A Microporous Aluminosilicate with 12-, 12-, and 8-Ring Pores and

Jun 5, 2017 - Synthesis of new zeolites with controlled pore architectures is important in the field of catalysis and separation related to chemical t...
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A Microporous Aluminosilicate with 12‑, 12‑, and 8‑Ring Pores and Isolated 8‑Ring Channels Naoto Nakazawa,† Takuji Ikeda,*,‡ Norihito Hiyoshi,‡ Yuka Yoshida,† Qiao Han,† Satoshi Inagaki,† and Yoshihiro Kubota*,† †

Division of Materials Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡ National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan S Supporting Information *

ABSTRACT: Synthesis of new zeolites with controlled pore architectures is important in the field of catalysis and separation related to chemical transformation, environmental protection, and energy-saving. Zeolites containing channels of different sizes in the same framework have been desirable. We report here the synthesis and structure of a novel aluminosilicate zeolite (designated as YNU-5), the first zeolite containing interconnected 12-, 12-, and 8-ring pores, as well as independent straight 8ring channels. The synthesis procedure is quite simple and consists of conventional hydrothermal conditions as well as readily available starting materials. The framework structure is stable enough and Si/Al ratio is controllable between 9 and 350. Determination of the crystal structure is performed by utilizing X-ray diffraction-based techniques, revealing 9 independent tetrahedrally coordinated atoms. This robust structure is expected to be industrially valuable and several unusual combinations of composite building units are of considerable interest in an academic sense. The new zeolite YNU-5 is promising catalyst for the production of useful light olefins such as propylene and butylenes in the dimethyl ether-to-olefin reaction, when the Si/Al ratio is properly tuned by dealumination through simple acid treatments.

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INTRODUCTION Zeolite is crystalline aluminosilicate composed of TO4 (T = Si or Al) tetrahedral units that share the corner oxygens. To date, 232 frameworks have been approved by International Zeolite Association (IZA); among them, materials with large or extralarge, multidimensional pores are especially promising from the viewpoint of catalytic applications or separation, because such large pores are capable of adsorbing bulky molecules and allowing them to diffuse inside pores. Typical synthetic techniques of the zeolite classically used inorganic alkaline medium.1 Afterward, the use of simple organic cations like tetraalkylammoniums as organic structure-directing agents (OSDAs) has expanded the variation of frameworks as well as chemical compositions.1 Furthermore, OSDAs with more sophisticated structures have been used, and numbers of new zeolites have been discovered one after another. In the meantime, the correlation between pore systems and OSDA structure has become clearer.2,3 Zeolite sciences regarding synthesis and structure are still developing, and the history and state-of-art technologies are well documented in some review papers.4−9 In recent years, the main issues in zeolite synthesis are as follows:6 (i) exploitation of OSDAs for new materials,7 (ii) the synthesis of zeolites without OSDAs,10,11 (iii) the synthesis of very hydrophobic materials,12 (iv) conversions of two-dimensional (2D) to 3D materials and vice versa, including © 2017 American Chemical Society

the assembly−disassembly−organization−reassembly (ADOR) protocol,13 (v) hierarchically organized materials,14 (vi) chiral materials,7,15−17 and (vii) direction of tetrahedral atoms to specific framework positions.18−20 In part of these investigations, many new germanosilicate zeolites with large pores have been synthesized by using bulky and rigid OSDAs.7 Such OSDAs tend to have complicated structure, which often cause the issue of cost. When using bulky and rigid OSDAs, typical zeolite products are pure silica or high-silica boro- or aluminosilicate materials, which are generally hydrophobic; formation of Al-rich zeolite is rather difficult. Although the synthesis of germanosilicate is an important issue, there are drawbacks such as low hydrothermal stability that could be a critical problem in industrial applications. In this sense, zeolite synthesis that uses conventional OSDAs like the tetraalkylammonium cations are still meaningful even now. In fact, MFIand *BEA-type zeolites obtained by using tetrapropylammonium and tetraethylammonium, respectively, are indispensable catalytic materials for modern industrial processes. Zeolite synthesis using the ADOR protocol is flourishing in producing new frameworks. It takes advantage of the weakness of germanosilicate. In fact, the ADOR technique afforded IPC-2 Received: April 7, 2017 Published: June 5, 2017 7989

DOI: 10.1021/jacs.7b03308 J. Am. Chem. Soc. 2017, 139, 7989−7997

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Journal of the American Chemical Society (OKO),21 IPC-4 (PCR),21 IPC-6,22 IPC-7,22 IPC-9,23 and IPC-1023 from UTL-type germanosilicate, and IPC-1224 from UOV-type germanosilicate. This technique has potential for targeting other new zeolites and could enable the preparation of functional materials. Nevertheless, it is still clear that aluminosilicate is more important than germanosilicate and borosilicate in industry, because the functions of zeolites as ion-exchangers and solid acid catalysts are caused by the Al in the framework. In other words, crystallization procedures incorporating tetrahedral atoms other than Al make the synthesis expensive or make the zeolite products not stable enough to sustain catalytic cycles. However, only a small number of multidimensional large-pore aluminosilicate-based frameworks have been approved by IZA among 106 new zeolite frameworks since 2000,25−28 that is, only three examples: MCM-6825,26 (MSE topology, 12−10−10-ring), ITQ-2727 (IWV topology, 12−12ring) and SSZ-6528 (SSF topology, 12−12-ring). On the other hand, 8-ring or 8-ring-containing zeolite frameworks such as CHA (8−8−8-ring) and MOR (12−8-ring) are recognized to be very useful for catalytic applications. Therefore, it is expected that the combination of 12-ring and 8-ring pores is a promising pore architecture. For the synthesis of zeolites with 8-ring- and 12-ring-containing pores (including cages or pockets in some cases), the charge density mismatch (CDM) approach is useful and interesting.29−31 In this situation, we have been successful in the synthesis of a new aluminosilicate zeolite with 12−12−8-ring (and an independent 8-ring) pore system by using dimethyldipropylammonium (Me2Pr2N+), one of the simple tetraalkylammonium-type compounds, as the OSDA. The new zeolite, YNU-5, crystallizes with a favorable chemical composition (Si/Al = 9), and the aluminum content is controllable through dealumination by means of appropriate acid treatment. We report herein the synthesis and detailed structure of YNU-5. In addition, dimethyl ether (DME)-to-olefin (DTO) reaction was investigated by using YNU-5 as a catalyst.

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treatments including ion-exchange and dealumination are described in Supporting Information. Material Characterization. The first screening of crystallinity and phase purity of each zeolite was performed by powder X-ray diffraction (XRD) on an Ultima-IV (Rigaku) using Cu Kα radiation operated at 40 kV and 20 mA. The Si/Al molar ratios in the bulk materials were measured by means of inductively coupled plasma-atomic emission spectrometry (ICP-AES, ICPE-9000, Shimadzu). The OSDA or adsorbed H2O contents of the as-made and calcined samples were determined by thermogravimetry and differential thermal analysis (TG-DTA, Thermo plus EVO II TG8120 (Rigaku) and TGDTA2000SA (Bruker AXS)). Argon gas adsorption−desorption isotherms at 87 K were obtained for samples pretreated under vacuum at 573 K for 12 h on a Autosorb 1-MP (Quantachrome) gas adsorption instrument. Specific surface areas (SBET) and micropore volumes (Vmicro) were calculated using the BET method and the t-plot method, respectively. Pore size distribution was analyzed by the nonlocal DFT algorithm using a kernel of spherical/cylindrical pore model. NMR Data Collection. 1H and 13C NMR spectra of organic compounds in solution were obtained on a DRX-500 (Bruker BioSpin) Fourier-transformed nuclear magnetic resonance (FTNMR) spectrometer operating at 500.13 MHz for 1H and 125.75 MHz for 13C. Solid-state magic angle spinning (MAS) NMR spectra were acquired using an AVANCEIII 400WB (Bruker BioSpin) operating at 400.13 MHz for 1H, 100.61 MHz for 13C, 105.84 MHz for 23Na, 104.26 MHz for 27Al, and 79.50 MHz for 29Si. All the MAS NMR spectra were recorded at room temperature in a 4.0 mm diameter ZrO2 rotor. 1H MAS spectra were collected using a recycle delay time of 4 s at a spinning rate of 14 kHz. {1H}-13C CP/MAS spectra were collected using a contact time of 1.5 ms and a recycle delay time of 4 s at a spinning rate of 14 kHz. 29Si dipolar-decoupling (DD) MAS NMR measurements were carried out using a π/4 pulse length and a recycle delay time of 30 s at a spinning rate of 6 kHz, and {1H}-29Si CP/MAS spectra were collected using a contact time of 4 ms and a recycle delay time of 4 s at a spinning rate of 6 kHz. 27Al doublefrequency-sweep (dfs) MAS NMR measurements were carried out using a recycle delay time of 2 s at a spinning rate of 14 kHz, and 23Na dfs MAS NMR spectrum was also measured using a recycle delay time of 1 s at a spinning rate of 10 kHz. The chemical shifts of observed nuclei were corrected using tetramethylsilane for 1H, 13C, and 29Si, 1.0 M NaCl aqueous solution for 23Na, and 1.0 M AlCl3 aqueous solution for 27Al as reference materials. SEM-EDX and STEM Data Collection. The crystal morphologies and chemical compositions were investigated by scanning electron microscopy (SEM) using a S-4800 (Hitachi High-Technologies) and energy-dispersive X-ray spectrometry (EDX) using the Quantax XFlash 6|100 system (Bruker AXS). Annular dark field scanning transmission electron microscopy (ADF-STEM) images were obtained using an ARM-200F electron microscope (JEOL) equipped with a CEOS probe aberration corrector and a cold field emission gun. Highresolution ADF images were obtained at 200 kV with a probe convergence semiangle of 24 mrad and a collection angle of 54−175 mrad. Obtained high-resolution images were treated with Local 2D Wiener Filter in an HREM-Filters Pro software (HREM Research Inc.) for noise removal. Low-magnification ADF images were obtained at 80 and 200 kV with a probe convergence semiangle of 7 mrad and a collection angle of 14−55 mrad. Image simulations were performed by the multislice method using a WinHREM software (HREM Research Inc.). For image simulations, isotropic atomic displacement parameters, Biso, of framework atoms and crystal thickness were fixed to 2 Å2 and 20 nm, respectively. Accurate X-ray Powder Diffraction Data Collection. For accurate structure analysis, high-resolution powder XRD data were collected at room temperature on a Bruker D8 Advance with Vαrio-1 diffractometer (Bruker AXS) in modified Debye−Scherrer geometry using Cu Kα1 radiation provided by a Ge(111) primary monochromator. The powder samples of as-synthesized and calcined form were packed into a borosilicate glass capillary with an inner diameter of 0.5 mm. The capillary glass tube containing calcinated YNU-5 was

EXPERIMENTAL SECTION

Synthesis of YNU-5. Detailed procedures of the preparation of Me2Pr2N+OH− as the OSDA as well as the postsynthetic treatments are described in Supporting Information. The OSDA is also available from Sachem, Inc. The new zeolite YNU-5 was synthesized as follows. Aqueous Me2Pr2N+OH− solution (2.097 mmol g−1, 16.21 g, 34.0 mmol), aqueous NaOH solution (3.200 mmol g−1, 9.37 g, 30.0 mmol), aqueous KOH solution (3.153 mmol g−1, 9.53 g, 30.0 mmol), and colloidal silica (Ludox AS-40, DuPont, 40 wt % SiO2, 21.39 g, 142.4 mmol) were mixed in a 150 mL Teflon beaker and stirred for 3 h on a hot plate (>353 K), allowing 20.26 g of water to evaporate. After cooling down to room temperature, 4.99 g of FAU-type zeolite (Tosoh HSZ-350HUA, Si/Al = 5.3) was added, and the mixture was stirred for 10 min. The resulting mixture, having the molar composition 1.0SiO2/0.025Al2O3/0.17Me2Pr2N+OH−/0.15NaOH/ 0.15KOH/7.0H2O, was placed in a 125 mL Teflon-lined autoclave within a convection oven and maintained at 433 K for 165 h. After the autoclave cooled to room temperature, the obtained solid was separated by filtration, washed several times with deionized water, and dried overnight. The as-synthesized YNU-5 zeolite was obtained as a white powder (6.84 g). To remove any OSDA occluded in the pores, the as-synthesized YNU-5 (5.02 g) was heated in a muffle furnace, raising the temperature from ambient to 823 K at a rate of 1.5 K min−1, and maintained at that same temperature for 6 h. Finally, the sample was cooled to room temperature to give the calcined product (6.84 g) as a white powder (Si/Al = 9). Procedures for postsynthetic 7990

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Journal of the American Chemical Society heated at 573 K for 1 h, then the tube was sealed quickly to keep dehydrated. Fourier Transform Infrared (FT-IR) Spectroscopy. For FT-IR spectroscopy, analysis was performed using a JASCO FT/IR-6100 spectrometer equipped with a Hg−Cd−Te (MCT) detector cooled by liquid nitrogen. More detailed procedure is described in Supporting Information. Catalytic Reaction. For the dimethyl ether (DME)-to-olefin (DTO) reaction, each zeolite catalyst was pelletized without any binder, roughly crushed, and then sieved to obtain 500−600 μm catalyst particles. Prior to running the reaction, 100 mg of catalyst particles was placed in a fixed bed reactor in an electric furnace. The temperature was raised to the pretreatment temperature with a ramp rate of 10 K min−1 under air flow (40 cm3 (NTP) min−1). After pretreatment at 823 K for 1 h, the temperature was maintained at 623 K under He flow (40 cm3 (NTP) min−1) so that W/F was adjusted to 20 g-cat h mol−1. DME (partial pressure, 5.0 kPa) was introduced into the top of the reactor (a down-flow quartz-tube microreactor with a 9 mm internal diameter) with He (40 cm3 (NTP) min−1). The reactants and products were analyzed on a DB-5 capillary column (Agilent Technologies; id 0.53 mm; length 60 m; thickness of the stationary phase 5.00 μm) and an HP-PLOT/Q capillary column (Agilent Technologies; id 0.53 mm; length 30 m; thickness of the stationary phase 40.0 μm) using a gas chromatograph (GC-2014 Shimadzu) with a flame ionization detector (FID). The conversion of dimethyl ether, the selectivity toward the products, the yield of products, and the material balance were calculated on the carbon basis of the inlet amount of DME. Some other technical remarks are contained in Supporting Information.

region (see Figure S1e,f) may be due to the effect of dehydration. After performing various postsynthetic treatments, high crystallinity was maintained. This finding indicates that YNU-5 has high structural stability. Additional synthetic conditions and postsynthetic treatments are described in Supporting Information. Characterization. Prior to X-ray structure analysis, chemical composition and local structures of constituent elements of the product were analyzed by NMR spectroscopy and EDX spectrometry. The Si/Al ratio was estimated as 9.0 by EDX analyses and as 9.4−10.2 by 29Si DDMAS NMR spectrum, which are consistent with the result of ICP-AES analysis. The K/Al ratio was estimated to be 0.55. Four resonance peaks attributed to Me2Pr2N+ cationic molecules were observed in 13C CP/MAS NMR spectra of as-synthesized sample. Although K/Na ratio is 1:1 in the starting gel composition, the amount of Na+ ion was very small in YNU5 (Na/K = 0.11). Nevertheless, the presence of Na+ ion with a single local structure was also confirmed by 23Na MAS NMR experiment as well. A sharp resonance peak attributed to a fourcoordinated Al site was detected in 27Al MAS NMR spectra of as-synthesized YNU-5; however, a small peak assignable to a six-coordinated Al site indicated that dealumination took place by calcination. In the 1H MAS NMR spectra of calcined YNU5, a resonance peak of O−H···O hydrogen bonding with a short O−O distance (ca. 2.8 Å) was detected,37 again suggesting the occurrence of dealumination (See Figures S2− S6). Determination of the Crystal Structure of YNU-5. The crystal structure of YNU-5 was elucidated by ab initio structural analysis from powder diffraction data. Lattice parameters, indices of the reflections, and possible space group were analyzed by using the program TOPAS5.38 “Observed” integral intensities, Iobs ∝ |Fobs|2 (F is structure factor), of all diffraction peaks were extracted by the hybrid-pattern decomposition method using the program RIETAN-FP.39 Obtained |Fobs|2 data were further analyzed by the maximum-entropy Patterson method40 using the program ALBA.39 An initial structural model was analyzed by the powder charge-flipping (pCF) method41,42 using the program Superflip43 and the direct method using the program EXPO201444 using the improved | Fobs|2 data set. Obtained initial model was refined by the Rietveld method.45 An extra-framework atom, that is, adsorbed H2O, K+, or Na+ ion, was found in an electron density distribution map obtained by the maximum-entropy method (MEM)46 analysis by using the program Dysnomia.47 All structural models and electron density maps were visualized by the program VESTA3.48 In assynthesized YNU-5, the distribution and conformation of OSDA cationic molecules were optimized by using the direct space method using the program FOX.49 In addition, the bond valence sum 3D mapping (BVS3D) for K+ ion was adopted in order to investigate a stable cation position, which is calculated for whole space in the unit cell of YNU-5 within a grid resolution of 0.1 Å.50 The BVS3D analysis was carried out by using the program PyAbstantia coded by Nishimura.51,52 The bond valence parameters are used by utilizing an expanded evaluation range of the bonding interaction; r0 = 2.132 and B = 0.37 for K−O bond.53 The framework structure of YNU-5 was analyzed by means of high-resolution powder XRD of the calcined sample with high crystallinity (Figure S7). It was found that the structure of YNU-5 was quite robust and hardly collapsed by acid treatment

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RESULTS AND DISCUSSION Synthesis of YNU-5. The synthesis of YNU-5 was performed under normal hydrothermal conditions within a narrow range of chemical composition by combining the structure-directing effect of the Me2Pr2N+, which is a cationic molecule, and the presence of alkali cations in the synthesis media. Key factors for the successful synthesis were the use of FAU-type zeolite as the source of Si and Al with a relatively small amount of water. When the input molar ratio of H2O/ SiO2 was 7, reproducible crystallization of pure YNU-5 was observed; however, when the ratio was greater than or smaller than 7, trace amounts of MFI or dense phase was contaminated, respectively. Although such a small ratio of H2O/SiO2 was sometimes essential for the crystallization of high-silica zeolites such as beta and SSZ-23 in fluoride media,32,33 the present case is not common because the typical H2O/SiO2 ratios in the normal alkaline media are 30−50. If such a range of H2O/SiO2 ratio (30−50) was applied, UZM-35 zeolite with MSE topology, one of the interesting frameworks,34,35 crystallizes as already known.36 More detailed synthetic behaviors will be reported elsewhere. The powder XRD patterns of the new zeolite YNU-5 in assynthesized and calcined forms are shown in Figure S1a,b. It is noticeable that the framework structure remains stable on calcination. The Si/Al ratio of the solid was lower than that used in the synthesis mixture (9 versus 20), which explains the nonquantitative yield (ca. 50%) of the product. The calcined sample (Si/Al = 9) was converted to H+ form via NH4+ form without change in the framework structure (Figure S1c,d). At the stage of NH4+ form, K+ and Na+ ions have already been almost removed. The Si/Al ratio was controllable between 9 and 350 by appropriate acid treatment. Even after deep dealumination with concentrated nitric acid solution (13.4 mol L−1), the framework structure remained (Figure S1e) and the dealuminated sample was thermally stable at 923 K (Figure S1f). The slight increase in intensity of peaks in the low-angle 7991

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Journal of the American Chemical Society using 0.1 M HCl aq. for 6 h at 313 K. Based on the indexing analysis, the most probable lattice constants and a unit-cell volume were a = 18.12 Å, b = 31.37 Å, c = 12.59 Å, α = β = γ = 90°, and 7157.2 Å3, respectively. Table 1 shows crystallographic Table 1. Powder XRD Experimental Conditions and Crystallographic Data for Aluminosilicate Zeolite YNU-5 sample form chemical composition FW space group a, Å b, Å c, Å β, deg V, Å3 λ, Å 2θ range, deg step size (2θ), deg observations contributing reflns refined params Rwp (Rietveld) Rp (Rietveld) RF (Rietveld) Rexp (Rietveld) χ2

as-synthesized Si108Al12O240K6.0· 6.0(Me2Pr2N+) 8230.6 C2/m (No. 12) 18.0862(5) 31.7418(9) 12.6133(5) 90.001(4) 7241.2(4) 1.540593 (Cu Kα1) 4.0−110 0.016346 6553 4666 323 0.0393 0.0292 0.0195 0.0122 6.4

calcined-dehydrated Si108Al12O240K6.3· 0.3(H2O) 7450.1 Cmmm (No. 65) 18.1053(3) 31.7362(5) 12.5763(2) 7226.3(2) ← ← ← 6553 2549 89 0.0486 0.0367 0.0196 0.0199 3.3

information and conditions of the powder XRD experiment. The most likely space group is determined to be Cmmm by considering a systematic absence. Besides Cmmm, space groups Cm2m, Cmm2, and C222, which are maximal nonisomorphic subgroups of Cmmm, can become candidates of structural analysis. However, initial phase determination based on these space groups failed. Next, we applied a lower space group C2/m as a pseudomonoclinic model to the same analytical procedure by using 2991 reflections (including 2441 overlapped reflections, d < 1.09 Å). The electron density map obtained by pCF analysis gave a major 16 tetrahedral T(Si or Al) and 26 O crystallographic sites (Figure S8). All T−T connectives indicate a novel microporous structure including 4-, 5-, 6-, 8-, and 12-rings. Thus, we constructed the complete framework structure by adding seven missing O sites, from a viewpoint of a general zeolite topology. By the symmetry operation, obtained C2/m framework model was reconstructed in Cmmm symmetry by using 9 T and 22 O sites reasonably. Figure 1A−C shows the crystal structure model of calcined YNU-5 obtained by Rietveld analysis. At all T sites, Si and Al were not distinguished, but T site was treated as a mixed atom of Si/Al = 9.0. All 8-rings are observed along the c-axis, and 12ring pores lie on the a−b plane (Figure 1D,E). As the most interesting feature, the framework topology of YNU-5 can be represented by the combination of two kinds of objects 1 and 2 (Figure 1F,G). These objects form two independent micropore systems. One is a cylindrical structure like a glass vase with an 8-ring straight channel along the c-axis, which can be formed by stacking of object 1. On the other hand, a 3D pore structure, which consists of a twin 8-rings and 12-rings, is formed between two neighboring objects 2. Both objects include various composite building units (CBU),54 that is, mor (8T), mtt (11T), fer (13T), mtw (14T), and d8r (16T). As far as the d8r is concerned, some frameworks with d8r, MER, MWF, PAU,

Figure 1. Crystal structure model of calcined YNU-5 viewed along the (A) [001], (B) [100], and (C) [010] directions. Panels D and E show the framework topology of YNU-5, which can be represented by two kinds of unique building units: (F) object 1 and (G) object 2. In both objects, various CBUs and a building part composed of 24T atoms are included.

RHO, SBE and TSC, are known as low-silica or nonsilicate materials, and they all have multidimensional pore systems. It is interesting to note that the linear arrangement of d8r in YNU-5 has some similarity to that of d6r in SSZ-65. In addition, an undefined building part composed of 24T atoms, which can be expressed as an expansion of the tiling of tber 18T, is included in object 1.8,54 In object 2, a twin 8-rings pore structure is composed of seven 4-rings and of four 6-rings. K+ ions are sited at the center of the 8-ring of the twin 8-rings pore structure (68% occupancy) and at the center of the new tiling arrangement (45% occupancy). The positions of K sites may be metastable a little which was suggested by the large value of |ΔV(A)| (ca. 0.75) at the K+ ion positions in the BVS3D map (Figure S9). 50 The atomic displacement parameter of K sites, B(K), converged to an unusually large value, therefore the B(K) was fixed to 10 Å2, suggesting disorder of K+ ion distribution (see Supporting Information). This fact may be related to the result of a BVS3D mapping. The framework structure composed of covalent bonds can be observed in an electron density distribution image by MEM analysis (Figure 2A). Although K sites are clearly seen by dehydration of the sample, the position of the Na site could not be determined unequivocally due to a very small content. A H2O site with a small occupancy was detected near the center of 12-ring in the MEM electron density map. The two 7992

DOI: 10.1021/jacs.7b03308 J. Am. Chem. Soc. 2017, 139, 7989−7997

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Figure 2. Detailed structural features of YNU-5 visualized crystallographically. (A) Electron density distribution of calcined YNU-5 calculated by the MEM analysis. (B−D) Two independent microporous systems: one is the 3D large micropore with intersection of 8ring and 12-ring, and the other is the straight channel of 8-ring micropore width. In panel A, equisurface level was set at 0.8 e/Å3.

Figure 3. Detailed structural features of YNU-5 visualized crystallographically. (A, B) Me2Pr2N+ cation distribution represented by two independent sites in micropores in as-synthesized YNU-5 estimated by the direct space method.

independent pore systems can be depicted by using a pseudopotential surface (Figure 2B−D). The large micropores with the pore diameter equivalent to the 12-rings are extended along [110] directions and cross mutually. At the cross-point, twin 8-rings are located and connect the adjacent large pores along the c-axis. The other straight channel with 8-ring pore size is isolated in the space between the large pore networks. In calcined YNU-5, based on the argon adsorption isotherms, the BET surface area and micropore volume was 402 m2 g−1 and 0.199 cm3 g−1, respectively. Pore size was estimated as 7.8 and 8.3 Å, both of which correspond to 12-ring (Figure S10). However, an expected small pore size attributed to 8-ring was not detected, probably due to the occlusion of the straight channel by a potassium ion. The surface area and pore volume would be underestimated, since the Ar gas is not adsorbed into the 8-ring straight channel. The crystal structure of as-synthesized YNU-5 was analyzed based on C2/m symmetry, because the Rietveld analysis based on Cmmm symmetry gave an inadequate conformation of Me2Pr2N+. As a result, it was found that two Me2Pr2N+ cationic molecule sites (OSDA 1 and 2) are located in the 12-ring micropores, and only K+ ions were incorporated in the 8-ring straight channels (Figure 3A,B)). This K site may be better “pore-filler” in the straight channels. The end of propyl groups of OSDA2 stuck into 8-rings of object 2 just like an anchor. The total number of Me2Pr2N+ cationic molecules was 6.0 per unit cell (ca. 10 wt %), which coincides with 5.9 molecules per unit cell estimated by the TG-DTA analysis (Figure S11). Meanwhile, calcined YNU-5 contains a large amount of H2O of approximately 11 wt %, which indicates hydrophilicity. The empirical structural composition was estimated as Si109.4Al10.6O240K6.0·6.0(Me2Pr2N+) for as-synthesized YNU-5 and Si109.4Al10.6O240K6.3·0.3(H2O) for the calcined, dehydrated form. Finally, Rietveld refinements (Figure 4) gave low Rfactors, suggesting good reliability, for as-synthesized and calcined samples (Table 1, Figure S5).

Figure 4. Observed pattern (red crosses), calculated pattern (light blue solid line), and difference pattern (blue) obtained by Rietveld refinement for (A) calcined and (B) as-synthesized YNU-5. The tick marks (green) denote the peak positions of possible Bragg reflections. The inset shows magnified patterns of the region above 33°.

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Journal of the American Chemical Society Direct Observation of YNU-5 by ADF-STEM. Figure 5 shows high-resolution ADF-STEM images of low and high

of {001} was also observed viewed along [100] direction (Figure S12). This tendency became more remarkable as the particle size increased. Meanwhile, the {110} plane form was observed in the small crystallite. Structural defects or vacancies were hardly detected inside the crystalline material. From the results of structure analysis and ADF-STEM observation, we assume that the objects 1 and 2 would play an important role in crystal growth of YNU-5 as a key fragmentation as illustrated in Figure 6. In Figure 5A, the

Figure 6. Schematic illustrations showing the outermost surface of the YNU-5 crystal constructed from two building units (objects 1 and 2) viewed along the (A) [001], (B) [100], and (C) [0.75, −0.25, 2.5] directions. Object 2 can be formed among the four objects 1. The layered structure can be formed by the two-dimensional array of objects 2. Again, objects 1 are stacked on the layer.

Figure 5. High-resolution ADF-STEM images of calcined YNU-5 viewed along the (A) [001], (B) [100], and (C) [110] directions. Each inset figure shows a simulation image based on the crystal structure model. Although all 8-ring micropores are observed along the c-axis only, 12-ring micropores are observed from lateral faces. Panel D shows low-magnification ADF-STEM images, which show the major facets with the plane forms of {100} and {130} that suggest the direction of crystal growth of YNU-5. Red arrows indicate part of the layered building unit.

outermost surface is filled by the closest packing of object 1. In addition, it can be seen that red-colored arrows in Figure 5 indicate a part of object 2. These findings suggest that twodimensional assembly and bonding of object 1 would form a layered structure, which consists of objects 2. Subsequently, next objects 1 are arranged on the new plane of the layered structure. We consider that K+ and Na+ ions will play a role as inorganic SDA for building of object 1, and Me2Pr2N+ cation may have a function that connects adjacent objects. Primary Evaluation of Acidic Property. As a fundamental characterization of solid-acid property, Brønsted and Lewis acid sites were detected by FT-IR spectroscopy for adsorbed pyridine on the YNU-5 (Si/Al = 8.4) prepared by calcination of

magnification of calcined YNU-5. The isolated 8-ring and the twin 8-rings and 12-rings are clearly seen in Figure 5A−C. Obtained images are compatible with the simulated image (inset figure); that is, there is no doubt in the reliability of the obtained structural model. In Figure 5D viewed along [001] direction, high crystallinity is maintained in the edge of crystallite, that is, a step structure per nanosized building unit was observed. It was found that the facet of crystallite mainly consists of two plane forms of {100} and {130} in large crystallite. The well grown surface attributed to the plane form 7994

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Journal of the American Chemical Society

locations of heavy coke formation can be the external surface or pore mouth of both 12-ring and 8-ring. By dealumination of parent YNU-5, the acid sites existing on the external surface or pore mouth (preferentially 12-ring) were selectively removed, while those on the internal surface were retained. The removal of acid sites on the external surface and pore mouth may suppress the consecutive reactions toward coke formation. However, the acid sites on the pore mouth of isolated 8-ring are still remaining and responsible for coke formation. This is inferred from the result of the reaction over YNU-5(61). When TOS was between 5 and 185, slight deactivation was observed, which suggests precoking to occur on the pore mouth of isolated 8-ring, and after complete deactivation of the 8-ring, the remaining acid sites inside 12-ring makes the YNU-5 a stable catalyst. When more dealuminated samples, YNU-5(110) and YNU5(160), were used as catalysts, the deactivation behavior was similar to that of the reaction over SSZ-13 (CHA), which is a representative high-silica zeolite with 8-ring pores that is known as a potential candidate for MTO reaction.59 The results of DTO reaction over SSZ-13 catalysts are shown in Figure S14 for a comparison purpose. It could be speculated that the acid sites in 12-ring pores of YNU-5, which are more readily removed than those in 8-ring pores, may be completely removed, and the remaining acid sites in 8-ring pores make the YNU-5(110) and YNU-5(160) act as an 8-ring zeolite. Nevertheless, there is a difference in product distribution between the reaction over YNU-5(110)/YNU-5(160) and SSZ13(20)/SSZ-13(100) (see Figure S14). The reaction over YNU-5(110) or YNU-5(160) tends to give more demanded propylene and butylenes, suppressing less demanded ethylene. Within the range of our investigation, the maximum yields of propylene and butylenes are 36-C% and 19-C%, respectively. This means that the new zeolite YNU-5 is promising catalyst for the production of useful light olefins in the DTO reaction, when the Si/Al ratio is properly tuned.

its ammonium form at 823 K. The result is shown in Figure S13. It is obvious that most acid sites are Brønsted sites. This is not always the case for other zeolites.55 The Brønsted acid sites of YNU-5 are recognized to be stable during calcination at 823 K, which is a favorable characteristic of this new zeolite. Although pyridine can adsorb on the acid sites at 12-ring and twin 8-ring pores, it is too large to enter into the isolated 8-ring channels. Thus, smaller probe molecules are to be employed to evaluate the acid sites inside the isolated 8-ring channels. Dimethyl Ether (DME)-to-Olefin (DTO) Reaction by Using YNU-5 Catalyst. One of the most suitable applications of YNU-5 is the use as a solid acid catalyst. This aluminosilicate material can be dealuminated by simple acid treatment to give samples with various Si/Al ratios while maintaining the framework structure. These samples were applied to the catalysts for dimethyl ether (DME)-to-olefin (DTO) reaction. This reaction as well as methanol-to-olefin (MTO) is important as an alternative means to the thermal cracking of ethane, supplied from nonpetroleum fossil resources such as natural gas or shale gas, because the thermal cracking does not satisfy propylene demand.56 The results of the DTO reactions over YNU-5 with various Si/Al molar ratios are shown in Figure 7. YNU-5 with Si/Al

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Figure 7. Dimethyl ether (DME)-to-olefin reaction over YNU-5 catalysts with Si/Al ratios of (a) 8, (b) 61, (c) 110, and (d) 160. Pretreatment conditions: 823 K, 1 h under air flow (flow rate, 40 cm3 (NTP) min−1). Reaction conditions: catalyst weight 100 mg; W/F = 20 g-cat h mol−1; pellet size, 500−600 μm; He flow rate, 40.0 cm3(NTP) min−1; reaction temperature 673 K.

CONCLUSION A microporous crystalline aluminosilicate with multidimensional and unusual pore architecture containing interconnected 12-, 12-, and 8-ring pores as well as independent straight 8-ring channels has been successfully synthesized for the first time by using a simple OSDA and FAU-type zeolite and designated as YNU-5. A key factor to the successful synthesis was the control of water amount in the synthesis mixture. The zeolitic material had large adsorption capacity and enough aluminum in its framework after removing the OSDA by calcination, and the aluminum atoms were removed by appropriate acid-treatment without framework collapse. The YNU-5 was indexed in the orthorhombic crystal symmetry with lattice constants a = 18.105 Å, b = 31.736 Å, and c = 12.576 Å with a unit-cell volume of 7226.3 Å3. It was found that the framework includes 9 independent tetrahedrally coordinated atoms. ADF-STEM observation clearly confirmed the framework topology. This robust structure is expected to be industrially valuable, and several unusually combined composite building units such as d8r, mor, fer, mtw, mel, mtt are of considerable interest in an academic sense. It is notable that a part of the pore architecture of YNU-5 has some similarity to that of mordenite, which is industrially a very useful zeolite, suggesting its potential for catalytic application. YNU-5 with Si/Al ratio around 9 just after crystallization was successfully dealuminated with simple acid treatments to give samples with various Si/Al ratios. They were

ratio of n is designated as YNU-5(n), and this rule applies to other zeolites. The conversion of dimethyl ether was high at a short time on stream (TOS) of 5 min over the parent YNU5(8) (difference in Si/Al between 8 and 9 is only from analytical fluctuation) that was protonated via ammonium ionexchange followed by pretreatment inside the reaction tube, and it decreased rapidly as the TOS increased. The rapid decrease in conversion was successfully suppressed by increasing the Si/Al ratio to 61. When the Si/Al ratio was increased to 110, the catalyst deactivation kinetics became rapid again, and further increase in the Si/Al ratio to 160 resulted in the same tendency. These results can be well understood based on the rough estimation of acid site distribution. It is an oftenobserved tendency in our investigation that the zeolite catalyst with an Si/Al ratio as small as 10 is subject to heavy coke formation, even if it has 12-ring pores,55,57 although the use of catalysts with Si/Al ratios smaller than 10 is possible under other conditions.58 A high density of acid site on YNU-5 could be responsible for the formation of coke by the consecutive reactions. One of the 7995

DOI: 10.1021/jacs.7b03308 J. Am. Chem. Soc. 2017, 139, 7989−7997

Article

Journal of the American Chemical Society

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actually applicable to the catalysts for the DTO reaction., and it was found that the dealuminated YNU-5 is promising catalyst for the production of useful light olefins such as propylene and butylenes when the Si/Al ratio is properly tuned. Introducing heteroatoms such as Ti into the framework is also promising and under investigation. Since the control and utilization of the dual micropore space in the current new framework structure are not limited to these examples, the material may have potential for various applications. The simplicity of synthetic procedure suggests a significant advantage to research and development.

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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/jacs.7b03308. Materials and Methods, Figures S1−S15, Table S1, and References S1−S7 (PDF) Crystal structure of as-synthesized YNU-5 (CIF) Crystal structure of calcined YNU-5 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Yoshihiro Kubota: 0000-0001-7495-9984 Author Contributions

N.N. and T.I. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported in part by the Japan Science and Technology Agency (JST) for the project of Creation of Innovative Functional Materials with Advanced Properties by Hyper-nanospace Design in the CREST program (project code JPMJCR1423).

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DOI: 10.1021/jacs.7b03308 J. Am. Chem. Soc. 2017, 139, 7989−7997