3a. I t is seen that the broad distribution of sites contains at least three separate distributions which peak at 1691, 1641, and 1625 cm.-l. On heating to 150" either in air or under vacuum, the spectrum shown in Figure 3b is obtained. This is identical with the spectra obtained by vapor deposition of the hexachloroacetone on the Aerocat Triple A surface at that temperature. Two interesting effects are observed. First, that the large peak at 1691 cm.-l is essentially removed: in other words, some of the weaker Lewis acid sites disappear. Second, new peaks are observed at 1625, 1636, and 1650 cm.-'. On further heating to 250" the peak at 1650 cm.-' disappears, and the distribution of sites appears to be much more symmetrical with peaks at 1638 and 1620 cm.-l and a shoulder around 1580 cm.-'. At 500" the peak at 1638 cm.-' disappears, and the majority of the hexachloroacetone has been desorbed leaving only two small peaks at 1583 and 1620 cm.-'. (8) R. N. Haszeldine and F. Nyman, J. Chem. SOC.,3015 (1961).
ADSORPTION OF HEXACHLOROACETONE ON ALUMINA-CONTAINING SURFACES
The spectra of hexachloroacetone adsorbed on alumina a t various temperatures are shown in Figure 4a, again using expanded scale. These can be compared with the spectra for Aerocat Triple A. At room temperature there are again three distributions which are attributed to hexachloroacetone chemisorbed on three sites of differing energy on the alumina surface. The peaks of these distributions occur at 1692, 1680, and 1585 cm.-'. On heating to 150" (Figure 4b), the peaks (1692 and 1680 cm.-l), due to the weaker acid sites, are lost and are replaced by a more symmetrical distribution centering around 1657 cm. -l. However, it is seen that this in turn is split into three distributions, the peaks of which occur at 1642, 1657, and as a shoulder a t 1693 cm.-'. As with the Aerocat Triple A, the peak at 1588 cm.-l, due to the strong acid sites, has been relatively unaffected by the heat treatment. At 250" (Figure 4c) the weaker Lewis acid complexes are again decomposed, and two peaks are observed at 1618 and 1583 cm.-'. It is to be noted that the peak at 1583 cm.-' changed little, if at all, in intensity during this series of heat treatments, whereas the other Lewis acid-carbonyl complexes are largely decomposed. Also, in comparing the spectra of alumina and Aerocat Triple A, it is to be noticed that there are two obvious differences, namely, the height of the peak a t 1583 em.-' and the unsymmetrical distribution of sites which occurs on the alumina at 250" compared with the much more symmetrical distribution found on the silicaalumina surfaces. The spectra of hexachloroacetone adsorbed on Aerocat 2000 cracking catalyst at various temperatures are shown in Figures Sa, b, and c. As might be expected from this mixture of synthetic and natural cracking catalysts, the spectra are very complicated, and a t 150' it is seen that there are a t least five separate site distributions, the peaks of which are observed a t 1587, 1618, 1640, 1657, and 1692 cm.-'. On heating to 200', however, the distribution of sites is more symmetrical, and two peaks are observed at 1615 and 1632 cm. --I.
Discussion An examination of the shifts of the fundamental carbonyl stretching frequency of the xanthone complexes obtained by Cook5 shows that the order of Lewis acidity for the boron halides is BIZ > BBr3 > BC13 > BF3. This series of Lewis acidities is the reverse of that which one would expect from application of normal electronegativity concepts.B This order, however, has been well documented by various author^.^^^ I n Figure 7 the positions of the carbonyl stretching frequency of xanthone complexes of the boron halides
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I)I200
13
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I5
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19
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21
E (e.v.1
Figure 7. Shift in carbonyl stretching frequencies of xanthone complexes of boron halides us. electronegativity of halogen atom.
200
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AI 01,
AIF3
I3
I
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I
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IS
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19
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I + E le.v.1
Figure 8. Shift in carbonyl stretching frequencies of hexachloroacetone complexes of aluminum halides us. electronegativity of halogen atom.
are plotted as a function of the Mulliken electronegativity of the appropriate halogen atom. (The Mulliken definition of electronegativity is equal to half the sum of the ionization potential and the electron affinity of the atom concerned.1° For convenience, the sum of the I.P. and E.A. has been taken as the ordinate in Figures 7 and 8.) From this plot it can be seen that the shift of the carbonyl stretching vibration is a function of the electronegativity of the substituent halogen atom, and, for the purposes of the ensuing arguments, this can be considered to be an approximately linear relationship. In order to extend these considerations to the alumina surfaces, a series of complexes was prepared between hexachloroacetone and the aluminum halides. The spectra of these complexes were examined in car(9) J. M.Miller and M. Onysachuck, Can. J. Chem., 41, 2898 (1963). (10) R.S. Mulliken, J. Chem. Phys., 3 , 573 (1935).
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bon tetrachloride solution, and the shift in the carbonyl stretching frequency was plotted as a function of the electronegativity of the halogen atom. This is shown in Figure 8. From this it can be seen that aluminum trifluoride does not undergo complex formation with hexachloroacetone, and there is, therefore, no change in the carbonyl stretching frequency, This is in character with the ionic structure of the compound and its general inability to form coordination complexes. Aluminum trichloride is a well-known Lewis acid and gives a complex with hexachloroacetone as do the tribromide and triiodide. As with the boron series, the tribromide is a stronger acid than the trichloride. In the case of the iodides, however, there is a difference in that aluminum triiodide is a weaker acid than the bromide. This can be traced to the increased ionic character of the aluminum iodide which is reflected in the different crystal structure. If the electronegativity concept can be applied to atoms other than in the halogen series, then two interesting features emerge as far as the alumina-containing surfaces are concerned. Firstly, as the Lewis acid sites on both silica-alumina and alumina are expected to consist of aluminum atoms surrounded by three oxygen atoms and as the oxygen atoms might be expected to differ little in electronegativity in both cases, then the Lewis acid sites on both these surfaces should be similar, and the spectra of hexachloroacetoneon such surfaces should show considerable similarities. Moreover, by inserting the value for the electronegativity of oxygen in Figure 8, it is seen that the observed shift should be about 170 cm.-l. Examination of the spectra shown in Figures 2 4 shows that these suppositions are essentially correct and that the peak of the distribution obtained for hexachloroacetone adsorbed on both silica-alumina and alumina at 250" is around 1620 cm.-'---a shift of 160 cm.-l from the free carbonyl vibration fre-
The Joiirnal of Physical Chemistry
NI. L. HAIRAND I. D. CHAPMAX
quency of hexachloroacetone in carbon tetrachloride solution. An examination of the spectra of hexachloroacetone adsorbed on Aerocat Triple A and alumina at 150" (Figures 3b and 4b) shows two distinct differences. First, the silica-alumina Lewis acid sites are apparently slightly stronger at this temperature than the alumina sites in that the peak of the distribution occurs around 1636 cm.-' in the case of the Triple A and at around 1657 cm.-' in the case of the alumina surface. However, on the alumina surface it is apparent that about 10% of the sites gives rise to a carbonyl peak at 1588 cm.-'. This shift of 200 cm.-l indicates that the Lewis acid sites are very strong in this case and more nearly comparable with the shift obtained for the boron halide complexes. From these results it is readily apparent that the nature of the sites on alumina-containing surfaces is varied and that there is indeed a wide distribution range of sites of varying Lewis acidity. By using a more refined technique it should be possible to obtain a quantitative measure of the number of sites available for complex formation. The low vapor pressure of hexachloroacetone makes vacuum manipulation of this compound difficult. However, trifluorotrichloroacetone has been used successfully, and quantitative data obtained with this ligand will be published. The results with hexachloroacetone indicate that the Lewis sites on the silica-alumina and alumina surfaces are in general very close in their acid strength. It has previously been proposed that protonic sites can be formed by reaction of a water molecule with a Lewis acid site to give an duminol grouping and a proton. Such a mechanism should be wholly dependent on the strength of the Lewis acid and, in view of the present results, should take place as easily on an alumina surface as on a silica-alumina surface. Protons have not been observed on alumina surfaces, and thus this mechanism must be considered inadequate in accounting for the Brghsted acidity of silica-alumina surfaces.
