A Nuclear Magnetic Resonance Study of Amorphous and Crystalline

Faculty of Physics, Babes¸-Bolyai UniVersity, 3400 Cluj-Napoca, Romania. E. de Boer and A. P. M. Kentgens*. Department of Physical Chemistry, NSR Cen...
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J. Phys. Chem. B 1999, 103, 7591-7598

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A Nuclear Magnetic Resonance Study of Amorphous and Crystalline Lanthanum-Aluminates D. Iuga and S. Simon* Faculty of Physics, Babes¸ -Bolyai UniVersity, 3400 Cluj-Napoca, Romania

E. de Boer and A. P. M. Kentgens* Department of Physical Chemistry, NSR Center, UniVersity of Nijmegen, ToernooiVeld 1, 6525 ED Nijmegen, The Netherlands ReceiVed: April 19, 1999

Lanthanum-aluminates of the composition (1 - x)Al2O3‚xLa2O3 (0 < x < 0.7) were prepared by sol-gel synthesis. Subsequent heat-treatment temperatures ranging from 200 to 1200 °C were applied. The resulting samples were characterized by X-ray powder diffraction and 27Al MAS NMR. The most complex spectra were analyzed by MQMAS NMR in order to get insight in the number of sites and the possible distribution of the NMR parameters in the amorphous samples. Analysis of the MQMAS spectra, taking the efficiency of this experiment for different sites into account, helped to resolve ambiguities in the MAS spectra, which could otherwise not be deconvoluted in an unique way. The analyses show that, except for the sample with the lowest La (x ) 0.083) concentration, hardly any pentacoordinated aluminum is observed over the whole temperature treatment range. For all samples the concentration of six-coordinated aluminum decreases as the heating temperature increases until the point where crystallization occurs. The concentration of 4-fold coordinated aluminum shows the opposite behavior. When crystallization occurs, a remarkable transformation between four- and six-coordinated aluminum takes place. Crystallization of the samples occurs at lower temperatures for samples with high amounts of lanthanum. At low La content crystallization is shifted to higher temperatures and thus the lower Al (four and five) coordinations are maintained over a larger temperature range. Storage of the samples in a water-saturated atmosphere leads to a conversion of four- to six-coordinated aluminum. This shows that the low-coordinated Al atoms are accessible to water molecules and must therefore be situated at the surface.

Introduction Porous mixed oxides based on aluminum oxide are widely used in heterogeneous catalysis. For specific applications such as an automobile converter catalyst1 it is important to preserve the porosity and surface area at high temperature, even at temperatures above 800 °C. The stabilizing effect of rare earths,1 particularly lanthanum,2,3 on the γ-alumina phase is well-known. The mechanism by which these rare earths shift the transition from γ- to R-alumina at higher temperature is still under discussion, however. The existing studies are limited to low levels of doping.4-7 So far, only two stable crystalline phases are identified in the Al2O3-La2O3 system; these were studied using NMR by Dupree et al.8 Their overall composition is 11Al2O3‚La2O3 s a hexagonal compound with a β-alumina or magnetoplumbite type structure, and LaAlO3sa cubic perovskite phase.9,10 Some variation in stoichiometry has been reported, however.11 A recent 27Al and 17O NMR study of the sol-gel synthesis of LaAlO3 revealed that the various components only react to form the perovskite at temperatures exceeding 800 °C.12 In the 11Al2O3‚La2O3 phase aluminum is mainly tetra- and hexacoordinated with oxygens, whereas only hexa-coordinated aluminum is encountered in the perovskite LaAlO3.8 The predominantly used synthesis method for porous oxides with high surface area is the sol-gel technique.13 Starting from * Authors to whom correspondence should be addressed. S. Simon, E-mail: [email protected]. A. Kentgens, E-mail: [email protected].

solution, heat treatment produces amorphous xerogels and finally crystalline oxides. The temperature and duration of the heat treatment control the porosity and crystallinity of these samples. The local order in different types of aluminates and its evolution during the synthesis process and the heat-treatment procedure can be studied by 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR). An important feature of 27Al solid-state NMR is the dependence of the isotropic chemical shift (δiso) on local Al coordination: AlO4 (∼80 to 45 ppm) and AlO6 (∼20 to -20 ppm). Usually the range of ∼45 to 20 ppm is taken to identify the AlO5 polyhedra.14 The range of experimental values is wider (∼52 to 14 ppm), however.15 Moreover, depending on the local symmetry the resonances are broadened and shifted (δqis) by the second-order quadrupolar interaction. This can hamper an unambiguous assignment and analysis of the spectra. A new method, Multiple Quantum (MQ)MAS NMR,16,17 that immediately gained great popularity in NMR research of half-integer quadrupolar nuclei, can overcome resolution problems and is thus a great help in the analysis and assignment of spectral components. Frydman and Harwood16 realized that the angular dependence of the 〈+m| T |-m〉 multiple-quantum transitions have a form similar to that of the central transition but with different zero-, second-, and fourthrank coefficients. Under MAS, the second-rank terms are averaged. Thus, the frequency spectrum of 〈+m| T |-m〉 multiple-quantum transition has the same (but scaled) shape as

10.1021/jp991257q CCC: $18.00 © 1999 American Chemical Society Published on Web 08/13/1999

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Figure 1. Heat treatment diagram.

the central 〈+1/2| T |-1/2〉 transition. This makes it possible to correlate multiple- and single-quantum transitions in a twodimensional experiment, leading to well-resolved spectra with an isotropic dimension free of anisotropic quadrupolar broadening. Multiple-quantum transitions cannot be detected directly by acquisition, they can be detected indirectly in a twodimensional experiment consisting of an excitation pulse, an evolution period, and a conversion pulse followed by the detection period. The first pulse excites many transitions in the quadrupolar spin system and therefore its phase is cycled in such a way to pick out the desired MQ-coherence. The second pulse converts 2m-quantum coherence to single-quantum coherence and the normal central-transition signal is acquired. Many improvements of this basic sequence have been proposed in order to obtain pure-phase spectra18-21 and gain sensitivity.22-26 MQMAS has been applied successfully for various nuclei in both crystalline and amorphous materials. In this paper we report X-ray diffraction, 27Al MAS and MQMAS NMR studies of amorphous and crystalline lanthanumaluminates prepared via sol-gel synthesis. The composition and preparation temperatures were varied over a broad range. The most complex spectra were analyzed by MQMAS NMR in order to get insight into the number of sites and the possible distribution of the NMR parameters in the amorphous samples. These results were used as a starting point for a quantitative analysis of the MAS spectra. Experimental Section Lanthanum-aluminate samples with composition (1 - x) Al2O3‚xLa2O3 (0 < x < 0.7) were prepared by thermal decomposition of aluminum and lanthanum nitrate mixtures sustained by the simultaneous oxidation of glycerol. After dissolving the components in a small amount of distilled water a clear solution was obtained at room temperature. By heating these solutions at 95 °C for ca. 1.5 h viscous gels were formed. Continuing the heat treatment at this temperature resulted in spongy, bulky, solid samples. MAS NMR measurements were carried out on samples heated in air for 30 min at various temperatures (Figure 1) in a

Figure 2. XRD patterns of the samples AlLa0.083, AlLa0.375, AlLa0.5, and AlLa0.7 after heat treatment at different temperatures. The symbols identify the reflections that could be assigned to the following phases: (O) Al11LaO18, (b) AlLaO3, (4) β-Al2O3, (×) La2O3.

cylindrical furnace. To keep hydration effects to a minimum, after each heating stage the samples were placed in airtight glass bottles and just before the beginning of the NMR measurements, rapidly transferred to the spinners. The samples are denoted AlLax-Ty where x is the lanthanum fraction in the sample and y refers to the treatment temperature Tt (y ) Tt/100). The MAS NMR measurements were carried out at room temperature on a Bruker AM-500 spectrometer equipped with a solid-state accessory, using a home-built probe head with a 5 mm Jakobsen MAS assembly. Usually a 1 µs excitation pulse (ν1 ∼ 35 kHz) was applied and spinning speeds up to 18 kHz were employed, adequate to ensure a quantitative interpretation of the data.27,28 Spectra are referenced with respect to an external Al(NO3)3 solution [Al(H2O)63+]. The z-filtered MQMAS NMR spectra18 were obtained on a Chemagnetics CMX-400 spectrometer in a 2.5 mm CP-MAS probehead using spinning speeds of 29 kHz and an rf-field strength of 180 kHz. MAS and MQMAS spectra were simulated using a program developed in MATLAB taking distributions in NMR parameters into account. Powder averages were performed by simultaneously incrementing the Euler angles.29,30 The X-ray diffraction patterns were recorded at room temperature with a Dron-type 3 X-ray powder diffractometer using Cr KR radiation. Results and Discussion XRD Characterization. After heat treatment at low temperatures the samples are amorphous. The stability of the amorphous phase at high temperatures depends strongly on the lanthanum content. As the lanthanum content increases, the temperature at which samples become crystalline decreases. The

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Figure 3. 27Al MAS NMR spectra of AlLa0.083, AlLa0.375, AlLa0.5, and AlLa0.7, obtained at 11.7 T. The spectra were recorded at room temperature after heat treatment at increasing temperatures. Spinning sidebands are indicated by (*). Two spectra were recorded in a Si3N4 rotor that contained an Al impurity (O).

X-ray diffraction patterns for the crystalline samples are shown in Figure 2. The sample AlLa0.083 is poorly crystalline even after heat treatment at 1200 °C, whereas in the AlLa0.7 sample a significant amount of fairly crystalline material is detected after heat treatment at 600 °C. The crystalline phases identified in the AlLa0.083-T12 sample are a distorted lanthanum hexa-aluminate structure (Al11LaO18, marked by O in the figure) and the perovskite AlLaO3 (marked b). An interesting evolution is observed for the AlLa0.375 sample, where the AlLaO3 phase (b) is gradually developing at the expense of β-alumina (β-Al2O3 (4)) showing that the excess aluminum is incorporated in the perovskite phase. Therefore the Al:La ratio in this structure must change and be different from 1:1. For the AlLa0.5 sample only the AlLaO3 perovskite phase develops, in accordance with the fact that Al and La are present in the synthesis mixture in 1:1 ratio. In case of the AlLa0.7 sample crystalline La2O3 (marked ×) is observed even at low temperatures. Subsequently a well-crystallized LaAlO3 phase (b) develops, in accordance with the observations by Bastow et al.12 Judging from the line width, the crystallinity of this phase seems higher compared to the AlLa0.5 sample. 27Al MAS NMR Spectroscopy. To characterize the Al coordination and the development of order in the amorphous phases, 27Al MAS NMR was used. Figure 3 shows the 27Al MAS NMR spectra for all solid samples as a function of heat treatment temperature starting at 200 °C. Below 200 °C the

NMR spectra show only one single peak at about 8 ppm corresponding to six-coordinated aluminum. This coordination is brought about by H2O molecules in solution, H2O and HOions in viscous gels, and OH- and O2- in the case of xerogels.13 For all samples the MAS spectra show the development of two new peaks, around 70 and 35 ppm, in the first stages of temperature treatment. The peaks between 0 and 20 ppm and 45 and 80 ppm can be straightforwardly assigned to six- and four-coordinated aluminum, respectively. The assignment of the signal intensity in the region between 20 and 45 ppm is somewhat ambiguous from MAS spectra alone. Generally resonances in this region are attributed to 5-fold-coordinated aluminum.14 An argument in favor of this reasoning is a previous study of aluminum-borates,31 where during the heat treatment the unresolved peak from this region in the amorphous samples transformed into the resonances of well-defined pentacoordinated aluminum sites in crystalline Al18B4O33 (A9B2).32 On the other hand, the signal intensity in the 20-45 ppm region may also be due to distorted 4-fold coordinated aluminum sites. Ray and Samoson observed this in a DOR study of zeolites.33 In a recent MQMAS study of alumino-silicate sol-gel materials it was unambiguously demonstrated that most of the intensity in the 20-45 ppm region of the MAS spectrum originated from distorted 4-fold-coordinated sites.34 Therefore, it is imperative to apply MQMAS to a number of key samples to assign the

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Figure 4. Stacked (a) and contour (b) representation of the 27Al MQMAS NMR spectrum of AlLa0.083-T6 clearly showing the presence of three lines assigned to four-, five-, and six-coordinated Al. In AlLa0.375-T6 only two resonances of tetrahedral and octahedral Al are observed. (c) Simulations of the spectra assuming a Gaussian distribution in isotropic chemical shift and quadrupole coupling constant. The solid line indicates the isotropic chemical shift line. The dashed line shows the direction of the anisotropic quadrupolar line broadening and the dotted-dashed line indicates the direction of the quadrupolar-induced shift.

MAS spectra of the amorphous lanthanum-aluminates, as will be described in the next section. At elevated temperatures the MAS spectra of all samples revert to mainly one resonance of a six-coordinated aluminum. In these spectra also a set of spinning sidebands (marked *) appear, originating from the 〈(1/2| T |(3/2〉 satellite transitions. This is a further indication that the Al surroundings are better ordered and we are dealing with crystalline materials in this case. Especially for the AlLa0.7-T12 sample both these

sidebands and the central transition line (at 12 ppm) are narrow, pointing to a symmetric octahedral surrounding for Al, as would be expected for a well-defined perovskite structure. Apparently, having an excess of La present helps to obtain this wellcrystallized AlLaO3 structure. Dupree et al.8 also observed an octahedral resonance with a small quadrupolar coupling constant (0.12 MHz) for AlLaO3. In their case this pure phase was obtained for a AlLa0.5 sample treated at 1775 °C. For AlLa0.5T12 and AlLa0.375-T12 we observe some line broadening of the

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Figure 5. The powder averaged, overall efficiency of the MQMAS experiment for spin I ) 5/2 as a function of the ratio of the quadrupole coupling constant and the rf-field strength (Cqcc/ν1), assuming a 180° excitation and a 60° conversion pulse.

central transition, together with a tiny amount of 4-foldcoordinated aluminum. This indicates that some disorder is still present in the crystalline phase. This might be caused by the suggested nonstoichiometry of the phases and/or the presence of some residual amorphous material. The AlLa0.083-T12 sample has the broadest lines of the samples treated at 1200 °C. It shows the presence of four- and six-coordinated aluminum in accordance with the observations of Dupree et al.8 for Al11LaO18. Also a tiny amount of five-coordinated Al (δiso ) 38 ppm) is observed in this spectrum. Five-coordinated Al can be present in the lanthanum hexa-aluminate if it has the magnetoplumbite

J. Phys. Chem. B, Vol. 103, No. 36, 1999 7595 structure, although the X-ray data are not entirely conclusive in this respect.11 Recently we were able to observe the resonance of five-coordinated Al in SrAl12O19 with a magnetoplumbite type structure. This was observed at an unusually low isotropic shift of 18 ppm.15 For the present sample we cannot exclude the possibility of the presence of some residual amorphous material causing the resonance of five-coordinated Al. 27Al MQ-MAS NMR Spectroscopy. As was mentioned in the previous section, a conclusive assignment of the overlapping MAS spectra is not possible. Therefore, MQMAS NMR spectra were recorded for the samples treated at 600 °C. As is shown in Figure 4, the AlLa0.083-T6 sample generates three resonances indicating that the sample contains significant amounts of four-, five-, and six-coordinated aluminum. For the other T6 samples the resonance of five-coordinated Al is almost absent, as is also shown for AlLa0.375-T6 in Figure 4. Only for the AlLa0.083 sample significant amounts of 5-fold coordinated aluminum are detected. Besides resolving the spectrum in clearly separated lines, the MQMAS experiment can give us information about the distribution in NMR parameters experienced by the various sites through the analysis of the line shapes.35,36 In the unsheared spectra presented here, the resonances are situated on the line ν1 ) 3ν2 in the absence of a quadrupolar interaction. The spectra are therefore plotted in such a way that this line is the diagonal of the 2D spectra. In the presence of a quadrupolar interaction the center of gravity of the resonance is shifted from the diagonal by the quadrupolar-induced shift (δQIS) in the direction ν1 ) (3/4)ν2, indicated in the spectra by a dotted-dashed line. Furthermore the lines are broadened by the second-order quadrupolar interaction along the direction ν1 ) (19/12)ν2, indicated by a dashed line in the spectra. In case of amorphous systems a distribution in the NMR parameters exists and the lines will be smeared out in the two-dimensional spectrum; e.g., if the lines are dominated by a distribution in isotropic chemical shift, the lines will be broadened in the direction parallel to the diagonal, whereas they will bent away from the diagonal if a wide distribution in quadrupole interaction exists.35 To model these distributions, two important points have to

Figure 6. The results of deconvolution of the MAS NMR spectra for (a) AlLa0.083-T6 and (b) AlLa0.375-T6 using the parameters obtained from the simulation of the MQMAS spectra.

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Figure 7. The fraction of four(0)-, five(g)-, and six(ª)-coordinated aluminum as a function of heat treatment temperature for (a) AlLa0.083, (b) AlLa0.375, (c)AlLa0.5, and (d)AlLa0.7.

be taken into account: (1) the nonuniform effect of the excitation and conversion pulse for sites experiencing different quadrupolar interactions, and (2) the shape of the distribution function for the NMR parameters. The effect of the excitation and conversion pulse was calculated by numerically diagonalizing the Hamiltonian containing the rf-field term and the first-order quadrupolar interaction, and evolving the density matrix for the employed pulse sequence as has been described in several publications.17,37,38 For the experimental conditions used in the present study, the overall efficiency of the MQMAS experiment in powdered samples is depicted in Figure 5 as a function of the ratio of the quadrupolar coupling constant and the rf-field strength (Cqcc/υ1). Clearly, the efficiency is strongly nonuniform; the signal intensities of sites with very small (Cqcc/υ1 ∼ 0) or very large (Cqcc/υ1 > 100) quadrupolar interactions will be significantly attenuated in the MQMAS experiment. Therefore, it is beneficial to use large rf-field strengths to avoid the attenuation of the signal of sites with large Cqcc. Obviously, in modeling the effect of a distribution in NMR parameters, the various sites in the distribution have to be weighted according to Figure 5. In the ideal case the existing distributions in the NMR parameters should be extracted from the spectra using a socalled inverse approach without putting in model information. Zwanziger suggested such an approach for DAS spectra.39 Considering the complexity of this procedure, no satisfactory algorithm has been developed for MQMAS spectra yet. So far,

distributions in quadrupolar interactions were modeled with simple Gaussian distributions in Cqcc and the asymmetry parameter η, or Gaussian distributions in the principal values of the electric-field-gradient tensor.40,41 However, no physical interpretation for such distributions is available. Structural variations in the materials, i.e., in bond angles and distances dictate the actual shape of the distributions. A more sophisticated method was introduced by Czjzek et al.42,43 This was recently applied in the analysis of MAS NMR spectra.44 A further point that has to be considered is the distribution in chemical shift and its possible correlation with the quadrupolar distribution. One can conclude that the extraction and interpretation of distributions in the NMR parameters of half-integer quadrupolar nuclei is to a large extent “terra incognita”. In the present study we have chosen to simulate the MQMAS spectra with simple Gaussian distributions in the NMR parameters with the sole purpose to finding a satisfactory description of the line shapes for the individual sites which can subsequently be used for a quantitative analysis of the normal MAS spectra. Using the MAS spectra alone, this is not possible because of the strong overlap of the lines. As can be judged from Figures 4 and 6 it is possible to find a set of parameters that gives a satisfactory description of both the MQMAS and the MAS line shapes. Quantitative Analysis of the MAS Spectra. As we got a good description of the line shapes for both MQMAS and MAS spectra, using Gaussian distributions in isotropic chemical shift (δiso) and quadrupolar coupling constant (Cqcc), all MAS spectra

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Figure 8. The average isotropic chemical shift of four(0)-, five(g)-, and six(ª)- coordinated aluminum for (a) AlLa0.083, (b) AlLa0.375, (c)AlLa0.5, and (d)AlLa0.7 used to simulate the MAS spectra. The bars indicate the width of the distribution.

were analyzed in this way to get the signal intensities for the individual sites. The results of this analysis are shown in Figure 7. From Figure 7 the following conclusions can be drawn: (1) Except for the sample AlLa0.083 the concentration of pentacoordinated aluminum is low over the whole temperature range. (2) The concentration of six-coordinated aluminum decreases as the heating temperature increases until the point where crystallization occurs. The concentration of 4-fold coordinated aluminum shows the opposite behavior. When crystallization occurs, a remarkable transformation between four- and sixcoordinated aluminum takes place. (3) Crystallization of the samples takes place at lower temperatures for samples with high amounts of lanthanum. At low La content, crystallization is shifted to higher temperatures and thus the lower Al (four and five) coordinations are maintained over a larger temperature range. The average isotropic chemical shift of four-(0), five-(g), and six-(ª)coordinated aluminum obtained from the simulation of the MAS spectra are depicted in Figure 8. The width of the distributions is also shown. The chemical shift values are in all cases in the normal ranges expected for the respective coordinations. The rather large widths in distribution for the amorphous samples indicate that there is considerable variation in the first and second coordination sphere of the aluminum atoms. This large width, in combination with the accuracy of the analysis, makes it impossible to draw definite conclusions about changes in the Al surroundings as a function of the treatment temperature.

The only obvious and expected change is the sudden drop in the width of the distribution upon crystallization, as the coordinations spheres are obviously much better defined in the crystalline phases. However, as was already remarked, even in the crystalline T12 samples, some disorder remains which is attributed to stoichiometry variations encountered in these phases. Hydration Effects. The amorphous samples treated at intermediate temperatures proved to be sensitive to hydration, similar to the previously studied amorphous aluminum-borates.31 This effect is illustrated in Figure 9 for AlLa0.7-T4. In the freshly prepared sample (Figure 8a) a considerable fraction of the aluminum is present in 4-fold coordinated sites. After exposing the sample to a moist atmosphere, a conversion from four- to six-coordinated aluminum is observed (Figure 9b). This shows that the low-coordinated Al atoms are accessible to water molecules which are used to achieve octahedral coordination. This effect appears to be mostly reversible as a renewed heating of the sample at 400 °C recovers the original situation (Figure 9c). Subsequent rehydration again leads to a shift toward hexacoordination (Figure 9d). It should be noted, however, that now the amount of five-coordinated Al has decreased, indicating that also some irreversible reordering of the surface has taken place. These experiments show that species with low coordination numbers, which are most interesting from a catalytic point of view, are situated mainly at the surface of the material.

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Figure 9. (a) 27Al MAS NMR spectrum of a freshly prepared AlLa0.7T4 sample. (b) After exposure of the sample to a water-saturated atmosphere most of the sites are converted to an octahedral coordination. Subsequent retreatment at 400 °C (c) followed by a rehydration (d) shows that this process is mostly reversible except for the amount of five-coordinated Al indicating that some irreversible reconstruction of the surface has taken place.

Conclusions The NMR results on amorphous and crystalline aluminumlanthanates prepared by the sol-gel procedure reveal that in dry gels, obtained after heat treatment at 200 °C, we have almost exclusively aluminum hexa-coordinated with hydroxyl groups. After heat treatment at temperatures between 400 and 600 °C, porous, amorphous samples are obtained with the majority of the Al sites in a tetrahedral coordination. For the samples with the lowest La content also a large amount of five-coordinated aluminum is present. In these samples crystallization occurs at higher temperatures as compared to the samples containing high amounts of lanthanum. The hydration effect demonstrates that these low-coordinated species are accessible to water molecules which are absorbed at the surface and thus increase the coordination number so that mainly six-coordinated Al is observed. Samples with a great number of lowly coordinated Al sites situated at the surface over an extended temperature interval should be interesting from a catalytic point of view. Acknowledgment. We thank Mr. H. Janssen, Mr. J. van Os, and Mrs. G. Nachtegaal of the NWO/CW HF-NMR Facility for their technical support and help with the NMR experiments. We are grateful to Mrs. M. Pop and Dr. Gh. Borodi for carrying out the XRD measurements. References and Notes (1) Catalyst and automobile pollution control; Crueg, A., Ed.; Elsevier: Amsterdam, 1992. (2) Schaper, H.; Amesz, D. J.; Doesburg, E. B. M.; van Reijen, L. L. Appl. Catal. 1984, 9, 129. (3) Kumar, K.-N. P.; Tranto, J.; Kumar, J.; Engell, J. E. J. Mater. Sci. Lett. 1996, 15, 266. (4) Ozawa, M.; Kimura, M.; Isogai, A. J. Mater. Sci. Lett. 1990, 9, 709.

Iuga et al. (5) Church, J. S.; Cant, N. W.; Trimm, D. L. Appl. Catal. A 1993, 101, 105. (6) Church, J. S.; Cant, N. W.; Trimm, D. L. Appl. Catal. A 1994, 107, 267. (7) Varquez, A.; Lopez, T.; Gomez, R.; Bokhimi; Morales, A.; Novaro, A. J. Solid State Chem. 1997, 128, 161. (8) Dupree, R.; Lewis, M. H.; Smith, M. E. J. Am. Chem. Soc. 1989, 111, 5125. (9) Wyckoff, R. Crystal structure; Interscience Publishers: New York, 1964. (10) Rooymans, C. J. M.; Rabenau, A. Crystal structure and chemical bonding in inorganic chemistry; North-Holland Publ. Comput.: Amsterdam, 1975. (11) Iyi, N.; Inoue, Z.; Takekawa, S.; Kimura, S. J. Solid State Chem. 1984, 54, 70. (12) Bastow, T. J.; Dirken, P. J.; Smith, M. E.; Whitfield, H. J. J. Phys. Chem. 1996, 100, 18539. (13) Brinker, C. J.; Scherer, G. W. Sol-gel science; Academic Press: New York, 1990. (14) Smith, M. E. Appl. Magn. Reson. 1993, 4, 1. (15) Jansen, S. R.; Hintzen, H. T.; Metselaar, R.; de Haan, J. W.; van de Ven, L. J. M.; Kentgens, A. P. M.; Nachtegaal, G. H. J. Phys. Chem. B 1998, 102, 5969. (16) Frydman, L.; Harwood: J. S. J. Am. Chem. Soc. 1995, 117, 5367. (17) Medek, A.; Harwood: J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117, 12779. (18) Amoureux, J. P.; Fernandez, C.; Steuernagel, S. J. Magn. Reson. Ser. A 1996, 123, 116. (19) Brown, S. P.; Heyes, S. J.; Wimperis, S. J. Magn. Reson. Ser. A 1996, 119, 280. (20) Massiot, D.; Touzo, B.; Trumeau, D.; Coutures, J. P.; Virlet, J.; Florian, P.; Grandinetti, P. J. Solid State Nucl. Magn. Reson. 1996, 6, 73. (21) Brown, S. P.; Wimperis, S. J. Magn. Reson. 1997, 124, 279. (22) Wu, G.; Rovnyak, D.; Griffin, R. G. J. Am. Chem. Soc. 1996, 118, 9326. (23) Ding, S. W.; McDowell, C. A. J. Magn. Reson. 1998, 135, 61. (24) Marinelli, L.; Frydman, L. Chem. Phys. Lett. 1997, 275, 188. (25) Kentgens, A. P. M.; Verhagen, R. Chem. Phys. Lett. 1999, 300, 435. (26) Madhu, P. K.; Goldbourt, A.; Frydman, L.; Vega, S. Chem. Phys. Lett. 1999, 307, 41. (27) Freude, D.; Haase, J. NMR basic principles and progress; SpringerVerlag: Berlin: 1993. (28) Kentgens, A. P. M. Geoderma 1997, 80, 271. (29) Cheng, V. B.; Suzukawa, H. H.; Wolfsberg, M. J. J. Chem. Phys. 1973, 59, 3992. (30) Conroy, H. J. Chem. Phys. 1967, 47, 5307. (31) Simon, S.; van Moorsel, G. J.; Kentgens, A. P. M.; de Boer, E. Solid State Nucl. Magn. Reson. 1995, 5, 163. (32) Massiot, D.; Muller, D.; Hubert, T.; Schneider, M.; Kentgens, A. P. M.; Cote, B.; Coutures, J. P.; Gessner, W. Solid State Nucl. Magn. Reson. 1995, 5, 175. (33) Ray, G. J.; Samoson, A. Zeolites 1993, 13, 410. (34) Peeters, M. P. J.; Kentgens, A. P. M. Solid State Nucl. Magn. Reson. 1997, 9, 203. (35) Kraus, H.; Prins, R.; Kentgens, A. P. M. J. Phys. Chem. 1996, 100, 16336. (36) Hwang, S. J.; Fernandez, C.; Amoureux, J. P.; Cho, J.; Martin, S. W.; Pruski, M. Solid State Nucl. Magn. Reson. 1997, 8, 109. (37) Wu, G.; Rovnyak, D.; Sun, B.; Griffin, R. G. Chem. Phys. Lett. 1995, 249, 210. (38) Fernandez, C.; Amoureux, J. P. Solid State Nucl. Magn. Reson. 1996, 5, 315. (39) Zwanziger, J. W. Solid State Nucl. Magn. Reson. 1994, 3, 219. (40) Meinhold, R. H.; Slade, R. C. T.; Newman, R. H. Appl. Magn. Reson. 1993, 4, 121. (41) Jager, C.; Kunath, G.; Losso, P.; Scheler, G. Solid State Nucl. Magn. Reson. 1993, 2, 73. (42) Czjzek, G. Phys. ReV. B 1982, 25, 4908. (43) Czjzek, G.; Fink, J.; Goetz, F.; Schmidt, H.; Coey, J. M. D.; Rebouillat, J.-P.; Lienard, A. Phys. ReV. B 1981, 23, 2513. (44) Bureau, B.; Silly, G.; Buzare, J. Y.; Emery, J.; Legein, C.; Jacoboni, C. Proceedings of the Joint 29th Ampere - 13th ISMAR International Conference, Berlin; 1998; 648-649.