J. Phys. Chem. 1995, 99, 14764-14770
14764
Pyridine Adsorbed on Na-Faujasite. 2. An FT-Raman and DRIFT Spectroscopic Study R. Ferwerda* and J. H. van der Maas Department of Analytical Molecular Spectrometry, Utrecht University, P.O. Box 80.083, 3508 TB Utrecht, The Netherlands Received: January 4, 1995; In Final Form: May 9, 1 9 9 9
To unravel the nature of the interactions of pyridine with nonacidic Na-faujasites, FT-Raman and DRIFT spectroscopies have been applied. The complementary character of these techniques provides further evidence for the location of pyridine adsorbed in the supercages. It appears that the preferential adsorption sites for pyridine change upon varying the loading of the zeolite. The occupied site of pyridine at low coverage (less than two molecules per supercage) is either in the 12-ring window or through an interaction with both the lone pair and the n-system of the aromatic. At medium coverage pyridine is adsorbed with its lone pair bonded to one of the four charge-balancing cations present in the supercage, possibly in a tetrahedral conformation. Adsorption of more pyridine molecules results in an enhanced perturbation of the "breathing" vibrations, shifting them to higher frequency.
TABLE 1: Activation and Method of Measurement for Different Na-Faujasites
Introduction Recently, we reported on the interaction of pyridine with Nafaujasites as determined with FT-Raman spectroscopy.' Since fluorescence, which is a major nuisance in Raman studies of zeolites when using a visible laser for excitation, appeared to be less of a problem with near-infrared radiation, the changes on varying the coverage could easily be monitored. It appeared that the Raman spectra of pyridine adsorbed on Na-Y and Na-X reveal apparent shifts of the bands in the ring-breathing region ( V I and ~ 1 2 upon ) evacuation. These have been attributed to changes in adsorption sites caused by traces of water. Owing to instrumental limitations, the zeolites were saturated with pyridine at room temperature, followed by evacuation at progressively higher temperatures in order to desorb aliquots of pyridine. Furthermore, pretreatment was limited to evacuation ovemight at 120 "C due to fluorescence problems? In this paper, a sequel study is presented in which the pyridine coverage of the zeolites was manipulated with a flow system, which facilitates pretreatment at higher temperatures. These studies enable us to further discuss the FT-Raman spectra and compare them with the diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy counterparts, which may provide a better insight into the nature of the association of pyridine with nonacidic faujasites. Experimental Section Materials. Three Na-faujasites, supplied by different manufacturers, were studied, viz., Nags-X (Union Carbide), Na56-Y (Crosfield Catalysts), and Na51-Y (AKZO). No special "cleaning" treatment was performed, and the catalysts were measured directly after in situ activation (Table 1). DRIFTS. Infrared spectra were recorded on a Perkin Elmer 1760 X FT-R instrument with a DTGS detector, equipped with an environmental diffuse reflectance infrared accessory (SpectraTech, Model 0030-100). The cell operates from room temperature up to 400 "C (temperature of catalyst bed; depth of 1 mm). Self-pressedKBr windows were used in the catalytic chamber. The energy throughput of the cell, relative to the open ~~
* Author
to whom correspondence should be addressed. Resent ad-
dress: AT1 Unicam, York Street, Cambridge CB1 2PX, United Kingdom. Abstract published in Advance ACS Abstracts, September 1, 1995. @
activation
T(T)
time
DesIAds
technique
flowlvac
120
ovemight ovemight ovemight ovemight 1h lh ovemight ovemight ovemight
D D A A D D D
Raman Raman Ra" DRIFT Raman DRIFT Raman Raman DRIFT
vacuum flow flow flow flow flow vacuum flow flow
150 200 200
470 400
120 200 200
A A
Diluted with germanium.
beam and measured with an alignment mirror at the sample position, was about 15%. This setup was used as background for the infrared measurements. Spectra were acquired by the accumulation of 128 scans recorded at a resolution of 2 cm-l (measurement time 18 min). The experiments on Na56-Y and Nags-X, both pretreated at 200 "C, were performed on the pure zeolite powders. NaslY, activated at 400 "C, was diluted with 50 wt % germanium powder (Highways International, 5 N) to decrease the specular reflectance component of the Si(A1)04 stretching vibrations, which would otherwise obscure the pyridine ring vibrations in the 1700- 1400 cm-I region. The amount of pyridine adsorbed on germanium was negligible. All infrared spectra were recorded at room temperature. The zeolites were heated (rate, 50 "CIS min, stepwise) in a 10% OdAr flow of 50 " i n (Hoekloos, purity, 5.0 N, passed through a "hydrosorb" trap before use) and activated for a fixed time (Table 1). After pretreatment the cell was cooled to room temperature in a stationary atmosphere and a spectrum was recorded. Adsorption experiments with pyridine (Aldrich 27,097-0, 99%) were performed by exposing the activated zeolite to a pyridine/Ar flow, using a simple saturator operating at 21 "C, at 300 "C for 5 min, followed by exposure to an Ar flow at the same temperature and cooling to room temperature in a static atmosphere before recording a spectrum. Next, this procedure was repeated at 250 "C. The adsorption temperature was continually lowered, and the experiment was
0022-3654/95/2099-14764$09.00/0 0 1995 American Chemical Society
Pyridine Adsorbed on Na-Faujasite
J. Phys. Chem., Vol. 99,No. 40,1995 14765
1
-
A
I10
Na-Y, 400 "C I
J
I
I
I
I
!
1700
I
1
1
t
I
,
l a
I
I
,
lux)
Wavenumber (cm-1)
,
,
,
,
1400
Figure 2. Ring-stretching region of the DRIFT spectra, after adsorption of pyridine onto Na-Y, activated at 200 "C, at the indicated temperature
("C) (KM = Kubelka-Munk units).
completed with pyridine adsorption at room temperature. The heating rate was 50 "C/3 min in a static atmosphere. For the desorption experiments, pyridine was allowed to adsorb onto the activated zeolite at room temperature for 5 min. Adsorption was followed by purging with Ar for 5 min in order to remove excess pyridine vapor, and a spectrum was recorded at room temperature. Subsequently, pyridine was desorbed by flowing Ar over the surface for 5 min at increasingly higher temperatures up to 350 "C. Pretreatment and pyridine experiments are summarized in Table 1. FT-Raman Spectroscopy. The vacuum experiments were performed during a previous study and are described in detail in Ferwerda et al.' Raman spectra throughout the flow experiments were recorded on a Raman module connected to a Perkin Elmer 1760 X FT-IR spectrometer. The Nd:YAG laser (excitation wavelength 1.064 pm, Spectron Physics) excited the sample with 310 mW of power. The InGaAs detector was operated at liquid nitrogen temperature to increase the signalto-noise ratio. Spectra were accumulated by co-adding 64 scans recorded at a resolution of 2 cm-' (measurement time 18 min). The sample was placed into a flow cell (sample bed of 5 mm) which was carefully inserted into a cell holder to increase repr~ducibility.*-~ Zeolites were pretreated at different temperatures (Table 1) in an OdAr flow (50 " i n ) . The heating rate was 50 "C/5 min stepwise. Adsorption and desorption were performed identically as described in the DRIFTS experiments, apart from the exposure time to either pyridine or Ar. Adsorption of pyridine was allowed for 20 min and exposure to Ar for 10 min. In order to investigate the influence of steric hindrance of the probe molecule, adsorption of 4-picoline (Cmethylpyridine, Aldrich 23,961-5, 99%) onto Na56-Y, activated at 150 "c, was investigated and compared with pyridine adsorption. Results DRIFTS. Figure 1 shows the OH stretching region of the faujasites after different pretreatment temperatures. It appears that following activation at 200 "C overnight, OH groups are still present at the surface. The band at 3695 cm-' and the broad band centered around 3400 cm-' are assigned to water coordinated to a sodium ~ a t i o n . ~The , ~ negative absorption around 3300 cm-I is attributed to Fermi resonance of the overtone of the bending mode of water with its stretching m ~ d e . ~The . ~ band around 3630 cm-I is ascribed to an OH group strongly influenced by the cations present in the lattice.6 Activation at 400 "C results in the disappearance of all structural
A
1700
1600
1m
1400
Wavenumber (cm-1)
Figure 3. Ring-stretching region of the DRIFT spectra, after adsorption of pyridine onto Na-X, activated at 200 "C, at the indicated temperature ("C) (KM = Kubelka-Munk units).
OH groups, except for a small absorption at 3740 cm-' assigned to silanol groups. Hydroxyl groups are necessary to terminate the faces of a zeolite crystal at positions where bonding would normally occur with adjacent tetrahedral aluminum or silicon ions. The spectra of the OH region are described extensively in Ward5g6and will not be discussed here. Adsorption of pyridine onto the zeolites activated at 200 "C leads to the spectra in the 1700-1400 cm-' region shown in Figures 2 and 3. The spectra are corrected for catalyst background after pretreatment. The spectra of pyridine on the Na-Y zeolite are characterized by bands at 1614, 1595, 1575, 1490, and 1443 cm-I. All bands shift slightly upon adsorption at different temperatures. The spectra of the Na-X zeolite show bands at 1606, 1591, 1573, 1490, and 1442 cm-I, and again small shifts are observed. The negative absorption at about 1645 cm-' observed in all spectra is attributed to the bending mode of residual water, its frequency being changed upon adsorption of pyridine. The other bands are assigned to pyridine interacting with an electron acceptor of moderate strength? Despite some minor variation in overall intensity, no dramatic differences are observed between adsorption and desorption experiments. Test experiments showed that identical results are obtained for the two Y zeolites, Na51-Y and Na56-Y. The infrared spectrum of pyridine adsorbed at room temperature onto the Na51-Y zeolite activated at 400 "C (Figure 4) reveals bands at the same positions as in Figure 2, although no negative absorption at 1645 cm-' is noticed. Upon desorption, the same shifts are once more observed.
Ferwerda and van der Maas
14766 J. Phys. Chem., Vol. 99, No. 40, 1995
1700
,
1600
,
!
1500
I
,
,
!
1400
%'avcnumbcr (cm-1)
Figure 4. Ring-stretching region of the DRIFT spectra after desorption of pyridine on Na-Y, activated at 400 "C, at the indicated temperature ("C) (KM = Kubelka-Munk units).
J
,
O
I
I
/
1
1050
I
I
,
1
I
1
1
1
lo00
Raman Shift (cm-1) Figure 6. Ring-breathing region of Raman spectra after adsorption of pyridine onto Na-X, activated at 200 "C, at the indicated temperature ("(3. TABLE 2: Calculated Coverages of Pyridine Adsorption/ Desorption onto Nonacidic Faujasites (moYsc = Molecules of Pyridine per Supercage). The Amounts Mentioned in the Column 9 Were Determined by Comparison of the Spectra Measured with the Flow Technique and the Ones Measured during the Vacuum Experiments. See Text for the Explanation of Wet and Dry temp @wet @dry sample i j (cm-') ("C) 0 (mglg) (moVsc) (moVsc) Na-Y
I
1
1050
1
1
1
1
l
I
l
I
J
I
I
I
l
l
loo0
Raman Shift (cm-1) Figure 5. Ring-breathing region of Raman spectra after adsorption of pyridine onto Na-Y, activated at 200 "C, at the indicated temperature
("0 FT-Raman Spectroscopy. The FT-Raman spectra of pyridine adsorbed onto Na-faujasites are characterized by bands at about 1035 and 1000 cm-' (Figures 5 and 6 ) , which are assigned to pyridine associated with a moderate electron pair a c ~ e p t o r .The ~ band maxima of both bands shift with varying pyridine coverage, the 1000 cm-' band substantially more than the one at 1035 cm-I. Adsorption of pyridine onto Na-Y at higher temperatures ('200 "C) results in two bands at about 1004 and 999 cm-I. Upon lowering the adsorption temperature to 200 "C, the latter band shifts to 998 cm-I, while the former simply decreases in intensity. Adsorption at even lower temperatures leads to a blue shift to 1001 cm-I. In the Raman spectra of pyridine on the Na-X zeolite, a band at 1001 cm-' is observed at low coverage. On reducing the adsorption temperature, the band shifts to 998 cm-I, and at even lower temperatures, a shoulder at about 1003 cm-' appears. Despite some minor differences in intensity, the same trends are observed in the pyridine spectra for adsorption and desorption experiments as well as for different pretreatment temperatures. Although the accuracy (ie.,exact position of the peaks) depends on both the resolution and the calibration, the precision (and thus the observed shifts) of Fourier transform infrared measurements is very high which is due to the Connes's advantage. In Fourier transform Raman spectroscopy, band positions are also
300- 180 180-100 100-15
8.2-5.0 5.0-2.8 2.8-0.4
6.1-3.6 3.6-2.0 2.0-0.3
isotherm experiments2
280-230
1.7-6.3
5.1-4.6
Na-X
260- 175 175-90 90-50
7.5-5.0 5.0-2.6 2.6-1.4
5.5-3.7 3.7-1.9 1.9-1.1
260-210
7.5-6.0
5.5-4.5
1001 998 1004 1003 998 1001
RT-100 150-200 250-300 RT-100 150-200 250-300
isotherm experiments*
TABLE 3: Different Probe Molecules Adsorbed onto Na-X and Na-Y Zeolites. The Last Three Columns Represent the Assumed Number of Pyridine Molecules per Supercage probe liquid H20-P CC4-P 0-2 2-4 4-6 998 1004 991 1001 pyridine (X) 991 998 998 1001 998 991 1003 pyridine (Y) 991 995 1005 1002 1007 1002 picoline(Y) 995 influenced by the position of the cell in the compartment. However, if the sample cell is carefully replaced in the compartment, high precision may be achieved. Therefore, we believe that the observed shifts in both the FTIR and in the FT-Raman experiments are significant and meaningful. Similar observations were made in the experiments performed in vacuum.' As described in Ferwerda et al.' this is probably caused by different bands at positions greater and smaller than the measured peak positions (which also explains the change observed in the fwhm of this band). With the aid of curvefitting routines it was possible to resolve the pattem around 1000 cm-l. Thus, the adsorption of pyridine onto Na-faujasites may be divided into three categories which reveal characteristic wavenumbers, temperature ranges, and coverages (Table 2). The spectra of 4-picoline adsorbed onto Na56-Y pretreated at 150 "C are shown in Figure 7a (Figure 7b shows the analogous experiment for pyridine). The same trend in band position is observed as in the pyridine experiments, although the band maxima are slightly different (Table 3).
J. Phys. Chem., Vol. 99, No. 40, 1995 14167
Pyridine Adsorbed on Na-Faujasite
3SO L
l
l
l
l
1
l
l
l
l
l
l
l
l
l
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1050
550
500
450
Raman Shift (Cm-l)
Raman Shift (cml)
Figure 9. Zeolite-bending region of Na-Y activated at 470 "C, following pyridine desorption at room temperature and subsequent desorption at the indicated temperature ("C).
lo00
1050
Raman Shift (cm-1) Figure 7. Ring-breathing region of Raman spectra after desorption of (a, top) picoline and (b, bottom) pyridine on Na-Y, activated at 150 "C, at the indicated temperature ("C).
temperatures below 200 "C, a band around 500 cm-I is observed, which is not affected by pyridine adsorption at room temperature (Figure 8). However, on pyridine desorption at temperatures above 250 "C, the band shifts to 517 cm-I, and at about 350 "C (very low pyridine coverage), this is accompanied by a decrease in intensity. The first effect (shift to 517 cm-I) is irreversible, Le., on pyridine re-adsorption at temperatures below 250 OC the band remains at the same position. The second one is reversible since the band reappears at 517 cm-' on re-adsorption at 300 OC (Figure 8). After activation at 470 "C the mode around 500 cm-' appears as a broad band centered around 507 cm-', which is composed of at least two contributions around 513 and 505 cm-' (Figure 9). Pyridine adsorption at room temperature results in a shift to 517 cm-I, while desorption at temperatures up to 300 "C hardly changes the spectrum. Activation of the Na-Y zeolite at 200 "C and subsequent pyridine adsorption at 300 "C results in a band at 517 cm-', which remains at this position upon subsequent adsorption at ever lower temperatures (not shown).
Discussion
550
500
450
Raman Shift (cm-1) Figure 8. Zeolite-bending region of Na-Y activated at 150 "C, following pyridine adsorption at room temperature and subsequent desorption at the indicated temperature ("C). Finally, pyridine was readsorbed onto the zeolite at 300 "C.
The framework vibrations of the Na-Y zeolite around 500 cm-I show some interesting features. Following activation at
In a previous paper' we reported on the changes in the Raman spectrum of pyridine when adsorbed onto Na-faujasite zeolites. We attributed the observed shifts to re-adsorbed water within the zeolite lattice modifying the adsorption sites. A model was proposed in which the heterocycle interacts with the zeolite lattice via water bridges. Samples activated at higher pretreatment temperatures (activation temperature was 120 "C) could not be studied due to fluorescence problems.* Since the work presented here was performed in a flow system, it was possible to pretreat the catalyst in an 0 2 / A r flow at elevated temperatures (470"C), burning off the fluorescent decompositionproducts.'o-12 After this pretreatment, the same features are observed in the pyridine-breathing region of the Raman spectra as from activation at 120 "C in vacuum or at 150 "C in a flowing atmosphere. The bending mode of the zeolite lattice around 500 cm-I, however, which is severely affected by the water content of the zeoliteI3 shows a deviating behavior upon pretreatment at various temperatures. As outlined in Ferwerda et a1.,I3 the variations in the Raman spectra of the framework vibrations of Y zeolites upon dehydration may be divided into two parts, viz., those at temperatures below and above 200 "C. Below 200
14768 J. Phys. Chem., Vol. 99, No. 40, 1995
"C,the Raman spectra are determined by the dehydration of the supercages, whereas at higher temperatures the sodalite cages are dehydrated resulting in a redistribution of the cations which affects the Raman spectra. It is also noteworthy that the amount of water in the pyridine used here should be negligible as ultradry pyridine was used. However, it seems difficult to prevent re-adsorption due to the use of a flow system.I4 We believe that, although water may affect the Raman spectra of pyridine, the reasons for the shifts of the pyridine ring vibrations are more complex than the model described in Ferwerda et al.' Water Content of the Faujasites. One of the major problems in understanding the influence of cations and adsorbed molecules on the Raman spectra is the incomplete knowledge of residual water in the zeolite under study. In our previous paper on pyridine adsorbed onto Na-faujasites,] we suggested that activation at 120 "C in vacuum would remove physisorbed water, whereas coordinated water would remain in the lattice. From the literature15 we know that much higher temperatures are required to dehydrate faujasites completely. Yet, one should realize that water located in the supercage is easier to remove than that present in the sodalite cage^.'^^^^ Hence, after evacuation at 120 "C overnight, water from the supercages will be desorbed, whereas the sodalite cages will still contain coordinated water, which is reflected in the band position of the bending mode of the zeolite framework, just under 500 cm-I.l3 The amount of residual water affects not only the possible sites for pyridine adsorption'but also the calculated amount of pyridine molecules per supercage in the gravimetrical experiments, as determined in Ferwerda et al.' Table 2 gives the number of pyridine molecules per supercage calculated for zeolites with coordinated water (wet) (Na,-faujasitey H2O) and for dehydrated (dry) zeolites (Na,-faujasite). It is obvious that even without taking re-adsorbed water into account the determination of the amount of adsorbed pyridine is subject to large errors. However, since at least 88% of the total amount of water in Y zeolites is present in the supercages and thus removed upon evacuation at 120 "C, we now believe that the calculations using the molecular mass of dry zeolites are more accurate than those for wet zeolites. Besides, the number of pyridine molecules present in the zeolite structure in isotherm experiments* as determined with the dry molecular mass approaches the expected values: the vapor pressure above the system equals that of liquid pyridine at loadings with more than five and one-half molecules per supercage, where the space is limited to five to six pyridine (or benzene) molecule^,'^*'^ and if less than four and one-half pyridine molecules are present they are strongly bound into the zeolite lattice resulting in a low vapor pressure (Table 2). Although we realize that our data are liable to errors, the amounts calculated via the dry zeolite agree well with the expected values, and we will therefore use these numbers in the determination of our model. High Pyridine Loading. From Table 2 it now becomes evident that pyridine is strongly bound within the voids of the zeolite up to four molecules per supercage. The band around 1000 cm-' in the Raman spectra shows a shift to higher frequency at loadings of more than four molecules per supercage. Moreover, these additional molecules can be removed from the cavities upon evacuation at room temperature.' This suggests that the supercage becomes too crowded at these loadings, resulting in an enhanced perturbation of the (ring) vibrations of the pyridine molecules. "Breathing" is becoming more difficult, and a blue shift, Le., to higher wavenumbers, is
Ferwerda and van der Maas observed (from 998 to 1001 (Na-Y) or 1003 cm-I (Na-X)). Since the supercage is overcrowded, the pyridine molecules lose rotational freedom. Therefore, the molecules may be desorbed at room temperature down to four molecules per supercage, after which elevated temperatures are necessary to desorb pyridine. The results presented here for pyridine may be compared with those of benzene adsorption onto faujasite zeolites. Freeman and UnlandZ0observed for benzene in faujasites that "the nature of the adsorbed benzene (molecules) begins to approach the liquid state" when more than three molecules are adsorbed within the supercage. Hence, in contrast to our findings on pyridine, the ring vibrations of benzene adsorbed into the supercage shift to the unperturbed frequencies with increasing loading. However, in the case of benzene a red shift is observed upon adsorption at low coverages as compared to pyridine for which a blue shift is monitored. This is a consequence of the different associations of these probe molecules with the lattice. The interaction of pyridine is mainly via the lone pair of the nitrogen, whereas benzene interacts via its n-system with the cations. For both probe molecules a blue shift is observed when more than four molecules are adsorbed into the supercage. Low Pyridine Loading. At relatively low coverage (less than two molecules per supercage) a high-frequency band shows up for all zeolites (Table 2). The (DRIFTS and Raman) experiments yield identical results for the intensity of this band upon adsorption or desorption of pyridine, revealing that the different associations are reversible. The most pronounced shift at low loading is observed in the DRIFT spectra of the band around 1450 cm-' of pyridine adsorbed onto the Na-X zeolite (Figure 3). While the band at 1446 cm-' completely disappears, a band at 1441 cm-' shows up. Two adsorption mechanisms are possible for loadings of less than two pyridine molecules per supercage. Firstly, in a study of pyridine adsorbed onto sodium-modified Silicalite, Matsumura et a1.21,22presented a model in which pyridine interacts with two sodium cations, viz., via the lone pairs of the nitrogen and via the x-system of the aromatic ring. Since the cations at site II in the supercage of Na-Y are separated by approx. 6 AZ3 and are known to be protruded by probe mo1ecules,I8 pyridine molecules (with a kinetic diameter of 5.9 A22,24)might be capable of adopting such a conformation. Since only four cations are present in a supercage, two pyridine molecules will occupy these sites. Secondly, it is interesting to compare the results presented here with those from XRD found by Goyal et al.25for pyridine adsorbed onto a dehydrated Y zeolite (Na56Y). They conclude that at loadings of one pyridine molecule per supercage the heterocycle preferentially adsorbs at the 12ring window site. Since each supercage has four 12-ring windows shared with the neighboring supercages, these sites will be full at a pyridine coverage of two molecules per supercage. In both cases, the third pyridine molecule entering the supercage will perturb this state. It seems reasonable that increasing the loading will also affect the pyridine molecules already present and that the molecules will be positioned in a conformation in which the total energy is minimized. Egerton et al.I9 concluded that all pyridine molecules are affected by the presence of the four charge-balancing cations present in the supercage and that they form sites for specific adsorption. Although we believe that their conclusions are correct, the wavenumber of 1002 cm-' in their work is in our opinion indicative of a loading of more than four pyridine molecules. We conclude that at a loading of four pyridine molecules per supercage, the probe will be associated with its lone pair to one of the four cations, possibly in a tetrahedral conformation, resulting in a band at 998 cm-I.
J. Phys. Chem., Vol. 99, No. 40, 1995 14769
Pyridine Adsorbed on Na-Faujasite The small wavenumber differences observed for pyridine adsorbed onto Na-X or Na-Y may be attributed to the excess cations present in the former zeolite. Na-X has four of the five and one-half Na+ ions fixed at site I1 in the supercage, and the other ones positioned at other locations, for example at site 111, which are not firmly attached to the cage These cations may cause a slight change of the electrostatic field within the zeolite, altering the frequencies of pyridine. Picoline Adsorption. If the apparent shifts of 4-picoline are compared with those of pyridine (Figure 7a and b), it appears that although the trends are identical, small deviations are observed in magnitude. Pyridine reveals positive shifts in the frequency of the ring-breathing mode of liquid-like pyridine of 12, 7, and 10 cm-' on increasing the coverage (Table 3). Picoline adsorption, on the contrary, yields values of 10,7, and 12 cm-', respectively. Since picoline has a methyl group, the position in the window site or with the two cations may be less favorable due to steric hindrance. Therefore, a smaller shift is observed. However, in the case of more than four molecules per supercage, the methyl group will further increase the "crowding" effect and a larger shift is observed. The Nature of the Interaction of Pyridine in NaFaujasites. The position of the pyridine bands in both the infrared and in the Raman spectra upon adsorption of pyridine onto the zeolites is assigned to interactions with a weak electron pair a c ~ e p t o r .This ~ may be either an OH group, which is not acidic enough to protonate pyridine, or a cation such as Na+. It seems rather difficult to distinguish between these two possibilities, because the shifts due to these interactions are very similar. Two features in the infrared spectra point in the direction of (at least a very important contribution of) association with sodium cations. Firstly, the intensity of the band at 1490 cm-' seems too high for a hydrogen-bonded species. In the infrared spectrum of pyridine adsorbed onto silica (hydrogen bonded), for example, its intensity is very low93l4and it has been found that the intensity depends on the accepting ~ a t i o n . For ~ ~ Na+, ,~~ a ratio of 0.25 for the intensities of the 1490 and 1445 cm-I bands has been reported,27 which is in good agreement with the spectra shown here. Secondly, the 8b mode of hydrogenbonded pyridine is usually found at about 1580 cm-', whereas this mode is observed at 1575 cm-I for (weak) Lewis-bonded specie^.^ Since we observe a band at 1575 cm-I, we conclude that pyridine probably interacts with a sodium cation. However, a third factor comes into play upon considering the bending region of Na-Y zeolites. When Na-Y is activated at 470 "C, which changes the water content of the sodalite cages as compared to pretreatment at 150 or 200 "C, the same spectral features are observed in the pyridine region of the Raman and infrared spectra. The zeolite-bending mode at about 500 cm-I, however, shows a deviating behavior. As outlined in Ferwerda et a1.,I3 this band is a measure of the dehydration of the supercage and the smaller cages. After activation at 150 "C, the sodalite cages are still hydrated (band at 500 cm-I, Figure S), but after pretreatment at 470 "C,water is also removed from these smaller cages (band at 507 cm-I, Figure 9). l'yridine adsorption onto Na-Y, pretreated at 470 "C, results in a shift of this mode to 517 cm-I, indicating that pyridine is capable of changing the interaction of the sodium cations with the ze01ite.I~ Two possible models have been postulated to explain this observation. Firstly, pyridine may interact with the cations directly, changing their location in the zeolite lattice. Secondly, the probe may perturb the residual water molecules in the sodalite cage, resulting in tum in a migration of the cations. Since a band at 507 cm-' is observed, we are positive that some
water is still present, which is possibly extracted from the smaller cage upon pyridine adsorption (remember that a completely dry Na-Y zeolite shows a band at 518 cm-I).l3 The shift to lower wavenumber after pyridine desorption at 350 "C sustains this idea: when almost all pyridine is desorbed, water originally rearranged by the high pyridine loading retums to its original position in the sodalite cages, causing a shift to lower wavenumbers. Furthermore, the position of this band directly after pretreatment at 150 "C (500 cm-I) indicates that even more water molecules are present in the structure. Since no differences in the ring-breathing regions are observed for either pretreatment method, we conclude that in both cases pyridine may interact with some of the residual water molecules too. The kind of interaction of pyridine within Na-faujasites is far more complicated than a simple model of one cation or OH group interacting with the lone pair. The molecules are possibly bonded within the zeolite structure and affected by several competitive interactions with cations, water, and zeolite walls.
Conclusions The combination of FT-Raman and DRIFT spectroscopy in the characterization of Na-faujasites, with pyridine as a probe, provides insight into the dynamics of the cations and adsorbed molecules in the cages. The adsorption of pyridine in the supercages may be divided into three steps. The probe molecule is positioned in the 12ring window sites or interacts via its lone pair and its n-system with the charge-balancing cations at low coverages (zero to two molecules per supercage). The molecules are probably bound in a tetrahedral configuration to the four charge-balancing cations in the supercage at medium coverages (two to four molecules per supercage). Finally, at high coverages (four to six molecules per supercage), the cages become overcrowded resulting in a further perturbation of the ring-breathing vibrations. The precise interaction of pyridine with either water molecules or cations is not yet clear, since pyridine adsorption appears to result in a redistribution of both water and sodium cations. The nature of the interaction of pyridine within the voids is probably more complex than the "straightforward" interaction of the lone pair of the heterocycle with an electron acceptor. Probably, pyridine is capable of rearranging the cations and/or residual water and will consequently adsorb in the energetically most favorable position.
Acknowledgment. Mr. P. Bentley (Southampton University, UK) is acknowledged for correction of the English writing of the manuscript. References and Notes (1) (a) Ferwerda, R.; van der Maas, J. H.; Hendra, P. J. J. Phys. Chem. 1993, 97, 7331. (b) Fenverda, R. Ph.D. Thesis, Utrecht University, 1994. (Copies of this thesis may be ordered from Dr. R. Ferwerda, ATI Unicam, York Street, Cambridge CBl 2PX, United Kingdom. E-mail: roel. ferwerdamati-unicam.co.uk.) (2) (a) Ferwerda, R.; van der Maas, J. H.; Hendra, P. J. Vib. Specfrosc. 1994,7, 37. (b) Ferwerda, R., PhD. Thesis, Utrecht University, 1994. (Copies of this thesis may be ordered from Dr. R. Fenverda, AT1 Unicam, York Street, Cambridge CB 1 2PX, United Kingdom. E-mail: roel.ferwerda@ ati-unicam.co.uk.) (3) Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectroscopy; John Wiley & Sons: New York, 1986. (4) Wames, G. M. Ph.D. Thesis, University of Southampton, 1991. ( 5 ) Ward, J. W. J. Phys. Chem. 1968, 72, 4211. (6) Ward, J. W. In Infrared Studies of Zeolite Surfaces and Sur$ace Reactions; Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976.
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