Langmuir 1987, 3, 549-555 metrical constraint leads to a low quadrupolar ~p1itting.l~ The present results indicate that the structuring effects of water are less important in lower water content samples. In these samples, the area per polar head group is smaller and thus the number of water molecules that can be associated with each polar head group is less than for the higher water content samples. Thus the following two effects could be important. First, the degree to which the water can impose a direction on the first C-C bond could be a function of the number of water molecules near the polar head group; with fewer surface water molecules per polar head group the waterllipid interaction may be reduced (compared to higher water content samples) thus leading to an average direction for the first C-C bond and an orientation for the water molecules which yields larger splittings for both water and polar region chain deuterons. Second, the lower area per polar head can constrict the freedom of the chain to spread out; thus it would be more
549
difficult for the first C-C bond to be parallel to the bilayer normal as this situation requires the polar end of the chain to be tilted with respect to the normal. The different results obtained with different water contents are interesting from another point of view. For potassium palmitate several phase transitions have been detected within the lamellar phase,"J5 and these could well be associated with changes in the lipidlwater interaction.
Acknowledgment. We thank A. Weaver for preparing the perdeuteriated potassium palmitate and the Natural Sciences and Engineering Research Council of Canada, NSERC, for financial support. E.J.D. wishes to acknowledge NSERC for the award of a Graduate Scholarship. M.Y.K. wishes to acknowledge UBC for the award of a University Graduate Fellowship. Registry No. Water, 7732-18-5;potassium palmitate, 262431-9.
Identification of Acetone Enolate on y-Alumina: Implications for the Oligomerization and Polymerization of Adsorbed Acetone Brian E. Hanson,*l* Larry F. Wieserman,lb George W. Wagner,la and Ruth A. Kaufmanlb Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, and Alcoa Laboratories, Aluminum Company of America, A h a Center, Pennsylvania 15069 Received November 3, 1986. In Final Form: February 20, 1987
.
In situ infrared spectroscopy was used for studying the adsorption of normal and isotopically labeled acetone on partially dehydroxylated alumina. Assignment of infrared bands to the surface species includes adsorbed acetone, acetone enolate, mesityl oxide, and isophorone. Acetone enolate is very reactive on the surface and is only observed during the initial adsorption of acetone at 300 K. For isotopically normal acetone, the enolate band is observed at 1595 cm-'. A small shift from this value is observed for acetone-d6, and [2J3C]acetone shows the C=C stretch of the enolate at 1570 cm-'. Exchange of residual protons from surface hydroxyl groups with deuterium of acetone-d, occurs concurrently with enolate formation.
One of the most fruitful methods for the investigation of catalytic oxides is the adsorption of probe molecules which react specifically with acid or base sites on the surface.l The surface reaction may be monitored by infrared spectroscopy or NMR, by observing changes either in the probe molecule or in the composition of the surface. Acetone is generally considered inappropriate as a probe molecule due to its chemical reactivity on metal oxide surfaces.l In our laboratory we have observed that acetone and other weak Lewis bases have a profound effect on the olefin metathesis activity of alumina-supported molybdenum complexes.2 A survey of the literature for the adsorption of acetone and its derivatives on metal oxides consistently implicates the presence of acetone enolate, however, it fails to convincingly identify this species spectroscopically. The present study was undertaken to provide a complete understanding of acetone reactivity on y-alumina and to attempt to identify acetone enolate as a reaction intermediate. A t temperatures greater than 473 K the major reaction of acetone on catalytic oxides is oxidation to a carboxylate (1) Knozinger, H. Adu. Catal. 1976,25, 184. (2) Wagner, G. W.; Hanson B. E., submitted for publication in Inorg.
Chem.
0743-7463/87/2403-0549$01.50/0
species? Substituted acetones, however, are more stable to oxidation and are used to probe acid sites on alumina surfaces. For example, the identification of at least three types of acid sites on y-alumina by using hexachloroacetone adsorption was r e p ~ r t e d . ~ Although hexachloroacetone reacts with the surface of y-alumina, reaction pathways involving formation of enolate species are not possible. Diisopropyl ketone and hexamethylacetone were also used to probe the nature of y-alumina surfa~es.~ Coordination at a Lewis acid site was revealed for hexamethylacetoneby a 39-cm-' shift in the carbonyl stretching frequency. In general, oxygen-containing organic molecules produce carboxylate complexes on metal oxide s u r f a ~ e swith ,~~~ oxidation as the primary reaction pathway at elevated temperatures. Adsorbed ketones and aldehydes are implicated as intermediates in the oxidation of alcohols. Direct adsorption of acetone on y-alumina reveals coordination at a Lewis acid site as a precursor to ~ x i d a t i o n . ~ Surface carboxylates are identified by infrared spectros(3) Fink,P. Reu. Roum. Chim. 1969, 14, 811. (4) Hair, M. L.; Chapman, I. D. J. Phys. Chem. 1965, 69, 3949. (5) Schulz, W.; KnBzinger, H. J. Phys. Chem. 1976,80, 1502. (6) Greenler, R. G. J. Chem. Phys. 1962,37, 2094. 0 1987 American Chemical Society
550 Langmuir, Vol. 3, No. 4, 1987
copy upon adsorption of acetone on silicon and titanium oxides? group I1 metal oxides? hydroxide-dopedalumina: and zirconium oxide.1° A surface enolate complex from acetone adsorption is tentatively assigned on MgO and NiO surfaces"J2 and r ~ t i 1 e . l ~ Miyata et a1."J2 and Griffiths and Rochester13assign bands at 1545 and 1430 cm-' to acetone enolate. These bands develop slowly in the infrared spectrum and remain at temperatures up to 473 K and after evacuation. This suggests that the surface species giving rise to these bands are very stable on the surface. However, the position of the bands is shifted significantly to lower wavenumbers from where stable enolates are o b s e r ~ e d . ~ ~ , ~ ~ Adsorption of acetone on silica,I4alumina and magnesia;15 and amorphous silica aluminals was used to determine the acid strength of surface hydroxyl groups on these surfaces. Adsorption at Brernsted acid sites was determined by shifts in the OH stretching mode of surface hydroxyl groups."j Reaction products from acetone adsorption on oxide surfaces, other than carboxylates, are generally rationalized by formation of acetone enolate interrnediate.l' An enolate intermediate supports the observation of deuterium exchange between acetone-d, and surface hydroxyls revealed by infrared spectroscopy.l8 Aldol condensation of acetone over MgO-A1203 catalysts yields mesityl oxide, mesitylene, phorone, and isophorone among other products." Similar products are observed by carbon-13 NMR upon the adsorption of acetone on Catapal a 1 ~ m i n a . l An ~ acetone enolate complex is suggested by others as a key intermediate in the surface reactions of a c e t ~ n e . ' ~An, ~acetone ~ enolate complex on the surface is postulated as a reaction intermediate for the formation of condensation products. Also it is likely to be identified as a transient species rather than the stable end product from the adsorption of acetone. 11-13 The catalytic synthesis of methyl vinyl ketone from acetone and methanol was reported to occur over ironmagnesium oxide.21 This reaction product implicates that an acetone enolate is formed as an intermediate. In this work, we present chemical and spectrometric evidence for the enolate of acetone on the surface of yalumina at 300 and 400 K as a reaction intermediate. Also, the major surface reaction at 400 K appears to be polymerization rather than formation of small molecules such as mesityl oxide or isophorone.
Hanson et al. Six way cross
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(7)Kiselev, A. V.;Uvarov, A. V. Surf. Sci. 1967,6, 399. (8)Tretyakor, N. E.; Filimonov, V. N. Kinet. Catal. 1970, 11, 815. (9)Deo, A. V.; Chuang, T. T.; Dalla Lana, I. G. J . Phys. Chem. 1971, 75, 234. (IO) Yamaguchi, T.; Nakano, Y.; Tanabe, K. Bull. Chem. SOC.Jpn. 1978,51,2482. (11)Miyata, H.; Toda, Y.; Kabokawa, Y. J . Catal. 1974,32, 155. (12)Miyata, H.; Wakamiya, M.; Kabokawa, Y. J. Catal. 1974,34,117. (13)Griffiths, D.M.; Rochester, C. H. J . Chem. Soc., Faraday Trans. 1 1978, 74,403. (14)Young, R. P.;Sheppard, N. J . Catal. 1967, 7,223. (15) (a) Lercher, J. A.; Noller, H. J . Catal. 1982,77,152;(b) Lercher, J. A.; Noller, H.; Ritter, G. J. Chem. SOC.,Faraday Trans. 1 1981,77,621. (16)Lercher, J. A.;Vinek, H.; Nollar, H. J. Chem. SOC.,Faraday Trans 1 1984,80,1239. (17)Reichle, W. T.J. Catal. 1980,63, 295. (18)Vinek, H.Z. Phys. Chem. 1980,120,119. (19)Bell, V. A.; Gold, H. S. J . Catal. 1983,79,286. (20)Pavlenko, N. V.; Tripol'skii, A. I.; Tel'biz, G. M.; Golodets, G. I. Teor. Eksp. Khim. 1986,21, 333. (21)Ueda, W.; Yokoyama, T.; Moro-Oka,Y.; Ikawa, T. J. Chem. SOC., Chem. Commun. 1984,39. (22)Willard, P.G.; Carpenter, G. B. J. Am. Chem. SOC.1986,108,462. (23)Lochmann, L.; De, R. L.; Trekoval, J. J. Organomet. Chem. 1978, 156,307.
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Figure 1. Schematic representation of the high-vacuum infrared cell for the in situ study of gas adsorption onto y-alumina.
Experimental Section The y-alumina studied was produced by the hydrothermal conversion of high-purity bayerite to boehmite in a Parr bomb. In a typical preparation the Parr bomb was charged with 100 g of bayerite and 10 g of water and heated to 510 K for 2 h. Boehmite was converted to y-alumina by heating to 820 K for 2 h in flowing air in 100-g lots. y-Alumina was confirmed by the X-ray diffraction pattern of the material. The alumina was sieved through 100 mesh; the surface area was determined to be 67 m2 g-'. Iron content of the material was determined to be 0.001% by elemental analysis. Acetone, HPLC grade; acetone-d,, 100.0 mol %; and iodomethane, 99%, were obtained from Aldrich Chemical Co. Acetone labeled with 13C at the carbonyl carbon (99 mol %) was obtained from Cambridge Isotopes. The solvents were stored over molecular sieves after opening. The liquids were outgassed prior to introducing their vapor to the IR cell by three freeze-pump-thaw cycles. All in situ infrared spectra were recorded on an IBM-98 FTIR spectrometer equipped with a liquid nitrogen cooled narrow-band MCT detector. The resolution was 4 cm-'; 128 scans were collected for each spectrum. A schematic representation of the high-vacuum cell used in this work is shown in Figure 1. The window materials consist of KRS-5 that were held in place with 21/8-in. diameter conflat flanges and sealed with viton-A O-rings. The vacuum cell is connected to a n Inficon Quadrex 200 quadrupole mass spectrometer. Data acquisition, temperature, and pressure programming are controlled by a Bruker Aspect 2000 computer. In a typical experiment, 15-25 mg of alumina powder was pressed at 33.5 MPa into a pellet 13 mm in diameter. The pellet was then mounted in the infrared cell where it was pretreated a t 700 K for 1 h a t lo4 torr. This treatment yields a y-alumina with a partially dehydroxylated surface. The pellet was then cooled in vacuo to the desired temperature and the appropriate liquid vapor was allowed to contact the wafer. In repeat experiments, absorbance values vary no more than 10% when corrected for the weight of the pellet.
Results Isotopically Normal Acetone. Infrared spectra of acetone adsorbed on y-A1203at 300 K are shown in Figure 2 as a function of acetone pressure. The pellet for this sample was preheated to 700 K under vacuum to yield a partially dehydroxylated surface. All spectra in the series are referenced to the pellet before exposure to acetone. Thus as acetone is introduced, the negative peaks centered
Langmuir, Vol. 3, No. 4, 1987 551
Identification of Acetone Enolate on y-Alumina c
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at 3750 cm-' are due to loss of isolated surface hydroxyl groups. With increasing acetone pressure, a broad band at 3500 cm-' grows in intensity. This indicates the concurrent formation of hydrogen-bonded surface hydroxyl groups with loss of isolated hydroxyl groups. The C-H stretching region is dominated by aliphatic C-H stretches; however, a small but significant band occurs above 3000 cm-'. This is consistent with a small amount of unsaturated hydrocarbon on the surface. After evacuation, most of the intensity above 3000 cm-' disappears. At pressures of 1torr and higher, gas-phase acetone is obserxed in the infrared spectra. The carbonyl stretch at 1738 cm-' is due to acetone in the gas phase. Comparison with the spectrum of gas-phase acetone allows the assignment of peaks at 1427 and 1368 cm-l to CH3 bending modes and the 1234-cm-' peak to the C-C stretch of acetone. These are not perturbed upon adsorption. In the carbonyl region, the most prominent band occurs at 1697 cm-' at a low pressure of acetone. The position of this band moves to higher wavenumbers with an increasing pressure of acetone until it appears at 1707 cm-l at 10 torr. A band assigned to liquid acetone is observed at 1711 cm-l. Additional bands are observed at 1632 and 1599 cm-' at low pressure (Figure 2A). As the pressure of acetone increases, the 1632-cm-' band increases in relative intensity with respect to the 1599-cm-l band (Figure 2B,C). The 1599-cm-' band shifts slightly to higher wavenumber with increasing pressure. Upon evacuation, the major band in the carbonyl region occurs at 1634 cm-' with a shoulder at ca. 1695 cm-' (Figure 2G). The total intensity of the C-H region compared to that of the carbonyl region is greater in the evacuated spectrum than observed at lower pressures (compare the 150 mtorr spectrum with the 5 X loi, torr spectrum, Figure 2D,G).
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Figure 3. Infrared spectra obtained upon adsorption of [213C]acetoneon y-alumina at 300 K. The pellet pretreatment was 700 K under vacuum: (A) 30 mtorr; (B) 100 mtorr; (C) 150 mtorr; torr. (D) 500 mtorr; (E) evacuated to 1 X
[2-13C]Acetone.The adsorption of isotopically labeled acetone (carbon-13 for carbonyl carbon) on y-alumina at 300 K results in the infrared spectra shown in Figure 3. The spectra are plotted from 2000 to 1000 cm-I to emphasize the shifts observed for the isotopically substituted material. In the carbonyl region, bands appear at 1655, 1601, and 1570 cm-' (Figure 3B). The high-frequency band shifts to 1663 cm-' at 500 mtorr, while the others shift to 1603 and 1583 cm-I (Figure 3D). The relative intensities are almost identical with the bands observed in Figure 2D at 1699, 1632, and 1601 cm-' at 150 mtorr. The shift to lower wavenumbers implicates that the carbonyl carbon-13 participates in these stretching modes. The 1447-cm-' band and the CH bending modes, 1364 and 1425 cm-I, are unshifted upon isotopic labeling with carbon-13. After evacuation, the most intense feature is the 1603-cm-' band with shoulders at 1650 and 1585 cm-l. Deuteriated Acetone. The infrared spectrum obtained upon adsorption of acetone-d, on y-alumina at 300 K indicates exchange of deuterium with hydrogen on surface hydroxyl groups. Surface deuteriation of surface hydroxyl groups is indicated by the broad region of positive intensity at 2800-2500 cm-' (gain of OD) and the broad region of negative intensity at 3500-3000 cm-' (loss of OH). Thus surface OH is being replaced by surface OD. Also, small peaks are observed at 2900-3000 cm-l, indicative of C-H bonds and isotopic mixing of C-D and C-H. In the carbonyl region, the most intense band appears at 1686 cm-' (Figure 4A) and shifts to higher wavenumber with increasing coverage. A band also appears at 1585 cm-' which moves to higher wavenumber with increasing acetone pressure. A band in the range of 1630-1640 cm-' never fully appears in this experiment as expected from the results in Figure 2. However, upon evacuation, the very broad band at 1600 cm-l (Figure 4F) has several components, and a shoulder appears at 1630 cm-'.
552 Langmuir, Vol. 3, No. 4 , 1987
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Figure 4. Infrared spectra for acetone-d8on y-alumina at 300 K (A) 5 mtorr; (B) 10 mtorr; (C) 100 mtorr; (D) 1 torr; (E) 7 torr; (F) evacuated to 1 X lo* torr. To distinguish kinetic or pathway differences between normal and acetone-d, adsorption on y-alumina, the pressure of acetone-d, was held constant while the carbonyl region was monitored as a function of time (Figure 5). The band a t 1641 cm-I increases at a slower rate in this experiment compared to the results shown in Figures 2 and 3. This is consistent with C-D bond cleavage in the formation of the species responsible for this band. The difference spectra shown in Figure 5 indicate that long exposures of y-alumina to acetone at 300 K lead to bands at 1445 and 1562 cm-l. Also, as the 1641-cm-' band grows, the 1690-cm-' band decreases in intensity. Methyl Iodide, Adsorption of acetone-d, on y-alumina at 400 K was followed by infrared spectroscopy as shown in Figure 6. Traces D, E, and F in Figure 6 were taken after exposure to CH31and represent difference spectra with trace C serving as the background. For exposure of y-alumina at 400 K with 1 torr of acetone-d,, bands are observed at 1690, 1639, 1609, and 1560 cm-'; the most intense of these is the 1639-cm-I band. The peak positions correlate well with results obtained at 300 K (Figures 4 and 5). The relative band intensities within the peak envelope are similar to those obtained for long exposure times (Figure 5E). The surface reactions are much faster and different species form a t 400 K than a t 300 K. After evacuation of the sample, the two most intense bands appear a t 1639 and 1560 cm-'. This sample was then treated with methyl iodide, CHJ, at pressures up to 100 torr. Clearly, the affected peaks upon adsorption of CH31 are those at 2280,1685, and 1636 cm-'. After evacuation, the temperature of the sample was increased (temperature-programmed desorbed, TPD) and the off gases were monitored with a quadrupole mass spectrometer. The major species observed in the mass spectrometer is acetoned6 (mass 64 amu). Very small peaks are present at 88 and 90 amu. These are assigned to the parent ions of perdeuterioisophorone and perdeuteriomesityl oxide and
1600
Figure 5. Evolution of infrared bands in the range 2500-1400 cm-' for acetone-d, (400 mtorr) on y-alumina at 300 K: (A) 5 min; (B) 1 h; (C) 2 h; (D) 4 h; (E) 21 h. (F)-(I) represent difference spectra, (B) - (A), (C) - (B), (D) - (C),and (E)- (D), respectively.
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Figure 6. Infrared spectra obtained upon adsorption of acetone-d6 at 400 K, (A)-(C). (D)-(F) were obtained after addition of CHJ, using spectrum C as a reference. (A) 1 torr of acetone; (B) 10 torr of acetone; (C) evacuated to torr; (D) 5 torr of Me1 torr at 300 K; (F) evacreferenced to (C); (E) evacuated to uated to torr at 700 K. are seen in other TPD experiments a t high spectrometer gain. At the spectrometer sensitivity required to see these
Langmuir, Vol. 3, No. 4, 1987 553
Identification of Acetone Enolate on y-Alumina
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