Article Cite This: Chem. Mater. 2017, 29, 9247-9255
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Seeding Bundlelike MFI Zeolite Mesocrystals: A Dynamic, Nonclassical Crystallization via Epitaxially Anisotropic Growth Hongbin Zhang, Hongxia Zhang, Yang Zhao, Zhangping Shi, Yahong Zhang, and Yi Tang* Department of Chemistry, Laboratory of Advanced Materials, Fudan University Library, Collaborative Innovation Center of Chemistry for Energy Materials and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China S Supporting Information *
ABSTRACT: Direct synthesis by assembly of precursor nanoparticles is a promising strategy for preparing distinct mesoscopic-structured crystals, especially when high controllability is realized. However, uncertain properties of amorphous precursors and inner complicacy of crystallization mechanisms hamper controllable synthesis of zeolite mesocrystals. Here, we develop a saltaided seed-induced organic-free method to facilely synthesize anisotropic MFItype nanorod-bundle zeolite mesocrystals. An epitaxial, anisotropic assembly and crystallization of precursor particles on seed crystals is successfully achieved via a distinctively dynamic, nonclassical process, from relatively disordered to ordered attachment (OA), triggering an enhanced one-dimensional (1D) growth, thus constructing a unique core−shell−shell structure. This work sheds new light on the insights of both zeolite mesocrystal properties and a nonclassical crystallization mechanism. With an understanding of the mechanism, this nonclassical process can be exploited to systematically tune mesocrystal properties and create zeolite materials with novel or enhanced physical and chemical performance.
1. INTRODUCTION As one kind of important open framework material, zeolites as well as their crystal engineering are of great interest for both fundamental and applied science.1−4 However, goal-oriented synthesis of zeolite crystals with fine-tuned mesoscopic structures has still been a challenge, especially via a simply regulated crystallization process based on understanding of the mechanism.5−8 Currently, nonclassical crystallization (i.e., growth by the addition of particle species, rather than monomers) has been proposed as one of the dominant mechanisms for elaborating on the zeolite formation process.9,10 Moreover, such a process can be regulated to construct diverse mesoscopically structured crystals (i.e., “mesocrystals”), which has been achieved in some metal oxides, magnetite, proteins, and so on.11−13 However, probably because of the inner complicacy of zeolite crystallization and the unknown coexisting mechanisms (classical and nonclassical ones),14−17 the comprehensive investigation of nonclassical crystallization and the deep insight of its role are still lacking for zeolite mesocrystal design and synthesis with diverse applications. For controlled assembly leading to anisotropic mesocrystals, we may tailor the nonclassical crystallization behaviors in two ways: (1) altering the properties of precursor particles, and (2) tuning the manner of aggregation or attachment. In zeolite synthesis, aluminosilicate precursor particles potentially serve as the main building blocks, while their metastable and evolutive characters, combined with uncontrolled size and shape, state © 2017 American Chemical Society
(solid or liquidlike), microstructure (amorphous or crystalline), and interactions, add a new level of complexities to control the assembly behaviors during crystallization.9,14,18−20 Nonetheless, the particle properties and their manner of assembly may be tailored by the presence of additional species, such as welldesigned organic templates, special modifiers, or other additives.21−26 As an alternative, template-directed routes have been postulated to fabricate self-assembled hierarchal zeolites,21−24 in which special organic templates not only facilitate the formation of microporous topology, but also take the role of tailoring the assembly and evolution process.21,22 Additionally, selective addition of some organic growth modifiers with organic templates could potentially modulate the nonclassical process via subtly inhibiting or promoting crystal growth.25,26 Despite these achievements, so far little is reported for synthesizing anisotropic zeolite mesocrystals and studying its distinctive properties; more importantly, the crystallization mechanism still needs to be clearly revealed. Meanwhile, the synthesis flexibilities, costs, and health−safety− environment (HSE) issues should also be considered. As a facile, low-cost, and green alternative, a seed-induced route offers a powerful method to produce zeolites with wellcontrolled sizes, shapes, compositions, and structures.27−29 In our recently reported “salt-aided seed-induced synthesis” route, Received: July 24, 2017 Revised: September 30, 2017 Published: October 10, 2017 9247
DOI: 10.1021/acs.chemmater.7b03121 Chem. Mater. 2017, 29, 9247−9255
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grade, Shanghai Chemical Co.); tetrapropylammonium hydroxide, 25% w/w aqueous solution (TPAOH, analytical grade, Yixing Dahua Chemical Co., Ltd.); and deionized water. The following chemicals were used as reagents for alkaline posttreatment: sodium hydroxide (NaOH, analytical grade, Shanghai Chemical Co.); sodium bicarbonate (NaHCO3, analytical grade, Shanghai Chemical Co.); and deionized water. In addition, a conventional ZSM-5 (denoted as CZSM-5, purchased from Nankai Catalyst Company, Tianjin, China.) is adopted as the reference sample for the comparison, and some of its physicochemical properties are listed in Table S1. 2.2. Zeolite Synthesis. The silicalite-1 seeds were first synthesized by a clear solution method, and the obtained silicalite-1 suspension was directly used as seed without further treatment.34 A mixture in absence of an organic template with a composition of nSiO2/nAl2O3/ nNa2O/nNaF/nH2O = 10/0.12/1.8/6/660 was first prepared, and then the preprepared silicalite-1 seed with the crystal size of normally ca. 200 nm was added. In detail, 40 wt % silica sol was slowly added to 10 wt % NaOH solution under mechanical stirring, and then 5 wt % Al2(SO4)3·18H2O solution was added dropwise to make solution A. Meanwhile, NaF was mixed with the remaining water to produce solution B. Afterward, solution B was added dropwise to solution A under stirring. After 30 min of aging, the seed solution was added dropwise, and the quantity typically equaled to 7.0 wt % of total SiO2 weight in the starting gel. The resulting gel solution was aged at room temperature under stirring for normally 3 h, and then heated up to 140 °C for 24 h. After hydrothermal treatment, the solid products were separated by filtration and washed with deionized water. For investigation of the crystallization process, a series of MFI-BZM samples with different crystallization periods (t) were synthesized, where t equals 0, 3, 4, 4.5, 5, 6, 8, 12, and 24 h. Some referential samples were also synthesized by altering the mixture composition, mainly involving seed size (50, 120, and 300 nm), NaF/Si ratios (0, 0.3, and 0.9), and reaction temperatures (T; 100, 120, and 180 °C). The detailed sample data are shown in Table 1. In addition, alkali
we observed the occurrence of epitaxial and particle-attached growth on seed crystals (Scheme 1).30−33 With the addition of Scheme 1. Evolution of Precursor Particles and Their Distinctive Attachment Processes on Seed Crystals during the Nonclassical Crystallization To Construct Diverse Mesocrystals
KF, it tends to form “wormlike” particles (tens to hundreds of nanometers) which stack closely on the seed surface (Scheme 1a), and then the denser aggregations undergo a dissolution− recrystallization process to produce a single crystal with abundant more-or-less spherical mesopores.30,31 After the addition of tetrapropylammonium (TPA+), many smaller spheroidal nanoparticles (20−30 nm) can be formed around seeds, and then their attachment and postcrystallization take place on seeds (Scheme 1b), finally producing a lattice-matched spherical nanocrystallite assembly.30,32 On the basis of these results, we think it is possible to manipulate the particle properties and attach aspects to achieve ordered assembly on seed crystals, for example, the case shown in Scheme 1c. For this goal, herein, we selectively add a small amount of inorganic NaF with 200 nm silicalite-1 seeds into the aluminosilicate gel through extensive variations of synthesis conditions. Afterward, an ordered and anisotropic assembly of amorphous and evolutive precursor particles are successfully achieved during crystallization, thus constructing an anisotropic and self-assembled MFI-type bundlelike zeolite mesocrystal (MFI-BZM). Through systematically detecting the internal microstructure and investigating its detailed dissolution and crystallization processes, the formation pathway of MFI-BZM is deeply understood, and a putative particle-based crystallization mechanism is proposed to guide the related morphology modulation of distinctive mesocrystals. Furthermore, the detailed physical/chemical properties and catalysis/adsorption performances of MFI-BZM are also investigated not only to determine their possible application but also to confirm the effectivity of this specific one-dimensional (1D) stacking nanostructure.
Table 1. Initial Synthesis Compositions or Conditions and Corresponding Product Phase in Synthesis sample numbera
seed sizeb (nm)
NaF/Si
T (°C)
product phase
1 2 3 4 5 6 7 8 9 10
200 50 120 300 200 200 200 200 200 200
0.6 0.6 0.6 0.6 0 0.3 0.9 0.6 0.6 0.6
140 140 140 140 140 140 140 100 120 180
MFI MFI MFI MFI MFI MFI MFI Amc MFI MFI
a
Aside from the typical sample (no. 1), some referential samples (nos. 2−10) were also synthesized by altering either mixture composition (seed size, NaF/Si) or crystallization temperature. bThe silicalite-1 seeds were used. cAm = amorphous phase. etching was carried out by the following procedure: (1) etching the uncalcined products, 0.5 g of products was dispersed in 20 g of 0.2 mol/L NaOH aqueous solution and reacted at 80 °C for up to 1 h under mechanical stirring; and (2) etching the intermediates, 0.5 g of intermediates was dispersed in 20 g of 6.5 wt % NaHCO3 aqueous solution and reacted at 80 °C for up to 48 h under mechanical stirring. 2.3. Zeolite Characterization. Morphological information was obtained by scanning electron microscopy (SEM; Hitachi S-4800) and transmission electron microscopy (TEM; JEOL JEM-2011 and FEI Tecnai G2 F20 S-Twin). Chemical analyses were performed by X-ray fluorescence (XRF) with a Bruker-AXS spectrometer. The N2- and Arsorption isotherms were measured by a Micromeritics ASAP-2010 instrument at liquid-nitrogen temperature and by an Autosorb IQ2
2. EXPERIMENTAL SECTION 2.1. Materials. The following chemicals were used as reagents for zeolite syntheses: sodium hydroxide (NaOH, analytical grade, Shanghai Chemical Co.); silicon(IV) oxide, 40% in H2O colloidal dispersion (SiO2, analytical grade, Alfa Aesar); aluminum sulfate (Al2(SO4)3·18H2O, analytical grade, Shanghai Chemical Co.); sodium fluoride (NaF, analytical grade, Shanghai Chemical Co.); and deionized water. The following chemicals were used as reagents for silicalite-1 seed syntheses: tetraethyl orthosilicate (TEOS, analytical 9248
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Chemistry of Materials analyzer at liquid-argon temperature, respectively. The mercury intrusion porosimetry was performed with a Micromeritics Autopore IV 9500 instrument following in situ sample evacuation to characterize the information for mesopores with an open structure. The crystalline structures were characterized by X-ray diffraction (XRD) on a Bruker D8-Advanced diffractometer. The magic angle spinning nuclear magnetic resonance (MASNMR) experiments were performed on a Bruker DSX 300 spectrometer. The acid amount and strength were determined by NH3-temperature-programmed desorption (NH3TPD) using a Micromeritics AutoChem 2920 analyzer. The number of accessible acid sites was determined by a nonaqueous titration on a potentiometric titration meter (ZDJ-5, Shanghai Leici Instrument Factory) using tert-butylamine (solvent diameter of ca. 0.68 nm) as the titrant. 2.4. Zeolite Catalysis and Adsorption Test. The catalytic activities of the samples toward 1,3,5-triisopropylbenzene and isopropylbenzene (TIPB/IPB) cracking were tested in a pulse microreactor. The sample (30 mg) was preheated at 500 °C for 1 h before reaction. N2 with a flow rate of 40 mL/min was used as the carrier gas; 0.4 μL of TIPB/IPB was injected for each test. The reaction was performed at different temperatures to detect the activities. The products were analyzed by an online gas chromatograph (Agilent, Model 5820) equipped with a flame ionization detector. The measurement of o-xylene and p-xylene adsorption in zeolites was performed using a computer-controlled intelligent gravimetric analyzer (IGA, Hiden Analytical Ltd., Warrington, UK). The sample was degassed under a vacuum of less than 10−3 Pa at 673 K for 2 h prior to the adsorption measurement.
Figure 1. Microscopy characterizations of bundlelike MFI zeolite mesocrystal. (a) FESEM and (b) TEM image showing the side view of crystals. (c) Magnified TEM and SAED image (inset) of an individual nanorod-bundle particle. (d) HR-TEM image of zeolitic nanorods. (e) HR-TEM image of the enlarged area in red box and corresponding FFT image (inset). (f) HAADF-STEM image of an individual particle. (g) Si and (h) Al mapping images at the same area. Scale bars are (a, b) 400, (c, f−h) 250, (d) 10, and (e) 5 nm.
3. RESULTS AND DISCUSSION 3.1. Morphology and Mesocrystal Structure of MFIType Zeolite Nanorod-Bundle Assemblies. Field-emission SEM and TEM images of the typical sample (no. 1) are shown in Figure 1a and Figure 1b−e, respectively, and the powder XRD pattern (Figure S1) confirms its MFI-type framework. This crystal displays an unusual bundle-shaped mesoscopic structure (Figure 1a,b), which is composed of many parallelarranged rodlike nanocrystals with the widths of ca. 20−30 nm (Figure S2) but with the lengths extending to hundreds of nanometers (aspect ratio > 10). The high-resolution TEM and related SAED/FFT data (Figure 1c−e, and Figure S3) indicate that all of these nanorod branches have a high crystallinity and yield a single crystal-like diffraction pattern, confirming their ordered crystallographic alignment. Moreover, these nanorods all spread along the [001] direction (i.e., c-axis, Figure 1c,e), illustrating the characteristic anisotropy. Meanwhile, close observation (Figure 1c,f) tells us that these rodlike nanocrystals seem to epitaxially and orientationally grow from a spheroidal high-silica core of ca. 200 nm (i.e., seed crystal, both have comparative size), which is confirmed by obvious Si enrichment in the core area (Figure 1g) but a uniform distribution of Al (Figure 1h). Thus, the ordered assembly and crystallographic register are both achieved epitaxially on the seed crystals, meanwhile constructing an anisotropic MFI-type bundlelike zeolite “mesocrystal” (MFI-BZM). Different from the rod- or needlelike crystals often observed in 1D channelled zeolite (e.g., zeolite LTL, TON, MTW, MOR) in which the micropores extend along the longest crystal axis,35−37 it is still a difficulty for zeolites of 2D or 3D pore systems to fabricate 1D stacking nanostructures, especially under nanoscale control. Moreover, the outcome of MFI-BZM crystals fulfills the demand of MFI crystal engineering with highly reduced a-/b-axis pore dimensions. Meanwhile, detailed observations are carried out on the poorly formed crystals to obtain deep insights. TEM images
(Figure 2a, and Figure S4a) exhibit apparent nanoparticleassembled properties, especially near the core area (see the
Figure 2. Some distinct evidence, indications, and consequences of nonclassical crystallization. TEM images of (a−c) poorly formed MFIBZM, and (d−f) alkali-etched MFI-BZM. The inset of part e shows an etched-out pore-size distribution referring to parts e and f and Figure S4e, while the inset of part f shows a corresponding FFT image. 9249
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Chemistry of Materials dashed cyclic area). Around the seeds, the initially formed part of the mesocrystal is rather disordered (closely stacked nanocrystals), while further growth results in more and more ordered domains (orderly arranged nanorods), thus forming a specific core−shell−shell (silicalite-1@nanocrystal ZSM-5@ nanorod ZSM-5) structure. Moreover, some features further prove that these nanorods are probably made by nanoparticle assembly, such as (1) the appearance of rounded protrusions on some zeolite nanorods (see the arc area in Figure 2b,c, and Figure S4b) with a comparable size of ca. 20−30 nm; and (2) the retention of dislocated nanocrystals at the end of nanorods with an apparent grain boundary (see the round area in Figure 2c, and Figure S4c), that may form because of misaligned attachment. These mesocrystal features exceed the scope of classical mechanisms, but provide important evidence for a nonclassical process referred to as “crystallization by particle attachment” (CPA).9,14 3.2. Deep Understanding of the Formation Pathway of MFI-BZM by Investigating its Dissolution and Crystallization Processes. Through the deconstructing of MFI-BZM by NaOH etching, the preferential dissolution of the interfaces has been observed between intergrown crystals and grain boundaries (Figure 2d−f, and Figure S4d−f). In detail, the relatively disordered region around the seed core (Shell-I) is seriously segmented and dissolved (Figure 2d, and Figure S4d, in which the seed cores are marked in dashed lines), while the outer nanorod shell (Shell-II) does not dissolve easily. This observed effect is probably connected with the chemical compositions of the core (TPA+-containing silicalite-1 seeds) and shells (randomly distributed, more siliceous MFI nanoparticles in Shell-I and ZSM-5 nanorods in Shell-II), and their relative sizes, in which the dissolution rate decreases with the increase of Al content38,39 and the presence of TPA+.40 More interestingly, abundant uniform mesopores, or a so-called “mosaic structure”,41 are observed within these nanorods (Figure 2d−f, and Figure S4d−f). Closer inspection reveals that these voids seem to have a near spherical shape with the size of 3−10 nm (Figure 2e,f). Although highly segmented and etched, these zeolite nanorods still hold a high crystallinity and consistent orientation, and some etched nanorods seem to display a chainlike structure (see the dashed-arc area in Figure 2f). Considering the grain boundaries, mutual order, and uniformity of void size, shape, and orientation, this further indicates that these zeolite nanorods are inherently made of several orientationally attached nanoparticles.41,42 Since the solubility of the less-ordered phase is also higher than that of the highly crystalline phase,43 it is reasonable to assume that the mesopores (voids) on the rods are formed by dissolution of partially crystallized or high-silica particle species of 3−10 nm. Furthermore, on the basis of the related literatures,20,44−46 these ultrasmall and spheroidal particles probably formed by a stepwise aggregation of 3−5 nm primary precursor species, and participated in the formation of the nanorods. To gain further, deeper insights into mesocrystal formation, we carried out the direct investigations of textural evolution at different time points, as shown in Figures 3 and 4. For a typical synthesis, the entire crystallization process has completed within only 8 h, with a key transformation of 3−6 h (Figure 3i and Figure S5). FESEM images (Figure 3ii) clearly show a chain of seed-mediated crystallization events, including the evolution of precursor particles (Figure 3iia,b), the deposition and reorganization of them on the seed surface (Figure 3iic), the formation of short zeolite nanorods (Figure 3iid), and
Figure 3. (i) Crystal growth kinetic curve according to the XRD data in Figure S5. (ii) FE-SEM images of the products crystallized for (a) 0, (b) 3, (c) 4, (d) 4.5, and (e) 5 h.
subsequent growth of nanorods (Figure 3iie). First, we focus on zeolite precursor particles. Prior studies have shown that they may evolve in both size and microstructure along with the elevated temperature or longer time.14−16,20,25,44 The schematic in Figure 4i outlines the putative pathways of precursor evolution in this work. TEM images (Figure S6) of the sample aged for 3 h at room temperature reveal the initial formation of relatively monodisperse spheroidal particles with a diameter of ca. 10−20 nm. In consideration of the etched small domains in the mesocrystal (3−10 nm, Figure 2e,f), these 10−20 nm particles seem to be composed of several 3−5 nm primary particle species.20,44−46 After 3 h of heating, we observe the formation of wormlike particles (WLPs, Figure 4iia,b) probably via the aggregation of 10−20 nm secondary particles.14−16,20,25 In addition, the assembly/crystallization aspect is also critical. Focusing on the key 3−4.5 h, TEM images show that an external layer of closely packed small particulates (10−20 nm) is first formed on seed surfaces at 3 h (Figure 4iia,b). Along with particle-attached growth, postattached particulates begin to rearrange so that they can share the common crystal lattice (Figure 4iic,d; HR-TEM image of Figure S7a). Afterward, oriented attachment seems to be triggered at ca. 4.5 h, and the adjacent coaligned nanocrystals are epitaxially fused together from the inner region to constitute a single domain (Figure 4iie,f; HR-TEM image of Figure S7b). Although the direct attachment of WLPs seems to occur (Figure 4iib), it is more reasonable that the first step in the formation of MFI-BZM is the attachment of secondary precursor particles on the seed surfaces, or rather, after WLP attachment, they further evolve to the secondary precursor particles, followed by postattachment and recrystallization. TEM images (Figure 4iia,b) show that the width of the WLPs (about 30−50 nm and greater) is considerably larger than the 9250
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Figure 5. (a) Morphology and anisotropy of the as-synthesized MFItype bundlelike zeolite mesocrystals (MFI-BZM), (b) their distinctive core−shell−shell mesoscopic structure (silicalite-1@nanocrystal ZSM5@nanorod ZSM-5), and (c) the detail of the seed-induced dynamic particle-attached mechanism.
have participated in crystallization (Figure 4i), including primary precursor species (3−5 nm), secondary precursor species (10−20 nm), and wormlike particles (30−50 nm and greater). It seems that the WLPs do not directly participate in the formation of either Shell-I (nanocrystals in Figure 5b) or the nanorods in Shell-II (outer shell in Figure 5b), but the WLPs are the source of the secondary precursor species (Figure 5c1). Actually, the nanorods are formed by attachment of the discrete secondary precursor species, which are probably partially crystallized at the time of attachment (left side of Figure 5c1; e.g., at t = 4 h, see Figure 4iic,d, and Figure S7a), and then, the formation of a crystalline phase in the secondary precursor species and their aligning to form the epitaxial fused nanorods are simultaneous processes (right side of Figure 5c1; e.g., at t = 4.5 h, see Figure 4iie,f, and Figure S7b). Throughout the particle-based crystallization, the added seeds play several key roles under the aid of NaF (Figure 5c1): (1) directing the evolution of precursor particles (PPs) on the interface, (2) as “substrate” to dynamically deposit or relax these PPs, and (3) inducing the epitaxial orientation and crystallization. Controlled experiments indicate that no ZSM-5 phase can be obtained in the absence of seeds (Figure S1). In addition, the structure of the mesocrystal can be well-tuned by altering the seed size. Specifically, smaller seeds (50 or 120 nm) can induce the formation of more and smaller nanocrystallites but cannot act as “substrate” to support epitaxial oriented attachment (Figure 6a,b), while larger seeds (from 200 to 300 nm) can produce larger bundle-shaped crystals composed of thicker nanorods (Figure 6c compared with Figure 1a). These results may be connected with the surface area available for the attachment of the secondary precursor species. When the seed size is smaller, more of the secondary precursor species are spent for the attachment to form the Shell-I, so that the rest of the precursor species is not sufficient for the further epitaxial growth to form nanorods in Shell-II. Meanwhile, the addition of NaF indeed favors the formation of a nanorod-bundle morphology (Figures 6d−f and 1a). Compared with the case of adding KF in our previous works,30,31 the aggregation degree of PPs is lower, and their local ordering seems to be higher in the NaF system, which may contribute to the occurrence of oriented attachment (OA).
Figure 4. (i) Proposed pathways of various precursor particles during MFI-BZM crystallization;. (ii) TEM images of the key points crystallized for (a, b) 3, (c, d) 4, and (e, f) 4.5 h.
width of the nanocrystal in Shell-I or nanorods in Shell-II (ca. 20 nm). In addition, because of the irregular and even ramified shape of WLPs, one cannot expect their parallel arrangement on the seed crystals to crystallize to 1D nanorods, and the SEM images (Figure 3iib−d) show that the WLPs are separated and randomly rather than orderly arranged. Moreover, after mild etching of the intermediates by NaHCO3, the 10−20 nm particulate-assembled structure is exhibited more clearly at 4 h (Figure S8a,b), which further confirms the above results. The subsequently attached domains seem to become bigger, faceted, and anisotropic at 4.5 h (Figure S8c,d). 3.3. Putative Crystallization Mechanism of MFI-BZM and Its Morphology Modulation. These above observations not only reveal the anisotropic core−shell−shell structure of bundlelike zeolite mesocrystal, but also suggest a distinctively seed-induced dynamical nonclassical mechanism, which is illustrated in Figure 5. Different particle-based growth units 9251
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with the evolution of PPs, in which non- or semi-OA mainly occurs at the earlier stage; afterward, relatively more ordered OA takes the major share to trigger the oriented 1D growth of nanorods (Figure 5c2). This sequential order should result from the thermodynamic or kinetic alteration during particle attachment, including concentration, microstructures, size distribution of precursor particles, and their diffusion, relaxation, etc.9,50 On the basis of the role of temperature on thermodynamics and kinetics, the length, thickness, and completeness of zeolite nanorods can also be controlled by varying the crystallization temperature (Figures 6g−i and 1a). Along with increased temperatures, it may be that the increasing rate of WLP decomposition and the subsequently increasing rate of PP formation facilitate the epitaxial growth and formation of nanorods, so that leads to the increasing of nanorod size. 3.4. Physical/Chemical Properties and Catalysis/ Adsorption Performances of MFI-BZM. Furthermore, the distinct anisotropic nanostructure of MFI-BZM with highly reduced [100]/[010] dimensions can largely minimize the micropore diffusion path and increase (100)/(010) surface area to improve sorbate access to pores. In addition, it also guarantees a highly crystalline MFI structure (XRD pattern in Figure S1), well-preserved framework state of Si and Al (NMR spectra in Figure S9), and even gives rise to abundant mesopores (Vmeso,N2 = 0.21 cm3/g) with size mainly ranging from 5 to 60 nm (N2 and Ar sorption data in Figure 7a, Figure S10, and Table S1). More importantly, almost all of these mesopores are open inter-nano-crystal mesopores (ca. 86%, Vmeso,Hg = 0.18 cm3/g) with good accessibility (Hg intrusion data in Figure 7b and Table S1), indicating their loose packed structure of nanorod branches, which is consistent with our TEM, SEM, and HAADF-STEM observation data. Compared
Figure 6. FESEM images of the zeolite products synthesized with (a− c) different seed sizes (50, 120, and 300 nm), (d−f) NaF/Si ratios (0, 0.3 and 0.9), and (g−i) reaction temperatures (temp = 100, 120, and 180 °C). For synthesis details, see Table 1.
In the absence of organic cations, only Na+ is recognized as the “structure forming” agent and is thus able to direct the MFI structure, while K+ has the “structure breaking” ability,47−49 which may determine their differences in the local ordering of secondary precursor species. Meanwhile, it seems that the presence of F− ions facilitates decomposition of WLPs during hydrothermal treatment and thus (re)transformation into the secondary precursor species, in view that the WLPs aggregate themselves in the absence of F− ions to form condensed aggregates without15,16,25,26 or with45 seed addition. On the other hand, it is found that a seed-mediated nonclassical crystallization process proceeds dynamically along
Figure 7. (a) N2 sorption isotherms at 77 K. (b) Hg intrusion curves from 20 to 420 MPa. (c) NH3-TPD profiles. (d) Potentiometric titration curves with tert-butylamine for (black) C-ZSM-5 and (red) MFI-BZM. Herein, the mesopores measured by Hg intrusion are open pores with good accessibility, which can be penetrated by Hg from the outside at high pressure. For the acidity characterization, NH3 can enter the micropore channels of MFI zeolites, whereas tert-butylamine (solvent diameter of ca. 0.68 nm) can only bind exclusively to the external surface or micropore mouth. 9252
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Figure 8. (a) Conversion of (i) TIPB and selectivity to (ii) IPB versus reaction temperature over (black) C-ZSM-5 and (red) MFI-BZM. Star points in part ai indicate the conversions of direct cracking of IPB. (b) Adsorption amount and (inset) kinetics curves of (i) o-xylene and (ii) p-xylene over (black) C-ZSM-5 and (red) MFI-BZM.
with a conventional ZSM-5 catalyst (C-ZSM-5), MFI-BZM possesses more accessible surface acid sites to bulky molecules (see tert-butylamine titration data in Figure 7d), despite the similar total acid amount of both samples (see NH3-TPD data in Figure 7c). Benefiting from the increased external surface active sites, highly enhanced intra-micro-pore diffusion, and well-preserved intrinsic acidity, MFI-BZM exhibits distinctive advantages in the cracking of triisopropylbenzene (TIPB) and isopropylbenzene (IPB), as well as the adsorption of o-xylene and p-xylene (Figure 8, and Figure S11). Figure 8a shows not only the enhanced conversion of bulky TIPB (Figure 8ai) on MFI-BZM, but also the deep cracking of smaller intermediate IPB (Figure 8aii). Meanwhile, the direct IPB cracking is also enhanced (see star points in Figure 8ai). Furthermore, much faster adsorption is achieved on MFI-BZM for both o-xylene and p-xylene (insets in Figure 8bi,ii), and more interestingly, an obvious increase of adsorption amount is observed for o-xylene with relatively larger size (Figure 8bi). The detailed explanation for the relationship of reaction-adsorption data with structure features can be found in the related section in the Supporting Information.
shell−shell structure, which seems to be determined by the interplay of thermodynamics and kinetics. Meanwhile, this mechanistic study enables us to regulate well the morphology or structure of the mesocrystal by only simply varying synthetic conditions. This finding would not only provide a practical route to control the nonclassical pathway for fine-tuning the properties of the mesocrystal, but also expand our knowledge on zeolite growth mechanisms.
4. CONCLUSION In summary, we have developed an effective approach of a saltaided seed-induced route for the facile and organic-free synthesis of anisotropic MFI-type bundlelike zeolite mesocrystals (MFI-BZM). The MFI-BZM possesses a wellmaintained crystallinity, intrinsic acidity, and dramatically shortened microporous channels (ca. 20 nm) along a-/b-axes, and exhibits remarkable advantages in molecule adsorption and the diffusion-controlled catalytic reaction. Moreover, on the basis of structure features and formation behavior of MFI-BZM, we discovered a distinct seed-modified nonclassical pathway which involves an epitaxial and dynamic self-assembly of precursors on the seed surface. Along with the evolution of precursor particles, non- or semioriented attachment (OA) first occurs, then followed by OA-based 1D (one-dimensional) growth. Such a hierarchical and multistep assembly crystallization pathway leads to a unique disordered-to-ordered core−
Corresponding Author
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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.7b03121. XRD patterns; SEM, FE-SEM, TEM, HR-TEM, and SAED images; 29Si and 27Al MAS NMR spectra; and catalytic selectivity to DIPB and benzene (PDF)
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AUTHOR INFORMATION
*E-mail:
[email protected]. ORCID
Yang Zhao: 0000-0002-2675-6434 Yahong Zhang: 0000-0003-1148-949X Yi Tang: 0000-0002-1463-9927 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by National Key Basic Research Program of China (2013CB934101), NSFC (21433002, 21573046, 21473037, and U1463206), Sinopec (X514005), and National Plan for Science and Technology of Saudi Arabia (14-PET827-02). 9253
DOI: 10.1021/acs.chemmater.7b03121 Chem. Mater. 2017, 29, 9247−9255
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Chemistry of Materials
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DOI: 10.1021/acs.chemmater.7b03121 Chem. Mater. 2017, 29, 9247−9255
