Solid State NMR Techniques Study the Structural Characteristics of As

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Solid State NMR Techniques Study the Structural Characteristics of As-Synthesized ITQ-33 Zhanpei Zhang, Youmin Guo, and Xiaolong Liu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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The Journal of Physical Chemistry

Solid State NMR Techniques Study the Structural Characteristics of As-synthesized ITQ-33

Zhanpei Zhang1,2, Youmin Guo2*,

Xiaolong Liu1*

1. State Key Laboratory Magnetic Resonance and Atomic Molecular Physics, Wuhan Center for Magnetic Resonance, Key Laboratory of Magnetic Resonance in Biological System, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China. 2. School of Physics and Materials Science, Anhui University, No.111 Jiulong Road, Hefei, 230601 China

*Corresponding authors: [email protected]; [email protected]

ACS Paragon Plus Environment


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ABSTRACT:

Multinuclear solid state NMR techniques have been applied to study the structural characteristics of extra-large-pore zeolite ITQ-33. Through analysis of 2D

29

Si{19F}

HETCOR NMR spectra, the configurations of Ge-D4R units in ITQ-33 can be confirmed to have the most separation between Si and Ge atoms. Because F anions are not in the center of D4R units for shorter Ge-F bond lengths and Ge-D4R units with Ge-F bonds in the crystals are related by mirror symmetries, 29Si NMR signals of D4R units are magnetically inequivalent, therefore two 19

F-29Si CP/MAS and

19

29

Si peaks are observed in

F-29Si HETCOR spectra. The formation of specific D4R

configurations proves that Ge atoms and F- anions play important structural directing roles in the formation process of zeolite ITQ-33.

27

Al 5QMAS experimental results

confirm that a major amount of Al atoms is incorporated into the 3-ring in the framework. 1D 13C−27Al S-RESPDOR experimental results show that Al atoms in the framework are spatially close to the methyl groups of HM2+ cations in the 18-ring channels. Therefore it can be suggested that the delicate electrostatic balances between the negative charge centers such as F- anions in Ge-D4R units and Al atoms in 3-ring in the zeolite framework and the HM2+ cations orient the formation of ultra-large pores (18R) in zeolite ITQ-33. Our observations could be helpful in the design and synthesis of new extra-large pore zeolites.

1. INTRODUCTION


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In the last two decades, tens of germanosilicate zeolites with new frameworks have been discovered by Corma’s group at Instituto de Tecnologia Quimica (ITQ) in Spain.1 These new zeolites have been synthesized under a low H2O/TIV(Si and Ge) ratio condition with fluoride anions and/or germanium atoms introduced into the framework，and most of them feature low framework densities, regular pore structures and high pore volumes, which are critical to the potential applications of this kind of material in the petrochemistry and refining industry.1-2 For a majority of Ge-ITQ topologies, the Ge atoms and fluoride anions are believed to play structural directing roles by orienting the formation of small units such as D4R units in the framework through the analysis of the

19

F and

29

Si MAS NMR.3 However, the underlying

chemistry is not clear because the specific configurations of D4R units are not well determined. As strong acid catalysts, zeolites are mainly used in the field of petrochemical industry

including

fluidized

catalyst

cracking

(FCC),

hydrocracking,

hydroisomerization, aromatics alkylation, reforming, xylene isomerization, dewaxing and chemical synthesis.4 In refineries, the major challenge zeolites face in fluid catalytic cracking (FCC) is the cracking of heavy molecules.5 Although they might be cracked on the external surfaces of the crystalline zeolites, the large molecules cannot enter the zeolites because the pore sizes are mostly smaller than 1 nm. Although the very high surface areas and ordered frameworks of zeolites endorse them with very good shape selectivity, they also result in severe diffusion limitations.5-6 Diffusion limitations make it difficult for the reactants to access the active sites and, accordingly,



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lower the whole reaction rate. An effective strategy to overcome the mass transport problem is synthesizing zeolites with the pores larger than 1 nm, which would be able to accommodate bulky reactants and improve the diffusion rate. Significant efforts have been made in the last two decades to develop larger porous solid acids with strong acidity.

However, so far, only a few zeolites such as UTD-17, ECR-348,

CIT-59,OSB-110, SSZ-5311, SSZ-5911, ITQ-1512, ITQ-3313, ITQ-3714,

ITQ-4015,

ITQ-4316, ITQ-4417, ITQ-5318 and ITQ-5419, have the extra-large pores. Commonly, bulky molecules were needed as the organic structure directing agents (OSDA) to fill the pore cavities and direct the formation of large pore systems. However, the large OSDA molecules make zeolites suffer from poor hydrothermal stability, low acidity and unidirectional pore systems.13 ITQ-33, however, shows outstanding catalytic properties on catalyzing bulky molecules and especially demonstrating high selectivity in cumene synthesis.20 The catalytic properties of H-ITQ-33 in the methanol to hydrocarbons (MTH) reaction demonstrate that H-ITQ-33 is an active catalyst

in

this

reaction

and

is

fully

re-generable

during

three

deactivation/regeneration cycles showing good hydrothermal stability.21 The good catalytic properties and hydrothermal stability should have some tight connections with the framework properties. But ITQ-33, the extra-large pore zeolite with three dimensional 18 × 10 × 10 channels, was synthesized with a relative small molecule (hexamethonium cations, HM2+) and showed good stability.13 The HM2+ molecule has the two tri-methyl ammonium heads and the six hydrocarbon chain; and the tri-methyl ammonium heads


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are the smallest organic ammonium structure-directing cations. Normally the framework density (FD) can be used to express the porosity of zeolite framework and the framework density decreases with increasing pore volumes. The framework density decreases as the number of the small rings, such as four membered ring (4-ring) and three membered ring (3-ring) in the tetrahedral networks increases,22 which means that a large number of 4-ring and 3-ring units are needed to synthesize the extra-large pore systems. In ITQ-33 structures there is a large number of double four-member ring (D4R) units; thus the requirement of having a large number of 4-ring units has been satisfied. As well known, the fluoride anions and germanium atoms are believed to promote the formation of D4R units; but it is still not clear how the large numbers of D4R and 3-ring units direct the formation of the extra-large pores in ITQ-33. Therefore, understanding structure characteristics of zeolite ITQ-33 can be very useful in designing new zeolites with the extra-large pore channel systems. Since Ĉejka and co-worker developed a strategy ADOR (Assembly, Disassembly, Organization, Reassembly) to generate new zeolite topologies through deleting the D4R units by using HCl acid to hydrolyze Si-O-Ge bonds,23-26 understanding the configurations of Ge included D4R units has become more and more important. Since many germanosilicate zeolites, especially ITQ series zeolites, have the layer-like topologies linked by Ge included D4R units, this ADOR strategy could be adopted as a general method to delaminate these germanosilicate zeolites. Through periodic density functional theory calculations, Deem and coworkers demonstrate that Ge



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atoms are positioned in the D4R cages of UTL such that the average T-O-T angles in the zeolite is at a minimum.27 Tuel and co-workers further supposed that some of the T-O-T bridges in D4R units were purely siliceous and resistant to degermanation by treatment by HCl acid, which leads to the conclusion that the D4R configurations in germanosilicate zeolites play a vital role in the degermanation process towards the formation of new topologies.28 The high-resolution solid-state NMR spectroscopy has proven itself a powerful technique to characterize and elucidate the structural properties of zeolites; therefore, multiple solid-state NMR techniques have been performed to characterize the specific elements and their special correlation in the configurations of small units. The hexamethonium cation (HM2+), a flexible organic structural directing agent, is the structural directing agent (SDA) for the synthesis of ITQ-33, which has produced eight phases such as ZSM-4829, EU-129, ITQ-1330, ITQ-1731, ITQ-2232, ITQ-2433, ITQ-3313, and IM-1034. However, the crystallization mechanisms of these zeolites are still a mystery. Most of them are rich with D4R units in the framework. In this work, the structural directing roles of HM2+ cations have been studied through 13C-｛27Al｝ RESPDOR NMR experiments. It is the first time

13

C-｛27Al｝ RESPDOR NMR

techniques have been used to characterize the spatial correlations between the HM2+ cations and the framework Al atoms. The wide range of pore systems formed by the same organic structural directing agent imply that the specific spatial interactions between the framework negative charges with HM2+ cations orient the formation of specific zeolite topologies. A comprehensive understanding of the structure properties


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of ITQ-33 could provide new ideas when designing and synthesizing new extra-large-pore zeolites.

2. EXPERIMENTAL DETAILS

ITQ-33 was synthesized using homemade hexamethonium cations as an organic structure directing agent in combination with fluoride anions, germanium and aluminum. The gel was prepared by dissolving germanium oxide in an HM(OH)2 solution; aluminum isopropoxide and tetraethylorthosilicate (TEOS) were then hydrolyzed in that solution. The mixture was stirred to fully eliminate the alcohols, then HMBr2 solution was introduced into the mixture and the gel was stirred to reach the proper H2O amount; finally, a proper amount of HF solution was added. The composition of a typical synthesis gel was as follow: 0.66 SiO2: 0.33 GeO2: 0.15 HM(OH)2 : 0.10 HMBr2: 0.3 HF: 0.05 Al2O3: 3 H2O. The gel was transferred to and crystallized in Teflon-lined stainless steel autoclaves at 448 K for 21 days. The solid was then recovered by filtration, washed with distilled water, and dried at 383 K. Homemade Hexamethonium dibromide (HMBr2) was prepared by reaction of dibromohexane with trimethylamine in ethanol. To verify the structures of ITQ-33, powder X-ray diffraction patterns were measured by X-ray diffraction on a Rigaku Ultimate VI diffractometer using Cu Kα radiation. Diffractogrammes were collected between 4o and 40o (2θ) with steps of 0.02 and 1 s per step. The microscopic features of synthesized ITQ-33 were examined



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by scanning electron microscopy (SEM). Images of the samples were obtained using a Hitashi SU-1510 Microscope. High magnetic field solid state NMR data was carried out at 14.1 T on a wide-bore Agilent 600 MHz NMR spectrometer equipped with a 3.2 mm probe head and powders were spun at 16 kHz. 19F SSNMR data was collected using a one-pulse sequence with 3 µs pulses and 10 s delay, and

19

F-29Si CP/MAS experiments were

performed using a standard cross polarization sequence with 5 µs pulses and 10 s delay. The incorporation of fluoride ions in the D4R units of ITQ-33 zeolites has been investigated using

19

F-29Si CP/MAS and two-dimensional

19

F-29Si heteronuclear

correlation (HETCOR). Since the magnetization is transferred through the dipolar coupling, the CP period acts as a filter which only allows the 29Si that are located in the near vicinity of the 19F spin to be detected. 29Si and 19F chemical shifts referred to tetramethylsilane (TMS) and CFCl3, respectively. The contents of Ge, Al and Si were determined by inductively coupled plasma optical emission spectroscopy (HORIBA Jobin Yvon Activa ICP-OES). 27

Al MAS NMR and 5QMAS experiments were performed on a Bruker Avance III

500 spectrometer (11.7 T) operating at 132 MHz for alumina using a 4.0 mm and 1.9mm MAS probe at magic angle spinning rates of 12.5 kHz and 40 kHz, respectively. Chemical shifts were referenced relative to an aqueous 1.0 M Al(NO3)3 solution and the relaxation delays were 1s.

The

13

C-{27Al} S-RESPDOR

experiments were performed on a Bruker Avance III 500 MHz spectrometer with a 4 mm probe under 12.5 kHz MAS spinning rate. SR4 dipolar re-coupling was carried


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on the 13C channel. Continuous-wave 1H decoupling with an amplitude of 80 kHz was used during SR4, while a SPINAL-64 (small phase incremental alternation with 64 steps) decoupling with an amplitude of 50 kHz was used during acquisition. A saturation pulses on

27

Al channel with an amplitude of 71 kHz and length of 80 µs

were utilized to interfere the 13C−27Al dipolar interactions. The recycle delay was set to 2 s and 1024−4096 scans were accumulated. The contents of Ge, Al and Si were determined through inductively coupled plasma optical emission spectroscopy (HORIBA Jobin Yvon Activa ICP-OES). All quantum chemical calculations were carried out with the Gaussian 09 programs.35 Calculations of the EFG tensors were done based on the models generated by crystallographic data from IZA database. 26T and 24T models were used to explore the Al quadrupolar coupling constant (QCC) of T4 and T3 sites of ITQ-33 zeolite. The geometry optimizations were performed at the ωB97XD level using the 6-31G (d, p) basis set for all atoms. All atoms are relaxing during the optimizations. Based on the optimized structures, the EFG tensor components were calculated at the level of ωB97XD/6-311+G(d, p). The calculated electric field gradient (EFG) has been diagonalized to yield three eigenvalues in its principle axis system (PAS) and are expressed by a traceless, second rank tensor with the principle components ∣Vxx∣≤ ∣Vyy∣≤∣Vzz ∣. The asymmetry parameter (ηQ) and the quadrupolar coupling constant (QCC) are related to the EFG eigenvalues in its PAS by the following equations: ηQ =

| |



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QCC =

eQ ∙ V h

where e is the electron charge, Q is the nuclear electric quadrupole moment (27Al: Q = 0.14 x10-28 m2) for Al and h is the Planck constant.

3. RESULTS AND DISCUSSION

The structure of obtained ITQ-33 has been characterized by powder X-ray diffraction in Figure 1a, which is in agreement with the reported diffraction data from references.13, 20 The morphology of crystals is pictured by SEM in Figure 1b, which is also similar with the reported ITQ-33 SEM pictures.36 The chemical composition of calcined ITQ-33 is Si/Ge = 1.86 and Si/Al = 5.9. In zeolite ITQ-33, there are three D4R units per unit cell, but the configurations of D4R in ITQ-33 are not clear. Therefore, it should be very important that the structural characteristics of Ge-containing D4R units (D4R configurations) are determined in order to fully understand the structural direction roles of D4R units during the ITQ-33 crystallization process. For Ge-included ITQ-33 zeolites, both Ge and F oriented the formation of D4R units in the framework and the F anions are located inside these D4R units. Therefore, the F anions can be used as the probe nuclei to characterize different Ge-D4R units, and the

19

F NMR signals can actually give important

structural information about the Ge-D4R units. The 19F MAS NMR spectra of ITQ-33 zeolites are shown in Figure 2a. The spectrum consists of one peak at 7.4 ppm, and this signal can be assigned to [5Si, 3Ge] or [4Si, 4Ge] D4R units according to


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assignment from the other Ge-included zeolites enrichened with D4R units in their structures such as ITQ-73,ITQ-1330, ITQ-1731 with signals at 8.0 ppm. The 19F to 29Si cross-polarization (CP) MAS technique was further performed to selectively provoke the

29

Si signals of atoms close to the fluoride ions in D4R. The

19

F-29Si CP/MAS

spectrum (Figure 2b) shows that two 29Si NMR signals were observed. Then 19F-29Si HETCOR method was further employed to study the configurations of Ge-containing D4R units in zeolites. This 2D

19

F-29Si HETCOR spectrum was acquired by

introducing an evolution time into the cross-polarization sequence of two heteronuclei with the best contact time to transfer polarization from

19

F to

29

Si spatially; then the

spatial correlation of them can be built. In Figure 3, two resolved signals have been observed and the two contours circled and are highly symmetric, which implies that the configuration of D4R is highly symmetric. Only the D4R configuration with the most separated Si and Ge distribution can match this high symmetry requirement. It is abnormal to observe two

29

Si signals, because the Si atoms in D4R units are

crystallographically equivalent in the common situation and normally only one signal should be observed. But the short Ge-F bond length that make the F anions deviate from the center and tend towards to Ge in D4R cage to form a penta-coordinated [GeO4/2F] unit,37-39 which made the F-Ge bonds in the D4R configurations be related by the mirror symmetry and have two directions (up and down) in the framework. The up and down F-Ge bonds in the framework made the

29

Si signals of D4R units

magnetically inequivalent, therefore two signals are observed. The slightly different



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intensities of two

29

Si signals in

19

F-29Si CP/MAS and HETCOR spectra may come

from non-perfectly crystallized zeolites, which are also seen in SEM pictures. The ITQ-33 structure contains four symmetrically independent T sites, which are expressed as T1, T2, T3 and T4 with site multiplicity 24:12:6:4. The T sites are shown in Figure 4, and Ge atoms are mainly in T1 sites. If the Al atoms are located at T2 site, it is spatially too close to the negative charge (F- anion) in D4R cage, which is not energetically favorable for the Coulomb repulsion. Therefore, it is reasonable to suggest that the T2 sites are composed of Si or Ge. Because the D4R can connect to T2 sites with no change in bond length and acceptable distortion bond angles,36 it could be proposed that the T2 sites are mainly composed by Si atoms or both Si and Ge atoms with 1:1 ratio. But if the ratio of Si/Ge is 1:1 at T2 sites, then the whole Si/Ge ratio of ITQ-33 will be (12T1+6T2+6T3+4T4)/(12T1+6T2) = 1.56, which is over the limitation (the maximum Si/Ge ratio in ITQ-33 is 2); therefore, the T2 sites are mainly composed of Si atoms. T3 and T4 sites properly accommodate Al atoms. Al atoms in 3-ring of zeolites is also observed in ZSM-18, in which Al atoms are also in 3-ring and provide the negative charges.40 Al atoms in 3-ring (T3 sites) units in ITQ-33 framework are more structurally and energetically favorable because of the more flexible O-Al-O bond angles in 3-ring.41 As it is well known, incorporating trivalent cations in zeolite framework positions is the most important issue for pursuing its catalytic applications. For ITQ-33 zeolites, the Al atoms not only directed the framework formation, but also introduced acidic properties in the final solids.

27

Al MAS NMR spectra of ITQ-33 in Figure 5a give a


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broad line shape with the center at 54 ppm. This resonance corresponds to the tetrahedral aluminum and could be used to assign Al located at some T positions. However, neither

29

Si nor

27

Al MAS NMR spectra have resolutions high enough to

differentiate the different resonances of crystallographically nonequivalent T sites in the complex zeolite systems for the disorderly arranged Si and Al atoms in the framework. The second-order broadening and line shift due to quadrupolar interactions made it more difficult to resolve the 27Al resonances. Because the line in the

27

Al MAS spectrum is broad and spectral de-convolution does not suffice for

deducing reliable structural information, 5QMAS NMR experiment was employed to resolve the different Al sites to detect the highly resolved signals. This two-dimensional multiquantum MAS (MQMAS) NMR experiment of quadrupolar nuclei was first proposed by Frydman and Harwood42 and further developed by Fernadez and Amoureux43. A precise setting of experimental parameters has been optimized to perform five-quantum MQMAS experiments on zeolite. The advantage of 5QMAS experiments is the increase of the frequency dispersion along the indirect detection axis, which enhances the resolution. The 2D 5QMAS NMR spectrum in Figure 5b presents two separate contour plots with big different intensities. In this 5QMAS spectrum, the quadrupolar interaction is refocused and the isotropic direction is free from isotropic quadrupolar interactions; thus the quadrupolar line shape is seen only in the F2 direction and the quadrupolar-induced shift is visible in both directions. When the Al resonances in zeolite framework experience a large quadrupolar interaction, the ridges parallel to the axis in the F2 dimension are



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reflected in the spectra. The effects of different quadrupolar coupling constants and isotropic chemical shifts of Al sites have been clearly reflected in 2D MQMAS NMR spectrum, and the short-range orders around the aluminum atoms can be determined. Because the intensities of 2D peaks indicate the relative amounts of Al at the T sites and T3 site is more favorable than T4 sites for accommodating Al atoms, the signals in wide and intense contour are Al atoms located at T3 sites, and the signals in narrow and the weak contour is from Al atoms located at T4 sites. If the Al atoms are mainly at T3 sites, the Si/Ge ration is (12T1+12T2+3T3+4T4)/(12T1+3T3) = 2, which is consistent with the Si/Ge ratio in ITQ-33 crystals.

This occupation referring to the 5QMAS

SSNMR results did help to confirm the site location of Al atoms. In this work, the 2D 5QMAS spectra were sheared so that the F1 axis is the isotropic chemical shift dimension and the F2 axis contains the second-order quadrupolar line shape. The isotropic chemical shift (δISO) and the quadrupolar-induced shift (δQ) can be derived from the values read from the spectra according to the following equations:44 δF1 = δISO - (10/17)δQ

(1)

δF2 = δISO + δQ

(2)

In which δF1 is the isotropic shift position along the F1 axis and δF2 is the center of gravity position along the F2 axis of the lines in the 2D spectra. Then the second-order quadrupole effect (SOQE) and the nuclear quadrupolar coupling constant (QCC≡2πe2qQ/h) can be derived from can be derived from the following equations:


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SOQE =

√ ϑ δ

SOQE = QCC!1 +

− δ

η%' 3

(3) (4)

The 2D coordinates of the centers of gravity of the typical lines (δF1 and δF2), isotropic chemical shifts (δISO), quadrupolar-induced chemical shifts (δQ), and second-order quadrupolar effect (SOQE) values of Al at T3 and T4 sites are given in Table 1. The quadrupolar parameters obtained from the calculation demonstrated that the SOQE values spread over a wider range and can be divided into two types of Al with values at around 1 MHz and 2.4 MHz. These values are in agreement with the typical quadrupolar coupling constants of tetrahedral Al in zeolite framework ,which are of the order of 1-2MHz.45 Because the bond angle of Al at T3 site is smaller than that at T4 site, the T3 sites have a higher quadrupolar coupling constant than T4. The obtained experimental results are agreement with that SOQE (T3) is bigger than SOQE (T4). The assignments can also be assisted by Density Functional Theory (DFT) calculations of the electronic field gradient of Al at T3 and T4 sites. Quantum chemical calculations have become an important complement to experimental SSNMR studies by providing suitable information about the local structure of Al sites. Theoretical calculations were employed to obtain the electric field gradients (EFG) of Al at different sites with specific coordination, which allow a better understanding of the tetrahedral framework aluminum sites in zeolites. The calculated electric field gradients (EFG) and asymmetric parameter (ηQ) allow a better understanding the tetrahedral framework Al sites in zeolites. The models based on the X-ray of ITQ-33



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were employed in quantum chemical calculations of the NMR interaction tensors. According to reference papers,

13, 36

the possible coordination of T3 (Al) could be

Al(OGe)1 (OSi)3, or Al(OGe)2 (OSi)2 even Al(OSi)4, but the T4 sites have been surrounded by more Ge atoms and the coordination could be Al(OGe)1 (OSi)3 or Al(OGe)2 (OSi)2. The results of these calculations are presented in Table 2. Referring to the theoretical calculation values, the coordination types of Al atoms at T4 sites are Al(OSi)4 and Al(OGe)1(OSi)3, while those of Al atoms at T3 sites are Al(OGe)1(OSi)3 and Al(OGe)2(OSi)2. The deviations between the experimental data and those determined through quantum chemical calculations at T3 sites, are expected to be inherent in the calculations, even though our models were deduced from available crystallographic data and are theoretically optimized structures, although the T3 sites in the triangle ring may not have that large structural flexibility. The incorporation of surrounding lattice effects should not be trivial in obtaining reliable EFG tensors. However, the calculated SOQE values of Al at T3 almost double the values at T4 and this trend in the calculated SOQE is in agreement with the experimental results. Thus, the calculated results further confirmed our proposition that the tetrahedral Al atoms are mainly at T3 sites (3-ring) in the framework of zeolite ITQ-33. According our above observations, we proposed the structure schematic of ITQ-33 in Figure 4. In ITQ-33, the 18-ring channel is along the same direction as the six composed columns and the Al included 3-ring units are important for stabling the 18-ring channels. Therefore the interactions between Al atoms in 18R channels and the positive charge parts of HM2+ (hexamethonium) cations should be strong. However,


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the observation of connectivity and proximity between Al and C nuclei is limited by the designations of current NMR probes for the close Larmor frequencies. Recently this instrumental limitation has been circumvented using diplexers,

13

C−27Al

correlations have been correlated by solid-state NMR spectroscopy.46-47 In this work one-dimensional (1D)

13

C−{27Al} symmetry-based rotational-echo saturation-pulse

double-resonance (S-RESPDOR) experiments were performed to probe the spatial interactions between HM2+ cations and the Al atoms in the framework. Comparing between the two spectra observed with (S) and without (S0) 13C−{27Al} S-RESPDOR makes it possible to clarify the spatial proximity of between the different carbon species in HM2+ molecules with the aluminum nuclei. Figure 6 shows the

13

acquired with and without

C MAS spectra of HM2+ in ITQ-33 zeolite which were 13

C−27Al dipolar dephasing.

The comparison spectrum

demonstrates that the signal of methyl groups of HM2+ is mainly subject to a strong 13

C−27Al dipolar dephasing decreasing at about 33%, which could be ascribed to the

methyl group in 18-ring channel which are spatially close to the Al atoms at T3 sites. But the 13C−27Al dipolar dephasing of methylene groups are trivial, which means the body part of HM2+ molecule is away from the Al atoms in 3-ring units. This observation confirmed that there are specific spatial electrostatic interactions between the Al sites in 3-ring units and the methyl groups of HM2+ (-N(CH3)3) in 18-ring channels. Understanding the crystallization mechanism of industrially important zeolites is of great scientific significance because it can provide structural information crucial to fully adjusting their physicochemical properties and catalytic functions,



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which should also be very helpful in improving the performance of these materials. In the last two years, solid state NMR techniques have been used to study the spatial correlations between the defect sites and TPA+ cations.

48-49

The existence of

specific spatial correlations between the charged pore walls and the structure of occluded organic molecules is the key to understanding the nature of structure direction of OSDAs. Therefore, the specific spatial correlation between the two methyl groups of HM2+, instead of methylene chain, and the Al atoms in the framework is very important to fully understanding the crystallization mechanism of ITQ-33.

4. CONCLUSION

The solid-state NMR experimental results demonstrate that the formation of specific building units such as [4Si, 4Ge]-D4R and [2Si, 1Al]-3-ring are the key factors in forming the extra-large 18-ring pores in zeolite ITQ-33. The

19

F-29Si

HETCOR spectrum shows that the configurations of D4R units are highly symmetric. Because the F anions are not in the centers of D4R units for the shorter Ge-F bond lengths, the two 19

29

29

Si NMR signals of D4R units are magnetically inequivalent, therefore

Si peaks with almost equal intensities are observed in

F-29Si HETCOR spectrum. 5QMAS

27

19

F-29Si CP/MAS and

Al NMR experiments of ITQ-33 and

theoretical calculation results confirmed that Al atoms are mainly in the 3-ring units. 1D

13

C−{27Al} S-RESPDOR experimental results clearly demonstrate that the Al


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atoms in 3-ring are spatially close to the methyl groups in HM2+ molecules. Therefore, it can be suggested that the specific spatial electrostatic interactions between the SDA cations and the negative charge centers such as F- anions in D4R and Al atoms associated with the Coulomb interaction between the specific Al-3-ring units and F-D4R units orient the formation of ultra-large pores (18R) in zeolite ITQ-33. The above experimental results confirm that not only the organic structural directing agents (HM2+ molecules),

but also the inorganic elements such as F, Al and Ge

atoms play important structure direction roles during the crystallization process of zeolite ITQ-33.

ACKNOWLEDGEMENTS This work was supported by the National Natural Foundation of China (Grants 21303253 and 21673282)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]



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List of figures and tables: Figure 1 ： a) XRD patterns of as-synthesized ITQ-33; b) SEM pictures of as-synthesized ITQ-33. Figure 2. a) 19F MAS and b) 19F-29Si CP/MAS spectrum of as-synthesized ITQ-33 obtained at 14.1 T on a wide-bore Agilent 600 MHz NMR spectrometer. Figure 3. 19F-29Si HETCOR NMR spectrum of as-synthesized ITQ-33 obtained at 14.1 T on a wide-bore Agilent 600 MHz NMR spectrometer. Figure 4. a) 27Al MAS NMR spectrum, and b) 5QMAS NMR spectrum of as-synthesized ITQ-33. Figure 5. A Schematic of molecular structure properties of ITQ-33. Figure 6. 13C MAS NMR spectra for HM2+ in ITQ-33 acquired at 10 kHz MAS. The top and medium lines represent the spectra observed without (S0) and with (S΄) 13 C−{27Al} S-RESPDOR dipolar dephasing accumulated with a recoupling period of τ = 4 ms, respectively. The bottom line represents the different spectrum (∆S) of S0-S΄. Table 1. The 5QMAS NMR experimental data of ITQ-33. Table 2. Theoretical calculated asymmetry parameters (η) and the calculated quadrupolar coupling constants (QCC) and SOQE (MHz) values of Al atoms located at T4 and T3 sites.



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Figure 1 ： a) XRD patterns of as-synthesized ITQ-33; b) SEM pictures of as-synthesized ITQ-33.


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Figure 2. a) 19F MAS and b) 19F-29Si CP/MAS spectrum of as-synthesized ITQ-33 obtained at 14.1T on a wide-bore Agilent 600 MHz NMR spectrometer.



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Figure 3. 19F-29Si HETCOR NMR spectrum of as-synthesized ITQ-33 obtained at 14.1T on a wide-bore Agilent 600 MHz NMR spectrometer.


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Figure 4. A Schematic of molecular structure properties of ITQ-33.



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Figure 5. a) 27Al MAS NMR spectrum, and b) 5QMAS NMR spectrum of as-synthesized ITQ-33.


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Figure 6. 13C MAS NMR spectra for HM2+ in ITQ-33 acquired at 10 kHz MAS. The top and medium lines represent the spectra observed without (S0) and with (S΄) 13 C−{27Al} S-RESPDOR dipolar dephasing accumulated with a recoupling period of τ = 4 ms, respectively. The bottom line represents the different spectrum (∆S) of S0-S΄.



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Table 1. The 5QMAS NMR experimental data of ITQ-33. Al sites T4

T3

δF1 52.64 53.72 61.75 63.08 64.22

δF2 51.95 53.01 59.04 60.35 61.52

δQ 0.43 0.45 1.71 1.72 1.70


δISO 52.38 53.46 60.75 62.07 63.22

SOQE4-5 1.11 1.12 2.20 2.21 2.19

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Table 2. Theoretical calculated asymmetry parameters (η) and the calculated quadrupolar coupling constants (QCC) and SOQE (MHz) values of Al atoms located at T4 or T3 sites. Al sites with their coordinated types T4 Al(OGe)1(OSi)3 Al(OGe)2(OSi)2 Al(OSi)4 T3 Al(OGe)1(OSi)3 Al(OGe)2(OSi)2

η

QCC(MHz)

SOQE(MHz)

0.09 1.00 0.56 0.73 0.12

1.34 0.55 3.38 3.15 4.37

1.34 0.64 3.55 3.42 4.38



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TOC Graphic


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Solid State NMR Techniques Study the Structural Characteristics of As

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