Insights into the Topotactic Conversion Process from Layered Silicate

Mar 3, 2013 - *E-mail: [email protected] (W.Z.); [email protected] (X.B.). ... cations (CTA+) replacing the original template were intercalated into ...
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Insights into the Topotactic Conversion Process from Layered Silicate RUB-36 to FER-type Zeolite by Layer Reassembly Zhenchao Zhao,† Weiping Zhang,*,‡ Pengju Ren,† Xiuwen Han,† Ulrich Müller,# Bilge Yilmaz,⊥ Mathias Feyen,# Hermann Gies,§ Feng-Shou Xiao,○ Dirk De Vos,∇ Takashi Tatsumi,∥ and Xinhe Bao*,† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ‡ State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China # BASF SE, Chemicals Research and Engineering, 67056 Ludwigshafen, Germany ⊥ BASF Corporation, Chemicals Research and Engineering, Iselin, New Jersey 08830, United States § Institute für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Germany ○ Department of Chemistry, Zhejiang University, Hangzhou 310028, China ∇ Centre for Surface Chemistry and Catalysis, K. U. Leuven, Leuven, Belgium ∥ Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama, Japan S Supporting Information *

ABSTRACT: Layered RUB-36 and PREFER (lamellar precursor of ferrierite) are the precursors of CDO and FER-type zeolites, respectively. Both are composed of the same ferrierite (FER) layer building blocks. Topotactic conversion from RUB-36 to pure silica zeolite ZSM-35 has been demonstrated in the presence of a surfactant cetyltrimethylammonium hydroxide (CTAOH). The transformation mechanism of this process was revealed, for the first time, by the detailed investigations of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal analysis, and one- and two-dimensional (2-D) solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) as well as theoretical simulations. During swelling at room temperature, cetyltrimethylammonium cations (CTA+) replacing the original template were intercalated into FER layers to expand the interlayer distance remarkably and consequently to destroy the strong hydrogen-bonding interactions between the layers. 2-D 1H−29Si heteronuclear correlation (HETCOR) NMR indicates that the surfactant polar heads approximate the FER layers in swollen RUB-36. After deswelling, only a small amount of CTA+ cations with long tails lay in the void space between the FER layers. The Monte Carlo simulations on the deswollen RUB36 further elucidate the occlusion of CTA+ cations in the pre-10 member ring of the layered ferrierite precursor, which may act as the structure-directing agent for the formation of FER-structured zeolite. The FER layer reassembly from the alteration of CTA+ conformation at the interlayers is of key importance to the topotactic transformation of RUB-36 to FER-type zeolite by the dehydration-condensation reaction. This may open up more applications in the lamellar zeolite system by the layer restacking approach. KEYWORDS: topotactic conversion mechanism, layered zeolite precursor, RUB-36, FER-type zeolite, layer reassembly

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INTRODUCTION Zeolites are a kind of crystalline microporous materials widely used in catalysis, adsorption, and separation for their distinct pore structures and properties.1−4 Generally, zeolite frameworks are formed by the inorganic species assembly in the presence or absence of organic species during the synthesis process. While another bottom-up concept that zeolites can also be derived by topotactic conversion from their layered precursors expands this area as some resultant new structures may not be obtained by the direct synthesis. For example, zeolite MCM-22, ferrierite, sodalite, RUB-24, CDS-1(RUB-37), and RUB-41 can be obtained from their corresponding layered © XXXX American Chemical Society

precursors MCM-22P, PREFER, RUB-15, RUB-18, PLS1(RUB-36), and RUB-39, respectively.5−13 Meanwhile, these layered precursors can be treated by silylation, acidation, alkaline swelling, and then delamination or pillaring resulting in new structures to extend their applications.14−20 One of the representative building blocks is the ferrierite (FER) layer. These layered precursors such as PREFER, MCM-47, ERS-12, PLS-3, RUB-36, and PLS-4, which are all composed of FER Received: September 27, 2012 Revised: February 19, 2013

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solution (4 wt % solution in water). The mixture was stirred for 48 h, then filtered and washed with deionized water, and finally dried at RT, and 0.83 g of swollen RUB-36 was obtained. The deswelling process was conducted by mixing 0.20 g of swollen sample with a mixture of 6 mL of ethanol and 10 mL of 1 M HCl solution and then stirred for 12 h at RT. The product was also recovered by filtration, repeated washing with deionized water, and dried at RT, and 0.09 g of deswollen RUB-36 was obtained. Calcination of obtained samples was conducted at 550 °C in static air for 4 h. The calcined deswollen RUB36 has the BET surface area of 340 m2/g, which is a little bit smaller than that of FER-type zeolite reported in the literature directly derived from its layered precursor.13 Characterizations. XRD patterns were collected on the Rigaku D/ MAX-2500 X-ray diffractometer with Cu Kα radiation in the 2θ range of 2−50° and scan rate of 5°/min. Solid-state 29Si MAS NMR measurements were performed on the Varian Infinity plus-400 spectrometer equipped with a 5.0 mm MAS probe. The spectra were recorded at 79.5 MHz with 1H decoupling at a spinning rate of 6 kHz, 100−2048 scans, and 10−120 s recycle delay. The chemical shifts were referenced to tetramethylsilane, and the spectra were fitted by the software DMFIT2011.39 1H MAS NMR spectra were recorded on Bruker Avance III-600 spectrometer at 600.1 MHz with a 2.5 mm MAS probe at a spinning rate of 30 kHz, 32 scans, and a recycle delay of 5 s. The estimated sample temperature inside the rotor is about 60 °C as calibrated by Pb(NO3)2 with a spinning rate of 30 kHz. The chemical shifts were referenced to tetramethylsilane. 13C CP/MAS NMR spectra were recorded on the same instrument at 150.9 MHz with a 4 mm MAS probe, a spinning rate of 12 kHz, 1024 scans, a contact time of 5 ms, and a recycle delay of 2 s. The chemical shifts were referenced to adamantane with the upfield methine peak at 29.5 ppm. The two-dimensional 1H−29Si HETCOR NMR spectra were acquired on the same probe at a spinning rate of 12 kHz, and t1 increment was set to 83.33 μs. The TPPI method was used in the 2D data acquisition and processing. SEM images were obtained using the FEI Quanta 200FEG scanning electron microscope, operating at an accelerating voltage of 20 kV, and samples were coated with gold before imaging. Thermogravimetric (TG) analyses were performed on a TA Q-600 analyzer with a temperature-programmed rate of 10 K/min under an air flow of 100 mL/min. The N2 adsorption measurements of calcined deswollen RUB-36 was carried out on a Micromeritics ASAP 2020 analyzer at 77.35 K after the sample was degassed at 350 °C under vacuum for 10 h. Theoretical Simulations. Three typical FER layer stacking models were proposed based on experimental results (details see the Supporting Information). The configurational bias Monte Carlo (MC) simulations were performed by sorption modules implemented in Materials Studio.40,41 CVFF force field was used for all elements, and the atom charges were calculated by the charge-equilibration method.42,43 The charge of CTA cations was assigned to be +1, and the framework of layered precursor was negative charged to neutralize CTA cations. The simulation contained two steps: 1) CTA+ insertion into the framework by loading 0.5 molecules per cell and 2) relaxing the structures by 200,000 MC moves to make CTA+ find the most stable configuration inside the layered precursors, and then the interaction energies were calculated (details see the Supporting Information).

layers with different organic moieties or stacking orders, can be transformed to FER- or CDO-type zeolites upon thermal calcination.7,13,21−23 In addition, Eilertsen et al. demonstrated that PREFER precursor could be swollen and delaminated to UCB-2 by using a combination of bromide, fluoride, and chloride anions as well as CTA and tetrabutyl ammonium cations.24 At the same time, the FER type zeolites have been extensively explored in the catalytic process such as olefin isomerization, dimethyl ether carbonylation, DeNOx, etc.25−28 We reported a new interlayer-expanded RUB-36 zeolite named as COE-4 showing the two-dimensional 10-ring porous structure by the silylation reaction of silanol groups on the interlayer surface with dichlorodimethylsilane, and these materials with the substitution of Si with Al, Ti atoms were shown to be active catalysts in isomerization reaction and olefin oxidation with unique selectivity.29−31 It is found that closer contacts between silanol groups and careful control of the layer stacking sequence are critical to succeed in the topotactic conversion.32 However, there are relatively few reports on the detailed investigation of topotactic transformation from one layered precursor to another by the layer reassembly due to the strong hydrogen bonds between the original layers. Especially, the local interactions between the inorganic building blocks and organic moieties have not been well elucidated.33 These interactions are of key importance to drive the topotactic conversion and to form a given zeolite phase. Solid-state NMR is a powerful tool to explore the local environments and interactions in porous materials.34,35 The 2-D HETCOR experiments have been well established to probe the spatial connectivity through heteronuclear dipolar interactions.36−38 1 H− 29 Si as well as 1 H− 27 Al HETCOR NMR nicely demonstrated the molecular proximities between protons of organic moieties in the structure-directing agent (SDA) and the surface silica of the inorganic wall in layered silicate surfactant mesophases.36,37 In this study, the topotactic conversion from layered RUB-36 to pure silica zeolite ZSM-35 with FER structure was accomplished in the presence of cetyltrimethylammonium hydroxide (CTAOH). The interactions between the FER layers and organic moieties during the layer reassembly process were investigated, for the first time, in detail by one-dimensional multinuclear solid-state NMR, twodimensional 1H−29Si HETCOR NMR, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The structure-directing role of the organic moieties was also illustrated by the combination of solid-state NMR and theoretical Monte Carlo (MC) simulations. The topotactic conversion process through the layer reassembly was proposed for the zeolitic lamellar precursors in order to gain more insights into the zeolite formation mechanism.

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EXPERIMENTAL AND COMPUTATIONAL

Sample Preparation. The layered silicate RUB-36 is composed of FER layers stacking in translation symmetry with diethyldimethylammonium (DEDMA) intercalated between the layers. It was prepared similarly to our previous procedures using diethyldimethylammonium hydroxide as the structure-directing agent (DEDMAOH, 20 wt % solution in water, Sachem Inc.).29 In general, it was crystallized from the gel with a composition of SiO2:0.5 SDA:10 H2O. Aerosil 200 was utilized as the silica source. Crystallization was carried out in an autoclave without stirring for 14 days. The resulting product was filtered, washed with deionized water, and dried at 100 °C. RUB-36 was swollen by CTAOH at room temperature (RT). Typically, 0.50 g of RUB-36 was dispersed in the 35.0 g CTAOH

RESULTS AND DISCUSSION XRD and SEM. Figure 1a shows the XRD pattern of layered zeolite precursor with a prominent peak of 200 diffraction at 2θ = 7.9° (d = 11.1 Å) as the preferred orientation, which confirms the RUB-36 lamellar structure composed of ferrierite sheets.10,13 This layered precursor can be swollen to RUB36SW by the surfactant CTAOH at room temperature as shown by the XRD pattern in Figure 1b consisting of decreased 200 peak intensity and shift of the layer-related 100 diffraction to 2.9° (d = 30.4 Å). The high order diffractions can also be B

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peak intensity at low-angle part and broad diffractions at highangle part. This means the deswelling process is necessary to obtain the layered PREFER-1 precursor. Considering the structure of RUB-36 and PREFER, we may find that there is a sliding in the stacking order of FER layers between them. As mentioned above the FER layers of PREFER-1 take a similar stacking order as PREFER but a smaller layer distance, the topotactic conversion may occur relatively easily to form FERtype zeolite by dehydration-condensation of the silanol groups on the neighboring layers over the acidic deswelling process. While for swollen RUB-36, the stacking order is disturbed due to the longer layer distance, and therefore the calcined swollen RUB-36 almost exhibits the amorphous properties. It can be concluded from XRD results that the layered RUB-36 precursor can be swollen to RUB-36SW by CTAOH and then deswollen in acid condition at room temperature to form a lamellar precursor of ferrierite, PREFER-1. The stacking order of FER layers in RUB-36 alters over the swelling and deswelling processes, which is crucial to the topotactic transformation to form FER-type zeolite ZSM-35 in the following calcination. The macroscopic morphologies of samples during above treatments were also followed by SEM. In Figure 2a, the parent sample RUB-36 shows typical plate-like shape, and there is no significant morphology change during swelling and deswelling processes (Figure 2b-c). As shown in Figure 2d, the calcined PREFER-1 i.e. zeolite ZSM-35 has nearly the same feature as parent RUB-36, suggesting that it was formed in a topotactic manner without collapse of the mother silicate layer. However, the calcined RUB-36SW exhibits differently curved and wrinkled flake morphology (Figure 2e and Supporting Information Figure S1). Interactions between the Inorganic and Organic Moieties. The local structure variations of inorganic species in the FER layers during the swelling, deswelling, and calcination processes were investigated by one-dimensional 29 Si MAS NMR. As shown in Figure 3, all spectra are composed mainly by resonances of Q3 and Q4 sites (Qn stands for X4‑nSi[OSi]n, X = OH or O−) at ca. −103 to −112 ppm.29,45,46 The spectrum of RUB-36 in Figure 3a shows at least two types of Q3 groups: one at ca. −102 ppm and the other at ca. −104

Figure 1. XRD patterns of the solid samples: (a) RUB-36; (b) RUB36SW; (c) deswollen RUB-36, i.e. PREFER-1; (d) calcined PREFER-1 i.e. ZSM-35; (e) calcined RUB-36 i.e. RUB-37; (f) calcined RUB36SW.

resolved to 500, indicating the FER layers are well preserved in high stacking order. At the same time, the high-angle diffractions are weakened a lot, which implies the FER layers are swollen by CTAOH completely. After deswelling in acid solution (Figure 1c), the layer-related diffraction shifts back to 8.3° (d = 10.7 Å), and the layer distance is a little shorter than the original RUB-36, meanwhile the high-angle diffractions could not be well resolved. As a consequence of the lack of crystallinity, the sample pattern in Figure 1c could not be used to identify the phase. However, after calcination of the deswollen RUB-36, the pattern in Figure 1d has well resolved diffractions, which is identified as zeolite ZSM-35 with the FER structure.44 This structure is completely different from calcined RUB-36 i.e. zeolite RUB-37 with the CDO structure (Figure 1e).10,13 So, the sample deswollen RUB-36 in Figure 1c could be a lamellar precursor of FER-type zeolite (named PREFER-1) which is similar to PREFER although the interlayer distance is smaller (ca. 2.3 Å) than that of PREFER.24 For the calcined swollen RUB-36 in Figure 1f, there is a remarkable decrease of

Figure 2. SEM images of the solid samples: (a) RUB-36; (b) RUB-36SW; (c) PREFER-1; (d) calcined PREFER-1; (e) calcined RUB-36SW. C

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Figure 4. 1H MAS NMR spectra of the solid samples: (a) RUB-36; (b) RUB-36SW; (c) PREFER-1; (d) calcined PREFER-1. Asterisk denotes the NMR probehead background.

deconvolution and integration of the 1H MAS NMR spectrum, which is in accordance with the model of RUB-36.33 However, this peak disappears for the swollen and deswollen samples, which confirms the success of expanding the FER layers by destroying their original strong hydrogen-bonding interactions over the swelling process. The peak positions of RUB-36SW and PREFER-1 in Figure 4b-c are almost the same at ca. 0.9, 1.2, and 3.2 ppm. The peaks at 0.9 and 1.2 ppm can be attributed to terminal and middle protons of cetyl groups, and 3.2 ppm belongs to protons of carbon directly connected to the nitrogen in the swelling agent CTA+ cation.37 The broad peak at ca. 5.2 ppm may come from surface silanols and/or adsorbed water species.13,37 In our case, this signal should be assigned more likely to the surface silanols because its intensity is much weaker for the swollen RUB-36 in alkaline condition and recovers to much stronger for the deswollen PREFER-1 in acidic condition. After calcination, signals of CTA+ cations and surface silanols on the layers disappear completely (Figure 4d), and there are only residual signals of isolated, hydrogenbonded, and dual silanol groups on the zeolite ZSM-35 at 1.7, 2.2, and 3.2 ppm, respectively. 28 This indicates that accompanying with the removal of the structure-directing agent the layered FER precursor was transformed to zeolite ZSM-35 through the dehydration-condensation of the neighboring silanols. 2D 1H→29Si HETCOR NMR was performed to explore the interactions between the inorganic and organic species during the swelling and deswelling processes. The space proximity between 1H and 29Si sites can be obtained by tuning different contact time. The spectra of swollen RUB-36 with contact time of 1 to 6 ms are shown in Figure 5a-b. The projection of 29Si dimension shows two main resonances near −103 and −112 ppm, which is similar to above one-dimensional 29Si MAS NMR. However, there are two obvious inequivalent Q3 sites with a main peak at −103 ppm and a shoulder at −105 ppm. These two inequivalent sites might result from the different interactions between CTA+ and FER layers. For swollen RUB36, no cross-peak between Q3 sites and Si−OH protons of significant intensity is observed in Figure 5a-b. This suggests that Q3 sites are the Si−O− type instead of the Si−OH one, in agreement with the pH conditions used in the swelling process. With a longer contact time of 6 ms (Figure 5a), the cross-peaks show higher intensities between 3.2 ppm in 1H dimension and −103, −112 ppm in 29Si dimension. This indicates the close space proximity between the polar head groups of CTA+ and

Figure 3. 29Si MAS NMR spectra of the solid samples: (a) RUB-36; (b) RUB-36SW; (c) PREFER-1; (d) calcined PREFER-1.

ppm. There are also two distinguishable Q4 sites at ca. −111 and −114 ppm, respectively, which are not as well distinguishable as in the literature.10 For the swollen sample in Figure 3b, the peaks of Q3 and Q4 sites become a little symmetric, which means the sites become more equivalent. It can be explained by the loss of original strong interactions as the layer distance expanded over swelling. The ratio of different silicon sites can be calculated by the deconvolution and integration of the spectra. As shown in Figure 3a-b, the ratio of Q4/Q3 decreases from 2.8 to 1.5, and minor Q2 groups appear because of the swelling in the high alkaline condition, which may lead to partial breakage of the framework Si−O−Si bond. Interestingly, the Q4/Q3 ratio returns to 2.6 after deswelling (Figure 3c). The FER layer may self-repair during the deswelling process by silanols condensation in acid condition. There is no significant change in the Q4/Q3 ratio of PREFER-1 compared to parent RUB-36, while two inequivalent Q4 sites become obvious (Figure 3c), one prominent peak at ca. −114 ppm and the other at ca. −111 ppm. The different peak shape compared with RUB-36 may result from the alterations of FER layer stacking order and the related interaction between FER layers and organic moieties after the treatments. Upon calcination the amount of silanol groups decreases drastically in Figure 3d, as most of the Q3 sites were condensed into the Q4 sites to form the framework of FER structure in zeolite ZSM-35, which is similar to the previous report.13 1 H MAS NMR was also conducted to investigate the organic parts of these solid samples. As shown in Figure 4a, there are two main peaks at ca. 1.4 and 3.1 ppm for templated RUB-36, which are attributed to the methyl species in ethyl groups and methyl/methylene species directly connected to the nitrogen of the structure-directing agent DEDMA+ cation.13 There is also a weak signal at 16.4 ppm that appeared in the layered silicate RUB-36. The assignment of this signal in 1H NMR to the strong hydrogen-bonded silanol groups between the adjacent FER layers is generally accepted in the literature.13,47−49 This peak could not be well resolved by the IR spectroscopy because it shifts to the low wavenumber regions for the layered materials.50 The content of this peak is about 6% after D

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Figure 5. 1H−29Si HETCOR NMR spectra and their one-dimensional projections of the solid samples with different contact time: (a) RUB-36SW, ct = 6 ms; (b) RUB-36SW, ct = 1 ms; (c) PREFER-1, ct = 6 ms; (d) PREFER-1, ct = 1 ms.

5d) the cross-peaks of the nonpolar alkyl part with Q3, Q4 sites are as strong as those of polar head parts with Q3, Q4 sites, which is completely different from the feature of swollen RUB36 in Figure 5b. This demonstrates that besides the polar head groups the nonpolar alkyl part of CTA+ also approximates the FER layers. It can be inferred that the CTA+ cations with a long tail should lie in the void space between the FER layers in the deswollen RUB-36. So, the local interactions of CTA+ cations with FER building blocks are much stronger in sample PREFER-1 than in swollen RUB-36, which is beneficial for driving the topotactic conversion from RUB-36 to a given zeolite ZSM-35. Role of Organic Species. Organic moiety changes during swelling and deswelling processes were investigated by 13C CP/ MAS NMR with the spinning rate of 12 kHz and an optimized contact time of 5 ms to achieve better resolution. For sample

the Q3, Q4 sites of the FER layer. There are also weak crosspeaks between 1.2 ppm in 1H dimension and −103, −112 ppm in 29Si dimension, which may indicate the longer distance between the nonpolar alkyl chain of CTA+ and the inorganic FER layer. These weak cross-peaks become even weaker with a shorter contact time of 1 ms (Figure 5b), which further reveals the longer distance between them. For the deswollen sample, i.e. PREFER-1, the intensity of cross-peak between the proton at near 5.2 ppm and Q3 sites at −103 ppm becomes much stronger in the HETCOR NMR spectra as the SiOH groups recovered during the deswelling process under acid treatment. With a longer contact time of 6 ms (Figure 5c) not only the polar head groups but also the nonpolar alkyl part of CTA+ in the 1H projection shows a much stronger cross-peak correlation with Q3 and Q4 sites in the 29Si projection. Even with a shorter contact time of 1 ms (Figure E

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So, 13C CP/MAS NMR and TGA measurements demonstrate that CTA+ cations replacing DEDMA+ thoroughly were intercalated into the layered RUB-36 during the swelling process, while most of the CTA+ cations were excluded upon deswelling, and the residual ones may act as the structuredirecting agent (SDA) for the formation of zeolite ZSM-35. The exact location and conformation of organic species inside the layered PREFER-1 are not so straightforward from the solid-state NMR. Theoretical computations can provide us deeper understandings of the molecular structures, energetics, and dynamics, etc., which are in favor of the experimental studies.53−58 In order to further evaluate the role of CTA+ cations in the formation of zeolite ZSM-35, we optimized the geometries of layered PREFER-1 precursor by the Monte Carlo simulations. As demonstrated in Figure 8, the most possible

RUB-36 (Figure 6a) the peaks at 9.2 and 59.2 ppm are attributed to the methyl and methylene species of ethyl groups

Figure 6. 13C CP/MAS NMR spectra of the solid samples: (a) RUB36; (b) RUB-36SW; (c) PREFER-1.

in DEDMA+, respectively. The peak at 52.6 ppm is ascribed to the methyl groups directly connected to the nitrogen atom in DEDMA+.13,51 The spectra of RUB-36SW and PREFER-1 in Figure 6b-c show the same feature with no obvious observation of DEDMA+. The peaks at ca. 14−33 ppm are attributed to the cetyl group of CTA+, and the peak at ca. 55 ppm is due to the methyl groups directly connected to the nitrogen atom. These findings suggest that DEDMA+ cations are incorporated into the interlayer of RUB-36, and CTA+ cations are introduced in swollen/deswollen RUB-36. To further exclude the existence of original template DEDMA+ in swollen RUB-36, solution 13C NMR was conducted for the dissolved RUB-36SW (see Supporting Information Figure S2). It is found that there only exist the 13C signals from the CTA+ in swollen RUB-36. This means the original template of RUB-36 was excluded completely during the swelling process. TGA curves in Figure 7a-c indicate the weight loss of these samples in the following

Figure 8. Optimized structure models of PREFER-1: (a) and (b) represent structural drawing projection along the (011) and (101) directions, respectively.

framework model of PREFER-1 is that the CTA+ surfactant lies in the void space between the FER layers with stagger arrangement viewed along the (011) direction i.e. CTA+ cation is occluded in the pre-10 member ring channel viewed along the (101) direction because it has the lowest stabilizing energy to accommodate the surfactant (see Supporting Information Table S1). This is in good agreement with the above 1H−29Si HETCOR NMR result that both the polar head groups and the nonpolar alkyl part of CTA+ approximate the FER layers. So, solid-state NMR combining with Monte Carlo simulation verify that the FER layer reassembly from the alteration of CTA+ conformation between the layers is favorable for the directing FER-structured zeolite. All these results can allow proposing the schemes shown in Figure 9 for the topotactic conversion of layered RUB-36 to FER-type zeolite. The translation symmetrically stacked FER layers with strong hydrogen-bonding interactions in RUB-36 are swollen by the intercalation of CTA+ cations to expand the interlayer distance remarkably and make the original interactions between FER layers and organic templates lost. Over deswelling, most intercalated CTA+ cations are extracted, but the FER layer distance recovers in a mirror symmetric stacking order with the remaining CTA+ cations occluded in the pre-10 member ring channel as the structure-directing agent. Upon calcination, FER and CDO-type zeolites can be obtained, respectively, by dehydration-condensation of the neighboring silanols from the layered RUB-36 precursors with or without swelling/deswelling.

Figure 7. TGA curves of the solid samples: (a) PREFER-1; (b) RUB36; (c) RUB-36SW.

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order: RUB-36SW > RUB-36 > PREFER-1. There are two main weight loss zones for swollen and deswollen RUB-36: one from 100 to 200 °C due to the CTA+ surfactant decomposition, the other above 200 °C due to the surfactant combustion.52 There is no weight loss of DEDMA in deswollen RUB-36 i.e. PREFER-1, and the lowest weight-loss sample is PREFER-1.

CONCLUSIONS Topotactic conversion from layered RUB-36 precursor to pure silica FER-type zeolite by reassembly of FER layers without significant structure degradation was accomplished in the presence of the surfactant CTAOH. The transformation F

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Figure 9. Schematic image of topotactic conversion of layered RUB-36 to FER- or CDO-type zeolites.

mechanism was demonstrated for the first time. Over swelling, the interlayer distance is expanded remarkably by the intercalation of CTA+ cations, and the surfactants replacing the original template take a lamellar stacking with polar head groups approximating tightly the FER layers as demonstrated by the two-dimensional 1H−29Si HETCOR NMR. After deswelling, most intercalated CTA+ cations are extracted, while the remaining organic surfactants with long tails lie in the void space between the FER layers. The theoretical simulations further indicate that the CTA+ cations are occluded in the pre10 member ring of deswollen RUB-36 acting as the structuredirecting agent for the formation of FER-structured zeolite. The FER layer reassembly from the alteration of CTA + conformation between the layers plays a critical role in the topotactic transformation of RUB-36 to zeolite ZSM-35 by dehydration-condensation of the neighboring silanols. This may be extended to other layered zeolite precursors to obtain different structures by the layer restacking approach.

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support of National Natural Science Foundation of China (No. 21173029), the Program for Liaoning Excellent Talents in University, and the Fundamental Research Funds for the Central Universities in China.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of calcined RUB-36SW, liquid 13C NMR spectra of dissolved RUB-36 and swollen RUB-36 in HF solution, and details on the theoretical calculations by Monte Carlo simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.Z.); [email protected] (X.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Su Xiong of DICP for assistance with N2 physisorption. We also thank one of the anonymous reviewers for the helpful discussions. This work was supported by the INCOE (International Network of Centers of Excellence) project coordinated by BASF SE, Germany. W.Z. thanks the G

dx.doi.org/10.1021/cm303131c | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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