An Infrared Spectroscopic Study of Acetone and Mesityl Oxide

The quantitative determination of the coverage of the acid sites by the reagent and the .... The Journal of Physical Chemistry B 1999 103 (10), 1664-1...
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An Infrared Spectroscopic Study of Acetone and Mesityl Oxide Adsorption on Acid Catalyst A. Panov and J. J. Fripiat* Department of Chemistry and Laboratory for Surface Studies, University of WisconsinsMilwaukee, P.O. Box 413, Milwaukee, Wisconsin 53201 Received December 11, 1997. In Final Form: April 17, 1998 The interactions between the acetone carbonyl and the carbonyl and double bond of mesityl oxide and the Bro¨nsted and Lewis acid sites of acid catalysts have been studied by Fourier Transform infrared (FTIR) spectroscopy. The acetone carbonyl interactions with bridging acidic OH and with bridging weakly acidic OH can be distinguished from one another semiquantitatively. This was substantiated by a quasi oneto-one relation between the number of bridging OH in a cluster Si-nAl containing only one Al and the number of strongly hydrogen-bonded acetone. The distinction between Bro¨nsted and Lewis sites is not easy with acetone, while it is with mesityl oxide. The system acetone-mesityl oxide adsorbed an acid catalysts is rather unique, since the reagent and the reaction product of the condensation reaction are qualitatively and quantitatively detectable by FTIR spectroscopy. The quantitative determination of the coverage of the acid sites by the reagent and the reaction product was a prerequisite to the study of the reaction presented elsewhere.

Introduction The aldol condensation of acetone on acid catalyst provides an interesting subject of study for infrared and NMR spectroscopists. Acetone (Ac) and its main condensation product, mesityl oxide (MO), are easily characterized and the strength of their interaction with electron acceptor sites, Bro¨nsted or Lewis acidic centers, is measurable from the shift of characteristic spectral lines. MO’s specific spectral features, for example, the CdO and CdC stretching vibrations, are better resolved, making the measurement of shift and specific absorbance less ambiguous than with Ac. Thus, the unique advantage of the system is to simultaneously provide a reagent (Ac) and a reaction product (MO), which can be identified separately, and the possibility to appreciate the strength of the interaction with the catalyst center responsible for the transformation. It is because of this double advantage that the study of the acetone condensation was carried out by IR spectroscopy on a set of zeolites, dealuminated zeolites, and alumina, which had been thoroughly characterized as to the nature and the content in Bro¨nsted and Lewis acid sites. The study of the transformation of acetone into mesityl oxide and higher condensation products will be reported in a following paper. Numerous papers dealing with the adsorption of acetone on various catalysts and catalyst supports have been published over the last 30 years. The sole originality of this paper is found in the use of catalysts that were characterized in such a way as to allow for a quantitative study. Earlier studies have focused on the exploration of the Lewis acidity,1 while in recent studies, the center of interest has been the interaction with Bro¨nsted sites. Besides IR spectroscopy,2-6 the other physical tool that * To whom correspondence should be addressed. (1) Hair, M. L. In Infrared Spectroscopy in Surface Chemistry; M. Dekker, Inc.: New York, 1967; p 149. (2) Nova´kova´, J.; Kubelkova´, L.; Bosa´cˇek, V.; Mach, K. Zeolites 1991, 11, 135. (3) Kubelkova´, L.; C ˇ ejka, J.; Nova´kova´, J. Zeolites 1991, 11, 48. (4) Kubelkova´, L.; Nova´kova´, J. Zeolites 1991, 11, 822. (5) Floria´n, J.; Kubelkova´, L. J. Phys. Chem. 1994, 98, 8734. (6) Kubelkova´, L.; Kotrla, J.; Floria´n, J. J. Phys. Chem. 1995, 99, 10285.

has been extensively used is the 13C nuclear magnetic resonance.7-11 The extent of the proton transfer from the Bro¨nsted sites to the carbonyl is small for Ac and almost complete for MO (absorbed on HZSM-5).10 Biaglow et al.7,8 reported that at coverage below one molecule Ac/Bro¨nsted sites, the hydrogen-bonded absorption complex is stable and Ac has a low mobility.9 At higher coverage the mobility is increased, making possible the bimolecular condensation reaction. MO has a much lower mobility than Ac. The interaction of Ac-Lewis sites has drawn less attention, although Schultz and Kno¨zinger12 pointed out that the CO stretch vibrations of diisopropyl ketone interacting with Lewis sites on δ Al2O3 or AlCl3 are similarly shifted. The spread in frequency, as appreciated by the spectral bandwidth, may be representative of the distribution of acid strengths. The adsorption of mesityl oxide by an acidic catalyst has been studied more as a byproduct of the condensation reaction than for its own sake.3,10,11 It will be shown here that the IR spectra of MO contain, in fact, more information than those of adsorbed acetone. MO is a stronger base than Ac.10 The shifts of the CdC and the CdO stretches have been observed to be equivalent, maybe because of the conjugation. In addition, the CdC stretch of MO on Bro¨nsted sites appears in a spectral window, allowing an easy deconvolution. We do not intend in this paper to examine the spectral modifications of the lattice OH vibrations due to hydrogen bonding. The large number of papers published on that aspect do not provide keys to quantitative measurements, which are of first importance to the goal of this contribu(7) Biaglow, A. I.; Gorte, R. J.; White, D. J. Phys. Chem. 1993, 97, 7135. (8) Sˇ epa, J.; Lee, C.; Gorte R. J.; White, D.; Kassab, E.; Evleth, E. M.; Jessic, H.; Allavena, M. J. Phys. Chem. 1996, 100, 18515. (9) Biaglow, A. I.; Gorte, R. J.; Kokotailo, G. T.; White, D. J. Catal. 1994, 148, 779. (10) Biaglow, A. I.; Sˇ epa, J.; Gorte, R. J.; White, D. J. Catal. 1995, 151, 373. (11) Xu, T.; Munson, E. J.; Haw, J. F. J. Am. Chem. Soc. 1994, 116, 1962. (12) Schulz, W.; Kno¨zinger, H. J. Phys. Chem. 1976, 80, 1502.

S0743-7463(97)01359-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/28/1998

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Table 1. Characterization of the Samples Used in This Studya sample

USY

USY5F

USY10F

HZSM

HM

DHM700

HY

alumina

Si/Al (NMR) Si/Al (ca.) Lewis sites (mmol/g) Bro¨nsted sites (mmol/g) Vac/VN2 Q1 (mmol/g)

4.9 2.6 1.43 1.42 0.52 1.38

14.0 2.6 0.78 0.62 0.41 1.87

59.4 2.6 0.33 0.23 0.3 0.27

18 14.9 ∼0 0.83 0.59 0.75

9.8 5.2 0.38 1.25 0.67 1.12

15.0 5.2 0.82 0.83 0.31 0.85

2.55 2.55 n/a >1.6 n/a >1.6

0.4

a NMR Si/Al ratio (framework components) are obtained from 29Si NMR and Si/Al (ca.) is obtained from chemical analysis. Number of Bro¨nsted and Lewis sites obtained from NH3 chemisorption: refs 14 and 15, Q1: calculated amount of Bro¨nsted sites (eq 1), Vac/VN2: ratio of the saturation volumes of acetone (at room temperature), and of N2 at -196 °C. This ratio is suggested to represent the availability of the sites.

tion. Recent information on the triplet (A, B, C) of OH bands in strong to moderate hydrogen bonds on surfaces have renewed the interest in these vibrations. Pelmenschikov et al.16 have suggested that these bands or pseudobands can show up in the 2900 (A), 2400 (B), and the 1900-1300 cm-1 (C) domain in the interaction of zeolite OH groups with acetone. The very broad C band would overlap with the vibrational bands of adsorbed acetone or mesityl oxide. With a gas-phase proton affinity of 196.7 kcal/mol-1 such as that of acetone or of THF (196 kcal/ mol-1), Paze´ et al.17 have observed the C band in the 1400 cm-1 region on acid zeolites. Thus, the eventual effect of this band will have to be considered. Experimental Section Catalyst. All catalysts used for the absorption studies have been thoroughly characterized earlier from the point of view of surface area, porosity, catalytic performance, and nature of the acid sites. To save time and space only a brief notice will be given here, the complete literature being readily available. The nanosized alumina rich in coordination defects has been prepared by a sol-gel process13 calcined at 600 °C and its Lewis acidity carefully characterized.14,15,18 The total number of Lewis sites is 0.4 mequiv/g and the surface area is between 300 and 350 m2/g. USY is a commercial (PQ) catalyst. USY5F and USY10F are the results of further USY dealumination using NH4F.19 In USY5F both the numbers of Bro¨nsted and Lewis sites are reduced to less than 50% of the initial content in USY, whereas in USY10F the reduction reaches about 80%. The HZSM-5 sample is from PQ and it was prepared as indicated in ref 14. The preparation and characterization of HM and of DHM700 by calcination of HM at 700 °C have been described in ref 18. Table 1 shows the Si/Al ratios obtained from 29Si NMR and chemical analyses. To minimize the dealumination process and, therefore, the number of Lewis sites, HY, HM, and HZSM5 were deammoniated at 400 °C in the IR cell before the adsorption study. The dealuminated samples were dried in the IR cell by outgassing at 500 °C for 5 h under a 10-5 Torr residual pressure. Note that one test for the absence of Lewis sites in the zeolites is the absence of AlOH vibrations in the 3600 cm-1 region. Only the OH stretching band(s) of bridging Si-OH-Al must be observable. Practically, this situation is achieved for samples deammoniated in the IR cell and never reexposed to atmosphere. For the sake of clarity, Table 1 shows the number of Bro¨nsted and Lewis sites obtained from ammonia chemisorption.14,18 Reagents. Acetone (94.9%) and mesityl oxide (99%) were Aldrich products which were dried over 4A molecular sieve (Ac) or MgSO4 (MO) and purified by repeated freezing and thawing. (13) Coster, D.; Fripiat, J. J. Chem. Mater. 1993, 5, 1204. (14) Yin, F.; Blumenfeld, A. L.; Gruver, V.; Fripiat, J. J. Phys. Chem. B 1997, 101, 1824. (15) Coster, D.; Blumenfeld, A. L.; Gruver, V.; Fripiat, J. J. J. Phys. Chem. 1994, 98, 6201. (16) Pelmenschikov, A. G.; van Wolput, J. H. M. C.; Ju¨nchen, J.; van Santen, R. A. J. Phys. Chem. 1995, 99, 3812. (17) Paze´, C.; Bordiga, S.; Lamberti, C.; Salvalaggio, M.; Zecchina, A. J. Phys. Chem. B 1997, 101, 4740. (18) Gruver, V.; Fripiat, J. J. J. Phys. Chem. 1994, 98, 8549. (19) Panov, A. G.; Gruver, V.; Fripiat, J. J. J. Catal. 1997, 168, 321.

Instrument. IR spectra were obtained with a Perkin-Elmer 1800 FTIR operating in the absorbance mode. The spectral resolution was 2 cm-1. One hundred scans were accumulated. The stainless steel cell is fitted with KBr windows and the sample can be outgassed up to 600 °C or it can be cooled at -190 °C. The weight of the pellets is about 14 mg/cm2. All spectra were recorded at room temperature unless otherwise indicated. The temperature of the sample is read by a small thermocouple touching the material.

Results The CO stretching bands in gaseous Ac and MO and the CdC stretching in gaseous MO are shown in Figure 1A, the pressure in the cell being 5 Torr. The optical path is 10 cm. The room-temperature spectra of Ac adsorbed on alumina are shown in Figure 1B, while the spectra recorded at -60 °C are shown in Figure 2A. When Figures 1B and 2A are compared to the spectra of MO adsorbed at room temperature on the same alumina (Figure 2B), it becomes clear that condensation of acetone into MO occurred very rapidly (less than 20 min) even at the lowest Ac pressure. At -60 °C, the slow development of the band at 1602 cm-1 (to be assigned later on to CdC stretch in MO adsorbed on Lewis sites (L)) indicated that condensation occurred to a much less extent. It should be emphasized that on zeolites at room temperature, the only sign of the onset of acetone condensation could be a weak band near 1584 cm-1 that increases with the pressure. The formation of water resulting from the condensation will be studied in the paper devoted to the study of the reaction. The spectra obtained with alumina are the simplest to deconvolute, because the baseline does not have to be corrected. Unfortunately, for zeolites the proximity of the SiO stretching below 1300 cm-1 requires such a correction. It was carried out by assuming a linear increase of the background between 1775 and 1325 wavenumbers. The justification of the procedure is in the leveling to absorbance below 0.2 in the 1500 cm-1 region, observable in Figures 2-6. The background correction of the quality shown in Figures 2-6 was considered suitable for deconvolution. At this stage we should reconsider the influence of the “C” band16,17 in the spectrum of acidic OH interacting with acetone (gas-phase proton affinity of 196.7 kcal/mol-1). In Figure 3A the 4000 to 1300 cm-1 region is shown for HM with three acetone converages. The corresponding spectra in the region of interest, between 1750 and 1300 cm-1 are shown in Figure 3B. In Figure 3A, except for the subtraction of the gas-phase spectrum, the spectra have not been manipulated. In Figure 4B the spectrum of the bare zeolite has been subtracted and the linear correction for the background has been applied. The “C” band should overlap with the whole spectrum in 3B, especially in the region below 1500 cm-1. Yet, the C band or pseudoband16 does not appear to influence the

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Figure 1. (A) Spectra of gaseous (5 Torr) acetone (s) and mesityl oxide (- - -). (B) Room-temperature spectra observed for alumina exposed to doses corresponding to 0.1, 0.2, 0.4, 0.6, 1.0, 1.5, 2.1, 3.1, and 4.1 mmol of gaseous Ac per gram catalyst.

Figure 2. (A) Spectra observed for alumina cooled at -60 °C and exposed to doses corresponding to 0.1, 0.2, 0.4, 0.6, 1.0, 1.5, 2.1, and 3.1 mmol of Ac/g. (B) Room-temperature spectra of alumina exposed to doses corresponding to 0.2, 0.4, and 0.6 mmol of mesityl oxide/g of catalyst.

spectra in Figure 3B, except perhaps for a moderate increase of the absorbance at about 1500-1550 cm-1. As demonstrated later this increase does not interfere appreciably with the measurement of the integrated absorbance of the CO stretching band nor with the linearity of the variation of the integrated absorbance of the CH bending bands with the dose of adsorbed acetone (not shown). The deconvolution of the spectra was obtained with a Peakfit program, bands being located at the positions indicated in Figures 2-6. An example of deconvolution in Gaussian functions is represented in parts A and C of Figure 7, while the integrated peak areas are plotted vs the dose of gaseous acetone, represented as the number of millimoles per gram of catalyst introduced into the cell in parts B and C of Figure 7. The integrated intensities (or areas) of the bands at 1680 and 1650 cm-1 increase

pseudolinearly (intercept ∼0) up to a dose corresponding to about 1.5 mmol adsorbed/g and then they progressively level out. The bands at 1712 cm-1 in Figure 7B and at 1701 cm-1 in Figure 7D have a very low intensity at low dose, but their intensity starts increasing in the region where those of other bands level out. Our interpretation is that the bands at 1650, 1680, and 1700 cm-1 in Figure 7C are attributable to chemically adsorbed acetone, while those at 1712 or 1701 cm-1 (Figure 7D) are probably due to physically adsorbed species for which saturation hardly occurs. In Figure 8 the integrated absorbances of the whole acetone stretching band between 1730 and 1630 cm-1 is plotted vs the dose of acetone introduced into the cell. Up to about 1 mmol/g the variation is linear and is the same for all zeolites. The specific absorbance for strongly absorbed acetone can be calculated from the slope of the linear regression, which

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Figure 3. Different treatment of the room-temperature spectra of HM exposed to increasing doses of acetone: (A) no correction; (B) background correction. Gas-phase spectrum is always subtracted. In B the doses were 0.1, 0.2, 0.4, 0.6, 1.0, 1.5, 2.1, 3.1, and 4.1 mmol of Ac/g of catalyst. In Figure 3A the doses were 0, 1, and 3 mmol of Ac/g of catalyst.

Figure 4. (A) HZSM5 exposed to increasing doses of acetone: 0.1, 0.2, 0.4, 0.6, 1.0, 1.5, 2.1 3.1, and 4.1 mmol of Ac/g. (B) HM exposed to 0.1, 0.3, and 0.5 mmol of MO/g.

is 6.0 ( 0.5 cm/µmol. This value was used in this work to calculate the number of acetone molecules absorbed on Lewis or Bro¨nsted sites. Discussion and Band Assignment. Besides the CdO and CdC stretching vibrations, which are the most important for this work, CH bending modes are also observed between 1470 and 1320 cm-1. The band at ∼1450 cm-1 in parts A and B of Figure 1 is characteristic of mesityl oxide, while the shoulder at 1471 cm-1 seems to be due to an aromatic condensation product.23 The assignments suggested by others for the CdO and CdC vibrations are (20) Dao, T. V.; Kitaev, L. E.; Topchieva, K. V.; Kubasov. A. A.; Ratov, A. N. Vestn. Mosk. Univ., Khim. 1971, 26, 719. (21) Hong, Y.; Gruver, V.; Fripiat, J. J. J. Catal. 1996, 161, 766. (22) McManus, J. C.; Harano, Y.; Low, M. J. Can. J. Chem. 1969, 47, 2545.

summarized in Tables 2 and 3. The assignments coming from the present contribution and the width at half-height (whh, cm-1) obtained from the Peakfit program using Gaussian functions are indicated there. As far as acetone (Ac) is concerned, our assignments and the assignments of others differ significantly for CO on Lewis sites and CO on Bro¨nsted sites. The spectra in Figure 2A obtained by adsorbing Ac on an alumina having about 0.4 mequiv Lewis sites per g, undoubtedly, shows one band relatively narrow (whh ) 27 cm-1) at 1702 cm-1. There is a much weaker band at 1602 cm-1, namely, at the frequency of the strongest band in adsorbed mesityl oxide (see Figure 2B), indicating a very limited trans(23) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Methuen; London, John Wiley: New York, 1958; p 72.

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Figure 5. Room-temperature spectra of HY exposed to doses corresponding to 0.1, 0.2, 0.4, 0.6, 1.0, and 1.5 mmol of Ac (A) and 0.1, 0.3, and 0.5 mmol of MO (B) per gram of catalyst.

Figure 6. (A) Room-temperature spectra of USY exposed to doses corresponding to 0.1, 0.2, 0.3, 0.5, 1.0, 2.1, 3.1, and 4.1 mmol of Ac and 0.1, 013, and 0.5 mmol of MO (B) per gram of catalyst.

formation at -60 °C. In addition, from the estimate of the specific absorbance, the corresponding number of chemisorbed Ac, calculated as the number of molecules remaining adsorbed after outgassing for 30 min at -60 °C, is in very reasonable agreement with the number of Lewis sites on this alumina. Some assignments obtained for different catalysts may overlap, for instance, the band at 1690-1702 cm-1 assigned to CdO on Lewis sites and the band of CdO interacting with SiOH at 1700-1705 cm-1. Such overlap means that the strength of these interactions is similar. When such overlap occurs on the same material, the band cannot be used for quantitative estimates. The assignment of a band at ∼1700 cm-1 to Ac interacting weakly with silanol (whh) ) 22 cm-1) comes from the observation that the intensity of the OH stretching at 3740 cm-1 decreases as adsorption of acetone

at ∼1700 cm-1 increases (parts B and D of Figure 7). This band is easily observed in USY and HZSM-5 zeolites. In HM in which the extent of dealumination is small (Figure 4A), the main CO stretching vibrations of absorbed Ac are at 1679 and 1645 cm-1 with widths of 28 and 49 cm-1, respectively. These bands are most probably attributable to two different kinds of Bro¨nsted Si-OHAl sites with different distributions of acid strengths, the most shifted one being the strongest.12 These are also observable in HZMS-5 and USY (Figures 3B and 6A). The width of the CO stretching bands of absorbed acetone and their frequency support the early conclusion by Hair1 that acetone is not a good probe for acid sites. It would be more meaningful to consider a continuum in the distribution of the strengths of electronacceptor sites with which the carbonyl interacts, the spread of stretching frequency being from about 1700 to 1650 cm-1. Lewis and Bro¨nsted sites are included in this broad

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Figure 7. Examples of deconvolution of the CdO stretching band of adsorbed Ac in Gaussian components with parameters indicated in Table 2 and evolution of integrated absorbance of Gaussian lines with increasing doses of acetone introduced into the IR cell: (A, B) USY; (C, D) HM.

Figure 8. Variation of the integrated absorbance of CdO stretching band (1630-1730 cm-1) of absorbed acetone with increasing amounts of acetone introduced into the IR cell: (0) USY; (]) HZSM-5; (*) HM; (O) HY; (º) DHM700; (9) USY10F; ([) alumina.

distribution. However, the electron-acceptor strength of the Lewis sites seems weaker than that of the Bro¨nsted sites. As stated earlier, mesityl oxide is a better probe than acetone for acid sites because of the sensitivity of the conjugated CdC to the interaction of CdO with the electron acceptor. As shown in Figure 9, the relative shifts of the CO and CC vibration are affected to the same extent by this interaction. In addition, the bands are resolved better (Table 3), because they are separated in frequency, as shown by the comparison of Tables 2 and 3. The agreement with the data in ref 3 is also quite good. In Figure 9 the graphic representation of the relative frequency shift, 100(νperturbed - νunperturbed)/νunperturbed, with respect to the nature of the vibrational band and the nature of the absorbent summarizes the data in Tables 2 and 3.

The shifts of the CdO and/or CdC vibration bands scale as the strength of the Bro¨nsted sites, HZSM-5 and HM being the strongest and HY being the weakest. The strength seems to be related, for instance, to the activity of HZSM-5, HM, USY, and HY for the steady-state isomerization of pentane at 285 °C. The activity scales as 100, 185, 13, and 0.85, respectively. If the cracking was considered, the scaling would be 100, 9.5, 3.5, and 0.21.21 The width of the bands gives some information on the distribution of the sites. Table 3 shows that the B2 Bro¨nsted sites have the narrowest distribution. Earlier studies on the low-temperature adsorption of carbon monoxide on acid or acid and dealuminated zeolites had shown two CO stretching bands with frequencies at 2175 ( 2 cm-1 or at 2163 cm-1 in HZSM-5 and USY.24 In dealuminated HY the broad CO stretching encompasses the two bands, while in dealuminated H-mordenite, a broad band was observed at 2170 cm-1, irrespective of the temperature. These observations suggested that carbon monoxide interacts with two kinds of OH,24 and they can be related to the distribution of the bridging Si-OH-Al among three populations, depending upon the number n of next-nearest-neighbor Al atoms (0 < n < 3). Isolated Al sites (n ) 0) are bearing the most acidic OH.25,26 The first and second population (n ) 1) are in cluster Si-1Al while the third one is in cluster (n > 1) either Si-2Al‚‚‚Si4Al. Only the first and second populations are actual Bro¨nsted sites, in the sense that upon exposure to NH3 they transform into NH4. The OHs in cluster (Si-nAl), n > 1, are not Bro¨nsted sites (no proton transfer to NH3) but they can hydrogen bond NH3. Here, we suggest that the B1 band at ∼1650 cm-1 can be assigned to the carbonyl of acetone in a H-bonded complex with the most acidic OH, those designated as the Q1 sites. Q1/Si is easily calculated from the NMR (Si/Al) (24) Gruver, V.; Panov, A.; Fripiat, J. J. Langmuir 1996, 12, 2505. (25) Blumenfeld, A. L.; Coster, D.; Fripiat, J. J. J. Phys. Chem. 1995, 99, 15181. (26) Blumenfeld, A. L.; Fripiat, J. J. Top. Catal. 1997, 4, 119.

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Table 2. Assignment Suggested for the CdO Stretching Vibration Bands of Acetone Adsorbed on Acid Catalysts assignment

band, cm-1

CdO, physisorbed CdO on Lewis sites CdO in acetone interacting with boron atoms in impregnated silicas CdO interacting with bridging OH B1 and B2 (B1 ) Bro¨nsted sites)a CdO on SiOH

1720 1678-1682 1670-1665 1671-1658

3 3 22 3

1710-1715

16, 22, 24

a

this work, cm-1

ref

whh, cm-1

1714-1720 1690-1702

12-15 27

1652-1655 (B1) 1679-1682 (B2) 1700-1705

44 27 22

This is explained later and justified in Figure 7.

Table 3. Comparison between the Parameters Suggested in Reference 3 and Those Obtained in This Work for the CdO and CdC Stretching Vibrations in Adsorbed Mesityl Oxide samplec

frequency CdO (cm-1)

whh CdO (cm-1)

specific absorbance CdO (cm-1/µmol)

frequency CdC (cm-1)

whh CdC (cm-1)

specific absorbance CdC (cm-1/µmol)

B (HM) B (ZSM-5a) B1 (HY) B2 (HY) B (USY) L (alumina) L (USY) Bro¨nsteda in HZMS-5 Lewisb

1617 1617 1631 1659 1632 1673 1676 1618 1650

43 43 24 18 30 21 21 n/a n/a

4.1 ( 0.4 4.1 ( 0.4 1.6 ( 0.1b 1.6 ( 0.1b n/a 1.6 ( 0.1 n/a n/a n/a

1557 1557 1565 1594 1565 1589-1603 1604 1555 1581

71 71 57 31 55 42 42 n/a n/a

10.0 ( 0.5 10.0(O.5 10.0 ( 0.5b 10.0 ( 0.5b n/a 5.2 ( 0.4 n/a n/a n/a

a

Reference 3. b Sum of B1 plus B2. c B: Bro¨nsted sites (B1) or weakly acidic bridging OH, B2. L: Lewis sites.

Figure 9. Relative frequency shifts observed for the CdO stretching band of Ac on Bro¨nsted sites or of the CdO and CdC stretching bands of MO on Lewis sites (L) or of CdO and CdC stretching bands of MO on Bro¨nsted sites, indicated as CdO, CdC.

ratio R.14

Q1/Si ) (1 - R-1)3R-1

(1)

The number of Bro¨nsted sites in the studied samples, obtained from IR absorption (Brt), NH4+ deformation band, are in good agreement with Q1. The linear regression between the number of acetone in the B1 band and Brt is excellent, but the slope is 0.4 instead of 1. Such a factor exceeds the experimental error, which is estimated to be about 10%. This could be explained, as suggested in similar circumstances,21 by the reduced availability of the Bro¨nsted sites to the reagent compared to their availability to NH3. As shown in Figure 10, the intensity of the B1 band correlates well with the number of Bro¨nsted sites

(from NH3 absorption) or with the number of Q1 sites, corrected by the ratio (volume acetone, mL, absorbed at saturation at room temperature)/(volume of N2, mL, adsorbed at saturation at -196 °C). This ratio, shown in Table 1, can be considered as an approximate measurement of the sites’ availability. Similarly, the sum of the acidic and nonacidic OH corrected by the same factor correlates well with the sum of the intensity of the B1 and B2 bands. This is the same as saying that the number of framework aluminum correlates with (B1 + B2), which is evident! Thus, B2 correlates with the number of nonacidic OH or with (FAl-Q1). This correlation supports the distribution of the lattice OH groups among two populations. This distribution was first suggested by the NMR study of the 29Si rotational echo double-resonance.25,26 Such

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Figure 10. (A) Correlation between the amount of acetone on B1 sites and the number of Bro¨nsted sites measured by FTIR of absorbed ammonia (B NH4), or the number of Q1 sites calculated from eq 1. (B) Correlation between (B1 + B2) integrated area and the total amount of framework aluminum (or total number of bridging OH).

an agreement between two different techniques, one including the deconvolution of infrared bands, is worth emphasizing. The CdO and CdC band assignments of absorbed MO shown in Table 3 are in good agreement with those in ref 3 (see two bottom rows), which were obtained for Bro¨nsted sites in HZSM-5. The bands assigned to Lewis sites in ref 3 were obtained on a calcined HZSM-5 and their frequencies are 24 cm-1 lower than those we have obtained on amorphous or on nonframework aluminas in USY. Two bands are observed for CO and CdC of MO on Bro¨nsted sites in HY. As mentioned earlier and displayed in Figure 9 the relative low-frequency shift of the CdC and CdO band on Bro¨nsted sites reaches 5% in HZMS-5 and HM, while it is less than 3% on HY on what we suggest calling the B2 sites, by analogy with the acetone carbonyl assignment. On B1 the shift is the same as that on the Bro¨nsted sites in USY. Contrary to what is reported for acetone in Table 3, it is only on HY that two MO CdO bands are observed. Note that the frequency difference is 28 cm-1, for example, about the same as that observed with acetone (27 cm-1) (Table 3). With MO as a probe, the CdO and CdC stretching are shifted more in HZSM-5 or in HM than in USY and HY. An interesting question regarding mesityl oxide is about the nature of the bond interacting with either the Bro¨nsted or Lewis sites. The π electrons as well as the carbonyl are candidates. The fact that the stretching CO and CdC band shifts are the same suggests that conjugation in delocalizing the electron makes the two possibilities equivalent. Unfortunately, it was impossible to calculate the intensity of the stretching CdO or CdC bands at saturation and to estimate the correction for the site availability for MO. The CdC stretching vibration is the most intense of those observed for the mesityl oxide and it will be used to quantify the kinetics of the acetone condensation in a forthcoming paper.24 Finally, we show in Figure 11C the reconstruction of the spectrum of mesityl oxide adsorbed on USY. Note that Figure 11C does not result from the deconvolution of the experimental spectrum (symbols). It was obtained by weighing the contributions of MO absorbed on Lewis sites (Figure 11A) or of MO absorbed on Bro¨nsted sites (Figure 11B) on alumina and HM, respectively, according to the population of these sites in USY. Considering that we deal with three different solids, the agreement is quite satisfactory, suggesting that the spectral parameters

Figure 11. Example of deconvolution of the CdC and CdO stretching bands of mesityl oxide absorbed on alumina (A), HM (B), and USY (C). The spectrum in C (solid line) is a reconstructed spectrum obtained by weighing the components 1 and 2 of spectrum A and 3 and 4 of spectrum B according to the number of Bro¨nsted and Lewis sites. The circles represent the experimental IR spectrum.

(frequency, specific absorbance, whh) reported in Table 3 are acceptable approximations. In addition, the assignment to the nature of the adsorption sites must also be correct. In conclusion, as for probing the surface acidity, acetone and mesityl oxide offer distinct advantages and inconveniences. The distinction between Bro¨nsted and Lewis sites is better achieved by MO, because the CdO and CdC frequencies are better separated. The magnitude of the relative frequency shifts for these vibrations scales as the activity or acid-catalyzed reaction (n-pentane isomerization). The quantitative aspect is achieved better when acetone is used, because the higher vapor pressure and smaller molecular dimensions allow the saturation of the available sites with acetone to be reached more easily. Satisfactory quantitative agreements with the number of Bro¨nsted sites have been achieved with acetone. Similar agreements probably exist for Lewis sites, but for the latter, mesityl oxide is preferred. The band characteristic

3796 Langmuir, Vol. 14, No. 14, 1998

of acetone on Lewis sites is in a region where it may overlap with other spectral features (Table 1). This is not the case for the CdC vibration of MO on Lewis sites. Conclusions The system acetone (Ac)-mesityl oxide (MO) adsorbed on acid catalysts is rather unique, since the reagent (Ac) and the reaction product (MO) are qualitatively and quantitatively detectable by IR spectroscopy and their interactions with Bro¨nsted and Lewis acid sites are distinguishable. The band assignments suggested in this work differ from those published earlier. In particular, the CO stretch

Panov and Fripiat

frequency of acetone interacting with weakly acidic (B2) OH has been confused, according to us, with the band assigned to CO interacting with Lewis sites. As far as mesityl oxide is concerned the discrepancies are less pronounced, probably because the bands are better separated. The kinetics of the acetone condensation has been studied, and it will be published elsewhere.27 Acknowledgment. This work has been made possible by DOE Grant DOE-FG02-90 ER1430. LA971359C (27) Panov, A. G.; Fripiat, J. J. J. Catal., in press.