CP-MAS NMR Studies of Promoter

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J. Phys. Chem. B 2003, 107, 14171-14175

14171

Multinuclear Solid-State MAS/CP-MAS NMR Studies of Promoter (Phosphate)-Enhanced Crystallization of Siliceous MCM-41 S. C. Laha,† M. D. Kadgaonkar,† A. Anuji,‡ S. Ganapathy,‡ J. P. Amoureux,§ and R. Kumar*,† Catalysis DiVision and Physical Chemistry DiVision, National Chemical Laboratory, Pune 411 008, India, and LCPS (CNRS-8012) UniVersite de Lille-1, VilleneuVe d’Ascq, F-59652, France ReceiVed: March 31, 2003; In Final Form: September 25, 2003

Promoter (phosphate)-enhanced crystallization of MCM-41 has been studied by multinuclear solid-state NMR spectroscopy with use of 13C, 29Si, and 31P as probe nuclei. The structure building Q4 units, in addition to the silanol nests built by the Q2 and Q3, are identified and quantified through 29Si MAS NMR. A measure of the relative amount of the silanol nests, present over the T-sites formed, was determined from the ratio (Q2 + Q3)/Q4 and is found to be smaller in the mesoporous material prepared by the phosphate promoter assisted synthesis. Signal evolution for the Q3 and Q4 resonances in 1H-29Si CP dynamics experiments shows that cross-polarization buildup rates (TIS-1) are unchanged for the end material prepared with phosphate promoter. 13 C signal evolution in 1H-13C CP dynamics experiments for the surfactant cetyl trimethylammonium bromide (CTABr), used as a template, is markedly different when it is confined within the mesopores of MCM-41. An enhanced mobility for the surfactant molecule is inferred and is consistent with the liquid crystal templating mechanism proposed earlier for the formation of mesoporous MCM-41 materials. Further, 31P MAS experiments show that there is no incorporation of the phosphate promoter within MCM-41 during the hydrothermal synthesis.

Introduction The entry of M41S1,2 in the world of material science and catalysis3 has given a major thrust to various aspects of the synthesis of these materials, particularly MCM-41 analogues, to extend the possibilities of exploiting their very high surface areas (>1000 m2 g-1), controlled and narrowly distributed pore sizes (2-10 nm), and other interesting structural and textural properties. Our recent report on the promoter-induced synthesis of MCM-41 materials,4 using a catalytic amount of group VA, VIA, and VIIA oxyanions as promoter, suggests that the formation of these materials is quite a bit faster in the presence of promoters than in their absence. The promoter action occurs essentially through a reduction in the induction period of the nucleation and long-range ordering processes, which favorably reduces the synthesis time.5,6 While enhanced crystallization of zeolitic material with promoters is very attractive as a time and cost saving exercise, the role of these promoters during synthesis requires further studies. Challenges are especially posed for the structural characterization of the newly synthesized materials since it is important to verify that the resulting end material indeed conforms to the parent topology and has the desired structural and textural properties. The enhanced crystallization in the presence of promoter and the structural integrity of the material can be discerned from powder XRD methods since the peak positions and intensities can be indexed and used to confirm whether the XRD pattern is the same as in the material synthesized without a promoter. While XRD methods are clearly * To whom correspondence should be addressed. Fax: 91-20-5893761 or 5893355. E-mail: [email protected]. † Catalysis Division, National Chemical Laboratory. ‡ Physical Chemistry Division, National Chemical Laboratory. § LCPS (CNRS-8012) Universite de Lille-1.

superior, they are, however, restricted to well-crystallized samples and one may encounter difficulties at early reaction times when the onset of nucleation and crystallization has just begun. NMR spectroscopy provides a convenient and elegant approach since it can be employed to progressively monitor and study the formation of various species and inspect the events at various stages of zeolite formation. Such NMR experiments can be performed in situ or ex situ on samples quenched at different reaction times during the entire synthesis period. The main advantage with NMR is that molecular level insights are provided through a choice of different NMR-active probe nuclei. As we had shown5 before, liquid-state NMR experiments can be successfully employed to monitor the various precursors (Q0, Q1, Q2, Q3, and Q4) in the solution phase and study the rate at which the structure building Q4 species disappear in the liquid state 29Si NMR spectra. Measurement of the ratio I(Q4)/∑Ii(Qi) (i ) 0, 1, 2, 3, 4) as a function of the reaction time indeed allows us to monitor the kinetics of crystal growth and hence determine the exact reduction of time in a promoter-assisted synthesis.5,6 Solid-state NMR experiments involving Magic Angle Spinning (MAS) are equally attractive and are complementary to the liquid-state experiments. The main advantage with the solid-state NMR method is that it affords the detection and characterization of various insoluble species in the condensed solid phase and, further, complications from exchange dynamics generally do not exist. Although one may employ MAS to enhance signal resolution, cross-polarization techniques can be additionally sought to increase sensitivity. Moreover, by performing the cross-polarization dynamics experiments, the relevant CP parameters can be determined and compared for the end materials prepared by conventional and promoterassisted hydrothermal synthesis. In the context of multinuclear solid-state NMR, the choice of 29Si and 13C as probe nuclei allows the study of the zeolitic

10.1021/jp030409l CCC: $25.00 © 2003 American Chemical Society Published on Web 11/27/2003

14172 J. Phys. Chem. B, Vol. 107, No. 51, 2003 and template environments, respectively. 29Si MAS experiments often provide adequate signal resolution so that the tetrahedral silicate species and silanol groups can be identified and quantified. By recording 29Si MAS spectra as a function of reaction time, the growth pattern of a solid can be monitored and used as a marker to study the influence of promoters during synthesis. Similarly, 13C CP-MAS NMR provides valuable insights on the formation of the zeolite with the required topology since organic molecules, used as templates, act as poredirecting agents. The use of 13C CP-MAS NMR has been demonstrated in the phosphate promoter assisted synthesis of Silicalite-1 with TPA ion as the template.6 As a further aid in studying the formation of the framework structure during hydrothermal synthesis, the dynamics of 1H-29Si and 1H-13C cross-polarization7 can be monitored and quantitatively analyzed in MAS NMR studies. In CP experiments, the cross-polarization (TIS-1) and rotating frame relaxation (T1F-1) rates can be determined through CP dynamics experiments and data analysis. These parameters, in turn, can be related to local structure and dynamics. Finally, the choice of 31P as a probe nucleus, by virtue of its high natural abundance, provides adequate detection sensitivity in 31P MAS experiments to determine whether there is any incorporation of phosphorus in the zeolite in a phosphate promoter assisted synthesis. The present study deals with a multinuclear (13C, 29Si, and 31P) solid-state NMR study of MCM-41, which was synthesized by conventional method (without promoter) (WOP) and with a phosphate promoter (WP). 29Si MAS NMR is mainly used to monitor the structure building Q4 species and the silanol nests (Q2 and Q3) and to calculate the effective ratio (Q2 + Q3)/Q4, which is a measure of the relative amount of the silanol nests over the T-sites present in the structure. 1H-13C and 1H-29Si cross-polarization dynamics experiments have been additionally employed for the determination and quantitative comparison of TCH and T1FH parameters for the end materials prepared by conventional and phosphate promoter assisted synthesis. Finally, 31P MAS NMR has been used to determine whether any implanting of the phosphate promoter in the mesopores of MCM-41 occurs or not during the hydrothermal synthesis. Experimental Section Materials. Sodium silicate (Na2SiO3, 28.48% SiO2, 9.02% Na2O, and 62.5% H2O), fumed silica, a 25 wt % aqueous solution of tetramethylammonium hydroxide (TMAOH), cetyltrimethylammonium bromide (CTABr), sodium chloride (NaCl, s.d. Fine Chem., India), and sodium dihydrogen phosphate (NaH2PO4 promoter) were used for the preparation of Si-MCM41 samples. Synthesis of Si-MCM-41: Purely siliceous MCM-41 samples were synthesized in the presence and the absence of promoter (NaH2PO4), using Na2SiO3 and SiO2 in a Na2SiO3:SiO2 molar ratio of 0.124:1 as the different silica sources.4 In a typical preparation, the two different silica sources (Na2SiO3 and fumed SiO2) were suspended in an aqueous solution of TMAOH and then aqueous solutions of CTABr and NaH2PO4 (promoter) were added in succession to the synthesis mixture to obtain the final molar gel composition of the Si-MCM-41-WP (S-WP) sample as 1SiO2:0.11Na2O:0.08TMAOH:0.21CTABr:125H2O:0.1NaH2PO4. The standard Si-MCM-41-WOP (S-WOP) sample was prepared in the absence of promoter, using an aqueous solution of NaCl instead of NaH2PO4 to maintain the same Na+/SiO2 molar ratio in both cases. The gel mixtures were refluxed at atmospheric pressure under stirring at a constant temperature of 100 °C for a period of 8-16 h depending upon the synthesis

Laha et al.

Figure 1. Powder XRD spectra of MCM-41 prepared with phosphate promoter (8 h) (A) and MCM-41 prepared by conventional hydrothermal synthesis without promoter (16 h) (B).

conditions. The solid products thus obtained were washed thoroughly first with deionized water and then with acetone, dried at 353 K for 4 h, and used for NMR analyses. The powder XRD spectra of the samples were recorded between 1.5° and 10° (2θ) with a scanning rate of 1 deg/min on a Rigaku Miniflex diffractometer with Cu KR radiation (λ ) 0.15406 nm). The samples were also characterized by BET surface area measurements, UV-vis, and FT-IR. Multinuclear solid-state NMR: 29Si MAS, 1H-29Si/1H13C CP-MAS, and CP dynamics experiments were performed on a Bruker MSL-300 NMR spectrometer at the Larmor frequencies of 59.595 and 75.432 MHz for 29Si and 13C, respectively. The chemical shift values (in ppm) were calculated with TMS as reference for both 29Si and 13C measurements. Sample spinning was kept at 4 kHz. 31P MAS experiments were performed at a spinning speed of 8 kHz on a Bruker DRX-500 NMR spectrometer. For the 29Si Bloch decay experiments a 45° flip angle pulse of 2.5 µs duration and a recycle delay of 3 s were used. The CP match conditions were established at applied r.f. fields of 50 and 46 kHz for the 1H-29Si and 1H-13C CPMAS experiments, respectively. Results and Discussion The powder XRD spectra of the fully synthesized samples, prepared with phosphate promoter, are compared with that of MCM-41 prepared by conventional synthesis (Figure 1). An identical XRD powder pattern is noticed for the end materials which were prepared after a reaction time of 16 h without promoter and 8 h with phosphate promoter. Both the XRD patterns indicate a strong [100] reflection peak with two small peaks at identical 2θ values, which can be indexed on a hexagonal lattice characteristic of MCM-41 material.2 The BET surface area obtained for Si-MCM-41-WP (8 h) and Si-MCM41-WOP (16 h) samples was 1480 m2 g-1 and 1296 m2 g-1, respectively. 29Si MAS NMR. 29Si MAS NMR spectra of samples quenched at reaction times of 0, 2, 4, 8, and16 h in the absence of promoter (S-WOP) and 0, 2, 4, 6, and 8 h in the presence of phosphate promoter (S-WP) are shown in Figure 2. Three signals at δ - 92, - 101, and -110 ppm (with respect to TMS) are readily assigned,8,9 respectively, to the geminal silanol sites (SiO)2Si(OH)2 (Q2) (which are either single or hydrogen bonded9) and isolated silanol sites of type (SiO)3SiOH (Q3) and (SiO)4Si (Q4), representing Si-O-Si linkages of the framework. Independent 1H-29Si CP experiments show pronounced signal enhancement for the Q2 and Q3 sites which have attached protons [(OH)2/(OH)].

Phosphate-Enhanced Crystallization of Siliceous MCM-41

Figure 2. Bloch decay 29Si MAS NMR spectra of as-synthesized MCM-41 as a function of the reaction time in the phosphate promoter assisted hydrothermal synthesis (left) [(A) 0 h, (B) 2 h, (C) 4 h, (D) 6 h, and (E) 8 h] and in the conventional hydrothermal synthesis without any promoter (right) [(F) 0 h, (G) 2 h, (H) 4 h, (I) 8 h, and (J) 16 h].

TABLE 1: Peak Areas Estimated from Deconvolution of 29Si MAS Spectra sample S-WOP

S-WP

reaction time, h

Q2

Q3

Q4

(Q2 + Q3)/ Q4

0 4 8 16 0 2 4 8

3.7 11.2 13.6 0.0 6.3 11.0 4.0 0.0

51.7 51.2 43.3 56.0 39.1 39.0 45.0 41.8

44.6 37.6 43.1 44.0 54.6 50.0 51.0 58.2

1.24 1.67 1.32 1.27 0.83 1.0 0.96 0.72

The relative intensities of Q2, Q3, and Q4 species among samples quenched at different reaction times allow us to estimate the degree of condensation of these species in the conventional

J. Phys. Chem. B, Vol. 107, No. 51, 2003 14173 and promoter-mediated synthesis. The peak intensity data for Q2, Q3, and Q4, obtained by deconvolution of 29Si MAS spectra, and calculated values of the ratio (Q2 + Q3)/Q4 are given in Table 1. For the synthesis carried out in the absence of promoter (Figure 2, right), (Q2 + Q3) species occur with comparably more intensity. A striking observation noticed in the 29Si MAS spectra (Figure 2) is that the value of the (Q2 + Q3)/Q4 ratio, which represents the relative population of the silanol groups over the tetrahedral Si sites present, is 1.27 and 0.72 for the conventional and phosphate promoter assisted synthesis of fully ordered SiMCM-41 (WOP) and Si-MCM-41 (WP) samples, respectively. The data presented in Table 1 clearly indicate that the condensation of Q3 species into Q4 species is facilitated in the presence of promoter even in the initial stages of the formation of MCM41 samples. During hydrothermal synthesis, the formation of condensed silicate materials involves an initial induction period during which the different soluble anionic silicate species (monomers, dimmers, double four-ring, etc.) are formed and these species subsequently undergo condensation to form the condensed silicate material Si-MCM-41. 13C CP-MAS NMR. Figure 3 shows the 13C CP-MAS NMR spectra recorded on as-synthesized samples at different reaction times in the conventional (right) and phosphate promoter (left) assisted synthesis of MCM-41. For the template molecule, namely, cetyltrimethylammonium bromide (CTABr), 13C resonances are observed at the chemical shifts 66.3, 53.6, 32.4, 30.4, 26.8, 23.0, and 14.0 ppm and are readily assigned to the different carbons in the structure (Figure 3). These chemical shift values are found to be identical with those observed for the neat template. In the study of template environment by 13C CP-MAS NMR, the topological fit of the template molecule within the zeolite channels can be discerned from the 13C spectra. It was shown10 that the locking of tetrapropylammonium ion (TPA) at the intersection of the straight and sinusoidal channels of Silicalite-1 could be inferred from the splitting of the methyl

Figure 3. 13C CP-MAS NMR spectra for the template cetyltrimethylammonium bromide (CTABr) in as-synthesized MCM-41 as a function of reaction time in the phosphate promoter assisted hydrothermal synthesis (left) [(A) 0 h, (B) 2 h, (C) 4 h, and (D) 8 h] and in the conventional hydrothermal synthesis without any promoter (right) [(E) 0 h, (F) 4 h, (G) 8 h, and (H) 16 h].

14174 J. Phys. Chem. B, Vol. 107, No. 51, 2003

Laha et al.

Figure 4. Evolution of the 29Si signal as a function of contact time under matched Hartmann-Hahn conditions in the 1H-29Si CP-MAS experiment. Experimental data points for the 29Si resonance at δ (with respect to TMS) -101 p (Q3) (O) and -110 ppm (Q4) (b) in the assynthesized MCM-41 materials, without promoter (16 h) (A) and with phosphate promoter (8 h) (B), are shown. The continuous lines denote theoretically calculated curves using the CP parameters given in Table 2.

TABLE 2: Cross-Polarization Parameters from 1H-29Si CP Dynamics Experiment sample; reaction time

TSiH (ms) Q3 Q4

S-WOP; 16 h S-WP; 8 h

1.5 1.4

2.0 1.9

T1FH (ms) Q3 Q4 11.5 4.8

10.0 4.1

τm (ms) Q3 Q4 3.5 2.4

4.0 2.7

I(τm)/I0 Q3 Q4 0.737 0.602

0.669 0.515

carbon signal of TPA in 13C CP-MAS spectra.11 However, in the case of MCM-41, the hexagonal structure of MCM-41 is built around the surfactant template (CTABr) molecules through a liquid crystal templating (LCT) mechanism and the micellar structure of the surfactant is believed to remain intact in the as-synthesized material.2 Further, the increased molecular mobility for the long hydrocarbon chain in the micellar structure would tend to average out orientational effects, if any, imposed on the template by the mesopores, resulting in a motional averaging for the chemical shielding tensor. Hence it is less likely that one would observe any signal splitting or change in chemical shifts for the carbon signals in the 13C CP-MAS spectra of MCM-41. On the contrary, 1H-13C CP-dynamics experiments, which we discuss below, show significant differences in the values for the cross-polarization parameters (TCH, T1FH) for the organic template residing within the mesopores of MCM41. Dynamics of 1H-29Si and 1H-13C Cross-Polarization. 1H29Si and 1H-13C cross-polarization dynamics experiments have been used to determine the CP parameters for the end materials prepared by conventional and promoter-assisted synthesis. While the 1H-29Si CP dynamics experiment allows the MCM-41 surface and framework species to be studied, the 1H-13C CP dynamics experiment, on the other hand, enables us to study

Figure 5. Evolution of the 13C signal as a function of contact time under matched Hartmann-Hahn conditions in the 1H-13C CP-MAS experiment. Experimental data points for the 13C resonance at δ (with respect to TMS) 30.4 (A) and 26.8 ppm (B) for the template in the as-synthesized MCM-41 materials, without promoter (16 h) (b) and with phosphate promoter (8 h) (2), are shown. Experimental data for the neat surfactant (CTABr) are also shown (O). The continuous lines denote theoretically calculated curves, using the CP parameters given in Table 3.

TABLE 3: Cross-Polarization Parameters from 1H-13C CP-Dynamics Experiments 13C

sample; reaction time

CP parameter

32.4

CTABr (neat)

TCH (µs) T1FH (ms) τm (µs) I(τm)/I0 TCH (µs) T1FH (ms) τm (µs) I(τm)/I0 TCH (µs) T1FH (ms) τm (µs) I(τm)/I0

40 1.3 144 0.895 160 13.3 716 0.948 140 13.5 646 0.953

S-WOP; 16 h

S-WP; 8 h

signal (ppm) 30.4 26.8 50 1.2 166 0.871 140 14.7 658 0.956 130 13.6 610 0.956

30 1.4 118 0.919 90 5.3 373 0.932 90 4.7 363 0.926

the surfactant molecule and thus probe the template environment. The time evolution12 of the signal in each case is determined by the corresponding cross-polarization buildup rate (TIS-1) and the rotating frame spin-lattice relaxation rate (T1FH)-1, which are determined by an analysis of CP signal intensities. It is also useful to deduce the contact time that yields the optimal signal, namely, τm ) [TIS/{1 - TIS/T1FH}] ln(T1FH/TIS). The results of 1H-29Si and 1H-13C CP dynamics experiments are presented in Figure 4, Table 2, Figure 5, and Table 3, respectively. The Q3 and Q4 CP buildup rates are identical for the end materials (16 h for S-WOP and 8 h for S-WP). TIS values are longer for Q4 sites since they do not have attached protons. A 2-fold reduction in T1FH is noticed for the end material from promoter-assisted synthesis (S-WP-8), which is in accord with the smaller number of silanol protons, especially those of hydrogen-bonded Q2 species,9 acting as “relaxation sink” for the decay of the spin-locked 1H magnetization. The shift in optimal contact time is mainly influenced by this change in T1FH

Phosphate-Enhanced Crystallization of Siliceous MCM-41 and is further reflected as a decrease (18 to 23%) in crosspolarization efficiency for the end material (S-WP-8) (Table 2). 1H-13C CP dynamics experiments depict a striking difference in the CP behavior for the template, being inherently rapid for the neat template (TIS ) 30-50 µs) and longer (90-160 µs) when present in MCM-41. For the template as neat solid, strong C-H dipolar interactions govern a rapid 1H-13C CP transfer, while on the other hand, the slow CP transfer for the template in MCM-41 implies a reduction in the C-H dipolar interaction, thus evidencing a larger flexibility for the template molecule within the mesopores of MCM-41. More significantly, the rotating-frame relaxation time T1FH, determined on the carbon resonances at 32.4 and 30.4 ppm, is an order of magnitude longer when the template is confined in MCM-41. Since surfactant molecules are arranged in a micellar array2 and guide the formation of mesopores in MCM-41, we ought to expect that the mobility for the long hydrocarbon chain of the surfactant molecules would be enhanced in the end materials. This, in turn, would drive the motional dynamics to shorter correlation time and cause T1FH to increase, as we indeed observe for the assynthesized end materials S-WOP-16 and S-WP-8. We also note that there is no significant change in the TIS values, irrespective of whether MCM-41 is prepared in the presence of promoter or not. The shift in optimal contact time (τm) to longer values, noticed for the template in S-WOP-16 and S-WP-8, is mainly influenced by the increase in TIS and T1FH. There is only a marginal increase in cross-polarization efficiency since TIS , T1FH holds for the surfactant, both in its neat form and in the as-synthesized end materials S-WOP-16 and S-WP-8. 31P MAS NMR of samples corresponding to the initial reaction product (0 h) or the fully crystallized MCM-41 indicate the total absence of any phosphorus signal, thus evidencing the lack of phosphorus incorporation in the mesoporous structure at any time during hydrothermal synthesis. 31P MAS NMR has served to indicate the catalytic role played by the phosphate promoter to merely enhance the synthesis rate. Conclusions The XRD as well as 29Si MAS NMR results indicate the accelerated formation of ordered Si-MCM-41 samples in a phosphate promoter assisted hydrothermal synthesis. Quantitative measurement of signal intensities show that in the presence of promoter the relative amount of silanol groups present is lower than that of those obtained in the absence of promoter under the same reaction conditions. The accelerated formation of MCM-41 structure due to phosphate promoter is also borne out from cross-polarization dynamics experiments. The respective cross-polarization buildup rates (TIS-1), determined for the silanol (Q2, Q3) and framework (Q4) silicon sites and the

J. Phys. Chem. B, Vol. 107, No. 51, 2003 14175 different carbon environments of the surfactant cetyltrimethylammonium bromide (CTABr), are found to be nearly equal in the end materials prepared by conventional and promoterassisted synthesis. The slower cross-polarization buildup rate for the template in the as-synthesized MCM-41, over that observed for the neat surfactant, indicates enhanced mobility upon confinement in the mesoporous structure of MCM-41. The CP dynamics results are in accord with a micellar arrangement for the surfactant molecules in the as-synthesized MCM-41 materials and are consistent with the liquid crystal templating mechanism proposed earlier for the formation of M41S materials. We have inferred from 31P MAS experiments that there is no incorporation of the phosphate promoter within MCM-41 during the hydrothermal synthesis. Acknowledgment. The authors would like to thank the IndoFrench Center for Promotion of Advanced Research, New Delhi for support of project 2105-1. S.C.L. thanks the Council of Scientific and Industrial Research, India for a research fellowship. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartulli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) (a) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368. (b) Corma, A.; Navarro, M. T.; Pariente, J. P. J. Chem. Soc., Chem. Commun. 1994, 147. (c) Corma, A.; Navarro, M. T.; Pe´rez-Pariente, J.; Sa´nchez, J. Stud. Surf. Sci. Catal. 1994, 84, 69. (d) Kloetstra, R. K.; van Bekkum, H. J. Chem. Res. 1995, 26. (e) Corma, A.; Marty´nez, A.; Marty´nezSoria, V.; Monton, J. B. J. Catal. 1995, 153, 25. (f) Sayari, A. Chem. Mater. 1996, 8, 1840. (4) Laha, S. C.; Kumar, R. Microporous Mesoporous Mater. 2002, 53, 163. (5) Kumar, R.; Bhaumik, A.; Ahedi, R. K.; Ganapathy, S. Nature 1996, 36, 298. (6) Rajamohanan, P. R.; Mukherjee, P.; Ganapathy, S.; Kumar, R. Stud. Surf. Sci. Catal. 2001, 135, 196. (7) Kolodziejski, W.; Klinowski, J. Chem. ReV. 2002, 102, 613. (8) (a) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1981, 103, 4263. (b) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1983, 87, 5516. (c) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982, 86, 5208. (d) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 3767. (e) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 1487. (f) Maciel, G. E.; Sindorf, D. W.; Bartuska, V. J. J. Am. Chem. Soc. 1980, 102, 7606. (g) Maciel, G. E.; Haw, J. F.; Chuang, I.; Hawkins, B. L. J. Am. Chem. Soc. 1983, 105, 5529. (9) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Millar, G. T.; Zhu, H. Y. J. Phys. Chem. B 1997, 101, 6525. (10) (a) Boxhoorn, G.; Van-Santen, R. A.; Van-Erp, W. A.; Hays, G. R.; Huis, R.; Clague, D. J. Chem. Soc., Chem Commun. 1982, 264. (b) Nagy, J. B.; Gabelika, Z.; Deroune, E. Zeolites 1983, 3, 43. (11) Engelhardt, G.; Michel D. High-Resolution Solid State NMR of Silicates and Zeolites; Wiley: New York, 1987; p 349. (12) Mehring, M. Principles of High-Resolution NMR in Solids, 2nd ed.; Springer-Verlag: Berlin, Germany, 1983; p 136.