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Cite This: Chem. Mater. 2018, 30, 1839−1843
Synthesis of Lamellar Mesostructured ZSM-48 Nanosheets Yunjuan Zhang,† Yanhang Ma,*,‡ and Shunai Che*,†,§ †
School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China ‡ School of Physical Science and Technology, ShanghaiTech University, 393 Middle Huaxia Road, Shanghai 201210, People’s Republic of China § School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, People’s Republic of China S Supporting Information *
A
that combine the aforementioned SDA (HM or HXN) and certain hydrophobic centers could direct the synthesis of lamellar structured ZSM-48. Here, we attempted to design a surfactant that could produce lamellar mesostructured ZSM-48 nanosheets by introducing benzophenanthrene into the hydrophobic core as well as six 1quaternary ammonium, 6-tertiary amine diaminohexane chains as the heads of the surfactant to form (C6H2)3−(O−CnH2n− N+(CH3)2−C6H12−N(CH3)2 (Br−))6 (denoted BPTn−6−0), in which the six quaternary groups were connected by different alkyl chain lengths of n = 4, 6, 8, 10 and 12 carbons (see Table S1, Figure S2). The benzophenanthrene core and alkyl chains could self-assemble to form a lamellar structure through π−π interactions and van der Waals interactions, while the six quaternary groups stabilized the lamellar micelle structure, and the six tertiary amine groups directed the ZSM-48 zeolite framework formation. The lamellar assembly of the surfactants would disrupt the zeolite growth as well as adjust the interlamellar spacing, leading to the formation of lamellar mesostructured ZSM-48. Figure 1a shows the low-angle powder X-ray diffraction (PXRD) pattern of the as-prepared sample templated by BPT10−6−0. The two well-resolved peaks at 2θ = 1.24° and 2.38° were indexed to the first- and second-order reflections of the lamellar mesostructure with a d-spacing of 7.1 nm. After removal of the templates by calcination, the layered zeolites collapsed into multilayers with randomly distributed mesopores between the layers. The high-angle PXRD pattern shows only several broad peaks, which are attributed to the crystalline of the ZSM-48 framework, as reported previously.26 The fact that most of the peaks in the high-angle range (Figure 1b) are broad is consistent with the lamellar structural feature. After calcination, the relative peak intensity of 002 reflection increases, indicating that some zeolite layers could connect with each other and form bulk ZSM-48. This is also consistent with the following TEM studies of the calcined sample. Of course, the coherence length of the calcined sample along the c axis is still short as the morphology of nanosheets maintained (Figure S3). Moreover, some zeolite layers could also be randomly orientated after calcination. However, limited
luminosilicate zeolites are well-known for their microporous crystalline structures, which are widely used in catalysis, adsorption, separation and gas storage.1−5 However, the microporous structure often goes hand in hand with the hindered diffusion of molecules into and out of the pores, which causes rapid deactivation.6−9 To alleviate the diffusion limitation problem, hierarchical structures containing both mesopores and microspores, which can decrease the diffusion length, have been introduced in zeolite systems.10,11 Nanosheets with ordered lamellar mesostructures or disordered assemblies are one of the most widely and intensively studied hierarchical zeolites. So far, many aluminosilicate zeoliteseparated nanosheets, such as, MWW,12−14 MFI,15−18 MEL,19 FAU,20 TON21 and MOR22 nanosheets, have been successfully synthesized through swelling/delamination, dual templates and bifunctional template methods. All of these nanosheet zeolites were formed by breaking the relatively few covalent bonds along the disconnection direction of the zeolites. Lamellar mesostructured MFI zeolites were first reported by Ryong et al. using a bifunctional template method,17 in which the researchers used designed gemini-type surfactants that consisted of multiple quaternary ammonium head groups and a hydrophobic alkyl tail to direct the formation of single-unit-cell MFI nanosheets. Furthermore, single-crystalline lamellar mesostructured MFI nanosheets were obtained from surfactants with aromatic tails, in which π−π interactions and geometry matching between the template molecule and the zeolite framework were the key factors for the synthesis of the lamellar structure.18,23 It is worth noting that among the aluminosilicate zeolite nanosheets, the lamellar mesostructure has only been found in MFI zeolite nanosheets. ZSM-48 is a high-silica zeolite with an *MRE framework consisting of tubular 10 T atom pores along the a-axis and a relatively weak link in the middle of the 10 T atom pores (two Si−O−Si bonds per unit cell along the c-axis).24−26 This structural feature of ZSM-48 resembles that of the MWW zeolites, which also have a weak connection along the c-axis, and MWW nanosheets have been successfully synthesized by breaking the framework in the ab plane using a bifunctional template method14 (see Figure S1 for details). It has also been reported that conventional ZSM-48 zeolites can be obtained by using hexamethonium (HM) bromide or 1,6-hexanediamine (HXN) as the structure-directing agent (SDA).27−30 Taking the structural features of ZSM-48 and the bifunctional template method into consideration, we can speculate that surfactants © 2018 American Chemical Society
Received: January 11, 2018 Revised: February 27, 2018 Published: March 1, 2018 1839
DOI: 10.1021/acs.chemmater.8b00146 Chem. Mater. 2018, 30, 1839−1843
Communication
Chemistry of Materials
Figure 1. Structural characterization of the LMZN templated by BPT10−6−0. Low- (a) and high-angle (b) PXRD patterns (Cu Kα radiation λ = 0.154 18 nm) of the as-prepared (black) and calcined (red) ZSM-48 nanosheets, indicating a highly ordered lamellar mesostructure and the ZSM-48 zeolite framework. Low- (c) and high-magnification (d) SEM images of the as-prepared samples revealing a flower-like morphology composed of alternating zeolites layers (as indicated by the arrow in panel d) and interlamellar spaces.
Figure 2. HRTEM images and nitrogen isotherm of the LMZN. Low(a) and high-magnification (b) TEM images taken along the common a-axis of a sliced as-prepared sample. Low- (c) and high-magnification (d) TEM images taken along the common b-axis of a sliced asprepared sample. The corresponding FDs were inserted over the TEM images. Low-magnification TEM image (e) taken along the common b-axis of a sliced calcined sample. (f) N2 adsorption−desorption isotherm and pore size distribution curve (inset) obtained from the desorption branch of the calcined LMZN.
information and considerable reflection overlapping caused difficulties in solving the structure of the LMZN. Scanning electron microscopy (SEM) images (Figure 1c,d) show that the sample is composed of slightly curved nanosheets, and some of the nanosheets agglomerate into flower-like particles with a diameter of a few micrometres while others scatter randomly. High-magnification SEM images show that the nanosheets are composed of regularly arrayed ultrathin zeolite layers (∼3 nm) as indicated by the arrow (Figure 1d). Some scattered nanosheets are detected during the lowmagnification inspections (see Figure S4). High-resolution transmission electron microscopy (HRTEM) images of sliced thin sections of the as-prepared and calcined samples are presented in Figure 2. The asprepared sample shows the uniformly stacking of zeolite layers in LMZN. Each nanosheet was composed of alternating an ∼3 nm-thick zeolite layers and ∼4 nm-thick surfactant micelles (Figure 3a,c). Because the coherence length of the sample along the c-axis was short, no selected area electron diffraction of the ordered nanosheets was obtained. A good match was obtained when the ZSM-48 framework31 was superimposed on the HRTEM image, and corresponding Fourier diffractograms (FDs) are consistent with this result. The high structural coherence of ZSM-48 sheet along the baxis is revealed in Figure 1b,d. The superimposed model was composed of three fused 10-MR tubules, which corresponded to a 1.5 unit cell dimension along the c-axis (c = 20.14 Å). The breaking of the zeolite framework occurred in the middle of the 10-MR rings along the c-axis, where has the smallest number of bonds per area (two Si−O bonds per unit cell) in the whole framework, resulting in integral multiples of the length of half
the unit cell along the c-axis (Figure 3b,d). Discrete spots in FDs appeared along the c*-axis indicate the spatial correlation of zeolite layers in some local areas. The obtained interlayer spacing is slightly different from the results obtained from the PXRD data, which may be due to the structural changes that occur in the surfactant micelles under the high-vacuum conditions of TEM. After calcination, the zeolite layers aggregated as the BPT template molecules were removed. Most of the layers collapsed into a single crystalline structure by forming new Si−O−Si bonds (Figure 3e, Figure S5), while some of the layers became curved during calcination due to the large interlayer spacing and chain length, resulting in the formation of some randomly distributed mesopores (see Figure S6). The N2 adsorption−desorption isotherm of the calcined LMZN shows a profile similar to that of conventional bulk ZSM-48 due to the crystalline microporous structure.32 The N2 adsorption−desorption isotherm of the calcined LMZN exhibits a type IV curve with an H3-type hysteresis loop.33,34 The adsorbed volume of N2 for the calcined sample is approximately 30 cm3 g−1 at a relatively low pressure (P/P0 < 0.02), which is smaller than that of the conventional ZSM-48 zeolite (∼40 cm3 g−1), indicating that the sample contains fewer zeolitic microspores. The calcined LMZN sample exhibited a clear hysteresis loop at a relative pressure range of 0.5−1.0, indicating that a small amount of mesopores were randomly distributed between the collapsed ZSM-48 lamellae. The Brunauer−Emmett−Teller (BET) surface area and the micro- and mesopore volumes of the LMZN were measured to 1840
DOI: 10.1021/acs.chemmater.8b00146 Chem. Mater. 2018, 30, 1839−1843
Communication
Chemistry of Materials
Figure 3. Schematic representation of the formation of the LMZN. Molecular structure of the surfactants used in this work (a). Ultraviolet−visible absorption spectra of the as-prepared template molecules (BPT10−6−0 in the liquid (i) and solid forms (ii)) and the as-prepared LMZN (iii) (b). Arrangement of the benzophenanthrene groups from adjacent ZSM-48 layers in the ab plane (c) and template molecules located in the interspacees along the c-axis (d).
be 244 m2 g−1, 0.05 cm3 g−1 and 0.19 cm3 g−1, respectively. The BET surface area is larger than that of conventional ZSM-48 (152 m2 g−1),32 while the micropore volume is nearly twothirds that of conventional ZSM-48 (0.09 cm3 g−1). The pore size distribution obtained from the desorption branch shows a relatively broad peak cantered at 12 nm, indicating the presence of randomly distributed mesopores after calcination. Moreover, the synthesis using surfactant molecules with different carbon chain lengths of 4, 6, 8 and 12 was performed (see Figures S7−10, Table S2). By increasing the carbon chain length to n = 12, a less ordered LMZN with a broad peak in the low-angle PXRD as observed (see Figure S7). The decrease in the ordering may originate from the long chains of the BPT12−6−0, which have difficulty in maintaining long-range order. A proper chain length of n = 8 and 10 can produce ordered LMZN products (see Figure S8). The SEM and TEM images and the N2 adsorption isotherm of these two samples have similar features (Figures S7−10). Further shortening the chain length to n = 4 and 6 resulted in the formation of phyllosilicate, which was probably due to the low flexibility of BPT4−6−0 and BPT6−6−0 under the hydrothermal conditions. With a decrease in the chain length, the rigidity of the molecular structure increases, making it difficult for the six alkyl chains to maintain their reversed parallel stretching structure, so the template molecules cannot cooperate effectively to break the 10-MR units that form the LMZN. In addition, the larger molecular structure makes it difficult to enter the 10-MR units, suggesting that these two surfactants cannot act as structure-
directing agents for conventional ZSM-48 zeolite. The various structural features of the samples indicate that the carbon chain length of the surfactant plays an important role in the structure formation. The position of the benzophenanthrene groups in the center of the micelles was confirmed by the electron charge distributions based on the low-angle XRD data (see Figure S11). The intact configuration of the BPT10−6−0 template without decomposition in the LMZN was confirmed by 13C nuclear magnetic resonance (NMR) spectra (see Figure S12). Thermogravimetric analysis (TGA) of the LMZN showed a rapid total weight loss (50%) above 200 °C (see Figure S13) due to decomposition of the template. The 50% surfactant content suggests that the SiO2/surfactant molar ratio is 44.5, corresponding to one template molecule per unit cell. The assembly status of the template molecules in and out of the zeolites was further studied by UV−visible adsorption spectroscopy (Figure 3b). In a dilute water solution, the BPT10−6−0 molecules were isolated from each other, resulting in absorption bands from the molecule itself at 278 and 315 nm. In the crystalline solid state, the packing of the template molecules was very tight and regular, and the benzophenanthrene groups participated in π−π stacking, which shows broad peaks centered at 301 and 328 nm. The redshifts in the absorption bands reveal the assembly behavior of the template molecules, as a shorter distance between the benzophenanthrene groups was obtained. Both LMZN and the template molecules (solid form) showed a broad peak at 274 nm, which 1841
DOI: 10.1021/acs.chemmater.8b00146 Chem. Mater. 2018, 30, 1839−1843
Communication
Chemistry of Materials Notes
belongs to a single benzophenanthrene group in the solid form. The presence of a single benzophenanthrene group in the LMZN may due to the outermost less ordered template molecules that are attached to the surfaces of the zeolite nanosheets, as the nanosheets are quite thin, which leads to a high proportion of template molecules on the outermost layers. Taking the structural features of ZSM-48 determined from the XRD patterns and the TEM images as well as the TGA data into consideration, we proposed that the material is composed of ZSM-48 lamellae wherein the template molecules align along the c-axis of the ZSM-48 framework with the tertiary amine heads located at the center of the broken 10-MR channels. The template molecules attach to the ab plane, which hinders the growth of the zeolite framework, resulting in a lamellar structure along the c-axis (Figure 3c,d), and the benzophenanthrene groups in the hydrophobic center form a stable network through strong π−π stacking. From the top view of the benzophenanthrene groups along the c direction, the arrangement of the benzophenanthrene groups from adjacent ZSM-48 unit cells in the ab plane can generate a plane composed of strong π−π interactions between benzophenanthrene groups. In this way, all the benzophenanthrene groups possess an offset conformation with a well-defined network that possesses a 3.6 Å π−π interaction distance between the benzophenanthrene cores, which geometrically matches the ZSM-48 framework and produces ZSM-48 layers. As the template layers provide interlamellar support, template removal was expected to lead to condensation of the ZSM-48 layers, consistent with the results obtained from the low-angle XRD patterns and TEM images of the calcined samples. Approximately 1/3 of the 10MR channels in the zeolite layers are split by the template molecules, resulting in an ∼1/3 micropore volume decrease. This result is in accordance with the micropore volume (0.05 cm3 g−1/0.09 cm3 g−1) obtained from the N2 adsorption isotherm (see Figure S14 for a schematic illustration). We present the first synthesis of lamellar mesostructured ZSM-48 nanosheets with a high surface area by introducing benzophenanthrene groups into the hydrophobic tail of the surfactant. The strong self-assembly ability and highly ordered orientation of the benzophenanthrene groups through π−π stacking stabilize the lamellar micelle structure. The stable lamellar structure and the weak link along c-axis of the ZSM-48 zeolites were speculated to be the key elements for the formation of the LMZN. We expect that our findings will provide new insight into the molecular factors governing the formation of inorganic−organic mesophase and microporous materials, which would address new challenges arising from our increasing need to introduce mesopores into bulky substrates.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21533002, 21571128), the National Excellent Doctoral Dissertation of P. R. China (201454) and the Young Elite Scientist Sponsorship Program by CAST (2017QNRC001).
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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.8b00146. Detailed XRD, SEM and TGA results (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*Shunai Che. E-mail:
[email protected]. *Yanhang Ma. E-mail:
[email protected]. ORCID
Shunai Che: 0000-0001-7831-1552 1842
DOI: 10.1021/acs.chemmater.8b00146 Chem. Mater. 2018, 30, 1839−1843
Communication
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