Steaming of Zeolite Y: Formation of Transient Al Species - The

The π/2 and π proton pulses were 2.5 and 5 μs, respectively. 1H{27Al} spin−echo double resonance MAS NMR experiments were performed according to ...
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J. Phys. Chem. B 2001, 105, 1135-1139

1135

Steaming of Zeolite Y: Formation of Transient Al Species Bart H. Wouters, Tiehong Chen, and Piet J. Grobet* Center for Surface Chemistry and Catalysis, Kardinaal Mercierlaan 92, 3001 HeVerlee, Belgium ReceiVed: April 27, 2000; In Final Form: October 16, 2000

Steamed zeolite Y samples were examined by 27Al MAS NMR, 1H spin-echo editing MAS NMR, and 1 27 H Al} double-resonance MAS NMR in order to evaluate the dealumination process during steaming. Framework-related Al-OH groups are observed in the 1H MAS NMR spectrum after steaming at low partial pressure (3166 Pa). These species manifest themselves in the 27Al MAS NMR spectrum as a sharp signal due to the octahedrally coordinated species. Ammonia adsorption converts the coordination of these Al species from octahedral to tetrahedral. Steaming at high partial pressure generates a 27Al MAS NMR spectrum that does not show the sharp line due to the octahedrally coordinated framework-related Al-OH. Therefore, the framework-related Al-OH are considered as intermediate species in the full hydrolysis of Al from the framework.

Introduction

Experimental Section

Steaming or hydrothermal dealumination is a well-established procedure for the ultrastabilization of zeolite Y.1,2 The ultrastabilization is commonly performed at a high partial pressure of steam (P ≈ 1013 hPa) and temperatures exceeding 500 °C.3 During this stabilization process, Al atoms are expelled from the zeolite lattice and form nonframework Al species. The resulting vacancies can be refilled with Si atoms. The destruction of entire sodalite cages is a possible Si source for the “healing” of defect sites. This ultrastabilization of zeolite Y has extensively been studied by means of solid-state nuclear magnetic resonance spectroscopy (NMR).4-7 It is commonly accepted that octahedrally coordinated Al species belong to nonframework Alcontaining phases. This interpretation was recently questioned.8-13 Octahedrally coordinated Al species were found that are attached to the zeolite lattice. These Al species are converted to a tetrahedral coordination by the adsorption of ammonia. The possibility of realumination of nonframework Al was excluded. In our previous work on the hydration of zeolite Y at room temperature,8 Al-OH groups were found that are attached to the zeolite framework (framework-related or transient Al-OH species). These resulted from a partial hydrolysis of the framework Al-O bonds due to the interaction of water with Bro¨nsted acid sites in the zeolite. Additional water molecules interact coordinatively with the Al-OH to form a frameworkrelated octahedrally coordinated Al complex. In the model of Wang et al. describing the steaming of zeolite Y, similar transient Al species were proposed which are considered as intermediate states in the complete removal of Al from the lattice.14 In this paper, we extend the observation of the transient Al-OH groups to steamed zeolites. Since the Al-OH result from a partial hydrolysis of Al-O bonds and since the degree of dealumination in steaming processes is proportional to the partial pressure of steam,14,15 it is necessary to work at low water vapor pressures to be able to observe the intermediates in this hydrolysis process.

Sample Preparation. A sodium Y zeolite (Zeocat Si/Al ) 2.65) was ion exchanged three times in a 1 M solution of NH4Cl (Acros) under reflux. Subsequently, it was washed with deionized water until it became chloride-free. The residual sodium content was less then 0.5% of the initial concentration. Then the zeolite powder was granulated, and 1 g of it was transferred in a tubular quartz reactor. The catalyst bed had a diameter/height ratio of 8. The samples were steamed with a low steam pressure of 3166 Pa by saturating a N2 flow with water vapor at 298 K. The saturated N2 flow was passed through the zeolite bed at rate of 80 cm3 min-1. The temperature of the reactor was ramped at a rate of 5 K min-1 from room temperature to the final temperature of steaming. The samples were steamed for 4 h. The steaming temperatures were 673 and 823 K, respectively. After steaming, the samples were flushed with dry N2 for 10 min to remove the traces of water from the steaming process. Subsequently, the samples were cooled to room temperature and transferred into the glovebox where the NMR rotors were filled for the 1H MAS NMR measurements. A second part of the steamed samples were placed again in the reactor tube. The samples were reheated to 388 K, and a gas flow (80 cm3 min-1) of ammonia was passed over the zeolite bed for 1 h. Afterward, the samples were equilibrated in a desiccator over a saturated solution of NH4Cl, i.e., a relative humidity of 79%. A third sample was steamed at 673 K and 8 104 Pa for 4 h. After steaming, the sample was equilibrated in a desiccator over a saturated solution of NH4Cl. NMR Spectroscopy. 1H and 29Si MAS NMR spectra were obtained on a BRUKER AMX300 spectrometer with a magnetic field strength of 7 T. The 29Si MAS NMR measurements were recorded with π/4 pulses and a recycle delay of 20 s. 1H spinecho MAS NMR experiments were performed using a Hahn echo sequence (π/2, τ-delay, π, τ-delay) with appropriate phase cycling.16 The τ-delay was adjusted to a multiple of the MAS rotor period. The acquisition was initiated at 2τ. The π/2 and π proton pulses were 2.5 and 5 µs, respectively. 1H{27Al} spinecho double resonance MAS NMR experiments were performed according to the technique of Beck et al.17 For both proton

* Corresponding author. Tel: ++32 (0)16 32 16 10. Fax: ++32 (0)16 32 19 98. E-mail: [email protected].

10.1021/jp001620p CCC: $20.00 © 2001 American Chemical Society Published on Web 01/18/2001

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TABLE 1: Si/Al Ratio Calculated from the 27Al and 29Si MAS NMR Spectrum 27Al

sample Y673 S Y673 SA Y823 S Y823 SA

NMRa

T site X site O site inv (Al/uc) (Al/uc) (Al/uc) Al (Al/uc) Si/Al 39.1 51.0 24.4 33.0

7.7 7.2

11.1 1.6 9.9 1.8

2.4 0.0 10.6 10.6

3.91 2.76 6.87 4.81

29Si

NMR Si/Al 3.03 2.84 3.30 3.27

a Calculated from the intensity of the 27Al MAS NMR spectrum relative to NH4Y (Zeocat Si/Al ) 2.65; 52.6 Al/uc).

measurements, the recycle time was set to 10 s. The quantification of the proton intensity of the 1H spin-echo MAS NMR spectra was done according to ref 8. Chemical shifts were referenced to tetramethylsilane (TMS) for both the 1H and 29Si NMR spectra. For all the above-mentioned measurements, the MAS rotors were spun at 10 kHz. A BRUKER MSL400 spectrometer (9.4 T) was used to record the 27Al MAS NMR spectra of hydrated samples. The pulse length was 0.6 µs, which corresponds to π/18 pulses for nonselective excitation. The repetition time was set to 0.1 s, and 3000 scans were accumulated. An aqueous solution of Al(NO3)3 is used as the shift reference. The MAS frequency was 12 kHz for the aluminum measurements. The Al intensities are compared relative to the parent NH4Y sample that has 52.6 Al atoms per unit cell. The 2D MQ(3Q) MAS NMR spectra were obtained using the three-pulse, amplitude-modulated split-t1 sequence described by Brown and Wimperis in their Figure 14a.18 This pulse sequence provides directly a sheared spectrum that contains isotropic resolution along the F1 dimension. For the MQ measurements on the BRUKER MSL 400 (9.4T), 512 spectra were acquired in the F1 dimension. An increment of 10 µs between two successive spectra was used. The pulse lengths were optimized to obtain maximal signal intensity in the time domain. Pulse lengths of 3.5, 1.5, and 0.6 µs for the three successive pulses were applied. Sample Encoding. The samples are labeled as follows: S is used to indicate the steaming of the samples and A the ammonia treatment. The temperature of steaming is added as a superscript to the encoding of the sample. HP is added for the steaming at 8 104 Pa. Results and Discussion Low-Temperature Steaming: Sample Y673 S. The 27Al MAS NMR spectrum of Y673 S exhibits sharp bands at 60 and 0 ppm because of tetrahedrally and octahedrally coordinated Al, respectively. When the 60 ppm Al intensity is considered as a measure for the framework Al content, a significantly higher Si/Al ratio is found for the steamed sample Y673 S in comparison to the ratio determined by means of 29Si MAS NMR (Table 1). In analogy to our results on the room-temperature hydration of zeolite Y,8 a conversion of the octahedrally coordinated Al to a tetrahedral coordination is obtained when ammonia vapor is adsorbed on the sample (Figure 1). Here this treatment changes slightly the 29Si MAS NMR spectrum (Figure 2), which is reflected by a small reduction of the Si/Al ratio determined from this spectrum. However, similar Si/Al ratios are found after the ammonia treatment by either of the MAS NMR methods (Table 1). Due to the low amount of silanol groups present in Y673 S (Table 2), the best estimation of the Si/Al ratio for Y673 S is obtained from the 29Si MAS NMR spectrum. Therefore, the

Figure 1.

27

Al MAS NMR spectra of (a) Y623 S and (b) Y623 SA.

Figure 2.

29Si

MAS NMR spectra of (a) Y623 S and (b) Y623 SA.

TABLE 2: Proton Distribution (H+/uc) in the Steamed Zeolite Y Samples SOD SUP Al-OHa Al-OHb Si-OH Al-OHb sample (4.6 ppm) (3.6 ppm) (3.1 ppm) (2.5 ppm) (1.8 ppm) (0.5 ppm) Y673 S Y823 S a

25.0 18.9

12.3 8.7

6.7 7.8

3.0 8.3

3.6 2.8

1.0 3.0

Framework-related or transient Al-OH. b Extraframework Al-OH.

initial discrepancy in Si/Al ratio observed for Y673 S can be accounted for if part of the octahedrally coordinated Al atoms are considered as framework-bound species, as we indicated in our previous paper.8 This framework-bound octahedral Al is transformed to a tetrahedral coordination by means of ammonia adsorption. 1H spin-echo editing MAS NMR measurements were conducted on the steamed sample (Figure 3). The spectra with an echo delay of 0.1 and 5 ms are displayed. Besides the proton lines of the Bro¨nsted acid protons (4.6 and 3.7 ppm) and the silanol groups (1.8 ppm), signals at 3.1, 2.5, and 0.5 ppm protrude. The connectivity of the protons to aluminum was established by applying 1H{27Al} double resonance MAS NMR (Figure 4a). All proton lines except the silanol band at 1.8 ppm have disappeared, which implies that the related protons are in close vicinity of Al. Extraframework Al-OH species in zeolite Y are generally found at 2.5 ppm in the 1H MAS NMR spectrum.16 The 0.5 ppm band is assigned to Al-OH groups that are, as a consequence of the low chemical shift, in a rather unperturbed environment. Extralattice aluminum species located in the

Steaming of Zeolite Y

Figure 3. 1H spin-echo MAS NMR of Y673 S (a) with a delay of 0.1 ms and (b) with a 5 ms delay of the echo time.

Figure 4. (a) 1H{27Al} spin-echo double resonance MAS NMR, (b) 1 H spin-echo MAS NMR, and (c) the difference of Y673 S (5 ms delay of the echo time).

supercages of zeolite Y are reported at low-temperature steaming conditions.19-21 Because of the open framework of zeolite Y, it can be expected that the supercages can host hydroxyl-bearing aluminum species without perturbing the proton by a hydrogen bonding mechanism. The presence of the 0.5 and 2.5 ppm proton band emphasizes that the dealumination has formed extraframework species. The 3.1 ppm signal that was seen after hydration at room temperature and succeeding evacuation was previously assigned to framework-related Al-OH groups.8 These framework-related or transient Al-OH were postulated by Wang et al. as intermediate Al species in the hydrolysis process of the framework Al.14 A very low water partial pressure of 3166 Pa was used in this work to steam the sample. This prevents severe dealumination14,15 and facilitates the observation of possible transient species. Therefore, on the basis of the position of the proton signal, its connectivity to Al (Figure 4), and the very low vapor pressure used from steaming, the 3.1 ppm band in Y673 S is assigned to framework-related Al-OH species. The total number of Bro¨nsted acid protons determined from the 1H spin-echo MAS NMR spectrum is 37.3 H+/uc (SOD + SUP), while 6.7 transient Al-OH groups are calculated (Table 2). Thirty-nine framework Al atoms per unit cell in a fully coordinated lattice position are measured by 27Al MAS NMR (Table 1). The small discrepancy that is observed between the

J. Phys. Chem. B, Vol. 105, No. 6, 2001 1137

Figure 5.

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Al MAS NMR spectra of (a) Y823 S and (b) Y823 SA.

aluminum and proton content can be due to the formation of a small amount of cationic Al species in the sample. From the 27Al MAS NMR line width of the hexacoordinated aluminum species in Y673 S, the cationic species cannot be excluded. It was shown before that the transient Al groups correspond well in number with the amount of Al that can be transferred form octahedral to tetrahedral coordination by means of ammonia adsorption.8 For Y673 S, the octahedral aluminum that can be transformed to tetrahedral coordination by means of ammonia adsorption (9.5 Al/uc, Table 1) deviates from the amount of framework-related Al-OH species (6.7 H/uc, Table 2). The realumination seen by 29Si MAS NMR after the ammonia treatment can be mentioned as a possible explanation. The main influence of the ammonia adsorption, however, remains the changing of the aluminum coordination due to the removal of water molecules out of the coordination sphere of Al-OH. High-Temperature Steaming: Sample Y873 S. Steaming zeolite Y at 823 K at a partial pressure of 3166 Pa generates an aluminum NMR spectrum that exhibits 3 lines (Figure 5). The typical lines at 60 and 0 ppm appear, of which the coordination is tetrahedral and octahedral, respectively. In between them, a broad, tailing band centered around 32 ppm is observed (Xsite). Fairly narrow bands for the tetrahedral and octahedral signal are seen. The 60 ppm signal is consequently due to the framework Al atoms accompanying the Bro¨nsted acid sites. The assignment of the 32 ppm band is not straightforward. Both distorted tetrahedral sites22 as pentacoordinated Al species23 were ascribed to the signal. Ray et al. emphasize the change in coordination number of the latter signal with increasing dealumination of the sample.24 The Al species go from distorted tetrahedra to a 5-fold coordination. Similar conclusions were obtained for mordenite by means of MQ MAS NMR.25 Multiple quantum MAS NMR was therefore applied to elucidate the coordination number of the 32 ppm signal in the 27Al MAS NMR spectrum of Y823 S. In the two-dimensional 27Al MQ MAS NMR spectrum (Figure 6), three signals are detected. Line A corresponds to the 60 ppm signal in Figure 5 and is therefore related to the tetrahedral framework Al. The octahedral Al species are found at the position C, while the 32 ppm line in the MAS NMR spectrum relates to the band B in the 27Al MQ MAS NMR spectrum. Slices through the individual signals in the MQ MAS NMR spectrum can be used to deconvolute the one-dimensional MAS spectrum26 (Table 1). From the 27Al MQ MAS NMR spectrum, it is clear that the signal B is formed by highly distorted Al sites, which implies a high quadrupolar

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Figure 8. 1H spin-echo MAS NMR of Y823 S (a) with a delay of 0.1 ms and (b) a 5 ms delay of the echo time.

Figure 6. (bottom) 27Al MQ MAS NMR of Y823 S. Asterisks (**) indicate the spinning sidebands. (top) 27Al MAS NMR with MQ slices through the individual signals.

Figure 7.

29Si

MAS NMR spectra of (a) Y823 S and (b) Y823 SA.

coupling constant. A coupling constant of 5-6 MHz and an isotropic chemical shift of 60 ppm are calculated for this line by the simulation of the one-dimensional MAS NMR spectrum. The isotropic position of the line suggests a 4-fold coordination. The nature of the 32 ppm line, i.e., framework or nonframework, remains the unanswered question. Generally, the 30 ppm aluminum NMR line is attributed to nonframework aluminum species.22,24,27-30 However, this assignment was questioned by Remy et al. and van Bokhoven et al.31,32 Our combined NMR approach cannot irrefutably solves this problem since vast amounts of NMR-invisible aluminum are calculated. A pronounced dealumination of the sample is observed by 29Si MAS NMR (Figure 7, Table 1). A low amount of silanol is seen by 1H MAS NMR (Table 2). Furthermore, the -110 ppm signal due to amorphous silica has a low intensity (3%). These two arguments indicate that an accurate Si/Al ratio is

also obtained here from the 29Si MAS NMR spectrum. The degree of dealumination is increased compared to Y623 S, which is expected when the temperature of steaming is raised.14 The classical ammonia treatment was also performed for the Y823 S sample. The resulting 27Al MAS NMR spectrum is given in Figure 5. As was observed in all the previous experiments, the Al of the sharp line at 0 ppm in the 27Al MAS NMR spectrum is converted to Al species with a tetrahedral coordination. The 32 ppm signal, however, is not influenced by the treatment (Table 1). For Lewis acid species, an increased coordination number is expected when bases are adsorbed.33 This indicates that the aluminum species comprised by the 32 ppm band do not coordinatively interact with the ammonia molecules. This can either be due to their inaccessibility or to the fact they are not Lewis acidic. Furthermore, the silicon speciation, as determined by the 29Si MAS NMR spectrum is not influenced by the ammonia adsorption. This implies that no realumination of the sample occurred under these conditions. The proton concentration of the Y823 S sample was determined by means of 1H spin-echo MAS NMR. Because of the existence of a vast amount of extraframework Al-OH groups at 2.5 and 0.5 ppm (Figure 8), the earlier presented spin-echo editing technique cannot resolve the typical 3.1 ppm signal clearly. By deconvolution of the different 1H spin-echo editing MAS NMR spectra, it is nevertheless deduced that an additional signal at 3.1 ppm is needed to produce consistent fittings for all the spectra. The fitting parameters for the SOD, SUP, and 3.1 ppm proton line were taken from the Y673 S sample, whereas only the intensity of the lines was varied. The resulting proton distribution is summarized in Table 2. Considering again the relative concentrations of the band that was previously assigned to the framework-related aluminum hydroxyl groups (Table 2) and the reversible part of the octahedral Al signal (Table 1), we encountered a close correspondence again. This points to the presence of the transient Al-OH groups. For industrial applications, however, the steaming is performed at a high partial pressure.3 Under these conditions, the octahedrally coordinated Al species form a broad, dispersed band (( 1600 Hz) (Figure 9b),22,30 which is distinctly different from the sharp signal (( 400 Hz) due to framework-bound or transient octahedral Al (Figure 9a). This implies that the Al species differ from the transient Al-OH groups. The Al-OH groups are only present in samples steamed with low water vapor pressure. Therefore, it is suggested that the framework-bound Al-OH

Steaming of Zeolite Y

Figure 9.

27

Al MAS NMR spectra of (a) Y673 S and (b) Y673 S (HP).

groups are intermediate species in the dealumination process. High partial pressures of water will further hydrolyze these AlOH species and form extraframework Al. This observation is in agreement with the dealumination model of Wang et al., which was determined by means of a kinetic study of the dealumination.14 Conclusion By the application of a multinuclear NMR approach, we were able to show the presence of framework-related Al-OH groups in steamed zeolite Y samples. These particular Al-OH species are stemming from a partial hydrolysis of framework Al-O bonds and is evidenced by a peak at 3.1 ppm in the 1H MAS NMR spectrum. These framework Al-OH sites can host water molecules, giving rise to octahedrally coordinated Al species in the 27Al MAS NMR spectrum at 0 ppm, which are different from the octahedrally coordinated, extraframework Al species at the same position. At higher partial pressures of steam no indication of the presence of these framework-related species are found. These leads to the conclusion that the transient Al species are related to the initial stage of the dealumination of zeolite Y during steaming. References and Notes (1) McDaniel, C. V.; Maher, P. K. U.S. Patent 3,292,192, 1966; U.S. Patent 3,449,070, 1969. (2) Stach, H.; Lohse, U.; Thamm, H.; Schirmer, W. Zeolites 1986, 6, 74. (3) Beyerlein, R. A.; Choi-Feng, C.; Hall, J. B.; Huggins, B. J.; Ray, G. J. Topics Catal. 1997, 4, 27.

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