J. Phys. Chem. 1995,99, 12327-12331
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Sorptive Properties and Reactivity of Cloverite Single Crystals Studied by in Situ FTIR Microscopy G. Muller,+G. Eder-Mirth,+H. Kessler,' and J. A. Lercher*,t University of Twente, Department of Chemical Technology, Christian Doppler Laboratory for Heterogeneous Catalysis, Postbox 21 7, 7500 AE Enschede, The Netherlands, and Laboratoire de Mattriaux Mintraux, URA du CRNS 428, Ecole Nationale Suptrieure de Chimie, 3 Rue Alfred Wemer, F-68093 Mulhouse Cedex, France Received: May 24, 1 9 9 9
The nature, strength, and reactivity of the sorption sites of single crystals of the microporous gallophosphate cloverite were studied in siru by IR microscopy. Two kinds of structural hydroxyl groups of high (Ga-OH groups) and moderate (P-OH groups) Brgnsted acid strength were identified. Upon sorption of polar molecules, however, a partial opening of the Ga-0 bond next to the structural hydroxyl group occurs, leading to a concerted Brgnsted (Ga-OH) and Lewis (Ga ion) type interaction of the sorption site with the probe molecule. This change in coordination of the Ga ion upon sorption of polar molecules leads to an unusual reactivity. Methanol, for example, reacts with different surface species in cloverite, which lead to different reaction products. While the weakly hydrogen-bonded molecules interacting with the P-OH groups desorbed unreacted at 300 K, the molecules sorbed at the Ga-OH groups reacted to either dimethyl ether (characteristic for Bronsted acid sites) or formaldehyde (characteristic for Lewis acid sites). The stability of the cloverite structure was found to be strongly dependent on the coordination of the Ga ions. While the microporous structure is stable, when the structural rearrangement of the Ga ions is confined to a 5-fold (as it occurs in the as-synthesized form or after sorption of larger polar molecules) or a 4-fold (activated form, in inert atmosphere) coordination, it collapses upon sorption of small polar molecules which force the Ga ion in an octahedral (6-fold) coordination. This occurs, for example, with sufficient concentrations of water at 300 K and with ammonia at elevated temperatures.
Introduction
Experimental Section
Cloverite is a microporous gallophosphate which possesses a complex interrupted framework built of double four-rings (D4R) with alternating Ga and P atoms. These D4Rs form aand rpa-type cages, each of them containing a F- ion in the center (counterion quinuclidine-H+) in the "as-synthesized" form. In "as-synthesized" cloverite, Ga is pentacoordinated, whereas P assumes tetrahedral coordination. Two nonintersecting 3-dimensional pore systems exist. The large pore system consists of supercages with a diagonal diameter of 2.9 nm (being the largest cavities in molecular sieves known so far) connected by 20-membered rings (free diameter = 1.32 nm) showing the shape of a cloverleaf. The shape of the window is due to terminal OH groups (attached to one type of D4Rs) protruding into the window (see Figure 1). The small pore system (eightmembered rings, free diameter -0.4 nm) runs through the aand rpa-type cages (see Figure l).1-4 Up to now, most reports on cloverite deal with the characterization of the lattice in the "as-synthesized" form by solid state MAS2-7 and static NMR,5 XPS,6FTIR,4,6 Raman: TGA, DTA,2 and TGA-MS, TGA-DSC4 Rather limited information, however, is available on the sorption and catalytic properties of cloverite.1-4,6 The aim of the present study is to evaluate not only the nature, strength, and stability of the acid sites by sorption of probe molecules but also their reactivity in selected heterogeneously catalyzed reactions. The use of single crystals allows to avoid any ambiguities in relation to amorphous materials present.
Materials. The samples used were single crystals with pseudohexagonallsquareshape with a diameter between 80 and 140 pm. The polycrystalline material had a particle size of -1 pm. For the synthesis, the molar starting composition was 1 Ga203: 1 P2O5:6 quinuclidine:0.75 HF:64 H20, where gallium sulfate was used as source for Ga203. The mixture was heated at 423 K for 24 h in Teflon-lined stainless steel autoclaves.' Sorption Experiments. Probe molecules used were the weak bases toluene and benzene, the moderately basic alcohols (methanol, ethanol, propanol), and the strong bases ammonia and pyridine. The experiments were carried out in a vacuum cell equipped with IR transparent windows attached to the stage of a Bruker IR microscope. The spectrometer used was a Bruker IFS 88 (4 cm-' spectral resolution). The template was removed by heating the sample with 10 Wmin up to 800 K in air, holding this temperature for 2 h, and subsequently evacuating the system to mbar and cooling it to 300 K. The sorbates were introduced into the reaction cell via a gas inlet system held at 300 K. The partial pressure was held constant until sorptioddesorption equilibrium was achieved and was then increased in steps from to 1 mbar. After sorption at 1 mbar, the system was again evacuated to mbar for 90 min, followed by a temperature-programmed desorption (heating rate 10 Wmin, final temperature between 800 and 900 K). Sorption and desorption were followed in situ by time-resolved FTIR microscopy. For probing the stability against water, additional experiments were carried out in a CSTR type microreactor equipped with IR transparent windows (for details see ref 8). The template of the molecular sieve was removed by heating the sample with 10 Wmin up to 800 K in synthetic air (150 d l m i n ) , holding
XRD,'32,495
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@
University of Twente. Laboratoire de MatCriaux MinCraux. Abstract published in Advance ACS Absrracrs, July 15, 1995.
0022-365419512099-12321$09.0010
0 1995 American Chemical Society
12328 J. Phys. Chem., Vol. 99, No. 32, 1995
Muller et al.
B
I /
structural 1/' hydroxyl groups
Figure 1. Visualization of the structure of cloverite (A), the channel system (note that the channels are shown with their free diameter) (B), and a D4R with two structural OH groups (C). absorbance [arb. units]
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activated "as synthesized" cloverite cloverite
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Figure 2. IR spectra of the "as-synthesized" cloverite and cloverite activated at 800 K for 90 min in air at 300 K.
this temperature for 2 h, and cooling to 400 K. Then, the flow was switched to pure He (150 mWmin). Water was applied to the He stream by means of a syringe pump; the partial pressures were adjusted in the range from 0.1 to 10 mbar. The experiments were carried out in the temperature interval from 300 to 800 K. By means of a dead volume free four-port crossover valve, the flow was switched between pure He and He containing water. The sorption was followed in situ by timeresolved FTIR microscopy. The polycrystallinematerial was pressed into self-supporting wafers which were treated like the single crystals. These samples were measured in another high-vacuum cell which was placed in the Bruker IFS 88 spectrometer (for details see ref 9). This cell was connected to a Balzers QMG 410 mass spectrometer which was used to monitor the gas phase composition during the temperature-programmed desorption.
Results and Discussion The spectrum of the activated cloverite (Figure 2) showed two absorption bands at 3702 and 3675 cm-l, which are assigned to OH stretching vibratiom6 The band at 3675 cm-l (low frequencyLF-OH band) is observed in the "as-synthesized" form as well as (with higher intensity) in the calcined form (see Figure 2) and is assigned to P-OH groups placed in
the 20-membered rings. This assignment is in accordance with absorption maxima reported for P-OH groups in SAPO5 (3670 cm-l)lo and Gap04 (3675 cm-l).ll The second OH band (3702 cm-'), however, is more difficult to attribute. On the one hand, Hair and Hertl12 reported an additional absorption maximum for silica-supported H3P04 at 3700 cm-l which they assigned to geminal OH groups attached to phosphorus. On the other hand, quite controversial results are reported in the literature for the absorption maxima observed for Ga-OH groups. For surface OH groups of a-Ga203 an absorption maximum at 3640 cm-l was reported,I3 and for Ga203 supported on HZSM-5 an additional band at 3660 cm-' was observed; however, its intensity was not related to the Ga loading.l4 For Ga-OH groups of extraframework gallium oxide species two absorption bands are found at 3670 and 3700 cm-l.15 Therefore, we rather tend to attribute the band at 3702 cm-l (high frequency/HF-OH band) to Ga-OH stretching vibrations. This assignment differs from that given earlier in the communication of Barr et aL6 and is supported by recent literature data on Ga-OH groups of extralattice gallium oxide.l5 In addition, the sorption experiments with probe molecules described in detail below agree well with this assignment. The absorption bands in the spectral region below 2500 cm-' are attributed to overtones and combinations of the lattice vibrations of the gallium phosphate. Sorption of WaterBtability of the Structure. The lattice of the activated cloverite is stable under vacuum as well as in air (at atmospherical partial pressures of water) between 400 and 950 K. Sorption of water at 300 K took place on both the LF- and HF-OH bands and gave rise to new bands at 3448, 3038 (broad, attributed to OH stretching vibrations of water), and 1630 cm-l (H-0-H deformation vibration of water). At partial pressures of water up to 0.1 mbar, only a fraction of the OH groups of cloverite were covered with water molecules (even after equilibration over 15 h). These molecules could be removed upon evacuation without altering the structure of the molecular sieve. After increasing the pressure to 0.5 mbar, however, the spectral features described above could not be observed anymore, and a broad absorption between 3600 and 2100 cm-' dominated the spectrum. Under these conditions, a coverage higher than one molecule sorbed per structural OH group was obtained. After reaching such a high loading, the structural
J. Phys. Chem., Vol. 99, No. 32, 1995 12329
Reactivity of Cloverite Single Crystals absorbance [arb. units] 0.7 250
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Figure 3. IR spectra of benzene sorbed on cloverite at 0.1 and 1 mbar.
hydroxyl groups could not be regained upon evacuation or heating of the sample. The IR spectrum of the material obtained after these treatments showed a OH band with low intensity around 3675 cm-l. The change in the IR spectrum of the sample was furthermore paralleled by a clearly visible volume contraction of the crystal. Note that a P-OH band at 3675 cm-' was also observed for dense GaP04.I' Thus, we conclude that the crystal structure of the molecular sieve was changed upon water sorption at 300 K leading to the formation of amorphous GaP04.3 We suggest that the collapse of the lattice structure is affiliated with the adsorption of more than one water molecule per Ga-OH group. It should be emphasized that the activated sample is unstable in ambient atmosphere at room temperature. Sorption of Benzene and Toluene. Sorption of the apolar molecules benzene and toluene was studied in order to evaluate the acid strength of the structural hydroxyl groups. It is known that the magnitude of the red shift of the VOH upon adsorption of a donor molecule is directly proportional to the acid strength of the OH group.'2 The wavenumber shifts detected for benzene were approximately 250 (HF-OH) and 220 cm-I (LF-OH). Compared to the 260 cm-l observed upon sorption of benzene on the SiOHAl groups of HZSM-5, these shifts indicate high and moderate acid strength of the OH groups. For benzene as well as toluene, the relative coverage of the two OH groups was nearly the same with a slight preference for the lowfrequency OH group. As the aromatic molecules interacted with both types of Bransted acid sites, we assume that all sites are located in the large pore channel system, because the free diameter of the small pore channel system is smaller than the kinetic diameter of the aromatic ring. The IR spectra of sorbed benzene (partial pressure of 0.1 mbar) showed strong absorption bands at 3092,3071, and 3037 cm-I (attributed to the CH stretching vibrations of the aromatic ring) and at 1497 cm-I (assigned to ring vibrations); weak bands appeared at 1958, 1814 (combination vibrations), and 1617 cm-l (ring vibration).I6 The broad band centered at 3455 cm-l is attributed to OH stretching vibrations of cloverite perturbed by hydrogen bonding to benzene (see Figure 3).17,18 The spectrum of cloverite recorded after contacting it with 0.1 mbar of toluene showed intense new IR bands at 3086,3061, and 3027 cm-l (CH stretching vibrations of the ring), at 2923 and 2871 cm-l (CH stretching vibrations of the CH3 group), at 1604 and 1495 cm-l (ring vibrations), and at 1463 (ring vibrations CH3 deformation) and 1380 cm-l (CH3 deformation). Weak IR bands appeared at 1941, 1858, and 1803 cm-l (overtone and combination vibrations of toluene).I6.l7 The broad band of the OH vibrations of the molecular sieve hydrogen bonding to toluene was found at 3425 cm-1,17.18 234
+
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1600 1500 1400 wavenumber [ licm]
wavenumber [I/cm]
Figure 4. IR spectra of pyridine and ammonia sorbed on cloverite at
0.1 mbar (L = Lewis type interaction, B = Briinsted type interaction).
It should be noted that only one broad band of perturbed OH vibrations was observed, although benzene and toluene interacted with both OH groups of cloverite. This indicates that the broad band contains contributions of both hydroxyl groups engaged in hydrogen bonding. At 300 K, the sorption of the aromatic molecules was completely reversible. At higher partial pressures (0.5 mbar, corresponding to an approximate coverage of one molecule benzeneltoluene sorbed per hydroxyl group) additional bands appeared at 3620 (benzene) and 3610 cm-' (toluene). Based on the absorption maximum, this band can only be assigned to an OH stretching vibration. One might speculate that the band results from a red shift of one of the OH bands of cloverite, indicating very weak interactions between the aromatic and the OH groups because of sterical constraints. Sorption of Ammonia and Pyridine. Sorption of ammonia and pyridine was performed to check whether or not the hydroxyl groups were strong enough to protonate the two bases. The spectra of ammonia and pyridine interacting with cloverite are shown in Figure 4. Both probe molecules gave rise to bands typical of Brmsted (absorption bands assigned to protonated NH321 at 1450 cm-l and the pyridinium ion19320with the characteristic IR band at 1543 cm-l) and Lewis type bonded species (IR bands of coordinatively bound species at 1616 cm-' for NH320 and 1613 cm-' as main feature for pyridine,'9-20 respectively). With increasing coverage, the absorption bands of Lewis and Bransted bonded species increased in parallel in intensity, while the intensity of the two OH groups decreased. At a partial pressure of 0.1 mbar, all OH groups were covered. After evacuation to loF6mbar for 90 min, the IR spectra did not change, indicating strong interaction of the base molecules with cloverite. During temperature-programmed desorption (TPD), NH3 and pyridine showed different thermal stability. Whereas the LFOH group was nearly completely recovered at 900 K in the case of pyridine, it was only partially restored in the case of NH3. For both molecules, the HF-OH group could not be restored under these conditions. Treatment of the loaded catalyst in synthetic air at 800 K, however, led to a complete recovery of the HF-OH group when pyridine was adsorbed but was unsuccessful in the case of ammonia. The fact that the structural Ga-OH groups of cloverite could not be regained after sorption of ammonia, even after a severe thermal treatment, led us to conclude that a structural change might have occurred upon interaction of NH3 with the Ga-OH groups. Further support for this suggestion is given by Patarin et ~ l . who , ~ reported an amorphization of cloverite after a stepwise thermal desorption of ammonia (measured by XRD).
Muller et al.
12330 J. Phys. Chem., Vol. 99, No. 32, 1995 Compared to other acidic molecular sieves, the temperature of the desorption maximum for pyridine and NH3 found with cloverite was very high. Sorbed pyridine for example desorbs at 850 K from HZSM-5.22 As HZSM-5 is known to possess very strong Bronsted acidic bridging hydroxyl groups, we conclude that the Bronsted acidity of cloverite does not account alone for the strong sorption of NH3 and pyridine. The unusual high stability of the base moleculekloverite systems is, thus, proposed to be caused by a concerted interaction of Bronsted and Lewis acid sites. This concept of a concerted interaction between Lewis and Brgnsted acid sites implies that upon sorption one of the Ga-0 bonds is partially opened and that the primary interaction takes place between the then accessible Ga cation and the base molecule. This might induce a rehybridization of the Ga cation from tetrahedral symmetry first to a 5-fold coordination and then to complete octahedral (corresponding to 6-fold) coordination. The 6-fold coordination should be easily possible for the smaller ammonia molecules, as they can enter also the small pore system (free inner diameter -0.4 nm) and coordinate to the Ga ion through the D4R (see Figure 1). For the larger pyridine molecules a breaking of the Ga-0 bond should be also possible. However, the additional interaction of a second molecule via the small pore system cannot be achieved, leading only to a 5-fold coordination of the Ga upon sorption of pyridine. Sorption of Alcohols. To test whether the concept of a concerted interaction between Lewis and Bronsted acid sites is also applicable to the interaction with other polar molecules, we investigated the sorption and reactivity of the weak bases methanol, ethanol, and propanol. In contrast to the other molecules, alcohols sorbed first on the HF-OH (Ga-OH) groups and only after nearly complete coverage of these sites on the LF-OH (P-OH) groups. The preferential uptake of the molecules on the HF-OH group compared to the LF-OH group decreased in the order methanol >> ethanol > propanol. The IR spectra of methanol sorbed on the two OH groups were quite disparate, indicating different types of interaction with the two types of hydroxyl groups. This is clearly seen in the variation of the absorption maxima and the relative intensities of the CH stretching bands as the coverage increased (see Figure 5). At low coverage, when interaction occurred exclusively with HF-OH (Ga-OH) groups, bands at 2940 and 2828 cm-' (attributed to the asymmetric and symmetric methyl stretching vibration, r e s p e ~ t i v e l y ~ ~dominated -~~) (species I). After longer equilibration times, additional bands at 2950 and 2838 cm-l appeared (assigned to species 11). With increasing coverage, bands attributed to a hydrogen-bonded species (species 111) sorbed on the LF-OH (P-OH) groups gained in intensity (3000,2962,2923 (shoulder), and 2855 cm-*). These hydrogenbonded molecules desorbed upon evacuation at 300 K, whereas the molecules sorbed at the HF-OH groups remained unaffected. The marked shift of the absorption maxima of the CH stretching vibrations of methanol sorbed on the HF-OH groups compared to methanol sorbed on weakly acidic materials like Si0226 (3001, 2958, 2925 (shoulder), and 2856 cm-l) cannot be explained by a hydrogen-bonding interaction of methanol to the Ga-OH groups. Taking into account that the absorption maxima of the sorbed methanol (species I) are comparable to those of methanol sorbed on Rb+ cations in the basic zeolite Rb-X, we attribute this species (I) to be strongly bound to a Lewis acid site. As Lewis type bonded species were only observed after sorption of methanol and other polar molecules like NH3 and pyridine but not with apolar molecules like
absorbance [arb. units] 0.7 1
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Figure 6. Visualization of the interaction of methanol with a HFOH (Ga-OH) hydroxyl group. Note: italic letters denote the atoms of methanol and normal letters those of cloverite.
benzene and also not with the higher alcohols, it is proposed that strong polarity is required for this type of interaction. It is suggested that a Ga-0 bond connecting the Ga with the neighboring PO4 tetrahedra is partially opened upon sorption of MeOH, leading to a coordinative bonding of the alcohol oxygen with the Ga cation (Lewis electron pair donor-acceptor type interaction) and hydrogen bridge bonding between the hydrocarbon part of the alcohol and the HF hydroxyl group (see Figure 6). This model is consistent with the parallel observation of a decrease in the intensity of the Ga-OH stretching vibrations and a Lewis type interaction upon sorption of polar molecules like methanol, ammonia, or pyridine. The spectra of the less polar alcohols, i.e., ethanol and propanol, suggest only hydrogen-bonding interactions between their hydroxyl groups and the OH groups of cloverite. Upon evacuation at 300 K, almost complete desorption of the sorbed alcohol molecules form the P-OH groups was observed. It is interesting to note that during TPD the two methanol species (I 11) interacting with the Ga-OH groups desorbed reactively as dimethyl ether (maximum of the rate of desorption 500 K) and formaldehyde (670 K) with distinctively different surface species as precursors. The surface species (11) with characteristic bands at 2950 and 2838 cm-l led to the formation of dimethyl ether, whereas the absorption bands at 2940 and
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J. Phys. Chem., Vol. 99, No. 32, 1995 12331
intensity [arb. units] dimethylethcr formaldehyde [de451 [de=30]
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Figure 7. IR spectra of methanol sorbed on cloverite during TPD, assigning the surface precursors and gas phase mass spectra of dimethyl ether and formaldehyde during TPD (starting at 300 K with 10 Wmin to 800 K).
2828 cm-' indicate a surface species (I) leading to the formation of formaldehyde (see Figure 7). The higher alcohols sorbed on the Ga-OH groups showed IR spectra characteristic for only one type of hydrogen-bonded species. Ethanol and propanol desorbed partially unreacted, partially after dehydration as ethene and propene.
Conclusions The nature, strength, and reactivity of the sorption sites in the molecular sieve cloverite were assessed by in situ IR microscopy. Cloverite possesses two kinds of Bransted acid sites, Le., structural Ga-OH groups with strong acidity and structural P-OH groups with moderate acid strength. All of them are located in the 20-membered rings. Sorption of apolar molecules like benzene and toluene was found to be reversible at room temperature, indicating only weak hydrogen bonding with the OH groups of cloverite. Upon sorption of polar probe molecules, a unique behavior of this molecular sieve was observed. The interaction with these molecules involves at least a partial opening of Ga-oxygen bonds next to the structural Ga-OH groups, leading to a strong Lewis acidic character of the molecular sieve. The %fold coordination of the Ga ion is changed upon the sorption process. Whether or not this change in the coordination of Ga leads to the collapse of the lattice seems to depend upon the ability of the polar molecule to force the Ga cation into a complete 6-fold coordination. This is only possible for molecules which can access the small pore system and coordinate via the D4Rs to the Ga cations like water (at 300 K) and ammonia (at higher temperatures). This explains why only the sorption of these small polar molecules leads to an amorphization of the molecular sieve. Methanol and especially pyridine, however, are too large to enter the small pore system and coordinate via the D4Rs and can, thus, only induce 5-fold coordination of the Ga cation. This results in a strong Lewis type interaction but does not lead to a destruction of the crystal structure of cloverite. Note that the structure of cloverite is also stable in theas-synthesized form, where Ga is also present in 5-fold coordination.
The concerted interaction of Bronsted and Lewis acid sites with the probe molecules leads to a completely unusual reactivity of this molecular sieve. As shown for methanol, different sorption species were formed at the structural hydroxyl groups of cloverite. While the weakly hydrogen-bonded molecules interacting with the P-OH groups desorbed unreacted at 300 K, the molecules sorbed at the Ga site reacted to either dimethyl ether (characteristic for Bronsted acid sites) or formaldehyde (characteristic for Lewis acid sites). The formation of these two products could be directly correlated with the presence of two different surface precursor species.
Acknowledgment. We gratefully acknowledge the financial support by the Christian Doppler Society. The graphic displays shown in Figures 1 and 6 were generated from the INSIGHT I1 molecular modeling system (BIOSYM Technologies Inc.). References and Notes (1) Esterman, M.; McCusker, L. B.; Baerlocher, C.; Merrouche, A,; Kessler, H. Nature 1991, 352, 320. (2) Merrouche, A.; Patarin, J.; Kessler, H.; Soulard, M.; Delmotte, L.; Guth, J. L.; Joly, J. F. Zeolites 1992, 12, 226. (3) Patarin, J.; Schott, C.; Merrouche, A.; Kessler, H.; Soulard, M.; Delmotte, L.; Guth, J. L.; Joly, J. F. In Proceedings of the 9th International Zeolite Conference; von Ballmoos, R., et al.; Butterworth-Heinemann: Oxford, 1993; p 263. (4) Bedard, R. L.; Bowes, C. L.; Combes, N.; Holmes, A. J.; Jiang, T.; Kirkby, S. J.; Macdonald, P. M.; Malek, A. M.; Ozin, G. A,; Petrov, S.; Plavac, N.; Ramik, R. A.; Steele, M. R.; Young, D. J . Am. Chem. SOC. 1993, 115, 2300. (5) Zibrowius, B.; Anderson, M. W.; Schmidt, W.; Schuth, F.-F.;Aliev, A. E.; Harris, K. D. M. Zeolites 1993, 13, 607. (6) Barr,T. L.; Klinowski, J.; He, H.; Alberti, K.; Muller, G.; Lercher, J. A. Nature 1993, 365, 429. (7) Bradley, S.; Howe, R. F.; Hanna, J. V. Solid State Nucl. Magn. Reson. 1993, 2, 37. (8) Muller, G.; Narbeshuber, T.; Mirth, G.; Lercher, J. A. J . Phys. Chem. 1994, 98, 7436. (9) Jentys, A.; Warecka, G.; Lercher, J. A. J . Mol. Catal. 1989, 51, 309. (10) Halik, C.; Lercher, J. A.; Mayer, H. J . Chem. Soc., Faraday Trans. 1 1988, 84, 4457. (11) Krvmova, V. V.; Kitaev, L. E.; Kubasov, A. A.; Gryaznova, Z. V.; Eshchenko, L. S.; Pechkovskii, V. V. Vestn. Mosk. Univ., Ser. 2: Khim. 1979, 20 ( 5 ) . 476. (12) Hair, M. L.; Hertl, W. J . Phys. Chem. 1970, 74, 91. (13) Mbriaudeau, P.; Primet, M. J . Mol. Catal. 1990, 61, 227. (14) MCriaudeau, P.; Naccache, C. Appl. Catal. 1991, 73, L13. (15) Bemdt, H.; Martin, A,; Kosslick, H.; Liicke, B. Microporous Mater. 1994, 2, 197. (16) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; Chapman and Hall: London, 1975; pp 73-96. (17) Mirth, G.; Lercher, J. A. J . Phys. Chem. 1991, 95, 3736. (18) Jentys, A.; Lercher, J. A. Stud. Sut$ Sei. Catal. 1989, 46, 585. (19) Parry, E. P. J . Catal. 1963, 2, 371. (20) Connel, G.; Dumesic, J. A. J . Catal. 1987, 105, 285. (21) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966. (22) Lercher, J. A.; Rumplmayr, G. Appl. Catal. 1986, 25, 215. (23) Pohle, W.; Fink, P. Z. Phys. Chem. (Munich) 1978, 109, 77. (24) Mirth, G.; Lercher, J. A. Stud. SUI$ Sci. Catal. 1991, 61, 437. (25) Aronson, M. T.; Gorte, R. J.; Fameth, W. E. J . Catal. 1987, 105, 455. (26) MOKOW,B. A.; Thomson, L. W.; Wetmore, R. W. J . Catal. 1973, 28, 332.
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