Engineering Fractal MTW Zeolite Mesocrystal: Particle-Based

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Engineering Fractal MTW Zeolite Mesocrystal: Particle-Based Dendritic Growth via Twinning-Plane Induced Crystallization Yang Zhao, Zhaoqi Ye, Lei Wang, Hongbin Zhang, Fangqi Xue, Songhai Xie, Xiao-Ming Cao, Yahong Zhang, and Yi Tang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01547 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Crystal Growth & Design

Engineering Fractal MTW Zeolite Mesocrystal: Particle-Based Dendritic Growth via Twinning-Plane Induced Crystallization Yang Zhao,a Zhaoqi Ye,a Lei Wang,a Hongbin Zhang,b * Fangqi Xue,a Songhai Xie,a Xiao-Ming Caoc Yahong Zhang,a and Yi Tanga.* a

Department of Chemistry, Laboratory of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai, 200433, China.

b

Institute for Preservation of Chinese Ancient Books, Fudan University Library, Shanghai, 200433, China.

c

Key Laboratory for Advanced Materials, Center for Computational Chemistry and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237, China. ABSTRACT: Constructing super-structured crystalline materials by crystal engineering is an attractive object for miscellaneous fields of researchers spanning biomimetic to catalytic materials. Zeolite is a kind of important crystalline catalyst, and super-structured zeolite has a great potential for widespread applications. However, the ambiguous crystallization mechanisms hamper the effective and scientific fabrication of super-structured zeolite with exceptional properties. Herein, a fractal super-structured MTW zeolite with mesocrystal side-branches is prepared via a nanoparticle-based nonclassical pathway with twinning-plane induced crystallization, which is distinct from the formation of general mesocrystal via crystal-crystal oriented attachment. Deformed atomic connection at a specific crystallographic plane contributes to the production of side-branches. Moreover, this intriguing morphology could be regulated merely via adjusting the crystallization kinetics based on the unequivocal nonclassical crystallization mechanism. It will open a new avenue for design and synthesis of targeted crystals with superstructure and extraordinary properties.

Design and synthesis of materials with intricate architectures and fascinating properties is a promising subject for material scientists.1 Recently, constructing superstructured crystals becomes a vital goal in this field with the aid of crystal engineering technology,2,3 and some elegant achievements have been emerging by advisably manipulating the crystallization kinetic process.4-6 Zeolite, as a type of microporous aluminosilicate crystalline materials, is extensively utilized in the fields from energy catalysis to environmental protection.7 Super-structured zeolites have been believed with great potentials for multifarious applications due to their extraordinary physicochemical properties.8,9 However, it is still a great challenge for artistically fabricating targeted zeolite with superstructure via predesigned crystal engineering routes because of the huge gaps between the comprehensive understanding of formation mechanisms and experimental manipulation of the crystallization processes.

cules, nonclassical crystallization mechanism via 3-D nanoparticle attachment process14 has been recently underscored to explicate the formation of some zeolites with complicated morphologies.11,15,16 For example, Rimer et al. have revealed the dual mechanisms of classical and nonclassical crystallization for silicalite-111 and SSZ-13, and the morphology of SSZ-13 could be regulated by adding different modifiers.15 Such mechanisms were also extended to LTL zeolite by Rimer et al.,16 bundle-like MFI zeolite by Tang et al.17 and microporous silicoaluminophosphates by Zhu et al.18,19 Very recently, we have demonstrated the intertwined classical/nonclassical crystallization mechanisms for MTW zeolite using TEAOH as structure directing agent (SDA), and the morphology/mesoporosity can be facilely modulated merely by scientifically adjusting the crystallization kinetics based on unambiguous crystallization mechanisms.20 These works present a bright future to construct zeolite with distinctive morphology and superstructure via employment of nonclassical crystallization pathway.

To this end, increasing attentions have been being attracted in the investigation of zeolite crystallization mechanisms.10-13 In contrast to the classical growth pathway based on two-dimensional (2-D) monomer-bymonomer incorporation of single atoms, ions or mole-

MTW zeolite possesses intergrowth of monoclinic and orthorhombic polymorphs and unidimensional 12member-ring pore along b-axis,21 and is an important catalyst in various reactions involving bulky molecules.22 Various morphologies of it have been reported, such as

INTRODUCTION

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shuttle-like,23 cubic,24 pseudo-hexagonal21 and so on, whereas the reasons behind remains elusive. In our recent work, the crystal structure of hexagonal plate and screwed MTW zeolite has been successfully uncovered and MTW zeolite was regarded as an excellent example to discern the crystallographic basis for super-structured zeolite thanks to its special crystalline symmetry and 1-D micropore channel.25 Despite the exciting advances for understanding the accurate crystal structure hidden in the fantastic morphology, the formation mechanism remains equivocal as a next important aspect waiting for solutions, which could trigger us precisely regulate this amusing morphology by ingeniously controlling the crystallization kinetic process. In this context, the super-structured MTW zeolite with much enriched dendritic mesocrystal branches is controllably prepared in aluminosilicate system, and an evident superiority of the fractal MTW zeolite is confirmed by the catalytic reactions involving bulky reactant molecules. The unequivocal indicators of nanoparticle-based nonclassical crystallization pathway are captured by directly observing the static images of the final product and monitoring the time-resolved zeolite growth process. Importantly, coincidence boundary induced crystallization rooted on a specific crystallographic plane is revealed as a crucial factor for the growth of mesocrystal main stem and side-braches in the dendritic morphology. This formation pathway is distinct from those of general mesocrystals resulting from the oriented attachment via rotation and reconstruction of adjacent crystallites.26,27 Finally, the fractal superstructure can be regulated via adjusting the kinetic processes depending on the unambiguous crystallization mechanism.

METHODS. Materials. All the chemicals were directly used without any purification. 1,5-dibromopentane, 1,6-dibromohexane, 1methylpyrrolidine, 1-methylpiperidine, acetonitrile, toluene and methyltriethylammonium chloride (MTEACl) were purchased from Aladdin Reagents Co., Ltd. Aluminum sulfate octadecahydrate (Al2(SO4)3•18H2O), sodium hydroxide (NaOH), ammonium nitrate (NH4NO3), benzyl alcohol and anisole and were provided by Sinopharm Chemical Reagents Co., Ltd. Silica sol (LUDOX HS-40, 40 wt% SiO2 solution) was obtained from Sigma-Aldrich. Low density polyethene (LDPE) was purchased from Alfa Aesar (Stock #42607). Synthesis of SDAs. 1 mol of 1,5-dibromopentane was dissolved in 100 ml of acetonitrile/toluene mixture (1:1, v/v), and o the mixture was heated to 70 C, then 0.1 mol of 1methylpyrrolidine was added dropwise. The obtained mixo ture was refluxed at 70 C for 120 h under magnetic stirring. After cooling to room temperature, the product was filtered and purified, then the obtained intermediate and 0.1 mol of 1methylpiperidine were mixed with 100 ml of acetonitrile and o refluxed at 70 C for 72 h under magnetic stirring. Finally, the precipitate products were obtained by filtration and purification (denoted as SDA-1, Cy6-C5-Cy5). Purities of the prod13 ucts were confirmed by C NMR spectra. For the synthesis of SDA-2 (Cy6-C6-Cy5), similar procedures were performed,

but 1,5-dibromopentane is substituted by 1,6-dibromohexane. The SDA-3 (Cy5-C5-Cy5) was prepared referring to the litera25 ture. Preparation of MTW zeolite. A mixture solution with the compositions of Al2O3/120SiO2/36NaOH/6SDA/12000H2O was prepared by adding the stoichiometric sodium hydroxide (10 wt% in water), aluminum sulfate octadecahydrate (5 wt% in water), SDA and silica sol (40 wt% in water) in deionized water. After aging for 3 h at ambient temperature, the mixture was transferred into an autoclave, and then the autoo clave was sealed and heated to 160 C under static conditions for 72 h. The product was isolated by centrifuging, washing and drying. In order to investigate the crystallization mechanisms of MTW zeolite in this system, the intermediates were also extracted. Similar procedures are employed for changing the synthesis parameters. The conventional MTW zeolite was prepared using methyltriethylammonium chloride (MTEACl) 28 as SDA referring to the previous report. H-MTW zeolite is prepared prior to catalytic reactions. In details, after removo ing the SDA by calcination at 550 C for 15 h, the samples o were ion exchanged with 1M NH4NO3 at 80 C for three times o and then were treated at 550 C for 6 h. Molecular mechanics simulation method. Molecular mechanics simulation was conducted to study the location and interaction energy of SDA in MTW framework using Sorption and Forcite programs implemented in Materials Studio 29 suite. The MTW zeolite framework was downloaded from IZA website, and the atomic charge distributions of SDA were obtained from DFT calculations by using B3LYP hybrid functional associated with 6-31++g(d,p) basis sets and the ESP method. The charge of framework oxygen was fixed to 0.3, and the charge of framework silicon was calculated to keep total charge neutrality. Firstly, the SDA was docked in the MTW framework by Monte-Carlo simulations implemented in Sorption program. Then CVFF forcefield was used to obtain the location and interaction energy by Simulated Anneal implemented in Forcite program. In details, the initial and mid-cycle temperature of simulated annealing is 300 and 700 K, respectively, and it was repeated for 10 times with the heating ramps per cycle of 20 K using NVT ensemble for each heating step. The framework of MTW zeolite was fixed and periodic boundary conditions were applied for all calcu30 lations. The interaction energy was calculated by subtracting the energy of SDA and MTW zeolite in the vacuum from the total energy with the unit of kcal/mol per unit cell. Characterization. The phase of products were confirmed by X-ray diffraction (XRD) on a Bruker D8-Advanced diffractometer with Cu Kα (40 mA, 40 kV). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to monitor the morphology and structure of products on a Hitachi S-4800 and JEOL JEM2011/Tecnai G2 F20 S-Twin instruments, respectively. The ultrathin cross sections were prepared on a UC-7 (Leica) ultramicrotome by using a DIATOME 45° diamond knife at a −1 27 13 cutting speed of 0.5 mm s . Al/ C magic angle spinning 27 13 nuclear magnetic resonance ( Al/ C MAS NMR) was conducted on a Bruker DSX 300 spectrometer. Liquid nuclear magnetic resonance was performed on a 500MHz Bruker AVANCE III HD NMR spectrometer. The composition of the products were detected by energy dispersive spectrometer


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Crystal Growth & Design (EDS) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Thermogravimetric analysis (TGA) was conducted on a SDT Q600 thermal analysis instrument with o a heating rate of 10 C/min under the condition of air from o room temperature to 850 C. CHN elemental analysis of the products was carried out on a vario EL Elemental Analyzer. The physical parameters of products were analyzed by N2 adsorption-desorption experiment on an autosorb iQ-2 instrument, and the pore size distribution of samples was calculated employing non-local density functional theory (NLDFT) method by the model: N2 at 77 K on silica with cylindrical pore, adsorption branch. Raman spectra were recorded on a Horiba Jobin Yvon XploRA confocal spectrometer with a 532 nm laser. NH3 temperature programmed desorption (NH3-TPD) was conducted on a Micromeritics AutoChem II 2920 instrument to obtain the acidities of the catalysts. Low density polyethene (LDPE) cracking. As reported in 31 our previous work, the zeolite powder were thoroughly mixed with LDPE with a weight ratio of 1/10 (catalyst/LDPE), and then the mixtures were analyzed on a TG analyzer with a o ramp rate of 10 C/min under N2 condition (50 ml/min). Pyrolysis Gas Chromatograph-Mass Spectrometer (Py-GC/MS, 7890B-5977B) was used to on line analyze the product distributions of cracking reaction at the specific temperature, where the corresponding weight loss is 50% in TG curves. Benzylation of anisole. Friedel-Crafts alkylation of anisole with benzyl alcohol was conducted in an autoclave, 30 mg of zeolite powder were mixed with 0.338 g of benzyl alcohol and 5 g of anisole, and then the reactions were performed at 120 o C for 2 h. After cooling to room temperature, the products were separated by centrifugation, and were analyzed using a gas chromatograph (GC9560) equipped with a SE-30 capillary column and flame ionization detector.

RESULTS AND DISSCUSION Preparation of fractal MTW zeolite with mesocrystal side-branches. MTW zeolite is obtained with high yields of ca. 90% under a static hydrothermal condition at 160 o C for 72 h using the homemade SDA-1 (Cy6-C5-Cy5, Scheme S1). The zeolite phase is confirmed by XRD pattern, which displays the characteristic peaks of MTW zeolite (Figure S1) with ca. 90% monoclinic and 10% orthorhombic phases referring to the simulated XRD patterns.25 Interestingly, SEM images show a unique dendritic morphology with a mean size of ca. 5 μm (Figure 1A and S2A). The dendritic particulates consist of a main stem and many side-branches, and from a closer inspection in highresolution SEM (HRSEM) image, it is manifested that they are bundle-like structure composed of co-aligned nanodomains and present a rough nanogranular surface (Figure 1B and S2B-D). Obviously, the fractal MTW zeolite displays four different hierarchies of (1) crystalline nanograin, (2) extended nano-rod, (3) bundle-like branch/stem and (4) dendritic particulate. They seem to be bridged by programmed self-assembly to form an ordered entity. Meanwhile, the dendritic morphology can be observed from TEM and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imag-

es (Figure 1C and S3a-b). It is noteworthy that both of the stem and branches exhibit a single-crystalline feature in

Figure 1. (A, B) SEM images; (C) high-angle annular dark field (HAADF) image; (D, E) the corresponding selected area electron diffraction (SAED) patterns; (F, G) ultrathin specimen TEM images viewed along [010] direction and the inset of F is the corresponding SAED and (H) inverse Fast Fourier transform (IFFT) images of obtained MTW zeolite after heated for 72 h. The arrows indicate the stacking faults.

selected area electron diffraction (SAED) patterns viewed along [001] zone axis, indicating that the main stem and each branch is mesocrystal sharing a common c-axis. Likewise, all of the bundle-like branches extend along baxis as substantiated by the SAED patterns, and the coaligned lattice fringes could be clearly observed along [010] direction with some protrusions from HRTEM images (Figure 1C-E and S3a-c). On the other hand, there is a similar angle of ca. 65o between the main stem and all of the side-branches as measured from both the morphological images in SEM/TEM and [010] zone axis in the corresponding SAED patterns. The second-generation sidebranches could further be identified on the firstgeneration branches, and the crystalline features of them are the same as the first-generation ones (Figure S2 and S3). Furthermore, the frequent stacking faults in ac plane can be discerned in the ultrathin specimen TEM images viewed along [010] zone axis (Figure 1F-H and S4), and the overlapped two sets of sharp diffraction grids in corresponding SAED patterns (Figure 1F, inset) clearly indicate the frequently stacking faults are direction-specific owing to the mirror-symmetrical intergrowth of monoclinic MTW domains via orthorhombic region.21,25,32,33 Actually, a planar pseudo-hexagonal stars of MTW zeolite with strange and well-defined angles of 4×6466o+2×50-51o has been found at the end of last century by Ritsch et al., and they attributed the appearance of this



morphology to the too tight-fit interaction of SDA with MTW framework.21 Nevertheless, this speculation seems difficult to elaborate the crystallographic origination of this morphology with very specific angles. Fortunately, the hidden mystery was uncovered in our most recent work, where the structural model of the hexagonal plate MTW zeolite was remarkably proposed. It declared that these branches atomically connected together via a coincidence crystallographic plane.25 Thanks to the “deformed twinning”, herein, we harvest a dendritic MTW zeolite with a highlighted main stem and more enriched branches in a dilute system using SDA-1, which exhibits a “treelike” morphology. The crystallographic model for this MTW zeolite (Figure S5) is distinguished from those in the reports on other super-structured zeolites, such as repetitive branching MFI nanosheet8 and house-of-cards FAU nanosheet.9 The twins in former (MFI) are believed to be connected by MEL phase located at the crossing region,8 while the branching in latter (FAU) is thought to originate from EMT island nucleation taking place at the edge of the nanosheets.9 HAADF-STEM mapping images of Si, Al and O elements reveal highly uniform distribution of each element throughout the whole dendritic crystal (Figure S6). The molar ratio of silicon and aluminum atoms is 46 determined by EDS, which is almost the same as the result of ICP-AES. Considering that all aluminum atoms are located in the zeolite framework as corroborated by 27Al MAS NMR with only a signal at ca. 55 ppm (Figure S7), these results represent the homogeneous distribution of acid sites in this zeolite. 13C MAS NMR spectrum of asprepared zeolite is almost consistent with the liquid 13C NMR spectrum of the SDA-1 (Figure S8), implying that the SDA-1 is intact without Hoffmann degradation. This is also supported by the result of CHN elemental analysis with a C/N molar ratio of 7.7, which is close to the theoretical value of 8 for such SDA. Additionally, a weight loss of ca. 9.9 wt% from 200 to 800 oC in the TGA curve can be assigned to the decomposition of SDA (Figure S9). According to these results, it can be calculated that there is approximate one SDA in each of MTW zeolite unit cell, and the location as well as conformation of SDA molecules in the framework are estimated by molecular mechanics simulation as displayed in Figure S10 A-C. N2 adsorption isotherm of the sample after removing SDA shows a large uptake and an evident hysteresis loop at low and mediate relative pressures (P/P0), respectively, meaning the co-existence of micro- and mesopores in the dendritic crystals (Figure S11A). The mesopore size distribution estimated by NLDFT method exhibits a wide range of mesopore diameter from 2 to 20 nm (Figure S11B), which is ascribed to the arrangement of nanocrystals in the dendritic MTW zeolite, consistent with the observations of SEM images. On the other hand, comparing with the conventional MTW zeolite without branches (Figure S12), this dendritic zeolite exhibits a superiority in the catalytic reactions involving bulky molecules, such as cracking of low density polyethene (LDPE, Figure S13) and benzyla-

tion of anisole with benzyl alcohol (Table S1) although they have the similar acidity (Figure S14). Collectively, the accurate structural basis for the superstructured MTW zeolite has been definitely proclaimed, in which the dendritic morphology is derived from the atomic connection at twinning plane {310}. Such MTW zeolite exhibits a superior catalytic performances. However, as another very important aspect, the formation process or crystallization mechanism in nanoscale for dendritic MTW so far remains evasive, which would seriously hinder scientific and effective regulation of this attractive morphology via adjusting its growth process. Crystallization mechanism of fractal MTW zeolite. The dendritic growth has been proverbially reported to form some crystals other than zeolite in a nonequilibrium system, such as snow,34 rutile TiO235,36 and so on. The intricate morphology is thought to be hardly generated solely by a classical crystallization pathway, i.e. crystallization by incorporation of simple species such as single atom, ion and molecule, and therefore a non-classical pathway was proposed based on nanoparticle-orientated aggregation to reveal these processes in the recent works.35,36 By comparing the phenomena in these works, several curial indicators or evidences of nonclassical crystallization can also be excavated from the complicated and unusual morphology of resultant fractal MTW zeolite here with large amount of mesocrystal side-branches and internal


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Crystal Growth & Design Figure 2. SEM images of MTW zeolite synthesized with different time. (A) 20 h; (B) 24 h; (C and D) 26 h; (E and F) 28 h and (G and H) 30 h.

24 h has the similar intensity with that of the final crystalline product (Figure S18), suggesting that the WLPs may be locally ordered rather than completely amorphous.

Figure 4. TEM images of MTW zeolite after crystallization for 28 h. The red circles indicate small nanoparticles.

Figure 3. TEM images of MTW zeolite synthesized with different time. (A and B) 26 h; (C and D) 28 h and (E and F) 30 h. The insets of (B) and (D) are the SAED patterns of the corresponding branches.

mesopores, rough surfaces with apparent nanogranular assemblies, and some round protrusions at the edge of particles (Figure 1 and S3). In order to further substantiate this mechanism as a dominant crystallization pathway, the time-resolved intermediates are isolated and characterized to detail the formation process of the dendritic twinned MTW zeolite. As shown in Figure S15, the XRD peaks of MTW zeolite are firstly detected after hydrothermal treatment for 20 h. Two populations of particles can be observed at this time (Figure 2A and S16A): small worm-like nanoparticles (WLPs, as main particulate) and large particles with some branches (as small fraction). HRSEM image reveals that the large particulate is composed of small nanoparticles (Figure S16B). With prolonging the heating time to 24 h more and larger particulates with dendritic morphology generate (Figure 2B). Meanwhile, numerous small nanoparticles are observed around and on the surface of these large dendritic particulates (Figure S16C and S16D). SAED patterns reveal that these small WLPs are amorphous, whereas the main stem of the dendritic particle has presented as single crystalline feature of MTW zeolite (Figure S17A and S17B). However, Raman spectrum of product at

As crystallization proceeds, zeolite particulates become more sophisticated with increasing branches accompanying the continuous depletion of small nanoparticles in the time interval from 26 to 30 h (Figure 2C, 2E and 2G). Meanwhile, some small nanoparticles can be observed on the surface as well as at the tip of branches (Figure 2D, 2F and S16E), as corroborated by TEM images (Figure 3, S19A and S19B). A closer examination clearly shows that the branches are made up of small nanocrystals (Figure S17C and S17D), and their crystallographically oriented arrangement along [010] direction can be deduced from the single-crystalline feature in SAED pattern (Figure 3B and 3D, insets). Moreover, the Si/Al ratios of WLPs and dendritic zeolite crystals are 40 and 47 determined by EDS, respectively. They are close to that of the final products, provisionally implying that these small WLPs directly assemble to form the dendritic MTW zeolite. The details of crystallization behaviors can be better comprehended from fine structure of the intermediate at 28 h (Figure 4). The crystallographically identical lattice fringes penetrating the adjacent crystallites could be clearly observed in different regions of one branch from HRTEM images. Moreover, amorphous or short-range ordered nanoparticles could also be discovered at the edge of branches (Figure S19C and S19D). It predicates that small nanoparticles gradually attach to the preformed larger particulates during the formation of dendritic MTW zeolite, and the crystalline particulates could induce the crystallization of subsequently attached nanoparticles. According to the crystallographic structure of MTW zeolite, two plausible modes are proposed for atomic connections at {310} planes during the induced crystallization (Figure S5 and Scheme S2): One of them is the mutual alignment along b-axis (Mode i), which produces a crystal with single crystal feature. Whereas the



other would give rise to a ca. 65o angle between two baxes when observed from [001] direction (Mode ii) and result in the generation of branches via deformed twinning.25 Consequently, the dendritic MTW zeolite crystals

Scheme 1. (A) Putative crystallization mechanism of fractal MTW zeolite via a nonclassical pathway with twinning-plane induced crystallization. (B) Two plausible modes of atomic connections at (310) plane during the induced crystallization. can be formed via cooperation of these two modes of twinning-plane induced crystallization. With further prolonging the crystallization time, all of the discrete WLPs disappear, and the dendritic zeolite with intercrystallite mesopores (Figure 3E and 3F) and clean surfaces (Figure 2G and 2H) is fabricated. Meanwhile, the second or even more generations of branches emerges. At 72 h, the coalescence of adjacent nanocrystals occurs, producing a slightly dense fractal MTW zeolite (Figure S3a). On the basis of experimental results, a plausible crystallization pathway of dendritic MTW zeolite is illustrated in Scheme 1. Huge amounts of small amorphous (or locally ordered) WLPs firstly form in the induction period, and then these WLPs gradually self-assemble via nanoparticle attachment process. The as-formed zeolite crystal (or nuclei) can induce the co-aligned crystallization of subsequently attached nanoparticles along , and the deformed atomic connection at {310} plane results in the formation of side-branches with a fixed geometry of ca. 65o between the main stem and branches (Mode ii). Simultaneously, the amorphous or locally ordered nanoparticles could continuously assemble to the branches for the growth of the second or more generation branches by the similar crystallization modes. In addition, the coalescence of adjacent nanocrystals occurs to slightly mend the intercrystalline interstices. Finally, a fractal twinned MTW zeolite with mesocrystal side-branches is harvested by the

cooperation of two modes of induced crystallization. In general, such a super-structured MTW zeolite can be constructed via a nanoparticle-based nonclassical pathway14

Figure 5. (A) N2 adsorption isotherms and (B) mesopore size distributions derived from NLDFT of products synthesized with different time. The isotherms and the pore size distribution curves have been vertically moved.

accompanied by the twinning-plane induced crystallization. The evolutions of physical parameters of intermediates also corroborate the nonclassical crystallization pathway. As shown in Figure 5A for N2 adsorption experiments, a hysteresis loop appears at the relative pressure (P/P0) higher than 0.8 after crystallization for 20 h, which is ascribed to the random accumulation of small WLPs. The wide range of pore size distribution (Figure 5B) calculated by NLDFT method confirms this hypothesis, and it can also be deduced from SEM images in which lots of small nanoparticles are existent. With crystallization time elapse to 30 h, the hysteresis loop moves to a lower relative pressure and the pore size distribution becomes narrower (Figure S5B). Meanwhile, the BET surface areas and microporous volumes gradually increase (Table S2), implying the improvement of crystallinity. In contrast, the mesoporous volumes monotonously decrease because that the oriented alignment of nanocrystals in each branch is more compact than random accumulation of small amorphous nanoparticles. After crystallization for 72 h, both of external surface areas and mesoporous volumes slightly decrease (Table S2) due to the occurrence of coalescence between adjacent nanocrystals. These results are in good agreement with the observations of SEM and TEM images, further verifying that the crystallization of


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Crystal Growth & Design dendritic MTW zeolite mainly follows a particle-based nonclassical crystallization pathway. Regulating the branches of fractal MTW zeolite. We then try to engineer the growth of dendritic MTW zeolite based on its crystallization mechanism by adjusting the

solid is collected. However, there is a slight influence on the products when the temperature is increased to 180 oC. On the other hand, if the alkalinity is gradually enhanced, smaller particles with a few branches are collected due to the faster crystallization rate, which is not favorable for the generation of branches (Figure S21 and S22 G-J). Consequently, we can optionally regulate the branches of fractal MTW zeolite by controlling the crystallization kinetics.

CONCLUSION

Figure 6. SEM images of products synthesized with different H2O/SiO2 ratios.

induced crystallization kinetics. H2O/SiO2 ratio was proved as a vital role during zeolite crystallization.20 The complexity of dendritic MTW zeolite can be well regulated by tuning H2O/SiO2 ratio. All of the products are pure MTW zeolite under H2O/SiO2 ratios ranging from 20 to 200 (Figure S20), but their morphologies are distinctive. With the increase of H2O/SiO2 ratio, more side-branches are generated accompanying the growth of crystal size (Figure 6). On the other hand, the substitution of SDA-1 with Cy6-C6-Cy5 (SDA-2) or Cy5-C5-Cy5 (SDA-3, Scheme S1) can also lead to the formation of dendritic MTW, while SDA-2 can produce the dendritic crystal with smaller nanoparticle size and fewer branches (Figure S21 and S22 A-D). Longer linked methylene chain of SDA-2 may account for this case. The slightly lower interaction energy of SDA-2 (-161.3 kcal/mol) than that of SDA-1 (-169.8 kcal/mol) may indicate that the latter can better fit into MTW framework than the former one (Figure S10). Comparatively, if the crystallization is conducted in a concentrated system (H2O/SiO2 = 20), the faster crystallization process presents, and nanoparticle aggregated MTW without branches is harvested using diquaternary ammonium salts (SDA-2 and SDA-3) as SDAs (Figure S23), probably because of low probability for exposed coincidence boundary on preformed crystal nuclei with relatively higher crystallinity. Moreover, temperature and alkalinity are also two important parameters for zeolite synthesis. When the crystallization temperature is decreased to 140 oC, amorphous

In conclusion, the dendritic fractal MTW zeolite with enriched mesocrystal side-branches is prepared using the homemade diquaternary ammonium salts as SDAs, and such super-structured MTW zeolite is superior to its unbranched analogue in catalytic reactions of bulky molecules. Likewise, the indicators of nanoparticle-based nonclassical crystallization are authenticated by directly observing the final products and tracing the dynamic crystallization process. These evidences signify that the dendritic super-structured MTW zeolite is fabricated via attachment of locally ordered nanoparticles accompanying twinning-plane induced crystallization process. Deformed twinning at {310} plane is responsible for the formation of side-branches. On the other hand, based on an in-depth understanding of the crystallization mechanism, the sidebranches of MTW zeolite could be regulated by tailoring the crystallization process. This work provides a promise for engineering the crystal with desired morphology and superstructure via nonclassical crystallization pathway. Alternatively, it is also a fascinating instance of nonclassical crystallization for zeolite, which maybe is suitable for other zeolites to elaborate their formation processes. Our findings indicate that there still are copious knowledges of crystal formation that should be studied for better designing and preparing excellent crystals.

ASSOCIATED CONTENT Supporting Information. Schemes, XRD patterns, SEM/TEM and HAADF-STEM images, TGA result, NMR spectra, structure models, N2 adsorption isotherms and mesopore size distributions, NH3-TPD profiles and the results of catalytic reactions are included. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Present Address Lei Wang is currently at Dalian Institute of Chemical Physics, Chinese Academy of Science.

Note



The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Key Basic Research Program of China (2013CB934101), NSFC (21433022, 21573046, 21473037, and U1463206). Yang Zhao acknowledges Mr. Pengcheng Tu and Peicheng Wang for the nice help for Molecular mechanics simulations.

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Crystal Growth & Design

For Table of Contents Use Only Engineering Fractal MTW Zeolite Mesocrystal: Particle-Based Dendritic Growth via TwinningPlane Induced Crystallization Yang Zhao,a Zhaoqi Ye,a Lei Wang,a Hongbin Zhang,b * Fangqi Xue,a Songhai Xie,a Xiao-Ming Caoc Yahong Zhang,a and Yi Tanga.*

A super-structured fractal MTW zeolite with dendritic and mesocrystal branches is controllably fabricated via a nanoparticle-based nonclassical pathway accompanying two modes of twinningplane induced crystallization.


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Engineering Fractal MTW Zeolite Mesocrystal: Particle-Based

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