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(12) Mitchell, P. G.; Sneddon, J.; Radziemski, L. J. Appl. Spectrosc. 1987, 4 1 , 141. (13) Yudelevich, I. G.; Cherevko, A. S.; Engelsht. V. S.;Pikaiov, V. V.; Tagiltsev. A. P.; Zheenbajev, Zh. Spectrochlm. Acta 1984, 398, 777. (14) Derie, R. Anal. Chim. Acta 1984, 166, 61. (15) Slinkman. D.; Sacks, R. Appl. Spectrosc. 1990, 4 4 , 76. (16) Slinkman, D.; Sacks, R. Appl. Spectrosc. 1990, 4 4 , 83. (17) Slinkman, D.; Sacks, R. Anal. Chem. 1990, 62, 1656. (18) Rettberg, T. M.; Holcombe, J. A. Anal. Chem. 1986, 58, 1462. (19) Langmyhr. F. J. Anayst 1979, 104, 993. (20) L'vov, B. V. Talanta 1976, 2 3 , 109. (21) Vollkopf, U.; Grobenski, 2 . ; Tamm, R.; Welz, 6. Anahst 1985, 710, 573. (22) Chakrabarti, C. L.; Karwowska, R.; Hollobone. B. R.; Johnson, P. M. Spectrochim. Acta 1987, 428, 1217.
(23) Bhkemore, W. M.; Casey, P. H.; Collie, W. R. Anal. Chem. 1984, 56, 1376. (24) Aziz, A.; Broekaert. J. A. C.: Leis, F. Spectrochim. Acta 1982, 3 7 8 , 369. (25) Reisch, M.; Nickel, H.; Mazurkiewicz, M. Spectrochim. Acta 1989, 4 4 8 , 307. (26) Greene, B.; Mitchell, P. G.;Sneddon, J. Spectrosc. Lett. 1986, 19 (2), 101. (27) Brewer, S. W., Jr.; Sacks, R. D. Anal. Chem. 1988, 6 0 , 1769 (28) Slinkman, D.; Sacks, R. Appl. Spectrosc., in press. (29) Slinkman, D.; Sacks, R. Appl. Spectrosc., in press.
RECEIVED for review August 9, 1990. Accepted November 5, 1990.
Quantitation of Acidic Sites in Faujasitic Zeolites by Resonance Raman Spectroscopy Robert D. Place' and Prabir K. Dutta* Department of Chemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, Ohio 43210
This paper examines the selective excitation of the Raman spectra of dye molecules adsorbed on acidic zeolite surfaces. By taking advantage of the strongly allowed transitions In these dye molecules (large extinction coefficients) and the different absorption maxima of the conjugate acid and base forms of the dye, selective enhancements of the Raman bands specific to each form can be obtained. The focus has been on the dye molecule, 4 4 phenyiazo)diphenyiamine (PDA), adsorbed onto the faujasitic zeolite, Nay. A calibration curve of Raman Intensity (peak area) versus number of protons In supercages was obtained. Because of the inner finer effect, at loadings significantly greater than 1 proton per supercage, the Raman intensity was found to decrease. The sensitivity of the Raman method at low proton loadings appears to be considerably better than the typical infrared methods used to estimate acidity on catalyst surfaces.
INTRODUCTION The acidic properties of zeolites play a central role in their catalytic behavior in a wide variety of chemical- and petroleum-related processes ( I , 2). Considerable research has been done in developing methods for measurement of distribution of acidic functionalities in these and other solid materials. Amongst the classical methods for measuring acidic properties of solid acid surfaces are color changes of indicators adsorbed on surfaces and butylamine titration of the surface in the presence of Hammett indicators (3,4). Infrared spectroscopy of basic molecules such as ammonia and pyridine adsorbed onto the acidic sites also provides for quantitative estimation of Bronsted and Lewis acid sites (5). The attractive feature of the Hammett indicator method is that the distribution of acid strengths in zeolites can be studied by choosing indicators of various pKa's. The cautionary aspect of this method is to choose dyes that can penetrate into the zeolite supercages through the 12-membered-ringopenings (-7-8 A). However, there are several shortcomings of the Hammett indicator approach. First, in order for the dyes to be successfully used, I Permanent address: Department of Chemistry, Otterbein College, Westerville, OH 43081.
there must be a perceptible change in color, implying that the spectral shift between the acid and the base forms be significant. This is especially a problem with lower pKa (el) indicators. In addition, there is often a change in color due to adsorption effects alone (6-8). These problems can be circumvented to some degree by obtaining the electronic spectra of the solid (9,lO). However, because of significant band broadening on the solid surfaces, often there is overlap in the electronic spectra between the acid and basic forms. New electronic bands may also appear in the spectrum, e.g., 4-nitrotoluene exhibits only one band at 380 nm in acidic solutions, but three bands were reported a t 300,340,and 500 nm on an acidic aluminosilicate surface (11). We reasoned that it may be possible to use the resonance Raman effect in order to distinguish between the acid and base forms, as well as to quantitate the amount of acidic form (and hence the acidity of the solid surface) from the Raman signal. In this paper, we illustrate this principle by using the dye molecule 4-(phenylazo)diphenylamine(PDA, see below) and various loadings of protons for the acidic form of zeolite Y.
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Resonance Raman spectroscopy involves excitation into an electronic band of the chromophore, which can result in enhancement of the Raman signal by orders of magnitude. Since the Raman spectrum is a vibrational signature of the chromophore and the Raman bandwidths are typically 10 cm-I, this technique provides the potential for high selectivity and sensitivity (12). The latter can be particularly high in the case of dye molecules, since the extinction coefficient, or absorptivity, of these molecules is large and the resonance Raman intensity scales as the square of this parameter (transition moment terms in the numerator for the dipole-allowed transition in the A term) (13). In principle, this makes it possible to examine zeolite materials with low levels of acidity. In addition, since laser excitation is readily available in a continuously tunable fashion ranging from the near ultraviolet to the visible, the choice of dye molecules is also clearly no longer restricted (14). We illustrate below some of these features of resonance Raman spectroscopy using the dye
0003-2700/91/0363-0348$02.50/0 0 1991 American Chemical Society
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molecule PDA and zeolite Y. The choice of this dye molecule was dictated by several factors: these include the size of this molecule ( - 7 X 24 A), which allows it to penetrate into the zeolite cages, and also the fact that its acidic and basic forms have well-separated absorption maxima a t 540 and 410 nm, respectively (IO), and are therefore readily accessible with Ar and Kr ion lasers. The focus in this preliminary paper is primarily to illustrate that resonance Raman spectroscopy can be developed as a quantitative technique to examine acidic sites on solid surfaces in general. EXPERIMENTAL SECTION Zeolite Y was obtained from Union Carbide (LZY-52) and was ion-exchanged with 1 M NaCl to ensure that all cationic sites were occupied by Na+ ions and that excess OH- ions were removed from the sample. These samples were then ion-exchanged with NH,Cl to provide loadings of 0.61, 0.95, 1.3, 1.7, 3.5, and 6.4 protons per supercage as validated by analysis for Na+ ions in the wash solutions using atomic emission spectroscopy. The choice of these loadings was made on the basis of the sensitivity of the pyridine IR method and is discussed in the Results and Discussion section. All other chemicals were used as received, including the from dye, 4-(pheny1azo)diphenylamine(C6H5N2C6H4NHC6H5) Aldrich. Each of the different NaNH4Y samples was pressed into a 12-mm (125-mg)pellet at 3000 psi and was heated to 500 OC over a 2-h period. The sample was maintained at this temperature for 2.5 h under vacuum of lo4 Torr in order to drive off NH3 and replace each NH4+ with H+ and generate the various proton loadings of NaHY. After 2 h at 500 "C, 1 atm of oxygen gas was added for 15 min to oxidize and vaporize impurities that were then evacuated. The sample treatment necessary to obtain Raman spectra from zeolites has been described in the literature (15). After the samples were cooled quickly to room temperature, pure ethanol was distilled onto and covered the pellet. The sample was allowed to equilibrate at room temperature overnight under 1 atm of nitrogen. This was done to ensure that ethanol was present through the zeolite, thus facilitating the movement of the dye. After this step, the zeolite sample was always maintained in a drybox or a sealed container until after the Raman and electronic spectra were recorded. Each zeolite sample was pulverized under ethanol, and then 5 mL of 7.1 x IO4 M solution of the indicator in ethanol was added for 1.5 h. The excess ethanol was then evaporated off, and as the last traces of ethanol were removed, the yellow zeolite samples turned to the final purple intensity in less than 1 min. The zeolite was then reground, pressed into a pellet, cut in half, and clamped into a Raman cell along with half a pellet of KNO, of the same thickness, which acted as an external standard. The Raman cell was rotated at 30 rev/s while the spectrum was collected, thus ensuring that the laser sampled each half of the pellet about 50% f 2% of the time. A number of scans were recorded with the laser focused on different positions of each pellet and then summed to produce the quantitative spectra. The Raman spectra were collected by excitation with 514.5-nm radiation from an Ar ion laser (Spectra Physics 171) or 406.7 nm radiation from a Kr ion laser (Coherent Innova K100). The scattered light was filtered through a Spex 1403 double monochromator and detected with an RCA C31034 GaAs PMT. The diffuse reflectance electronic spectra were recorded on a Shimadzu (Model 265) spectrometer. The integrity of the zeolite sample throughout the experimental procedure was verified by powder diffraction patterns with a Rigaku D/Max-B diffractometer. The infrared spectra were obtained on self-supporting 20-mg wafers in a transmission anaerobic cell on a Perkin-Elmer 1600 FTIR spectrometer. RESULTS AND DISCUSSION Figure 1 shows the resonance Raman spectrum of the dye molecule in the free base (2% KBr pellet) and protonated forms (acidified with 1 M H,S04 in methanol). Figure 2 shows the electronic spectra of these species and includes the excitation laser frequencies used to produce the resonance Raman spectra. Comparison of Figures 1 and 2 confirms clearly that the Raman spectra of the base and acid forms are distinct.
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Figure 1. Resonance Raman spectra of PDA (A) in its acidic form (acidified methanol, excitation 514.5 nm) and (B) as a free base (2% KBr pellet, excitation 406.7 nm).
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Figure 2. Electronic spectra of (A) acidic and (B) basic forms of the dye. (The Raman excitation lines are also shown in the figure.) This is a reflection of the fact that the electronic structure of the molecule undergoes considerable changes upon protonation. In the basic form of the molecule, the electronic band a t 410 nm has been assigned to a charge-transfer transition from the amino group to the azo group (16). Raman and IR data for a variety of azo compounds have been reported, along with band assignments (17). Two of the prominent bands a t 1143 and 1431 cm-' in Figure 1B (base form) are assigned to C-N and N=N stretches, respectively. The other bands a t 1189, 1311, 1469, and 1600 cm-' have been assigned to the phenyl ring modes. Considering the chargetransfer nature of this electronic band, it is not surprising that the most strongly enhanced Raman band is the N=N stretching of the azo group. Upon protonation, both the electronic and resonance Raman spectra exhibit considerable changes. There are two sites of protonation on this molecule, the azo group and the secondary amine. Primarily betause of the long wavelength shift of the electronic band and the increase in absorptivity, the azo group has been thought t o be more likely the center for protonation since resonance structures, such as drawn below, delocalize the positive charge (18). D
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Resonance structures are not possible if the amino group is protonated. The disappearance of the N=N stretching band in the acid spectrum a t 1434 cm-' also supports this site for protonation. The new set of bands observed in the Raman spectra are also more appropriate for the charge-delocalized structure shown above. The band at 1632 cm-' is typical for a C=N stretch, and the most prominent band at 1285 cm-' is characteristic of quinone-like structures (19). The only major band that appears to remain close to the same frequency in both the acid and base forms is the band at 1596 cm-' and is characteristic of monosubstituted benzene. It is quite clear from Figure 1 that, by selective choice of excitation wavelength, distinct resonance Raman spectra of acid and base forms can be readily observed. In order to illustrate the selectivity of the Raman spectra on the zeolite surface, we treated a sample of zeolite HY with excess indicator. Figure 3 shows the resonance Raman spectra obtained at excitation wavelengths of 406.7 and 514.5 nm, in resonance with the basic and acidic forms of the indicator molecule, respectively. Comparison with Figure 1will confirm that we are indeed observing separately both the basic and acidic forms of the molecule on the same zeolite sample simply by virture of choice of the excitation wavelength. Neither spectrum contains the Raman features assigned to the other form. Shifts of up to -12 cm-' are observed in some of the Raman bands upon interaction with the zeolite surface as compared to spectra in Figure 1 of the dye in solution or on the solid KBr. The final part of this study concentrated on the ability to correlate the Raman intensities of the protonated form of the dye on NaHY to the amounts of acidic groups introduced into the zeolite. These acidic groups were introduced by partial exchange of Na+ ions by NH,+ followed by calcination to generate NaHY. In order to exploit the sensitivity of resonance Raman spectroscopy, the loading levels were chosen to be in the range of 1 proton per supercage. At these loadings, the signals for the pyridinium ion in the conventional pyridine IR method is low. Figure 4 compares with the IR spectra for pyridine on completely exchanged HY and NaHY partially exchanged with a loading of 0.95 proton per supercage. The band at 1545 cm-' due to protonated pyridine and indicative of Bronsted acidity is considerably weaker in the NaHY sample because of the low loading of protons (20). As a matter of fact, the NaHY intensity at 1545 cm-' is the same as that of a totally unprotonated sample. Since it is unlikely that more than one dye molecule can fit into a supercage, it was appropriate to keep the loading
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Figure 4. FTIR spectra of pyridine absorbed on (A) HY and (B) NaHY (0.95 proton per supercage).
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Figure 5. Diffuse reflectance electronic spectra of PDA adsorbed on NaHY. The proton loadings were (A) 0, (B) 0.61, (C) 1.7, and (D) 6.4 protons per supercage.
levels around 1 H+ per supercage. However, the observations during the reaction of the dye with acidified zeolites indicate that the protons may have considerable mobility (21). For example, when ethanolic solutions of the dye are in contact with the NaHY sample, the zeolite retains the yellow color characteristic of the basic form of the dye. Only after all the ethanol is removed from the sample does the blue color of the protonated dye develop. This clearly indicates that the acidic sites generated at these low loadings (1H+ per supercage) are strong enough to protonate C2H50H(pK = 2.3). The fact that the ethanol was indeed protonated was confirmed by replacing ethanol with benzene as solvent, in which case the purple color of the protonated dye was observed immediately after contact with NaHY. With benzene as a solvent, irregular intensities were observed for Raman bands, and that was the motivation for changing the solvent to ethanol. The reason for this is unclear but could relate to the difficult transport of the polar dye molecule in the strongly polar environment of the zeolite by the hydrophobic solvent, benzene. The protonation of ethanol and its subsequent migration through the zeolite system appears to result in more uniform reaction with the
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Figure 6. Resonance Raman spectra of PDA on NaHY. The spectra have ail been normalized to the KNO, peak at 1049 cm-'. The proton loadings are (A) 0.61, (B) 0.95, (C) 1.3, (D) 1.7, (E) 3.5, and (F) 6.4 protons per supercage. i i r
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ACKNOWLEDGMENT We thank Wayne Turbeville for his helpful assistance and ideas in the data collection and analysis. R. Place also thanks Otterbein College for a sabbatical leave which allowed time for this work to be completed. LITERATURE CITED
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external standard, and all of the spectra have been normalized to this peak. What is apparent from the Raman intensities of the acidic form is that they initially increase with the number of acidic sites in the zeolite. The integrated areas under the strongest Raman band (1281 cm-') as a function of the amount of acidic groups are plotted in Figure 7. The other bands show similar trends. Even though the electronic spectrum shows an increase in the intensity of the 540-nm band due to the acidic form of the dye, the decrease in Raman intensity beyond 1.3 protons per supercage arises from the self-absorption of the Raman scattered photon by the higher surface concentration of absorbing dye molecules. Quantitation by addition of an internal standard would alleviate this problem (22). However, since an activated zeolite is very reactive, the complications of interactions with the internal standard would have to be taken into consideration. We are presently examining such systems. However, the data in Figures 6 and 7 show that it is indeed possible to obtain selective resonance Raman spectra by choosing the proper excitation wavelength and use Raman intensities to construct calibration curves. Presently, efforts are underway to study dyes with lower pK,'s that exhibit significant overlap in the electronic spectrum before and after protonation, with particular emphasis on benzalacetophenone (pK, -5.6).
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Figure 7. Plot of the integrated Raman peak area intensities of the 128 1-cm-' band versus proton loading. dye molecules than in the case of benzene. Figure 5 shows the diffuse reflectance spectra of the dyezeolite system for NaHY with various degrees of protonation. In the absence of protons on the zeolite, the electronic spectrum resembles that of the free base form (Figure 2B), except that the 410-nm peak is considerably broadened and shifted slightly. With increassing Hf concentration on the zeolite, the band at 540 nm due to the acidic form is observed and increases with increasing H+ loading on NaHY. Figure 6 shows the resonance Raman spectra at various loadings. The peak at 1049 cm-' is due to the NO3- ion, which is used as the
(8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
Olson, D. H.; Haag, W. 0.; Lago, R. M. J . Catal. 1980, 67, 390. Ward, J. W. Appl. Ind. Catal. 1984, 3 , 271. Benesi, H. A. J . fhys. Chem. 1957, 61, 970. Beaumont, R.: Barthomeuf, D. J . Catal. 1972, 27, 45. Ward, J. W. J . Catal. 1968, 70, 34. Deeba, M.; Hall, W. K. J . Catal. 1979, 60.417. Deeba, A. K.; Curthoys, G. J . Chem. Soc.. Faraday Trans. 1 1983, 29, 147. Kladnig, W. F. J . F'hys. Chem. 1979, 83, 765. Drushei, H. V.; Sommers, A. L. Anal. Chem. 1968, 38, 1723. Anderson, M. W.; Klinowski, J. Zeolites 1986. 6 , 150. Tape, J.; Tsuruya, T.; Sato, T.; Yoneda, Y. Bull. Chem. SOC. Jpn. 1972, 45, 3409. Long, D. A. Raman Spectroscopy; McGraw-Hill: New York, 1977. Albrecht, A. C.; Hutley, M. C. J . Chem. fhys. 1971, 55, 4438. Asher, S. A.; Johnson, C. R.; Murtaugh, J. Rev. Sci. Instrum. 1983, 5 4 , 1657. Zaykoski, R.; Dutta, P. K. Zeolites 1968, 8 , 179. Griffiths, J.; Rwspeikar, 6. J . Chem. SOC.,ferkin Trans. 7 1976, 42. Hacker, H. Spectrochim. Acta 1965, 2 1 , 1989. Liler, M. Adv. fhys. Org. Chem. 1975, 7 1 , 308. Lopex-Garriga, J. J.; Babcock, G. T.; Harrison, J. F. J . Am. Chem. SOC. 1986, 708, 7261. Liengne, B. V.; Hall, W. K. Trans. Faraday SOC. 1967, 62, 3232. Lohse, U.; Stach, J.; Thamm, H.; Schirmer, W.; Isirikjan, A. A,; Regent, N. I.; Dubinin, M. M. i'. Anorg. Allg. Chem. 1980, 460, 179. Shriver, D. F.; Dunn, J. B. R. Appl. Spectrosc. 1974, 28, 319.
RECEIVED for review August 21,1990. Accepted November 16, 1990.
