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Synthesis of Lamellar Mesostructured ZSM-48 Nanosheets Yunjuan Zhang, Yanhang Ma, and Shunai Che Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00146 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 1, 2018
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Chemistry of Materials
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 (P.R. China) $ School of Physical Science and Technology, ShanghaiTech University, 393 Middle Huaxia Road, Pudong, Shanghai, 201210 (P. R. China) % School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, China, 200092 (P. R. China) ABSTRACT: Lamellar mesostructured ZSM-48 (*MRE framework, polymorph 6) nanosheets (LMZN) were synthesised using an amphiphilic template consisting of six tertiary amine-terminated alkyl chain branches bonded to one benzophenanthrene hydrophobic centre. The hydrophobic zones formed by self-assembly of the benzophenanthrene tails through π-π interactions disrupted the extension of the zeolite framework along the c-axis, while the six tertiary amine head groups directed the formation of the ZSM-48 zeolite. By changing the length of the alkyl chain between the benzene rings and the quaternary ammonium groups, LMZNs with different interlamellar spacings were obtained. The weak link of the ZSM-48 zeolite framework along the c-axis and the rationally designed structure-directing agent are proposed to be the principal elements for the formation of the LMZN zeolites.
Aluminosilicate 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 zeolite-separated nanosheets, such as, MWW12-14, MFI15-18, MEL19, FAU20, TON21 and MOR22 nanosheets, have been successfully synthesised 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 synthesised 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 that combine the aforementioned SDA (HM or HXN) and certain hydrophobic centres 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-CnH2nN+(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 stabilised 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 BPT106-0. The two well-resolved peaks at 2θ = 1.24° and 2.38° were indexed to the 1st- and 2nd-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
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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 ze- olite 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
Figure 1. Structural characterisation of the LMZN templated by BPT10-6-0. Low- (a) and high-angle (b) PXRD patterns (Cu Kα radiation λ = 0.15418 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 Figure 1d) and interlamellar spaces.
randomly orientated after calcination. However, limited information and considerable reflection overlapping caused difficulties in solving the structure of the LMZN. Scanning electron microscopy (SEM) images (Figure 1c and 1d) 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 low-magnification 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 as-pre -pared 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). Since
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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.
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 highmagnification (d) TEM images taken along the common b-axis of a sliced as-prepared 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.
The high structural coherence of ZSM-48 sheet along the b-axis was revealed in the Figure 1b and 1d. 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 highvacuum 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
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Chemistry of Materials 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
