Solid State NMR Spectroscopy Studies of the Nature of Structure

Jun 12, 2017 - In Figure 3, 1H–13C CP/MAS NMR spectra of TPA-F-ZSM-5 show that the intensity of the left signal (δ = 11.6 ppm) is always higher tha...
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Solid State NMR Spectroscopy Studies of the Nature of Structure Direction of OSDAs in Pure-Silica Zeolites ZSM‑5 and Beta Xiaolong Liu* and Qing Luo 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 S Supporting Information *

ABSTRACT: 2D 1H DQ-SQ NMR spectra of zeolites TPA-OHZSM-5 and TEA-OH-ZSM-5 clearly demonstrate the specific spatial correlations between the SiO−··HOSi hydrogen bonds within the framework and the alkyl chains of TPA+ and TEA+. For zeolite TPA-OH-ZSM-5,the 2D 1H DQ-SQ NMR spectrum shows that the SiO−··HOSi hydrogen bonds within the framework are spatially close to the methyl groups of TPA+ cations. For zeolite TEA-OH-ZSM-5, the 2D 1H DQ-SQ NMR spectrum shows that the SiO−··HOSi hydrogen bonds within the framework are spatially close to both the methyl and methylene groups of TEA+ cations. These observations imply that the position and distribution of the negative charge centers such as F anions, SiO−··HOSi hydrogen bonds and T3+ atoms in the MFI framework are influenced by TPA+ or TEA+ cations for the strong electrostatic interactions. By analyzing the variable contact time 1H−13C CP/MAS NMR experimental results, the 13C signal with δ = 10.6 ppm can be assigned to the methyl groups of TPA+ cations located in zigzag channels and the 13C signal with δ = 11.6 ppm can be assigned to the methyl groups of TPA+ cations located in straight channels. Both 2D 1H DQ-SQ NMR spectrum and 1H−13C CP/MAS NMR spectra of TEA-OH-Beta show that the SiO−··HOSi hydrogen bonds within the framework are spatially further from the alkyl chains of TEA+ in Beta than those in ZSM5, which indicates that van der Waals interactions play the dominant roles during the crystallization process of zeolite Beta. According to our NMR observations, it can be inferred that the nature of structure direction of OSDAs roots in the complex relationship between van der Waals interactions and electrostatic interactions in the inorganic−organic composites formed in the induction period.

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INTRODUCTION Zeolites are crystalline aluminosilicates with multidimensional pore and cavity architectures. 1−3 They are important heterogeneous catalysts, and their properties depend on their long-range organization and the spatial distribution of active sites: the framework properties strongly affect their catalytic performance, and the negative charges (T3+ sites; Al or B atoms) in the crystalline framework provide the stabilization for the protons or metal ion counter species which are the active sites for catalytic reactions. The distribution of T3+ sites in zeolite channel systems is not random and commonly controlled by the zeolite synthesis conditions, especially by the specific organic structural directing agents (OSDAs). Therefore, understanding the formation mechanisms of zeolites is of great academic and industrial importance, as it can provide critical information for controlling their physicochemical properties and catalytic functions. High-silica zeolites are important heterogeneous catalysts in petrochemical industries, for instance, high silica Beta (BEA*) and ZSM-5 (MFI) structural type zeolites are well-known for their excellent catalytic performances on the production of cumene and olefins (MTO (methanol-to-olefin)), respectively.4,5 Organic structure-directing agents have often been © XXXX American Chemical Society

employed to synthesize zeolites, especially with high silicon-toaluminum ratios.6 Both zeolite ZSM-5 (MFI) and Beta (*BEA) are model materials in studying the effects of organic structure directing agents on zeolite nucleation and crystallization for their specific structures and important industrial applications. There are two models for describing the structural directing roles of OSDA molecules: one is the isolated model, in which the single TEA+ or TPA+ cation is typically occluded in the zeolite cavities as an isolated cation and located at the channel intersection of MFI zeolite; unlike this isolated mode of OSDA cation in MFI zeolite, a cluster model, in which a cluster of six tetraethylammonium (TEA+) cations play the structural direction, has been proposed for the crystallization of zeolite Beta (*BEA-type zeolite).7 A schematic of the structure direction of OSDA cations in (a) isolated and (b) clustered modes is shown in Figure 1. In the isolated model, OSDA is confined in the zeolite cavities as an isolated cation or molecule. For example, the TPA+ cation is located at the channel intersection of ZSM-5 Received: April 9, 2017 Revised: June 4, 2017

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DOI: 10.1021/acs.jpcc.7b03350 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Schematics of structure direction models of OSDA in (a) ZSM-5 (isolated) and (b) Beta (clustered).

(MFI) zeolite.8 One theory is that a geometric fit between isolated TPA cations and zeolite cavities make the zeolites reach the maximum stabilization energy,9 but the formation of siliceous zeolites is controlled kinetically and the most stable crystalline phase does not need to be formed. Unlike an isolated mode of the structure direction of TPA+ for ZSM-5 zeolite, the structure formation of zeolite beta (*BEA) seems to be oriented by a cluster of TEA+ cations.7,10 Through Raman spectroscopy, Okubo and his co-worker found out that the induction period, during which the formed inorganic−organic (TEA+/aluminosilicate) composites are gradually transformed into the zeolite Beta like structure, is the crucial step for the crystallization of zeolite Beta. They also identified that the clustering and conformational rearrangement of TEA+ is strongly related with the formation of micropores and the incorporated Al in the aluminosilicate frameworks, which reveal that the conformation of TEA+ clusters plays a critical structural directing role in the formation process of zeolite Beta. By using solid-state NMR techniques, Davis and co-workers confirmed that the short-range intermolecular interactions (on the order of van der Waals contact distances) are established by forming the inorganic−organic composites during the heating of the zeolite synthesis gel prior to the fully development of long-range order ZSM-5 crystalline structure.11 These types of noncovalent intermolecular interactions are the key factors to yield the inorganic−organic composites during the preorganization process of silicate species. But alone, the van der Waals interactions in the inorganic−organic (TPA+/silicate) composites cannot explain why the four same propyl chains of the TPA+ cations can orient the formation of the paralleled straight channels and zigzag channels in the crystal structures of ZSM-5. Through using 1H double quantum NMR spectroscopy, the interaction between the SiO−··H−OSi hydrogen bonds within the pore wall of ZSM-5 and the methyl groups of the propyl chains of TPA+ cations has been observed.12,13 The specific spatial correlation implies that the electrostatic interactions between OSDA cations and the SiO−··H−OSi hydrogen bonds should play more important roles in orienting the crystallization process. It is well-known that TEA+ cations can be used to prepare zeolite beta when the synthesis temperature is low (≤140 °C), and zeolite ZSM-5 when the synthesis temperature is high (>140 °C). Although the dimension and configuration of the

organic structural directing agent strongly influence the channel properties and topologies of the final zeolites, the structure directing behaviors of TEA+ cations imply that the nature of structure direction of OSDAs is not just about the structure properties of OSDAs themselves. Using Raman spectroscopy, Weckhuysen and co-workers found out that the conformer distribution of TEA+ cations occluded in the inorganic−organic composites are different for preparing MFI (tg.tg) and *BEA (tt.tt) zeolites, and the inorganic−organic composites have similar structure characteristics with their final zeolites.14 Because the Raman spectra cannot provide the stereo correlation between the charged frameworks and OSDA molecules, solid-state nuclear magnetic resonance (SSNMR) spectroscopy is a wonderful tool to study the spatial correlations among the nuclei. The correlation between the alkyl chains of the OSDA cations and the channel systems of formed zeolites is the key to understanding the nature of structure direction of OSDAs in the formation of zeolites; therefore, SSNMR techniques have been employed in this work to find out the spatial relationship between the charged framework and OSDA cations in zeolites. Meanwhile, the assynthesized ZSM-5 and Beta crystals have been characterized by powder X-ray diffraction (XRD) and 1H/19F MAS NMR spectroscopy, and the morphologies of them were pictured through scanning electron microscopy (SEM).

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EXPERIMENTAL SECTION

TPA-OH-ZSM-5 crystals were prepared following the hydroxide route using tetraethyl orthosilicate (TEOS, Aldrich), alkalifree TPAOH solutions (1 M in water) and water. The gel composition is SiO2:0.4TPAOH:40H2O, then the final mixture was transferred into a Teflon-lined stainless-steel autoclave and heated at 175 °C under static conditions for 14 days. After crystallization, the autoclave was rapidly cooled and the zeolite was recovered by centrifugation, washed with distilled water and dried at 60 °C. TPA-F-ZSM-5 was prepared almost the same as the synthesis of OH-ZSM-5, except HF acid was added into the sol−gel to make the mixture have a composition with SiO2:0.4TPAOH:0.4HF:20H2O. For synthesizing the TEAOH-ZSM-5 crystals, the gel composition is SiO2:0.4TEAOH:40H2O, and pure silica ZSM-5 seeds were added onto the gel with 10 wt % and then the final mixtures were transferred into a Teflon-lined stainless-steel autoclave and heated at 175 °C under static conditions for 20 days. B

DOI: 10.1021/acs.jpcc.7b03350 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C However, for preparing TEA-OH-Beta, the gel with SiO2:0.4TEAOH:40H2O composite was heated at 140 °C under static conditions for 30 days. Solid-state 1H−13C cross-polarization magic angle spinning (CP/MAS) and high power decoupling (hpdec) NMR experiments of as-synthesized TPA-ZSM-5 zeolites were performed on a Bruker AVANCE-III 500 spectrometer using a 4.0 mm MAS probe with a spinning rate of 10 kHz, a 1H π/2 pulse length of 3.85 μs, a TPPM 1H decoupling of 65 kHz and a recycle delay of 5 s. 1H MAS and 2D 1H DQ-SQ MAS NMR spectra were carried out in a 1.9 mm MAS probe on a Bruker AVANCE-III 500 spectrometer with a sample spinning rate of 40 kHz, a 1H π/2 pulse length of 1.75 μs and a recycle delay of 2 s. 1H−13C CP/MAS NMR experiments of TEA-OH-ZSM-5 and TEA-OH-Beta were performed on a Bruker AVANCE-III 500 spectrometer using a 1.9 mm MAS probe with a sample spinning rate of 30 kHz, a total of 30000 scans were accumulated with a recycle time of 5 s. 19F MAS NMR spectra were carried out in a 2.5 mm MAS probe on a Bruker AVANCE-III 500 spectrometer with a sample spinning rate of 20 kHz, using a 19F π/2 pulse length of 3.50 μs and a recycle delay of 10 s. The overall pulse phases for the excitation period were phase cycled according to the time-proportional phaseincrement (TPPI) procedure. The chemical shifts of 1H and 13 C were referenced to TMS and 19F chemical shift was referred to CFCl3. The crystallinity of the zeolites was determined through Xray diffraction (XRD) on a Rigaku Ultimate VI diffractometer (40 kV, 40 mA) using Cu Kα (λ = 1.5406 Å) radiation. Diffractogrammes were collected between 4° and 40° (2θ) with steps of 0.02° per second. Scanning electron microscopy (SEM) experiments were performed on Hitachi SU-1510 electron microscopes.

Figure 2. 2D 1H DQ-SQ MAS NMR spectrum of as-made TPA-OHZSM-5.

and co-workers through two-dimensional 1H−2H CPMAS NMR correlation spectroscopy.19 The correlated signals in Figure 2 show that the SiO−··H−OSi hydrogen bonds is spatially close to both the methyl groups of TPA+ in the zigzag channels and in the straight channels similarly, which suggests that the negative charges should be located at the points that have almost the same perpendicular distances to the zigzag and straight pore walls. But it is commonly observed that the methyl carbon resonance in the 1H−13C CP/MAS NMR spectrum of TPA in OH-ZSM-5 is splitting. The split methyl group of TPA+ is dependent neither on the zeolite Si/Al ratio nor on the nature of the alkali cations. The splitting methyl carbon resonances have been attributed to the different channel environments and the closeness between two methyl heads in the same channels,20−22 but this interpretation is obviously questionable. According to 2D 1H DQ-SQ MAS NMR experimental results, it is more reasonable to suppose that the electrostatic interactions between the cationic TPA+ and the defect sites in different ZSM-5 channels could be one of the sources to cause the split. In this work, CP (cross-polarization) technique with various contact times is applied to investigate the reason for the split methyl groups of TPA+ cations. As the simplest kinetic case with both 1/2 spin species, the behavior of CP intensity I(t) versus contact time (t) in the 1H−13C CP NMR kinetics can be described as I(t)= I0[1 − exp(−t/TCH)], in which TCH depends on structural parameters, i.e. the internuclear distances (r) between the carbon and adjacent proton(s). At the earliest time, the carbon magnetization develops rapidly, but then gets slowly and ultimately proton spin−lattice relaxation determines the magnetization evolution.23 In order to avoid other unwanted affective factors such as the relaxation of protons and carbons, only earliest contact time data are used to study the influence of the H−C pairs with different internuclear distances on the carbon signal intensities. In Figure 3, 1H−13C CP/MAS NMR spectra of TPA-F-ZSM-5 show that the intensity of the left signal (δ = 11.6 ppm) is always higher than the right one (δ = 10.6 ppm). Because the signal intensity is verse with the bond lengths, it can be informed that the signal at 10.6 ppm is corresponding to a relative longer C−H bond length than that with a signal at 11.6 ppm. As we know, the pore diameters of zigzag channels (5.1 × 5.5 Å) are smaller than those of the straight channels (5.3 × 5.6 Å),

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RESULTS AND DISCUSSION As we know, both TPA+ and TEA+ cations can be used to synthesize ZSM-5.15,16 Although the high silica ZSM-5 zeolites demonstrate a close geometric correspondence between the pore architecture and occluded cations, the spatial correlations between the charges framework and the OSDAs are still not fully studied. As such, the crystallization mechanism of ZSM-5 is still mysterious for scientists. In order to find out how the TPA+ and TEA+ cations with four same alkyl (propyl or ethyl) chains orient the formation of straight and sinusoidal channels, 2D 1H DQ-SQ MAS NMR experiments have been performed. 2D 1H DQ-SQ MAS NMR method is a powerful tool to probe homonuclear proton−proton spatial proximities and allows to for the observation of resonances with very close or even identical isotropic chemical shifts.17 In the 2D DQ-SQ spectrum the spectral span of the DQ dimension is twice that of the SQ dimension, i.e. the chemical shift value of a correlated peak in the DQ dimension is the sum of the isotropic chemical shifts of the two same or different spins in the SQ dimension. The highly resolved NMR signals in Figure 2 provide abundant information about the proton proximities in ZSM-5. In order to obtain the better DQ dipolar recoupling, SPIP sequence is employed in this 1H DQ-SQ MAS experiments.18 In Figure 2 the pair signals at (10.2, 11.2) and (1.0, 11.2) clearly demonstrate that the SiO−··H−OSi hydrogen bonds within the framework are spatially close to the methyl groups of TPA+ cations. Actually, the spatial proximity between the organic SDA cations and the inorganic charge centers (SiO−·· H−OSi) in as-made zeolites has also been observed by Lobo C

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H species from intermolecular C···H sources start to show their effects on the signal intensities because a long contact time allows for more efficiency in transferring a long-distance proton dipole. When the contact time approaches to 1000 μs, the signal intensity at 10.6 ppm is higher than that at 11.6 ppm. Therefore, it can be deduced that the protons of SiO−··H−OSi hydrogen bonds within the framework are spatially closer to the methyl groups of TPA+ in zigzag channels than those in straight channels. The signal intensity changes caused by the conflicting effects of the intramolecular C−H bonds and the intermolecular C···H further confirm that the SiO−··H−OSi hydrogen bonds within the framework are spatially close to the methyl groups of TPA+ in the channels. These specific spatial proximities between the TPA+ cation with the charged framework made the electrostatic interactions play the dominant roles in the formation of zigzag channels and straight channels in ZSM-5. In order to further confirm that the signal intensity differences of the methyl groups of TPA+ are from the intramolecular and the intermolecular proton dipolar transformation, high power decoupling 13C SSMAS NMR (hpdec) experiments have been performed. This pulse sequence can get rid of the dipolar effects from the protons. Parts a and b of Figure 5 show that the intensities of the two signals from the

Figure 3. 1H−13C CP/MAS NMR spectrum of as-made TPA-F-ZSM5 with various contact times (a = 100 μs, b = 300 μs, c = 500 μs, d = 700 μs, e = 800 μs, f = 900 μs, g = 1000 μs). The methyl signals of TPA are highlighted.

and the smaller pore diameters will make the negative charges within the framework are closer to the methyl groups of TPA+ in zigzag channels. Then the stronger electronic interactions stretch the C−H bonds of methyl groups of TPA+ in the zigzag channels a little bit longer than that in the straight channels. Therefore, the signals of the methyl groups of TPA+ in Figure 3 can be assigned: the methyl group with higher signal at 11.6 ppm is in the straight channels for the shorter C−H bond length and the methyl group with lower signal at 10.6 ppm is in the zigzag channels for the longer C−H bond length. Other than zeolites TPA-F-ZSM-5, in which the intramolecular C−H bonds are the only source to provide the proton dipolar transformation, zeolites TPA-OH-ZSM-5 have the protons in SiO−··H−OSi hydrogen bonds within the framework as the other proton dipolar transformation source. 1 H−13C CP/MAS NMR spectra of as-made TPA-OH-ZSM-5 with variety contact times in Figure 4 demonstrate the conflict affection of the intramolecular C−H bonds and the intermolecular C···H on the signal intensities of methyl groups. When the contact time is lower than 800 μs, the H sources from C−H bonds play the dominant roles in the dipolar transfer; however, as the contact time is higher than 800 μs, the

Figure 5. 13C hpdec/MAS NMR spectrum of as-made TPA-OH-ZSM5 (a) and TPA-F-ZSM-5 (b).

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methyl groups in TPA-OH-ZSM-5 and TPA-F-ZSM-5 are the same. These signal intensity differences of the methyl groups of TPA+ of zeolite ZSM-5 in 1H−13C CP/MAS NMR spectrum has previously been attributed to the experimental error.21 Our 13 C spectra clearly demonstrate that the different signal

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Figure 4. H− C CP/MAS NMR spectra of as-made TPA-OH-ZSM5 with various contact times (a = 100 μs, b = 300 μs, c = 500 μs, d = 700 μs, e = 800 μs, f = 900 μs, g = 1000 μs). The methyl signals of TPA are highlighted. D

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The Journal of Physical Chemistry C intensities are coming from the proton dipolar transformation, not “experimental error”. Davis and his co-workers suggested that the key initial step for the structure direction of TPA+ is the formation of TPA+silicate composites, and the van der Waals interactions between the silicate species and alkyl chains of TPA+ are the driving forces in forming the inorganic−organic composites.11,24 As we know, controlling the self-assembly process of the local silicates species is one of the key challenges for the formation of zeolites with unique channel systems and structural properties. Our research results suggest that the strong electrostatic interactions between the SiO−··H−OSi hydrogen bonds within the framework and the methyl groups of TPA+ are the dominant interactions in orienting the self-assembly process of the silicates species to form the inorganic−organic composites with the MFI-like structures. As we know, the TEA+ cations can also function as a structural directing agent in synthesizing zeolite ZSM-5. The spatial correlations between the structural defect sites within the framework and the TEA+ cations are also worth studing. 1H DQ-SQ MAS NMR experiments have been performed to investigate the spatial proximity of protons in the TEA+ cations with the protons in the SiO−··H−OSi hydrogen bonds. The 1H DQ-SQ MAS NMR spectrum in Figure 6 demonstrates that the signal at (10.0, 11.2) and (10.0, 13.2)

Figure 7. 1H−13C CP/MAS NMR spectrum of as-made TEA-OHZSM-5.

TPA+ that caused the split signals, instead of the different environments of zigzag channels and straight channels.20−22 Weckhuysen and co-workers also presented that the large stabilization energy differences provided by the electrostatic interactions between the charged framework and TEA+ cations made the TEA+ cations adopt the specific conformer in the framework.14 A schematic is drawn in Figure 8 to demonstrate the spatial correlations between the SiO-··H−OSi hydrogen bonds within

Figure 8. Schematic of spatial correlations between OSDA cations in the cross sections and the charged MFI framework: (a) TPA+ cations in TPA-ZSM-5 channels and (b) TEA+ cations in TEA- ZSM-5 channels. Figure 6. 2D 1H DQ-SQ MAS NMR spectrum of as-made TEA-OHZSM-5.

the framework and TPA+ and TEA+ cations. This schematic clearly demonstrates that the electrostatic organic−inorganic interactions influence the location of SiO−··H−OSi hydrogen bonds in the zeolite framework. Normally, two synthesis routes have been adopted to prepare zeolites: one uses OH− anions as mineralizers at high pH values (>10) and the other uses F− anions as mineralizers at lower pH values (7−8). When zeolites were prepared through the hydroxide synthesis condition, smaller crystals were obtained and the positive charges of OSDAs are compensated by framework connectivity SiO−··H−OSi hydrogen bonds.24,25 But the larger crystals are formed under the fluoride route with the absence of the framework defects and the positive OSDA charges are compensated by fluoride anions located in the specific subunits in the framework.26 Both the SiO−··H−OSi hydrogen bonds within the framework and F− anions play the role of negative charge centers to counterbalance the positive charges from the OSDA cations.27 Lots of zeolites can be

are corresponding to the spatial correlations between the protons from the SiO−··H−OSi hydrogen bonds within the framework and the alkyl (both methyl and methylene) groups of TEA+, respectively. Because the other symmetric signal pairs are covered by the not yet evolved alkyl signals of TEA+, they are not shown in the spectrum. According to our observations and discussions about TPA-OH-ZSM-5, we can tell that the carbon signals from both methyl and methylene groups of TEA+ are split for the strong electrostatic interactions from the spatial proximity. The 1H−13C CP/MAS NMR spectra of TEAOH-ZSM-5 in Figure 7 clearly demonstrate that the carbon signals from both the methyl and methylene groups of TEA+ are split. This observation further confirms that it is the strong electrostatic interactions between the SiO−··HOSi hydrogen bonds within the framework and alkyl chains of TEA+ and E

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The Journal of Physical Chemistry C prepared by both the basic and the neutral synthesis routes, and it is commonly observed that the number of the SiO−··H−OSi hydrogen bonds within the framework or F anions in the structures decreases as the number of T3+ sites in the zeolite framework increases.28 Therefore, by studying the spatial correlation between the OSDAs and the SiO−··HOSi hydrogen bonds within the framework, the position and distribution of T3+ atoms in the zeolite framework could be inferred indirectly. According to the 2D 1H DQ-SQ MAS NMR spectrum of TPAOH-ZSM-5 and TEA-OH-ZSM-5, we can suggest that the position and distribution of T3+ atoms in the MFI framework are at the centers between the channels for matching the specific spatial proximities between the SiO−··H−OSi hydrogen bonds within the framework and the methyl groups of TPA+. Because both the methyl and methylene groups of TEA+ cations are spatially close to the SiO−··H−OSi hydrogen bonds within the framework, 2D 1H DQ-SQ MAS NMR spectrum of TEA-OH-ZSM-5 tell that the position and distribution of T3+ atoms in the MFI framework are away from the centers between channels, and they are closer to the channels’ intersections than those in TPA-ZSM-5. For zeolite ZSM-5, the TPA+ or TEA+ cation are isolated in the intersection of channels, but for zeolite Beta,29 the TEA+ cations seems to be clustered in the cavities formed by channels.,7 2D 1H DQ-SQ MAS NMR spectrum of TEA-OHBeta in Figure 9 demonstrate the spatial correlation between the protons in the TEA+ cations with the protons in the defect sites (SiO−··H−OSi).

Figure 10. 1H−13C CP/MAS NMR spectrum of as-made TEA-OHBeta.

SiO−··H−OSi hydrogen bonds within the framework with the alkyl groups of TEA+ are weak over long distances. The signal pair at (3.4, 4.8) and (1.4, 4.8) in Figure 9 corresponds to the spatial closeness between methyl and methylene groups of TEA+, which means the distance between TEA cations is short. This observation is consistent with TEA+ cations aggregating to form a TEA+ cluster to direct the formation of zeolite Beta.7 The electrostatic interactions and van der Waals interaction between the TEA+ clusters and the charged framework coexist in the inorganic−organic composites, but van der Waals interactions, not the electrostatic interactions, play the dominant structure direction roles to organize the silicate species around the TEA+ cluster to form zeolite Beta. The structure direction roles of OSDAs are performed by the interactions between the OSDA cations and the charged framework in the inorganic−organic composites, rather than the structure properties of OSDAs themselves. The complex relationship between van der Waals interaction and electrostatic interaction in the inorganic−organic composites is the fundamental reason for the different zeolite structures.

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CONCLUSIONS The highly resolved 1H DQ-SQ correlation spectra of TPAOH-ZSM-5 and TEA-OH-ZSM-5 confirmed that the SiO−·· H−OSi hydrogen bonds within the MFI framework are spatially close to the alkyl chains of TPA+ and TEA+ cations. Variable contact times 1H−13C CP/MAS NMR experiments further confirm that the SiO−··H−OSi hydrogen bonds within the framework not only are spatially close to the alkyl chains of TPA+ and TEA+ cations but also are the reasons causing the splitting of methyl groups of TPA+ cations. After analyzing the variable contact times 1H−13C CP/MAS NMR experiments of TPA-OH-ZSM-5 and TPA-F-ZSM-5, the signal at 10.6 ppm can be assigned to the methyl group in the zigzag channels and the signal at 11.6 ppm can be assigned to the methyl group in the straight channels These observations clearly demonstrate that the electrostatic interactions between the negative charges within the framework and the methyl groups of TPA+ cations are the dominant interactions that direct the crystallization process of zeolite ZSM-5. But for the structure direction of TEA+ to *BEA zeolite, 1H DQ-SQ correlation spectrum and 1H−13C CP/MAS NMR spectrum show that the electrostatic interactions between the OSDA cations and the charged framework are relativly weaker than those in MFI zeolites, and thus the van der Waals interactions between OSDA clusters and the framework in the

Figure 9. 2D 1H DQ-SQ MAS NMR spectrum of as-made TEA-OHBeta.

Unlike the strong correlation signals between the protons of the SiO−··H−OSi hydrogen bonds within the framework with the alkyl groups of TEA+ in zeolite TEA-OH-ZSM-5, the correlation signals are rather weak and difficult to observe for zeolite TEA-OH-Beta. The observation reflects that the distances from the SiO−··H−OSi hydrogen bonds within the framework to the alkyl groups of TEA+ of zeolite TEA-OHBeta are longer than those in zeolite TEA-OH-ZSM-5, which caused the less effective recouplings among the protons. 1 H−13C CP/MAS NMR spectrum of TEA-Beta zeolites in Figure 10 demonstrate that no carbon signals are splitting. This observation further confirms that the interactions between the F

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Article

The Journal of Physical Chemistry C

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inorganic−organic composites play the dominant structure directing roles during the crystallization process of zeolite Beta. We can suggest that the nature of structure direction of OSDAs is mainly original from the complex relationship between van der Waals interactions and electrostatic interactions in these inorganic−organic composites, other than the structure properties of OSDA themselves. If the inorganic− organic electrostatic interactions determine the self-assembly processes, an isolate model is fit to describe the zeolite crystallization mechanism, but when the van der Waals interactions are the dominant interactions, a clustered model is more reasonable.

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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.7b03350. Additional information about the SEM pictures, XRD patterns, and NMR spectra of as-made zeolites (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(X.L.) E-mail: [email protected]. ORCID

Xiaolong Liu: 0000-0002-7346-7846 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Foundation of China (Grants 21673282, 21473246, and 21475147).

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DOI: 10.1021/acs.jpcc.7b03350 J. Phys. Chem. C XXXX, XXX, XXX−XXX