J. Phys. Chem. 1995,99, 13763-13768
13763
Mobility of the Acidic Proton in Brsnsted Sites of H-Y, H-Mordenite, and H-ZSM-5 Zeolites, Studied by High-Temperature lH MAS NMR Priit Saw,* Tiit Tuherm, and Endel Lippmaa Institute of Chemical Physics and Biophysics, Akadeemia 23, EE0026 Tallinn, Estonia
Kari Keskinen and Andrew Root Neste Oy, Technology Centre, P.O. Box 310, SF-06101 Porvoo, Finland Received: May 12, 1995; In Final Form: July 28, 1995@
The mobility of the acidic proton of the Bransted site in three zeolites has been investigated by means of high temperature (up to 660 K) MAS Nh4R. At high temperatures, spinning sidebands (SSB), caused by the dipolar interaction between the acidic proton and the neighboring aluminum nucleus, gradually disappear. This means that the acidic protons become mobile. At the same time protons from terminal SiOH groups remain stationary over the time scale of the spinning speed. Fitting of the temperature dependence of the second moment of the SSB pattern with known models enables the estimates for the activation energy of proton mobility to be given. These were found to be 45, 54, and 61 kJ/mol for HZSM-5, H-mordenite, and H-Y, respectively. In our interpretation the activation energy corresponds to the average proton affinity difference of oxygen atoms in the first coordination sphere of the aluminum atom at the Bransted center.
Introduction Zeolites are porous aluminosilicates used in petrochemistry as shape-selective catalysts. At the present time the main industrial applications of zeolites (cracking, isomerization, alkylation, etc.) involve their acidic properties. Protonic acid centers (Bronsted acid sites) are generated when negative charges, in excess due to the replacement of Si04 tetrahedra by A104 tetrahedra in the framework, are neutralized by protons. It is known that the acidity of Bronsted sites differs considerably from one type of zeolite to another, but understanding the reason for this at the atomic level has emerged only recently. It was generally believed that the activity of acidic zeolite catalysts depended on the proton affinity of the bridging hydroxyl oxygen atom, but Kramer et al. have shown that the rate of the deuterium-hydrogen exchange reaction depends on the difference in proton affinity of this oxygen atom and the next.‘ Quantum chemical calculations have shown that the proton affinity of different oxygen sites is strongly related not only to the chemical composition of a zeolite but also at least to the same extent to its s t r u c t ~ r e . ~Redondo .~ et al. have found a direct relationship between the experimental T-0-T angle and the proton affinity. This gives a new meaning to the “breathing motions” of a zeolite lattice simulated by Deem et al.4 and implies that proton affinity of one distinct oxygen may change in time. This kind of modulation of proton affinity of neighboring bridging oxygens may cause proton mobility, and it is reasonable to expect that a smaller proton affinity difference (lower energy barrier) produces higher mobility. To study the hopping of protons in Bronsted sites and to compare the results with earlier studies, we have chosen three different zeolites in their protonic forms: H-Y, H-mordenite, and H-ZSM-5. The catalytic activity of H-Y can be as much as 2 orders of magnitude lower than that of H-ZSM-5: while H-mordenite has similar but slightly weaker acid sites than H-ZSM-5.6 Thus the mobility of the acid sites in H-ZSM-5 and H-MOR is expected to be similar to but different from H-Y. Solid-state NMR spectroscopy has become a common tool _ _ _ _ _ _ ~ @
Abstract published in Aduance ACS Abstracts, September 1, 1995.
0022-365419512099-13763$09.0010
for zeolite characterization. Relaxation and line-width studies have been employed to investigate the proton mobility and determine the Al-H+ distan~e.’.~It is proposed that the higher the jump frequency (inverse mean lifetime at lattice oxygen atom), the higher the acid strength of the proton.’ In their review article Pfeifer et aL8 have used wide-line variable temperature (VT) NMR to study OH mobility in H-Y zeolite and concluded that it is mainly determined by residual amounts of ammonium or other basic molecules, which act as “transport vehicles”. To separate the signals of different OH groups from each other, one has to use MAS NMRa9 IH MAS NMR sideband analysis can give valuable information about the geometry and location of the bridging OH groups.I0 Hightemperature MAS NMR has been mainly used to study catalytic reactions in situ,‘ but we have chosen this method to study zeolite acidity in situ. In this paper we shall focus on proton dynamics of Bronsted acid sites in pure zeolites without any kind of adsorbed molecules. High-temperature MAS NMR will be employed to estimate energy barriers and hopping frequencies.
Experimental Section High-Temperature MAS Probe. The VT CPMAS NMR probe used here was home built and tested up to 680 K. The maximum spinning speed obtained was 3.5 kHz, and the 90” pulse length was 6 ,us. The samples were contained in conventional 5 mm 0.d. NMR tubes to a depth of 10 mm, while the sample tube itself was sealed to a length of 50 mm. This means that the sample is easily exchangeable as in conventional liquid NMR systems. The heater power at 680 K was 210 W, and the temperature gradient over the sample was not more than 5 K at 579 K (this can be reduced if the sample is shorter). The temperature gradient was measured by monitoring the disappearance of the satellite transition bands in the 23Na NMR spectrum of a static NaN03 powder near the melting point of 579 K.25 Our spinning module is shown in Figure 1 and resembles an ultralow-temperature MAS probe described by Hackman et al.13 The construction of our module is based on two simple principles: the heatable volume is separated from 0 1995 American Chemical Society
Letters
13764 J. Phys. Chem., Vol. 99, No. 38, I995 Results and Discussion
COOLING GAS
5 /'
/ ,'/','
/
'\, THERMOCOUPLE
\ COOLING GAS
HEATING GAS
Figure 1. Sketch of the spinner assembly of the high temperature MAS probe. Components are labeled as follows: 1, rf (sample) coil; 2, stator (Macor); 3, rotor (Macor); 4, glass sample tube and sample; 5, glass dewar.
TABLE 1: Silicon-to-Aluminum Ratio in the Framework (SVAI) and Framework-to-Nonframework Aluminum Ratio of Samples (Accuracy Better Than. &lo%) (AIF/AINF) ~
samples
Si/AI
AIF/AlNF
HZSM-5 H-MOR
38 7 3
25
H-Y
5 2
the spinning system and RF coil and the mass of the rotor has to be much bigger than the mass of the glass tube holding the sample. The first principle mqans that heater power is considerably reduced (compared with 1000 W in other designsI2), and there is no need for special measures to protect the magnet from excessive heating (water jacket, ceramic foam). RF circuits remain at near room temperature (RT), which makes the RF tuning of the probe more stable, and experimental temperatures can be reached more quickly. The only drawback is that the filling factor of the coil is poor, but at higher temperatures this is compensated for by a better Q factor of the coil. The second principle means any asymmetry in the glass tube and sample is small compared to the whole rotor and no special mechanical devices are needed to seal the glass tubes, an experienced glass blower is enough. Two rubber O-rings were used to hold the sample tube and absorb shock waves that are generated when starting and stopping the spinning. Samples. H-ZSM-5, H-MOR, and H-Y samples were of uncommercial origin. The samples were characterized with 29Si and 27Al MAS NMR (Table 1). Prior to IH NMR experiments all samples were dehydrated in 5 mm 0.d. conventional NMR glass tubes for 6 h at 683 K under a vacuum better than Torr. The temperature was raised 3 Wmin. After dehydration, samples were cooled to RT in 5 min and sealed under vacuum. Before and after the NMR experiment the sample tube vacuum was tested with highfrequency glow discharge, and it was never lower than Torr. NMR Spectroscopy. All IH MAS NMR spectra were acquired at 200 MHz on a Bruker CXP200 spectrometer using single pulse excitation (30" pulse and 10 s relaxation delay, 300 scans) and sample spinning at 2.0-2.4 kHz. The background signal from the glass dewar and the sample tube was acquired separately for every temperature and subtracted from the spectra. Spectra were simulated with the Bruker Linesim program. All 27AIand 29SiMAS NMR spectra were acquired on a Bruker AM500 at 11.7 T field with a home-built probe. Prior to measurements samples were kept at 75% relative humidity for 48 h. All spectra were acquired using single-pulse excitation (27Al: 10"pulse and 0.1 s delay; 29Si: 30" pulse and 10s delay) and fast sample spinning (15 kHz).
Lines in the IH MAS NMR spectra of zeolites are according to their chemical shift (CS, relative to TMS) assigned as follows: 8*9ammonium ions (6.4-7.5 ppm), Brgnsted acid sites (5.63.6 ppm), AlOH groups at nonframework aluminum (2.5-3.5 ppm), nonacidic, terminal SiOH groups on the surface of zeolite crystallites and crystal defect sites (1.O-2.2 ppm). Our spectra exhibit two main peaks in the center band corresponding to Brgnsted sites at 4.3 ppm and terminal SiOH groups at 1.8 ppm, plus spinning sidebands (SSB) associated with them (Figure 2). MAS NMR experiment allows us to separate signal intensities coming from these two major OH groups, which is impossible in case of nonspinning samples. SSB-s belonging to bridging OH groups (Brensted sites) are of major importance for this study, since they describe dipolar interaction between acidic +H and A1 atom.1° This interaction and also the distance between +Hand Al in a catalytic center is known to be roughly the same for all three types of zeolites under study (0.24-0.25 nm),Io a remarkable fact, if one considers the big differences in acidity and catalytic activity of these materiak5q6 At 298 K the distribution of +H site signal intensity between central line and SSB-s is the same for all the three samples HZSM-5, HMOR, and HY (Figures 2 and 3). For all samples the relative intensity of SSB-s equals ca. 70% at 298 K and is in good agreement with the data published earlier,24taking into account that the spinning speed is roughly 2.2 kHz and the second moment of the spectrum from static sample is about (4-6) x lo8 s-* (homonuclear magnetic dipolar interaction and anisotropy of chemical shielding are included).IO When we increased the temperature, the SSB-s gradually disappeared (Figures 2 and 3A). This means that the dipolar interaction between the acidic proton and Al atom of a Brensted site became modulated at the same or higher rate as the spinning-speed (2.2 kHz), i.e. the acidic protons became mobile. Upon lowering the temperature we obtained the starting spectra (298 K) again-the effect is thus reversible. For HZSM-5 the SSB-s disappeared already at 478 K, for HMOR at a slightly higher temperature (568 K), while for HY it happened at 658 K. We have explicitly shown that mobility of an acidic proton in a Brensted site decreases if we go from HZSM-5 to HMOR and HY zeolite, and now we have spectroscopic means to probe this mobility in situ. The question remains, what causes this mobility? It is not NH4+, because at 500 MHz (spectra not shown), where the resolution is better no peaks at 6.4 ppm could be found. To show that it is not water, released upon recombination of terminal OH groups, the relative intensity from SiOH groups versus temperature was plotted and within experimental error it remained constant (Figure 3B). To verify that SiOH groups are not taking part in any kind of exchange, the relative intensity of the SSB-s of SiOH groups versus temperature was plotted (Figure 3C). Again, within experimental error, it remained constant. The conclusion is that in the case of our experimental conditions (no adsorbed basic molecules), the cause for acidic proton mobility must lie in the structure of the acidic site and the zeolite itself. Knowing the structures of zeolites and also the structure of the Brgnsted site itself, we can consider two possible types of motion for the acidic protons: 1. Proton exchange between Brflnsted sites, which means isotropic translational motion. 2. Small amplitude reorietational jumps between oxygens surrounding the aluminum atom in the Brensted site. The first possibility has to be ruled out on the following grounds. Proton affinity of oxygens in the first coordination sphere of aluminum is about 13 eV,1-3 which means that one
Letters
J. Phys. Chem., Vol. 99, No. 38, 1995 13765
HMOR
ppm
io . jo
o
.
.io
-40
ppm 40
.
io . i . -io
. -io’pdm
HY
io
zo
i
.
-20
-40
Figure 2. Single-pulse ‘H (200 MHz) high-temperature MAS NMR spectra of HZSM-5, HMOR, and HY. For clarity spinning sidebands (SSB-s) of H+ signal of HMOR at room temperature are marked with 4. Excitation 30°,relaxation delay 10 s, background signal is subtracted.
cannot release them by thermal activation. Our experiments have shown that in HY zeolite, where the concentration of Bronsted sites is about 10 times bigger than in HZSMJ, proton mobility is smaller than in HZSM-5. Of course, one possibility is, that there exist paired Bronsted sites (on the opposite walls of a channel). In that case, to explain our experimental results, all Bronsted sites have to be in a paired configuration in HZSM-5 and HMOR, which seems quite unrealistic.Proton exchange between Bronsted sites may be the type of motion in HY zeolite, but in HZSM-5 and HMOR this is not a possible mechanism for mobility. This leaves the second possibility. The optimum bridging OHTOangle is 90-loo”, * and this gives us about 0.24 nm for the bridging oxygen-oxygen distance. The bridging oxygenproton bond length is about 0.10 nm, and if one includes the Madelung field, generated by the zeolite framework, into numerical calculations, the bridging oxygen-proton bond length may be even 0.15 nm.I4 Experimental evidence for the
existence of this 0.15 nm configuration can be found in the recently discovered “low temperature” acidic site with a chemical shift of 7.0 ppm,lS.l6which is described as a bridging OH group interacting with a neighboring oxygen atom. The distance of 0.24 nm between the bridging oxygens may be further reduced by thermal motion of the framework. The ZSM-5 framework for example, undergoes a monoclinicorthorhombic phase transition in the temperature range 333423 KI7 and is known for its softness. Therefore, we cannot rule out the possibility that acidic protons can move from one bridging oxygen to the other, either by hopping over the potential barrier or tunneling through the barrier. Our results show (Figure 3) that this process is triggered in HZSM-5 at lower temperatures than in HMOR and HY. An important parameter, governing this process, is the difference in proton affinities of neighboring oxygens in the Bronsted site; it is energetically more favorable for a proton to jump between oxygens with equal proton affinity. In the fiist model we describe our process as
Letters
13766 J. Phys. Chem., Vol. 99, No. 38, 1995
B
m +-HMOR T
t ' 'l' W "+I
1
I
I
01
300 400 500 600 700
TI
300 400 500 600 700
300 400 500 600 700
Temperature / K
Temperature / K Temperature / K Figure 3. Relative intensity of spinning sidebands (SSB-s) of H+ signal (broad component at higher temperatures is considered as a center band) versus temperature (A); relative intensity of SiOH signal versus temperature (B); relative intensity of SSB-s of SiOH signal versus temperature (C).
A
B n 600
-\
A
300
400
500
600
700
Temperature, K Figure 5. Second moment of SSB pattern of HZSM-5, HMOR and
HY versus temperature and the least squares fit (solid line).
Figure 4. Sketch of the catalytically active BNnsted site and relevant two-site potential well. Proton is hopping between sites A and B. Path (a) for proton mobility is described by first model and path (b) by second model (see text).
a proton jumping in a potential well with two nonequivalent sites (Figure 4), where H i s the height of the energy barrier and d is the proton affinity difference. This model and relevant changes in the second moment of an NMR line upon increasing temperature have been studied by Latanowicz et al.I8 We use their results with slight modifications to extract coarse estimates
for H and d from experimental data. To simplify the problem, we assume that there exists only one correlation time ZC. In the following we assume that at low spinning speeds the sideband envelope (Figure 2) mirrors the powder pattern of a nonspinning sample and show that this simplification gives us results comparable with earlier simulations.10-21*22 To study the collapse of the SSB envelope upon increasing temperature, we treated the SSB pattern as a histogram (SSB intensity versus its frequency) and plotted the second moment of this distribution against temperature (Figure 5). The second moment of a resonance line of a powdered sample without magic angle spinning is given by19v20
where Wxt2is the second moment of homonuclear interaction with protons statistically distributed at distances larger than ~ A I H , Mtl" is the second moment of the dipolar interaction between A1 and H+,and is the second moment of chemical shift anisotropy (CSA). For HZSM-5,HMOR, and H Y these
e
J. Phys. Chem., Vol. 99, No. 38, 1995 13767
Letters
TABLE 2: Results of Least-Squares Fit. First Model: Proton Jumps over the Potential Barrier. Second Model: Proton Penetrates through the Barrier or There Is No Barrier parameters used first model second model sample 106AM*/s-2 106M2/ s-2 i0-9to,bs H,kJ/mol d, kJ/mol d, kJ/mol HY 585 45 2 61 f 1 Of2 61 f 1 HMOR 494 20 2 54f 1 O f 1 54f 1 HZSM-5 523 20 lkl 45 2 2 45 f 2 Constants used in least-squares fit. At first the fit was made with three parameters (to,H,and 4, then to was fixed to the average value of all three samples (all values were in the range (1 -5) x s) and data were fitted with two parameters: H and d. Decreasing 50 leads to higher values of H.
*
moments have been determined from an analysis of SSB pattems'0-2'-22and the resultant second moments (M2) are respectively (5.3-5.5) x los, 5.4 x lo8, and (5.4-6.9) x lo8 C2. One can see (Figure 5) that second moments of our SSB distribution at 298 K match these numbers quite well. At higher temperatures M2 levels off to between 0 and 1 x lo8 s - ~(Figure 5 ) . This is because the short-range heteronuclear dipolar coupling and CSA interactions are averaged out at higher temperatures, but long-range homonuclear interaction is not. The proton does not change its position with respect to other protons far away from it; it only circulates between four oxygens that are located around aluminum. If it circulated between two oxygens only, the dipolar interaction would have been only partly averaged out and we would not have seen the increase of the central line at higher temperatures (Figure 2). The final value of M2 correlates well with the value of fl;' determined by Hunger et a1.,I0 (0.2-1.2) x lo8 s - ~ . To fit our experimental data, we use eq 59 in ref 18 with modifications, which consider that at a high temperature M;IH and are averaged to zero and M2 levels off to
esA
My:
where
(3) a = exp(d/RT)
+ eSA; fi
(4)
and &I2 = MFH = 1.6 kHz. The results of the least-squares fit are shown in Table 2. The calculation of zc from eq 3 for HZSM-5 at 500K yields 35 ps. If we compare our experimental spectra and their correlation times zc with calculated MAS NMR spectra of a zeolite in case of thermal m o t i ~ n ,we ~ ~notice , ~ ~ a qualitative similarity and correlation times match quite well. So we have reason to think that the decrease of the second moment of the SSB distribution reflects a decrease in the second moment of dipolar interaction with the accuracy good enough to make coarse estimates of the potential well parameters Hand d (Table 2). One can see from Figure 5 that HZSM-5 and HMOR fit our model better than HY. The cause for this are probably two distinct Bronsted sites in large and small cavities of HY zeolite (not resolved in our spectra, because of low spinning speed and low magnetic field), which implies that our assumption of one correlation time is not correct in the case of HY zeolite. To our surprise, the average proton affinity difference is between the limits of 0-2 kJ/mol, which differs by more than an order of magnitude from 60 kJ/mol, predicted elsewhere.'-3 The situation is different, if we assume that the proton is able to
penetrate through the potential barrier (or there is no barrier) and energy is needed only to overcome the proton affinity difference (d). Thus instead of eqs 2 and 3, we useZo
where
and the activation energy corresponds to the proton affinity difference (6).The results of the least squares fit for dare the same as for H in Table 2, but zo equals s. Our second model gives a very good match with earlier numerical and experimental results.'-3 The gap between activation energies of HZSM-5 and HY zeolites (ca. 16 kJ/mol), correlates quite well with the difference (ca. 10 kJ/mol) in the additional energy barrier for D/H exchange in HZSM-5 and HY, found by Kramer et al.' This additional barrier is induced by the increased proton affinity difference between the neighboring oxygen sites. I According to our calculations (Table 2) the activation energy for proton mobility in HZSM-5, HMOR, and HY zeolites is about 45, 54, and 61 kJ/mol, respectively. We think that this activation energy corresponds approximately to the average proton affinity difference of neighboring oxygen atoms in the Bransted site. We assume that the mobility of the proton in the Bransted site of dehydrated zeolites must also be detectable by 27AlNMR. At lower temperatures the proton is bound to one oxygen atom only and the oxygen atoms surrounding the aluminum atom are unequal, causing a strong electric field gradient at the aluminum site (quadrupole coupling constant of 13-16 Mhz) and yielding a very broad line in 27Al NMR spectrum.26 At elevated temperatures the proton mobility makes the oxygen atoms equal on the NMR time scale (milliseconds) and the quadrupole coupling constant must decrease. We hope these assumptions will be proved in the very near future. After the preparation of this letter a paper by Baba et al.27 was brought to our attention, concerning a similar investigation as the present one. Baba et al. studied the dependence of the line width of the central line of Bronsted acid sites in 'H MAS Nh4R spectra with temperature. HZSM-5 ( S U N = 21) showed a remarkable increase of the line width upon increasing temperature, but for HMOR and HY the changes were smaller because the samples were heated only to 373 K. The activation energy for proton mobility in HZSM-5 (SUA1 = 21) was estimated to be 11 kJ/mol, and it was shown that upon increasing the SUA1 ratio the mobility of the proton decreases. Our estimate of 45 Wmol for HZSM-5 (Si/Al = 38) is higher because of the higher SUA1 ratio. On the whole our results are consistent with the results given by Baba et al. but give a more complete description of this phenomenon because of the broader temperature range used.
13768 J. Phys. Chem., Vol. 99, No. 38, 1995 Conclusions With a new type of high-temperature MAS NMR probe it was possible to monitor proton mobility in catalytically active Bronsted acid sites of three different zeolites over the temperature range 300-660 K. At elevated temperatures protons start hopping between oxygen atoms that surround aluminum in the first coordination sphere. This process manifests itself in a gradual decrease of SSB-s in the MAS NMR spectrum. The mobility of protons, consequently also acidity, decreases from HZSM-5 to H-mordenite and HY zeolite. This result is in line with earlier calorimetric s t u d i e ~ . Fitting ~ , ~ experimental results to a simple two-site potential well model gave estimates for the activation energy, which in our interpretation corresponds to the average proton affinity difference between neighboring oxygen atoms. The activation energy for proton delocalization increases, if we go from HZSM-5 to HMOR, and HY (45, 54, and 6 1 kJ/mol, respectively). The characteristic correlation times for proton mobility at elevated temperatures are between 30-50 ps. High-temperature MAS NMR of zeolites in sealed glass ampules allow us to study in one experiment such diverse qualities as flexibility of the zeolite framework and proton affinity of oxygen atoms at the Bronsted site, thus leading to a better unerstanding of dynamics of the catalytically active site. Further studies of one type of zeolites with different SUA1 ratio are needed to give further insight into this phenomenon. T I measurements would also give valuable information about the process.
Acknowledgment. We thank Estonian Scientific Foundation for supporting this research through Grant No. 546. References and Notes (1) Kramer, G. J.; van Santen, R. A,: Emels, C. A,; Nowak, A. K. Nature 1993, 363, 529. (2) Kramer, G. J.; van Santen, R. A. J. Am. Chem. Soc. 1993, 115, 2887. (3) Redondo, A.; Hay, P. J. J. Phys. Chem. 1993, 97, 11754.
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127, 51. (6) Vedrine, J. C.; Auroux, A.: Bolis, V.; Dejaifve, P.; Naccache, C.: Wierzchowski, P.: Derouane, E. G.; Nagy, J. B.: Gilson, J.-P.; van Hoof, J. H. C.; van der Berg, J. P.; Wolthuizen, J. J. Catal. 1979, 59, 248. (7) Mestdagh, M. M.; Stone, W. E. E.: Fripiat, J. J. J. Phys. Chem. 1972, 76, 1220, J . Catal. 1975, 38, 358; J. Chem. Soc., Faraday Trans. 1 1976, 1, 154. (8) Freude, D.; Oehme, W.; Schmiedel, W.; Staudte, B. J. Catal. 1974, 32, 137. Pfeifer, H.: Freude, D.; Hunger, M. Zeolites 1985, 5, 274. (9) Engelhardt, G.: Jerschkewitz, H.-G.; Lohse, U.; Sarv, P.; Samoson, A.; Lippmaa, E. Zeolites 1987, 7, 291. (10) Hunger, M.; Anderson, M. W.; Ojo, A,; Pfeifer, H. Microporous Mater. 1993, 1, 17. (1 1) Munson, E. J.; Kheir, A. A,: Lazo, N. D.; Haw, J. F. J. Phjs. Chem. 1992, 96, 7740. (12) Oliver, F. G.; Munson, E. J.: Haw, J. F. J. Phys. Chem. 1992, 96, 8106. (13) Hackman, A.; Seidel, H.; Kendrick, R. D.; Myhre, P. C.; Yannoni, C. S. J. Magn. Reson. 1988, 79, 148. Myhre, P. C.; Webb, G. G.; Yannoni, C. S. J. Am. Chem. SOC. 1990, 112, 8991. (14) Cook, S. J.; Chakraborty, A. K.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1993, 97, 6679. (15) Beck, L. W.; White, J. L.; Haw, J. F. J. Am. Chem. SOC. 1994. (16) Brunner, E.; Beck, K.; Koch, M.; Pfeifer, H.; Staudte, B.; Zscherpel Stud. Sut$ Sci. Catal. 1994, 84, 357. (17) Hay, D. G.; Jaeger, H. J. Chem. Soc., Chem. Commun., 1984, 1433. (18) Latanowicz, L.; Andrew, E. R.; Reynhardt, E. C. J. Magn. Reson. A 1994, 107, 194. (19) Brunner, E.: Freude, D.: Gerstein, B. C.; Pfeifer, H. J. Magn. Reson. 1990, 90, 90. (20) Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, 1962. (21) Fenzke, D.; Hunger, M; Pfeifer, H. J. Magn. Reson. 1991, 95,477. (22) Freude, D.:Klinowski, J.; Hamdan, H. Chem. Phys. Lett., 1988, 149, 355. (23) Fenzke, D.; Gerstein, B. C.: Pfeifer, H. J. Magn. Reson. 1992, 98, 469. (24) Pfeifer, H. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3777. (25) Jager, C.; Scheler, G. Exp. Techn. Phys. 1984, 32, 315. (26) Emst, H.; Freude, D.: Wolf, I. Chem. Phys. Lett. 1993, 212, 588. (27) Baba, T.; Inoue, Y.; Shoji, H.; Uematsu, T.: Ono, Y. Microporous Mater. 1995, 3, 647.
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