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J. Phys. Chem. B 2001, 105, 4374-4379
FTIR Study on Molecular Motions of Benzene Adsorbed in ZSM-5 Zeolite: Role of Charge-Balancing Cations and Pore Size A. Sahasrabudhe, V. S. Kamble, A. K. Tripathi, and N. M. Gupta* Applied Chemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, India ReceiVed: January 30, 2001
Occlusion of benzene in NaZSM-5 zeolite is investigated using in situ FTIR spectroscopy as a function of substitution of Na+ with group IIA cations. At least four pairs of overlapping vibrational bands were observed in the region of out-of-plane C-H bending vibrations (2000-1800 cm-1) on adsorption of benzene in NaZSM-5 at room temperature. Whereas two pairs of these bands, e.g., one pair at around 2007 and 1986 cm-1 and the other at 1969 and 1956 cm-1, correspond to the 1960 cm-1 band of liquid benzene, the other two pairs, e.g., at 1874 and 1852 cm-1 and 1831 and 1810 cm-1, appear in place of the 1815 cm-1 band of liquid benzene in this region. No measurable difference was observed in the frequencies of these bands for adsorption in cation-exchanged samples, suggesting that any specific interaction between cations and benzene molecules is small compared to the effect of benzene-benzene interaction. These multiple bands are therefore attributed to the existence of at least two distinct clustered states of benzene, localized at intersections and in the straight channels of NaZSM-5, respectively. While the frequency of these bands remained unchanged, the intensity of the lower frequency side pair (i.e., 1969, 1956 cm-1 and 1831 and 1810 cm-1) was found to be very sensitive to the nature of the charge-balancing cation and followed a trend NaZSM5 < CaZSM5 > SrZSM5 > BaZSM5, similar to that followed by the pore volume of exchanged samples. These two pairs of bands are therefore identified with the benzene clusters encapsulated in straight zeolitic channels where most of the balancing cations are located. Dose-dependent measurements have shown that such benzene clusters may form at loading as low as ∼1.6 molecules/uc; when a larger fraction is located at intersection sites and at the same time a small fraction also exists in the straight or sinusoidal channels. The concentration in the later locations grows with the increasing benzene loading. Considering these results and in view of the fact that no frequency shift or band splitting was observed in the in-plane C-H/C-C and fundamental ν19 stretching vibrations of adsorbed benzene, we infer that the benzene molecules are packed side by side with their planes parallel to the zeolite channel, the intermolecular interaction occurring through π-electron cloud.
Introduction
Experimental Section
We demonstrated in previous studies that the small molecules such as CO and CO2 occlude in the form of molecular clusters (dimers, trimers, etc.), when adsorbed in the cages of X-, Y-, and ZSM-5 type zeolites at room temperature.1-3 These inferences were based on the shift in the frequency of vibrational bands of encapsulated gases as a function of adsorbate pressure, rise in temperature, evacuation, and also on using labeled gases. The potentials associated with the charge-balancing cations and the size and structure of pores are shown to play a vital role in the binding together of these cluster molecules within the zeolitic cavities.4,5 In continuation, we have now performed in situ FTIR spectroscopy studies on the adsorption of benzene in NaZSM-5 with an objective to monitor the effect of benzene loading and of substitution of Na+ by cations of alkaline earth metals on the binding states and the transport of C6H6 molecules in zeolitic pores. The in-plane C-H/C-C stretches (3100-3000 cm-1), out-of-plane C-H vibrations (2000-1810 cm-1), and the fundamental ν19 C-C stretch (1479 cm-1) of adsorbed benzene molecules are followed systematically because of the reported sensitivity of these IR bands to the chemical and geometrical nature of an adsorbent zeolite.6 * Corresponding author. Fax: 091-22-5505151. E-mail: nmgupta@ magnum.barc.ernet.in.
Details of the zeolites studied and a high-temperature, highpressure stainless steel cell used for recording the IR spectra of sample wafers (70-80 mg, 2.5 cm diameter) are given elsewhere.5 The physical characteristics of the samples used are given in Table 1. The samples were heated in situ at 625 K under vacuum for 24 h and complete removal of any occluded moisture was ascertained before recording a background spectrum. The pretreated sample was exposed at room temperature to 10 to 1000 µmol g-1 doses of benzene, drawn from a benzene (6 mol %) + argon mixture. A Mattson model Cygnus-100 FTIR spectrometer equipped with a DTGS detector was used in transmission mode and 300 scans were collected for each spectrum at a resolution of 4 cm-1. Frequencies reported in different spectra were reproducible with in (2 cm-1. The values given in parentheses in some of the figures represent the absorbance value of individual IR band and are taken as a measure of relative intensity. No IR bands due to free benzene were noticed, indicating a complete uptake of benzene in the sample for the dose ranges used in the study. Benzene was distilled twice before use and was free from any contamination and moisture.
10.1021/jp010381i CCC: $20.00 © 2001 American Chemical Society Published on Web 04/21/2001
Benzene Adsorbed in ZSM-5 Zeolite
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4375
Figure 1. C-H out-of-plane IR bands for loading of (b) 1.6, (c) 3.2, and (d) 6.5 benzene molecules per unit cell of NaZSM-5. Spectrum (a) shows comparative IR bands of liquid benzene in this region.
Figure 3. C-H out-of-plane IR bands for adsorption of 3.2 benzene molecules per unit cell of different cation-exchanged NaZSM5 zeolite. (a) NaZSM-5, (b) CaZSM-5, (c) SrZSM-5, (d) BaZSM-5.
Figure 2. Effect of benzene loading on the relative intensity of different vibrational bands in C-H out-of-plane region. (a) I1850 /I1810, (b) I1887/ I1957.
Figure 4. Loading dependence of intensity ratio of 1855 and 1810 cm-1 bands for adsorption of benzene in different cation-exchanged ZSM-5 zeolite. (a) CaZSM-5, (b) SrZSM-5, (c) BaZSM-5.
Results
The relative intensity of these bands exhibited a strong dependence on the nature of the charge-balancing cation also. Figure 3 presents typical IR bands of the NaZSM-5 sample exchanged with group IIA cations after exposure to 496 µmol g-1 (∼3. 2 molecules /uc). There are two salient features that are noticeable in this figure. We see no shift in the frequency of the individual IR bands on cation exchange, contrary to what has been reported for benzene adsorption in cation exchanged samples.6 Second, the absorbance of individual bands and their relative intensity showed a progressive change for a change in charge-balancing cation from Ca2+ to Ba2+. In the typical case of 1852 and 1810 cm-1 bands, the dose dependence of the intensity of these peaks is plotted in Figure 4 for a comparison. It is seen that this intensity ratio follows a trend Ca2+ > Sr2+ > Ba2+. This ratio decreases in each case with benzene loading, the effect again being most pronounced in the case of Ca2+exchanged sample (Figure 4a). A similar trend is also noticed for the intensity ratio of the other two bands. In-Plane C-H/C-C Stretching Vibrations. Figure 5A shows the IR bands due to fundamental C-H stretching (ν20) and combination C-C stretching vibrations of benzene adsorbed (6.5 molecules/uc) in cation-exchanged zeolites. No shift was observed in the frequency of these bands for benzene loadings in range of 10- to 1000 µmol g-1. The intensity of these bands, however, increased progressively with benzene loading and the extent of this increase depended upon the nature of cation. Thus,
Out-of-Plane C-H Vibrational Bands. Spectra (b)-(d) in Figure 1 show out-of-plane C-H vibrational bands in the 21001750 cm-1 region when NaZSM-5 was exposed at room temperature to benzene doses in the range 245 to 990 µmol g-1, corresponding to a loading of around 1.6 to 6.5 molecules per unit cell (uc). In spectrum (a) of this figure are plotted IR bands of liquid benzene in this region. A comparison shows that instead of 1960 and 1815 cm-1 band of liquid benzene (Figure 1a), we observe multiple bands in the case of benzene adsorbed in zeolite. The relative intensity of these overlapping bands depended considerably on the amount of benzene adsorbed. From the change in relative intensity of these bands as a function of benzene dose, we may divide these multiple bands into at least two sets. In the 2050-1900 cm-1 region we have one set of two bands at around 2007 and 1986 cm-1 and a second set at 1969 and 1956 cm-1 (Figure 1 c, d). In the lower frequency region we similarly see the presence of two sets, one comprisied of bands at 1874 and 1852 cm-1 and another set of two bands at 1831 and 1810 cm-1. The intensity of the second set of bands increased in each case with the increased amount of benzene adsorbed (Figure 1b-d). Figure 2 gives the representative dose dependence of intensity ratio of 1852 and 1810 cm-1 bands at one hand and that of 1986 and 1956 cm-1 bands on the other.
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Figure 5. (A) C-H stretching (ν20) and combination C-C (ν1 + ν6 + ν19 and ν19 + ν8) vibrations, and (B) in-plane C-C stretching vibration (ν19) of benzene for adsorption of 6.5 benzene molecules per unit cell of different cation-exchanged ZSM-5 zeolite samples. (a) NaZSM-5, (b) CaZSM-5, (c) SrZSM-5, (d) BaZSM-5.
the intensity of the three bands i.e., 3093, 3073, and 3039 cm-1, in this region increased considerably after exchange of Na+ with Ca2+. For the samples exchanged with other group IIA cations, the intensity followed a trend Ca2+ > Sr2+ > Ba2+. These data are shown in Figure 6A for different loadings of benzene. It may be noted that the disparity in the peak intensity increases progressively with increasing benzene loading. Fundamental ν19 C-C Stretching Vibration. Figure 5B shows the variation in the intensity of fundamental C-C stretching vibrational band (ν19, 1479 cm-1) of benzene adsorbed in cation-exchanged ZSM-5. As in the case of Figure 6A, the intensity of the 1479 cm-1 band also depended on the nature of charge-balancing cation, for the experiments carried out under identical loading conditions. These data are shown in Figure 6B. Thus the absorbance of this band increased considerably when Na+ was exchanged with Ca2+ (spectrum b). For the other samples with group IIA cations, the intensity of this band followed a trend Ca2+ > Sr2+ > Ba2+ (spectra b-d). The variation in the intensity of this band as a function of benzene loading is shown in Figure 6B for different cation-exchanged zeolites. Again, the data of this figure show that the difference in the intensity of this band in cation-exchanged ZSM-5 grows with the increasing loading of benzene. Effect of Evacuation. Spectra a-d in Figure 7 present IR bands in the 2050-1750 cm-1 region when NaZSM-5 sample loaded at room temperature with ∼7 benzene molecules/uc was evacuated for different periods of time. These data show that while the set of IR bands at 2007 and 1986 cm-1 decreased only marginally, the intensity of bands at the lower frequency side i.e., at 1969 and 1956 cm-1, decreases considerably. A similar trend may be noticed in the other region where the bands at 1874 and 1852 cm-1 show only marginal change while the bands at 1831 and 1810 cm-1 decrease substantially during 2 h of evacuation. This is also important to note that the trend seen in Figure 7 is almost the reverse of that observed for increasing benzene loading (e.g., Figure 1). A similar trend of change in peak intensity was observed in the case of cation-exchanged samples as well. Thus, the ratio of 1852 and 1810 cm-1 bands (i.e., I1852 /I1810), which was found to decrease from 5 to 0.9 with an increase of benzene loading from 250 to 986 µmol g-1 in the case of CaZSM-5 (Figure 4), reverted to a value of ∼3 on subsequent pumping. Hydroxy Region Bands. IR spectrum of NaZSM-5 as well as of cation-exchanged samples showed the presence of three
Figure 6. Relative absorbance of (A) 3039 cm-1, and (B) 1479 cm-1 bands as a function of benzene loading in cation-exchanged ZSM-5 zeolite. (a) NaZSM-5, (b) CaZSM-5, (c) SrZSM-5, (d) BaZSM-5.
Figure 7. C-H out-of-plane vibrational bands of benzene after adsorption of benzene (6.5 molecules/uc) in NaZSM-5 followed by pumping for different periods of time. (a) 5 min, (b) 30 min, (c) 1 h, (d) 2 h.
overlapping ν(OH) bands at around 3738, 3675, and 3612 cm-1 after outgassing at 625 K for 24 h. Exposure to benzene at room temperature resulted in marginal decrease in the intensity of
Benzene Adsorbed in ZSM-5 Zeolite
Figure 8. Infrared spectra of CaZSM5 (a,b) and SrZSM-5 (c,d) zeolites before (a,c) and after adsorption of 6.5 molecules of benzene per unit cell.
these bands for loading of up to 7 benzene molecules per unit cell in different cation-exchanged samples. No shift was observed in the frequency of these bands. At the same time, the development of two very weak and broad IR bands at ca. 3510 and 3255 cm-1 was observed, their frequency again remaining unchanged on cation exchange. These data are shown in Figure 8 for benzene loading of ∼6.5 molecules/uc in two typical cation-exchanged samples, viz. CaZSM-5 and SrZSM5. IR spectra of corresponding unexposed samples are also given in this figure for a comparison. Discussion The adsorption of benzene in zeolites has been investigated widely and reviewed.6 In the case of adsorption in super cages of Faujasite-type zeolites, the C-H out-of-plane (ν5 + ν17) and (ν10 + ν17) bending vibrations are found to show a shift of around 20 to 40 cm-1 on adsorption of benzene as a function of substitution of Na+ with alkaline and alkaline earth cations. No shift was observed in the ν19 C-C streching vibrations. On the basis of these results it was proposed that the cations in zeolites serve as adsorption sites, where benzene molecules are bonded through their π -electrons, giving rise to a “specific interaction”.6-10 Thus, Dzwigaj et al.11 reported splitting of CH out-of-plane bands into two pairs on adsorption of benzene on zeolite-β, where the low-frequency pair of bands is assigned to benzene interacting with cations through the π-cloud and highfrequency pair of bands to CH weak interaction with the framework oxygen. In a similar study on HEMT and NaEMT zeolites, Barthomeuf and co-workers12 reported a red shift (∼345 cm-1) in hydroxy region bands for adsorption of ca. 12.5 molecules per unit cell, in addition to multiple vibrational bands in out-of-plane C-H stretching region. These observations have been interpreted in terms of an interaction of the π-cloud of benzene ring with the zeolite protons or with the chargebalancing cations. From the extent of shift in hydroxy reigon bands the acidic character of a sample is assessed. Broad and weak vibrational bands at 3250 and 3505 cm-1, as observed in our study (Figure 8), have also been reported by Busca et al.13 for adsorption of benzene over HZSM-5 zeolite and have been attributed to perturbation of bridging OH groups and terminal silanols. In a recent FT-Raman spectroscopy study, Huang and Havenga14 have demonstrated a split in all the fundamental vibrations of benzene for moderate to high loadings of benzene in siliceous ZSM-5, which has been attributed to the existence of two crystallographically nonequivalent benzene molecules
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4377 on the framework. On the basis of the relative intensity of these bands, the location of benzene molecules inside the zeolite channels at various loading levels have been determined. It is shown that all the guest molecules reside at the channel intersections for the loading range of 1 to 4 molecules per unit cell. In the intermediate (5 to 6 molecules/uc) and in high loading range of 7 to 8 molecules/uc, the guest molecules have an access to zigzag channels and to midsections of the straight channels, respectively, in addition to channel intersections. An X-ray diffraction study of benzene adsorbed in zeolite ZSM-5, has similarly demonstrated that for less than four molecules per unit cell, the benzene is confined to the intersections of the straight and sinusoidal channels and a considerable disorder exists.15 At the higher benzene coverage (∼7.6 molecules/uc), benzene molecules are located in the intersections and the sinusoidal channel, with a smaller number in the straight channels. In line with these observations, Mentzen and Lefebvre16 used power XRD technique to show that the benzene molecules first occupy the channel intersections while both the straight and zigzag channels remain empty. Some contradictory observations have also been reported about the migration and localization of benzene in ZSM-5. Thus, 2H NMR studies by Portsmouth et al.17 indicated that at low loadings ( SrZSM-5 > BaZSM-5 (Figure 4). Data in Table 1 reveal that this intensity trend has a striking relationship with the
Sahasrabudhe et al. TABLE 1: Specific Surface Area and Pore Volume of Cation-Exchanged Zeolite ZSM-5 zeolites cation exchange (%) surface area (m2 g-1) pore volume (m3 g-1)
NaZSM-5 CaZSM-5 SrZSM-5 BaZSM-5 454 0.26
57 624 0.29
50 358 0.23
67 334 0.19
change in pore volume of ZSM-5 on cation exchange, which also follows a similar trend, i.e., NaZSM-5 < CaZSM-5 > SrZSM-5 > BaZSM-5. It is thus apparent that the sets of bands at 1970 and 1955 cm-1 and 1836 and 1810 cm-1 are due to occlusion of benzene in those pore where charge-balancing cations are located, i.e., in the straight channels. Vibrational spectra in Figure 5 reveal that contrary to data in Figures 1 and 3, no frequency shift or band splitting occurs in the in-plane fundamental C-H stretching (ν20) and combination C-C vibrations of benzene molecules in the 3200-3000 region and also in the fundamental ν19 C-C stretching vibration (1480 cm-1), irrespective of the cation and the amount of benzene loaded. This may be interpreted in terms of packing of benzene molecules one by the side of other and with their planes parallel to the walls of zeolitic channels. In this mode of packing, benzene molecules will experience immense π-π bond interaction, resulting in splitting of only out-of-plane C-H vibrations. This type of stacking will also result in the compression of C-H bonds, in increase in the effective force constants, and hence in an upward shift in frequency of splitted out-of-plane C-H vibrational bands. This is in accordance with the results of this study (Figures 1 and 3). The exchange of cation is known to influence the channel size and hence pore volume, which in turn will lead to a change in the number of trapped benzene molecules at a particular location resulting thereby in increased intensity of IR bands, as is also reflected in data of Figures 4 and 6. A similar behavior is also expected as a function of benzene loading, as is indeed seen in Figures 1 and 2. Furthermore, this concept of benzene packing in zeolitic channels is in line with the observed formation of benzene clusters even for low loadings of benzene (Figure 1b). In the alternative mode when the plane of the benzene ring is perpendicular to the walls of a zeolite channel, the intermolecular interaction of the benzene molecules is expected to be weaker due to large channel lengths, thus ruling out the possibility of their clustering/condensation. Also, for this mode of bonding, one would expect significant changes in frequencies of the inplane (specially C-H stretching) vibrations also, as is not the case. Furthermore, in the suggested parallel mode of adsorption an interaction of π-cloud with some of the framework hydroxyl groups may lead to a perturbation resulting in a band shift as seen in Figure 8 and reported earlier by several research groups. Data in Figure 7 show that the lower frequency side cationsensitive pair of IR bands, arising due to benzene clusters in straight channel, are removed quickly on pumping while the intensity of the other two pair of bands is affected only marginally. It may therefore be inferred that the transport of benzene clusters located at channel intersections is hindered compared to those located in straight and or zigzag channels. The plots in Figure 6A,B show that the intensity/absorbance of the in-plane vibrational bands is related to the amount of benzene adsorbed; which in turn depends on the pore volume. A trend in the intensity of these bands i.e., NaZSM-5 < CaZSM5 > SrZSM5 > BaZSM5 is similar to that reported above for the out-of-plane bands. It is well-known that the size of the charge-compensating cation in a zeolite and the degree of ion-exchange have profound influence on the amount of guest molecules adsorbed by the framework.23 These observations
Benzene Adsorbed in ZSM-5 Zeolite therefore suggest that the intensity of well-resolved in-plane vibrational bands of adsorbed benzene may serve as a means of evaluating subtle changes in pore size/volume in such microporous zeolitic materials. Conclusions We may draw the following conclusions from these studies. Benzene molecules adsorbed at room temperature in the channels of ZSM-5 experience zeolitic wall pressure, resulting in significant intermolecular interaction that gives rise to a highly compressed or clustered state. Benzene clusters occluded in different locations of ZSM-5, i.e., straight and interconnecting zigzag channels and their intersections, give rise to distinct pairs of out-of-plane C-H stretching vibrations. The relative intensity of these bands represents the concentration of benzene entrapped in these locations. Our results reveal that even for loadings as low as ∼1.6 molecules/uc a small fraction of benzene may exist in the straight channels of ZSM-5 even though a larger fraction is localized at intersections. With increase in loading, the benzene molecules migrate to straight and connecting channels only after intersection locations are completely filled. The benzene molecules are stacked in zeolite channels with their plane parallel to wall, the intermolecular interaction occurring through π-electron cloud. The intensity of the well-resolved in-plane vibrational bands varies with the amount of benzene loaded and may serve as a measure of zeolitic pore volume. References and Notes (1) Kamble, V. S.; Gupta, N. M.; Kartha, V. B.; Iyer, R. M. J. Chem. Soc., Faraday Trans. I 1993, 89, 1143.
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4379 (2) Shete, B. S.; Kamble, V. S.; Gupta, N. M.; Kartha, V. B. J. Phys. Chem. B 1998, 102, 5581. (3) Shete, B. S.; Kamble, V. S.; Gupta, N..M.; Kartha, V. B. PCCP 1999, 1, 191. (4) Kamble, V. S.; Gupta, N. M. J. Phys. Chem. B 2000, 104, 4588. (5) Kamble, V. S.; Gupta, N. M. PCCP 2000, 2, 2661. (6) Barthomeuf, D. Catal. ReV.-Sci. Eng. 1996, 38, 521. (7) Primet, M.; Garbowski, E.; Mathieu, M. V.; Imelik, B. J. Chem. Soc., Faraday Trans. I 1980, 76, 1942. (8) Angell, C. L.; Howell, M. V. J. Colloid Interface Sci. 1968, 28, 279. (9) Coughlan, B.; Carroll, W. M.; Malley, P. O.; Nunan, J. J. Chem. Soc., Faraday Trans. 1 1981, 77, 3037. (10) Ward, J. W. J. Catal. 1968, 10, 34. (11) Dzwigaj, S.; de Mallman, A; Barthomeuf, D. J. Chem. Soc., Faraday Trans. 1990, 86, 431. (12) Su, B. L.; Manoli, J-.M.; Potvin, C.; Barthomeuf, D. J. Chem. Soc. Faraday Trans. 1993, 89, 857. (13) Armaroli, T.; Trombetta, M.; Alejandre, A. G.; Solis, J. R.; Busca, G. Phys. Chem. Chem. Phys. 2000, 2, 3341. (14) Huang, Y.; Havenga, E. A. J. Phys. Chem. B 2000, 104, 5084. (15) Goyal, R.; Fitch, A. N.; Jobic, H. J. Phys. Chem. B 2000, 104, 2878. (16) Mentzen, B. F.; Lefebvre, F. Mater. Res. Bull. 1997, 32, 813. (17) Portsmouth, R. L.; Duer, M. J.; Gladden, L. F. J. Chem. Soc., Faraday Trans. 1995, 91, 559. (18) Shah, D. B.; GuO, C. J.; Hayhurst, D. T. J. Chem. Soc., Faraday Trans. 1995, 91, 1143. (19) Schroder, K. P.; Sauer, J.; Leslie, M.; Catlow, C. R. A. Zeolites 1992, 12, 20. (20) Hollenberg, J. L.; Dows, D. A. J. Chem. Phys. 1962, 37, 1300. (21) Mair, R. D.; Hornig, D. F. J. Chem. Phys. 1949, 17, 1236. (22) Smith, J. V. In Zeolitic Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171, American Chemical Society: Washington, DC, 1976; p 47. (23) Abrams, L.; Corbin, D. R. J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 21, 1.
