27Al MAS NMR Studies of HBEA Zeolite at Low to ... - ACS Publications

May 17, 2017 - Mary Y. Hu,. † ... 19.97 T. High field 27Al MAS NMR spectra acquired at 19.97 ... to deconvolute the 27Al SP MAS spectra acquired at va...
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Al MAS NMR Studies of HBEA Zeolite at Low to High Magnetic Fields

Jian Zhi Hu,*,† Chuan Wan,†,∥ Aleksei Vjunov,†,∥ Meng Wang,†,∥ Zhenchao Zhao,† Mary Y. Hu,† Donald M. Camaioni,† and Johannes A. Lercher*,†,‡ †

Institute for Integrated Catalysis, and Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory Richland, Washington 99354, United States ‡ Department of Chemistry and Catalysis Research Institute, TU München, Lichtenbergstrasse 4, 85748 Garching, Germany S Supporting Information *

ABSTRACT: 27Al single pulse (SP) MAS NMR spectra of HBEA zeolites with high Si/Al ratios of 71 and 75 were obtained at three magnetic field strengths of 7.05, 11.75, and 19.97 T. High field 27Al MAS NMR spectra acquired at 19.97 T show significantly improved spectral resolution, resulting in at least two well-resolved tetrahedral-Al NMR peaks. Based on the results obtained from 27Al MAS and MQMAS NMR acquired at 19.97 T, four different quadrupole peaks are used to deconvolute the 27Al SP MAS spectra acquired at various fields by using the same set of quadrupole coupling constants, asymmetric parameters and relative integrated peak intensities for the tetrahedral Al peaks. The line shapes of individual peaks change from typical quadrupole line shape at low field to essentially symmetrical line shapes at high field. We demonstrate that, for fully hydrated HBEA zeolites, the effect of secondorder quadrupole interaction can be ignored, and quantitative spectral analysis can be performed by directly fitting the high field spectra using mixed Gaussian/Lorentzian line shapes. Also, the analytical steps described in our work allow direct assignment of spectral intensity to individual Al tetrahedral sites (T-sites) of zeolite HBEA. Finally, the proposed concept is suggested to be generally applicable to other zeolite framework types, thus allowing a direct probing of Al distributions by NMR spectroscopic methods in zeolites with high confidence.

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INTRODUCTION Zeolites, porous crystalline tectosilicate materials, such as the H form Beta zeolite (HBEA), are promising catalysts for the liquid phase conversion of biomass-derived molecules to automotive range hydrocarbon fuels.1,2 Recently, significant efforts have been undertaken to characterize this class of materials in assynthesized and postsynthetic modified forms, e.g., steam3,4 or hydrothermally5 treated in liquid water, which is a favorable solvent for upgrading of biomass pyrolysis oils and ligninderived components.6,7 The siliceous lattice of zeolites often incorporates heteroatoms, such as Al, that are in tetrahedral coordination and create local negative charges, which are in turn compensated by counterions.8 In the case when a proton acts as a charge-balancing ion, the zeolite becomes a solid acid catalyst exhibiting Brønsted acidity.9 However, the specific location of the Brønsted acid sites and their distribution among the different crystallographic positions of a zeolite has for a long time remained ambiguous. Lippmaa et al., the pioneers in this field, have demonstrated that an empiric relation exists between the observed nuclear magnetic resonance (NMR) chemical shift and the geometry, specifically the Al−O−Si angle, of the zeolite Al tetrahedral sites (Al T-sites).10 Sklenak et al. suggested that the individual Al T-sites can be mapped using NMR spectroscopy by assigning the peak positions of the © 2017 American Chemical Society

different Al-species based on the isotropic chemical shift prediction from density functional theory (DFT).11 Vjunov et al. have previously proposed a general strategy for the characterization of the acidic Al tetrahedral sites in HBEA invoking a combination of extended X-ray absorption fine structure (EXAFS) and 27Al MAS NMR spectroscopies supported by theoretical calculations.12 Because the EXAFS analysis provides insight regarding the Al coordination geometry up to 3−4 atom shells from the absorber atom, in combination with molecular dynamics (MD) EXAFS calculations, it allows discrimination of different Al environments among different secondary building units of the zeolite framework.11 Similarly, the 27Al NMR chemical shift values for the different zeolite Al T-sites can be calculated and used to fit the experimental NMR spectra.11 Albeit Vjunov et al. have demonstrated using a set of lab-synthesized and commercial HBEA zeolites that the distribution of Al among the different zeolite T-sites has little effect on the per-site catalytic activity for liquid phase acid catalyzed reactions,13 determining and understanding the Al siting in zeolites aids both fundamental Received: April 13, 2017 Revised: May 16, 2017 Published: May 17, 2017 12849

DOI: 10.1021/acs.jpcc.7b03517 J. Phys. Chem. C 2017, 121, 12849−12854

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The Journal of Physical Chemistry C

11.75, and 19.97 T on spectrometers with the corresponding Larmor frequencies of 78.2, 130.3, and 221.4 MHz, respectively. All single pulse spectra were acquired using a single pulse sequence at pulse angles of 10, 45, and 90 deg, respectively, and recovery delay of 1 s. The spectra were accumulated from 10000 scans using a 7.5 mm probe at a sample spinning rate of 5 kHz at 7.05 T, 4096 scans using a 4 mm probe at a sample spinning rate of 14 kHz at 11.75 T, and 5000 scans using a 3.2 mm probe at a sample spinning rate of 20 kHz at 19.97 T. 27Al 3QMAS NMR spectra were acquired using the z-filter 3QMAS pulse sequence using a 3.2 mm probe with sample spinning rate of 20 kHz at a 19.97 T spectrometer. The optimized pulse widths were p1 = 5.0 μs, p2 = 1.7 μs, and p3 = 20.0 μs. In the hypercomplex 3QMAS experiment, 960 transients with a 0.3 s recycle delay and 246−284 evolution increments were used. Spectral widths for the F2 (acquisition) and F1 (evolution) dimension were 416.7 and 40 kHz, respectively. All spectra are externally referenced (i.e., the 0 ppm position) to a 1 M Al(NO3)3 aqueous solution. The DMFIT 2011 program was used to simulate the spectra and fit the peaks.23

studies of aluminosilicate nucleation and growth during synthesis14 as well as provides additional insights toward improving zeolite hydrothermal stability by controlling the concentration and location of framework defects.15 Because Al is a spin 5/2 nucleus, the 27Al MAS NMR spectrum is often greatly affected by the second-order quadrupole interactions.16,17 For example, for HBEA zeolites with different degrees of pore hydration prior to the NMR measurement, the Al T-sites experience quadrupole interactions with quadrupole interaction constants ranging from 3 to 17 MHz, thus causing a modification of the observed spectral line shapes, e.g., from very sharp (fully hydrated samples) to extremely broad (fully dehydrated samples) 27Al MAS NMR peaks.18−21 Previous examples of high-quality NMR spectra for zeolites relevant for biomass conversion reactions were obtained at high fields of either 900 or 850 MHz, because under the high field conditions, it was assumed that the line broadening due to the second quadrupole interaction in fully hydrated HBEA zeolites could be ignored and the 27Al MAS NMR spectra could be deconvoluted with simple Gaussian/ Lorentzian line shapes to determine the Al T-site distributions.5,12 While this qualitative hypothesis may be correct, there is a lack of spectroscopic evidence supporting this pragmatic approach. In this contribution, we report a comprehensive 27Al MAS NMR investigation of two HBEA zeolites with Si/Al ratios of 71 and 75, respectively, which have similar physicochemical properties, yet somewhat different Al distributions, using different magnetic fields varying from 7.05 to 19.97 T. The effect of the second order quadrupole interactions on the individual line shapes at different field strengths are demonstrated, and a comparison of quadrupole line and mixed Gaussian/Lorentzian line fitting for high field 27Al MAS NMR spectra is reported. We demonstrate that the simple Gaussian/Lorentzian line shape is appropriate for fitting the 27 Al MAS NMR peaks of the fully hydrated HBEA samples measured at 19.97 T (high field, 850 MHz spectrometer). We also provide the necessary high-field MAS NMR-based calibration that enables reliable quantification of Al species distributed among different T-sites of zeolite HBEA. The provided approach is suggested to be applicable to other zeolite framework types upon performing the respective tetrahedral Al peak-mapping and NMR calibrations similar to the approach described in this contribution.

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RESULTS AND DISCUSSION Al MAS NMR spectra of two HBEA zeolites (HBEA150a, Si/ Al = 71, and HBEA150b, Si/Al = 75) acquired at different (7.05−19.97 T) magnetic fields are shown in Figure 1. Both 27

Figure 1. 27Al MAS NMR spectra of HBEA zeolites at magnetic field strengths from 7.05, 11.75, to 19.97 T: (a) HBEA150a and (b) HBEA150b. Asterisks denote background signal from the empty rotor.

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EXPERIMENTAL SECTION Zeolite Samples. The NMR measurements at different magnetic fields are performed using two different batches of HBEA150, with Si/Al = 71 (HBEA150a) and 75(HBEA150b), respectively. Both zeolites were obtained from Clariant in Hform. HBEA150a (the older batch) was calcined at 500 °C in a 100 L/min flow of dry air for 6 h prior to use. HBEA150b (a newer batch) was used as received. Prior to MAS NMR measurements, all zeolites are stored for 48 h in a desiccator over a saturated Ca(NO3)2 aqueous solution in order to achieve full pore hydration, thus leading to improved NMR spectral resolution due to minimization of distortions in the Al tetrahedral.22 Zeolite Characterization. The Si and Al concentrations in the studied HBEA150 were determined from atomic absorption spectroscopy (AAS) on a UNICAM 939 AA-Spectrometer. 27 Al MAS NMR Measurements. 27Al single pulse MAS NMR experiments were performed at a magnetic field of 7.05,

samples show typical signals around 50−60 ppm assigned to the tetrahedrally coordinated framework Al and a signal at ∼0 ppm attributed to the octahedrally coordinated extra-framework Al, which is in agreement with previous reports for HBEA.24 There is no obvious change of the narrow lines of extraframework Al for the spectra acquired at different magnetic fields. In contrast, at low field of 7.05 T, the tetrahedral Al atoms observed between ∼50−60 ppm appear in NMR as heavily overlapped signals that are not distinguishable from one another. At 11.75 T, a shoulder peak appears on the lower field side, i.e., at ∼59 ppm. At 19.97 T, for both HBEA150 samples two tetrahedral Al peaks are well resolved. This result indicates that the overlapped peaks in 27Al spectra acquired at low magnetic field mainly result from the broadening effect of second order quadrupole interactions.25 12850

DOI: 10.1021/acs.jpcc.7b03517 J. Phys. Chem. C 2017, 121, 12849−12854

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The Journal of Physical Chemistry C Since the second-order quadrupole interactions are inversely proportional to the strength of the magnetic field,26 the quadrupole interactions are significantly decreased at high field. With improved spectral resolution at high field, it can be unambiguously concluded that, while the studied samples have very similar framework Al concentrations, the two zeolites exhibit somewhat different Al distributions with HBEA150a containing more Al T-sites that result in a signal at ∼60 ppm. The 27Al MQMAS NMR spectra of the two HBEA zeolites are shown in Figure 2 and indicate that Al T-sites mainly

Table 1. Detailed Simulation Parameters Obtained from the Sliced Spectra of 3QMAS Spectra Shown in Figure 2 HBEA150a δiso (ppm)a

lb (ppm)b

CQ (MHz)

ηQ

59.3 59.3 58.6 58.3 55.2 54.9 54.3 53.8

2.7 2.5 1.6 1.5 1.2 1.2 1.3 1.4

2.5 2.5 2.5 2.5 1.8 1.8 2.4 2.4

0.40 0.40 0.40 0.40 0.55 0.55 0.40 0.40

δiso (ppm)a

lb (ppm)b

CQ (MHz)

ηQ

59.5 59.4 58.5 58.3 55.2 54.8 54.1 53.6

2.8 2.4 1.7 1.6 1.1 1.2 1.2 1.3

2.6 2.6 2.5 2.4 1.8 1.8 2.3 2.3

0.30 0.30 0.20 0.20 0.60 0.60 0.40 0.40

HBEA150b

Isotropic chemical shift values obtained by fitting each sliced spectrum. bLine broadening applied on top of the ideal quadrupolar line shapes obtained from the fitting.

a

respectively; (3) For the downfield peak centered at ∼59 ppm along the F1, there is only one CQ that is identified as ∼2.5 ± 1 MHz. Based on the presented results of the MQMAS NMR spectra fitting, we can now accurately deconvolute using similar quadrupole interactions each of the two broad peaks at ∼60 ppm and ∼55 ppm in the 1D MAS NMR spectra acquired at 19.97 T field. The fitting results are shown in Figure 3. The resulting isotropic chemical shifts, the quadrupole interaction constants (Qcc), the asymmetry parameter ηQ and the relative ratios of the peak areas for the four peaks are summarized in Table 2, and the simulation parameters of Gaussian/Lorentzian line shapes are reported in Table S1. To validate the fitting results, the same fitting parameters were used to fit the respective lower field MAS NMR spectra. The simulated spectra reproduced well the features of the experimental spectra at 7.05 and 11.75 T fields. We note, however, that the peaks in the spectra acquired at low fields (7.05 and 11.75 T) are significantly broadened and show typical quadrupole line shapes due to quadrupole interactions. The high field can significantly suppress or eliminate the quadrupole interaction as is shown in the peak deconvolution (Figure 3). While Table 2 reports the fit parameters obtained for the bestfit, there are two more possible fits that need to be considered: one that assigns ∼2% of total spectral intensity to the upfieldmost peak of the tetrahedral Al NMR region and one that assigns ∼1% of the total spectral intensity to the downfieldmost peak of the tetrahedral Al NMR region. These fits are shown in Figure S1 and Figure S2 with respective fit parameters reported in Table S2 and Table S3 of the SI, respectively. The best-fit is determined based on the fit residual values and the fraction of the experimental spectrum that could not be fit. The comparison of the three outlined fits are reported in Table S4 in the SI. To validate the feasibility of fitting the peaks of the high field 27 Al MAS NMR spectrum using simple Gaussian/Lorentzian line shapes, we attempted to simulate the high field spectra with

Figure 2. 27Al MQMAS NMR spectra of HBEA zeolites on the right, and the representative slices (black) parallel to the F2 (acquisition) dimension at selected F1 (the isotropic chemical shift) dimensions with the fitting lines (red) on the left: (a) HBEA150a and (b) HBEA150b. No line broadening was applied before Fourier Transformation in both the F2 and F1 dimensions.

distribute near 58.5 and 55 ppm along the isotropic dimension (F1) with broader features along the acquisition dimension (F2). Apparently, there are gradual shifts along the F1 dimension for the two major peaks due to chemical shift distributions, which is reasonable considering there are nine crystallographically different T-sites in the HBEA crystal structure27 with very close chemical shifts and the fact that the samples are composed of ∼50/50 mixtures of polymorphs A and B, as determined from XRD.5,12 The representative slices parallel to the F2 dimension of MQMAS NMR spectra at selected F1 chemical shift positions were extracted and then fit to obtain the isotropic chemical shifts and quadrupole interaction constants. The results are summarized in Table 1. Three major conclusions can be made from the 2D sliced fitting: (1) There is clearly a distribution of isotropic chemical shifts for each of the two major peaks at 19.97 T; (2) For the upfield peak centered at ∼55 ppm along the F1, two CQ values are identified with the values of about 2.4 and 1.8 MHz, 12851

DOI: 10.1021/acs.jpcc.7b03517 J. Phys. Chem. C 2017, 121, 12849−12854

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The Journal of Physical Chemistry C

Table 2. Comparison of Spectral Simulation Parameters Reported in Figure 3 HBEA150a magnetic field (T)

δiso (ppm)a

7.05

59.6 58.5 55.4 54.5 59.5 58.4 55.0 54.3 59.3 58.1 55.0 54.2

11.75

19.97

Figure 3. Deconvoluted 27Al MAS NMR spectra of HBEA zeolites at magnetic field strengths from 7.05, 11.75, to 19.97 T by using quadrupole line shapes: (a) HBEA150a and (b) HBEA150b. The 19.97 T spectra were also deconvoluted by mixed Gaussian/ Lorentzian (G/L) line shape: 19.97 T (G/L). Experimental spectra (black), simulated spectra (red), deconvoluted peaks (green dashed), and the difference (blue) are shown.

3.3 3.3 1.5 3.1 1.4 1.9 1.2 2.0 1.6 1.8 1.6 1.9 HBEA150b

ηQ

% total intensityc

2.5 2.5 1.7 2.4 2.5 2.5 1.8 2.4 2.5 2.6 1.9 2.4

0.40 0.30 0.60 0.40 0.40 0.40 0.50 0.40 0.40 0.40 0.55 0.40

9.7 40.4 39.6 10.3 9.4 41.2 39.1 10.3 9.4 (9.2G/L) 40.5 (41.5G/L) 39.5 (39.5G/L) 10.6 (9.8G/L) % total intensityc

magnetic field (T)

δiso (ppm)a

lb (ppm)b

Qcc (MHz)

ηQ

7.05

59.8 58.8 55.3 54.6 59.4 58.3 55.1 54.3 59.4 58.4 55.1 54.3

3.7 3.9 1.9 3.6 2.0 2.7 1.4 1.6 1.8 2.3 1.6 1.9

2.6 2.5 1.6 2.3 2.6 2.5 1.8 2.3 2.6 2.5 1.8 2.3

0.30 0.20 0.65 0.40 0.30 0.20 0.60 0.50 0.30 0.20 0.70 0.50

11.75

19.97

mixed Gaussian/Lorentzian line shapes considering the existence of chemical shift distributions. Albeit the simulated peak centers from simple Gaussion/Lorentzian line shapes do not rigorously represent the isotropic chemical shift positions, the spectral features are well reproduced, and the quantitative results related to the Al T-site distribution are almost the same as the best-fit quadrupole line shape simulations (Figure 3). The obtained fits demonstrate that a simple Gaussian/ Lorentzian line shape fitting generates a good fit for all fully hydrated HBEA zeolites with different Si/Al ratios and also determines similar Al T-site distributions (based on peak areas) as those obtained from rigorous quadrupolar line shape fitting. Interestingly, the minimum number of peaks necessary to obtain a good spectral fit, which is based on the residual χ2 values, is four in all cases for both the quadrupolar and the simple Gaussian/Lorentzian line shape fitting. Also, we note that while the peak isotropic shifts vary slightly (0.2−0.4 ppm on average) from fit to fit, the peak area (integral) under the peaks is in quantitative agreement between the two methods for all samples. The slight deviations in the position is attributed to multiple factors, which are both Al T-site-specific, e.g., the extent of quadrupolar interaction for each Al-species,28 as well as sample-specific, e.g., the extent of structural defects in the zeolite framework may have subtle electronic effects on Al shielding observed by the NMR spectroscopic methods.29 Most importantly, the above-mentioned fits confirm the assumptions previously made by Vjunov et al. when performing fits to the NMR spectra acquired for different HBEA zeolites5,12,13 and experimentally demonstrate that the Gaussian/Lorentzian line shapes are adequate for fitting the 27Al NMR spectra acquired at high-field (i.e., at or above 850 MHz).

Qcc (MHz)

lb (ppm)b

7.6 29.2 50.9 12.3

7.3 28.7 51.1 12.9 7.3 29.8 50.6 12.3 (6.5G/L) (28.4G/L) (51.3G/L) (13.8G/L)

Isotropic chemical shift values obtained by fitting each sliced spectrum. bLine broadening applied on top of the ideal quadrupolar lineshapes obtained from the fitting. cG/L denotes percentage derived by deconvolution using mixed Gaussian/Lorentzian line shape. a

For magic angle spinning quadrupolar nuclear magnetic resonance, with nuclear spin I > 1/2, the difference between true chemical shift (δiso) and center of gravity of experimental line (δexp) is related to three parameters:26 the quadrupolar coupling constant CQ, the asymmetry parameter ηQ and the Larmor frequency ω0. CQ is a measure of the strength of the quadrupolar interaction and ηQ is a measure of the deviation of the electric field gradient from axial symmetry. The equation for the difference is shown below. δiso − δexp =

1 ⎛ 1 2 ⎞⎡ 3⎤ ⎜1 + ηQ ⎟⎢I(I + 1) − ⎥ ⎠⎣ 30 ⎝ 3 4⎦ ⎡ ⎤2 3CQ ⎢ ⎥ ⎣ 2I(2I − 1)ωo ⎦

(1)

27

According to the equation, for Al, when the spectrum was acquired at 19.97 T, which means ω0 = 221.4 MHz, the difference will be less than 1.1 ppm when CQ is less than 2.6 MHz and the line broadenings for our fitting are more than 1.6 ppm. In this particular case, the center of a Gaussian/ Lorentzian line almost represents the true chemical shift δiso. This equation principally explains why the Gaussian/Loretnzian line shapes are adequate for fitting the NMR spectra acquired at high-field. 12852

DOI: 10.1021/acs.jpcc.7b03517 J. Phys. Chem. C 2017, 121, 12849−12854

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The Journal of Physical Chemistry C The final question addressed in this manuscript is whether the fitted peaks can be assigned to certain Al-species and thus be used to determine the Al distribution in a zeolite, e.g., HBEA. We note that albeit the quadrupolar and Gaussian/ Lorentzian line shapes provide the same quantitative answer in respect to peak area, to our best knowledge there are no experimental references for the individual Al T-sites in zeolites and their respective chemical shifts, and hence theoretical calculations are necessary to attempt peak assignment. Previously, Sauer et al. demonstrated that small Al-clusters can be used as representative species to calculate the isotropic chemical shifts of tetrahedral Al-species, e.g., those similar to the zeolitic T-sites in MFI-type zeolite.12 Vjunov et al. further developed this concept for zeolite HBEA by performing a periodic DFT calculation to optimize the zeolite unit cell with incorporated Al and then use the geometry of the extracted Al(O4Si4O12H12)−1 cluster to calculate the isotropic chemical shift values for the 9 T-sites of HBEA.12 The resulting values are referenced to aqueous Al3+ as well as zeolite Mordenite (MOR) followed by a fit of the experimental spectrum with 9 peaks representing the Al T-sites that could be present in the sample. Vjunov et al. have also assumed all tetrahedral Al peaks have the same Gaussian/Lorentzian line shape as well as halfwidth and only vary in intensity.12 Interestingly, the authors found that for the same HBEA150b sample as in this work, the spectral fit using the DFT-calculated peak positions, and assumptions outlined above, the best-fit was obtained using a combination of four peaks, identical to our findings. In addition, we note that our peak-assignments and fitted parameters are not constrained by a theoretical model, which in turn demonstrates that the similar result is obtained using two completely independent methods. The latter in turn suggests that the proposed conceptual pathway allows an assignment of the tetrahedral Al spectral intensity to individual Al T-sites of a zeolite upon performing the calibration described in this paper. Combining the results from this work and those reported by Vjunov et al.,12 a general strategy for determining Al-T site distribution on HBEA and similar zeolites is proposed. (i) Conduct DFT NMR calculations on all the Al-T sites using the same strategy as described previously by Vjunov et12 to obtain the theoretical 27Al isotropic chemical shift, i.e., the δiso for each of the Al-T site. Here the selection of the reference is critically important for accurately calculating the value of the theoretical δiso. This can be done by calculating the absolute shielding of each Al-T site on a model zeolite with experimental isotropic chemical shifts close to those of HBEA and then using the calculated absolute shielding as a second reference to the known experimental isotropic chemical shifts with respect to 1 M Al(NO3)3 aqueous solution (0 ppm). In Vjunov et al’s work,12 mordenite was chosen to be the model system, as it has 4 Al-T sites that are basically equivalent as well as experimental isotropic shifts close to those of the Al-T sites in HBEA. (ii) Perform quantitative single pulse 27Al MAS NMR experiments at high magnetic field with field strength of 19.97 T or greater on fully hydrated zeolites. (iii) Use a minimum number of quadrupoler lineshapes to fit the single pulse spectra and estimate the displacement between the isotropic chemical shift position and the center of the gravity for each peak, i.e., determining the value of Δ= δiso − δexp for each line shape. Then obtain an average value of Δ among the various lineshapes. (iv) Use a group of Gaussian/Lorentzian lineshapes with the peak centers locked at δexp = δiso − Δ, where δiso is the DFT calculated isotropic chemical shifts. Deconvolute the Al-T

site peaks to obtain the distribution of the integrated peak intensity, i.e., the Al-T site distribution. Note that during spectral deconvolution, the line width for all the peaks is locked the same, and the differences between the peak centers are locked to those determined by theoretical calculations. The parameters that are allowed to change are the peak intensity of each peak, the line width, and the relative ratio of Gaussian and Lorentzian components. Minor parallel shifts for all the δexp are also allowed to take account of the difficulties both in accurately calculating the theoretical isotropic chemical shifts and in determining Δ.

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CONCLUSIONS Al single pulse MAS NMR spectra of HBEA zeolites with high Si/Al ratios were obtained at different magnetic field strengths. High field 27Al MAS NMR spectra show significantly improved spectral resolution. Based on the high field 27Al MAS and MQ MAS NMR, four different peaks are deconvoluted from 27Al MAS spectra at various field strengths using the same set of quadrupole coupling constants, asymmetric parameters and relative integrated peak intensities for the tetrahedral Al peaks. The line shapes of individual peaks change from typical quadrupole line shape at low field to symmetrical line shape at high field. We conclude that for fully hydrated HBEA zeolites and similar zeolites with qudrupolar coupling constants of, e.g., ∼ 3 MHz or less, the effect of second order quadrupole interaction can be ignored and quantitative results can be derived by directly fitting the high field (∼19.97 T or higher) spectra using mixed Gaussian/Lorentzian line shape, supporting the strategy reported in prior publications.5,11,12 Finally, the proposed spectroscopic concept is demonstrated to allow an assignment of Al distribution among zeolite T-sites with high confidence. The concepts outlined and described in this contribution are suggested to be generally applicable to other zeolite framework types, thus, allowing one to directly probe and quantify Al populations in porous crystalline aluminosilicates. 27

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ASSOCIATED CONTENT

S Supporting Information *

The fitted parameters and the fits of the measured 27Al MAS NMR spectra are supplied as Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03517. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jian Zhi Hu: 0000-0001-8879-747X Chuan Wan: 0000-0002-8226-7619 Meng Wang: 0000-0002-3380-3534 Donald M. Camaioni: 0000-0002-2213-0960 Johannes A. Lercher: 0000-0002-2495-1404 Present Address §

Dr. Zhenchao Zhao currently holds a position at the Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS), 457 Zhongshan Road, 116023 Dalian P.R. China. 12853

DOI: 10.1021/acs.jpcc.7b03517 J. Phys. Chem. C 2017, 121, 12849−12854

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The Journal of Physical Chemistry C Author Contributions

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These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences (BES FWP #47319). All of the NMR experiments were performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research, and located at Pacific Northwest National Laboratory (PNNL). PNNL is a Multi-Program national laboratory operated for the DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.

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ABBREVIATIONS MAS, magic angle spinning; NMR, nuclear magnetic resonance; MQ, multiple quantum; SP, single pulse; DFT, density functional theory; EXAFS, extended X-ray absorption fine structure

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DOI: 10.1021/acs.jpcc.7b03517 J. Phys. Chem. C 2017, 121, 12849−12854