Investigation of Aluminum Site Changes of Dehydrated Zeolite H-Beta

Nov 17, 2014 - In this study, quantitative 27Al MAS and MQ MAS NMR investigations probing aluminum site changes of fully dehydrated H-Beta zeolite wit...
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Investigation of Aluminum Site Changes of Dehydrated Zeolite H‑Beta during a Rehydration Process by High-Field Solid-State NMR Zhenchao Zhao,†,‡ Suochang Xu,†,§ Mary Y. Hu,† Xinhe Bao,‡,§ Charles H. F. Peden,† and Jianzhi Hu*,† †

Pacific Northwest National Laboratory, Richland, Washington 99352, United States Collaborative Innovation Center of Chemistry for Energy Materials, Xiamen University, Xiamen 361005, People’s Republic of China § Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Aluminum site changes of dehydrated H-Beta zeolite during a rehydration process are systematically investigated by 27Al MAS and MQ MAS NMR at a high magnetic field up to 19.9 T. Benefiting from the high magnetic field, more detailed information is obtained from the considerably broadened and overlapped spectra of dehydrated H-Beta zeolite. Dynamic changes of aluminum sites are demonstrated during the rehydration process. In completely dehydrated H-Beta, invisible aluminums can reach 29%. The strength of quadrupole interactions for framework aluminum sites decreases gradually during the progressive water adsorption process. Extra-framework aluminum (EFAL) species, that is, penta- (34 ppm) and octahedral- (4 ppm) coordinated aluminum atoms, rise initially with increasing amount of water adsorption, and finally change into either tetracoordinated framework or extra-framework aluminum in water saturated samples, with the remaining octahedrally coordinated aluminum lying at 0 and −4 ppm, respectively. Quantitative 27Al MAS NMR analysis combined with 1H MAS NMR indicates that some active EFAL species formed during calcination can reinsert into the framework during this hydration process. The assignment of aluminum at 0 ppm to EFAL cation and −4 ppm to framework aluminum is clarified for H-Beta zeolite.

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the framework/extra-framework aluminum species.5,8,11−14 In particular, one-dimensional MAS and two-dimensional double/ multiple quantum magic angle spinning (DQ/MQ-MAS) 27Al NMR have been used extensively to study the local structure of aluminum sites in zeolites.11,15,16 Various aluminum sites with coordination numbers ranging from tetra, penta, to octahedral, their spatial proximity and interchangeability have been well studied.11 However, most of these 27Al NMR investigations are using partially or completely hydrated zeolites. During realistic catalytic processes, the activated zeolites are often in H-form and dehydrated state. In dehydrated H-form zeolites, Al−O bond stretching due to highly electrophilic acid protons and/or loss of ligands such as H2O or OH groups results in a distorted aluminum coordination environment, which produces strong

INTRODUCTION Zeolites with their distinct acidic and shape selective catalytic properties are used widely in cracking, isomerization, and alkylation associated with petrochemical industries. 1−3 Brönsted and Lewis acid sites, as the active centers in zeolites, play a crucial role in these catalytic reactions. It is generally accepted that the bridging OH group between framework Al and Si atom accounts for the Brönsted acid site. Both framework, that is, partially hydrolyzed three-coordinated aluminum, and EFALs such as Al3+, Al(OH)2+, Al(OH)2+, AlOOH, Al(OH)3, and Al2O3 with coordinatively unsaturated sites, have been proposed as the Lewis acid center.4−8 Besides their role as Lewis acid sites, these EFALs may also significantly enhance the stability and performance of zeolites.9,10 Therefore, various analytical tools including solid-state nuclear magnetic resonance (NMR) spectroscopy, infrared spectroscopy, X-ray diffraction, X-ray adsorption near-edge spectroscopy, and computational methods have been employed to investigate © XXXX American Chemical Society

Received: October 2, 2014 Revised: November 17, 2014

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723 K (0.0334 K s−1) under a vacuum of ∼10−6 Torr for 16 h followed by 1H MAS NMR measurements. Ammonium ion exchange of the calcined and water adsorption saturated H-Beta was conducted by dispersing 0.2 g of zeolite powder into 50 mL of 0.1 M NH4NO3 aqueous solution at room temperature. After being stirred for 1 h, the solid samples were recovered by repeated centrifugation and deionized water washing, and finally dried under vacuum (∼7 Pa) at room temperature. Characterization. 1H and 29Si MAS NMR measurements were carried out on a Varian 500 MHz NMR spectrometer using a 4.0 mm MAS probe with a sample spinning rate of 10 kHz at resonance frequencies of 500.1 and 99.3 MHz, respectively. 1H MAS NMR spectra were recorded using a pulse width of 1.8 μs for a π/2 pulse, and 128 scans were accumulated with a 5 s recycle delay. Chemical shifts are referenced to tetrakis(trimethylsilyl)silane at 0.25 ppm as a second reference to TMS (0 ppm). Adamantane was used as a standard sample for 1H MAS NMR quantification. 29Si MAS NMR spectra were recorded using a pulse width of 2.5 μs for a π/2 pulse with 1H TPPM decoupling, and 180−1024 scans were accumulated with 10−360 s recycle delay. Chemical shifts are referenced to TMS (0 ppm) using tetrakis(trimethylsilyl)silane with the downfield peak at −9.8 ppm as a second reference. 27 Al MAS NMR experiments were performed on a Varian Inova 850 MHz NMR spectrometer at a magnetic field of 19.9 T, equipped with a 3.2 mm MAS probe. The spectra were acquired at 221.41 MHz using a pulse width of 0.299 μs for a π/4 pulse at a sample spinning rate of 20 kHz ± 1 Hz, 1000− 5000 scans, and a 0.7 s recycle delay. 27Al 3Q MAS NMR spectra were acquired using the z-filter 3Q MAS pulse sequence at the same sample spinning rate.32,33 The optimized pulse widths were p1 = 2.4 μs, p2 = 0.9 μs, p3 = 10.0 μs. In the hypercomplex 3Q MAS experiment, 2048−38 400 transients with a 0.1 s recycle delay and 14−128 evolution increments were used depending on the line width along the isotropic dimensions (F1). Spectral widths for the F2 (acquisition) and F1 (evolution) dimension were 100 and 40 kHz, respectively. All spectra are externally referenced (i.e., the 0 ppm position) to a 1 M Al(NO3)3 aqueous solution. The DMFIT 2011 program was used to simulate the spectra and fit the peaks.34 For 1H and 27Al NMR quantitative analysis, all weights of the samples loaded into the rotors were recorded, and three spectra were acquired to check the stability of the spectrometer. All of the other experimental conditions were set to be the same for serial samples analysis.

quadrupole interactions for the aluminum atoms, and thus significantly broadens the 27Al NMR spectrum, and even renders their NMR features invisible.4,17−21 Indeed, it has been previously reported for dehydrated zeolites that the quadrupole coupling constants (Qcc) of framework/extra-framework aluminums cover a range of 5−16 MHz, exhibiting broad overlapped lines in 27Al NMR spectra that are, again, even invisible in low-field 27Al MAS NMR experiments.4,18,19,21,22 As such, although different framework/extra-framework aluminum sites have been proposed, direct investigations of these sites, accounting for both Brönsted and Lewis acidity in dehydrated zeolites by 27Al NMR, are still a great challenge. Thus, most 27Al NMR studies of zeolites utilize rehydrated samples to reduce the quadrupole interactions, therefore, and enhance the spectral resolution. To bridge the gap between dehydrated and rehydrated samples, aluminum site changes of dehydrated zeolites after water, ammonia, or pyridine, etc., guest molecules adsorption have been investigated.23,24 Visibility of aluminum is greatly improved in these cases due to the significant reduction of Qcc by adsorption of the guest molecules, and, at the same time, the reversibility from octahedral to tetra-coordinated framework aluminum was able to be observed in zeolites Y, Beta, ZSM-5, etc.17,25,26 However, the properties of different aluminum sites during gradual water adsorption processes have been rarely investigated for dehydrated zeolites, and the assignment of the aluminum sites near 0 ppm as either due to extra-framework or framework aluminum, especially in Beta zeolite, is still ambiguous, and even contradictory.27−29 Benefiting from the recent availability of high-field (up to 21.1 T) NMR spectrometers, 27Al NMR spectra with significantly enhanced spectral resolution can be obtained because the effect of the second-order quadrupole interactions is inversely proportional to the strength of the magnetic field. In this way, high resolution and quantitative 27Al NMR spectra at high fields for zeolites and alumina have been obtained.30,31 Until now, high-field 27Al MAS and MQ MAS NMR investigations of dehydrated zeolites have only been reported for dehydrated Y zeolite with relatively high aluminum content (Si/Al = 5) on a 17.6 T NMR spectrometer.4 In this study, quantitative 27Al MAS and MQ MAS NMR investigations probing aluminum site changes of fully dehydrated H-Beta zeolite with relatively high Si/Al ratios (>10) during progressive water adsorption processes were carried out for the first time at high magnetic fields up to 19.97 T, and the attributions of octahedrally coordinated aluminums are clarified.

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EXPERIMENTAL SECTION Sample Preparation. NH4-Beta (Si/Al = 12.5 and 19, Alfa Aesar) zeolites were calcined at varying temperatures (823− 1023 K) (0.0334 K s−1) for 5 h to obtain H-Beta zeolite. These samples were subsequently dehydrated at 723 K (0.0334 K s−1) under a vacuum of ∼10−6 Torr for 16 h. Portions of the dehydrated samples were transferred into NMR sample rotors inside a nitrogen glovebox for NMR measurements. Other portions of the dehydrated samples were exposed to a water adsorption treatment. In particular, water adsorption saturated samples were obtained by placing dehydrated zeolite samples in a desiccator containing saturated NH4Cl aqueous solution for 2 days. The amount of water adsorbed in these samples was quantified by 1H MAS NMR (see Supporting Information Figure S1). The high temperature (1023 K) calcined and water adsorption saturated H-Beta zeolites were also dehydrated at

RESULTS AND DISCUSSION Al MAS and MQ MAS NMR. Figure 1 shows 27Al MAS NMR spectra of H-Beta zeolite (Si/Al = 19) calcined, dehydrated and with different degrees of water adsorption. The amount of visible aluminum sites was obtained by integration of the entire spectral peak area, and the relative visibilities for different samples were normalized to the water adsorption saturated zeolite assuming 100% visibility. The calcined sample with water content of 21 H2O/unit cell (u.c.) shows mainly broad and sharp lines at ∼55 and 0 ppm, corresponding to tetrahedrally and octahedrally coordinated aluminums, respectively (Figure 1a), and almost all of the aluminum atoms are visible. The broad peak at ∼55 ppm is attributed to distorted tetra-coordinated framework aluminum due to strong quadrupole interactions in H-Beta zeolite, while B

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water adsorption saturated samples from dehydrated (Figure 1e) and freshly calcined (Figure 1f) samples. Because of the lack of spectral resolution in 27Al MAS NMR of dehydrated samples, it is difficult to obtain information on the changes of different aluminum sites during the water adsorption process. Therefore, 27Al MQ MAS NMR was conducted for the dehydrated samples with varying levels of water exposure. The resultant 2D spectra are shown in Figure 2 and Supporting Information Figure S3. The isotropic chemical shifts (δiso) and Cq of representative aluminum sites were obtained by fitting 1D spectra using the DMFIT program on sliced spectra that were extracted parallel to the F2 (acquisition) dimension and at various F1 (isotropic) dimensions, with the results listed in Table 1. For the dehydrated zeolite, the dramatically broadened spectrum is again clearly evident in Figure 2a. At least two tetrahedrally coordinated aluminum sites are distinguishable. The relatively narrow peak (IV1) with δiso = 61.7 ppm and Cq = 6.5 MHz is assigned to the framework aluminum sites near counterions, such as residual sodium cation.4,36 The broad peak (IV2), represented by a distribution of varying Cq for different sites, with the largest Cq reaching 17 MHz, is consistent with the reported large Cq for tetrahedral aluminum sites in dehydrated H-form zeolites.19,37 Weak dispersed signals (V) corresponding to penta-coordinated aluminums are observed near 34 ppm, and are more evident in the H-Beta zeolite with higher aluminum content (Si/Al = 12.5) or after calcination at higher temperatures (see Supporting Information Figure S3). This site is attributed to extra-framework Al(OH)2+ or Alx+ species tightly coordinated with framework oxygens.4,7 Almost no signal from octahedrally coordinated aluminum is found, even in the sample with higher aluminum content (Supporting Information Figure S3). With a small amount of water adsorption (1.2 H2O/u.c., Figure 2b), the peak near 55 ppm changes clearly, and the signal intensity of penta-coordinated aluminum site is enhanced. At the same time, the signal due to octahedrally coordinated aluminum (VI1) appears with δiso = 3.6 ppm and Cq = 5.4 MHz, which is attributed to hydrated Al(OH)3.7,23 Although the sites that turn into these new visible octahedral or penta-coordinated aluminums are still unknown, they should result from the changes of coordination numbers due to water adsorption. All of these changes contribute to about 8% recovery of NMR visibility of aluminums. With additional water adsorption (5 H2O/u.c., Figure 2c), strong quadrupole interactions are decreased for the framework aluminums, along with further increases of the signal intensities of penta- and octahedrally coordinated aluminums. Moreover, besides the broad signal (VI1) in the octahedrally coordinated region, a new sharp peak (VI2) with δiso = 0.5 ppm and Cq = 3.4 MHz appears. Finally, when water adsorption is saturated (71 H2O/u.c., Figure 2d), two tetra-coordinated aluminum sites are evident in the tetrahedral coordinated zone, and the strong quadrupole interactions are completely eliminated. Indeed, the strong quadrupole interactions in H-form zeolites can only be effectively eliminated by a sufficient amount of water to form hydrogen-bond networks with the zeolite protons.38 It is also clearly demonstrated in Figure 2d that two octahedrally coordinated aluminum sites exist with the same isotropic shift in the F1 dimension for the water saturated sample. The new broad peak (VI3) with δiso = 0.9 ppm and Cq = 5.8 MHz corresponds to the signal near −4 ppm in the onedimensional spectrum (Figure 1e). The disappearance of penta-

Figure 1. 27Al MAS NMR spectra of H-Beta zeolite (Si/Al = 19) with different amounts of water adsorption: (a) calcined (21 H2O/u.c.), (b) dehydrated (0 H2O/u.c.), (c) 1.2 H2O/u.c., (d) 5.0 H2O/u.c., (e) saturated water adsorption of (b) (71 H2O/u.c.), and (f) saturated water adsorption of (a). The asterisks denote spinning side bands.

the assignment of the relatively sharp peak near 0 ppm is not clear according to previous controversial reports.28,29,35 Currently, there are two major opinions, including octahedrally coordinated framework aluminum due to partial breakage of the Si−O−Al bond, while the other opinion attributes it to octahedrally coordinated extra-framework aluminum due to dealumination.27−29 After dehydration, only a broad signal exists, covering a range from ∼65 to −20 ppm (Figure 1b). The dramatic broad line indicates that strong quadrupole interactions are presented in dehydrated zeolites, with the strong electrophilic proton distorting the coordination environment of framework aluminum atoms.19,21 The disappearance of the peak near 0 ppm may be due to the loss of H2O or OH groups during dehydration. Moreover, after dehydration, visible aluminums are decreased to about 71% as compared to the parent sample. It means that the dehydration treatment results in some 27Al NMR invisible aluminums even at high magnetic fields. After water adsorption of 1.2 H2O/u.c. (Figure 1c), there is no significant change of the line shape, but the visibility of Al recovers somewhat to 79%. As more water is adsorbed, up to 5 H2O/u.c. (Figure 1d), the peak near ∼55 ppm reemerges albeit overlapped with the broad signals, and at the same time the octahedrally coordinated aluminum near 0 ppm becomes evident. At this stage, the 27Al NMR-visible aluminum sites reach 90%. After saturated water adsorption (71 H2O/u.c., Figure 1e), the spectral resolution is greatly enhanced. Two tetrahedrally coordinated Al peaks are clearly distinguishable at 54 and 57 ppm, attributed to T1-T2 and T3-T9 sites of Beta zeolite, respectively.27 Moreover, besides the sharp peak at 0 ppm, a new broad peak near −4 ppm appears. Clearly, this signal arises as a result of the large amounts of adsorbed water, which may lead to the hydrolyzation of framework Si−O−Al bonds. The reversibility of dehydration and rehydration can be confirmed by the observation of identical spectral peaks for the C

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Figure 2. 27Al MQ MAS NMR spectra of H-Beta zeolite (Si/Al = 19): (a) dehydrated, (b) 1.2 H2O/u.c., (c) 5.0 H2O/u.c., and (d) saturated water adsorption of (a) (71 H2O/u.c.). The asterisks denote spinning side bands.

Table 1. Isotropic Chemical Shift (δiso) and Second-Order Quadrupole Interaction Constant (Cq) of Each Aluminum Species in H-Beta Zeolite (Si/Al = 19) during Rehydration Process IV1

IV2

V

sample

δiso/ppm

Cq/MHz

δiso/ppm

Cq/MHz

dehydrated 1.2 H2O/u.c. 5.0 H2O/u.c. 71 H2O/u.c.

61.7 60.9 59.2 59.5

6.5 6.5 3.4 3.8

70.0 68.9 61.5 54.7

17.6 16.8 12.5 2.7

VI1

VI2

δiso/ppm

Cq/MHz

δiso/ppm

Cq/MHz

34.0

7.3

3.7 4.7

5.4 6.1

VI3

δiso/ppm

Cq/MHz

δiso/ppm

Cq/MHz

0.5 0.5

3.4 2.3

0.9

5.8

Scheme 1. Schematic Structures of Octahedrally Coordinated Aluminums: (a) Framework Aluminum, (b) Extra-Framework Aluminum Hydroxide, and (c) Extra-Framework Aluminum Cations

coordinated Al and the octahedrally coordinated Al(OH)3 features will be discussed in detail below. In summary, it can be concluded that water adsorption not only decreases the

quadrupole interactions, but also changes the coordination status of aluminum, especially for the EFALs. D

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decrease of tetra-coordinated aluminum after the first addition of NH4NO3 solution (Figure 3a′,b′) is also observed. It can be explained that the decreased tetra-coordinated aluminum may be extra-framework aluminum as counterion, which was exchanged out by the ammonium cation and turned into octahedrally coordinated aluminum cation. The net increase of 10% aluminum at 0 ppm mostly results from the 8% decrease of aluminum at −4 ppm. The relative broad signal near −4 ppm, which also existed in the samples calcined at lower temperature (723 K, spectra are not shown) after saturated water adsorption, should be attributed to partially hydrolyzed framework aluminum. The assignment can be further confirmed by the significant enhancement of SiOH signal in 29 Si MAS NMR after saturated water adsorption (Supporting Information Figure S4) due to hydrolyzation of the Si−O− Al(Si) bond, and it has also been observed previously in steaming treated H-Beta during the dealumination process.27,28 Our results clearly indicate that the framework aluminum at −4 ppm is not stable and ready to be stripped off from the framework when treated by NH4NO3 solution. Dynamic Changes of EFAL Species. NH4-Beta zeolites with different Si/Al ratios (12.5, 19) were calcined at a relatively higher temperature (1023 K) to make the changes of EFALs much more significant. The deconvoluted spectra of calcined samples and after saturated water adsorption are shown in Figure 4. Both calcined samples show a significant amount of EFALs at 34 and 4 ppm (Figure 4a,c). However, after saturated water adsorption, almost all of these EFAL species disappear for H-Beta (Si/Al = 19), as is shown in Figure 4d, while for the sample with higher aluminum content (Si/Al = 12.5), there is a significant amount of EFALs remaining at 34 and 4 ppm, and also a weak signal of (3.3%) distorted tetra-

Attribution of Octahedrally Coordinated Aluminum Species. Generally, the sites near 0 ppm are attributed to octahedrally coordinated aluminums in hydrated zeolites. Three kinds of octahedrally coordinated aluminums (Scheme 1) are proposed to explain the peaks near 0 ppm. According to the above 27Al MAS and MQ MAS NMR analysis, these octahedrally coordinated aluminums have NMR features at 4, 0, and −4 ppm, respectively. The peak at 4 ppm is observable in MQ MAS NMR spectra at intermediate water levels, and is attributed to hydrated Al(OH)3. This peak is much more evident for H-Beta with higher aluminum content (Si/Al = 12.5) after calcination at higher temperature (Supporting Information Figure S3), and thus very likely due to dealumination.7,23 The aluminum species with a sharp feature at 0 ppm in H-Beta was previously deemed as octahedrally coordinated framework aluminum resulting from threecoordinated aluminum during calcination, which became octahedrally coordinated aluminum when exposed in air.27 This attribution was supported by disappearance of the 0 ppm signal and enhancement of framework aluminum through ammonia treatment or ion exchange.17,27,29,35 However, it was also claimed as extra-framework aluminum.28 In light of the spectral analysis, the sharp symmetric peak at 0 ppm should be the aluminum species in high symmetrically coordinated environment, similar to Al(H2O)63+, while the framework aluminum species (Scheme 1a) is in asymmetrically coordinated environment showing relatively large Cq (Figure 2d). We carried out ammonium ion exchange for the calcined or water adsorption saturated H-Beta. Indeed, the peak at 0 ppm is almost disappeared and −4 ppm is also decreased (Figure 3b−

Figure 3. 27Al MAS NMR spectra of H-Beta zeolite (Si/Al = 19): (a) calcined, (b) NH4NO3 solution exchange of (a), (c) water adsorption saturated of (a), and (d) NH4NO3 solution exchange of (c). (a′) The same as c, (b′) first adding 20 μL of 0.1 M NH4NO3 solution, and (c′) second adding 20 μL of 0.1 M NH4NO3 solution. The dashed lines represent the simulated peaks: a′ tetra (54 and 57 ppm) 73.5%, 0 ppm 3.9%, −4 ppm 22.5%; b′ tetra (54 and 57 ppm) 69.7%, 0 ppm 9.9%, −4 ppm 20.3%; c′ tetra (54 and 57 ppm) 71.6%, 0 ppm 13.7%, −4 ppm 14.6%. The asterisks denote spinning side bands.

d). To quantify the relative changes of aluminums, we directly added a small amount of NH4NO3 aqueous solution into the hydrated sample in the rotor after each 27Al MAS NMR measurement, as is shown in Figure 3a′−c′, and the relative amount of different aluminum sites was obtained by deconvolution. The intensity of the peak at 0 ppm increases significantly with more NH4NO3 solution added. Thus, this peak should be attributed to aluminum cations, and the absence of 0 ppm peak for the ion exchanged sample (Figure 3b,d) is due to that it is exchanged out by ammonium cation. A little

Figure 4. Deconvoluted 27Al MAS NMR spectra of H-Beta: (a) Si/Al = 12.5 calcined at 1023 K, (b) saturated water adsorption of (a), (c) Si/Al = 19 calcined at 1023 K, (d) saturated water adsorption of (c). Dashed lines represent the simulated peaks. Black and red lines represent the experiment and simulated spectra, respectively. Visibilities of aluminum are above 94% for all of the samples. E

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OH groups at 4.0 ppm for the sample over hydration treatment is still observed, especially for the samples with Si/Al = 19 (Figure 5c,d), which is in accordance with the results derived from 27Al MAS NMR. It means after saturated water adsorption some EFAL species may reinsert into the framework to form framework aluminum producing more Brönsted acid sites as compared to the parent calcined sample. This process is similar to the previously reported reversible dealumination-realumination of Beta zeolite in pH value controlled suspension.42 Thus, the increased tetra-coordinated aluminums are composed of both framework and extra-framework aluminums. For H-Beta with lower aluminum content, the EFAL species may be more mobile and active, and thus more penta- and octahedralcoordinated EFALs change into tetra-coordinated framework/ extra-framework aluminums. For the sample with higher aluminum content, more EFAL species produced may block the channel, which decreases the mobility these EFAL species, and finally some of the penta- and octahedral-coordinated EFALs remain after saturated water adsorption. In both cases, it is clearly indicated that water plays an important role in the dynamic changes of these EFALs.

coordinated EFAL near 46 ppm is observed (Figure 4b). The visibilities of aluminum in calcined and water adsorption saturated samples are all above 94%, so the changes of different sites can be quantified by 27Al MAS NMR. Both hydrated samples show a significant amount of octahedrally coordinated framework aluminum near −4 ppm due to the hydrolyzation of Si−O−Al bond (Figure 4b,d). Moreover, there is no significant change for the framework aluminum at 55 ppm with a relatively narrow peak, but there are about 11% and 19% increases of the other kind of tetra-coordinated aluminum showing relative broad lines near 55 ppm for hydrated H-Beta zeolite with Si/Al ratios 12.5 and 19, respectively, which may result from the decreased aluminums near 34 and 4 ppm. The increased tetracoordinated aluminums may be attributed to framework or extra-framework aluminums, which are not well distinguished even in 27Al MQ MAS NMR spectra (Supporting Information Figure S3d,f). It has been demonstrated by theoretical calculation that penta-coordinated AlOH2+ may change into tetra-coordinated status by adsorbing one water molecule and coordinating with two framework oxygen atoms.7,39 Under this condition, water may also enhance the proton transfer from Brönsted acid site and result in the reaction between Al(OH)3 and proton, and finally make it change into tetra-coordinated status. Otherwise, if it is attributed to framework aluminum, there should be an increase of the amount of Brönsted acid site after saturated water adsorption. Thus, 1H MAS NMR experiments were conducted for the corresponding dehydrated samples, and the spectra are shown in Figure 5. All of the

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CONCLUSIONS Aluminum site changes of dehydrated H-Beta zeolite during a rehydration process are investigated by 27Al MAS and MQ MAS NMR at high field. EFALs in H-Beta zeolite exhibit dynamic changes during progressive rehydration. In dehydrated zeolite, about 29% aluminums are invisible, and the peaks of framework aluminums are dramatically broadened with Cq values reaching 17 MHz. No octahedrally coordinated aluminum sites and very weak signal of penta-coordinated sites are demonstrated in 27Al MQ MAS spectrum. During the rehydration process, octahedral (4 ppm) and penta- (34 ppm) coordinated extra-framework aluminum sites increase initially as some invisible aluminum species change into the penta- and octahedrally coordinated species, and finally disappear by changing into tetra-coordinated framework/extra-framework aluminums after saturated water adsorption. Quantitative 27Al MAS NMR analysis combined with 1H MAS NMR indicates some EFAL species can reinsert into the framework during the rehydration process due to their high mobility and activity, especially in H-Beta with lower aluminum content. The attribution of octahedrally coordinated aluminum sites is clarified; that is, 0 and −4 ppm are attributed to extraframework aluminum cations and partially hydrolyzed framework aluminum, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 5. 1H MAS NMR spectra of dehydrated H-Beta zeolites: (a) Si/Al = 12.5 calcined at 1023 K, (b) saturated water adsorption of (a), (c) Si/Al = 19 calcined at 1023 K, and (d) saturated water adsorption of (c).

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H MAS NMR spectra of dehydrated and progressive rehydrated H-Beta zeolites, 27Al MQ MAS NMR spectra of dehydrated and water adsorption saturated H-Beta zeolites with different Si/Al ratios, and 29Si MAS NMR spectra of dehydrated, calcined, and water adsorption saturated H-Beta zeolites. This material is available free of charge via the Internet at http://pubs.acs.org.

samples over hydration treatment show a significant increase of signals of SiOH groups near 1.8−2.2 ppm due to the hydrolyztion of the Si−O−Si(Al) bond (Figure 5b,d).40 Especially, for H-beta with a higher Si/Al ratio, it is similar to zeolite Y in hydrolysis of Si−O−Si domains for sample with a high Si/Al ratio when treated under steaming condition.41 The signals of bridging OH groups at 4.0 ppm are very weak due to severe dealumination, and part of the protons were exchanged by EFAL cations.6 However, signal enhancement of bridging

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. All of the NMR experiments were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research, and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the DOE by Battelle Memorial Institute under contract DE-AC0676RLO 1830.

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