Intergrown Zeolite MWW Polymorphs Prepared by the Rapid

Nov 10, 2015 - This raises the bar of the structural characterizations and remains an enormous challenge to understand the synthesis conditions and th...
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Intergrown Zeolite MWW Polymorphs Prepared by the Rapid Dissolution−Recrystallization Route Le Xu,†,§ Xinyi Ji,† Jin-Gang Jiang,† Lu Han,*,‡ Shunai Che,‡ and Peng Wu*,† †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, PR China ‡ State Key Laboratory of Metal Matrix Composites, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China § College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China S Supporting Information *

ABSTRACT: Zeolites with intergrown structures are particularly interesting because they often exhibit unique performance in heterogeneous catalysis. This raises the bar of the structural characterizations and remains an enormous challenge to understand the synthesis conditions and the formation mechanisms of such intergrown materials. Herein, a novel intergrown zeolite (ECNU-5) was successfully synthesized via a rapid dissolution− recrystallization (RDR) route, which reorganized the conventional MWW layer stacking into two new different polymorphs, ECNU5A and ECNU-5B. Structure elucidation indicates both polymorphs are reconstructed from the same MWW layer but are different in relative shift between adjacent layers. ECNU-5 is the first structure-determined zeolite with interrupted structure that MWW layers shift in the horizontal direction, in which the two polymorphs are never predicted before and are additional members of the MWW family. The unique geometry mismatch between the organic structure-directing agent (OSDA) and inorganic silicate framework is ascribed to causing the zeolite layer shift. Moreover, the implementation of silylation technique readily expanded the interlayer pore of as-made ECNU-5, producing the interlayer-expanded zeolite (IEZ-ECNU-5), which maintained the original stacking sequence of MWW sheets.

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synthesis conditions and the formation mechanisms of intergrown materials. The three-dimensional (3D) MWW zeolites are useful industrial catalysts that are typically derived from hydrothermally synthesized two-dimensional (2D) layered precursors MCM-22,16−18 or in 3D form MCM-49.19 The 3D materials comprise MWW layers with a thickness of 2.5 nm ordered stacking.20 Playing a central role in the field of layered zeolite materials, the MWW layers are also amenable to postsynthetic modifications, causing mismatch between adjacent layers.21−26 Moreover, materials with an unknown stacking mode of MWW layers can also be obtained by hydrothermal synthesis, such as EMM-10P,27 MCM-56,28,29 and SSZ-70.30,31 The materials with MWW layers stacking disorder often exhibit unique catalytic performance. Postsynthesized MCM-56 analogues containing Al, Ti, or Sn have proved to be more active toward bulky molecules than corresponding zeolites with MWW structure either as solid acids or selective oxidation catalysts.22 Although simplified geometries are proposed to image the stacking modes of MWW layers in these materials,27 visible

eolites with well-defined crystalline structures and unique microporosities are employed as useful adsorbents, ion exchangers, and heterogeneous catalysts in the petrochemical industry.1−3 Over the past decades, zeolite chemistry has experienced tremendous development and prosperity. Despite the large number of zeolites with novel topologies that have been reported,4−8 the discovery of novel materials with unusual structural features is always attractive. A class of faulted structures consisting of variable polymorphs is particularly interesting. For instance, zeolite beta, which contains an intergrowth of two similar polymorphs, A and B, has been widely used in catalytic reactions such as hydrocarbon conversion, alkylation, and acylation of aromatics.9,10 Other polymorphs in the same beta family, such as beta polymorphs C, D, and E, have also been discovered.11−13 Recently, Corma and Zou reported a new complex zeolite ITQ-39, which is an intergrowth of three polymorphs that are built from the same layer but stacked in different ways. With stacking faults, ITQ-39 serves as a promising catalyst in the alkylation of aromatics with olefins, a process that is of great interest in industry.14,15 The promising applications of intergrown zeolites have inspired further research on the associated synthesis chemistry. However, this raises the bar of the structural characterizations and remains an enormous challenge to understand the © XXXX American Chemical Society

Received: May 30, 2015 Revised: November 9, 2015

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Figure 1. PXRD patterns of (A) as-synthesized, (B) calcined samples obtained during ECNU-5 crystallization with different synthesis times, and (C) recrystallization time-dependent solid product yields (a) the parent ITQ-1 sample, after recrystallization for (b) 1 h, (c) 2 h, (d) 3 h, (e) 6 h, (f) 9 h, (g) 12 h, and (h) 24 h. silica ECNU-5 synthesis. IEZ-ECNU-5 was synthesized according to the literature.34 In a typical procedure, ECNU-5P (1 g) was mixed with 30 g of aqueous solution of 2 M HNO3 and 2 mmol Me2Si(OEt)2. The mixture was placed in a 100 mL Teflon-lined stainless-steel autoclave, and then silylation was performed at 423 K for 15 h. After silylation, the solid was collected by filtration and rinsing with ethanol and distilled water and then dried at 373 K. The silylated product was calcined at 823 K for 6 h, and IEZ-ECNU-5 was obtained. Characterization. Powder X-ray diffraction patterns (PXRD) were gathered using a Rigaku Ultima IV diffractometer equipped with a Cu Kα radiation source (λ = 1.5406 Å, 35 kV, 25 mA). High-resolution PXRD data were collected at room temperature on a PANalyticalX’Pert PRO diffractometer with an X-ray wavelength of 1.5406 Å. To improve the preferred orientation of the sample, the transmission geometry was used with a 0.5 mm glass capillary sample holder. Further details about the diffraction experiment are summarized in Table S1. SEM images were captured with a Hitachi S4800 microscope. TEM images were collected with an FEI G2F30 with an accelerating voltage of 300 kV. N2 adsorption isotherms were measured at 77 K, and the pore size distribution was calculated by the Horvath−Kawazoe method based on the data collected by Ar adsorption at 87 K on a Micromeritics ASAP2020 adsorption instrument. IR spectra were collected on a Nicolet Nexus 670 FTIR spectrometer at room temperature using the conventional KBr technique without evacuation. Liquid-state NMR spectra were collected on a commercial Bruker-300 spectrometer (300 MHz for 1 H NMR, 75.4 MHz for 13C NMR). Solid-state 29Si and 13C MAS NMR spectra were recorded on a Varian Model VNMRS-400 MB spectrometer under a one-pulse condition and cross-polarization, respectively. Thermogravimetry (TG) and derivative thermogravity (DTG) analysis was carried out on a Mettler TGA/SDTA 851e instrument. Structure Analysis. On the basis of the HRTEM images of the sample, a structure model was derived and subjected to geometry optimization using the quantum mechanical code Dmol3 in the Materials Studio suite of programs. The exchange-correlation functional was expressed by the generalized gradient corrected (GGA) functional by Perdew−Burke−Ernzerh parametrization. Detailed parameters used in the DIFFaX+ simulation are listed in Table S2. Crystallographic image processing on HRTEM images was carried

evidence of the real structures is still lacking. Given that MCM22 aluminosilicate and Ti-MWW titanosilicate, both with an ordered 3D MWW structure, have already found applications in the important industrial processes of benzene alkylation and ketone ammoximation,32,33 it is expected that zeolite catalysts composed of MWW sheets stacked in novel fashion but with an elucidated crystalline structure can be designed. Herein, we report the synthesis of novel intergrown zeolite MWW polymorphs (ECNU-5, name after East China Normal University) through a rapid dissolution recrystallization (RDR) route in which an unusual structural reorganization occurred from the regular stacking sequence of MWW nanosheets. Structure elucidation indicates that ECNU-5 is a new intergrown zeolite with two different polymorphs (ECNU-5A and ECNU-5B) sharing the common MWW layer but distinguished by their stacking sequences and interlayer connection modes. ECNU-5 is the first structure-determined zeolite with MWW layers stacking disorder. Both of the polymorphs, ECNU-5A and ECNU-5B with the interrupted structures, are the additional members of the MWW family.

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EXPERIMENTAL SECTION

Synthesis of Organic Structure-Directing Agent. The OSDA employed in this study was 1,3-bis(cyclohexyl) imidazolium hydroxide (IM+OH−). The synthesis procedure and the characterization of the organic compound are presented in the Supporting Information. Synthesis of Intergrown Zeolite ECNU-5 and IEZ-ECNU-5. Intergrown pure-silica zeolite ECNU-5 was synthesized by using 1,3bis(cyclohexyl)imidazolium hydroxide (IM+OH−) as the OSDA. In a typical synthesis, the ITQ-1 silicate (its detailed synthesis procedure is shown in the Supporting Information) was added to an aqueous solution of the OSDA under stirring at room temperature for 1 h. The molar composition of the mixture was 1.0 SiO2:0.5 IM+OH−:25 H2O. Then, the mixture was transferred into an autoclave and further reacted at 443 K for 24 h under stirring. ECNU-5P was recovered by filtration and dried at 353 K. The OSDAs used were removed by calcination at 873 K for 6 h. For synthesized Al-containing ECNU-5, NaAlO2 (Na2O: 37.5%, Al2O3: 44.3%) was used as the aluminum source, and the molar composition is 1.0 SiO2:0.5 IM+OH−:25 H2O:0.005 Al2O3. The other procedures are the same as those of pureB

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Figure 2. Scheme description of ECNU-5 crystallization process. out using the program CRISP.35 The simulations of the TEM images were performed using the MacTempas software.

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RESULTS AND DISCUSSION Zeolite Synthesis. We synthesized ECNU-5 from a 3D MWW silicate (well-known as ITQ-1) in which MWW nanosheets were assembled in an ordered manner. The powder X-ray diffraction (PXRD) pattern of the parent ITQ-1 is shown in Figure 1Aa. The calcined MWW silicate was recrystallized in an aqueous solution of 1,3-bis(cyclohexyl) imidazolium hydroxide (IM+OH−) as OSDA (Figures S1−S3) with a molar composition of 1.0 SiO2:0.5 OSDA:25 H2O. The MWW crystals were quickly dissolved to a great extent in the basic medium as its framework significantly collapsed within 1 h at 443 K, providing a solid yield of only 17.8% (Figure 1Ab,C). Prolonging the treatment time to 2 h gave rise to a lamellar precursor (Figure 1Ac). After further hydrothermal treatment for 24 h, the dissolved silica species were recrystallized with the aid of IM+OH−, forming a lamellar precursor ECNU-5P that possessed structural features different from those of the MWW lamellar ones, and a greatly increased solid yield of 92.3% was obtained (Figure 1Ad−h,C). ECNU-5P was clearly synthesized via the dissolution of MWW crystals and OSDA-assisted recrystallization of degraded silicate species (Figure 2), in contrast to the delamination, interlayer expansion processes, and the recently proposed ADOR process in which the zeolite sheets remain almost intact from the onset of treatment.21,36−38 The recrystallization and crystal growth rate was extremely fast, mostly because the dissolved silicate fragments, still containing the basic building units of the MWW structure, may act as seeds to accelerate the crystallization process. This type of RDR process for synthesizing ECNU-5 did not occur when the 3D MWW silicate was replaced with MFI-type Silicalite-1 or highly dealuminated MOR and BEA* zeolites as starting sources (Figure 3). Moreover, the ECNU-5 and 3D MWW silicate showed the same FT-IR spectra in the fingerprint region of Si− O−Si vibrations (Figure S4), implying that ECNU-5 possesses building units similar to those of the MWW topology. Unlike the MWW precursor ITQ-1P, which exhibited two well resolved 101 and 102 reflections over a 2θ range of 8−10.5° in its PXRD pattern (Figure S5a), ECNU-5P displayed a broad peak (Figure 1Ah). Subsequent calcination induced interlayer condensation and converted the precursor to 3D structured ECNU-5, as the layer-related reflection at 2θ values of 3.38 and

Figure 3. PXRD patterns of samples prepared from different starting materials. (A) Pure-silica ITQ-1 (MWW), (B) pure-silica S-l (MFI), (C) dealuminated mordenite with Si/Al = 182 (MOR), (D) dealuminated Beta with Si/Al > 500 (BEA*), (a) starting material, (b) as-synthesized material, and (c) further calcined product. Crystallization conditions: 1.0 SiO2:0.5 OSDA:25 H2O; temp., 443 K; time, 24 h.

6.76° disappeared (Figure 1Bh). Moreover, the abovementioned broad peak at 2θ = 8−10.5° became sharper, but it still resembled that of ECNU-5P and was not split into two resolved peaks even when the PXRD pattern was captured at a high resolution (Figure 4d). The PXRD patterns of ECNU-5 resemble those of previously reported EMM-10P,27 MCM56,28,29 and SSZ-70.30,31 The MWW layer in EMM-10P is believed to be twisted off-register or disordered in-plane but still hydrogen bonded through silanols, resulting in lacking the discrete interlayer reflections, especially 101 and 102 (Figure S6). On the other hand, the partial delaminated zeolite MCM56 comprised a collection of MWW monolayers, misalignment, and unbound except for possible coincidental cross-linking39 (Figure S6). Moreover, the PXRD pattern of ECNU-5 is nearly the same as that of SSZ-70, but the ECNU-5P containing OSDA shows a broad peak with more low intensity in 2θ range C

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layered structure, and the building layers were shifted along the stacking axis, giving rise to several twinning planes (Figure 6a). The layers can be A, B, or C depending on the origin of the structure. Both ABAB... stacking and ABCABC... stacking sequences between layers were observed. The selected-area electron diffraction (SAED) pattern shows both sharp spots and streaks, indicating the existence of a faulted structure, similarly to the diffraction patterns of other intergrown zeolites such as beta9,10 and ITQ-39.15 The sharp diffraction spots in the SAED patterns can be indexed as h0l (h = 3n, n = 0, 1, 2, ...), indicating the presence of shears of ±1/3a* (a* denotes the reciprocal lattice vector) stacking along the c-axis.40 The averaged structural features were obtained from Fourier transforms of the HRTEM images from the ABAB... stacking part (indicated by the white box) using the crystallographic image processing software CRISP,35 for which only translational symmetry is applied (p1 symmetry). The averaged image shows the similar contrast of the MWW layer along the b-axis ([1̅21̅0]MWW direction); however, the layers are stacking with a shifted feature. Meanwhile, the structure shows nearly no difference from the conventional MWW structure along the [0110̅ ]MWW direction from both the SAED pattern and the HRTEM images (Figure 6b and Figure S8). Besides, the SAED pattern taken from [0001]MWW direction shows hexagonal symmetry, which is similar to the MWW structure but different in the diffraction intensity (Figure S8). The corresponding Fourier diffratograms (FDs) of the ABAB... stacking domain are shown in Figure S9, indicating the extinction conditions for the reflections of [hh̅0l: none, hh2̅h̅l: l = even; 000l: l = even], suggesting three possible space groups (SGs): P63mc, P6̅2c and P63/mmc. From the averaged HRTEM images of the two projections, plane groups of p2gm and p2mm have been assigned, which verified the space group of the crystal to be P63/mmc. The three-dimensional (3D) electrostatic potential map is obtained from combining the HRTEM images along the [1̅21̅0] and [011̅0] directions. The amplitudes and phases (with amplitudes > 3% of the strongest reflection) have been extracted from the FDs using the software CRISP35 and merged into a 3D data set by scaling the amplitudes with common reflections.15,41 The 60 unique reflections with amplitudes >5% of the strongest reflection (Table S3) have been employed to calculate the 3D electrostatic potential map φ(x,y,z) using the software VESTA.42 Figure 7a presents the reconstructed 3D map of one unit cell. As expected, the reconstructed structure shows the typical MWW layer construction. By shifting the MWW layer along the ab-plane, the structure shows a good agreement with the reconstructed volume (Figure 7b). These results indicate that the ECNU-5 consists of the MWW layers as the building layer; however, the MWW layers are stacking along the c-direction in different ways. A structural model was built to interpret the faulted structure derived from the MWW layers. The conventional MWW structure has been described previously; the structure is hexagonal with the SG P6/mmm.20 The side pockets, which feature a 12-membered ring (MR), occur on both sides of the MWW layer and form a large MWW cage (Figure 8a). It is worth noting that the structure is formed by joining different layers along the c-axis by single Si−O−Si bridges in a Kagomé net, and there is no shift between the adjacent layers in the MWW structure (Figure 8b). Notably the layers can be shifted along the ⟨101̅0⟩ direction with 1/3 unit cell in ECNU-5, and these Si−O−Si bridges can be reconnected. If the positions of the silanol groups are

Figure 4. PXRD patterns of (a) ECNU-5P, (b) IEZ-ECNU-5P, (c) IEZ-ECNU-5, and (d) ECNU-5 captured at high-resolution.

of 8−10.5° compared with SSZ-70 before calcination. The proposed structure model suggests that these materials (EMM10P, MCM-56) are likely not ordered in terms of vertical MWW layer alignment but feature interlayer linkages or unknown connections. Therefore, these imply that ECNU-5 may contains MWW layers with intergrown stacking sequences. On the other hand, the 2D precursor of ECNU-5P was readily transformed into an interlayer-expended-zeolite (IEZ-ECNU5) by silylation with the monomeric silane (OCH2CH3)2Si(CH3)2. After silylation and calcination, the reflections related to the expanded interlayer were clearly observed at 2θ values of approximately 3.38 and 6.76° (Figure 4c), implying that IEZECNU-5 contained larger pore openings between adjacent layers than ECNU-5. Nevertheless, IEZ-ECNU-5 still showed the same broad reflection as ECNU-5 over the 2θ range of 8− 10.5°, indicating that the faulted layer stacking remained intact during interlayer expansion. ECNU-5 exhibited a platelet-like morphology, with the primary particles measuring 2−3 μm in width and ∼100 nm in thickness, and hexagonal surface growth steps associated with layer stacking in the ab plane could be clearly observed. The side surface of the plates also revealed information pertaining to the layered crystal growth. These results demonstrate that crystal growth occurred by the stacking of layers along the caxis, yielding a product different from the starting material in terms of both morphology and particle dimension (Figure 5 and Figure S7). As MWW zeolites are promising catalysts in industry, it is important to elucidate the structure of the materials. Structure Analysis. To investigate the structure of ECNU5, transmission electron microscopy (TEM) investigations were carried out on a pure silicate sample. The high resolution TEM (HRTEM) image of an ECNU-5 crystal exhibits the stacking of

Figure 5. SEM images of (A) 3D MWW silicate and (B) ECNU-5. D

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Figure 6. HRTEM images and the corresponding ED patterns of (a) ECNU-5 taken from [1̅21̅0]A/[010]B direction, (b) ECNU-5 taken from [0110̅ ]A/[130]B direction, and (c) IEZ-ECNU-5 taken from [1̅21̅0]A/[010]B direction. The averaged HRTEM images are obtained from the image in the white box after Fourier transformation using the crystallographic image processing software CRISP with only translational symmetry applied (p1 symmetry). The inset images in the upper images are the corresponding ED patterns and in the bottom are the structural model and simulated HRTEM images.

Figure 7. Representation of the 3D reconstruction and the structural model of ECNU-5A. (a) 3D structure (1 unit cell) reconstructed from the HRTEM images. (b) 3D structure overlaid with the structural model by shifting the MWW layers with 1/3 unit cell along ab-plane. (c) 3D structure overlaid with the optimized structural model.

marked by the letters A, B, and C in different layers, both the ABAB... and ABC... stacking sequences can be formed (Figure 8c). The ABAB... stacking is denoted as ECNU-5A, and ABCABC... stacking is named as ECNU-5B. The SG was determined to be P63/mmc, and the unit cell parameters were determined from the XRD pattern to be a = 14.16 Å and c = 49.66 Å for ECNU-5A (Table 1), whereas the SG and unit cell parameters of ECNU-5B were determined to be C2/m and a = 24.52 Å, b = 14.16 Å, c = 26.14 Å, and β = 108.2°, respectively (Table 1). Thus, the difference between two ECNU-5 polymorphs and the MWW structure lies in the relative shift

between adjacent layers. As a result of the shifted structures, the connectivity of the cages was greatly altered. The relative shift between MWW layers caused the 12-MR side pocket in the top layer to move to the center of three 12-MR pockets in the next layer (Figure 8d,e, indicated by the yellow spheres). Thus, they communicate with each other through rectangular 14-MR windows, forming an interlayer connected 3D network. Moreover, HRTEM images of the ECNU-5P and IEZECNU-5 are shown in Figure S10 and Figure 6c, respectively. An interlayer expanded pore structure in IEZ-ECNU-5 was clearly observed, in which the interlayer silicon pillars E

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Figure 8. Structural model of (a) MWW topology and the connectivity of the cages, (b) MWW view from the c direction, and (c) the stacking of the MWW layers with the shift of 1/3 unit cell along the ⟨101̅0⟩ direction. Structural model and the cage connectivity of (d) ECNU-5A and (e) ECNU5B.

Table 1. Unit Cell Parameters and Physical Properties unit cell parameters sample

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

SSAc (m2 g−1)

PVc (cm3 g‑1)

3D MWWa ECNU-5Ab ECNU-5Bb IEZ-ECNU-5Ab IEZ-ECNU-5Bb

14.21 14.16 24.52 14.16 24.52

14.21 14.16 14.16 14.16 14.19

24.95 49.66 26.14 55.16 28.77

90 90 90 90 90

90 90 108.2 90 106.5

120 120 90 120 90

440 430

0.177 0.192

448

0.197

a From ref 20 for ITQ-1. bObtained after geometry optimization from PXRD measurement. cSSA, specific surface area; PV, micropore volume measured by N2 adsorption at 77 K.

connected the adjacent MWW layers with similar ABAB... and ABCABC... stacking sequences in ECNU-5. These results coincided with that from PXRD analysis discussed before. The structures of the two polymorphs were subjected by geometry optimization with the quantum mechanical code Dmol3 in the Materials Studio software.43,44 The optimization was calculated using ECNU-5A because of its high symmetry. Then, the structure of ECNU-5B was obtained by coordination transformation from ECNU-5A. It is worth noting that the free silanol groups of ECNU-5A are straight because of the symmetry restriction, whereas those in ECNU-5B can be relaxed in the monoclinic unit cell. The structural models are overlaid on the averaged images in Figure 6 and the reconstructed unit cell in Figure 7c, and the results are very well consistent with the observed contrast and the reconstructed potential map, respectively. Classical Rietveld refinement for analyzing the structure of ECNU-5 was unfortunately precluded because of the faulted structure. Instead, the program DIFFaX+45 was employed to simulate the diffraction diagrams of various stacking disordered materials. The coordinates of the optimized tetrahedral network

were used for the simulation (Tables S4 and S5). According to the SEM observations, the sample width was set to 2 μm with the stacking of 40 layers (∼100 nm). A series of PXRD patterns were simulated with different ECNU-5A/ECNU-5B ratios (Figure S11). Excellent matching between the simulated and experimental high-resolution PXRD patterns could be obtained with 44% ECNU-5A (Figure 9). To further verify the structural model, the image simulation for HRTEM was performed by using MacTempas software, for which the averaged HRTEM image nicely corrsponds to that in the simulated image with the sample thickness of 10−15 nm and the defocus value of −45 to −55 nm (inset of Figure 6 and Figure S12). It can be observed that the contrast of the HRTEM images for ECNU-5 and IEZECNU-5 matches with the simulated images very well in the interlayer regions. The simulated ED patterns are also shown in Figure S8, which further proved our structural model. Structure Description. The two polymorphs have a similar framework density of 16.7 silicon atoms per 1000 Å3 and the same chemical formula [SiO1.986(OH)0.028] in calcined form. ECNU-5 has an independent sinusoidal 10-MR channel system within the MWW layers, which is the same as that of ITQ-1, F

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Figure 10. (A) Nitrigen adsorption isotherms on a semilogarithmic scale and (B) micropore size distribution from argon adsorption of (a) 3D MWW silicate, (b) ECNU-5, and (c) IEZ-ECNU-5. Figure 9. Comparison of the experimental high-resolution PXRD patterns of ECNU-5 with the pattern simulated with 44% ECNU-5A stacking of the building layers.

OSDA molecules filling the intracrystal voids. The ECNU-5 crystals grow with two adjacent MWW layers interacting with bulky organic cations. The positively charged organic cations well balance the negatively charged anionic framework, whereas the two cyclohexyl groups in OSDA may point toward the two side pockets in the upper and lower MWW layers. As the two cyclohexyl groups in IM+ cations possess nonlinear geometry, this unique spatial configuration is assumed to make the adjacent MWW layer shift along the ab-plane. The irregular stacking modes are subsequently dictated by the interaction between the OSDA and previous layers (Figure S16). Thermodynamically, the right- and left-shifted stackings may be equally favored, resulting in two polymorphs with similar component percentages (44% vs 56%). Additionally, introducing a catalytic active site, such as Al atoms, into the ECNU-5 framework was also successfully performed. As shown in Figure S17, the 3D form Al-ECNU-5 (Si/Al = 100) exhibited very similar PXRD patterns as those of pure-silica ECNU-5. These results indicated that the heteroatoms-containing ECNU-5 with shifted MWW structure may be a promising catalyst for catalytic processes.

whereas the other interlayer 2D channel systems of the polymorphs are completely different. The shifted structure of ECNU-5 causes half of the interlayer Si−OH groups to exist freely. The 29Si MAS NMR spectra were compared between 3D MWW silicate and ECNU-5 (Figure S13 and Table S8). In the case of 3D MWW silicate, the peak at −100.3 ppm, assigned to Q3 species [Si(OSi)3(OH)], accounts for 4.7% of the total Si signal intensity. The formation of Q3 species was due to a small fraction of the Si−OH defect groups, which are commonly developed in the synthesis with N,N,N-trimethyl-1-adamantammonium hydroxide as the OSDA.20 However, more defect silanols were present in ECNU-5. The silicon species assigned to the Q3 species at −100.2 and −97.2 ppm account for 3.1% and 5.9% of the total silicon configurations, respectively. The presence of more silanols than 3D MWW supports the notion that the interlayer connection of the ECNU-5 structure is more interrupted. These isolated and pendent silanols could be assigned to the T10 site for ECNU-5A and T3 site for ECNU5B, respectively. This type of crystallographically ordered silanol has been observed in the high-silica zeolites SSZ-7446 and EMM-23.47 Besides, ECNU-5A and ECNU-5B with shifted structures can be considered as analogous to the ERS-12 material, which possess disordered ferrierite structure.48 ECNU-5 exhibits a Brunauer−Emmett−Teller (BET) specific surface area (430 m2 g−1) comparable to that of the 3D MWW silicate (440 m2 g−1). The micropore size distribution determined by argon adsorption revealed that the interlayer pore size of ECNU-5 was slightly narrowed. However, the silylation of ECNU-5P caused an increase in interlayer pore size from 5.7 to 7.0 Å (Figure 10B). The expansion of the MWW interlayers led to an increased specific surface area and micropore volume for IEZ-ECNU-5 (448 m2 g−1, 0.197 cm3 g−1) (Table 1). Solid-state 13C MAS NMR reveals that the OSDAs maintained their molecular structure during hydrothermal synthesis (Figure S14). The weight percentage of OSDA in ECNU-5P was approximately 45.6 wt %, as determined by thermogravimetric analysis. It was observed that 91% of the OSDAs were removed below 480 °C (Figure S15), indicating that these relatively bulky organic molecules mainly exist in the interlayer spaces. In hydrothermal synthesis, the host silicate framework is constructed surrounding the guest organic cations, leading to novel zeolite architectures, with the organic

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CONCLUSIONS In summary, new intergrown zeolite ECNU-5 polymorphs have been synthesized by an RDR process. Because of the unique synthesis and OSDA employed, the MWW cages formed by the side pockets on both sides of the layer were split, and each side pocket was connected to the three neighboring side pockets in the adjacent layer, forming a new type of interlayer-connected porous network. With the newly formed interlayer pore windows different from those of 3D MWW, ECNU-5 may offer unusual shape selectivity. Introducing catalytically active sites (such as Al, Ti, and Sn sites) into the ECNU-5 zeolite framework by the proposed RDR method would give rise to novel metallosilicates with promising applications in solid acid catalysis and selective oxidation reactions.

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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.chemmater.5b03658. Experimental and characterization information (PDF) G

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P. Wu). *E-mail: [email protected] (L. Han). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Prof. Osamu Terasaki (Stockholm University, Sweden) for discussion in the structural solution of ECNU-5, Dr. Zheng Liu (AIST, Japan) for kind help in the simulation of HRTEM images, and Prof. Matteo Leoni (University of Trento) for the discussion in X-ray diffraction simulation using the DIFFaX+ program. We also acknowledge the financial supports from the NSFC of China (21373089, 21201120, 21533002), Programs Foundation of MOE of China (2012007613000), and SLAD (B409). Dr. Lu Han gratefully acknowledges the support from the National Excellent Doctoral Dissertation of PR China (201454).

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DOI: 10.1021/acs.chemmater.5b03658 Chem. Mater. XXXX, XXX, XXX−XXX