MAS NMR Studies on the Dealumination of Zeolite ... - ACS Publications

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J. Phys. Chem. B 2001, 105, 1770-1779

MAS NMR Studies on the Dealumination of Zeolite MCM-22 Ding Ma, Feng Deng,† Riqiang Fu,‡ Xiuwen Han, and Xinhe Bao* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, P.O. Box 110, Dalian 116023, P.R. China ReceiVed: September 29, 2000

The dealumination of Zeolite MCM-22 by calcination or hydrothermal treatment was investigated by conventional multinuclear solid-state NMR and checked by an ultrahigh-field NMR experiments (19.6 T) with a very fast spin rate (19.1 kHz). The presence or variation of different species (e.g. silanol nest, 4-coordinated framework, 6-coordinated and 5-coordinated extraframework aluminum) during or after dealumination was detected, and their changes were followed. Four tetrahedral framework aluminum sites were identified upon the 27Al MAS NMR experiments showing chemical shifts at 61.5, 57.0, 53.5, and 50.0 ppm, respectively. After hydrothermal treatment, aluminum at 57.0 ppm which corresponded to T3, T4, T7, T10, and T12 sites in the orthorhombic model is preferentially expelled from the zeolitic framework. Moreover, a dehydroxylation process without the release of four-coordinated framework Al also happened, resulting in the presence of a tricoordinated framework aluminum species, which, together with the formation of tetrahedral framework aluminum species, may serve as the Lewis acid site of the zeolite.

1. Introduction Zeolite MCM-22, which possesses an unique crystalline structure, combining the behavior of both the 10MR and 12MR systems,1-6 has been paid much attention for its prospects in many catalytic reactions, such as isomerization,7,8 etherification,9 disproportionation,10 cracking,11 alkylation,12 and, as we reported recently, methane dehydroaromatization.13 Its topological structure, suggested by Leonowicz et al.,2 is composed of an interconnected {435663[43]} building unit forming two independent pore systems: two-dimensional, sinusoidal, 10-ring interlayer channels and 12-ring interlayer supercages with a depth of 18.2 Å, with both accessible through 10-ring apertures.3,6,14-16 A number of studies reported in the literature deal with the structure characterization of MCM-22 and also the Si and Al atom distribution in the zeolites.3,17-25 These studies employed solid-state NMR spectroscopy and infrared spectroscopy. It is reported that seven Si sites and three Al sites were observed in the corresponding 29Si and 27Al MAS NMR spectra. They were attributed to different crystallographically nonequivalent framework atoms. It is well-known that the properties and the performance of zeolites usually depend on the state of the aluminum species in the zeolites, and therefore, the investigation of the state of aluminum is significant and interesting.26-28 Most authors observed the formation of Lewis acid site during the dealumination of zeolites,29-35 however, with different opinions about the nature of the Lewis site. For example, it was oscillatedly pointed to derive from tricoordinated framework aluminum,29 Al-O species on extralattice positions,32 or tricoordianted framework silicon,33,34 as well as from nonframework aluminum.35 The existence of tricoordinated framework aluminum is still unclear since some researchers observed that * Corresponding author. Telephone: 0086-411-4671991-551 (office). Fax: 0086-411-4694447. E-mail: [email protected]. † Present address: State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics, Chinese Academy of Sciences, Wuhan 430071, P.R. China. ‡ Present address: Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310.

dehydroxylation was always associated with the release of Al from the framework.31,37 Nevertheless, most of their interests are focused on aluminum-enriched zeolites (low Si/Al ratio), such as Y.33 Thus the second step in the two-step model described by Ku¨hl31 is easy to carry out. This, in the end, leads

to a lack of the tricoordinated aluminum in the dealuminated samples. However, in zeolites with high Si/Al ratio, a different situation may exist since there are not so many aluminum atoms in the system. NMR is a powerful tool for the identification of the variation of short-range ordering and the local symmetry of the zeolite structure. In this paper, the dealumination of MCM22 with calcination or hydrothermal treatment was investigated by 29Si CP/MAS, 27Al MAS, and 1H MAS as well as 1H{27Al} spin-echo double resonance. An ultrahigh-field NMR instrument (19.6 T) was also employed in order to shed light on what happened in the dealumination process. 2. Experimental Section Sample Preparation. MCM-22 zeolite was synthesized according to the procedures described in refs 1 and 5 using hexamethyleneimine (HMI) as the directing agent. Briefly, certain amounts of sodium meta-aluminate, sodium hydroxide (96% NaOH), R (containing ca. 85% HMI), silica (95% SiO2), and deionized water were added into a vessel under vigorous stirring for 30 min. The resulting gels were then introduced into stainless steel autoclaves and heated to 423 K until the gels crystallized completely. After quenching of the autoclaves in cold water, the samples were centrifuged at 5000 rpm, washed, and dried at 383 K overnight. To remove the template, the synthesized solid was calcined at 810 K for 24 h. After 3-fold successive 1 mol/L NH4NO3 exchanging, the samples were calcined at 773 K for 4 h and the proton form of zeolites was obtained. The ratio of Si to Al of the MCM-22 used in the present experiment was about 15. The MCM-22 zeolites exhibit a typical MCM-22 crystalline structure (MWW), as evidenced

10.1021/jp003575r CCC: $20.00 © 2001 American Chemical Society Published on Web 02/14/2001

Dealumination of Zeolite MCM-22 by XRD (D/max-rB diffractometer using Cu KR radiation), and no hybrid crystallites were observed. HMCM-22 was calcined at 873 K for 2 h in flowing dry air to obtain a calcined sample. The hydrothermal treated samples were obtained using a saturated water vapor at 723 and 823 K for 2 h, respectively. They were denoted as h1 and h2, respectively. Prior to the 1H MAS and some 27Al MAS NMR measurements, the samples were dehydrated at 673 K under a pressure below 10-2 Pa for about 20 h in a homemade apparatus,38 by which the treated sample can be filled in-situ into a NMR rotor, sealed, and transferred to the spectrometer without exposure to air. Weak rehydration was carried out by exposing the sample to the atmosphere for different periods of time (from 0.5 to 4 h). Adsorption of perfluorotributylamine (Acros Organics) was conducted by exposing the dehydrated sample to its saturated vapor at room temperature for 30 min and then degassing it at same temperature to remove the physical adsorbates. All other samples (for the routine 29Si MAS, CP/ MAS, and 27Al MAS NMR) were kept in a desiccator filled with aqueous NH4Cl solution for more than 72 h to get fully hydrated samples. NMR Measurements. NMR spectra were obtained mostly at 9.4 T on a Bruker DRX-400 spectrometer using 4 mm ZrO2 rotors. 29Si MAS NMR spectra were recorded at 79.5 MHz using a 0.8 µs (π/8) pulse with a 4 s recycle delay and 3000 scans. 1H f 29Si CP/MAS NMR experiments were performed with a 4 s recycle delay, 8000 scans, and a contact time of 1.5 ms. The magic angle spinning rate for all 29Si spectra was 4 kHz, and chemical shifts were referenced to 4,4-dimethyl-4-silapentanesulfonate sodium (DSS). 27Al MAS NMR spectra were recorded at 104.3 MHz using a 0.75 µs (π/12) pulse with a 3 s recycle delay and 1600 scans. A 1% aqueous Al(H2O)63+ solution was used as reference of chemical shifts, and samples was spun at 8 kHz. To distinguish the aluminum species in more details, an instrument with ultrahigh field of 19.6 T (Bruker DRX-833) was employed to generate the 27Al MAS NMR spectra. The measurements were performed with spin rate of 19.1 kHz, 2 µs excitation pulse, 5 s recycle delay, and 512 scans. 1H MAS NMR spectra were collected at 400.1 MHz using single-pulse experiments with π/2 pulse, a 4 s recycle delay. A [π/2-τ-π-τ-acquire] spin-echo pulse (π/2 ) 4.5 µs) was also used to acquire 1H spectra, and τ was set to one rotor period. 1H{27Al} spin-echo double resonance MAS NMR experiments were performed according to the method of Veeman et al.39 In the experiments, a [π/2-τ-π-τ-acquire] spin-echo pulse was applied to the 1H channel and aluminum was irradiated simultaneously during the first τ period. All the 1H spectra were accumulated for 200 scans and spun at 5 kHz. The chemical shifts were referenced to a saturated aqueous solution of DSS. The deconvolution of the spectra was conducted using the Bruker software. 3. Results and Discussion 3.1. XRD. The XRD pattern of the as-synthesized sample is in good agreement with those reported in the literature,1,2 proving high crystalline purity of the MCM-22 used. Moreover, the low-intensity backgrounds of the spectra from the samples after calcined or hydrothermal treatments (Figure 1) revealed no significant loss of crystallinity after those certain treatments. 3.2. 29Si MAS and 1H f 29Si CP/MAS NMR Spectra. The 29Si MAS spectrum of the parent MCM-22 zeolite shown in Figure 2 a consists of five main peaks at -120.0, -114.7, -112.6, -106.6, and -100.0 ppm (accuracy (0.2 ppm). The first four are due to inequivalent framework T-sites of the

J. Phys. Chem. B, Vol. 105, No. 9, 2001 1771

Figure 1. Powder X-ray diffraction patterns of parent MCM-22 (a), hydrothermally treated at 723 K (b) and at 823 K (c) and calcined at 873 K (d).

Figure 2. 29Si MAS NMR spectra of parent zeolite (a), h1 (b), h2 (c), and MCM-22 calcined at 873 K (d).

zeolite, which correspond to Si(0Al) units and are the characteristic lines for MCM-22 or MCM-49. They were first reported by Kennedy et al.3 and have been observed by other authors.20-23 The peak at -100 ppm cannot be ascribed to a Si(0Al) site, for, in both of the structure models (P6/mmm and Cmmm), the chemical shift of Si(0Al) site cannot reach such a low field according to the qualitative correlations between 29Si chemical shifts and geometric data for zeolites.40-42 However, this value of chemical shift just falls into the region of the Si(1Al) sites. This can be verified by the progressive decrease of the intensity of this line after successive dealumination (Figure 2, from a to c). On the other hand, one cannot rule out the possibility of the

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Figure 3. Bloch-decay (BD) and cross-polarization (CP) NMR spectra of parent zeolite (a, a′) and h1 (b, b′).

Ma et al.

29

Si MAS

overlap of Si(3Si)OH with the Si(1Al) line. In the corresponding CP/MAS spectra, the lines at -100.0 and -106.7 ppm were pronouncedly enhanced with respect to other lines such as that at -112.1 ppm (Figure 3, a,a′). Essentially, only the species coupling strongly with hydroxyl protons, such as Si(3Si)OH, exhibit a distinct enhancement in the CP/MAS spectra.43 Thus, the peak at -100.0 ppm should arise, at least in part, from the silanol groups.21 It is also remarkable that the intensity of the corresponding CP/MAS peak showed a slight increase (Figure 3b) although the intensity of the peak at -100.0 ppm faded out after dealumination through hydrothermal treatment. This indicates that more silanol groups may be generated during the hydrothermal treatment at the same time as the loss of framework aluminum, which will be also seen in corresponding 1H MAS spectra (in sections below). Moreover, the relative enhancement of the peak at -106 ppm seems to indicate that the Si(0Al) site might be located in the vicinity of surface silanol groups.21 After hydrothermal treatment, the line widths of the Si(0Al) sites are narrowed (Figure 2a-c), and therefore, the broad shoulder peaks at -112 to -118 ppm appear to split into three distinct lines at -112.6, -114.7, and -117.5 ppm, respectively. For the dealuminated sample, at least five Si(0Al) peaks were resolved in the 29Si MAS spectra, which is in good agreement with the observation of other authors.3,24 Unlike for the hydrothermally treated sample, the analogue phenomena have not been observed on the calcined samples, and the decrease of the intensity of the peak at -100.0 ppm is also not apparent (Figure 2d). 3.3. 1H NMR Spectra. 1H MAS and 1H{27Al} Spin-Echo Double Resonance MAS NMR. The 1H MAS NMR spectra of parent zeolite MCM-22 and the samples treated by hydrothermal process and calcination are shown in Figure 4 a-d, respectively. Three bands at 6.0, 3.7, and 1.6 ppm from the parent MCM-22 sample can be resolved clearly, while a poorly resolved peak at about 2.2 ppm is present as a shoulder of the 1.6 ppm line. The peak at 3.7 ppm is commonly attributed to bridging OH groups (Bro¨nsted acid sites), and the resonance at 1.6 ppm, which is very intense in the present spectra, is mostly assigned to silanol groups, as demonstrated.44 The assignment of the signal at about 6 ppm is most oscillatory. Some authors suggested its appearance being due to the presence of residual ammonium ions introduced during the synthesis,45 whereas Beck

Figure 4. 1H MAS NMR spectra of parent zeolite (a), h1 (b), h2 (c), and MCM-22 calcined at 873 K (d). For each spectrum, 200 scans were accumulated.

et al. argued with another kind of Bro¨nsted site for the zeolite.46 Some experiments indicated that this resonance may also originate from the water molecule adsorption on the Lewis acid site as a new sort of Bro¨nsted acid site caused by an additional electrostatic interaction of the zeolite framework.47 In the present case, the shoulder at about 2.2 ppm may be attributed to OH groups at nonframework aluminum species or another kind of silanol group. To clarify this, 1H{27Al} spin-echo double resonance NMR experiments were performed. This method was originally introduced by van Eck et al.39 for establishing the connection between coupled nuclei, has been used in different systems by various authors,39,46,47,50 and is analogous to a dipolar dephasing experiment, as pointed by Fyfe et al.50 Essentially, the signals of proton groups that are strongly coupled with aluminum will be significantly suppressed while those that are not will be unaffected. Therefore, it offers a possibility to distinguish the OH signals from the species with or without connection to an Al atom. As shown in spectra of the parent zeolite (Figure 5 a), with Al irradiation, the lines at 6.0, 3.7, and 2.2 ppm were heavily suppressed while the peak at 1.6 ppm remains unchanged. Hence, we can deduce that the shoulder at 2.2 ppm should be from the hydroxyl protons associated with nonframework Al. After hydrothermal treatments, the lines at 3.6 and 6.0 ppm decrease (Figure 5b) and almost disappear (Figure 5c) in the spectrum of the h2 sample (treated at 823 K). Meanwhile, the intensity of the line at 2.2 ppm kept more or less unchanged at low hydrothermal temperature (h1 sample) and then greatly decreases when the temperature was up to 823 K (Figure 5b,c). The difference spectra of the spin-echo double resonance gives a tendency of the variation of the different proton signals with various treatments, but for a well-known reason (different echodampings caused by homonuclear dipolar interaction), it is not available to determine an accurate value of the intensity ratio for both sites with this method. However, quantitative results of different hydroxyl species can be obtained from careful deconvolution of the specific proton MAS NMR spectrum (Figures 4 and 6a). The number of OH groups/unit cell of the parent and treated MCM-22 zeolites are listed in Table 1 as estimated from the integrated areas of the deconvoluted lines in the relevant 1H MAS NMR spectra. As shown in Table 1, the parent zeolite possesses 3.7 bridging OH groups, 1.1

Dealumination of Zeolite MCM-22


Figure 5. 1H spin-echo spectra of parent zeolite (a), h1 (b), h2 (c), and MCM-22 calcined at 873 K (d). For each figure, the upper spectrum was recorded without 27Al irradiation, while the middle spectrum was with 27Al irradiation. Their difference spectrum is shown at the bottom. The spinning speed was set to 5 kHz, and each τ period was equal to one rotor period in the experiments.

TABLE 1: Proton Amount Determined by 1H MAS NMR of Different Samples

sample

no. of hydroxyl species/ unit cell ((0.3/uc)a Bro¨nsted sites

silanol

extraframework Al-OH

MCM-22 h1 h2

3.7 2.7 1.8

1.6 2.3 2.4

1.1 2.0 1.7

a Calculated from the intensity of deconvoluted 1H MAS NMR spectra.

extraframework Al-OH species, and 1.6 silanol group per unit cell. For the hydrothermal treated samples, its Bro¨nsted acid sites are gradually lost with the increase of treating temperature. At the treating temperature of 823 K, only 1.8 Bro¨nsted sites remained. This is similar to the trend observed by IR spectroscopy reported recently, although their treating temperatures were much higher.24 The variation of extraframework Al-OH species is interesting, as indicated in the different spectra of spin-echo double resonance. Its amount underwent a slight increase at first (h1 sample) and then shrinked (h2) clearly. It is not hard to

understand this behavior: in the mild dealumination process, the aluminum expelled from the framework may reside in the channels or cavities of the zeolite in form of monomeric or lowmolecular-weight oxide-aluminum species. Under a more vigorous dealumination condition, it will condense into polymeric aluminum species accompanied by the loss of hydroxyl groups.51,52 The later phase is, perhaps, boehmitelike or pseudoboehmite Al2O3 species.53 From Table 1, one may also notice the elevation of silanol groups after hydrothermal treatment. This is due to the formation of silanol nests that result from the expelling of aluminum atoms from the zeolite lattice. Thus, in the 29Si CP/MAS spectra of h1 sample, we observe that the peak at -100 ppm corresponding to silanol groups is heavily enhanced compared with that of the parent zeolite (Figure 3b,b′), though the chemical shift of Si(1Al) groups is in the identical position. It is noticed that the increase of the intensity of the silanol group is not in linear dependence of that of the expelled/ reduced Bro¨nsted sites (suggests that expelling of one framework aluminum atom will leave four silanol species54). This supports that the annealing or reoccupying of the vacancies by free Si atom occurred during the hydrothermal treatments, which was


Ma et al.

Figure 6. Deconvolution of the 1H MAS NMR spectra of parent MCM-22 before (a) and after (a′) adsorption of perfluorotributylamine. The corresponding spectra of the h1 sample are shown in (b) and (b′). Notice that (a) and (b) were drawn with different scales.

also observed by other authors in the calcined zeolites samples.51,55 On the other hand, the improved resolution of the 29Si MAS spectra of hydrothermally treated sample is probably due to this reason, perfection of the zeolite lattice. The 1H MAS NMR profile of the calcined sample is similar to that of the parent (Figure 4d). No obvious changes have been observed, except for a little decrease of both Bro¨nsted sites and extraframework aluminum. This suggests that MCM-22 zeolite is more resistant to calcination as compared with hydrothermal treatment, even at a higher (873 K) temperature. 1H MAS Spectra of Samples Loaded with Perfluorotributylamine. Perfluorotributylamine [(n-C4F9)3N] is a weak base with a diameter larger than the pore size of microporous zeolites such as ZSM-5 and Y zeolites or, in the present case, MCM-22. As we reported previously,56 it is a sensitive NMR probe that can be used to quantitatively determine the external acidity as well as the position of silanols and some nonframework Al species in zeolites by 1H MAS NMR spectroscopy. Figure 6 shows the 1H MAS NMR spectra of parent MCM-22 (curve a) and h1 sample (curve b) before (upper curve) and after (lower curve) adsorption of perfluorotributylamine. Quantitative deconvolutions of the corresponding spectra are plotted simultaneously. After adsorption of perfluorotributylamine the resonance position of silanols shifts 0.2 ppm to low field, revealing most of the silanols are located on the external surface and not at the lattice defects in the internal surface. This is coincident with the configuration feature of MCM-22. As reported by various authors,5,6 MCM-22 has thin plateletlike crystals with an extremely large external surface area. It has thin hexagonal crystal morphology probably with 1 µm/side along the crystallographic a and b directions and 50-100 Å thick in the c direction.6 Sometimes, it just consists of only one or two isolated layers as revealed by high-resolution transmission microscopy. Thus, we can imagine that more silanol may be present in MCM-22 as compared with other zeolites (e.g. ZSM-5). As presented in Figure 6a, the peak intensity at 3.7 ppm decreases,

TABLE 2: Concentration of External Hydroxyls Species of MCM-22 and h1 Determined by 1H MAS NMR of Samples Loaded with Perfluorotributylamine concn of external hydroxyl species (%)a sample

Bro¨nsted sites

MCM-22 h1

22 20

a

silanol extraframework Al-OH ∼100 64

70 35

Resulted from eq 1.

while that at about 6.0 ppm increases. This suggests that a portion of the Bro¨nsted acidic sites located on the external surface of the zeolites interact with the perfluorotributylamine to form a perfluorotributylammonium ion, of which the resonance signal appears at about 6.0 ppm. However, the Bro¨nsted sites on the internal surface are not accessible to perfluorotributylamine, and their chemical shift would remain unchanged. A similar variation is observed for nonframework Al-OH. After adsorption of perfluorotributylamine, a 0.3 ppm low-field shift of the signal at 2.2 ppm demonstrates that some of the nonframework Al exists on the external surface of the zeolites. The concentration of Bro¨nsted acidic sites or nonframework AlOH on the external surface of the zeolite can be calculated by

Cext.surf ) (1 - A1/A2) × 100%

(2)

where A1 and A2 denote the integral area of the peak at 3.7 or 2.2 ppm after and before adsorption of perfluorotributylamine, respectively. In this way, the concentrations of internal and external silanol species, Bro¨nsted sites, and extraframework Al-OH are determined, and the corresponding results are listed in Table 2. For the parent MCM-22 zeolite, nearly all the silanol species are located at the external surface and in the form of so-called terminal silanol groups, while only 64% of those in the hydrothermally treated sample (723 K) are on the outer surface.



Figure 8. 27Al MAS NMR spectra of parent zeolite (a), h1 (b), h2 (c), and MCM-22 calcined at 873 K (d). The spectra were recorded at 19.6 T, and the spinning speed is 19.1 kHz.

27Al

Figure 7. MAS NMR spectra (9.4 T) of parent zeolite (a), h1 (b), h2 (c), and MCM-22 calcined at 873 K (d). Before measurements, the samples were kept in a desiccator filled with aqueous NH4Cl solution for more than 72 h to get fully hydrated samples.

There are 1.48 external silanol groups/unit cell for the h1 sample. It is interesting to note that this value is coincident with that of the parent MCM-22 (1.6/unit cell). This suggests that almost all the terminal silanols were well retained in the hydrothermal treatments. The unshifted part of the resonance comes from the contributions of internal silanols residing in the defects created in the treatments. This is demonstrated by the use of perfluorotributylamine, terminal silanols, and internal silanols (silanol nest) that can be clearly distinguished. After dealumination (723 K), the concentration of external nonframework Al-OH decreased from 70% to 30%, whereas the amount of total nonframework Al-OH doubled (Table 1). This is evidence that the expelled aluminum does not migrate out and, on the other hand, stays in the supercage or sinusoidal channels. A similar feature has been observed on H-Y zeolite.53 The above discussion shows that MCM-22 is resistant to the calcination, while steaming at 723 K or higher will lead to a decrease of the amount of Bro¨nsted acidic sites. The aluminum expelled from the lattice mainly stays at the internal pore systems of the zeolite and will condense into polymeric aluminum species accompanied by the loss of hydroxyl groups with a higher treating temperature. Almost all the silanol species of the parent zeolite are located at the external crystal surface, while hydrothermal treatment gives rise to the internal silanol groups (silanol nest). The amount of the latter species will be lost during the course of treatments, indicating that annealing or reoccupying of the vacancies by free Si atoms occurred. As a result, perfection of the zeolite lattice will occur. 3.4. 27Al MAS and 1H MAS NMR of Progressively Rehydrated MCM-22. 27Al MAS NMR has been used as an efficient probe to determine the coordination and the local structure, as well as the geometry, of specific aluminum species in zeolites. Besides the extraframework octahedral aluminum at 0 ppm, two components with chemical shifts at 55 and 50 ppm can be resolved in the 27Al MAS spectra of MCM-22 zeolite.20-23,25 Figure 7 displays the 27Al MAS NMR spectra of parent MCM-22 and those after different treatments. It is easy to find a main peak at ca. 55 ppm with a shoulder located at 50 ppm

present in the 27Al MAS NMR spectra, which is consistent with those reported by other authors.20-23 With increase of the hydrothermal treating temperature, the peaks ascribed to framework tetrahedral aluminum decreases gradually, while a broad resonance at about ca. 30 ppm appears. In addition, the line widths of the four-coordinated Al are broadened which leads to the poor resolution of the resonances. At the same time, the area of peak at 0 ppm, which corresponds to six-coordinated extraframework aluminum, increased. It is well-known that the line width of 27Al NMR signals is very sensitive to the coordination environment around the Al atom. Steam treatment usually gives rise to the change of the coordination geometries of Al atoms, causing a distorted tetrahedral or octahedral environment. As a result, the quadrupole interactions of Al atoms largely increase and their MAS signals will be broadened, even beyond detection. This phenomenon has been observed by various researchers in other zeolites such as Y, ZSM-5, or mordenite.49,52 It must be noted that, among the framework aluminum lines, the decrease of that at 55 ppm is more apparent. To get further insight into the detail of dealumination process, an ultrahigh-field 27Al NMR experiment (19.6 T) was conducted. With the magnetic field changing from 9.4 to 19.6 T, three distinct components can be observed in the framework aluminum region, with their chemical shifts at about 61.5, 56.0, and 50.0 ppm (Figure 8). At the same time the very fast 19.1 kHz MAS speed of this experiment enables the quantitative determination of the relative Al amount.57 Deconvolution of the 27Al MAS NMR spectra gives the relative aluminum contents of the different framework tetrahedral Al species. The simulation spectra of parent HMCM-22 using four Gaussian lines was presented in Figure 9. We found that unless using a four-line simulation, the original spectra at 19.6 T cannot be matched successfully. Beside the lines at 50.0 and 61.5 ppm which can be distinguished in the original spectra, the 56 ppm resonance splits into two lines at 57.0 and 53.5 ppm, respectively. The number of Al resonances of MCM-22 has been a hot topic in recent literatures. Hunger et al.20 suggested that five signals are needed with the aim of keeping accordance with the five inequivalent T-sites in the 29Si MAS NMR, while two aluminum resonances were proposed as more reasonable by Kolodziejski et al.21 after using quadrupolar nutation 27Al NMR. The origins of these two lines are suggested to come from two sets of the framework tetrahedral sites, differentiated by their location in small and large atom rings and/or by their structural relation to two pore systems proposed for this zeolite. Very


Figure 9. The 27Al MAS NMR spectrum (19.6 T) of parent MCM-22 (a) can be simulated (b) using four Gaussian lines (c).

recently, Kennedy et al.25 presented detailed NMR studies on MCM-22 and MCM-49, where they observed three distinct tetrahedral Al sites by a 17.4 T instrument. By considering the correlation between chemical shift of tetrahedral Al and average Al-O-Si angles established by Lippmaa et al.,42 they attributed these three Al lines to aluminum in different groups of T sites which possess different Al-O-Si angles. In their interpretation of 27Al MAS NMR, they used the hexagonal model of MCM22 with respect to the orthorhombic model, whereas both their own X-ray diffraction and 29Si MAS NMR experiments support the latter. Using the average T-O-T angles in the orthorhombic model, a wider Al chemical shift is expected. This is consistent with our observation that the satisfactory fitting of the spectra could only use four lines in the present (higher field) 19.6 T spectrum. We believe that with the development of a more advanced instrument, the overlapped two peaks at 53.5 and 57.0 ppm could be better resolved. Moreover, one cannot exclude the possibility of finding more Al lines due to the different crystallographically nonequivalent T positions in this zeolite. After hydrothermal treatment, one can notice that the peak at 57.0 ppm preferentially decreased, while the intensities of other lines remain almost unchanged (Figure 8b,c). By deconvolution, the ratio of peaks at 61.5, 57.0, 53.5, and 50.0 ppm in parent HMCM-22 is 22:38:15:25, but for the h2 sample, it is 29:29:15:27 (that of the h1 sample is 26:32:16:26). This fact clearly indicates that the four-coordinated aluminum at 57 ppm, which corresponding to T3, T4, T7, T10, and T12 sites in orthorhombic model, is easier to release compared to those at other sites during hydrothermal treatments. This conclusion is different from the dealumination resulted from SiCl4 treatment. In that case, aluminum population of all T positions is decreased by a same extent. Maybe, the dealumination processes of MCM22 from hydrothermal and SiCl4 treatment are different in nature. Some authors found that the peak at 30-40 ppm is not an independent resonance but a low-frequency part of a secondorder quadrupole pattern (its high-frequency part is overlapped with the 60 ppm signal),58 while others believed that it was an independent signal and ascribed it to 5-coordinated nonframework Al.59 Through application of double rotation (DOR) NMR, which was designed to eliminate anisotropic quadrupolar interaction, or 1H f 29Al CP/MAS techniques, it was demonstrated that the peak at about 30 ppm is an independent signal. Moreover, it shows different rehydration behaviors in the 27Al MAS NMR spectra of samples with different water loading ( which will be discussed in detail in the following); thus, the former possibility can be ruled out.

Ma et al. The presence of a peak associated with 5-coordinated nonframework Al can be clearly observed in the 27Al MAS NMR spectra of dehydrated MCM-22 and the h1 sample (Figure 10 a and Figure 11 a). For the parent MCM-22, only a small shoulder is present whereas it is a sharp resonance in the sample steamed under 723 K. At the same time, a broad hump with a line width larger than 15 kHz is overlapped with the abovementioned peaks. Some authors reported that the appearance of the broad hump is due to the nonframework aluminum in low symmetry (NMR invisible) and it will merge into the lines at 0, 30, and 60 ppm for ultrahigh spinning speed (18 kHz),60 but strong heterogeneous interaction such as second-order quadruple interaction (Qcc > 15 MHz)61 should also be considered since our 19.1 kHz spin rate cannot fully eliminate the hump. The spectra in Figures 10 and 11 were drawn in two different modes: normalized intensity (left); absolute intensity (right). The intensities of five signals (61, 55, 50, 30, and 0 ppm) grow gradually at the expense of the broad hump upon rehydration, with the peaks at 50-61 ppm growing fastest and to the largest extent. Up to fully rehydration, a very weak broad hump can still be observed. If we compare the integral area of Figure 11a with Figure 11d, we will find that ca. 80% of the total Al species were “NMR invisible” after the dehydration. Apparently, it is the broad hump that is responsible for the “invisibility” of 27Al signals. 1H/27Al TRAPDOR NMR, established by Grey and Vega,61 has been used for the detection of aluminum species with large quadrupole coupling constant. Through this method, one of us pointed out that, in dehydrated zeolites, the broad hump comes from the contribution of the three Al species (Bro¨nsted and Lewis acid sites, nonframework Al species) while, in fully rehydrated zeolites, only the Lewis acid site (tricoordinated Al) is associated with the broad hump.49 This is due to the fact that the thermal treatment results in a partial transformation of 4-coordinated framework Al to tricoordinated framework Al and this process is irreversible. Our experimental results also support this finding. It is generally accepted that one bridging OH group (Bro¨nsted acid site) corresponds to one framework four-coordinated Al in the zeolite, which means that their concentrations should be equal. Two mechanisms were proposed for the dehydroxylation of zeolite during thermal or hydrothermal treatment: (1) Dehydroxylation occurs without dealumination of the framework Al, and tricoordinated Al is formed at the defect sites of the zeolite framework.29 (2) Dehydroxylation is always associated with the dealumination of framework Al.37 After the hydrothermal treatment, the number of bridging OH groups (determined by 1H MAS NMR) in the MCM-22 samples measured is decreased by 27% for the sample h1 and 51% for the sample h2 compared to the parent MCM-22, while the number of four-coordinated framework Al (determined by 27Al MAS NMR) decreased by 11% for the hydrated sample h1 and 29% for the hydrated sample h2. The discrepancy of the decrease of amount of Bro¨nsted sites and that of framework aluminum strongly suggests that a certain amount of the four-coordinated framework Al has been transformed to three-coordinated framework Al, and dehydroxylation occurs without dealumination during the hydrothermal treatment. As suggested by Uytterhoweven et al.,29 this kind of dehydroxylation will simultaneously produce the same amount of four-coordinated framework aluminum without connecting to hydroxyl groups, and the transformation is irreversible upon the dehydration/ rehydration. Thus, it is easy to understand why the decrease of the bridging OH groups is much larger than the decrease of the four-coordinated framework Al in our hydrothermally treated



Figure 10. 27Al MAS NMR spectra (9.4 T) of parent MCM-22 as a function of rehydration. From spectrum a to d, the rehydration degree is gradually increased. Spectrum d corresponds to a fully hydrated sample. The spectra on the left were drawn in a normalized intensity mode, while those on the right were drawn in an absolute intensity mode.

Figure 11. 27Al MAS NMR spectra (9.4 T) of h1 as a function of rehydration. From spectrum a to d, the rehydration degree is gradually increased. Spectrum d corresponds to a fully hydrated sample. The spectra on the left were drawn in a normalized intensity mode, while those on the right were drawn in an absolute intensity mode. The broad hump can be still observed in the fully rehydrated sample, which is believed to be the signal of tricoordinated framework Al species.

sample. Ku¨hl31 pointed out that the tricoordinated framework aluminum is in a metastable state and tends to transfer into the final extraframework state, a more stable state, with the help of nearby framework aluminum (eq 1). However for zeolite with high Si/Al ratio, especially the present MCM-22, one could hardly imagine the existence of Si(2Al) species. Therefore, the second step in eq 1 can hardly happen. It is of interest to note that, in the fully hydrated 27Al MAS NMR spectra of h1 sample, a broad hump can be clearly resolved (Figure 11d), while no such signal can be observed in that of the parent zeolite (Figure 10d). We believe that this signal is probably due to presence of

the tricoordinated framework aluminum species in a more distorted geometry. Therefore, the dealumination process of high siliceous zeolites such as MCM-22 is quite different from those of aluminum-enriched zeolites such as H-Y, where the dehydroxylation is always accompanied by dealumination.31,37 Of course, hydrothermal treatment transforms a small amount of the framework four-coordinated Al to nonframework Al species which resonate at 30 and 0 ppm in the 27Al MAS NMR spectra. The rehydration processes of the MCM-22 zeolites were also investigated by 1H MAS NMR. For the parent MCM-22 (Figure 12), the signals of Bro¨nsted sites and nonframework Al-OH


Ma et al.

Figure 13. 1H MAS spectra of h1 as a function of rehydration: (a) 0.15 H2O/uc; (b) 0.8/uc; (c) 1.6/uc; (d) 4.8/uc. Figure 12. 1H MAS spectra of parent MCM-22 as a function of rehydration: (a) 0.2 H2O/uc; (b) 0.4/uc; (c) 1.6/uc; (d) 4.8/uc; (e) fully hydrated.

are largely decreased when water loading increases from 0.2 to 0.4 per unit cell. This is probably due to formation of hydrogen bonding between adsorbed water molecules and the bridging OH groups or nonframework Al-OH. In addition, the 6.3 ppm signal is sharpened when the loading increases to 0.4H2O/uc, which can be interpreted as the adsorption of a small amount of water molecules on the Lewis acid sites.33,36,47 The Si-OH signal remained almost unchanged. Increasing the loading to 1.60 H2O/uc leads to a further decrease of the Bro¨nsted and nonframework Al-OH signals and an increase of the 6.3 ppm signal, and moreover two new siganls at 9.0 and 5.6 ppm can be observed. The former signal (9.0 ppm) can be assigned to the aluminum hexaaqua complexes Al(H2O)63+, and the latter (5.6 ppm), to coordination complexes between adsorbed water molecules and Bro¨nsted aid sites (or nonframework AlOH).33,62 When the water loading is increased to 4.8 H2O/uc, the intensities of the three signals increase significantly, with the 5.6 ppm signal growing the fastest. In addition, the line width of this signal is much broadened, probably due to a fast proton exchange between water molecules, bridging OH (or nonframework Al-OH), and the hydrogen-bonding complex.62,63 For fully rehydrated sample, mainly the broad signal at 4.7 ppm was observed, which can be attributed to physically adsorbed water.33 However, the peaks at about 9, 6.8, and 1.8 ppm still can be observed. Similar features were observed for MCM-22 steamed at 723 K, but for that the signals of the Bro¨nsted site and nonframework OH decrease slowly upon the rehydration (Figure 13). The reason is still unclear now, and further experiments are needed to clarify this issue. 4. Conclusions MCM-22 zeolite exhibits a different characteristic when it was calcined in air or experienced hydrothermal treatments at elevated temperature. As evidenced by responsive measurements of MAS NMR, the structure and framework aluminum as well as the Bro¨nsted acid sites of the zeolite do not alter much in calcination even to 873 K, while the hydrothermal treatment gives rise to a remarkable distortion of the framework Al (fourcoordinated) environment, as concluded from the broadening

and blurring of the corresponding resonances in 27Al MAS NMR spectra. Four kinds of framework tetrahedral aluminum sites, located at 61.5, 57.0, 53.5, and 50.0 ppm, respectively, were observed in the highest field 27Al NMR spectrometer currently available. The framework aluminum at 57 ppm is preferentially expelled from the zeolite lattice during the hydrothermal treatment. Meanwhile a 30 ppm line corresponding to fivecoordinated extraframework aluminum appears. These peaks together with that at 0 ppm will disperse into a broad hump in the dehydrated 27Al MAS NMR spectra due to the presence of a large second-order quadruple interaction. For a parent zeolite, all the silanol species are located at external surface while a hydrothermal treatment will lead to the formation of internal silanol nest and the expelling of lattice aluminum. The aluminum released from the lattice mainly stays in the internal pore systems (e.g. as five-coordinated extraframework aluminum) and then condenses into polymeric aluminum species, accompanied by the loss of hydroxyl groups under a more vigorous treatment condition. The hydrothermal treatment of MCM-22 is different from that of low Si/Al ratio zeolite such as Y: the second step in the dealumination model of Ku¨hl31 is hardly likely to happen in present case since there is not enough nearby aluminum present. Tricoordinated framework aluminum, which may serve as the Lewis acid site of the zeolite, is found to be formed during mild hydrothermal treatment, which leads to the discrepancy between 1H MAS and 27Al MAS NMR. And the peak at about 6.3 ppm in the 1H MAS NMR spectra of the weakly hydrated sample is attributed to the water adsorbed on the Lewis acid site. Accompanying the formation of the tricoordinated framework aluminum, the four-coordinated framework aluminum without connection to hydroxyl groups also appeared. Acknowledgment. We are grateful for the support of the National Natural Science Foundation of China and the Ministry of Science and Technology of China. The high-field NMR measurements were performed at the National High Magnetic Field Laboratory supported by the National Science Foundation Cooperative Agreement DMR-9527035 and the State of Florida. References and Notes (1) Rubin, M.; Chu, P. U.S. Patent 4954325, 1990. (2) Leonowicz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910.

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