Distribution of Aluminum over the Tetrahedral Sites in ZSM-5

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Distribution of Aluminum over the Tetrahedral Sites in ZSM-5 Zeolites and Their Evolution after Steam Treatment Julian Holzinger, Pablo Beato, Lars Fahl Lundegaard, and Jørgen Skibsted J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05277 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Journal of Physical Chemistry

Distribution of Aluminum over the Tetrahedral Sites in ZSM-5 Zeolites and Their Evolution after Steam Treatment

Julian Holzinger†, Pablo Beato*‡, Lars Fahl Lundegaard‡ and Jørgen Skibsted*,† †

Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University,

Langelandsgade 140, DK-8000 Aarhus C, Denmark ‡

Haldor Topsøe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark

____________________ *

Corresponding authors.

Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark. Tel: +45 8715 5946. E-mail: [email protected] (J. Skibsted). Haldor Topsøe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark. Tel: +45 2275 4681. E-mail: [email protected] (P. Beato).

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Abstract Zeolite ZSM-5 is one of the most widely used zeolites, in particular in heterogeneous catalysis. This work investigates the incorporation of Al in the silica framework of monoclinic ZSM-5 and the Al speciation during steam treatment for four ZSM-5 samples with different Si/Al ratios, using ultra-high field 27Al NMR (22.3 T), 29Si NMR, X-ray diffraction and IR spectroscopy. 27Al MQMAS NMR at 22.3 T allows identification and quantification of ten distinct tetrahedral framework resonances which are assigned to the crystallographically sites by their average T-OT angles, determined from powder XRD patterns for the same samples. A clear Al-site preference is observed, and found to be dependent on the Si/Al ratio of the parent zeolite. The framework sites facing the intersections in ZSM-5 are found to be most prone to dealumination whereas Al in the straight and sinusoidal-shaped channels are more stable towards steam treatment at high temperatures. Extra-framework Al species with five- and six-fold coordination and two distorted tetrahedral AlO4 sites have also been identified. The spectroscopic data are supported by catalytic activities from α-test cracking experiments which are in accordance with a re-insertion of Al in the framework at mild steaming and the onset of dealumination at higher temperatures.


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Introduction Zeolites are crystalline microporous aluminosilicates which find widespread applications as molecular sieves, detergents, adsorbents1 and as heterogeneous catalysts in a range of chemical industries2. In recent years, zeolites have also found important applications in the conversion of biomass3 and natural gas to more valuable hydrocarbons such as in the methanol-tohydrocarbons (MTH) processes4-6 and the oxidation of methane to methanol7-9. Their outstanding properties and performances are strongly related to their framework topology10 which is built-up by corner-sharing SiO4 and AlO4 tetrahedra. ZSM-5 belongs to the group of the so-called “big five” zeolites and is one of the commercially most widely used Si-rich (Si/Al > 12) zeolites.4,11 ZSM-5 exhibits MFI topology, which consists of intersecting straight and sinusoidal tenmembered ring channels of approx. 5.5 Å in diameter. In addition, the resulting 3D pore network contains larger spherical voids at the channel intersections of ~10 Å in diameter. The MFI topology is influenced by temperature, Si/Al ratio and the type of guest molecules and can exist in a monoclinic (P21/n symmetry) and an orthorhombic structure (Pnma symmetry) with 24 and 12 crystallographically distinct tetrahedral sites (T-sites), respectively.12-15 While a completely silicious zeolite has a neutral structure, the replacement of Si by Al creates a charge deficit which is counter-balanced either by a metal cation or a proton. The latter situation provides Brønsted acidity which is responsible for the materials catalytic activity. Thus, the number and strength of these acid sites is directly related to the amount of Al in the structure (i.e., the Si/Al ratio) and the location and distribution of Al over the framework. Along with the steric constraints caused by the framework structure, this leads to variations in selectivity and reactivity for the zeolites.16,17 Aluminum in the framework may also affect the resistivity of the structure towards coking and post-treatments such as dealumination or desilication.18,19 The key role of Al for the Brønsted-acid activity, selectivity and lifetime calls for the development of characterization tools which are able to experimentally determine the exact location of these active sites in the zeolite structure. A distinction between Al and Si in the Tsites by X-ray diffraction (XRD) is rather complicated as a result of high Si/Al ratios and the very similar scattering factors for these two neighbor elements in the periodic table. However, by


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the exchange of the proton in H-ZSM-5 with the much heavier tellurium or cesium cations (i.e., Tl-ZSM-5 and Cs-ZSM-5), it has been indirectly shown by XRD that the distribution of Al over the T-sites is not random.20-22 The same indications have been obtained in studies of the distribution of Al on a long-range scale (Al zoning) by energy dispersive X-ray spectroscopy (EDX)23 and atomic probe tomography (APT)24. Recent investigations of identical zeolite types, prepared by different synthesis methods, have shown that the distribution of Al over the T-sites can vary significantly. Experimental evidence of Al site preferences in framework sites are reported for zeolite-beta from extended X-ray absorption fine structure (EXAFS) and

27

Al

magic-angle spinning (MAS) NMR studies25,26, and for ferrierite27 as well as ZSM-528,29 from 27

Al multiple-quantum (MQ) MAS NMR. 27Al NMR has the advantages of high sensitivity and

the reflection of the nearest local environments in its chemical shift and quadrupole coupling parameters. This was successfully demonstrated for the first time by Sarv et al.30, who identified up to three 27Al NMR resonances and assigned them to different T-sites in ZSM-5. In a similar study31, two distinct 27Al resonances were assigned to specific T-sites in the orthorhombic ZSM5 structure, utilizing a relationship between 27Al chemical shifts and average T-O-T bond angles (determined by XRD). In a later work, Sklenak et al.32,33 employed a combination of density functional theory (DFT) calculations and samples, and identified

27

27

Al MQMAS NMR on a set of 18 different ZSM-5

Al resonances from 12 framework sites, which were assigned to the

distinct T-sites in the monoclinic structure. Most recently, as-synthesized ZSM-5 (i.e., including tetrapropylammonium ions in the structure) has been investigated by a combination of DFT calculations and two-dimensional 29Si–27Al NMR experiments by Dib et al.34, who proposed the preferential incorporation of Al in four of the 24 tetrahedral sites, assuming that DFT can provide calculated 27Al and 29Si chemical shifts of very high precision. Dealumination of the zeolite framework, for example by water vapor treatment at high temperatures (i.e., steaming), can have a remarkable effect on the performance of zeolites35-38. Furthermore, such post treatment processes are often used to increase the thermal stability and catalytic activity prior to their use in industrial reactors. The removal of Al from the tetrahedral sites results in new species including distorted four-fold, penta-coordinated or octahedral Al sites, as clearly revealed by 27Al MAS NMR39-44. These species can be present as so-called extra-


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framework aluminium (EFAl) or as broken framework sites partly bonded to hydroxyl groups and water. The formation mechanisms and detailed structure of these species are far from understood, due to the lack of experimental methods which are able to provide the necessary information on the local structure. However, the structure and formation of these EFAl species and distorted framework sites have been addressed in several theoretical studies, often in combination with NMR spectroscopy.39,40,45-48 In the present work, we have selected four ZSM-5 samples with different Si/Al ratios and dealuminated them via steam treatments at different temperatures (300 – 500 oC) and times (2h – 48h). All samples have been analyzed in detail, utilizing the complementary techniques of solidstate

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Al and

29

Si MAS NMR spectroscopy, powder XRD diffraction, FT-IR spectroscopy,

thermal analysis and elemental atomic spectroscopy (ICP-EOS). In addition, α-test experiments have been performed as they give information about the effect of steaming and dealumination on the zeolites catalytic activity. In particular, 27Al MAS and MQMAS NMR spectra, obtained at an ultra-high magnetic field (22.3 T), provide a new level of high resolution for these materials, allowing identification and quantification of 10 different tetrahedral Al sites. These resonances are assigned to the distinct tetrahedral sites in the monoclinic structure of ZSM-5 using refined powder-XRD data obtained for the actual samples of ZSM-5. The direct comparison of NMR and XRD data for the same samples enhances significantly the reliability of the assignments compared to earlier 27Al NMR studies, where XRD data from the literature was used, potentially containing template molecules or different water contents. The quantification of the framework Al resonances, following the dealumination under different steaming conditions, has not been achieved before and provides a unique tool to follow the dealumination as function of Si/Al ratio and steaming conditions, as demonstrated in this work.

Experimental Materials Four commercial ZSM-5 samples, denoted as samples A – D, with bulk molar Si/Al ratios of 15 (A: CBV-3024), 40 (B: CBV-8014), 50 (C: PZ-2/100) and 140 (D: CBV-28014) were obtained from Zeolyst International (Conshohocken, PA, USA, samples A, B, and D) and


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Zeochem AG (Uetikon, Switzerland, sample C). The zeolites were received in their ammonium form and transferred into H-ZSM-5 zeolites by calcination in ordinary air at 500 oC for 5 hours. The parent calcined samples A and B have been subjected to steam treatment at 300, 350, 400, 450 and 500 °C for 5 hours and at 500 oC for 48 hours, whereas samples C and D have only been exposed to the harshest steam treatment at 500 oC for 48 hours. The samples were placed in a homebuilt steaming chamber (10 cm diameter) inside of a furnace and heated initially from room temperature to 100 oC employing a heating rate of 1.5 oC/min without steam, and subsequently to the maximum temperature using the same heating rate and 100% water steam with a flow of 2 mL/min. The temperature was kept at the maximum temperature for 5 h (or 48 h). Afterwards the water steam was turned off and the sample was naturally cooled down in the furnace to room temperature. After calcination and steam treatment, all samples have been stored at ambient conditions prior to the NMR studies. Thus, these measurements have been performed on samples in a hydrated state, unless otherwise stated. The bulk Si/Al ratios of all samples were determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) for acid-digested samples with an Agilent 720 ES ICP-OES instrument.

Powder-XRD, TGA, FT-IR and α-test Powder XRD patterns were obtained with a PanAlytical X’PertPro diffractometer (Cu Kα) in Bragg-Brentano configuration using variable divergence slits. Data were analyzed by the TOPAS software and a structural model including soft constraints on T-O distances and O-T-O angles. Thermogravimetric analyses were performed with a NETZSCH TG 209 Libra instrument. The samples were analyzed in the temperature range 30 - 550 °C using a heating rate of 20 °C/min, with a 6-hour plateau at 430 °C, 1-hour plateau at 550 °C, and a N2 flow of 20 mL/min. FT-IR experiments were performed on a Bruker Vertex 70 spectrometer with a HgCdTe (MCT) detector operating in transmission mode. Prior to the measurement, 20 mg of each sample were pressed into a self-supporting wafer and dehydrated under dynamic vacuum at 400 °C for 14 hours using a heating rate of 3 °C/min. Catalytic activities were investigated by determining the n-hexane cracking activities, also known as the α-test.49 The conversion of n-hexane was measured by mass spectrometry on


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sample amounts of 300 mg in the temperature range of 300 - 600 °C, using a heating rate of 5 °C/min, a constant pressure of 1.0 atm and a constant carrier gas-flow rate of 100 mL/min (nhexane saturated helium at 16 °C). The loaded plug flow reactor was initially flushed with nhexane saturated helium and feed levels were measured before and after the reaction to track any change in the feed level concentration. Assuming a first-order reaction, the cracking-rate constant can be determined from the conversion. With these data, an Arrhenius plot is produced and a linear fit is made in the temperature region of the most rapid increase in the conversion. For each sample, the α-value was obtained from the cracking-rate constant at 538 °C (= 1000 °F) relative to the value of a standard. In our case the standard is the parent calcined H-ZSM-5 CBV8014 (sample B calc.) measured under the same experimental conditions.

NMR spectroscopy Single-pulse

27

Al MAS and triple-quantum MAS (MQMAS) NMR spectra have been

acquired at ambient temperature on a wide-bore Varian Direct-Drive VNMR-600 spectrometer (14.1 T), using a homebuilt CP/MAS probe for 4 mm o.d. PSZ (partially stabilized zirconia) rotors and a spinning frequency of νR = 13.0 kHz, and on a narrow-bore Bruker 950/54 us2 spectrometer (22.3T), using a triple-resonance (1H-X-Y) 2.5 mm MAS probe and a spinning frequency of νR = 30.0 kHz. The single-pulse 27Al MAS experiments employed relaxation times of 0.5 – 1.0 s and short rf pulses of 0.5 µs for rf field strengths of γB1/2π = 60 kHz (14.1 T) and 100 kHz (22.3 T), corresponding to π/16 and π/10 flip angles, respectively. The MQMAS spectra at 22.3 T were obtained with the z-filter three-pulse sequence50, employing hard excitation (π) and conversion (π/3) “liquid” pulses (γB1/2π = 100 kHz), a soft, solid π/2 z-pulse (γB1/2π = 6.5 kHz), relaxation delays of 0.5 s, and identical spectral widths of 30 kHz in both dimensions. Pure absorption-mode 2D spectra were obtained by the hypercomplex approach for data acquisition and the isotropic spectra were obtained by a shearing transformation during the 2D Fourier transformation of the experimental time-domain data.51 The F1 dimension of the MQMAS spectra are referenced according to the triple-quantum shift = ( + (3 − /((1 + with being the resonance frequency, the carrier frequency, and = 19/12 for a

spin I = 5/2 nucleus in a 3QMAS experiment.51 The 27Al{1H} CP/MAS NMR experiments were


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performed on a Bruker 700 spectrometer (16.4 T), employing a 1H-X-Y 4 mm MAS probe and a spinning frequency of νR = 15.0 kHz. The initial 1H excitation pulse used a field strength of γB2/2π = 100 kHz (τ90 = 2.5 µs), whereas the contact-time field strengths were γB2/2π = 41 kHz (1H) and γB1/2π = 4.0 kHz (27Al) in order to match the modified Hartmann-Hahn condition for a quadrupolar under MAS, i.e., (, = ( + 1/2 , ± ∙ . The CP contact times were 0.2 and 0.4 ms and relaxation delays of 1.5 or 2 s were used. The 27Al chemical shifts are referenced to an aqueous solution of 1.0 M AlCl3·6H2O. For all single-pulse

27

Al MAS NMR spectra, a

spectrum of an empty rotor, recorded under the exact same experimental conditions, has been subtracted in order to remove a few broad, low-intensity resonances from the rotor and probe. The

29

Si MAS NMR spectra were recorded on a Varian INOVA-300 (7.05 T) and a Bruker

400 (9.4 T) spectrometer, using homebuilt CP/MAS probes for 7 mm o.d. PSZ rotors and a spinning frequency of νR = 7.0 kHz. The single-pulse spectra employed a π/4 excitation pulse (τp = 3.0 µs for γB1/2π = 40 kHz), a relaxation delay of 30 s and typically 2048 scans. The 29Si{1H} CP/MAS NMR experiments (7.05 T) used a 4-s relaxation delay and an initial 1H π/2-excitation pulse for a rf field strength of γB2/2π = 42 kHz whereas the contact-time field strengths were γB2/2π = 42 kHz (1H) and γB1/2π = 35 kHz (29Si) for contact times of 0.5 – 5.0 ms. The

29

Si

chemical shifts are referenced to neat tetramethylsilane (TMS) using a sample of β-Ca2SiO4 ( δ = -71.33 ppm) as a secondary reference.

Results and Discussion Structural model: orthorhombic versus monoclinic symmetry The calcined ZSM-5 samples are highly hygroscopic, as seen by thermal analysis (Fig. S1), which shows that the calcined samples contain 22 ± 3, 20 ± 2, 15 ± 1, and 5 ± 1 water molecules per unit cell (Si96-xAlxHxO192) for samples A – D or on average 4.5 ± 1.0, 9.0 ± 1, 9.0 ± 0.5 and 7.5 ± 0.5 water molecules per Al site, respectively, when stored and handled under ambient conditions. Powder X-ray diffractograms have been obtained for all samples, where patterns of the A and B series of steamed ZSM-5 are shown in Figs. S2 and S3. Refinements of the data for the A and B series with simulations of the structure, including water molecules inside the channels for compensation of residual charge density, reveal that all samples in these two series


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contain ZSM-5 in its monoclinic form (P21/n symmetry), which includes 24 distinct Si tetrahedral sites with equal occupancies (Fig. 1). The monoclinic symmetry is identified by a closer inspection of the peak at 24.5° (Fig. S3), which has an asymmetric shape for the calcined parent samples and is divided into two peaks for the steamed samples, representing clear evidence for the monoclinic symmetry. The splitting of the peak reflects a small decrease in unitcell volume and a slightly increasing β-angle at an increasing degree of dealumination for the samples steamed at increasing temperature. As a consequence, a decrease of all Si-O-Si bond angles is observed (Fig. S4). Application of the orthorhombic symmetry in the simulations of the calcined and mild-steamed samples also results in a reasonable match to the XRD patterns. However, the resulting Si-O-Si angles, from which 29Si NMR chemical shifts can be calculated53 (-109 to -119.5 ppm) do not match with the experimental

29

Si chemical shifts reported for

samples in the orthorhombic symmetry (-112 to -117 ppm)54. Furthermore, the 29Si NMR spectra obtained in the present work reveal chemical shifts (-109.5 to -117.5 ppm) in the range of the monoclinic symmetry (-110 to -117 ppm)54, which supports the assumption of having all the samples in the monoclinic symmetry. Considering the monoclinic symmetry of ZSM-5, the Tsites T4, T10, T16 and T22 (sinusoidal channels) and T8, T11, T20 and T23 (straight channels) are not facing the channel intersections.

Parent samples – 27Al NMR 27

Al MAS NMR spectra of the four parent calcined samples at 14.1 T and 22.3 T are shown

in Figure 2 and allow at first sight an identification of central-transition resonances from four-, five- and six-fold coordinated Al at roughly 55, 30 and 0 ppm, respectively. Sample D with the highest Si/Al ratio contains exclusively Al in tetrahedral coordination whereas low-intensity resonances from octahedrally coordinated Al are observed for samples B and C with the narrow peak at 0.0 ppm being ascribed to mobile Al(H2O)3+ species in the framework cavities. In contrast to samples B – D, the zeolite with the lowest Si/Al ratio (sample A) contains appreciable amounts of five- and six-fold coordinated Al which originate from non-framework aluminum. The linewidth of the tetrahedral resonance is slightly larger for sample A (full width at half maximum, FWHM = 7.8 ppm) as compared to samples B – D for which the linewidths are in the


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range FWHM = 5.3 – 5.8 ppm. A comparison of the spectra at 14.1 T and at the very high magnetic field of 22.3 T reveals a significant improvement in resolution, especially for the separation of the resonances from tetrahedral and five-fold coordinated Al (sample A). The resonance from the tetrahedral framework sites are roughly observed over the same chemical shift range at the two magnetic fields, demonstrating that these sites exhibit small quadrupole couplings that only result in a small second-order quadrupolar broadening even at 14.1 T. Thus, the increased resolution for the tetrahedral peaks at 22.3 T mainly reflects increased chemical shift dispersion, as shown recently for a cement mineral with two Al sites and very similar, small quadrupolar couplings.55 To further improve spectral resolution and to identify different tetrahedral Al sites,

27

Al

MQMAS NMR measurements at 22.3 T have been performed for the mild steamed sample A (300/5), the calcined parent samples B – D and the harsh steamed (500/48) samples A - D. As an example, the 27Al MQMAS NMR spectrum of the calcined parent sample B is shown in Figure 3, illustrating the spectral region for tetrahedrally coordinated Al. The propagation of a narrow contour diagonal to the F1 and F2 dimensions of the MQMAS spectrum, parallel to the pure chemical shift line (PQ = 0), and shifted only slightly (< 0.5 ppm) to lower frequency in the F2 dimension, demonstrates the presence of a rather small second-order quadrupolar broadening. This strongly suggests that the diagonal width of the contour is governed by resonances from a number of Al sites with different chemical shifts, each exhibiting small quadrupolar couplings (vide infra). Similar contours are observed in the 27Al MQMAS NMR spectra for the other ZSM5 samples (Fig. S5), both the calcined parent samples and the samples after harsh steam treatment. A careful analysis of the topographical 2D intensities in the chemical shift range of 50 - 70 ppm in these MQMAS NMR spectra allows identification of a number of individual signals. The peak positions are established by reading out where signals appear and disappear by scanning through different slices in the F1 dimension. The procedure provides centers of gravity for the individual sites in the F1 and F2 dimensions of the MQMAS NMR spectra, i.e. ! and

"! , respectively. These positions are further refined in simulations, first of the F1 slices at the

actual chemical shifts and then of the summed F2 projections from the 2D spectra. However, the intensities in the MQMAS NMR spectra are not quantitative reliable, as they depend on the


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applied rf field strengths and the size of the quadrupole interactions.56 Thus, the quantitatively reliable central transitions, observed in the single-pulse

27

Al MAS NMR spectra, are finally

simulated to obtain the relative intensities for the individual sites (Table 2). Using this approach for all the acquired 27Al MQMAS spectra at 22.3 T allows identification of ten narrow peaks and two broad resonances from tetrahedral

27

Al sites (Fig. S5). For the most silicious sample (D,

Si/Al=140) only nine narrow peaks have been identified, whereas the two broad sites are observed only for the harsh steamed samples and for sample A 300/5. The narrow sites originate from the tetrahedral framework sites of the ZSM-5 structure as the electronic surroundings of these sites are very symmetric and hence, result in rather small quadrupolar couplings. Thus, the narrow resonances from the framework sites are simulated with Voigt-type functions. The centers of gravity in the F1 and F2 dimensions of the MQMAS NMR spectra can for a spin I = 5/2 nucleus be expressed as (in ppm)51,57: #

(

! = − $% − &&' ( ) " ∙ 10

(

*+

"! = $% − ( ) " ∙ 10&'' *+

(1) (2)

Thus, the isotropic chemical shift ($% ) and the quadrupolar product parameter (./ = 0/ (1 + 1" /3 /" ) can be calculated from these frequencies, according to the expressions '

$% = "! − ! "# "# . = 23−

&&'

#

4 3! + 34 $% 4 ∙ 105-

(3) (4)

Average values of the ! and "! shifts for the ten narrow resonances, obtained by the analysis of all the acquired MQMAS spectra, are summarized in Table 1 and from these values, the corresponding δiso and PQ values are calculated for the distinct framework sites. The isotropic chemical shifts are observed over a 6.5 ppm range and differ by about 0.6 – 1.0 ppm for the individual sites. These differences in isotropic chemical shift are clearly larger than the calculated average standard deviations of 0.1 - 0.3 ppm for the δiso values. The quadrupolar product parameters are in the range of 1.3 - 2.1 MHz with average standard deviations of 0.3 –


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1.0 MHz. The high uncertainties of the ./ values reflects that the centers of gravity in the F1 and F2 dimensions of the MQMAS NMR spectra are only affected to a very small extent by secondorder quadrupolar effects at very high magnetic fields (c.f., eqs. (1) and (2)), implying that the individual sites cannot be distinguished based on the PQ values determined from the present experiments. However, the magnitudes of the quadrupole couplings (PQ) are very similar to the values, PQ = 1.3 – 2.4 MHz, which can be calculated from the

27

Al NMR data reported by

Sklenak et al.32,33 for monoclinic ZSM-5, who identified 12 distinct T-sites in an analysis of 18 ZSM-5 samples obtained by different synthesis methods and Si/Al ratios. The observed PQ range of 1.3 – 2.1 MHz corresponds to full widths of the central transition centerbands of 0.35 – 0.92 ppm at 22.3 T, considering the second-order quadrupolar broadening and assuming ηQ = 0 (i.e., CQ = PQ). This range is well below the linewidths of about 2 – 3 ppm, observed for the selected slices in the anisotropic dimension of the MQMAS NMR spectra (Figure 3), demonstrating that the contours in the MQMAS spectrum is overwhelmingly dominated by differences in chemical shifts. This fact implies that narrow lines must be used in the simulations of the slices in the F1 dimension as illustrated in Figure 3. Our approach is further supported by the simulation of the contours in the

27

Al MQMAS NMR spectrum of

sample A 300/5 (Fig. 4), using the δiso and PQ for the 10 tetrahedral sites for this sample and the DMFIT software.58 The very good agreement between the contours in the experimental and simulated MQMAS spectra justifies the use of 10 distinct Al(IV) sites. The summations onto the F1 and F2 dimensions could be simulated individually using a smaller number of broader resonances. However, the combination of such peaks in two dimensions would not be able to reproduce the overall narrow contours observed along the PQ = 0 line in the MQMAS NMR spectra. This is clearly evident from the simulation of the MQMAS NMR spectrum of sample A 300/5 in Figure S6 which employs the δiso and PQ parameters derived from simulations of the F1 and F2 projections of the experimental spectrum by two distinct resonances. For the 12 distinct T-sites, identified by Sklenak et al.32,33 in an analysis of 18 different monoclinic ZSM-5 samples, eight of them exhibit

27

Al isotropic chemical shifts in

the same range as observed for the ZSM-5 samples studied in this work, whereas the remaining three sites possess δiso values in the range 60 – 64 ppm and one site a lower value of 50.0 ppm.


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Consideration of the six sites with δiso in the 52 – 58 ppm range from the work of Sklenak et al.32,33 in a simulation of the contours for the 27Al MQMAS NMR spectrum for sample A 300/5 results in a simulation (Fig. S6c), which gives a less convincing agreement with the experimental spectrum (Fig. S6a), as compared to the simulation with the 10 sites identified in the present work (Fig. S6b).The 10 resonances for the framework Al sites reported in Table 1 can also provide a satisfactory simulation of corresponding spectra obtained at lower magnetic field, as illustrated in Fig. S7 by the 27Al MQMAS NMR spectrum of calcined sample B at 14.1 T. The contour from the framework resonances deviates somewhat from the pure chemical shift line, as a result of increased quadrupolar broadening; pure second-order quadrupolar broadening gives full linewidths of 0.9 – 2.3 ppm at 14.1 T considering the PQ values in Table 1. Thus, it becomes obviously more difficult to identify the individual peaks a lower magnetic field without consideration of the second-order quadrupolar broadening. However, in a

27

Al MAS NMR

spectrum at 14.1 T Yokoi et al.28 identified five out of 12 framework Al sites for orthorhombic ZSM-5 in a similar manner as used in the present work.

Assignment of 27Al NMR resonances The 27Al isotropic chemical shifts determined for the 10 framework resonances from the highfield

27

Al MQMAS NMR spectra (22.3 T, Table 1) fall in a narrow range of 6.5 ppm (52.0 to

58.4 ppm), which is in good agreement with

27

Al chemical shifts reported in earlier studies of

ZSM-5 samples.23,28-31,34,38,41,59-64 Sklenak et al.32,33 have investigated 18 different ZSM-5 samples (framework Si/Al = 15 – 140) in their hydrated state by 27Al MQMAS NMR (11.7 T) and identified a total of 12 Al framework sites with a significantly larger variation in δiso(27Al) values (50.0 – 63.5 ppm, Fig. 5). These peaks were assigned to the tetrahedral sites in monoclinic ZSM-5 (P1 symmetry) using QM/MM calculations of the 27Al chemical shieldings for the bare structure of ZSM-5 without cations and water molecules but with geometry optimization for each introduction of an Al atom in a single tetrahedral site. These values are also shown in Fig. 5 and are observed to fit the experimental shift range, although the center of gravity is somewhat shifted to higher frequency for the calculated values, as compared to those determined experimentally. Most recently, Dib et al.34 have calculated the 27Al chemical shifts for ZSM-5 in


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its as-synthesized form with the tetrapropylammonium template molecule (monoclinic, P21/n) by geometry optimized DFT calculations and the CASTEP code. This resulted in a wide chemical shift range (45 – 64 ppm, Fig. 5), most likely reflecting strong effects from the template molecule, whereas their experimental δiso(27Al) values, determined from

29

Si –

27

Al D-HMQC

NMR experiments, fall in a much narrower range (51.1 – 54.1 ppm). The two sets of calculated 27

Al chemical shifts (Fig. 5) indicate that an assignment of the resonances is not straightforward

and that cations, water molecules and template molecules may have an impact on the chemical shifts. The ZSM-5 samples studied in the present work have been analyzed and refined by powder Xray diffraction in their hydrated state, i.e., identical to the samples studied by

27

Al NMR.

Thereby, mean T-O-T bond angles (Table 1) have been calculated from the crystal structures obtained by simulations of the XRD patterns for the actual ZSM-5 samples (Fig. S4). Using these angles along with the correlation between bond angles for the distinct T-sites,

%$27

Al isotropic chemical shifts and mean T-O-T

Distribution of Aluminum over the Tetrahedral Sites in ZSM-5

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