Bolaform Molecules Directing Intergrown Zeolites

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Bolaform Molecules Directing Intergrown Zeolites Xiaoli Jia, Yunjuan Zhang, Zheng Gong, Bo Wang, Zhiguo Zhu, Jingang Jiang, Hao Xu, Huai Sun, Lu Han, Peng Wu, and Shunai Che J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02946 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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The Journal of Physical Chemistry

Bolaform Molecules Directing Intergrown Zeolites Xiaoli Jia†,#, Yunjuan Zhang†,#, Zheng Gong#, Bo Wang$, Zhiguo Zhu$, JinGang Jiang$, Hao Xu$, Huai Sun#, Lu Han*,#,%, Peng Wu*,$ and Shunai Che*,#,% #

School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240 (P.R. China)

%

School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, China, 200092 (P. R. China) $

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai, 200062, (P.R. China)

ABSTRACT: Understanding of epitaxial and rotational intergrowth is particularly important for the design and synthesis of hierarchically structured zeolites. Herein, we propose a new route for fabricating hierarchical MTW and MFI-type zeolites with intergrown structures by tuning the methylene chain lengths between the two N-heterocycles in bolaform quaternary ammonium structure-directing agents (SDAs). MTW-type zeolites with 66.4º rotational intergrowth and MFI-type zeolites with 90º rotational intergrowth were synthesized with the methylene chain lengths of 4-6 and 7-9, respectively. A possible theoretical evidence of the geometrical match between SDAs and zeolite structures was proposed by the binding energy obtained by molecular mechanics calculations. It is speculated that the bolaform SDAs are located in the straight 12-membered ring (MR) channels along the b direction of MTW topology, directing the intergrown structure by sharing the (310) plane. The bolaform SDAs located in the intersections of straight 10-MR channels and sinusoidal 10-MR channels of MFI topology directed the intergrown structure by sharing the (010) plane. The hierarchical zeolite catalysts prepared using this strategy, characteristic of hybrid microporous and mesoporous porosities, proved to be useful for the tandem cracking reactions involving bulky molecules.

INTRODUCTION Zeolites are widely used as adsorbents, ion-exchangers and catalysts in petrochemical industry because of their crystalline frameworks and unique microporosities.1-4 However, the zeolites with single micropores usually suffer from serious molecular diffusion problems and fail to catalyse organic molecules with large dimensions. This drawback can be partially solved by shortening the effective diffusion pass lengths5-7 or introducing mesopores into zeolite particles.8 In the past decades, many approaches, including hard-9-14 and soft-templating methods,15-20 have been proposed to synthesize hierarchical zeolites. Among these solutions, the fabrication of intergrown zeolites is of interest not only because they often exhibit unique performance characteristics in heterogeneous catalysis21, 22 but also because they provide inspiration for the design and synthesis of hierarchically structured porous materials.

formed with 90° rotational intergrowths, in which the (h00) faces are epitaxially overgrown on the (0k0) faces. Self-pillared MFI nanosheets with repetitive branching were synthesized using tetrabutylphosphonium (TBP) hydroxide as the SDA.26 Similarly, plate–like ZSM-5 structures with a sequential intergrowth were constructed using dimers of TPA cations as the SDAs.30, 38 Mesoporous FAU-type nanosheets were obtained by soft-templating technique with organosilane surfactants, such as 3-(trimethoxysilyl) propyl hexadecyldimethyl ammonium chloride (TPHAC).34 The progress has been realized using well-designed mesoporogens composed of hydrophobic benzenes and bolaform hydrophilic SDAs generating ordered mesoporous zeolites with house-of–cards-like morphology and stable mesostructure even after calcination.17, 18 The MTW zeolites contain a large-pore system composed of one-dimensional 12-MR channels along the b-axis with a window opening of approximately 5.6 × 7.7 Å, which attracted industrial interest as catalysts due to their special microporous structures.39-41 The hierarchical MTW with a multi-dimensional mesostructure improves the performance of its one-dimensional micropore system. Several papers reported the large crystal MTW zeolite with one node intergrown,42-44 but only one paper described the intergrown structure, in which the monoclinic polymorph B with a stacking sequence of abcabc twining at every

The presence of branched structures (by twinning, rotational intergrowths, or polytypic overgrowths) is a common phenomenon in inorganic crystals and zeolites.23, 24 Epitaxially and rotationally overgrown zeolites, e.g., MFI/MEL,25-27, FAU/EMT28, 29, MFI,30-32 AFI,33 FAU34, 35 and MWW,36 have been studied extensively. For example, an MFI-type ZSM-5 zeolite usually presents a classic 90º rotational intergrowth (twinning or multiple) structure.37 MFI zeolites with straight 10-MR channels along the b-axis and sinusoidal 10-MR channels along the a-axis are often ACS Paragon Plus Environment

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stacking sheet resulted in the orthorhombic polymorph A with a stacking sequence of abab.45 It is known that the N-hetrocycles connected by methylene chains can be used as SDAs in the synthesis of kinds of zeolites.46-49 It is also known that the bolaform-type molecules possess the ability to direct the intergrowth of zeolite structures.17, 18, 30, 38 By changing the length of the carbon chains between the two N-heterocycles, it was possible to geometrically match different zeolite topologies.50-54 To the best of our knowledge, no successful methods for creating hierarchically intergrown structures of MTW zeolites have been reported. Herein, we attempted to construct hierarchically intergrown MTW zeolite structures by using diquaternary N-methyl pyrrolidiniums linked by different methylene chains (C4H8-N+(CH3)-CnH2n-N+(CH3)C4H8 (2Br-), denoted as CNMP-n, where n=4, 5, 6, 7, 8, 9) (Figure S1) as SDAs. The structure of different zeolites was characterized by powder X-ray diffraction (XRD), Scanning electron microscopy (SEM), High-resolution transmission electron microscopy (HRTEM) and nitrogen adsorptiondesorption isotherm. In addition, the possible role of the SDAs on the formation of different zeolites was discussed by binding energy calculated by molecular mechanics. EXPERIMENTAL SECTION Synthesis of bolaform molecules: In a typical synthesis, the divalent 1,5-bis (N-methylpyrrolidinium) pentane (CNMP-5) cations was prepared by refluxing 1,5-dibromopentane with an excess of 1-methylpyrrolidine in acetonitrile for 20 hours. The excess amine was removed by extraction with acetonitrile, and recrystallizations were performed in diethyl ether mixtures. By changing the reactant, CNMP-n was obtained using the same method. Synthesis of intergrown zeolites: The hydrothermal synthesis of zeolite materials followed the conventional methods as reported. Zeolites synthesized by using different SDAs were performed at the original gel composition of x SDA: 20 SiO2: 0.167 Al2O3: 4.2 NaOH: 720 H2O (2≤x≤ 4).. The colloidal silica solution and Al2(SO4)3∙18 H2O were used as silicon and aluminium source, respectively. Take the synthesis procedure by using the CNMP-5 for an example： 0.36 g bromide form CNMP-5, 0.06 g sodium hydroxide and 0.0315 g Al2(SO4)3.18 H2O were mixed in 3.87 g deionized water with vigorous magnetic stirring. After stirring about 1 h, 1.5 g colloidal silica was added drop wise. The mixtures were stirred for another 3 h. Crystallization process was carried out in Teflon-lined stainless steel autoclaves (15 ml) at 433K with the autoclaves set to tumbling at 30 r.p.m. After a desired period, the zeolite products were filtered, washed with distilled water and dried at 373 K overnight. The as-prepared zeolite products were calcined under 823 k for 8 h for the removal of organic species. Characterization: Powder XRD patterns were recorded on a Rigaku X-ray diffractometer D/max-IIIA equipped with a Cu Kα radiation source (40 kV, 30 mA). SEM was conducted on a JEOL JSM-7401F electron microscope with the landing energy of 1 kV with a specimen bias of -2 kV and JEOL JSM-7800F electron microscope with the landing

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energy of 0.3 kV with a specimen bias of -5 kV. HRTEM was performed using an FEI G2 F30 microscope operating at 300 kV (Cs = 1.2 mm, point resolution of 2.0 Å) and JEOL JEM-2100 microscope operating at 200 kV (Cs = 1.4 mm, point resolution of 2.3 Å). The nitrogen adsorption/desorption isotherms were measured at 77 K using an ASAP 2010 M+C analyser. Thermogravimetric analyses (TGA) were performed on TGA 7 thermogravimetric analyser (Perkin Elmer, Inc., USA). The heating rate was 20 °C/min. The element analysis of Si, Al and C, N was determined from the inductively coupled plasma analysis (ICP, PerkinElmer 3300DV) and Elementar Vario-ELIII IRMS analyser, respectively. The temperature-programmed desorption of NH3 (NH3-TPD) was performed on Micromeritica Auto Chem 2920. The IR spectra was collected on a Nicolet Nexus 670 FT-IR spectrometer in absorbance mode at a spectral resolution of 2 cm−1 using the KBr technique (3 wt % wafer). 13C solid-state MAS NMR spectra were recorded on a VARIAN VNMRS-400WB spectrometer under one-pulse conditions. 1H Nuclear Magnetic Resonance (NMR) spectra were recorded on a Varian MERCURY plus-400 (400 MHz) spectrometer, and the chemical shifts are reported in ppm relative to the residual deuterated solvent and the internal standard tetramethylsilane. RESULTS AND DISCUSSION Synthesis of intergrown MTW zeolites. MTW-type zeolites were readily crystallized at 433 K for 4 days with CNMP-4-6 as SDAs. The XRD patterns of the calcined samples indicate that MTW-type zeolites were obtained when the carbon chain length was between 4 and 6 (Figure 1). The refinement of calcined material was given in Figure S2 and Table S1, S2 to precisely analyse the data. The final results of the Rietveld analyses were Rwp = 13.4%, Rp = 16.3% and Rexp = 5.94%, indicating that the experimental powder pattern agrees well with the standard one. The XRD results verify that highly crystalline zeolites were produced and were free of other zeolite impurities and amorphous phase. The phase purity was also verified by corresponding SEM images (Figure 2).

Figure 1. Powder XRD patterns of calcined samples synthesized with CNMP-4 (a), CNMP-5 (b), CNMP-6 (c) and the black bar refer standard MTW zeolite copyright from IZA. All samples possess MTW-type framework.

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The SEM images show a morphological comparison of the zeolite samples synthesized with CNMP-4-6 (Figure 2). The MTW superstructures were all composed of specifically oriented assembled nanorods (Figure S3). As shown in Figure 2, for the MTW zeolites directed by CNMP-4-6, the morphology changed from relatively large fishbone-like with sparse intergrowth (CNMP-4) to small snow flower-like with dense intergrowth (CNMP-5-6), and the diameter of the intergrown nanorod changed from ~50 to ~40 nm. Compared to MTW-CNMP-4, MTW-CNMP-5 and MTW-CNMP-6 possess a greater degree of intergrowth, which suggests that only proper chain lengths are favourable for the formation of intergrown MTW zeolites. Methylene chains of 5 and 6 were more favourable for intergrowth. The time resolution of CNMP-5-MTW (Figure S4) indicated that the morphologies shown in Figure 2 were stable but not transient.

Figure 2. Low (a, c, e) and high (b, d, f) magnification SEM images of the calcined samples directed by CNMP-4 (a, b), CNMP5 (c, d), CNMP-6 (e, f), revealing that all MTW samples composed of intergrown nanorods.

The porosities of the hierarchically intergrown zeolites were characterized by nitrogen sorption/desorption measurements (Figure 3). All the samples (MTW-CNMP-4, MTWCNMP-5 and MTW-CNMP-6) exhibit a steep increase below P/P0= 0.02, indicating the existence of micropores of the zeolite structure. The adsorption amount of MTW-CNMP-4 remains almost no change at higher relative pressure. While the MTW-CNMP-5 shows the capillary condensation

with the hysteresis loop in relative pressure range of P/P0 = 0.4-0.9, indicative of the presence of abundant mesopores. The MTW-CNMP-6 exhibits hysteresis loop with a rapid increase at high relative pressure range close to P/P0 = 1, suggesting the porous structure formed by stacking of the nanoparticles. Accordingly, the pore size distribution curve of the MTW-CNMP-5 and MTW-CNMP-6 zeolites exhibits mesopores with pore size in a wide range of 5-25 nm. However, MTW-CNMP-6 zeolites exhibits much sharper mesopores distribution than MTW-CNMP-5, indicating the presence of relatively uniform and large amount of mesopores in MTW-CNMP-6. The textural properties of the samples are summarized in Table S3. The three samples possess similar micropore volumes but different mesopore volumes and Brunauer-Emmett-Teller (BET). Specially, MTW-CNMP-6 shows a higher BET surface area (263 m2/g) and a larger total pore volume (0.21 m3/g) than the other two samples. These results are in good agreement with the SEM characterizations shown in Figure 2. The mesoporousity of the samples is due to the voids formed by stacking of the nanorods. MTW-CNMP-4 has less intergrowth structure and the nanorods are closely attached to each other. For MTWCNMP-5, the stacking of the nanorods with more intergrowth structure create abundant mesopores. While the MTWCNMP-6 has smaller nanorods with large amount of intergrowth structure, leading to the disordered mesoporousity and macroporoustiy by stacking of the nanorods.

Figure 3. Nitrogen physisorption isotherms (a) and pore size distribution (b) of intergrown MTW, indicating that samples directed by CNMP-5 and CNMP-6 possess mesoporous structure owing to the assembly of nanorods with great degree of intergrowth and small crystal size.

TEM technique was used to further investigate the intergrowth structure in the MTW zeolites. The images of calcined CNMP-5 after slicing are shown in Figure 4. The lowmagnification TEM image (Figure 4a) taken along the caxis show the intergrowth structure with crossed branches. The selected-area electron diffraction (SAED) of the whole area shows a very complex pattern, corresponding to the overlapping of three SAED patterns. They are taken from the same [001] direction, however, two patterns are rotated ±66.4º from the intermediate one. Therefore, the SAED patterns are overlapped on both 310 and 3-10 reflections.



The simulated SAED pattern is shown in Figure 4c, which agrees very well with the observed pattern. The HRTEM image taken at the boundary of the main backbone and the side branch is shown in Figure 4d. The image composes of two structural domains sharing the common [001] direction but they differ by 56.8º from each other. The corresponding Fourier diffractograms (FDs) indicate that the two domains are sharing the (310) plane as the boundary. Therefore, the exception crystal morphology and the SAED pattern are due to the formation a contact reflection twin and the twin plane is (310). As the dihedral angle between (310) and (200) is 56.8º, it fits the observation of the 66.4º = 180º − 2 × 56.8º. Besides, it was also confirmed that the

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channel direction corresponded to the long axis of the nanorods, implying the growth velocity along the direction of the 12-MR channels was higher. Based on the observation, a structural model (Figure 4e and 4f) is built to explain the twinning, which the twin plane is in the middle of two (310) planes and the bondings can be perfectly reconnected. This is different from the previous observation by Terasaki et al.,45 which reported that intergrown MTW zeolite was formed by stacking disorders, in which the monoclinic polymorph B with a stacking sequence of abcabc twining at every stacking sheet resulted in the orthorhombic polymorph A with a stacking sequence of abab along the [010] direction.

Figure 4. HRTEM images of slices of the calcined intergrown MTW-CNMP-5 zeolite. Low- (a) and high-magnification (d) TEM images taken along the common c-axis of sliced calcined samples, showing the intergrown structure with crossed branches. The SAED (b) and simulated pattern (c) of the crystal shown in a, indicating the intergrowh of the MTW nanorods on {310} plane. Proposed structure model (e, f) for twined MTW zeolite by sharing the common (310) plane seen from different directions.

Synthesis of intergrown MFI zeolites. Figure 5 shows XRD patterns of the calcined samples directed by CNMP-7-9, which indicate that pure MFI zeolites were obtained. The SEM images (Figure 6a-f) clearly revealed that MFI-CNMP-79 zeolites possess 90ºrotational intergrown structure. The morphology changed from cuboid-like particles composed of non-uniform nanorods in diameter (approximately 100 nm for MFI-CNMP-7 and 50 nm for MFI-CNMP-8) to cross-like particles with limited intergrowth (CNMP-9). The most dense and greatest intergrowths were formed by CNMP-8. The difference between the CNMP-9-directed sample and the others is clear, as the former possessed less intergrowth structure. These results suggest that the number of methylene carbons between two N-heterocycles in the SDAs strongly alter the intergrowth structure.

The nitrogen adsorption/desorption isotherms of the calcined samples MFI-CNMP-7 and MFI-CNMP-8 experiences both a steep increase below P/P0= 0.02 and a capillary condensation with the H4-type hysteresis loop in relative pressure range of P/P0 = 0.4-0.99, revealed the existence of both micropores and macropores (Figure S5). For MFI-CNMP-9, it shows the microporousity below P/P0= 0.02 and a capillary condensation with the H3-type hysteresis loop in the high relative pressure region. All these samples show a rapid increase at high relative pressure range close to P/P0 = 1, suggesting the mesoporousity and macroporousity due to the stacking of the nanorods. The textural properties (Table S3) shows that the three samples possessed very close micropore volumes. However, MFI-CNMP-8 showed a higher surface area (406 m2/g) and larger mesopore volume (0.22


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m3/g) than the other two samples due to the dense intergrowth and relative small nanorods, as confirmed by SEM observations (Figure 6). MFI-CNMP-7 has less intergrowth structure and MFI-CNMP-9 shows dense structure without obvious porousity, while abundant mesoporousity was created by the intergrowth of nanorods in MFI-CNMP-8 structure. Therefore, MFI-CNMP-8 sample possessed a higher BET surface area and larger pore volume (Figure 6).

Figure 5. Powder XRD patterns of calcined samples directed by MFI-CNMP-7 (a), MFI-CNMP-8 (b), MFI-CNMP-9 (c), showing that all samples are MFI framework type zeolites.

Figure 6. SEM images taken at different magnifications of MFI-CNMP-7 (c, d), MFI-CNMP-8 (e, f) and MFI-CNMP-9 (g, h), revealing that all MFI samples composed of intergrown nanorods.

The internal structure of calcined MFI samples directed by CNMP-8 was also observed using HRTEM. Figure 7a shows the low-magnification TEM image taken from the top view of the cross-section of a cuboid particle. The 90º degree rotational relationship can be observed, which the branches are random distributed in the particle. Figure 7b shows the SAED pattern taken from the whole area. A fourfold symmetry can be observed, which is due to the 90º degree rotational intergrowth structure while sharing the common [001] direction of MFI, which the 200 and 020 reflections from the domains with perpendicular relationship are overlapped. Judging from the TEM image, the MFI nanocrystals are not with exactly 90o relationship, the angle variation made the diffusion and rotation of the high order reflections. Therefore, the simulated SAED pattern was performed by rotating the simulated patter and overlapped them within ±1.5o, which agrees with the observed SAED pattern very well (Figure 7c). The HRTEM image of the intergrowth boundary is shown in Figure 7d. The image shows two domains sharing the common [001] direction but differ by 90o from each other. The corresponding FDs also confirmed that the nanorods connected with each other with a 90° rotation. Therefore, it can be concluded that the (100) plane is overgrown on the (010) plane, namely, the sinusoidal 10-MR channels along the a-axis interconnect with the straight 10-MR channels along the baxis. The structural model of the intergrowth is shown in Figure 7e and 7f, the boundaries with full atomic connectivity can be achieved. Discussion for the formation of MTW and MFI and intergrown structure. It can be considered that bolaform molecules have ability to direct intergrown zeolites and the zeolitic framework type strongly associated with the methylene number between the two quaternary ammonium groups. The solid-state 1H-13C cross-polarization magic-angle spinning (CP/MAS) NMR spectrum of the as prepared MTW and MFI zeolites synthesized at 433K reveals that the CNMP-5 and CNMP-8 cations are occluded intact in the zeolite products (Figure S6, S7). To understand the formation mechanism and how the zeolite framework types depend closely on the methylene number between two Nheterocycles,, the relative stabilities/binding energies of MTW and MFI zeolites with different CNMP-n (Figure S8) were calculated using molecular dynamics (MD) simulations. The simulation models of MTW and MFI were constructed using experimental CHN (Table S4) and TG (Figure S9) data and the results are summarized in Table S4. The zeolite frameworks using in computational calculations were downloaded from IZA, which are composed of Si and O atoms. The calculation results indicate that the binding energies of CNMP-4 and CNMP-5 with MTW topology were lower than those with MFI. For CNMP8 and CNMP-9, the MTW zeolite energies were higher, as shown in Table S5. The binding energies of CNMP-6 and CNMP-7 with MTW were close to that with MFI. The calculated results were in good agreement with the experimental results, which provided a possible evidence for the geometric matching between the SDAs and zeolites.



When using CNMP-6 and CNMP-7 as SDAs, a mixture of MTW and MFI zeolites were obtained under Al-free conditions (Figure S10, S11), which is in accordance with the calculation result. However, CNMP-4~6 directed pure MTW zeolites and CNMP-7~9 directed pure MFI zeolites when aluminium was introduced into the synthesis system. It is worth noting that the morphology was also influenced by the composition of the reactants. The aluminium source is one of the crucial factors for the control of zeolite topology and phase purity that would be controlled by many thermodynamic and kinetic factors. The existence of aluminium facilitate the intergrown of zeolite crystals probably because the negatively charged Al-O tetrahedron favour the formation of defects. In this way, all samples formed greater degree of intergrowth with addition of aluminium than without it. To fully study the effect of Al on the “sudden change” of the zeolitic framework in the synthesis system, samples were prepared by changing Si/Al ratios in the range of 50200 (Figure S13-18). The phase “sudden change” just happened with Si/Al ratios below 60. With the Si/Al ratio above 60, CNMP-4~6 still directed MTW-type zeolite and CNMP-8~9 directed MFI-type zeolite, while CNMP-7 directed the mixture of the two types of zeolites. The existence of Al in zeolites could make different binding energy between SDA and MTW or MFI. It can be speculated that the negatively charged Al would be strongly interacted with SDA with shorter carbon chain in the framework of MTW than MFI, and vise versa with SDA with longer carbon chain,

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probably due to the special location of Al in the zeolite framework. Therefore, the existence of Al caused greater difference on the binding energy between SDA-MTW and SDA-MFI, as CNMP-6 directed MTW zeolite and CNMP-7 directed MFI zeolite. However, the effect of Al on the formation of zeolite both in theory and experimental needs to be further explored and improved. The effect of SDA structure on the formation of zeolite intergrowth can be explained in terms of geometric matching between the SDAs and zeolite frameworks. To further explore how the CNMP-n cations can induce crystallization intergrowth, the optimized distance between the two nitrogen atoms was calculated to be 6.3, 7.5, 8.7, 9.9, 11.3 and 12.5 Å for CNMP-4-9, respectively (Figure S18). For intergrown MTW structures sharing the (310) plane, the distance between the two quaternary ammonium molecules was estimated to be approximately 7.5 Å. The possible arrangement of SDA in an MTW framework is shown in Figure 8a and b. The CNMP-4-6 cations were parallel to the 12-MR straight channel but tune their arrangement at the intersection to adapt to the intergrown structure. The particular arrangement of SDAs in MTW zeolites and the certain length of the SDA molecules determined the intergrown structure. However, in comparison to CNMP-5 and CNMP-6, the methylene length of CNMP-4 is too short for the formation of intergrown MTW zeolites. Thus, CNMP-4 gave rise to less intergrown structure and smaller mesoporous volume, as confirmed by SEM images (Figure 2a) and textural properties (Table S3).

Figure 7. HRTEM images of slices of the calcined intergrown MFI-CNMP-8 zeolite. Low- (a) and high-magnification (d) TEM images taken along the common c-axis of sliced calcined samples, showing the intergrown structure with crossed branches. The SAED (b) and simulated SAED pattern of the crystal shown in a, indicating the 90 degrees rotational intergrowth relationship. Proposed structure model (e, f) for intergrown MFI zeolite, which the (100) plane is overgrown on the (010) plane by connecting the 10-MR channels along the a-axis with the straight 10-MR channels along the b-axis.


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Figure 8. Schematic illustration of SDA arrangement directing for intergrown channels of MTW (a, b) and MFI (c, d) zeolites. (a) Bolaform molecules arrangement in the intergrown MTW zeolite seen from the common [001] direction. (b) SDAs located in the straight 12-MR channel of MTW zeolite. (c) Bolaform molecules arrangement in the intergrown MFI zeolite seen from the common [001] direction. (d) SDAs located in the straight 10-MR channel of MFI zeolite.

In intergrown MFI zeolites, the quaternary ammonium could be located in the intersections along the straight and sinusoidal channels. As shown in Figure 8a and 8b, one quaternary ammonium of the bolaform-type molecule was located in the straight channel and the other in the sinusoidal channel, forming the intergrown MFI structure. However, it can be considered that the formation of intergrown structure needs higher combining energy than the crystal growth. Therefore, the MFI nanorods was thinner at the intersection and thicker at the other end to reduce the energy of the system (Figure 6, 7). The distance between the centres of two intersections was estimated to be approximately 11 Å. Therefore, the chain length of CNMP-8 (11.3 Å) is most compatible with an intergrown MFI zeolite structure, which was confirmed by SEM imaging (Figure 2e). For CNMP-7 (9.9 Å) and CNMP-9 (12.5 Å), the too short and too long carbon chain lengths, lead to the formation of a less intergrown MFI zeolite structure (Figure 2d, f), which caused smaller BET surface area and mesoporous volume (Table S3). It is speculated that the sudden change from MTW to MFI phase is probably because of the geometric matching. The optimized distance between the two nitrogen atoms for CNMP-6 (8.7 Å) was more matchable to intergrown MTW (7.5 Å) and CNMP-7 (9.9 Å) was more matchable to intergrown MFI zeolites (11 Å). Catalytic properties of intergrown MTW and MFI zeolites. In view of the novel intergrown structures generated by CNMP SDAs, catalytic cracking of 1,3,5-triisopropylbenzene (TIPB) was employed as a model reaction to assess the catalytic performance of hierarchical MTW and MFI zeolites. They were compared to conventional ZSM-5 and Beta at comparable Si/Al molar ratios of 38-40 (Table 1). The bulky TIPB molecules (dimension of > 0.9

nm) are not able to enter the micropores of zeolites; thus, their cracking would evaluate the contribution of the external surface area of materials. The main liquid products were diisopropylbenzene (DIPB) isomers together with isopropylbenzene (IPB) and benzene (BZ), which are produced by successive cracking. Benefitting from the mesoporosity of intergrown structures, H-MFI-CNMP-8 and HMTW-CNMP-5 were clearly more active than conventional H-ZSM-5 and H-Beta zeolites. The H-MFI-CNMP-8 catalyst exhibited six times higher TIPB conversion than the conventional H-ZSM-5. On the other hand, the selectivity for benzene and IPB were enhanced to 19.4% and 4.3%. The TIPB conversion over H-MTW-CNMP-5 was lower than HMFI-CNMP-8 but was still two-fold higher than conventional H-Beta with 3D 12-MR pores. Moreover, the deep cracking products (IPB and benzene) over H-MTW-CNMP-5 accounted for the highest value of 76.4% among the four catalysts investigated. Once the TIPB molecules were precracked on the open reaction spaces of mesopores, the subsequent cracking to smaller molecules occur easily within the 12-MR channels of MTW although only one dimensional. The relationship between TIPB conversion and product selectivities with time on stream (TOS) is shown in Figure S19. The TIPB conversion over hierarchical MFI and MTW zeolites decreased with prolonging reaction time but was still much higher than that of conventional ZSM-5 and Beta zeolites. The acidity determined by NH3TPD (Figure S20) and IR spectra (Figure S21） after pyridine adsorption were shown in SI, more acid sites of HMFI-CNMP-8 and H-MTW-CNMP-5 catalyst are accessible to the bulk TIPB molecule due to the presence of mesoporosity from intergrown structure.



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Table 1. Results of 1,3,5-TIPB cracking over various zeolites.[a] +

Catalyst

o + + r 1,3,5-TIPB 1,3-DIPB 1,4-DIPB

IPB

BZ

H-MFI-CNMP-8

H-MTW-CNMP-5

H-ZSM-5

H-Beta

Si/Al ratio[b]

39

38

40

40

Pore

3D 10-MR, mesop.

1D 12-MR, mesop.

3D 10-MR

3D 12-MR

TIPB conv. (%)

41.4

20.6

7.2

10.9

DIPB[c]

73.2

15.3

67.2

30.0

IPB

4.3

44.9

1.8

47.1

BZ

19.4

31.5

8.7

12.2

3.1

8.3

22.3

10.7

Product sel. (%)

[d]

Others

[a] Reaction conditions: cat., 0.2 g; TIPB feed rate, 1.7 mL h-1; N2, 30 mL min-1; temp., 573 K. [b] Measured by ICP. [c] Including 1,3- and 1,4-DIPB isomers. [d] TIPB isomers.

The performance of TIPB cracking could be also related to the acid site amount, strength and acid types of the zeolites. To clarify this issue, the zeolite acidity was determined by NH3-TPD (Figure S20 ) and pyridine-adsorption IR spectroscopy (Figure S21). The four zeolites, with similar Si/Al ratios, reasonably possessed very comparable amounts of total acid sites (Table S6). FT-IR spectra of adsorbed pyridine peak at 1544 cm−1 associated with pyridine adsorbed on a Brønsted acid site and a peak at 1490 cm−1 associated with adsorption on both Lewis and Brønsted acid sites, as well as a peak at 1454 cm−1 associated with pyridine adsorbed on a Lewis acid site. However, H-MFI-CNMP8 and H-MTW-CNMP-5 had relatively smaller amounts of strong acid sites than conventional ZSM-5 and Beta zeolites as evidenced by NH3-TPD profiles. Thus, the outstanding performance achieved on H-MFI-CNMP-8 and HMTW-CNMP-5 are assumed to be contributed by the unique mesoporosity that were not contained in ZSM-5 and Beta. The mesopores generated by intergrown structures would make the acid sites accessible to the bulk TIPB molecules more easily. CONCLUSION In conclusion, we demonstrated the synthesis of different intergrown zeolites could be realized using bolaform molecules as SDAs, including one with an MTW topology not previously reported. This finding is of particular interest because it demonstrates that bolaform molecules with different carbon chain lengths between the two quaternary ammoniums may geometrically match different zeolite topologies. We hope our strategy will provide a new methodology for the design and synthesis of hierarchical zeolite catalysts for processing bulky molecules.

ASSOCIATED CONTENT Supporting Information. Detailed computational simulations, Rietveld refinement of the PXRD data for MTW zeolites,

catalytic reactions, XRD, SEM, and elemental analysis results etc. This material is available free of charge via the Internet at http://pubs.acs.org.

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

ACKNOWLEDGMENT This work was supported by the National Basic Research Program (2013CB934101), the National Natural Science Foundation of China (21533002, 21571128) and the National Excellent Doctoral Dissertation of PR China (201454).

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Bolaform Molecules Directing Intergrown Zeolites

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