129Xe NMR Investigation of ETS-10 Titanosilicate Molecular Sieves

Feb 29, 1996 - This issue is systematically investigated as a function of proton exchange in the present work using 129Xe NMR spectroscopy. Different ...
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J. Phys. Chem. 1996, 100, 3713-3718 129Xe

3713

NMR Investigation of ETS-10 Titanosilicate Molecular Sieves Xiaolin Yang* and Ralph E. Truitt Engelhard Corporation, 101 Wood AVenue, Iselin, New Jersey 08830 ReceiVed: July 28, 1995; In Final Form: October 13, 1995X

A newly discovered large-pore titanosilicate molecular sieve, ETS-10, has found increasingly important commercial use such as a desiccant in chlorofluorocarbon-free air conditioners. Its useful properties are due to unique three-dimensional channel structure and charge distribution in the material. Although the framework structure of Na-ETS-10 was established recently, the effect of cations on the local pore structure remains to be described. This issue is systematically investigated as a function of proton exchange in the present work using 129Xe NMR spectroscopy. Different from the behaviors of conventional zeolites such as Y and ZSM5, the “porosity” of ETS-10 is significantly affected by proton exchange: the effective “pore” diameter and Xe uptake are both reduced due to the formation of defects in 12-member ring channels. Xe adsorption isotherms reveal that energetically different adsorption sites coexist in the 12-member ring channels of NaETS-10. Proton exchange reduces the difference by proton delocalization.

Introduction ETS-10 titanosilicate is a member of a class of new molecular sieves.1 These materials do not behave like conventional molecular sieves for catalysis and adsorption purposes and find novel applications such as for manufacturing chlorofluorocarbon (CFC)-free air conditioners based on evaporative and desiccant cooling.2 A crystal structure model of ETS-10 has been proposed by Anderson et al. based on a detailed chemical analysis, structural modeling, XRD, HREM, and a highresolution MAS 29Si NMR study.3 It is believed that the framework of ETS-10 consists of corner-sharing tetrahedrally coordinated silicon and octahedrally coordinated titanium linked through bridging oxygens. The pore system is a threedimensional 12-member ring channel network and displays a considerable degree of disorder. Figure 1 shows the local atomic connectivity and the channel structure of the framework of ETS-10. In ETS-10, every Ti atom is connected octahedrally to four other Si and two Ti atoms through oxygen bridges. The net charge on a TiO6 unit is -2, which is balanced by extraframework cations, such as Na+ and H+. Because Ti is only part of the small 7-member ring channels, water molecules in the wider 12-member ring channels do not bond to the framework as strongly as in conventional aluminosilicate zeolites, so that much lower drying temperatures are possible for ETS-10.4 On the other hand, because of the long-range electronic interactions induced by the negative charges, bonding between water in the 12-member ring channels and the framework is not as weak as in silica where there are no charge centers. Thus, a type I water adsorption isotherm can be achieved in ETS-10.5 These properties have made ETS-10 economically competitive to replace CFCs for manufacturing air conditioning systems. To understand the pore structure and the interaction between adsorbed molecules and the sieve is of fundamental importance in the study and application of ETS-10. 129Xe NMR is wellsuited for these purposes. Since the pioneering work by Fraissard et al.,6-10 Ripmeester et al.,11,12 Jameson et al.,13,14 and other research groups, 129Xe NMR has been extensively used in the pore structure characterization of various microporous materials, especially zeolites. Several excellent review * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, February 1, 1996.

0022-3654/96/20100-3713$12.00/0

Figure 1. Schematics of ETS-10 framework structure and pore systems, based on ref 3.

articles have summarized the development, theories, and various applications of 129Xe NMR.15-18 Xe has an atomic diameter of 4.4 Å, which restricts its entry to only the supercages of faujasites and channels of other zeolites. It is an inert gas so that chemical bonding with zeolite can be avoided. 129Xe has a spin quantum number of 1/2 and a relatively high natural abundance of 26.4%. It resonates at a fairly high frequency with reasonable short relaxation time inside zeolite pores. It has a large molecular polarizibility so that its chemical shift is very sensitive to its surrounding electronic environment. All these factors make 129Xe NMR a sensitive probe for investigating pore structure changes, partial channel blockages, and cation effects. This information is difficult to obtain by conventional gas adsorption methods, such as BET. Using 129Xe NMR, we systematically studied the porosity of ETS-10 as a function of proton exchange in the present work. Fraissard et al. have systematically studied cation effects on the porosity of zeolite Y.7 They found that the chemical shift is practically independent of the number and the nature (Na or H) of the cations present at room temperature. This was explained by the argument that the Xe-Na or Xe-H interactions are so weak that their mean electric field effects are negligible at 25 °C. However, we found in the present work that proton exchange has a dramatic effect on the 129Xe NMR chemical shift and adsorption isotherm of ETS-10. This new © 1996 American Chemical Society

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TABLE 1: Elemental and Surface Area Analysis Data Summary sample

pHa

Ti

Na-ETS-10 ETS-10-1.74 ETS-10-1.16 ETS-10-0.58 H-ETS-10

11.04 8.0 5.0 4.0 2.5

1 1 1 1 1

atomic ratio Na Si 2.14 1.74 1.16 0.58 0.18

surface areab (m2/g)

5.16 5.15 5.23 5.17 5.18

313 293 306 307 288

a pH value at which the proton exchange was performed. b The values are the micropore area per gram dried sample on a constant Ti-content basis. The sample were pretreated in a 10-2 Torr vacuum at 250 °C for 6 h. The cutoff diameter for micropores is 10 Å.

(2) Xe Adsorption. All the samples were equilibrated to room temperature and moisture and then dried under a 10-2 Torr vacuum at 300 °C for 24 h. Some structural destruction was observed for H-ETS-10 above 400 °C. TGA (thermal gravimetric analysis) profiles show that water is effectively removed at 300 °C. 129Xe NMR measurements as a function of dehydration temperature show that, after 200 °C, both Xe uptake and chemical shift do not change significantly. The Xe adsorption was made in a 10 mm NMR tube with PTFE valve (Aldrich) which allows Xe loading without exposing the sample to air. Xe uptake and 129Xe NMR were measured 15 min after the samples were equilibrated with the Xe gas at desired pressures. Xe (99.995%) from Liquid Carbonic, Inc., was used without further purification. Two independent methods were used to measure Xe uptake per gram of dried zeolite. The first uses an analytical balance to measure the weight gain due to Xe adsorption, WXe/Ws, where WXe and Ws are the weights of adsorbed Xe gas and the dried sample, after making a correction for Xe in dead space. The second directly measures the 129Xe NMR integrated intensity due only to the Xe adsorbed in the micropores, Int/Ws. Differences in Xe uptake were observed between the two methods, particularly at high Xe loading. However, as will be shown later, the chemical shift values extrapolated to zero Xe pressure (δ*) are almost identical between the two methods. (3) 129Xe NMR Measurements. All the 129Xe NMR spectra were taken at room temperature using a Varian Unity-400 spectrometer which is operated at 110.64 MHz for the 129Xe nucleus, using a Varian 10 mm probe. A single π/2 pulse was used for the excitation with 3.0 s recycle time, 3 µs dead time, and 128 scans. The chemical shift is referenced to Xe gas extrapolated to zero Xe pressure.19

Figure 2. XRD powder patterns of Na- and H-ETS-10.

Theory finding is discussed based on the unique framework structure and charge distribution of ETS-10. Experimental Section (1) Sample Preparation. ETS-10 samples were synthesized based on the original patents.1 In an effort to eliminate ambiguity resulting from the presence of other cations, a Naonly gel was used to crystallize a Na-ETS-10 starting material. XRD and 29Si NMR indicate that the Na-only ETS-10 has the same framework structure as those of Na- and K-containing ETS-10.1,3 Proton ion exchange was performed by routine cation-exchange procedures using sulfuric acid and a single batch of Na-ETS-10 starting materials. Long-range crystal structure and the phase of the samples were monitored by XRD. Conventional molecular sieve porosity was determined using standard BET surface area analysis. Si, Ti, and Na contents were measured by XRF (X-ray fluorescence) elemental analysis. Table 1 lists the elemental and BET surface area analysis results. The ideal molecular formula for Na-ETS-10 is Na2Si5TiO13.3,5 In the framework structure, the stoichiometry atomic ratio of Si to Ti is 5. The elemental analyses of Si/Ti for all the samples are in the range 5.15-5.23. Figure 2 shows XRD powder patterns of Na- and H-ETS-10, proton form of ETS-10. For H-ETS-10, although some peak intensities are reduced at larger diffraction angles possibly due to the removal of Na+ ions which are better diffraction centers than protons, no significant peak broadening was observed. This indicates that the crystallinity of H-ETS-10 is not significantly decreased. BET surface area analysis with N2 gas (Table 1) also shows that the micropores (total numbers of pores with diameter less than 10 Å) remain relatively constant as Na content is varied.

Because 129Xe NMR is an indirect method to measure the zeolite pore structure using probing molecules, it is necessary to outline a few fundamental relationships between 129Xe NMR observable parameters and zeolite pore structure. The most important 129Xe NMR parameter is the chemical shift extrapolated to zero Xe loading. According to Fraissard et al.,6-10 the measured chemical shift can be expressed as

δ ) δ0 + δs + δe + δXe[Xe] + δp

(1)

where δ0 is the reference, δs is due to the effect of the framework structure (pore void), δe is the contribution from electronic interactions between Xe atoms and cations, δXe is due to the interaction between Xe atoms, [Xe] is the number of Xe atoms adsorbed in a cavity, and δp is the term due to paramagnetic centers which are not present in ETS-10. Thus, at [Xe] ) 0, the measured chemical shift δ* is composed by only two terms:

δ* ) δs + δe

(2)

Fraissard et al. have experimentally established a quantitative relationship between δs and the mean free path l of Xe adsorbed in the cavities:8

δs ) (234 × 2.054)/(2.054 + l)

(3)

and l can be related to the diameter of the cavity:

l ) 0.5(Ds - DXe) (spheric)

(4a)

l ) Ds - DXe (infinity cylinder)

(4b)

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Figure 3. Effect of Xe loading and Na content on 129Xe NMR chemical shift. The fitted linear lines were drawn based on the first three data points at high Xe loading.

where Ds is the diameter of a spheric or cylindric pore and DXe is the diameter of Xe atom ) 4.4 Å. Thus, a measurement of δ* and δs leads to the diameter of the zeolite pore, assuming the zeolite pores are spheric or cylindric. Another important 129Xe NMR parameter is the slope of the plot of chemical shift vs WXe/Ws (or Int/Ws). The slope measures the total number of pores or pore volume accessible for Xe adsorption, assuming all the pores are the same size. WXe/Ws, the total number of Xe atoms adsorbed by a gram of zeolite, is proportional to the product of the total number of pores present in a gram of zeolite, [Z], and the number of Xe atoms adsorbed in a single cage, [Xe]:

WXe/Ws ) c[Z][Xe]

(5)

where c is a numeric constant depending on the zeolite nature. Substituting eqs 2 and 5 into 1

δ ) δ* + δXe{WXe/Ws}/{c[Z]}

(6)

Plotting δ vs WXe/Ws, the intercept gives δ* and the slope is

S ) δXe/{c[Z]}

(7)

For two samples of same zeolite, i.e., same pore diameter and length, the relative zeolite content in two samples is

[Z1]/[Z2] ) S2/S1

(8)

Fraissard and co-workers have used a similar slope approach to estimate the total number of supercages and crystallinity in pure zeolites.9 On the other hand, if the total number of pores are the same for the two samples while their pore sizes are slightly different, Z1/Z2 measures the ratio of their total pore volumes. Results 1. Effect of Xe Loading on 129Xe NMR Chemical Shift. All the 129Xe NMR spectra of ETS-10 samples consist of only one sharp peak. This is consistent with the structural model that Xe can only enter the 12-member ring channels.3,6 As shown in Figure 3, nearly a linear correlation was found between Xe loading and the chemical shift at different Na/Ti ratio. Deviation from the straight line is seen at lower Xe loadings. The nonlinear behavior has been seen in other zeolites when the Xe-Xe collision distribution is not symmetric due to effects such as partial pore blockage, presence of metal particles, coexistence of different cations, etc. In the present case, we speculate that the nonlinearity may be due to the presence of defects and different adsorption sites. This will be discussed later.

Figure 4. Dependence of δ* on Na/Ti: (a) extrapolating WXe/Ws to zero Xe pressure using the first three data points at high Xe loading, (b) extrapolating Int/Ws to zero Xe pressure using the first three data points at high Xe loading, and (c) extrapolating Int/Ws to zero Xe pressure using the last two data points at low Xe loading.

2. Effect of Na(H) Content on Chemical Shift at Infinite Xe Loading. Figure 4 shows the chemical shift extrapolated to zero Xe loading, δ*, as a function of Na(H) content expressed by Na/Ti ratio. δ* was obtained in three ways: (a) extrapolating WXe/Ws to zero Xe loading using the first three points at high Xe loading; (b) extrapolating Int/Ws to zero Xe loading using the first three data points at high Xe loading, and (c) extrapolating Int/Ws to zero Xe loading using the last two data points at low Xe loading. Two systematic trends are observed in Figure 4: (1) Almost identical δ* values are obtained for Xe adsorption measured by either weight gain or NMR signal intensity (curve a and b). The excellent agreement between the two independent methods serves as a self-consistent check. (2) Although the absolute values are different, all three extrapolating procedures show a similar correlation between δ* and Na/Ti (curve a and c): δ* increases as Na content decreases. In other words, the effective channel diameter decreases as more Na ions are replaced by protons. This is different from the behavior of other zeolites such as Y where Na content does not have any affect on δ*. Using eqs 3 and 4b, the effective channel diameter Ds is calculated based on assumptions that the pores are infinite long cylinders and δe is negligible. A value of 8.8-9.9 Å is obtained for Na-ETS-10, depending on how δ* was extrapolated. For H-ETS-10, Ds is in the range 6.8-7.2 Å, a reduction of about 2 Å compared with the Na form. We must point out here that the calculation is only a semiquantitative estimation because ETS-10 is not composed of ideal, infinity long cylinders of same diameters and length. The “pore diameter” deduced from the NMR data is merely a measurement of the mean free path of Xe in a cavity. 3. Effect of Na(H) Content on Xe Adsorption Isotherm. Figure 5 shows the Xe adsorption isotherms as function of Na/ Ti. Two trends emerge from the plots. First, the adsorption isotherms are not linear with Xe pressure: Xe are adsorbed rapidly at lower pressures, but the curve is concave to the pressure axis at higher pressures. At higher Na content this trend becomes more pronounced. Second, Na-ETS-10 has higher Xe uptake than its H-ETS-10 counterpart. As we will discuss later, the Xe adsorption isotherm can reveal useful information about the Xe adsorption site distribution in a sample. 4. Effect of Na(H) Content on 129Xe NMR Line Width. NMR spectral line width can provide useful information about local structure inhomogeneity and spin-spin interactions. Figure 6 plots the line width (full width at half-height) as a function of Xe loading for Na- and H-ETS-10. For Na-ETS-10, the line width increases with Xe loading. However, the line width of

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Figure 5. Xe adsorption isotherm as a function of Na/Ti.

Yang and Truitt

Figure 8. Effect of Na content on spin-lattice relaxation time T1.

Torr), however, only the positive magnetization could be measured, and the T1 value thus obtained is slightly smaller than the value at high Xe loading for the same sample. Since T1 depends much more on Na content than Xe loading, we focus here on the effect of proton exchange on the T1 values at high Xe loading. A T1 value of about 1 s is obtained for Na-ETS-10. As Na ions are replaced by protons, T1 is significantly reduced, see Figure 8. H-ETS-10 has a T1 about 2 orders of magnitude smaller than the Na form. Discussion Figure 6. Effect of Xe loading on

129

Xe NMR line width.

Figure 7. Effect of Na content on 129Xe NMR spectra of ETS-10 at 20 Torr of Xe pressure. The spectra are plotted in such a way that the main peak of each spectrum is at 0 Hz.

H-ETS-10 decreases with Xe loading. The line width of the partially proton-exchanged sample lies between that of Na- and H-ETS-10. A detailed look of the 129Xe NMR spectra at low Xe loading (20 Torr) reveals that, for the fully and some of the partially proton-exchanged samples, the “broader” peak actually consists of at least two closely separated peaks, shown in Figure 7. This suggests that the abnormal line width trend of H-ETS10 is partly due to the presence of at least two different pore structures. 5. Effect of Proton Exchange on Spin-Lattice Relaxation Time. To further look into the effect of proton exchange on spin relaxation properties of ETS-10, we measured the spinlattice relaxation times of the ETS-10 samples, using a standard inverse-recovery technique (180°-τ-90°). At high Xe loading (600 Torr), all the samples show the typical -M(τ) to 0 and then to +M(τ) pattern as τ increases, and an exponential relaxation behavior is observed. At low Xe loading (e.g., 100

To understand the 129Xe NMR results and their implications for ETS-10’s pore structures, we start the discussion with a closer look at the unique pore structure of ETS-10. ETS-10 has a wide-pore, three-dimensional channel structure. According to Kuznicki et al.6 and Anderson et al.,3 the channel structure of ETS-10 consists of four types: 3-, 5-, 7-, and 12-member rings. However, Xe can only get into the largest channels due to its size. ETS-10 is characterized by the presence of significant disorders and faults.3 This is partly due to the coexistence and random stacking of two polymorphs: polymorph A with chiral symmetry and polymorph B with C2/C symmetry. We found recently by a multinuclear NMR study that Na+ ions in Na-ETS-10 are located mainly in the 7-member ring channels to balance the negative charges, and terminal hydroxyl groups are present in both 12- and 7-member ring channels of H-ETS-10, indicating that defects are formed in ETS-10 after proton exchange.20 On the basis of the known structural information and the present 129Xe NMR data, we hope to shed more light on the pore structure and cation effect on ETS-10 by answering the following two questions: (1) Why does Na(H) content have a strong effect on δ* and Xe adsorption isotherm of ETS-10? (2) Why does proton exchange have such a dramatically different effect on the 129Xe NMR spectral line width and spin-lattice relaxation time? (1) 12-Member Ring Channels Reduction Due to More Local Defects and Structural Compactness. As predicted by eq 2, δ* is composed of two terms: δs and δe. An increase of δ* after proton exchange can be due to two factors: a reduction of effective pore size or the Xe-H interaction is significantly different from that of Xe-Na. It is unlikely that the Xe-H interaction in H-ETS-10 is dramatically different from that of H-Y zeolite, assuming protons are located in both zeolites at the bridging (-Si-OH-X-) positions. Anderson et. al. has indicated that a small amount of larger micropores of 18- and 32-member ring channels exist in Na-ETS-10 by a coalescence of two or four 12-member ring channels.3b 129Xe NMR measured channel diameter for Na-ETS-10 (8.8-9.9 Å) is close to the value derived from the adsorption of “plug-gauge”

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J. Phys. Chem., Vol. 100, No. 9, 1996 3717

molecules (8.0 Å)5 and is consistent with the size of a 12-member ring channel. The single NMR peak for Na-ETS10 indicates that the amount of channels larger than 12-member ring is not significant if they are present. One possible cause for the 12-member ring channel diameter reduction after proton exchange is the formation of local defects in the 12-member ring channels. The overlap of at least two signals in 129Xe NMR for H-ETS-10 (Figure 7) at low Xe loading indicates that 12member ring channels are no longer uniform. The presence of defects such as silanol OH groups in some of the 12-member ring channels reduces the mean free path of Xe atoms, and thus δ*. Another possible cause is the reduction of stackings after proton exchange, which makes the structure more compact. The peak broadening at low diffraction angle, e.g., about 6° in Figure 2, can be related to disorder or stacking structures.21 In H-ETS10, the peak intensity is decreased while the peak width remains relatively unchanged. This leads to the hypothesis that the proton exchange has reduced the stackings in ETS-10. A HREM study should provide more direct evidence for this hypothesis. 2. 129Xe NMR Line Broadening and T1 Reduction in H-ETS-10 Due to Local Defects and 1H-129Xe DipolarDipolar Interactions. An observed spin-lattice relaxation rate of adsorbed 129Xe molecules can be due to a combined effect of several different sites:22-23

1/T1(observed) ) a/T1(a) + b/T1(b) + ...

(9)

Figure 8 shows that 1/T1 levels off at both low and high Na content ends, an indication of at least two relaxation sites. A significant enhancement of spin-lattice relaxation rate can be related to more efficient relaxation centers such as defects or stronger dipolar-dipolar interactions. In the present case, we believe 1H-129Xe dipolar-dipolar interactions between defects silanol OH groups and Xe atoms is likely for speeding the spinlattice relaxation process. This is also consistent with the broader peak width of H-ETS-10. We did a high-power proton decoupling experiment on H-ETS-10 sample, but did not observe any peak-narrowing effect, possibly due to high mobility of Xe atoms. 3. Multiple Adsorption Sites Revealed by Xe Adsorption Isotherms. Because of the three-dimensional structure of ETS10 and the presence of disorders and faults, the total number of “pores” or “pore volume” become ill-defined. Instead, the shape of a Xe adsorption isotherm can reveal more information about cation-Xe interactions and total Xe uptake. For a supercage with energetically homogeneous sites, the Xe adsorption isotherm is usually linear with respect to Xe pressure. This is the case for NaX and NaY. A nonlinear adsorption isotherm, on the other hand, is observed when energetically different sites coexist in a supercage. The presence of multiple sites in a supercage also results in a nonlinear relationship between chemical shift and Xe uptake. For example, Boddenberg et al. have successfully used a three-sites model to simultaneously fit nonlinear Xe adsorption isotherm and chemical shift vs Xe uptake plots.24 The nonlinear behaviors of chemical shift (Figure 3) and Xe adsorption isotherm (Figure 5) are due to the inhomogeneous interactions between Xe atoms and different sites in ETS-10. Although a quantitative curve fitting, temperature variation, and structural modeling is out of the scope of this contribution, we would like to offer a qualitative structural insight of ETS-10 based on the experimental observations. The most striking difference between ETS-10 and other zeolites like Na-Y is that no cations are present in the 12-member ring channels. However, Na+ ions in the 7-member ring channel can have a

strong influence on the Xe atoms in the 12-member ring channels through walls, leading to energetically different adsorption sites, e.g., sites near walls versus those at the center. Xe atoms preferentially occupy those strong adsorption sites first, making the isotherm concave to the pressure axis and the initial adsorption increase steep for Na-ETS-10 (Figure 5). This also explains the fact that more curvature is seen in the plot of chemical shift vs Xe uptake for ETS-10 with higher Na content (Figure 3). The presence of energetically different adsorption sites in Na-ETS-10’s 12-member ring channels is entirely in agreement with the T1 relaxation data discussed in the above section. Logically, the increase of Xe adsorption isotherm linearity after proton exchange is due to the reduction of cationXe interaction (Figure 5), possibly accomplished by cation charge dilution through a process such as delocalization of protons. In a related contribution we further address the issue of proton delocalization in H-ETS-10.20 Conclusion ETS-10 is a new wide-pore titanosilicate sieve. Unique pore structure and charge distribution lead to its novel properties for new commercial applications. A detailed understanding of the pore system is of fundamental importance in exploring these properties and applications. We have three conclusions from this contribution: (1) 129Xe NMR results are consistent with the pore structure model established previously: (A) negative charges and Na+ ions are located in the 7-member ring channels while the 12member ring channels are empty; (B) the 12-member ring channels are the main pore system of ETS-10. (2) We found, however, that proton exchange has a significant effect on the porosity of ETS-10. The formation of defects and reduction of stackings in proton exchanged ETS-10 cause a significant reduction of Xe uptake and the effective channel diameter felt by Xe atoms. (3) Xe adsorption isotherm and spin-lattice relaxation reveal that energetically different adsorption sites coexist in the 12member ring channels for Na-ETS-10. Proton exchange reduces the energetic differences by decreasing the cation-Xe interaction of strong adsorption sites. The strong effect of cations on Xe adsorption and chemical shift in ETS-10 is different from previous observations for conventional aluminosilicate zeolites such as zeolite Y. It is the unique pore structure and charge distribution in ETS-10 that makes the difference. Acknowledgment. We are grateful to Prof. J. Fraissard and Prof. M. W. Anderson for useful discussions, and we thank Dr. Patrick Blosser and Mr. Thomas Gegan of Engelhard Corp. for sample preparation and the XRD data. References and Notes (1) (a) Kuznicki, S. M., U.S. Patent 4,853,202, 1989. (b) Kuznicki, S. M.; Thrush, K. A. EP Patent 0405978A1, 1990. (c) Kuznicki, S. M.; Thrush, K. A. U.S. Patent 5,208,006, 1993. (2) See, for example: (a) A special review titled: Looming Ban on Production of CFCs, Halons Spurs Switch to Substitutes, Chem. Eng. News 1993, NoV 15, 12-18. (b) Davis, R. J.; Liu, Z.; Tabora, J. E.; Wieland, W. S. Catal. Lett. 1995, 34, 101. (c) Carli, R.; Bianchi, C. L.; Ragaini, V. Catal. Lett.1995, 33, 49. (d) Robert, R.; Rajamohanan, P. R.; Hegde, S. G.; Chandwadkar, A. J.; Ratnasamy, P. J. Catal. 1995, 155, 345. (3) (a) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347. (b) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Malley, P. J. O.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Philos. Mag. B 1995, 71, 813. (c) Ohsuna, T.; Terasaki, O.; Watanabe, D.; Anderson, M. W.; Lidin, S. In Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., et al. Eds.; Elsevier: Amsterdam, 1994; p 413. (4) Garfinkel, H. M.; Kuznicki, S. M.; Thrush, K. A. WO Patent 9,300,152, 1993; EP 544892A1, 1993.

3718 J. Phys. Chem., Vol. 100, No. 9, 1996 (5) Kuznicki, S. M.; Thrush, K. A.; Allen, F. M.; Levine, S. M.; Hamil, M. M.; Hayhurst, D. T.; Mansour, M. Synth. Micropor. Mater. 1992, 1, 427. (6) Ito, T.; Fraissard, J. J. Chem. Phys. 1982, 76, 5225. (7) Ito, T.; Fraissard, J. J. Chem. Soc., Faraday Trans. 1 1987, 83, 451. (8) Demarquay, J.; Fraissard, J. Chem. Phys. Lett. 1987, 136, 314. (9) Springuel-Huet, M. A.; Ito, T.; Fraissard, J. P. In Structure and ReactiVity of Modified Zeolites; Jacobs, P. A., et al., Eds.; Elsevier: Netherlands, 1984; p 13. (10) Fraissard, J.; Ito, T.; Springuel-Huet, M. A.; Demarquay, J. In New DeVelopments in Zeolite Science and Technology,; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Elsevier: New York, 1986; p 393. (11) Ripmeester, J. A. J. Am. Chem. Soc. 1982, 104, 209. (12) Ripmeester, J. A. J. Magn. Reson. 1984, 56, 247. (13) Jameson, C. J.; Jameson, A. K.; Gerald II, R.; de Dios, A. C. J. Chem. Phys. 1992, 96, 1690. (14) Jameson, A. K.; Jameson, C. J.; Gerald II, R. J. Chem. Phys. 1994, 101, 1775.

Yang and Truitt (15) Reisse, J. NouV. J. Chim. 1986, 10, 665. (16) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350. (17) Dybowski, C.; Bansal, N. and Duncan, T. M. Annu. ReV. Phys. Chem. 1991, 42, 433. (18) Barrie, P. J.; Klinowski, J. Prog. in NMR Spectrosc. 1992, 24, 91. (19) Jameson, C. J.; Jameson, A. K.; Cohen, S. M. J. Chem. Phys. 1973, 59, 4540. (20) Yang, X.; Blosser, P. Zeolites, submitted. (21) Newsam, J. M.; Treacy, M. M. J.; Koetsier, W. T.; de Gruyter, C. B. Proc. R. Soc. London A 1988, 420, 375. (22) Forsen, S.; Hoffman, R. J. Chem. Phys. 1963, 39, 2892. (23) Smith, M. L.; Dybowski, C. J. Phys. Chem. 1991, 95, 4942. (24) (a) Grosse, R.; Gedeon, A.; Watermann, J.; Fraissard, J.; Boddenberg, B. Zeolites 1992, 12, 909; (b) Watermann, J.; Boddenberg, B. Zeolites 1993, 13, 427.

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