Local Structure of Framework Aluminum in Zeolite H−ZSM-5 during

J. Phys. Chem. B 2001, 105, 8143-8148

8143

Local Structure of Framework Aluminum in Zeolite H-ZSM-5 during Conversion of Methanol Investigated by In Situ NMR Spectroscopy M. Seiler, W. Wang, and M. Hunger* Institute of Chemical Technology, UniVersity of Stuttgart, 0-70550 Stuttgart, Germany ReceiVed: December 11, 2000; In Final Form: June 4, 2001

In situ MAS NMR spectroscopy under continuous-flow conditions and spin-echo NMR experiments have been applied to investigate the quadrupolar interactions of framework aluminum atoms in zeolite H-ZSM-5 at elevated temperatures and during conversion of methanol. The strength of the 27Al quadrupolar interactions of framework aluminum atoms in zeolites depends on the electric field gradient caused by the charge distribution in the local structure of the aluminum sites. Adsorption of methanol on bridging OH groups at 295 K under flow conditions leads to weak 27Al quadrupolar interactions, corresponding to a quadrupole coupling constant of QCC ) 4.4 MHz. After the temperature was raised to 573 K during methanol conversion, a strong decrease in the 27Al MAS NMR signal of framework aluminum at 54 ppm occurred due to a significant increase in the 27 Al quadrupolar interactions. Comparison of 27Al echo NMR spectra of zeolite H-ZSM-5 loaded with methanol and dimethyl ether and data given in the literature showed that this increase in the 27Al quadrupolar interactions is caused by the formation of dimethyl ether (QCC ) 11.2 MHz) and methoxy groups (QCC ) 16.2 MHz) at bridging OH groups. Experiments performed during purging zeolite H-ZSM-5 with dry nitrogen at 573 K indicated a high hydroxyl proton mobility which has, however, no influence on the local structure of framework aluminum atoms.

Introduction In the past decade, in situ MAS NMR spectroscopy has been developed into a powerful tool for the investigation of heterogeneously catalyzed reactions. In a growing number of studies (see, e.g., refs 1-5), this method has been applied to investigate reaction pathways and adsorbates, intermediates, and deposits formed on the surface of solid catalysts under batch and continuous-flow conditions. However, only scarce attention has been paid to the study of the catalyst framework under reaction conditions. In zeolites, the framework aluminum atoms contribute to the local structure of acidic hydroxyl groups formed at oxygen bridges between silicon and aluminum sites (SiOHAl). Due to the electric quadrupole moment, eQ, of the 27Al nuclei (spin I ) 5/2), the shape of the 27Al NMR signals depends strongly on the z-component of the electric field gradient, Vzz ) eq. This electric field gradient is caused by the charge distribution in the local structure of framework aluminum atoms, i.e., it depends on the Al-O bond lengths and O-Al-O bond angles. In hydrated zeolites, the interaction of water molecules with SiOHAl groups increases the local symmetry of framework aluminum atoms, leading to weak 27Al quadrupolar interactions, i.e., to a small quadrupole coupling constant, QCC ) e2qQ/h (h, Planck’s constant). Due to the small quadrupole coupling constant of tetrahedrally coordinated framework aluminum in hydrated zeolites H-Y and H-ZSM-5 (QCC ) 0.6-2.4 MHz),6-8 these materials can be investigated by 27Al MAS, DOR, and MQMAS NMR spectroscopy. On the other hand, the strong quadrupolar interactions of framework aluminum atoms in dehydrated zeolites H-Y and H-ZSM-5 (QCC ) 1316 MHz)9 and the rapid induction decay of the corresponding 27Al NMR signals require the use of spin-echo NMR experi* Corresponding author. Phone: +49/711/685-4079. Fax: +49/711/6854065. E-mail: [email protected].

ments.10 By application of a spin-echo sequence, it could be shown that the quadrupole coupling constant and, therefore, the local structure of framework aluminum atoms in dehydrated zeolites H-Y and H-ZSM-5 are sensitive to the chemical interaction of bridging OH groups with adsorbate molecules.11,12 Adsorption of strong bases such as ammonia or pyridine, which causes a proton transfer from the zeolite hydroxyl group to the adsorbed molecule, leads to a significant decrease in the quadrupole coupling constant from ca. 15 to 6 MHz. In contrast to this, no significant variation of the quadrupole coupling constant could be found after adsorption of benzene which interacts with bridging OH groups via hydrogen bonds.11 After adsorption of methanol on dehydrated zeolite H-ZSM-5, a narrow 27Al echo NMR signal of the framework aluminum corresponding to QCC values of 2.5-4.5 MHz was observed.12 Using density functional theory (DFT), Koller et al. calculated the electric field gradients of zeolite clusters consisting of a single-framework aluminum site surrounded by four SiO4 tetrahedra.13 These authors found that the formation of a bridging OH group perturbs the symmetry of the aluminum site by weakening the Al-O bond contributing to the Si-O-Al bridge. This leads to an increase in the electric field gradient, corresponding to a calculated QCC value of ca. 18 MHz. Addition of an ammonium ion to the zeolite cluster gave a calculated QCC value of 6.6 MHz, which agrees very well with the experimentally observed value of 6 MHz.11,13 Significant differences in the calculated QCC values were obtained for an interaction of the zeolite clusters with methoxonium ions (CH3OH2-zeolite, 8.2 MHz), methanol molecules (CH3OHzeolite, 15.5 MHz), and methoxy groups (CH3-zeolite, 16.2 MHz).13 For adsorption of methanol on the zeolite cluster, however, a rapid proton exchange between various oxygen positions was proposed, leading to an average of the electric field

10.1021/jp004455m CCC: $20.00 © 2001 American Chemical Society Published on Web 08/02/2001

8144 J. Phys. Chem. B, Vol. 105, No. 34, 2001 gradient and to a QCC value close to that obtained for methoxonium ions interacting with the zeolite cluster. In the present work, in situ MAS NMR spectroscopy has been applied to study the behavior of framework aluminum in zeolite H-ZSM-5 during conversion of methanol under continuousflow (CF) conditions. The reaction conditions used were equal to those in our recently published studies of “methanol-to-olefin” conversion on zeolite HZSM-5 and silicoaluminophosphates SAPO-34 and SAPO-18 investigated by in situ 13C CF MAS NMR spectroscopy.14 The interpretation of the CF MAS NMR spectra was supported by spin-echo NMR experiments on dehydrated zeolite H-ZSM-5 after adsorption of various reactants. Experimental Section Zeolite Na-ZSM-5 with the chemical composition Na4.2Al4.2Si91.8O192 was synthesized as described elsewhere.15 The ammonium form was prepared by a 4-fold ion exchange at 353 K in a 0.4 M aqueous solution of NH4NO3. After reaching an ion exchange degree of 98%, the zeolite powder was washed in demineralized water and dried at room temperature. The material was characterized by AES-ICP, XRD, 27Al, and 29Si MAS NMR spectroscopy, indicating that no dealumination occurred in result of the treatment. The NMR investigations were performed on a Bruker MSL 400 spectrometer at resonance frequencies of 400.1 and 104.3 MHz for 1H and 27Al nuclei, respectively. In situ 1H and 27Al MAS NMR investigations under continuous-flow (CF) conditions were performed with sample spinning rates of ca. 2.0 kHz using a modified DSI-740 7 mm STD MAS NB NMR probe, Doty Scientific Instruments, Texas, USA (see ref 16). 1H and 27Al CF MAS NMR spectra were recorded after excitation with π/2 and π/6 pulses and with repetition times of 5 s and 500 ms, respectively. 27Al echo NMR spectra were recorded after applying a phase-cycled echo sequence as described in ref 10 with a pulse delay of 20 µs and a repetition time of 500 ms. The spectra were referenced to tetramethylsilane for 1H and a 0.1 M aqueous solution of Al(NO3)3 for 27Al MAS NMR spectroscopy. Prior to the NMR investigations, the ammonium form of zeolite ZSM-5 was heated in a vacuum with a rate of 20 K/h up to the final temperature of 723 K. There, it was calcined at a pressure below 10-2 Pa for 12 h, leading to zeolite H-ZSM5. Before the in situ CF MAS NMR experiments, 250 mg of the calcined zeolite was filled into the MAS NMR rotor reactor under dry nitrogen in a glovebox and pressed to a cylindrical catalyst bed. After transferring the rotor into the MAS NMR probe, a second dehydration of the catalyst was performed at 673 K for 1 h under flowing nitrogen (30 mL/min). During the in situ CF MAS NMR experiments, dry nitrogen or nitrogen loaded with CH3OH, according to a modified residence time of W/F ) 25 g h mol-1, was injected into the 7 mm MAS NMR rotor reactor, applying the injection equipment described elsewhere.16 Via an exhaust tube on the top of the MAS NMR rotor, the probe was connected with the sampling loop of an on-line gas chromatograph HP 5890 (Hewlett-Packard) equipped with a Coating Poraplot Q capillary column (Chrompack Plot fused silica, length 50 m, inner diameter 0.32 mm). The exhaust flow was sampled and analyzed in steps of 20 min. Results and Discussion 1H

and 27Al CF MAS NMR Spectroscopy of Zeolite H-ZSM-5 during Conversion of Methanol. The 1H and 27Al CF MAS NMR spectra shown in Figure 1 were recorded

Seiler et al. during purging the zeolite H-ZSM-5 with dry nitrogen (a) and during injection of nitrogen loaded with CH3OH (b-e). The 1H MAS NMR spectrum in Figure la is that of a dehydrated zeolite H-ZSM-5, consisting of sideband patterns due to silanol groups and bridging OH groups at isotropic chemical shifts of 1.8 and 4.2 ppm, respectively.17 The absence of a signal in the simultaneously recorded 27Al MAS NMR spectrum is caused by the strong quadrupolar interactions of framework aluminum atoms in dehydrated zeolite H-ZSM-5 (QCC ca. 16 MHz), leading to quadrupolar patterns distributed over a spectral range of up to 1000 ppm.9-12 After starting the methanol flow at room temperature (Figure 1b), the 1H MAS NMR spectrum is dominated by signals at 3.7 and 8.8 ppm due to methyl groups of adsorbed methanol molecules and hydroxyl protons contributing to 2-fold hydrogen bound methanol complexes, respectively.18 The resonance position of the latter signal is affected by a rapid exchange of hydroxyl protons of methanol molecules H-bound to the zeolite framework and of bridging OH groups H-bound to the methanol molecules. Quantum-chemical calculations of zeolite clusters consisting of one SiOHAl group and one CH3OH molecule gave a mean 1H NMR chemical shift of the hydroxyl protons of 10.8 ppm.19 After raising the temperature to 473 K (Figure 1d), a resonance shift of the low-field signal to 10.9 ppm occurs which agrees very well with the above-mentioned theoretical value. Simultaneously, a methanol conversion of Xme ) 36% and a dimethyl ether yield of Ydme ) 33% were obtained by on-line gas chromatography. This agrees with our earlier in situ 13C CF MAS NMR investigation of “methanol-to-olefin” conversion on zeolite H-ZSM-5, which gave a strong 13C CF MAS NMR signal of dimethyl ether (δ13C ) 61 ppm) at this reaction temperature.14 After a further increase in the reaction temperature to 573 K (Figure le), in the lowfield range of the 1H MAS NMR spectrum, a broad signal occurs at ca. 4.5 ppm, corresponding to the chemical shift of methoxy groups on zeolite H-ZSM-5.20 The signal at 0.9 ppm is due to methyl groups of alkanes and alkenes adsorbed in the pores of the catalyst. By on-line gas chromatography, a methanol conversion of Xme ) 98% and yields of dimethyl ether, ethylene, propylene, and butene of 23.8%, 12.5%, 13.0%, and 9.9%, respectively, were determined. The 27Al CF MAS NMR spectra, simultaneously recorded during conversion of methanol on zeolite H-ZSM-5 at temperatures of 373, 473, and 573 K (Figure 1c-e, right), show a significant decrease in the signal of framework aluminum atoms at 54 ppm. On the right-hand side of these spectra, the integral intensities of the experimentally observed signals, Iexp, referred to the intensity of the 27Al MAS NMR signal at 54 ppm obtained at room temperature, Iexp ) 1.0 (Figure 1b, right), and the integral intensities, ICurie, expected according to Curie’s law (M0 ∝ 1/T) are given.21 These values give an evidence that the strong decrease in the 27Al MAS NMR signal at 54 ppm observed after raising the reaction temperature during methanol conversion on zeolite H-ZSM-5 cannot be explained by Curie’s law alone. Rather, it must be assumed that an increasing number of bridging OH groups exist in a chemical state, leading to strong quadrupolar interactions of the framework aluminum atoms hindering their observation by 27Al MAS NMR spectroscopy. 1H and 27Al MAS NMR Spectroscopic Study of the Mobility of Hydroxyl Protons in Zeolite H-ZSM-5 at Elevated Temperatures and of Their Influence on the Local Aluminum Structure. To support the interpretation of 27Al MAS NMR spectra recorded at elevated temperatures during methanol conversion on H-ZSM-5, the effect of hydroxyl mobility on the NMR spectroscopic behavior of framework

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Figure 1. 1H CF MAS NMR (left-hand side) and 27Al CF MAS NMR (right-hand side) spectra of calcined (673 K) zeolite H-ZSM-5, recorded under purging with dry nitrogen at room temperature (a) and during the injection of a continuous methanol flow (W/F ) 25 g h mol-1) at temperatures of 295 (b), 373 (c), 473 (d), and 573 K (e). By on-line gas chromatography, methanol conversions of 36% and 98% and DME yields of 33% and 24% were determined at 473 and 573 K, respectively.

aluminum atoms was considered more in detail. In Figure 2, 1H and 27Al MAS NMR spectra of the hydrated (a) and dehydrated (b-d) zeolite H-ZSM-5 recorded at temperatures of 295-573 K are shown. The 1H MAS NMR spectrum of the hydrated sample obtained at 295 K consists of a weak signal at 1.8 ppm due to silanol groups which are not effected by water. The strong line at 6.2 ppm is due to H-bound water molecules (4.6 ppm) involved in a rapid exchange with bridging OH groups (4.2 ppm) and hydroxonium ions (ca. 13 ppm).22 The 27Al MAS NMR signal at 54 ppm obtained for this sample (Figure 2a, right) agrees with that observed after adsorption of methanol (Figure 1b, right). In addition, a weak signal occurs at 0 ppm caused by Al(H2O)63+ complexes due to the presence of a small amount of extraframework aluminum. After dehydration and during purging the zeolite H-ZSM-5 with dry nitrogen at 295 K (Figure 2b, right), the intensity of the 27Al MAS NMR signal at 54 ppm is significantly decreased. This finding agrees with the strong quadrupolar interactions of framework aluminum atoms in dehydrated H-ZSM-5 hindering their observation by MAS NMR technique. The corresponding 1H MAS NMR spectrum (Figure 2b, left) consists of sideband patterns at 1.8 and 4.2 ppm due to silanol and bridging OH groups, respectively.17 After raising the temperature, an isotropic signal occurs at 3.8-4.5 ppm (Figure 2c,d, left) due to the highly mobile bridging hydroxyl protons, leading to a narrowing of the

sideband patterns observed at 295 K. A similar behavior of bridging OH groups in zeolites H-ZSM-5 at elevated temperatures was described by Baba et al.23 Interestingly, in the 27Al MAS NMR spectra of the dehydrated H-ZSM-5 recorded at 473 and 573 K during purging the sample with dry nitrogen (Figure 2c,d, right), no narrow signal of framework aluminum atoms adjacent to the highly mobile bridging hydroxyl protons occurred. 27Al Echo NMR Investigations of the Framework Aluminum in Dehydrated Zeolite H-ZSM-5 at Elevated Temperatures and after Adsorption of Reactants. 27Al NMR signals of framework aluminum atoms with quadrupole coupling constants in the order of 12-18 MHz show an induction decay of 5 µs. Due to the ring down time of the NMR probes, the detection of such framework species by 27Al NMR spectroscopy requires the application of a spin-echo sequence.10 The 27Al echo NMR spectrum of dehydrated zeolite H-ZSM-5 recorded at 295 K (Figure 3a) agrees with those obtained in earlier investigations.9,12 It consists of a broad quadrupolar pattern, QP, characterized by a quadrupole coupling constant of 14.0 MHz, and a narrow quadrupolar line, QL, with a QCC value of 5.7 MHz (see Table 1). The former signal which is due to framework aluminum atoms in the local structure of bridging OH groups has an intensity contribution of ca. 90%. The NMR parameters of signal QL indicates the presence of a weak amount

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Seiler et al.

Figure 2. 1H MAS NMR (left-hand side) and 27Al MAS NMR (right-hand side) spectra of zeolite H-ZSM-5, recorded in the hydrated state at room temperature (a) and in the dehydrated state at temperatures of 295 (b), 473 (c), and 573 K (d).

of framework aluminum atoms (ca. 10%) adjacent to cationic species such as sodium cations, residual ammonium ions, or cationic extraframework aluminum species. Adsorption of methanol leads to an 27Al echo NMR spectrum consisting of a single signal (Figure 3b) due to framework aluminum atoms in the local structure of bridging OH groups interacting with methanol molecules. The QCC value of 4.4 MHz allows the observation of the corresponding framework aluminum atoms by MAS NMR technique and causes the signal at 54 ppm shown in Figure 1b, right. After the temperature was raised to 473 and 573 K, a simultaneous decrease in the 27Al MAS NMR signal at 54 ppm (Figue 1d,e) and an increase in the formation of dimethyl ether (GC) were found. The influence of dimethyl ether on the 27Al echo NMR spectrum of framework aluminum in zeolite H-ZSM-5 is shown in Figure 3c. In contrast to the zeolite H-ZSM-5 loaded with methanol, the signal QP occurring after adsorption of dimethyl ether is significantly broader and corresponds to a quadrupole coupling constant of ca. 11 MHz. Hence, under reaction conditions leading to a rapid conversion of methanol to dimethyl ether, the strong 27Al quadrupolar interaction hinders the observation of framework aluminum atoms by MAS NMR technique and causes the decrease in their 27Al NMR signal in the in situ CF MAS NMR experiments. However, the effect of methoxy groups formed at these reaction temperatures, which cause a quadrupole pattern corresponding to a QCC value of ca. 16 MHz,13 also has to be considered. Their formation is indicated by the 1H MAS NMR signal at 4.5 ppm in the spectrum recorded at 573 K (Figure 1e). In our former study, it was shown that dimethyl ether molecules and methoxy groups are simultaneously formed on zeolite H-ZSM-5 at reaction temperatures of 473 and 573 K.14a

The reaction product dimethyl ether occurring at 61 ppm, however, was found to be the dominating species covering the weak 13C MAS NMR signal of methoxy groups at ca. 58 ppm. To study the influence of the hydroxyl proton mobility on the quadrupolar interaction of framework aluminum atoms in dehydrated H-ZSM-5 at elevated temperatures, the 27Al echo NMR spectrum of this material was recorded at 573 K (Figure 3d) during blocking the MAS NMR rotor in the DSI-740 7 mm STD MAS NB NMR probe. At this temperature, the mobility of the hydroxyl protons averages the dipolar interactions causing the 1H MAS NMR sideband pattern of bridging OH groups occurring at 295 K (Figure 1b, left) to an isotropic signal at 4.2 ppm (Figure 2d, left). The 27Al echo NMR spectrum in Figure 3d, on the other hand, shows only a weak narrowing of the signal QP, corresponding to a decrease in the quadrupole coupling constant from 14.0 MHz at 295 K to 12.4 MHz at 573 K (see Table 1). This indicates that the bridging hydroxyl protons do not jump between all four oxygen positions of the framework aluminum sites in the same manner which would average the electric field gradient, leading to a significant decrease in the quadrupole coupling constant. Rather, jumps between the two or three energetically favored oxygen atoms of aluminum sites or between different aluminum sites must be assumed. Conclusions In situ MAS NMR experiments under continuous-flow conditions and spin-echo NMR spectroscopy were applied to investigate the local structure of framework aluminum atoms in zeolite H-ZSM-5 during conversion of methanol at elevated

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Figure 3. 27Al NMR spectra of dehydrated zeolite H-ZSM-5, recorded at 295 K in the unloaded state (a), at 295 K after loading of one molecule CH3OH per bridging OH group (b), at 295 K after loading of one molecule dimethyl ether (dme) per bridging OH group (c), and at 573 K in the unloaded state using the spin-echo sequence of Kunwar et al.10 (d). The simulation of the quadrupolar patterns was performed using the Bruker software WINFIT.

TABLE 1: Data of Framework Aluminum in Zeolite H-ZSM-5 Obtained by a Simulation of the 27Al Echo NMR Spectra Shown in Figure 3 sample H-ZSM-5 at 295 K H-ZSM-5at 295 K, +1 CH3OH/SiOHAl H-ZSM-5 at 295 K, +1 dme/SiOHAl H-ZSM-5 at 573 K

signal

intensitya

QCCb

ηc

QL QP QL

13% 87% 100%

5.7 MHz 14.0 MHz 4.4 MHz

0.70 0.25 0.90

QL QP QL QP

18% 82% 15% 85%

5.2 MHz 11.2 MHz 5.7 MHz 12.4 MHz

0.80 0.20 0.50 0.30

a Accuracy: (5%. b Quadrupole coupling constant QCC ) (e2qQ)/ h, with the z-component, Vzz ) eq, of the electric field gradient, the electric quadrupole moment, eQ, of 27Al nuclei, and Planck’s constant, h. Accuracy: (0.2 MHz. 3 Asymmetry parameter η ) (Vxx - Vyy)/Vzz as a function of the x-, y-, and z- components of the electric field gradient. Accuracy: (0.05.

temperatures. 1H CF MAS NMR spectra recorded at elevated temperatures during conversion of methanol on H-ZSM-5 gave signals at 3.2-3.5 ppm and 8.8-10.9 ppm due to methanol and dimethyl ether. Simultaneously recorded 27Al CF MAS NMR spectra show a strong decrease in the signal of framework aluminum at 54 ppm after raising the temperature from 295 to 573 K, which is explained by a strong broadening of the 27Al MAS NMR signal due to quadrupolar interactions. To investigate the 27Al NMR signals occurring under these conditions, spin-echo NMR experiments were performed with dehydrated zeolite H-ZSM-5 in the unloaded state and after adsorption of methanol and dimethyl ether. While adsorption of methanol on zeolite H-ZSM-5 leads to a decrease in the quadrupole coupling constant of framework aluminum atoms from 14.0 to 4.4 MHz, the presence of dimethyl ether cause a quadrupolar pat-

tern according to QCC ) 11.2 MHz. Hence, the decrease in the 27Al MAS NMR signal of framework aluminum in zeolite H-ZSM-5 observed during conversion of methanol at 473 K could be due to the formation of dimethyl ether. In addition, the formation of methoxy groups has to be considered which cause a 27Al quadrupole coupling constant of 16.2 MHz. The 1H MAS NMR spectra obtained at elevated temperatures during purging zeolite H-ZSM-5 with dry nitrogen indicate a significant averaging of the dipolar interactions of bridging OH groups due to the improved hydroxyl proton mobility. In the simultaneously recorded 27Al MAS NMR spectra, however, no narrow signal of framework aluminum atoms occurred at 54 ppm. By 27Al echo NMR experiments, it was shown that raising the temperature from 295 to 573 K gives a only weak decrease in the 27Al quadrupole coupling constant of framework aluminum atoms in dehydrated H-ZSM-5 by ca. 2 MHz. This finding can be explained by jumps of the hydroxyl protons between two or three of the energetically favored oxygen atoms of each aluminum site or between different aluminum sites. In these cases, no decrease in the electric field gradients at the framework aluminum atoms is caused by the improved hydroxyl proton mobility. Acknowledgment. Financial support by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie, and MaxBuchner-Forschungsstiftung is gratefully acknowledged. References and Notes (1) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B. Acc. Chem. Res. 1996, 29, 259. (2) Anderson, M. W. Top. Catal. 1996, 3, 195. (3) Mildner, T.; Ernst, H.; Freude, D.; Ka¨rger, J.; Winkler, U. Magn. Reson. Chem. 1999, 37, S38.

8148 J. Phys. Chem. B, Vol. 105, No. 34, 2001 (4) Hunger, M.; Schenk, U.; Seiler, M.; Weitkamp, J. J. Mol. Catal. A: Chem. 2000, 156, 153. (5) Derouane, E. G.; He, H.; Derouane-Abd Hamid, S. B.; Lambert, D.; Ivanova, I. I. J. Mol. Catal. A: Chem. 2000, 158, 5. (6) Wouters, B. H.; Chen, T.; Goossens, A. M.; Martens, J. A.; Grobet, P. J. J. Phys. Chem. B 1999, 103, 8093. (7) van Bokhoven, J. A.; Roest, A. L.; Koningsberger, D. C.; Miller, J. T.; Nachtegaal, G. H.; Kentgens, A. P. M. J. Phys. Chem. B 2000, 104, 6743. (8) Lentz, P.; Carvalho, A. P.; Delevoye, L.; Fernandez, C.; Amoureux, J.-P.; Nagy, J. B. Magn. Reson. Chem. 1999, 37, S55. (9) Freude, D.; Ernst, H.; Wolf, I. Solid State Nucl. Magn. Reson. 1994, 3, 271. (10) Kunwar, A. C.; Turner, G. L.; Oldfield, E. J. Magn. Reson. 1986, 69, 124. (11) Hunger, M.; Horvath, T.; Engelhardt, G.; Karge, H. G. Stud Surf. Sci. Catal. 1995, 94, 756. (12) Hunger, M.; Horvath, T. J. Am. Chem. Soc. 1996, 118, 12302.

Seiler et al. (13) Koller, H.; Meiljer, E. L.; van Santen, R. A. Solid State Nucl. Magn. Reson. 1997, 9, 165. (14) (a) Seiler, M.; Schenk, U.; Hunger, M. Catal. Lett. 1999, 62, 139. (b) Hunger, M.; Seiler, M.; Buchholz, A. Catal. Lett., in press. (15) Ernst, S.; Weitkamp, J. Chem.-Ing.-Tech. 1991, 93, 748. (16) Hunger, M.; Seiler, M.; Horvath, T. Catal. Lett. 1999, 57, 199. (17) Hunger, M. Catal. ReV.sSci. Eng. 1997, 39, 345. (18) Anderson, M. W.; Barrie, P. J.; Klinowski, J. J. Phys. Chem. 1991, 95, 235. (19) Haase, F.; Sauer, J. J. Am. Chem. Soc. 1995, 117, 3780. (20) Bosacek, V.; Emst, H.; Freude, D.; Mildner, T. Zeolites 1997, 18, 196. (21) Slichter, C. P. Principles of Magnetic Resonance, 3rd ed.; SpringerVerlag: Berlin, 1996; p 163. (22) Batamack, P.; Doremieux-Morin, C.; Fraissard, J.; Freude, D. J. Phys. Chem. 1991, 95, 3790. (23) Baba, T.; Ono, Y. Appl. Catal. A: General 1999, 181, 227.