Probing Disorder in Al-ZSM-5 Zeolites by 14N ... - ACS Publications

Article pubs.acs.org/JPCC

Probing Disorder in Al-ZSM‑5 Zeolites by

14

N NMR Spectroscopy

Eddy Dib,† Tzonka Mineva,† Philippe Gaveau,† Emmanuel Véron,‡ Vincent Sarou-Kanian,‡ Franck Fayon,‡ and Bruno Alonso*,† †

Institut Charles Gerhardt de Montpellier, ICGM-MACS, UMR 5253 CNRS-ENSCM-UM, 8 rue de l’Ecole Normale, 34296 cedex 5 Montpellier, France ‡ CNRS, CEMHTI UPR 3079, Université d’Orléans, F-45071 Orléans, France S Supporting Information *

ABSTRACT: 14N solid-state NMR spectroscopy is used to investigate and quantify the nanometer scale disorder promoted by Al/Si substitution in ZSM-5 zeolites. After a preliminary characterization by SEM, XRD, and multinuclear (1H, 13C, 19F, 27Al, 29Si) solidstate NMR, the 14N MAS NMR spectra of a series of as-synthesized ZSM-5 zeolites containing various amounts of Al are analyzed. The 14N spinning sideband patterns are shown to evolve with the Si/Al ratio. The modeling of the NMR spectra allows one to estimate the local disorder arising from the Al site distribution within the tetrahedral sites of the zeolites, the variations of F locations, and the presence of silanol defects. The influence of the zeolite framework modifications due to Al/Si substitution on 14N NMR parameters is discussed on the basis of the results obtained with the Density Functional Theory periodic quantum chemical calculations augmented with an empirical London dispersion term. Analysis of the results highlighted the influence of CNC angle variations on the 14N quadrupole coupling constant distributions.

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of its high industrial interest3,15,16 and its inherent topological complexity.17 ZSM-5 zeolites are composed of interconnected AlO4 and SiO4 tetrahedra forming the MFI topology. The negative charges of AlO4 tetrahedra within the framework are balanced by the positive charge of the structure directing agent (SDA) used in the synthesis, which is often tetrapropylammonium (TPA).18 The isoelectronic Al3+ and Si4+ cations cannot be easily distinguished using X-ray diffraction, making determination of their precise localization in the framework challenging.19 Then, several attempts were made previously to describe the Al/Si disorder using mainly 27Al NMR methods, and valuable information was already obtained.20−22 In this work, we show that the local disorder in assynthesized ZSM-5 zeolites can be studied and quantified using 14 N NMR. Nitrogen-14 being a quadrupolar nucleus located at the center of the organic SDA, can provide detailed structural and dynamical information on the local order inside zeolite frameworks as recently shown for pure silica AST23 and MFI (silicalite-1) synthesized through different routes.24,25 This structural information is obtained through variations of the 14N quadrupolar interaction resulting from the coupling between the electric quadrupolar moment of the nitrogen-14 nucleus (spin I = 1, natural abundance of 99.6%) and the electric field gradient (EFG) produced by the surrounding charge distributions (electrons, nuclei). The quadrupolar coupling parameters (coupling constant, CQ; asymmetry parameter, ηQ),

INTRODUCTION Although the spatial positioning of atoms in crystals is governed by symmetry operations, ideal symmetric arrangements are rarely observed and local disorder might always exist. The longrange order in materials is well described by X-ray, neutron, and electron diffraction methods; however obtaining information about the short-range local order using these techniques is more difficult due to the intrinsic long-range and dynamical averaging, which may induce uncertainties in atomic positions.1 Zeolites are microporous silicon-based crystalline materials known for their wide applications in adsorption, ion exchange, and heterogeneous catalysis. Their properties are due to their finely tunable selectivity and activity, arising from the geometry and size of pores and crystals as well as the concentration and distribution of heteroatoms (Al, B, Ga, Ti, etc.) within the frameworks.2 Obtaining precise information about the spatial distribution of these elements in zeolitic tetrahedral sites is of paramount importance to understand and tune the final properties.3−12 Insights on the atomic local ordering or disordering (at the unit cell scale), which are hardly obtained using diffraction methods, are fortunately recovered by local spectroscopies such as NMR, one of the most used for this purpose nowadays.13 In silicon-rich zeolites, heteroatoms substitute a small fraction of the silicon atoms and are more or less randomly distributed, especially when several tetrahedral sites are available. At the scale of the crystal cell, they are considered as a source of disorder that broadens the NMR resonance thereby decreasing the spectral resolution.14 The ZSM-5 zeolite (Zeolite Socony Mobil Five, MFI topology) is one of the most interesting case systems because © XXXX American Chemical Society

Received: May 19, 2017 Revised: July 6, 2017 Published: July 7, 2017 A

DOI: 10.1021/acs.jpcc.7b04861 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C which define the width and the shape of the 14N resonances, might thus provide insights about the charge distribution at an intermediate length scale, typical of the pore dimension. In this study, we apply inter alia 14N solid-state NMR to probe the disorder in a series of ZSM-5 zeolites.

PerkinElmer apparatus under air with a 313−1173 K temperature ramp and a heating rate of 10 K·min−1. 1 H NMR. 1H NMR MAS single pulse spectra were recorded at ν0(1H) = 600.1 MHz on a Varian 600 spectrometer using 1.2 mm rotors spun at νMAS = 60 kHz. The π/2 flip angle (corresponding to a pulse duration of 2 μs) and recycling delays of 1.2 s were employed (enough to ensure proper longitudinal relaxation). Chemical shifts were referenced to external neat TMS. 13 C NMR. 13C{1H} NMR cross-polarization (CP) MAS spectra were recorded at ν0(13C) = 75.5 MHz on a Varian 300 spectrometer using 7.5 mm rotors spun at νMAS = 5 kHz. Experiments were conducted using a contact time τc = 5 ms, a linear ramp on the H contact pulse (10% slope), proton decoupling (CW) during acquisition (1H RF field strengths νRF = 50 kHz), and a recycle delay of 5 s. Chemical shifts were referenced to external neat TMS. 29 Si NMR. 29Si{1H} NMR CP-MAS spectra were recorded at 29 ν0( Si) = 59.6 MHz on a Varian 300 spectrometer using 7.5 mm rotors spun at νMAS = 5 kHz. Experiments were conducted using a contact time τc = 15 ms, a linear ramp on the H contact pulse (10% slope), proton decoupling (CW) during acquisition (1H νRF = 50 kHz), and a recycle delay of 5 s. 29Si{19F} CPMAS spectra were recorded at ν0(29Si) = 119.2 MHz on a Varian 600 spectrometer using 3.2 mm rotors spun at νMAS = 5 kHz, a contact time τc = 2 ms, a linear ramp on the F contact pulse (10% slope), proton decoupling (CW) during acquisition (1H νRF = 50 kHz), and a recycle delay of 5 s. Chemical shifts were referenced to external neat TMS. 19 F NMR. 19F Hahn echo MAS NMR spectra were recorded at ν0(19F) = 799.9 MHz on a Bruker Avance III 850WB spectrometer using 2.5 mm rotors spun at νMAS = 20 or 30 kHz. The echo delay was set to two-rotor periods and the 19F nutation frequency was 154 kHz (90° pulse length of 1.62 μs). A recycling delay of 10 s was employed, and 19F chemical shifts were referenced to neat CFCl3. 27 Al NMR. 27Al NMR MAS single pulse spectra were recorded at ν0(27Al) = 221.5 MHz on a Bruker Avance III 850WB spectrometer using 2.5 mm rotors spun at νMAS = 20 or 30 kHz. A pulse duration of 0.338 μs corresponding to a flip angle of π/18 and a recycling delay of 0.5 s were employed. We have verified that the 27Al NMR spectra are not influenced by hydration or dehydration processes. Chemical shifts were referenced to a saturated Al(NO3)3 solution (1 mol/L). The two-dimensional (2D) z-filtered multiple-quantum 27Al MQMAS experiments30,31 were performed at a spinning frequency of 30 kHz, using triple−quantum (3Q) excitation and reconversion pulse lengths of 4 and 1.25 μs, respectively (νRF = 98 kHz). The z-filter duration was set to one rotor period (33 μs), and the duration of the central transitionselective pulse was 13 μs (νRF = 8.6 kHz). A total of 45 rotorsynchronized t1 time increments32 of 66.6 μs (two rotor periods) were collected with a recycle delay of 1s. 14 N NMR. 14N NMR MAS single pulse spectra were recorded at ν0(14N) = 43.3 MHz on 600 Varian spectrometer using 9.5 mm rotors spun at νMAS = 2, 3, and 4 kHz. RF field strengths, νRF, were set to ca. 42 kHz, flip angles to π/4 (6 μs, bandwidth ∼85 kHz), and recycling delays to 0.5 s. Smaller flip angles were also tested (down to 2 μs, bandwidth ∼250 kHz). These conditions ensure a wide irradiation and proper longitudinal relaxation. The initial FID points contaminated by signal distortion were removed (left shift) such that Fourier transform started at the top of the first rotational echo

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METHODS Synthesis. ZSM-5 samples of the first series (A0−A5) were obtained in fluoride media from the following procedure adapted from previous works.26,27 Typically, 2.22 g of NH4F (Alfa Aesar, 98% pur.) were dissolved in 40 g of water in the presence of a certain amount of Al(OH)3 (varied so as to reach the desired Si/Al ratio) and 2.22 g of tetrapropylammonium bromide, TPABr (Aldrich, 98% pur.). After a complete dissolution, 4 g of fumed silica (Sigma, 99.8% pur.) was added slowly, and the mixture was homogenized with a spatula. The mixture was heated in a PTFE coated autoclave during 4 days at 175 °C. The obtained powder was filtered and washed abundantly with deionized water and dried at 80 °C overnight. Molar ratios: 1SiO2/yAl(OH)3/0.14TPABr/1NH4F/33H2O; y varies here according to Si/Al molar ratios in the gel (Table 1). Table 1 Si/Ala A0 A1 A2 A3 A4 A5

∞ 196 199 149 54 26

B0 B1 B2 B3 B4

∞ 276 245 162 30

Si/Fa 1st ZSM-5 series 31 27 34 39 49 99 2nd ZSM-5 series 26 31 32 36 39

Al/ucb

F/ucc

0.00 0.49 0.48 0.64 1.75 3.59

3.14 3.65 2.79 2.47 1.94 0.94

0.00 0.35 0.39 0.59 3.06

3.72 3.06 3.01 2.62 2.39

a

Atomic ratios from chemical elemental analyses. bAverage number of Al atoms per unit cell estimated from chemical elemental analyses using nAl/(nSi + nAl) × 96. cAverage number of F atoms per unit cell estimated from chemical elemental analyses using nF/(nSi + nAl) × 96.

For the second series of ZSM-5 samples (B0−B4), we have modified the source of Al (nitrate), the temperature (200 °C), and the duration (15 days) of the hydrothermal treatments, and the final molar ratios were 1SiO 2 /yAl(NO 3 ) 3 ·9H 2 O/ 0.08TPABr/0.04NH4F/20H2O. Other parameters are identical including the quantity of fumed silica (4 g). Characterization Details. Laboratory X-ray diffraction (XRD) was used to control the purity and determine the lattice parameters of the as-synthesized ZSM-5 zeolites depending of Al/Si ratio. Measurement were performed on a D8 Advance Bruker Bragg−Brentano diffractometer (Cu K1,2 radiation) equipped with a LynxEye XE detector. From powder diffraction data, cell parameter refinements were performed by the Lebail method28 using the Jana2006 software.29 Scanning electron microscopy (SEM) was undertaken using a HITACHI 4800 S microscope. The chemical analysis was undertaken at Service Central d’Analyses (CNRS, Solaize, France). The fluorine concentration was determined using a specific electrode. Thermogravimetric analyses were performed on a B


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The Journal of Physical Chemistry C maximum. 1H CW decoupling (νRF ≈ 25 kHz) was used during 14 N acquisition. For νMAS = 3 and 4 kHz, the number of acquired scans was in the 5000−10000 range; for νMAS = 2 kHz, the range was 10000−100000. 14N NMR chemical shifts were referenced toward solid NH4Cl spun at 3 kHz. For the magic angle setting, we use the 14N NMR signal of tetramethylammonium bromide as standard. All MAS frequencies were stable within 1−2 Hz. NMR Spectrum Modeling. 14N NMR spectra were fitted using a linear combination of two functions allowing us to model a single set of quadrupolar parameters (CQ, ηQ) and a distribution of quadrupolar parameters (following the Gaussian isotropic model within the Czjzeck model).33,34 These functions are implemented in the freely available DmFit software,35,36 and named QUAD1st and Q1-Cz respectively. The quadrupolar interaction is described at the first order (enough for the systems under study). The 2D 27Al MQMAS spectrum is fitted using mathematical functions that describe the distribution in the quadrupolar coupling parameters and the related EFG through the Czjzeck model (Gaussian Isotropic Model).33,34 1D 27Al spectra are fitted using either Czjzeck or pseudo-Voigt functions. 19F spectra are fitted using pseudo-Voigt functions. All the functions are available on the DmFit program above-mentioned. DFT-D Calculations. The initial crystallographic structure of the ZSM-5 zeolite was taken from available crystallographic data.37 The geometrical optimizations (atomic positions and unit-cell parameters) and NMR parameter calculations were carried out with the periodic ab initio Crystal09 program, based on atom-centered Gaussian orbitals.38 The computational method used for the present calculations is based on DFT, augmented with a pair wise addition of a London dispersion energy term (DFT-D) as implemented in Crystal09 program.39 Basis functions of double-ζ quality were used to describe all atoms as follows: 6-31d1 for O, N, C, and H,40 85-11G* for Al,41 and Pople’s basis set with polarization for Si.42 The generalized gradient-corrected PBE approximation was used as the exchange correlation (XC) functional. The nuclear quadrupolar interaction is described by the quadrupolar coupling constant, CQ, and the asymmetry parameter, ηQ, defined by the principal values, Vii, of the electric field gradient tensor (EFG) as follows: CQ = eQV33/ℏ and ηQ = (V11 − V22)/V33 where ℏ is Planck’s constant and Q is the nuclear quadrupolar moment (Q = 2.04 fm2 for 14N).43,44 The three principal values of the EFG tensor are defined with the following convention: |V33| > |V22| > |V11|. Computation of parameters such as CQ and ηQ therefore only requires calculating the components of the EFG tensor. It is worth noting that EFG is a property of the ground-state electronic wave function and that, unlike the chemical shift or the indirect spin−spin coupling, its computation does not require knowledge of any excited states. Thus, the quadrupolar NMR parameters can be computed with greater time efficiency, and accuracy, which will be basically determined by the accuracy of the electronic structure method and geometry optimization. In addition to the EFG tensor obtained by DFT-D, computations based on the so-called point charge model (PCM) have been carried out with the aim to distinguish between the influence of the Al position and the cation molecular structure on the 14N NMR quadrupolar coupling parameters. In the PCM, only the ion charges in their positions in the crystal lattice are considered, and the EFG tensor elements at the 14N nucleus are obtained from the equation:

n

VijPCM =

∑ k=0

⎞ eqk ⎛ 3xikxjk ⎜ 2 − δij⎟ 3 4πε0rk ⎝ rk ⎠

where i,j = 1, 2, 3; the sum is over all n ions with charges eqk and coordinates (x1k, x2k, x3k), distant at rk from the central nucleus N; ε0 is the vacuum dielectric constant. The EFG eigenvalues are obtained after diagonalization of the tensor in the principal system axis.45 Finally, the contribution of only the TPA molecular structure to the 14N quadrupolar coupling was also obtained considering isolated TPA molecules with the geometries that were optimized in the zeolite crystals. To this purpose, the cluster DFT approach using the deMon2k computer program46 was employed with the same XC functional and atomic bases, reported above.

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RESULTS AND DISCUSSION Preliminary Characterizations. We have synthesized a first series of ZSM-5 zeolites in fluoride medium by varying the Si/Al ratio in the initial gel (samples A0−A5). It is worth noting that the fluorine route was used to synthesize highly crystalline samples and to minimize the occurrence of additional silanolate and silanol defects, our main goal being to quantify the disorder coming from aluminum insertion only. Table 1 presents the Si/Al and Si/F molar ratios obtained by chemical analyses for the as-synthesized zeolites. Similar values were obtained by EDX (Table S1), and no significant Al zoning at the micrometer scale is therefore suspected in the series of samples. As expected from charge balance, we notice that the quantity of fluorine decreases when the content of aluminum increases. For each composition, we always observe by SEM a single population of micrometric crystalline particles with hexagonal prism morphology (see Figure 1 and Figure S1). Therefore, Al

Figure 1. SEM images (left) showing the typical crystal morphologies and corresponding X-rays diffractograms (right) of the samples A0 and A5.

and F atoms are certainly both present in the crystallites and both involved in the compensation of the positive charge of the SDA. When increasing the Al content, we notice a gradual decrease of the length/width aspect ratio and a modification in the shape of the particles, probably due to changes in the rates of crystal growth. The XRD patterns of the series of samples (Figure 1 and Figure S2) can all be assigned to as-synthesized ZSM-5 crystals C


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The Journal of Physical Chemistry C

Figure 2. 29Si NMR spectroscopy. (a) 29Si{1H} CP-MAS spectra for the series of as-synthesized zeolites (ν0(29Si) = 59.6 MHz, νMAS = 5 kHz). (b) 29 Si{19F} CP-MAS spectra of the A0 and A3 as-synthesized zeolites (ν0(29Si) = 119.2 MHz, νMAS = 5 kHz). The related 29Si{1H} CP-MAS spectra are presented with dashed lines for comparison.

with a Pn21a space group.37 No additional diffraction peaks coming from other phases or scattering peaks associated with amorphous phases were detected. The unit cell (uc) of assynthesized ZSM-5 (Pn2 1 a space group) contains 96 tetrahedral sites. It also accommodates here four TPA cations as verified by TGA (Figure S3). The number of Al and F per uc has been estimated for each sample from molar ratios (Table 1). The values agree roughly with (Al + F)/uc ≈ 4. The mismatches can be explained by the presence of impurities or defects like silanolates/silanols (cf. the peak at 10.2 ppm in 1H NMR spectra in the Figure S8) and by experimental errors made in the determination of Al and F contents. We notice also an increase in the cell volume when the Al content increases (∼12 Å3 per Al incorporated in the unit cell, Figure S2) in agreement with previous studies (∼13 Å3 per Al incorporated in the unit cell).47 These variations in crystal cell sizes, which can be explained by the difference between Al and Si atomic radii (1.18 and 1.11 Å respectively, difference in volume of about 1.15 Å3) and by the modifications induced in the bonding schemes and intermolecular interactions, provide evidence for the gradual Si/Al substitution inside the ZSM-5 structure. Multinuclear NMR Study. The 29Si{1H} CP-MAS NMR spectra depicted in Figure 2a show a gradual modification upon Al insertion with the appearance of 29Si signals related to Si tetrahedra bound to Al tetrahedra (δ range: −100 to −105 ppm) and the disappearance of the SiO4/2F− species (δ ≈ −125 ppm), accompanied by a broadening of the 29Si resonances likely due to SiOSi angular variations. This evolution of the 29Si NMR spectra with the Si/Al was observed previously on calcined zeolites, including ZSM-5 and discussed in terms of local disorder due to a nonordered distribution of Al sites.48−50 Spectral edition of the Si atoms in the vicinity of F atoms was achieved using 29Si{19F} cross-polarization (Figure 2b). The similarity between the 29Si{19F} CP MAS spectra obtained for

two samples containing or lacking Al (A0 and A3) shows that even in the presence of Al, the fluorine environment existing in the silicalite-1 (F atoms in the [415262] cage)51 can be preserved. However, the observed broadening of these 29Si peaks when the Al content increases also reflects a disordering effect in the vicinity of F atoms, which is also evidenced in 19F NMR spectra (vide inf ra). Gradual variations are also observed in the 27Al NMR spectra recorded at very high magnetic field (20 T) when the Al percentage in the zeolite framework is increased (Figure 3). The isotropic lines of the 27Al central transition can be assigned to Al tetrahedra sites inside the ZSM-5 framework.21,22,27 To increase the spectral resolution, we have recorded in parallel a 2D 27Al 3Q-MAS spectrum on the sample with the lowest Si/Al ratio (A5, Si/Al = 26) (Figure S4). The obtained 2D spectrum

Figure 3. 27Al NMR spectra for the series of as-synthesized zeolites (ν0(27Al) = 221.5 MHz, νMAS = 20 or 30 kHz). D


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Figure 4. 19F MAS NMR spectra for the series of as-synthesized zeolites (ν0(19F) = 799.9 MHz, νMAS = 20 or 30 kHz). (a) Full spectra showing spinning sidebands (*) and extra peaks for the more aluminated zeolites corresponding to fluorides of ammonia, silica, and alumina. (b) Expansion of the center band region (−55/−85 ppm range) and corresponding fits using three pseudo-Voigt functions. The G/L ratios are 0.5 for F1 (at −63.5 ± 1 ppm), 0.8 for F2 (at −64.5 ± 1 ppm), and 1.0 for F3 (at −79.0 ± 1 ppm). For each sample, the experimental spectrum is displayed on the top, the reconstructed spectrum on the middle, and the individual lines at the bottom.

and −90 ppm) becomes almost negligible for this sample although not null. Interestingly, for all the 19F spectra, the observed peak at −64 ppm is not totally symmetric. Moreover, its shape evolves with the percentage of the Al incorporated in the framework (Figure 4b). The introduction of aluminum in the zeolite framework leads therefore to a slight variation in the fluorine environment. To our knowledge, this has not been observed before. Focusing on the −60/−90 ppm region (Figure 4b), the spectra can be reconstructed using three pseudo-Voigt functions corresponding to three distinct signals: F1 at −63.5 ± 1 ppm, F2 at −64.5 ± 1, and F3 at −79.0 ± 1 ppm. The relative areas of these signals evolve differently with increasing Al percentage (Figure S6). The 13C{1H} CP-MAS spectra show the typical peaks of TPA in ZSM-5 (Figure S7).56 A broadening of these peaks with the increase in Al content can also be noticed specially for the Cγ (δ ≈ 10−11 ppm) and Cα (δ ≈ 65 ppm) signals. This can be related to a distribution of molecular conformations compared to the F-MFI where a unique conformation occurs as recently shown.24,25 Besides, the 1H NMR spectra show three main peaks located at 1.0, 1.8, and 3.2 ppm corresponding to the γ, β, and α protons of the TPA molecule, respectively (Figure S8). Other smaller peaks appear also at ∼10 ppm. These latter ones correspond to silanolate/silanol (Si−O−···HOSi) defects that may play a role in the compensation of the TPA charge as well, but their percentage is lower than 1%. All of these negative charges may participate differently in the electric field gradient (EFG) at the 14N site (vide inf ra). Determination of a Local Degree of Order by 14N NMR Spectroscopy. The 14N NMR spectra for the series of as-synthesized ZSM-5 are presented in Figure 5. We can observe that the envelope of the spinning side bands (SSBs) evolves gradually with the Si/Al ratio. While for the sample A0 (without Al), the SSB envelope resembles a spectrum produced

reveals the presence of at least three distinct 27Al resonances exhibiting distributions of 27Al isotropic chemical shifts, as indicated by the spread along the isotropic chemical shift axis (i.e., the diagonal of the 2D map). Cross sections along the MAS dimension also indicate a distribution of quadrupolar couplings and the 2D spectrum was modeled considering a Gaussian distribution of 27Al isotropic chemical shifts and a Czjzeck distribution (Gaussian Isotropic Model) of the quadrupolar parameters. The resulting average values of CQ(27Al) are relatively low (between 1.3 and 1.8 MHz), and in the 1D MAS spectra obtained at very high magnetic field, each 27Al resonance shows a pseudo-Voigt line shape with a broadening dominated by the distribution of the isotropic chemical shift. All 27Al 1D MAS spectra can be modeled with three signals using pseudo-Voigt (G/L = 0.5) functions. The relative intensities of these three 27Al signals (located at 51.5, 54.0, and 55.4 ± 0.2 ppm) vary gradually with Si/Al (Figure S5) suggesting the existence of specific variations in the distribution of Al sites over the 24 possible tetrahedral sites. This trend, which cannot be analyzed more precisely here, is consistent with other 27Al NMR data reported for Al-ZSM-5 zeolites.21,22 19 F NMR spectra recorded at high magnetic field are presented in Figure 4a. They show a main peak at −64 ppm assigned to fluorine atoms in the [415262] cage51 and a secondary one at −80 ppm assigned to fluorine with a slightly different environment that might be related to structure defects as discussed earlier.52 For the sample containing the lowest amount of fluorine (A5, with Si/F = 99), we observe extra peaks in the −115/−170 ppm range coming from secondary products assigned to fluoride salts of ammonia, silica, and alumina53−55 that have not been completely removed by filtration and washing. It is worth mentioning that the amount of F atoms inside this ZSM-5 zeolite (19F peaks between −60 E


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Figure 5. 14N NMR spectra for the series of as-synthesized zeolites (ν0(14N) = 43.3 MHz, νMAS = 2 kHz).

Figure 6. 14N spectrum modeling approach exemplified here for sample A0 (ν0(14N) = 43.3 MHz, νMAS = 2 kHz).

by one single electric field gradient (EFG) tensor interacting with the quadrupole moment of nitrogen-14, the spectrum of the sample A5 with the lowest Si/Al ratio (Si/Al = 26) can be solely modeled using a distribution of EFG. This distribution is the indirect signature of the distribution of local environments of which the consequences are also visible through the broadening of the 29Si and 19F peaks (vide supra). Being aware of the sensitivity of 14N quadrupolar parameters toward the spatial distribution of charges in its surroundings,57,58 we want to use it to quantify the disorder probed by 14N NMR in this series of ZSM-5 samples. We simplify the problem assuming that each 14N spectrum can be modeled considering two distinct functions (see Methods section). The first function corresponds to the single EFG determined in a pure and very ordered silicalite-1 (MFI topology without Al) leading to the parameters CQ = 53 kHz and ηQ = 0.3.24 The second signal corresponds to a distribution of quadrupolar parameters (Czjzeck model). The successful modeling of the 14N spectra for the six samples at three different MAS frequencies (νMAS = 2, 3, and 4 kHz) was achieved through a fitting procedure using a superposition of the two functions weighted by two coefficients a and b (see Figure 6 as an example of modeling). The average quadrupole coupling constant ⟨CQ⟩ corresponding to the distribution function was free to vary, while the single set of quadrupolar parameters was kept inside a restricted range (CQ between 53 and 58 kHz, ηQ between 0.3 and 0.4). The coefficient b is directly related to the relative quantity of 14N nuclei experiencing a distribution of EFG, different from that existing for 14N in highly ordered silicalite-124 reflected here by the coefficient a. Therefore, we consider here the values taken by b as a measure of the degree of local disorder probed by 14N NMR related to TPA molecules present in disordered domains. The results obtained from modeling 14N spectra recorded at νMAS = 2 kHz (Table 2) are considered to be more precise because of the higher number of MAS sidebands. They are nevertheless confirmed by the results obtained at the two other MAS frequencies (Table S3). From these results, we observe a global increase of the local disorder when the Si/Al ratio decreases (Figure 7a), and thus when the Si/F ratio increases

Table 2 A1

A2

A3

A4

A5

Si/Al ∞ 196 14 N spectrum modeling (νMAS = 2 kHz) single EFG coeff a 0.50 0.33 CQ, kHz 54 56 ηQ 0.3 0.3 Czjzeck coeff b 0.50 0.67 distribution ⟨CQ⟩, 64 59 kHz

Sample

A0

199

149

55

26

0.29 54 0.3 0.71

0.20 57 0.3 0.80

0.00 1.00

0.00 1.00

62

54

64

55

(Figure 8a). If we consider the ZSM-5 unit cell containing four TPA molecules, the evolution of local disorder with Al content is exemplified in Figure 7b (black squares). In this figure, the percentage of disorder probed by 14N is expressed by the number of TPA per uc present in disordered domains estimated from the percentages found by modeling. These values are then plotted against the number of Al atoms per uc obtained by elemental analyses. Clearly, the gradual insertion of Al atoms in the ZSM-5 crystal can be associated with a decrease of local ordering for the TPA molecules. The sample A0 containing no Al might be considered as the most ordered sample of the series with regard to the EFG present at the N sites. Indeed, the related b coefficient found by spectral modeling is the lowest, but it is not zero. The disorder probed by 14N NMR in this sample A0 is obviously not related to the presence of Al atoms and can be explained by the existence of different environments for the fluorine atoms in the [415262] cage related to the 19F signals F1 and F2 (vide supra). The first F1 signal is the narrowest and its relative percentage decreases with Al content. Besides, we found that there is near proportionality (black squares in Figure 8b) between the number per uc of F atoms leading to a 19F signal F1 (estimated from 19F NMR spectrum modeling and elemental analyses) and the TPA molecules present in ordered domains (estimated from coefficient a associated with 14N signal with a single EFG). As the proportionality factor is almost 1, we can identify F


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Figure 7. Effect of Al content on the local disorder probed by 14N NMR for the series of ZSM-5: A0−A5 (■) and B0−B4 (□). (a) Variation of the percentage of disorder as a function of the Si/Al ratio. (b) Variation of the average number of TPA molecules present in disordered domains per unit cell as a function of the average number of Al atoms per unit cell. Dashed lines are only guides for eyes.

Figure 8. Effect of F content of the local disorder probed by 14N NMR for the series of ZSM-5: A0−A5 (■) and B0−B4 (□). (a) Variation of the percentage of disorder as a function of the Si/F ratio. (b) Variation of the average number of TPA molecules present in ordered domains per unit cell as a function of the average number of F atoms corresponding to the 19F NMR F1 signals per unit cell. The dashed line is only a guide for eyes.

both 19F and 14N NMR signals to the same locally ordered environment in agreement with previous works.24 In order to verify the validity of these statements, we have analyzed a second and complementary series of ZSM-5 samples (B0−B4) also synthesized in fluoride medium with variable Si/ Al ratio (Table 1) but with different Al precursor, molar ratios, and hydrothermal conditions (see Methods section). This series presents similar NMR signatures to the first one. In particular, we found the same trend in the variations of the 14N SSB envelopes with the Si/Al ratio (Figure S9). Following the modeling procedure described above, the local disorder probed by 14N was estimated. The variations of TPA molecules per unit cell in disordered domains as a function of Al atoms per unit cell confirms the variations found for the first series of ZSM-5 zeolites (empty squares in Figure 7a). For this second series, the sample B0 containing no Al is more locally ordered (coefficient b = 0) than the sample A0. Accordingly, the 19F

NMR spectrum of this sample B0 does not present signals F2 and F3 related to F atoms in slightly different environments. Furthermore, the proportionality between the number of F atoms leading to a 19F signal F1 and TPA molecules present in ordered domains is confirmed (empty squares in Figure 8b). Origin of the Disorder Probed by 14N by Theoretical Calculations. In parallel to these experimental studies, we have performed DFT periodic calculations to study the effect of Al insertion on the 14N quadrupolar parameters. The starting structure used is issued from the study of Yokomori et al. on a ZSM-5 with a Si/Al ratio of 23.37 As it was not possible in this X-ray study to locate unambiguously the Al atoms, the atomic coordinates published are related to a purely silica zeolite. A preliminary optimization was thus undertaken using these structural data with F atoms added in the [415262] cage to compensate the positive charge of TPA present at the intersection of the sinusoidal and direct channels of the zeolite. G


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The Journal of Physical Chemistry C In the next step, the F atoms were removed and Si atoms sitting in one of the 24 possible positions were replaced by Al atoms. Twenty-four different ZSM-5 structures with Al atoms sitting in one of the possible positions were thus generated (see Figure 9

Figure 9. Perspective view along the straight channel of an optimized ZSM-5 structure (Al is sitting in position 1). Atoms’ colors are green (Al), blue (Si), red (O), pink (N), and gray (C, H).

Figure 10. 14N quadrupole coupling constants CQ calculated from the 24 optimized ZSM-5 structures containing one Al atom sitting in one specific site of the 24 specific tetrahedral sites. For each structure, the 14 N CQ values were calculated following the three approaches described in the text: CRY (periodic calculation considering the whole crystalline structure), PCM (calculation using only point charges), MOL (cluster calculation from the TPA molecule only).

for an example). No water molecules or hydroxy groups need to be considered here as their presence inside the zeolite and their effects on the related experimental NMR signals are not evidenced.59 Full DFT-D geometry optimizations allowed us to take into account all the intra- and intermolecular interactions occurring within the as-synthesized zeolite and to avoid artifacts related to the choice of truncated cluster models. Considering the extended periodic structure also allows optimization of the location and conformation of the TPA molecule. The optimized crystal cell parameters and relative energies are weakly influenced by the Al sitting position (see Table S4), in line with previous force field calculations60,61 and more recent QM/MM studies of the calcined ZSM-5 zeolites.22,62 In order to better understand the disorder probed by 14N NMR, the EFG tensors at N sites, and thus the quadrupolar parameters CQ and ηQ, were calculated from the optimized structures using three different approaches: (i) periodic calculations considering the whole crystalline structure (named CRY); (ii) point charge model calculations considering only positive and negative charges at the N and F atomic positions (named PCM); (iii) cluster calculations considering the TPA molecule only (named MOL). Comparison between the resulting EFG tensors allows discussion of the relative contributions of the molecule conformations and of the spatial distribution of charges to the EFG, since it was demonstrated that they both affect the 14N EFG tensor in tetra-nalkylammonium halide crystals.57 The CQ(14N) values obtained for the 24 structures are presented in Figure 10. We can observe that the 14N quadrupolar coupling parameters are very sensitive to the method employed to calculate the EFG tensors and to the Al sitting positions (Figure 10, Table S5). Using the first and more complete approach

(CRY), we obtained a wide range of CQ values (125−241 kHz) for the 24 structures. This first theoretical result could explain the distribution in quadrupolar parameters found experimentally for the highest Al contents as originating from a static disorder. As TPA molecules can be surrounded at the nanometer scale by Al atoms sitting in different crystal positions, there is indeed little chance to obtain the same EFG tensor for all TPA molecules. Besides, we notice that the calculated CQ values are larger than those determined from spectrum modeling (50−70 kHz range). We have also found such a gap between calculated and experimental CQ(14N) values in the case of TPA@silicalite-1 (ongoing work). The gap could be explained by the occurrence of motional dynamics at the temperature at which the NMR spectra are collected (T ≈ 300 K). Indeed, all theoretical calculations are done for T = 0 K and do not integrate motional effects that might affect calculated CQ(14N) values as shown for quaternary ammoniums in mesoporous materials.63 The modeling and understanding of dynamical effects inside the MFI topology is not a trivial task and is still under progress. However, one can presume that motions can slightly differ for TPA in the different environments and that dynamical disorder can also contribute to the measured distribution of 14N quadrupolar parameters. It is also worth noting that the CQ(14N) values calculated using the MOL approach (MOL) are always larger (around 10 times) than those obtained from the PCM approach (Figure 10). This may be seen as an underestimation of the distortion H


Article

The Journal of Physical Chemistry C of the 14N electronic orbital effects by the PCM method. However, the differences between the PCM and the MOL values prove the important role played by the conformational modifications around N sites on the global EFG tensor with respect to the spatial distribution of charges surrounding N sites. In order to quantify the angular distortions of the six CNC angles, we use an orientation order parameter, qTd, defined as the relative distortion of the CNC angles with respect to the perfect tetrahedral angles (109.47°).57,64 Figure 11 presents the

(ii) The geometrical modifications of the zeolites at shortrange scales (evidenced at long-range scale by the variations in cell parameters). This produces local variations in electron densities that might affect EFG at N sites, and indirectly this also leads to rearrangements of the TPA molecules to minimize the other intra- and intermolecular interactions. Therefore, both types of nanometer-scale disorder affect directly or indirectly the EFG at N sites and are thus probed by 14N NMR. Another noteworthy aspect, supported by 14N and 19F NMR data (signal shapes, modeling results), is the possible coexistence within the same ZSM-5 particles of crystal domains in which TPA molecules or F atoms can have either ordered or disordered environments. Further work (e.g., estimation of crystal domains from XRD using synchrotron sources) will be needed to deepen this aspect.

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CONCLUSIONS We have been able to probe the local disorder due to the insertion of Al atoms in ZSM-5 zeolites using 14N solid-state NMR. The gradual insertion of aluminum leads to a gradual modification of the 14N NMR spinning sidebands pattern. By fitting the 14N NMR spectra with two models corresponding to a single EFG or to a distribution of EFG at the N nucleus, we have evidenced a correlation between the number of Al atoms per unit cell and the number of TPA cations in disordered environments per unit cell. This effect can be rationalized using DFT calculations. After geometry optimization, the 24 ZSM-5 structures containing one Al atom in one of the 24 possible tetrahedral crystallographic sites lead to very different sets of 14N quadrupolar parameters (CQ, ηQ). The main factor affecting the EFG at the N nucleus is proven to be the CNC angles in TPA, which vary with the Al sitting position and are responsible of the deviation from Td symmetry for the [NC4] bonding units. Last, we want to mention that the results obtained here for ZSM-5 zeolites with variable Al content might be generalized to other heteroatoms like boron as additional experiments revealed a high similarity between the variations of 14N NMR spectra with the Si/B ratio and the variations presented here as a function of the Si/Al ratio.

Figure 11. Variation of the 14N quadrupole coupling constants CQ obtained from periodic calculations considering the whole crystalline structure of DFT-D optimized ZSM-5 structures as a function of (1 − qTd), the deviation from Td symmetry of CNC angles in TPA, qTd being the associated orientation order parameter (see text). The dashed line corresponds to a linear regression (R-squared value of 0.7435).

calculated CQ(14N) values (CRY approach) plotted as a function of (1 − qTd), which can be taken as a measure of the deviation from Td symmetry for the [NC4] bonding unit. The near proportionality between both sets of values demonstrates the importance of the angular distortion effect on the EFG at the N sites. Relationships between CQ(14N) and qTd have been demonstrated earlier for tetra-n-alkylammonium halide crystals.57 In this former work, it was also shown that the proportionality factor depends on the crystal nature. This is also confirmed here as this factor is about 30 times higher than in the case of TPABr crystal. Final Discussion. From the results above, we can state that the local disorder probed by 14N NMR is here related to TPA molecules that are not strictly in a unique environment. In that sense, local disorder can pre-exist without the insertion of Al atoms. Nevertheless, the substitution of Si by Al atoms has a clear disordering effect. There are two main types of interrelated disorder that can be induced by the insertion of Al: (i) The Al site distribution that leads to a distribution of charges. This affects directly the EFG at N sites (although in a moderate proportion as evidenced by the calculations with the PCM approach) but also indirectly by possible rearrangements of the TPA molecules (displacements, conformational changes) to minimize the electrostatic interactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04861. Additional information on the 1st series of ZSM-5 samples (molar ratio, SEM, XRD, TGA), complementary experimental solid-state NMR data (1H, 13C, 14N, 27Al), and calculated parameters (crystallography, 14N quadrupolar coupling parameters) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tzonka Mineva: 0000-0002-9156-2396 Bruno Alonso: 0000-0002-3430-1931 Notes

The authors declare no competing financial interest. I


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ACKNOWLEDGMENTS Thomas Cacciaguerra is kindly acknowledged for his help on SEM analysis. Financial support from the TGIR-RMN-THC FR3050 CNRS for conducting research is gratefully acknowledged.

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