A Combined Computational and Experimental 27Al NMR Study

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Article

Identification of Distinct Framework Aluminum Sites in Zeolite ZSM-23: A Combined Computational and Experimental Al NMR Study 27

Julian Holzinger, Malte Nielsen, Pablo Beato, Rasmus Yding Brogaard, Carlo Buono, Michael Dyballa, Hanne Falsig, Jørgen Skibsted, and Stian Svelle J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06891 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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The Journal of Physical Chemistry

Identification of Distinct Framework Aluminum Sites in Zeolite ZSM-23: A Combined Computational and Experimental 27Al NMR Study

Julian Holzinger†,||, Malte Nielsen‡,§,||, Pablo Beato§, Rasmus Yding Brogaard*,§, Carlo Buono‡, Michael Dyballa‡, Hanne Falsig§, Jørgen Skibsted*,† and Stian Svelle*,‡

†Department

of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University,

Langelandsgade 140, DK-8000 Aarhus C, Denmark ‡Center

for Materials Science and Nanotechnology (SMN), Department of Chemistry, University of

Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway §Haldor ||These

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

authors contributed equally to this work.

____________________ *

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). Center for Materials Science and Nanotechnology (SMN), Department of Chemistry, University of Oslo, N0315, Oslo, Norway. Tel: +47 2285 5454. E-mail: [email protected] (S. Svelle). Haldor Topsøe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark. Tel: +45 4191 8352. E-mail: [email protected] (R.Y. Brogaard).

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Abstract ZSM-23 (MTT) is a silicon-rich zeolite with one-dimensional, 10-membered ring channels, which has recently attracted interest as a promising catalyst in aromatic-free methanol-to-hydrocarbons conversion. To obtain a better understanding of the catalytic activity and ultimately to design a better catalyst, it is crucial to locate the active sites in the zeolite framework. This work investigates the tetrahedral aluminum framework sites in two zeolite H-ZSM-23 samples by experimental and computational 27Al NMR spectroscopy. 27Al MQMAS NMR experiments at six different magnetic fields (4.7 to 22.3 T) were utilized to resolve distinct Al sites. The detected tetrahedral framework Al sites were assigned to the specific tetrahedral sites in the crystal structure by DFT calculations of the 27Al chemical shieldings. A comprehensive investigation of the structural model, basis set and exchange-correlation potential used in the DFT calculations was performed. Two avenues were pursued for calculating the 27Al isotropic chemical shifts: the isolated-sites approach where clusters are extracted from large supercells with high Si/Al ratios and an approach targeting lower Si/Al ratios with a fully periodic model. It is found that for the ZSM-23 zeolites with Si/Al = 24 and 37 investigated here, the latter approach gives the best agreement with experiment.


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Introduction Zeolites are crystalline, microporous aluminosilicates with well-defined porous framework structures. As a result of these structures, they exhibit outstanding properties, which are utilized in a range of catalytic processes, such as in the methanol-to-hydrocarbons conversion processes.1-3 Their framework topologies are built up by corner-sharing SiO4 and AlO4 tetrahedra, where the replacement of Si by an Al atom generates a charge deficit which can be counter-balanced by a proton, producing a material with Brønsted-acid properties. In most zeolite structures there are more than one crystallographically distinct position in the framework structure, where Al can be located, and these distinct tetrahedral sites (T-sites) can have unique properties.4-7 27Al NMR spectroscopy may allow distinction of the framework Al sites,8-15 since the 27Al isotropic chemical shift and the quadrupolar coupling parameters depend on the local environment of the nucleus which varies slightly for the distinct tetrahedral framework positions. Generally, the resonances identified in a 27Al

NMR spectrum cannot be directly assigned to the individual T-sites. Thus, the isotropic

chemical shifts or quadrupolar coupling parameters are compared with structural parameters or calculated values using theoretical approaches. A simple option is the application of the linear correlation between mean T-O-T bond angles from XRD and

27Al

isotropic chemical shifts (𝛿𝑖𝑠𝑜

= 132 ― 0.5 ∙ < T - O - T, in ppm), proposed by Lippmaa et al.16 for framework aluminosilicates in general. For example, this correlation has been used for the assignment of T-sites in zeolite Beta and zeolite ZSM-5.15,17,18 However, the validity of this correlation is debated for highly siliceous zeolites, since theoretical calculations indicate that the bond angles also depend on the Si/Al ratio, the presence of extra-framework cations and the hydration level.10,19 Computational methods for calculating NMR parameters have been employed for the assignment of

27Al

NMR resonances in

several studies of zeolite structures. For example, Sklenak et al. 8,9,20 identified 27Al resonances from 12 different framework sites in a study of 18 different ZSM-5 samples and assigned these resonances to the distinct T-sites in monoclinic ZSM-5 by DFT/MM calculations. Dědeček et al. 21 have assigned

27Al

isotropic chemical shifts for ferrierite to individual T-sites and determined

relative occupancies of each site by simulating the

27Al

NMR spectra, and a similar approach has

been used for zeolite Beta by Vjunov et al.10 Framework Al Lewis sites and perturbed Al sites22 as well as the effect of Al-adjoined silanol-nests on 27Al and 29Si isotropic chemical shifts23 have also been studied by DFT/MM approaches, whereas Klein et al.24 have investigated distortion effects of



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different cations on the local environment of Al in chabazite. Moreover, it has been shown that the presence of two Al sites separated by a single Si atom can result in changes of the calculated 27Al isotropic chemical shift of up to 4 ppm.25 This result points out the importance of investigating isolated Al T-sites. Most recently, a combination of DFT calculations and two-dimensional 27Al

29Si–

NMR experiments has been used to investigate the preferential incorporation of Al in the

ZSM-5 framework structure.26 Despite the wide application of DFT calculations, it is not straightforward to assess their performance in zeolitic systems, and the accuracy of the calculated chemical shifts strongly depends on the proper choice of structural and potentially computational parameters. Cluster models of the zeolite framework have been chosen in most DFT studies for calculations of NMR chemical shieldings. Parameter choices like basis sets, exchange correlation potentials or cluster size have often not been motivated. Brouwer and Enright27 examined different Pople basis sets for calculations of 29Si isotropic chemical shifts for purely siliceous zeolites. However, the Pople basis sets are only one type of basis set and it is of interest whether the choice of other basis sets affects the results. Bull et al.28 have calculated

29Si

and

17O

NMR chemical shifts and

17O

quadrupolar

coupling parameters for zeolite siliceous ferrierite, employing different methods (Hartree-Fock and DFT), basis sets and cluster sizes, and they have shown that the structural model used in the calculations is the primary source of error. Bussemer et al. performed a convergence test for cluster sizes for calculations of

29Si

isotropic chemical shifts in purely siliceous zeolites.29 However, it is

not clear if their results can be extended to Al-containing zeolites since calculations of

27Al

isotropic chemical shifts in zeolites need the inclusion of a negative charge for each Al. This provides the motivation of the present work for a systematic investigation of the different parameters that enter DFT calculations of

27Al

isotropic chemical shifts for zeolites. As a

challenging model system, zeolite ZSM-23 has been chosen, which has recently attracted interest as a promising catalyst in the process of aromatic-free MTH conversion.2,30-32 Different synthesis approaches can have an effect on the catalytic performance and in this relation, it is of interest to investigate if the different protocols also affect the Al distribution over the distinct framework sites. So far, an accurate determination of the Al distribution in ZSM-23 has not been achieved, since only theoretical calculations of the stability of Al in the distinct sites of the MTT topology have been reported.33 In this work we calculate the 27Al isotropic chemical shifts from first principles and


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compare them with experimentally determined values from

27Al

NMR spectroscopy for an

assignment of the T-sites in zeolite ZSM-23. Experimental Materials Two ZSM-23 samples, denoted as samples A (synthesized with heptamethoniumbromide) and B (synthesized with dimethyl formamide), with bulk Si/Al ratios of 25 (A) and 37 (B) were synthesized with two different structure-directing agents and converted to H-ZSM-23 zeolites by ion exchange with NH4NO3 and calcination in ordinary air.32 After calcination, the samples have been stored at ambient conditions, and thus they were in a hydrated state prior to and during the NMR experiments. The Si/Al ratios of the ZSM-23 samples, given above, were determined by ICPOES, whereas the crystallite sizes were estimated to be 50 – 100 nm and 1 m for samples A and B, respectively. Powder X-ray diffraction patterns have been obtained for the two ZSM-23 samples, and a refinement of these data including water molecules in the structure shows that both samples are present in the orthorhombic form with Pmn21 symmetry (Figure S1; a = 21.642 Å; b = 11.178 Å; c = 5.052 Å). The synthesis and treatment of the zeolites SSZ-13 and SSZ-24 are described in the supplementary material.

NMR spectroscopy Single-pulse

27Al

MAS and

27Al

multiple-quantum MAS (MQMAS) NMR spectra have been

acquired at ambient temperature at six different spectrometers: (1) Varian Unity-plus 200 (4.7 T), homebuilt 7 mm CP/MAS probe (PSZ (partially stabilized zirconia) rotors), spinning frequency of νR = 7.0 kHz; (2) Varian Unity INOVA-300 (7.1 T), homebuilt 5 mm CP/MAS probe (PSZ rotors), νR = 10.0 kHz; (3) Bruker Avance II 400 (9.4 T), Bruker 1H-X-Y 4 mm MAS probe, νR = 12.0 kHz; (4) Varian Direct Drive VNMR-600 (14.1 T), homebuilt 4 mm CP/MAS probe (PSZ rotors), νR = 12.0 -13.0 kHz; (5) Bruker Avance III 700 (16.4 T), Bruker 1H-X-Y 4 mm MAS probe, νR = 15.0 kHz; and (6) Bruker Avance III 950/54 us2 (22.3 T), Bruker 1H-X-Y 2.5 mm MAS probe, νR = 30.0 kHz. The single-pulse

27Al

NMR experiments employed short rf pulses of 0.5 μs for rf field

strengths of B1/2 = 57, 50, 65, 60, 56 and 100 kHz (at 4.7, 7.1, 9.4, 14.1, 16.4, and 22.3 T, respectively) and relaxation delays of 0.5 – 1.0 s. The 27Al MQMAS spectra at 4.7 T and 7.1 T were obtained with the standard two-pulse MQMAS pulse sequence, employing hard excitation (π) and



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conversion (π/3) “liquid” pulses for rf field strengths equal to the single-pulse experiments. At the higher magnetic fields, the z-filter three-pulse sequence34 was utilized, using π excitation and π/3 conversion “liquid” pulses for rf field strengths equal to the single-pulse experiments, and a soft solid π/2 z-pulse for rf field strengths of B1/2 = 12, 10, 4 and 6.5 kHz (at 9.4, 14.1, 16.4, and 22.3 T). Relaxation delays of 0.5 and 1 s, and identical spectral widths of 3 x νR in both dimensions were used, except for the 22.3 T spectrum which employed spectral widths of 30 kHz. Two-dimensional (2D) absorption-mode spectra were obtained by a 2D Fourier transformation, including a shearing transformation prior to the transformation of the F1 dimension. The F1 dimension of the MQMAS spectra is referenced according to the triple-quantum shift, 𝛿3𝑄 = (𝜈𝑖 + (3 ― 𝑘)𝜈𝑐𝑓)/((1 + 𝑘)𝜈𝐿), where 𝜈𝑖 is the resonance frequency, 𝜈𝑐𝑓 the carrier frequency, and 𝑘 = 19/12 for a spin I = 5/2 nucleus in a 3QMAS experiment.35 The

27Al

chemical shifts are referenced to a 1.0 M aqueous

solution of AlCl3·6H2O. A spectrum of an empty rotor, recorded under the same experimental conditions, has been subtracted from all single-pulse

27Al

MAS NMR spectra to remove low-

intensity background signals from the rotor and the probe. 27Al

isotropic chemical shifts, 𝛿𝑖𝑠𝑜, and the quadrupolar product parameter, 𝑃Q = 𝐶Q 1 + 𝜂2Q/3 ,

are determined by linear regression utilizing the linear relationship between the frequencies of the center of gravities of the resonances in the F1 dimension, 𝛿𝐶𝐺 𝐹1 , determined at the different magnetic fields, and the applied magnetic field, 1/ 𝜐L2. From these correlations, 𝛿𝑖𝑠𝑜 and 𝑃Q are calculated from the intersection with the ordinate axis and the slope of the linear regression, respectively, according to the relationship35,36 17

3

𝑃Q 2

6 𝛿𝐶𝐺 𝐹1 = ― 31 𝛿𝑖𝑠𝑜 ― 1550 ( 𝜐L ) ∙ 10 .

(1)

The theoretical quadrupolar product parameters were calculated from the EFG tensor eigenvalues, Vii, obtained from the DFT structure optimization calculations. The eigenvalues fulfill the convention, |𝑉𝑧𝑧| ≥ |𝑉𝑦𝑦| ≥ |𝑉𝑥𝑥|, and the quadrupolar coupling constant and asymmetry parameter are calculated as follows: 𝐶𝑄 = 𝜂𝑄 =

𝑉𝑧𝑧e𝑄 ℎ

𝑉𝑥𝑥 ― 𝑉𝑦𝑦 𝑉𝑧𝑧

(2) (3)

where e is the elementary charge, Q the nuclear quadrupole moment (Q(27Al) = 0.1466 b37), and h Planck’s constant.


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Computational Details Structure relaxations have been performed using the Quantum Espresso code38 interfaced with the Atomic Simulation Environment.39 The BEEF-vdw exchange correlation functional40 was applied, employing a plane wave basis with a kinetic-energy cutoff at 700 eV and a charge-density cutoff at 7000 eV. These parameters were based on a convergence analysis of the geometry optimization combined with unit-cell relaxation showing convergence to 0.01 Å in any direction for purely siliceous unit cells. The Brillouin zone for the ZSM-23/MTT was sampled using 4x2x2, 2x2x2, 2x1x2 and 2x1x1 Monkhorst-Pack k-point grids41,42 for the unit cell and for the small, medium and large supercells, respectively. For the SSZ-13/CHA and SSZ-24/AFI unit cell relaxations, the Brillouin zone was sampled with 1x1x1 and 1x1x2 k-point grids, respectively. SSZ-24/AFI was also modelled as a 1x1x2 supercell, where only the  point was used to sample the Brillouin zone. The unit cell relaxations were started from structures obtained from the Database of Zeolite Structures,43 and did not enforce any point group symmetry on the structure. Unit cell optimizations as well as structure relaxations employed a force threshold of 0.01 eV/Å and a convergence threshold of 10-6 eV for electronic energies. Table S1 collects the obtained unit cell parameters. Supercells including Al were negatively charged (vide infra) and compensated by a jellium background to avoid divergence in the periodic calculations. SSZ-24 and SSZ-13 were modeled with Si/Al ratios of 46 and 35, respectively. The Si/Al ratios in the ZSM-23 models are mentioned below. The electric field gradient (EFG) eigenvalues (Vii) were obtained from the relaxed structure. Calculations of the chemical shielding tensors were performed using the software Gaussian09,44 applying an ultrafine grid. Cluster sizes, basis sets and XC potentials were varied and will be specified for each calculation. EFGs and chemical shielding tensors have also been calculated for periodic structures, using the gauge including projector augmented wave (GIPAW) method as implemented in the Quantum ESPRESSO-GIPAW 6.3 code.38,45-47 The GIPAW method can be considered as an extension of the PAW method and allows reconstruction of the all-electron wavefunction and of the expectation values from a pseudopotential calculation. This is essential for the accurate calculation of the response of the valence electrons in regions near the nuclei, which determine the NMR shielding.48 In this work, the ultrasoft GIPAW type pseudopotential was



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adopted in combination with the PBE functional to compute the exchange-correlation term. The kinetic energy cutoff for the wavefunctions was 73.5 Ry (1000 eV), the kinetic energy cutoff for charge density was set at 800 Ry and the convergence threshold for self-consistency was 10-10. The GIPAW calculations considered the small and medium supercells when deriving the isotropic shieldings of the Al atom in each of the seven T-sites. The GIPAW calculations employed a 3x3x3 k-point grid to sample the Brillouin zone both for the small ZSM-23 supercells and the SSZ-13 and SSZ-24 reference cells. We have confirmed that the obtained total energy and NMR parameter values were converged with respect to the energy cutoff, charge density cutoff and k-point sampling density. For the medium ZSM-23 supercells, full convergence was already achieved at 2x2x2 kpoints.

Model systems The ideal model systems are single-T-site zeolites with different

27Al

isotropic chemical shifts, as

this ensures a one-to-one correspondence between the sites considered in the experiment and the calculation. Additionally, the Al site should be isolated in the framework from other Al atoms to avoid interference, particularly for the clusters chosen in the calculations. Only a few single-T-site zeolites can be synthesized with high enough Si/Al ratios to assume that the Al sites are isolated. Therefore, SSZ-13 (high-silica Chabazite, CHA framework) and SSZ-24 (silica analog of AlPO4-5, AFI framework) have been chosen in this work as the single-T-site zeolites. ZSM-23 is chosen as the multiple-T-site zeolite, since it can be synthesized with a fairly high Si/Al ratio, to limit interaction between T sites. ZSM-23 exhibits the MTT framework topology which consists of straight, parallel and tear-drop shaped ten-membered ring channels with a size of 4.5 x 5.1 Å, as illustrated in Figure 1. This topology results in a 1D pore structure with 24 tetrahedral sites per unit cell and seven crystallographically distinct T-sites in the orthorhombic symmetry. Among those, T1 – T5 have a multiplicity of four, whereas it is two for T6 and T7. Moreover, T4 is the only framework site not facing the channel system.

Computational Results


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Each calculation of the NMR isotropic chemical shifts is divided into two parts: (1) optimization of the structure and (2) calculation of the NMR properties on the optimized structure. We have pursued two directions: first, an isolated-sites approach based on a large supercell with a very high Si/Al ratio, from which a cluster is extracted for the NMR calculations. Second, a fully periodic approach in which both structure relaxation and NMR calculations are done on smaller supercells approaching the experimental Si/Al values. For the isolated-sites approach we discuss relaxation of the structure, cluster design and the choice of variables for the calculation of the NMR parameters (basis sets and exchange-correlation (XC) potentials) in the following. This is followed by a presentation of the results obtained from the fully periodic approach. For the sake of simplicity, all obtained chemical shieldings in this section are referenced to the Al framework site in H-SSZ-13 (Si/Al=35) calculated with the exact same method, if not stated otherwise.

Supercell size and relaxation The structure of the zeolite is crucial for an accurate calculation of the

27Al

isotropic chemical

shifts, as the calculations are strongly affected by bond angles and bond lengths around the Al site.15,25 Structure relaxation is performed by DFT utilizing the BEEF-vdw XC-potential with a plane wave cut-off of 700 eV. BEEF-vdw has been shown to reproduce experimental zeolite lattice constants excellently,49 which is the only experimentally available structural parameter for directly judging the quality of the optimization. The importance of using a dispersion-corrected XC potential during relaxation has been highlighted by Ashbrook et al.50 The zeolite is in its H-form during the NMR experiments and hence the acid site could be considered to be protonated. However, bearing in mind that the samples are hydrated during measurement, the protons are solvated in water clusters. This was modelled by removing the proton from the acid site in the calculations, and the charge during the relaxation is hence -1 per acid site. This model has proven successful in previous computational studies,9,21 primarily because it has been observed experimentally that the type of counter ion does not appreciably affect the 27Al isotropic chemical shift in hydrated ZSM-5.9 In this work, a homogeneous positive background (cf. Computational Details) constitutes the counter charge. The charge of the Al site is distributed mainly on the second layer of oxygen, i.e., the third coordination sphere.7 This implies that in a periodic model at least four layers of Si-O (henceforth named Si layers) are required between Al sites to ensure that



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charges are not centered around the same Si atom, which would affect the structures of the sites. An illustration of the Si layers is shown in Figure S2, where three clusters have been constructed, containing one, two and three layers of Si-O around a central Al site. The ZSM-23 unit cell has dimensions of 5.3 Å x 22.1 Å x 11.4 Å and contains 24 T-atoms. Three different supercell sizes have been tested during relaxation. The smallest supercell contains 48 T-atoms (Si/Al = 47) and is a 2 x 1 x 1 repetition of the unit cell. In this supercell there are only three Si atoms between each Al atom in the shortest direction. The medium supercell contains 72 Tatoms (Si/Al = 71) and is a 3 x 1 x 1 repetition of the unit cell resulting in four Si atoms between each Al T-site in the shortest direction. The largest supercell contains 144 T-atoms (Si/Al = 143) in a 3 x 1 x 2 repetition of the unit cell, providing five Si between each Al T-site in the shortest direction. Figure 2 shows the resulting calculated 27Al isotropic chemical shifts of all T-sites in the MTT structure using the three different supercells. The relative as well as the absolute shifts clearly depend on the size of the supercell. The T1 – T6 shifts seem converged at the medium supercell size. However, the medium supercell results in a quite different chemical shift of the T7 site in comparison to the large supercell. The effect of the XC potential used for the relaxation on the resulting 27Al isotropic chemical shifts has been examined by relaxing the SSZ-13 structure with a 36 T-atom cell using BEEF-vdw, PBE and PBE+D2, all with a plane wave cut off of 700 eV. The resulting referenced chemical shifts using two Si layers and the pcsSeg-2 basis set, varied only 0.7 ppm between the calculations. We finally note that mean Si-O-Al angles of the supercells in general become smaller with decreasing supercell size and closer to the experimental T-O-T angles (Figure S3). This variation in Si-O-Al angles provides an explanation for the deviation of the calculated 27Al isotropic chemical shifts of the different supercell sizes and indicates that it is important to perform the calculations on a cell with a Si/Al ratio close to the experimental value. It also demonstrates that (very) large supercells are required to fully converge the geometries.

Cluster size The cluster approach for calculations of chemical shifts is the most prevalent in the literature.8,9,21,25,27 Brouwer et al.51 performed

29Si

chemical shielding calculations on a pure

siliceous zeolite using both a periodic and a cluster approach. Their findings showed that a periodic


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approach gave only slightly better results than using a cluster approach. However, their cluster only contains a single Si layer (three coordination spheres) around the atom of interest, which is equivalent to the smallest clusters considered in this study. A cluster of a given size is extracted from the relaxed periodic structure of the zeolite for the calculation of NMR chemical shifts. Convergence tests were performed for cluster sizes in

29Si

chemical shift calculations for purely siliceous zeolites29 and it was shown that at least four coordination spheres around the investigated site should be used. In the present case, the clusters are charged and therefore require additional convergence tests. The dependencies of the calculated NMR shifts for each acid site in ZSM-23 with increasing cluster sizes are shown in Figure 3. Instead of coordination spheres, Si layers are used as a measure. Thereby, termination of Si atoms with H atoms is avoided and the outermost Si layer is terminated with OH-groups. A cluster of n Si layers is constructed by including n layers of Si around the Al site, and the n+1 Si layer is replaced by hydrogens with a bond length of 0.96 Å to terminate the outer oxygen layer. The OH bond length is chosen in agreement with previous studies.10,27 The last layer is constructed in a way that Si is not replaced by two hydrogen atoms, i.e., if a Si atom in the n+1 layer terminates more than one oxygen, it is added to the cluster and terminated at the next Si atom. The procedure is illustrated in Figure S4. The order and the relative

27Al

isotropic chemical shift of each T-site changes

significantly upon going from a one- to a two-layered cluster (Figure 3). A change from two to three Si layers maintains the order of the T-sites, whereas the relative chemical shifts change slightly. The two-layered Si clusters have been applied in the remaining calculations in this study, analogously to previous studies.22-25 Table S2 contains information on the Si/Al ratios of the 1-, 2- and 3-layered Si clusters.

Basis sets Different basis sets have been employed to calculate NMR chemical shifts of zeolites in earlier studies, including TZVP8 and 6-311+G(d,p)10. These two commonly applied basis sets are compared to the pcsSeg-2 basis set, which is specifically optimized for the calculation of nuclear magnetic shielding constants.52 The resulting 27Al isotropic chemical shifts of the individual T-sites for zeolite ZSM-23 are shown in Figure 4, using the XC potential BLYP and two layers of Si around each Al atom. Absolute 27Al isotropic chemical shifts change up to 2.0 ppm, depending on



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the used basis set. However, the relative 27Al isotropic chemical shifts are not affected significantly. The order of the sites does not change between TZVP and pcsSeg-2, only the difference in

27Al

isotropic chemical shifts for individual T-sites varies, in contrast to 6-311+G(d,p), where T2 and T4 are interchanged.

XC potential In earlier studies, both BLYP8, B3LYP10 and PBE26,53 have been applied in calculations of NMR chemical shifts. The effect of these XC functionals on the chemical shift calculations is examined in Figure 5. PBE is a General Gradient Approximation (GGA), while BLYP and B3LYP are hybrid functionals, which have an empirical mixture of the GGA and Hartree Fock exchange functionals and a GGA correlation part. The resulting 27Al isotropic chemical shifts for all T-sites in ZSM-23 are shown in Figure 5, relative to the shift of H-SSZ-13, employing the three different XC potentials, the basis set pcsSeg-2, and a 2-layered Si cluster. A comparison of the resulting

27Al

isotropic chemical shifts clearly shows that the chosen XC functional does not have a significant effect. The absolute chemical shifts are slightly lower for B3LYP, but the relative shifts are unchanged.

Periodic approach Calculations of the

27Al

chemical shieldings have also been performed on periodic structures

employing the Quantum ESPRESSO-GIPAW code38,45-47. The relaxed small (Si/Al = 47) and medium (Si/Al = 71) supercells described above were applied in these calculations, as these cells have Si/Al ratios approaching the experimental values (Si/Al = 25 and 37). The results for the large supercell, including 144 T-atoms, are not reported, as the calculations were computationally prohibitive. Figure 6 shows the resulting 27Al isotropic chemical shifts of the individual T-sites in zeolite ZSM-23 from the periodic GIPAW calculations. It should be notated that the variations of relative chemical shifts by using the small and the medium supercells are very similar to those observed by changing the supercell sizes in the cluster calculations (Figure 2).

Summary of the computational results


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The supercell size used for structure relaxation has by far the largest impact on the resulting 27Al isotropic chemical shifts. Hence, in zeolites with Si/Al ratios high enough for the Al sites to be considered isolated, large supercells are required for structure relaxation. As a consequence, the cluster approach is the only option for the calculations because periodical calculations for NMR parameters using the large supercell are computationally prohibitive. The extracted cluster used for calculating chemical shieldings should contain at least two layers of Si around the Al site. In the present study, the chemical shifts employing the TZVP and pcsSeg-2 basis sets would lead to the same assignment of the T-sites, whereas 6-311+G(d,p) changes the order of the T-sites in the assignment. Surprisingly, the XC potential used in the calculations of the

27Al

isotropic chemical

shifts has no significant impact on the results. As the Si/Al ratio of the zeolite decreases, the Al sites can no longer be considered isolated; our results indicate that this is already the case for the ZSM-23 samples with Si/Al = 23 and 37 considered in this work (vide infra). Therefore, the small supercell with Si/Al = 47 has to be employed to achieve similar Al T-site densities. Interestingly, we obtain similar chemical shifts from the periodic and the cluster approach based on a relaxed structure of the small supercell. Thus, it seems that the geometric structure, which is the same in the cluster and periodic approaches, is more important than the electronic structure, which is truncated in the cluster compared to the periodic approach. Thus, to have similar Si/Al ratios in experiment and theory, the

27Al

isotropic chemical shifts

calculated with the periodic approach, using Quantum ESPRESSO-GIPAW and the small supercells, are employed to assign the experimental 27Al isotropic chemical shifts of the T-sites in ZSM-23.

Experimental Results Referencing Compounds The chemical shift in 27Al NMR experiments is usually referenced relative to aluminum ions in an aqueous solution ([Al (H2O)6]3+). This primary reference has been used in previous computational work.10,21 However, we have decided to use solids as secondary reference, like several authors before us.8,9,23,23,26 We chose single T-site zeolites, aiming to limit systematic



Page 14 of 40

differences between experiment and computation by using references as similar as possible to the system we are interested in. As mentioned above, the H-SSZ-24 and H-SSZ-13 zeolites have been chosen as references in the present study. 27Al MQMAS NMR spectra of the samples H-SSZ-13 and H-SSZ-24 have been acquired at magnetic fields of 7.1 T and 14.1 T (Figure S5) to verify the presence of a single framework Al site and to obtain the corresponding

27Al

isotropic chemical

shifts, used as references for the DFT calculations. In addition, the quadrupolar product parameters are obtained for both samples from the shifts (𝛿𝐶𝐺 𝐹1 ) in the isotropic dimension of the MQMAS NMR spectra, according to eq. (1). The observed resonance for H-SSZ-13 reveals an isotropic chemical shift of 𝛿𝑖𝑠𝑜 = 59.8 ppm and a quadrupolar product parameter of 𝑃𝑄 = 1.4 MHz, which is in good agreement with literature values.9 The corresponding values for the T-site in H-SSZ-24 are 𝛿𝑖𝑠𝑜 = 55.2 ppm and 𝑃𝑄 = 2.1 MHz. A reference isotropic shielding, 𝜎𝑟𝑒𝑓, is used to determine the calculated isotropic chemical 50 shifts, 𝛿𝑐𝑎𝑙𝑐 𝑖𝑠𝑜 , to improve the accuracy of the referencing, as proposed by Ashbrook et al. The value

𝑒𝑥𝑝 for 𝜎𝑟𝑒𝑓 is obtained from a plot of calculated (𝜎𝑐𝑎𝑙𝑐 𝑖𝑠𝑜 ) and experimental chemical shifts (𝛿𝑖𝑠𝑜 ) for the

model systems, which are fitted with a linear regression, 𝛿𝑖𝑠𝑜 = (𝜎𝑟𝑒𝑓 ― 𝜎𝑐𝑎𝑙𝑐 𝑖𝑠𝑜 )/𝑚, where 𝜎𝑟𝑒𝑓 and m are the intercept and the slope, respectively. For the two model compounds in the present work, the approach gives the relation, 𝛿𝑖𝑠𝑜 = (594.77 ― 𝜎𝑐𝑎𝑙𝑐 𝑖𝑠𝑜 )/1.52. The value of 1.52 for the m coefficient (ideal value of 1.0) reflects that the experimental 27Al isotropic chemical shifts of the T-sites in HSSZ-13 and H-SSZ-24 differ by 4.6 ppm, whereas the calculated values differ by about 8 ppm.

ZSM-23 27Al

MQMAS NMR spectra of ZSM-23 samples A and B have been acquired at six different

magnetic fields (4.7 – 22.3 T, Figures 7 and S6) in order to identify different tetrahedral framework Al sites. The two MQMAS spectra acquired at the highest magnetic field (22.3 T and 16.4 T) show a narrow contour diagonal to the F1 and F2 dimensions without any significant broadening in the F2 dimension. The linewidths observed for slices throughout the contours in the anisotropic dimension are about 2 ppm and 3 ppm at these magnetic fields. This demonstrates minor influences of secondorder quadrupolar broadening in the F2 dimension at these high magnetic fields, since these linewidths correspond to centerband widths for quadrupolar product parameters of 3.1 and 2.8 MHz for magnetic fields of 22.3 and 16.4 T, respectively. This implies that the diagonal width of the


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contour is governed by resonances from a number of individual Al sites with different chemical shifts. In contrast, at lower magnetic fields the line shape of the resonances is increasingly affected by the second-order quadrupolar interaction resulting in off-diagonal signals. The individual signals of the tetrahedral framework sites have been identified by a careful analysis of the topographical 2D intensities in the F2 chemical shift range of 40 – 70 ppm. The different spectral slices in the F1 dimension were scanned and peak positions were determined by reading out where distinct signals clearly appear and disappear. This procedure provides the centers of gravity (𝛿𝐶𝐺 𝐹1 ) for the individual sites in the F1 dimension of the MQMAS NMR spectra. Using this approach for all 27Al MQMAS NMR spectra acquired at the different magnetic fields allows identification of five resonances from tetrahedral Al sites in each sample (Al(1) – Al(5)). However, the overlap of the resonances in the two 27Al MQMAS NMR spectra recorded at 7.1 T (Figures 7 and S6) is rather large and prevents a proper distinction between the individual sites. The detected centers of gravity in the F1 dimension of the 27Al MQMAS NMR spectra are indicated in Figures 7 and S6. In addition, Figure S7 shows simulations of the F1 slices at the actual chemical shifts of the identified sites in the 27Al MQMAS spectrum at 22.3 T of sample A and simulations of the 2D contour plots of the

27Al

MQMAS

spectra acquired at 16.4 and 22.3 T. These simulations support the identification of five distinct tetrahedral resonances in the present analysis. We note that attempts to analyze the spectra in terms of seven distinct Al sites, corresponding to the number of T sites in the ZSM-23 structure, were not successful and considered as an over-interpretation of the actual experimental data. The triplequantum chemical shifts of the detected tetrahedral sites are plotted as a function of the applied magnetic field (1/ 𝜐L2) in Figures 8 and S8, utilizing the relationship of eq. (1). This includes an indication of the range of triple-quantum chemical shifts at 7.1 T, where a proper identification of the T-sites could not be achieved as a result of a significant overlap of the resonances. For both samples, the 𝛿𝐶𝐺 𝐹1 values are fitted with five linear regression plots, employing the assignments that give the highest possible correlation coefficient, R2, (R2 ≥ 0.981). 27Al isotropic chemical shifts and quadrupolar product parameters were obtained from the intersections with the ordinate axes and the slopes of the linear regressions, respectively. The isotropic chemical shifts of the individual sites observed for the two samples differ by a maximum of 0.2 ppm and the quadrupolar product parameters are also very similar. This strongly suggests that the resonances originate from the same framework sites and that the same framework sites are occupied by Al in the two samples. Thus, for



Page 16 of 40

comparison with the calculate values, averages of the experimental iso and PQ parameters for the two samples are used, as listed in Table 1. The experimentally determined iso and PQ parameters are employed in the simulations of the single-pulse

27Al

MAS NMR spectra in Figure 9. A quantitative estimation of the relative

tetrahedral Al intensities was only possible by simulation of the spectra acquired at 22.3 T, since the overlap of the peaks is very large at the other magnetic fields. Thus, the intensities of the five distinct framework sites, determined at 22.3 T, were used as fixed values in the simulations of the spectra acquired at lower magnetic fields. The simulations show good agreements with the experimental centerbands for the framework Al sites for all magnetic fields, which supports the identification of five distinct resonances and the intensities obtained from the simulations of the spectra at the highest magnetic field. Furthermore, the simulations suggest a non-random distribution of Al atoms over the distinct T-sites and in addition, a different distribution for sample A as compared to sample B, which may reflect the different synthesis approaches. There is a slight preference for the occupation of Al sites (2) and (4) in sample A with relative intensities of approx. 22% and 25%, respectively. However, in sample B there is a clear preference of Al site (4) (about 35%), whereas Al sites (3) and (5) are less preferred, as indicated by the relative intensities of 12.5% and 10%, respectively. Despite the different crystal size of the two samples (sample A: 50 – 100 nm; sample B: 1 m), no effects of the crystal sizes could be detected in any of the spectra, and thus we trust that the appearance of the 27Al NMR spectra is not affected by the actual crystal sizes in the present case. The experimental

27Al

isotropic chemical shifts, determined as an average for

the two ZSM-23 samples (Table 1), are illustrated in Figure 10 along with the calculated

27Al

isotropic chemical shifts using the different approaches.

Discussion Figure 10 shows the DFT-calculated

27Al

isotropic chemical shifts performed on the small

supercell (left) and referenced to both H-SSZ-13 and H-SSZ-24. The chemical shift differences between the individual sites fit very well, however, the calculated 27Al isotropic chemical shifts are in general about 1.5 ppm lower than the experimental values. The primary source of error resulting in this offset may be the referencing of the calculated chemical shielding values. The coefficient m


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for the reference chemical shielding should ideally be 1.0. In this study, m = 1.52, which reflects a discrepancy between experimental and calculated chemical shifts/shieldings for the two reference compounds. As noted above, the calculated chemical shieldings for the two reference compounds differ by 8 ppm, whereas the experimental 27Al isotropic chemical shifts show a difference of only 4.6 ppm. Especially the calculated chemical shielding for Al in SSZ-13 seems to be slightly off. Thus, the calculations based on the relaxed, optimized structure for SSZ-13 may be the main source responsible for these deviations and hence the offset of the calculated chemical shielding values. Nevertheless, a plausible assignment is achieved, assuming that the T-site with the highest calculated chemical shift corresponds to the highest experimental isotropic chemical shift, thus adding an offset of 1.5 ppm while retaining the same order of the sites. This proposed assignment is listed in Table 2. In the calculations, the isotropic chemical shifts of T1 and T3 as well as of T2 and T4 are almost identical, and thus they are combined in the assignment to the T-sites Al(4) and Al(3), respectively. This suggests that either one of these tetrahedral sites T1/T3 and/or T2/T4 are not occupied by Al atoms, or that they are all occupied with overlapping signals in the resonances Al(4) and Al(3), making a distinction impossible. For a more detailed analysis, the experimental and calculated quadrupolar product parameters, PQ, could in principle be considered (Tables 1 and 2). However, the experimental PQ values are affected by the water molecules present in the samples, whereas calculations did not include water molecules in the structure. This leads to the conclusion that Al atoms occupy at least five of the seven distinct tetrahedral framework sites in the ZSM-23 framework, preferably located at the tetrahedral site T7 (the multiplicity for T7 is only half of T15). T1, T3, T5 and T6 are less preferred and T2 and T4 are the least preferred occupied T-sites. From each of the T-site pairs, T2/T4 and T1/T3, one of them may not be occupied at all. This assignment utilized the results from the periodic calculations using the small supercell in order to have similar Si/Al ratios in experiment and calculations.. Moreover, the good agreement between the calculated 27Al isotropic chemical shifts from the cluster and periodic approach based on the small supercell (Figure 10, left) supports this approach, also showing that the geometric structure seems to be more important than the electronic structure. Furthermore, the use of small supercells is computationally less demanding than the application of large supercells. However, there are no obvious means to verify that calculations on a small supercell are more accurate. The



Page 18 of 40

results will be sensitive to the specific choice of the cell size used for the structure optimization and GIPAW calculations. The second option for the assignment of the experimental data utilizes cluster calculations on a large supercell (Figure 10, right; Table S4), mimicking isolated Al T-sites for very high Si/Al ratio zeolites. After adding an offset of 3.32 ppm to the referenced values, a similar assignment to the small supercell approach can be achieved, however, the two assignments are not fully identical. Although it is likely to converge to stable values for very large cells, the Si/Al ratio will not match a “real” material since many zeolites crystallize only within a limited Si/Al ratio range (e.g. ZSM-23). Moreover, large supercells are computationally not feasible using the periodic approach. For comparison, the resonance assignments based on the calculated

27Al

isotropic

chemical shifts are also compared with the assignments obtained using the mean Si-O-Al or T-O-T bond angles and the correlation by Lippmaa et al.,16 which has been used in earlier studies of 27Al chemical shifts for zeolites.15,17,18 The mean Si-O-Al bond angles were obtained from the bond angles of the relaxed unit cell from the DFT-optimized structures of the frameworks (Table S3), using the corresponding supercell. Furthermore, a set of mean T-O-T bond angles (Table S3) were obtained from structure refinements of XRD powder patterns of the studied samples. The chemical shifts calculated from the mean Si-O-Al angles of the tetrahedral sites obtained from the DFTrelaxed structures, included in Table 1 and Figure 10, are about 4 to 7 ppm lower than the experimental values. The order in chemical shifts of the T-sites is nearly the same as the DFTcalculated values using the large supercell, however in the assignment of the five detected resonances different T-sites coincide and will thereby lead to a different assignment, as illustrated in Figure 10. The mean Si-O-Al bond angles from the small supercell does not result in any comparable values for the isotropic chemical shifts. This shows that the mean Si-O-Al bond angles of isolated Al T-sites from the DFT-relaxed structures can lead to a rough assignment of the signals, however this assignment is not in full agreement with the assignment based on the obtained

27Al

isotropic chemical shifts by DFT calculations. The same applies to the mean T-O-T angles obtained from refined XRD patterns, although the calculated values are here in the same range as the experimental chemical shifts. Again, this approach results in a different assignment, as compared to the one obtained from the isotropic chemical shifts by DFT calculations, and in larger chemical shift spacings in comparison to the experimental results. For Si-rich zeolites, the T-O-T angles obtained from the refinement of XRD data are strongly dominated by the contributions from the Si-O-Si


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bond angles, thereby suppressing effects from the Al incorporation. This fact may explain the smaller dispersion in the 27Al isotropic chemical shifts obtained from the T-O-T angles as compared to those from the DFT calculated Si-O-Al angles (Figure 10), and it suggests that these angles are not the optimum structural parameter for assignment of

27Al

isotropic chemical shifts in silicious

zeolites.

Conclusions The aim of this study has been to identify distinct tetrahedral Al framework sites in zeolite ZSM23 by solid-state

27Al

NMR spectroscopy and assign them to the distinct tetrahedral sites in the

ZSM-23 structure based on DFT calculations of 27Al isotropic chemical shifts. It is found that the structural model is the most important parameter in the application of DFT calculations of NMR parameters on zeolites, in agreement with results for 29Si chemical shift calculations.28 Based on the examined cluster and supercell sizes for the Al environments in ZSM-23, we propose two options for calculations of

27Al

isotropic chemical shifts for zeolites. For intermediate/low Si/Al ratio

zeolites, it is recommended to use a fully periodic approach on a supercell that achieves agreement between experimental and theoretical Si/Al ratios. For the investigated ZSM-23 samples, a small supercell (2x1x1 repetition of the unit cell) containing 48 T-atoms was applied. For very high Si/Al ratio zeolites, the cluster approach using a large supercell for relaxation is preferred in order to simulate isolated Al T-sites. To meet the requirements of an isolated Al T-site, it is concluded that each tetrahedral Al site has to be separated by at least four Si layers in the supercell. For ZSM-23, this leads to a 3x1x2 repetition of the unit cell, including 144 T-atoms. Moreover, it is found that the applied cluster size in the calculations of the chemical shielding performed with Gaussian is very important. It is recommended that the zeolite structure should be relaxed with a large unit cell and the subsequent calculations of chemical shielding should be performed on an extracted cluster model consisting of at least two Si layers. An extensive study of two zeolite H-ZSM-23 samples, synthesized by different approaches, with similar bulk Si/Al ratios has been performed by the application of 27Al MQMAS NMR experiments at six different magnetic field strengths (4.7 – 22.3 T). Five tetrahedral Al framework sites with very similar NMR parameters have been identified in both samples showing that the same T-sites



are occupied by Al atoms in the two samples. Simulations of single-pulse

Page 20 of 40

27Al

NMR spectra

acquired at an ultrahigh field of 22.3 T, employing the determined NMR parameters, reveal that the two samples have a different and non-random distribution of Al over the distinct T-sites. A possible reason for this variation may be the application of different structure-directing agents in the synthesis of the samples. An assignment of the

27Al

isotropic chemical shifts for the five identified resonances from

tetrahedral Al sites has been achieved by comparison of the experimental and calculated

27Al

isotropic chemical shift NMR parameters. An offset of about 1.5 ppm had to be included for the calculated and referenced chemical shifts from periodic calculations using the small supercell to achieve nearly full agreement between experimental and calculated data. This indicates a systematic deviation of the computational model, which may be remedied by taking into account the effect of finite temperature on the zeolite structure.50 We note that the results from the cluster calculations using the large supercell (isolated Al T-site approach) lead to a slightly different assignment of the experimental data. Therefore, the choice of the approach and variables are crucial for the resulting calculations. For obtaining the most accurate results, in our view there is yet no objective criterion to select from. Hence, the computational methodology recommended here may be further improved to reach an unambiguous procedure for NMR resonance assignment of Al T-sites in zeolites.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Synthesis conditions for SSZ-13 and SSZ-24; table including unit cell parameters from unit cell optimizations; table including Si/Al ratios of the cluster models; table including Si-O-Al and T-O-T bond angles from DFT calculations and XRD analysis; table including an optional assignment based on cluster calculations using the large supercell; powder XRD patterns and structural refinements for sample A and B; illustration of clusters of different sizes; comparison of T-O-T bond angles from experiment and calculations; OH-termination procedure in the cluster modelling; 27Al

MQMAS NMR spectra (7.1 T, 14.1 T) for the samples H-SSZ-13 and H-SSZ-24;

27Al

MQMAS NMR spectra acquired at six different magnetic fields for sample B; 27Al MQMAS NMR spectra (16.4 and 22.3 T) for sample A, including the deconvolution of the five identified T-sites (22.3 T) and simulated contour plots; linear regression analysis of the 3Q-shifts from the

27Al

MQMAS NMR spectra of sample B.

Author information Corresponding authors Tel: +45 8715 5946; E-mail: [email protected] (J. Skibsted). Tel: +45 4191 8352; E-mail: [email protected] (R.Y. Brogaard). Tel: +47 2285 5454; E-mail: [email protected] (S. Svelle).

ORCID Julian Holzinger: 0000-0002-0530-8413 Pablo Beato: 0000-0003-0261-6944 Rasmus Yding Brogaard: 0000-0001-7882-9183 Carlo Buono: 0000-0002-5125-1868 Michael Dyballa: 0000-0002-8883-1145 Jørgen Skibsted: 0000-0003-1534-4466 Stian Svelle: 0000-0002-7468-5546



Page 22 of 40

Notes The authors declare no competing financial interest.

Acknowledgement Financial support was received via the European Industrial Doctorates project “ZeoMorph” (Grant Agreement No. 606965), part of the Marie Curie actions (FP7-PEOPLE-2013-ITNEID). The authors thank the Norwegian High Performance Computing program for a generous grant of computing resources under project no. nn4683k and the staff at the USIT center for support. The use for the facilities at the Laboratory for Solid-State NMR of Inorganic Materials, the Department of Chemistry, Aarhus University, sponsored by the Danish Research Councils and the Carlsberg Foundation (CF14-0138), is acknowledged. We thank for access to the 950 MHz NMR spectrometer at the Danish Center for Ultrahigh-Field NMR Spectroscopy (Ministry of Higher Education and Science grant AU-2010-612-181). Additionally, Andrea Molino and Katarzyna A. Łukaszuk are acknowledged for the synthesis of the H-ZSM-23 samples.


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(13) Pashkova, V.; Sklenak, S.; Klein, P.; Urbanova, M.; Dědeček, J. Location of Framework Al Atoms in the Channels of ZSM-5: Effect of the (Hydrothermal) Synthesis. Chem. – Eur. J. 2016, 22, 3937-3941. (14) Hu, J.Z.; Wan, C.; Vjunov, A.; Wang, M.; Zhao, Z.; Hu, M.Y.; Camaioni, D.M.; Lercher, J.A. MAS NMR Studies of HBEA Zeolite at Low to High Magnetic Fields. J. Phys. Chem. C 2017, 121, 12849-12854. 27Al

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Table 1 Calculated and experimental 27Al isotropic chemical shifts and quadrupolar product parameters for SSZ-13, SSZ-24 and the two ZSM-23 samples. Calculated results obtained from mean Si-O-Al and T-O-T bond angles and DFT calculations. DFT, periodic approach

DFT

XRD

(small supercell)

structure

structure

𝛿𝑖𝑠𝑜 zeolite

T-site

𝜎𝑖𝑠𝑜

𝛿𝑖𝑠𝑜

𝑃Q

A Combined Computational and Experimental 27Al NMR Study

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