3Q-MAS NMR Studies of High Silica USY Zeolites

May 17, 2002 - In high silica USY materials prepared by MSTP method, the two AlIV species resolved by 3Q-MAS experiments could be assigned to Al envir...
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J. Phys. Chem. B 2002, 106, 6115-6120 29Si

and

27Al

6115

MAS/3Q-MAS NMR Studies of High Silica USY Zeolites

Kishan U. Gore,† Anuji Abraham,‡ Suryakant G. Hegde,† Rajiv Kumar,† Jean-Paul Amoureux,§ and Subramanian Ganapathy*,‡ Catalysis and Physical Chemistry DiVisions, National Chemical Laboratory, Pune 411008, India, and LCPS-CNRS-8012, UniVersite de Lille-1, VilleneuVe d’Ascq, 59655 France ReceiVed: NoVember 27, 2001; In Final Form: February 7, 2002

High-resolution solid state 29Si, 27Al magic angle spinning (MAS) and 27Al triple quantum magic angle spinning (3Q-MAS) NMR spectroscopic techniques were used to characterize highly crystalline ultrastable Y (USY) zeolites having different framework Si/Al ratios, obtained by multistep-temperature-programmed (MSTP) steaming at different temperatures. The 29Si MAS NMR spectra of high silica USY show, in addition to the regularly ordered Q4(Si) framework sites (δ ≈ -105 ppm), the presence of distorted tetrahedral environments (δ ≈ -111 ppm), generated by the healing of the framework vacancies created by dealumination. Further, 27 Al MAS experiments show the presence of aluminum in four (AlIV), five (AlV), and six (AlVI) coordinations, whereas the multiplicity within AlIV and AlVI is revealed by 27Al 3Q-MAS experiments. Two different tetrahedral and octahedral Al environments are resolved in the samples subjected to increasing steaming severity. Quantification of all the Al environments is provided based on isotropic chemical shifts (δCS) and second-order quadrupole interaction parameters (PQ), which were derived by a graphical analysis of 3Q-MAS data. In high silica USY materials prepared by MSTP method, the two AlIV species resolved by 3Q-MAS experiments could be assigned to Al environments which are tetrahedrally connected to silicons in the original and healed regions of faujasite framework.

Introduction 29Si

27Al

and MAS NMR spectroscopy has been Solid state frequently used for the structural characterization of zeolites and related materials.1-4 Although 29Si MAS NMR experiments offer definite benefits of observing spin 1/2 nuclei, for which line broadening is effectively removed by sample rotation at the magic angle (θ ) 54.74°), 27Al MAS experiments, on the other hand, suffer from inferior spectral resolution due to secondorder quadrupolar broadening on the observed (-1/2, 1/2) “central transition”. Recently, Gola et al.,5 van Bokhoven et al.,6 and Fyfe et al.7,8 have employed 27Al 3Q-MAS NMR technique, developed by Frydman and co-workers9,10 and further improved by others,11-18 for the characterization of different Al species in the zeolites Y and ultrastable USY, a widely used cracking catalyst. It was shown by Fyfe et al.7,8 that in addition to the commonly observed tetrahedral and octahedral Al, so-called “NMR invisible”19 pentacoordinated Al was also detected. The effect of leaching agents (EDTA and AHFS) in the dealumination of steamed Y-zeolite was studied by Gola et al.5 The aluminum in framework and nonframework coordinations were fully characterized in H-USY and La(x)NaY materials by van Bokhoven et al.6 All of these USY zeolites were not highly dealuminated because their Si/Al ratios typically were in the range of 2.5 to 9.0 (2.54 for La(x) NaY,6 4.5 for H-USY,6 2.5-2.9 for steamed Y zeolites,5 and 9.0 for USY7,8). Recently, we have been using a Multi Step Temperature Programmed (MSTP) steaming procedure for dealumination and ultrastabilization of zeolite Y. The crucial exercise in this * To whom correspondence should be addressed. Fax: 91-20-5893234. E-mail: [email protected]. † Catalysis Division, National Chemical Laboratory. ‡ Physical Chemistry Division, National Chemical Laboratory. § LCPS-CNRS-8012, Universite de Lille-1.

steaming procedure is to carefully control the rate of dealumination in the initial period in order that the aluminum is not withdrawn too quickly from the framework, thus ensuring an arrest of the collapse of the framework structure. More importantly, MSTP procedure entails preparation of high silica USY samples with framework Si/Al molar ratio in the range of 8-26 without any significant loss of crystallinity. Further, in high silica USY there is an increased chance of observing 29Si signals due to the siliceous Q4(4Si, 0Al) type species belonging to the original and “healed” framework locations. Further, 27Al 3Q-MAS NMR studies of highly dealuminated USY materials would provide interesting and useful information about various Si and Al environments in USY samples prepared with varying framework Si/Al ratios. This manuscript deals with 29Si and 27Al MAS and 3Q-MAS NMR studies of hydrated form of high silica USY zeolites prepared carefully using MSTP steaming procedures. 29Si MAS NMR was used to characterize different Q4(4Si) silicon environments, whereas high field (11.7 T) 27Al MAS and 3Q-MAS NMR were used to distinguish nonequivalent aluminum sites in different coordinations (AlIV, AlV and AlVI) and to estimate their chemical shift and quadrupole interaction parameters. Experimental Section Preparation of USY Zeolites. Multi Step Temperature Programmed (MSTP) Steaming. In the ultrastabilization of Y zeolites, it is crucial to control the initial step in the steaming procedure in such a way that the rate of framework dealumination is not too fast compared to the rate at which the vacancies are filled by the insertion of silicon into the framework so that a partial collapse of the faujasite structure will be arrested. This underlying idea is fully implemented in the Multi Step Temperature Programming that we have employed in the preparation

10.1021/jp0143241 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/17/2002

6116 J. Phys. Chem. B, Vol. 106, No. 23, 2002

Figure 1. Multi Step Temperature Programmed (MSTP) steaming procedure used for the preparation of USY-I, USY-II, and USY-III materials (see text).

of USY materials. Because the amount of framework aluminum available at the beginning is large, the rate of dealumination has been kept low by using a gradual 30 min ramp and 30 min stay to reach the intermediate step of ultrastabilization in step I. Subsequent steps, namely, step II, step-III, and step IV, are used to further improve ultrastabilization. Such carefully steamed materials have been used for solid-state NMR spectroscopic characterization. All relevant details pertaining to the temperature programmed multistep steaming are provided in the scheme displayed in Figure 1. The starting material prior to step I is NH4-Y, obtained by repeated (3×) ion exchange (1N NH4NO3, 100 °C) of commercial Na-Y (Linde, SK-40). The end material obtained in each step was used as the starting material for the next step as shown in Figure 1. The steaming was done under stepwise temperature programmed procedure where water vapors were introduced at 100 °C and then the temperature was increased till the desired temperature (550, 700, 850, and 925 °C for USY-I, USY-II, USY-III, and USY-IV, respectively) was reached. The samples were maintained at the desired temperature for 4 h before switching off the electrical heating for cooling the samples down to 100 °C and then the injection of water was stopped. All the four samples (USY-I, USY-II, USYIII, and USY-IV) were then dried under air flow, followed by ammonium exchange using 1N ammonium nitrate solution for 4 h at 100 °C, washed repeatedly (5×) with copious amount of water, dried and calcined at 500 °C at ∼3 °C per minute in an air flow for 8 h to convert ammonium form into H-form. Various USY samples were obtained using the above MSTP steaming procedure. For comparison purposes, parent NH4-Y and commercial USY (Union Carbide, batch number: 966184061053-S-1) samples were also included in the present study. All the samples were characterized by powder X-ray diffraction (XRD, Rigaku D MAX III VC using a Ni-filter Cu KR radiation at λ ) 0.154 06 nm with graphite monochromator and silicon as an internal standard) and BET surface area measurements (Coulters, Omnisorp 100 CX, USA) along with 1D and 2D NMR measurements, details of which are given below. MAS and 3Q-MAS NMR. The NMR experiments were performed on Bruker MSL-300 and DRX-500 FT-NMR spectrometers. Whereas the 27Al MAS and 3Q-MAS experiments were carried out on the DRX-500 at 130.3 MHz, the 29Si MAS experiments were carried out on the MSL-300 at 59.6 MHz.

Gore et al. The high field spectrometer was preferred for 27Al to reduce the second-order quadrupolar broadening in the MAS spectra and hence facilitate the detection of nonequivalent aluminum environments. For 27Al MAS experiments, the 27Al r.f. pulse length was chosen to be very short, namely, 0.6 µs, taking into account the nutation behavior20 of the quadrupolar spins. A 30° flip angle pulse of duration 2.5 µs with a recycle delay of 3s was used for 29Si MAS experiments. Samples were spun at 10 and 3 kHz for the 27Al and 29Si MAS experiments, respectively. Chemical shifts are referenced to TMS for 29Si and [Al(H2O)6]3+ for 27Al. Typically 2000 and 6000 transients were accumulated for 27Al and 29Si experiments, respectively. Deconvolution of 29Si MAS spectra were carried out using Bruker Win-NMR PCsoftware. For the 27Al 3Q-MAS experiments, a three pulse sequence incorporating a z-filter16 was used. Here, the z-filter modification of the original two-pulse sequence enables the introduction of an intermediate coherence transfer pathway ((3Q f 0Q) prior to the conversion step (0Q f -1Q). Because equal jumps are involved for the echo and anti-echo coherence transfer pathways, a pure absorption mode 2D spectrum is obtained with negligible phase distortions. For the r.f. field used (νrf ) 60 kHz), the first and second pulses were individually optimized to give maximum efficiencies for the 0Q f (3Q coherence creation and the (3Q f 0Q conversion steps, respectively. The last conversion step (0Q f -1Q) to the observed (-1/2 T +1/2) central transition was achieved using a soft ‘central transition selective’ 90° pulse of duration 9 µs (νrf ) 9.2 kHz). The phase cycling was designed to select only the desired pathway, namely, (0) f ((3) f (0) f (-1), while eliminating coherence transfer from other pathways. The phase-sensitive 2D experiments were conducted using the hypercomplex States21 procedure, for which the phase of the first r.f. pulse was shifted by 30° between successive experiments. Typically, the 2D accumulations involved 1024 (t2) × 64 (t1) values, using 2400 scans for each of the t1 values with a t1 increment of 40 µs and a spinning speed of νR ) 13.5 kHz. A 2D FT followed by a shearing10,22 transformation gave pure absorption mode spectra in which the isotropic spectra were obtained by a projection of 2D data onto the δISO axis. Because we have not employed rotor synchronization during the evolution period t1, spinning sidebands (indicated by *) appear along the isotropic dimension. However, the sideband interference is minimal, allowing the center bands to be readily identified and quantified. Results and Discussion Figure 2 depicts the XRD curves of different FAU samples. The diffraction pattern corresponding to NH4-Y sample (spectrum A) exhibits the highly crystalline nature of the parent sample. Spectra B to F represent commercial USY (C-USY), USY-I, USY-II, USY-III, and USY-IV samples, respectively. It is clear from Figure 2 that except USY-IV (F) all other samples exhibit very high and comparable crystallinity. The BET surface area of NH4-Y, C-USY, USY-I, USYII, and USY-III zeolites were 820, 655, 555, 545, and 406 m2/ gm, respectively. Because USY-IV sample exhibited a significant collapse of faujasite structure, it was not included for BET surface area measurements. All the calcined zeolite samples used in the present study were in rehydrated form due to their exposure to ambient conditions. The framework Si/Al molar ratios, as determined by 29Si MAS NMR,1 were 2.4, 5.0, 8.7, 18.2, 26.3 and 31.0 for NH4-Y, C-USY, USY-I, USY-II, USY-III, and USY-IV samples. 27Al and 29Si MAS NMR. Figure 3 depicts 27Al (left) and 29Si (right) MAS NMR spectra of NH -Y, C-USY, USY-I, 4

Studies of High Silica USY Zeolites

Figure 2. XRD specta of (A) NH4-Y, (B) C-USY, (C) USY-I, (D) USY-II, (E) USY-III, and (F) USY-IV acquired on a Rigaku D MAX III VC powder X-ray diffractometer using a Ni-filter Cu KR radiation between 2θ ) 5° and 55° in steps of 1°/mt. Diffraction peaks related to the internal standard are indicated with *.

Figure 3. 27Al (left) and 29Si MAS (right) NMR spectra obtained from parent and thermally treated Y zeolites. (A) NH4-Y, (B) Commercial USY (C-USY), (C) NH4-Y steamed at 550 °C (USY-I), (D) NH4-Y steamed at 700 °C (USY-II), (E) NH4-Y steamed at 850 °C (USYIII), and (F) NH4-Y steamed at 925 °C (USY-IV).

USY-II, USY-III, and USY-IV samples (A-F). Although NH4-Y parent sample exhibits only one 27Al signal at ca. δ2 ≈ 60 ppm corresponding to Al in tetrahedrally coordinated (Td) framework positions, no signal for penta- and hexa-coordinated Al is seen, clearly suggesting the absence of extraframework Al in the parent sample. As the steaming temperature is increased (samples USY-I to USY-III), increasing emergence of the 27Al signals due to pentacoordinated aluminum (δ2 ≈ 30 ppm)7,8,19 and octahedral aluminum (δ2 ≈ 0 ppm) is noticed. However, quite interestingly USY-IV did not exhibit any appreciable amount of pentacoordinated aluminum (Figure 3F, left). The pentacoordinated Al is usually not seen, especially in experiments performed at lower B0 fields because the broad resonance at ∼30 ppm observed in the MAS spectrum has contributions from both broad tetrahedral and five-coordinate aluminum.7 Because in this region it was difficult to clearly identify the newly formed Al species, they have been collectively referred to as “invisible” aluminum in the literature.7,8,19 Compared to 7.1 T field, operations at 11.7 T are definitely rewarding since second-order quadrupolar broadenings are reduced by a factor of 1.66, whereas the chemical shift dispersion is enhanced by the same factor, both resulting in a

J. Phys. Chem. B, Vol. 106, No. 23, 2002 6117 resolution enhancement factor of 2.8, which is favorable for the revelation of five coordinate Al species. The larger signal intensity we observe for this signal at ∼30 ppm is mainly due to the large extent of dealumination on our MSTP prepared samples. We observe that in both the C-USY and USY-I (mild steaming conditions at 550 °C) not only considerable dealumination takes place, but the pentacoordinated Al is also created at the early stages of steaming. As seen in Figure 3, further stepwise steaming treatment is seen to increase the peak intensity for the signal at δ2 ≈ 30 ppm at the expense of decreasing intensity for that at δ2 ≈ 60 ppm, whereas the intensity of octahedral Al is affected to a much less degree (compare USYII, D and USY-III, E). These observations suggest that upon increasing dealumination (USY-I to USY-III) there is redistribution of the original tetrahedral framework Al population into increasing penta and hexa coordinated Al species. Beyond a steaming temperature of 850 °C, framework collapse is imminent as noticed from the loss of AlV signal and increased broadening for the AlVI resonance in USY-IV (925 °C steaming temperature). Thus, 27Al MAS results indicate that AlV species seem to impart stability to the FAU framework even at very high degree of dealumination. 29Si MAS NMR spectra of the parent NH -Y (A) and 4 steamed FAU samples (B-F) are shown in Figure 3 (right). On the basis of the multiplicity of silicon resonances, clearly noticed in spectra A-C, and their measured chemical shifts, we identify the different framework silicon Q4 environments, namely, Si(4Si, 0Al) [-105 ppm], Si(3Si, 1Al) [-101 ppm], Si(2Si, 2Al) [-95 ppm], Si(1Si, 3Al) [-89 ppm], and Si(0Si, 4Al) [-85 ppm].1 At 550 °C steaming (USY-I), 29Si MAS spectrum shows two dominant silicon resonances due to Si(4Si, 0Al) and Si(3Si, 1Al), along with less intense signals due to Si(2Si, 2Al) and Si(1Si, 3Al). For the sample steamed at 700 °C (USY-II) and 850 °C (USY-III), the Si(3Si, 1Al) and Si(2Si, 2Al) signals have comparably much weaker intensities and the dominant signal is the one due to Si(4Si, 0Al). Nevertheless, the 29Si MAS spectra of (A-F) could be deconvoluted to estimate framework Si/Al ratio. Increase in Si/Al from 8.7 (USY-I) to 26.3 (USY-III) indicates increasing dealumination with increasing steaming severity. Thus, in the present study, samples USY-II (Si/Al ) 18.2) and USY-III (Si/Al ) 26.3) are considerably more dealuminated than those studied earlier.4-8 We additionally observe a broader signal at ∼-111 ppm, whose identity is clearly revealed at higher steaming temperatures. Its occurrence within the -102 to -116 ppm range shows that it belongs to Si(4Si,0Al) type silicons,1 but however is quite different from the tetrahedral silicons of the regularly ordered original faujasite framework. We have also inspected the 29Si MAS spectrum (not shown) of an acid leached USY-III. We observe that this peak at -111 ppm also appears in the acid leached sample, although there is a very slight loss of the signal intensity in the spectrum for the acid leached sample. This leads us to conclude that the broad resonance at ∼-111 ppm must originate from new Si(4Si, 0Al) environments which are created in the steamed samples and not exclusively from extraframework Si species. During steaming, vacancies are created by the removal of framework aluminum to produce Q3 Si moieties which are subsequently converted into newly ordered Q4 type Si(4Si, 0Al) species upon healing of the faujasite framework. We ought to expect that such environments will be highly distorted. The geometrical distortions induced by steaming treatment would increase the mean T-O-T angle to cause an upfield resonance shift,23 in line with our observations (∆δ ) +6 ppm) and fully in support of the mechanism of ultrastabi-

6118 J. Phys. Chem. B, Vol. 106, No. 23, 2002 lization earlier proposed by Klinowski et al.2 We finally observe that in USY-IV, the peak appearing at ∼-111 ppm becomes quite broad and intense compared to USY-II and USY-III and is further accompanied by the loss of Si(3Si, 1Al) and Si(2Si, 2Al) environments. The consequent partial collapse of FAU structure in USY-IV sample (steamed at 925 °C) is strikingly evident from 29Si MAS experiments and is in accord with XRD (Figure 2 F) and 27Al MAS NMR (Figure 3 F, left) data. This has allowed us to select and study the intact USY samples, namely, C-USY, USY-I, USY-II, and USY-III by 27Al triple quantum MAS experiments which are discussed below. 27Al Triple Quantum MAS NMR. While27Al MAS NMR offers high detection sensitivity it unfortunately suffers from inferior spectral resolution due to residual second-order quadrupolar broadenings which are not eliminated at the magic angle. In general it is feasible to use 27Al MAS NMR to detect Al environments belonging to different coordinations. The 27Al MAS experiments conducted in USY materials have already enabled us to detect four, five, and six coordinate aluminum species. The main limitation with 27Al MAS, however, is the lack of fine resolution which has precluded the identification of nonequivalent aluminum environments within the above coordinations. The recently developed multiple quantum magic angle spinning (MQ-MAS)9-18 offers new opportunities in this direction. In this technique, the various orientation dependent second-order quadrupole frequencies of any symmetric multiple quantum (3Q, 5Q for 27Al) transition are correlated with those corresponding to the observed “central transition”. The anisotropic second-order quadrupolar broadenings can thus be eliminated in a 2D experiment designed to provide a frequency correlation of the multiple quantum evolution during t1 (δISO dimension) with the single quantum detection during t2 (δ2 dimension). As a further aid to quantification of the various isotropic aluminum resonances resolved by the MQ-MAS method, a graphical analysis24 of the 2D contour plot can be readily carried out. When the 2D spectrum is plotted with “normalized” δ2 and δISO ppm scales,24 the chemical shift (CS) axis (δCS) lies along a slope of unity, whereas the quadrupole induced shift (QIS) direction has a slope of -10/17. For each species, the projection of the center of gravity of the corresponding contour onto these axes yields the values for the chemical (δCS) and quadrupole induced (δQIS) shifts. From the latter, the second-order quadrupole parameter is calculated as PQ ) CQ (1+ η2/3)1/2 ) (ν0/300) (15 δQIS)1/2, where ν0 is the Larmor frequency. 27Al 3Q-MAS experiments are therefore clearly superior compared to the conventional MAS experiments because they not only lead to the detection of various Al environments in the structure under high-resolution conditions but also additionally allow us to estimate the above chemical shift and quadrupole interaction parameters. Thus, by employing 3Q-MAS experiments, we are in a position to identify the various nonequivalent Al sites in USY samples and also to provide a quantification of those sites. Accordingly, we have employed 27Al 3Q-MAS experiments in C-USY, USY-I, USY-II, and USY-III, and the corresponding 2D contour plots are shown in Figure 4 (A-D). The corresponding isotropic spectra, devoid of second-order quadrupolar broadening, are obtained by a projection of the 2D sheared frequency domain onto the δISO axis. These are shown in Figure 5 for the tetrahedral (left) and octahedral (right) regions. It is clear from the 2D contour diagrams that all the USY samples (except USY-IV), including the commercial USY (C-USY), exhibit more or less similar patterns for the AlIV,

Gore et al.

Figure 4. 27Al triple quantum magic angle spinning (3Q-MAS) spectra of (A) C-USY, (B) USY-I, (C) USY-II, and (D) USY-III. The chemical shift axis (δCS) is indicated on each figure. The isotropic shifts (δISO), chemical shifts (δCS) and quadrupole interaction parameters (PQ), deduced from a graphical analysis of the 2D contour data, are given in Table 1. The spinning sidebands are indicated by *.

Figure 5. 27Al isotropic spectra obtained by a projection of the sheared 2D data set onto the δISO axis. (A,E) C-USY, (B,F) USY-I, (C,G) USY-II, and (D,H) USY-III. Spectra on the left (A to D) and right (E to H) correspond to the isotropic signals detected by triple quantum experiment in the tetrahedral and octahedral regions, respectively.

AlV and AlVI species, although the AlV species are detected in the 3Q-experiments with much less intensity. Presumably, the low signal intensity for this AlV species which we observe in 3Q-MAS spectra arises from residual Al-H dipolar interactions which would dominate during the 3Q evolution period and cause a rapid t1 decay for this signal. It should be kept in mind that MQ-MAS method is not directly quantitative. Indeed, its efficiency depends on many parameters, such as quadrupole coupling constant, r.f. field used in the z-filter 3Q-MAS experiment and spinning speed employed. These were not identical in the four 3Q-MAS experiments. In our study of USY materials with differing (Si/Al)FW (5.0, 8.7, 18.2, and 26.3), 3Q-MAS experiments clearly reveal nonequivalent sites belonging to aluminum in different coordinations. With the resolution of two distinct aluminum environments in the 62 to 72 (AlIV) and 1 to 10 (AlVI) ppm ranges, and a lone signal in the 35 to 40 ppm (AlV) region, a total of five different aluminum environments are detected in C-USY, USY-I and USY-II from 3Q-MAS experiments (Figure 4).

Studies of High Silica USY Zeolites

J. Phys. Chem. B, Vol. 106, No. 23, 2002 6119

TABLE 1: Chemical Shift and Quadrupole Interaction Parameters Estimated from Graphical Analysis of 27Al 3Q-MAS Data of USY Zeolitesa sample

C-USY

USY-I

USY-II

USY-III

c

AL environment

δISO (ppm)b

δCS (ppm)

δQIS (ppm)c

PQ (MHz)d

AlIV(1) AlIV(2) AlV AlVI(1) AlVI(2) AlIV(1) AlIV(2) AlV AlVI (1) AlVI(2) AlIV(1) AlIV(2) AlV AlVI(1) AlVI(2) AlIV(1) AlIV(2) AlV AlVI

71.8 64.3 39.5 3.8 -0.3 71.8 63.4 37.1 3.9 0.9 72.2 63.3 43.0 6.6 0.5 73.4 64.8 43.7 9.7

60.2 62.4 35.1 2.4 -1.1 61.8 62.0 34.0 2.9 0.1 60.5 62.0 39.8 5.9 0.1 69.7 63.0 39.6 7.5

19.7 3.3 7.6 2.3 1.3 17.0 2.4 5.4 1.7 1.4 19.8 2.2 5.5 1.9 1.2 6.2 3.0 7.0 3.7

7.0 3.1 4.6 2.6 1.9 6.9 2.6 3.9 2.2 2.0 7.5 2.5 3.9 2.3 1.8 4.2 2.9 4.5 3.2

a With reference to [Al(H O) ]3+. b Obtained from isotropic spectra. 2 6 δQIS ) (17/27)(δ2 - δISO). d PQ ) CQ(1 + η2/3)1/2.

Inspection of Figure 5 shows that the two AlIV species are clearly depicted in USY-I, whereas their presence in C-USY, USYII and USY-III is confirmed by inspection of the 2D 3Q-MAS spectra at lower contour levels. The occurrence of the isotropic signal at δISO ≈ 35-40 ppm also confirms, based on its earlier assignment, that it belongs to the penta coordinated AlV species.7,8,19 The structural characterization of the various aluminum species is fully provided by the δCS and PQ values we have determined from graphical analysis for each of these five environments (Table 1). Further, these values determined by graphical analysis for the AlIV, AlV, and AlVI species are used to simulate the 27Al MAS spectra comprising the individual line shapes for these environments. This is shown in Figure 6 where we find a good match with the experimental MAS spectra obtained at two different magnetic fields, namely, 11.7 and 7.1 T for the highly siliceous USY-III. In the context of the very recent study of mildly steamed USY by others,4-6 a comparison of our δCS and PQ values with those reported will be in order. Recently, Fyfe et al.7,8 have carried out 27Al 3Q-MAS experiments on USY at the very high magnetic field of 18.8 T and have identified two distinct tetrahedral, one penta coordinated and one octahedral Al species in USY having Si/Al ) 9.1. A recent 3Q-MAS study by Menezes et al.,4 however, reports the observation of only tetrahedral (framework and nonframework) and octahedral aluminum species in USY. When compared with these reports, we find that our 27Al 3Q-MAS results on USY materials prepared at different steaming temperatures are generally in agreement with the findings of Fyfe et al.,7,8 although the striking observation from our 3Q-MAS studies is that the octahedral Al clearly shows a 2-fold multiplicity, not resolved in the 3Q-MAS study conducted at very high magnetic field (18.8 T).7,8 Our values of δCS and PQ for AlIV and AlVI species find an overall agreement with those reported by these two groups.4,7,8 Our PQ value of 6.9-7.5 MHz for the second tetrahedral species detected in C-USY, USYI, and USY-II is very similar to the PQ value reported by Fyfe et al.8 for USY with Si/Al ) 9.1. In the study of USY by Menezes et al.,4 this type of AlIV species, having a large quadrupole induced shift, was attributed to amorphous nonframework silica-alumina, the aluminum being in the tetra-

Figure 6. 27Al MAS spectra of USY-III, recorded at two different magnetic fields of 7.1 T (B) and 11.7 T (D). The corresponding computer simulated spectra, together with individual components, are shown in (A) and (C). These spectra were simulated using e2qQ/h ) 2.7, 2.1, 3.5, and 2.0 MHz; η ) 0.5, 0.1, 0.1, and 0.1 and δCS ) 63.0, 56.0, 37.0, and 5.5 ppm for AlIV(1), AlIV(2), AlV, and AlVI, respectively.

hedral coordination. In our study, based on the 29Si MAS data and the discussion we have presented, we believe that we are able to distinguish from 3Q-MAS experiments framework AlIV species tetrahedrally coordinated to silicons belonging to the original and unperturbed region of the faujasite framework, as well as the tetrahedral AlIV species coordinated to the new silicon species created by reoccupation of vacancies upon dealumination followed by healing of the distorted framework regions. As far as the two AlVI species detected in our 3QMAS studies, the poor resolution at the very high B0 field of 18.8 T is more likely to arise from the attenuation of the secondorder quadrupole interaction which would cause δQIS to have a much smaller value for these sites, thus decreasing the apparent resolution. We notice in Figure 5 (E-H) that increasing dealumination (increasing steaming) renders one of the two octahedral species, i.e., AlVI(1) (Figure 4) to be the dominant one. Further it is noticed that at the highest steaming temperature reached (850 °C, USY-III) severe line broadening ensues for AlVI followed by a shift of ∼6 ppm in the value of δISO. These observations are attributable to severe geometrical distortions which could result at high steaming temperature. The 3Q-MAS results support our overall view that the effect of steaming is to redistribute the aluminum from the framework into AlVI, AlV, and the extraframework AlIV environments without a collapse of the faujasite structure. Conclusions Multistep-temperature-programmed (MSTP) steaming of NH4-Y up to 850 °C ultrastabilizes the FAU structure (USY) with very high framework Si/Al ratio (∼26.3). 29Si and 27Al MAS/3Q-MAS solid-state NMR spectroscopy provide useful information for the characterization of steamed and dealuminated USY zeolites, which have been studied in materials having Si/Al of 5.0, 8.7, 18.2, and 26.3. The 29Si MAS NMR of highly crystalline high silica USY shows the regularly ordered Q4(Si)

6120 J. Phys. Chem. B, Vol. 106, No. 23, 2002 framework sites (δ ≈ -105 ppm) and distorted tetrahedral environments (δ ≈ -111 ppm), generated by the healing of the framework vacancies created by dealumination. The presence of different AlIV, AlV, and AlVI species is evidenced by 27Al MAS experiments. Quantification of the various nonequivalent Al sites, determined by 3Q-MAS NMR, is provided based on isotropic chemical shifts (δCS) and second-order quadrupole interaction parameters (PQ), which were derived by a graphical analysis of 3Q-MAS data. On the basis of the observation of two nonequivalent AlIV species and their quantification in terms of chemical shift and quadrupolar parameters, regular and distorted tetrahedral Al environments can be clearly distinguished from triple quantum MAS (3Q-MAS) experiments. These are thought to belong to the original and healed parts of the faujasite structrure. Acknowledgment. The authors would like to thank IndoFrench Center for Promotion of Advanced Research, New Delhi for support of an international collaborative project (No. 2105-1). J.P.A. would like to thank the Nord/Pas de Calais Region, FEDER European funding and the Bruker company. References and Notes (1) Engelhardt, G.; Michel D. High-Resolution Solid State NMR of Silicates and Zeolites; Wiley: New York, 1987, 149-275. (2) Klinowski, J.; Thomas, J. M.; Fyfe, C. A.; Gobbi, G. C. Nature 1982, 296, 533-536. (3) Ian, E. M.; Van Erp, W. A.; Gray, H. R.; Ton, C.; Rob, H.; Derek, C. H. J. Chem. Soc., Chem. Commun. 1982, 523-524. (4) Menezes, S. M. C.; Camorim, V. L.; Lam, Y. L.; San, G. R. A. S.; Bailly, A.; Amoureux, J. P. Applied Catalysis A: General. 2001, 207, 367377. (5) Gola, A.; Rebours, B.; Milazzo, E.; Lynch, J.; Benazzi, E.; Lacombe, S.; Delevoye, L.; Fernandez, C. Microporous and Mesoporous

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