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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Comparative Study on the Different Interaction Pathways between Amorphous Aluminosilicate Species and Organic Structure-Directing Agents Yielding Different Zeolite Phases Tadashi Umeda,† Hiroki Yamada,† Koji Ohara,‡ Kaname Yoshida,§ Yukichi Sasaki,§ Miku Takano,∥ Satoshi Inagaki,∥ Yoshihiro Kubota,∥ Takahiko Takewaki,⊥ Tatsuya Okubo,*,† and Toru Wakihara*,† †

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 Japan Research & Utilization Division, JASRI/Spring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan § Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan ∥ Division of Materials Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ⊥ Inorganic Functional Material Lab, Mitsubishi Chemical Group, Science and Technology Research Center, Inc., 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan ‡

S Supporting Information *

ABSTRACT: Zeolites possessing novel structures and compositions have been produced using guest organic species, which are called organic structure-directing agents (OSDAs). However, the specific roles of OSDAs have not been clarified yet. In this work, *BEA- and CHA-type zeolites are synthesized from the same starting inorganic materials but with different OSDAs, and the crystallization pathways are observed using various analytical techniques. During the synthesis of *BEA-type zeolite, the gradual evolution of the aluminosilicate structure leads to the transformation from an amorphous structure to a crystalline one. On the other hand, during the synthesis of CHA-type zeolite, a few CHA nuclei are formed in “partially-ordered amorphous species” and grow through the consumption of “buffer amorphous species” on the surfaces of CHA-type zeolite crystals. This clear difference in crystallization behaviors of the two zeolites under the same synthetic conditions is attributed to the differences in the structure-directing ability. This study demonstrates the potential of structure-directing agents to control the crystallization behaviors of zeolites and provides novel insights into zeolite crystallization behavior.

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INTRODUCTION Zeolites are microporous crystalline aluminosilicates that are widely used in industrial fields because of their unique microporous structures, high stabilities, and anionic frameworks. These properties are strongly related to the microscopic structures of zeolites, including pore architecture and framework atoms.1−6 To control the microscopic structures of zeolites, it is critical to understand their formation mechanisms. In particular, interactions between organic structure-directing agents (OSDAs) and (alumino)silicate species are key to understanding the mechanisms of zeolite formation. After hydrothermal synthesis, OSDAs are known to be encapsulated in cavities of crystalline zeolites as isolated cations or molecules.7 To design new zeolites with tailored structural and chemical properties, it is necessary to develop a molecularlevel understanding of zeolite formation, including specific roles of OSDAs. Zeolites are commonly synthesized under hydrothermal conditions from reactants including a silicon source, an aluminum source, an OSDA, and a mineralizer in water. © XXXX American Chemical Society

Zeolite synthesis is typically carried out under highly alkaline, high-temperature, high-pressure conditions, which make the characterization of the intermediate species during hydrothermal synthesis difficult. Moreover, the interdependences of various synthetic parameters (e.g., the initial compounds, synthesis temperature, mixing procedure, and synthesis time) limit our overall understanding of zeolite crystallization.1−6 Despite these obstacles, numerous studies have tracked the intermediates formed during zeolite crystallization to reveal the interactions between OSDAs and amorphous (alumino)silicate species. Recently, molecular-level studies on zeolite crystallization have revealed that zeolite crystallization does not follow classical nucleation theory.8 The crystallization of pure-silica MFI-type zeolite (silicalite1) using tetrapropylammonium cation (TPA+) as an OSDA was studied extensively as a model case for zeolite Received: August 4, 2017 Revised: September 26, 2017 Published: September 26, 2017 A

DOI: 10.1021/acs.jpcc.7b07745 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C crystallization. It was revealed that a TPA+−silicate composite was formed by the replacement of the hydrophobic hydration sphere around TPA+ by silicate species9−12 driven by van der Waals interactions between the alkyl chains of TPA+ and hydrophobic silica (enthalpic effects). Consequently, hydrated water molecules around the TPA+ molecule are released into bulk water (entropic effects). These TPA+−silicate composites assembled to form nanoparticle aggregates that contribute to both crystal nucleation and growth.13−17 This aggregative growth pathway was directly observed by in situ atomic force microscopy (AFM) and applied to other zeolite synthetic systems that contain OSDAs, including aluminosilicate systems.18−26 In aluminosilicate zeolites, introducing aluminum atoms into the silicate structure promotes the polycondensation of silicate species;23 therefore, approaches designed for pure siliceous systems that yield clear sol reactants are difficult to apply to general aluminosilicate crystallization systems.27−30 However, many studies have attempted to clarify the mechanisms of crystallization of aluminosilicate zeolites.27−38 For instance, considerable effort has been devoted to the crystallization of *BEA-type zeolite mediated by tetraethylammonium hydroxide (TEAOH). The similarities between the structures of the amorphous product and the final crystal have been discussed for this system, and clustered TEA+ molecules have been reported to have a structure-directing role in the crystallization of *BEA-type zeolite.39−41 Despite the previous efforts, elucidating structure-directing effects remains a challenging task because of the complexity of the amorphous materials involved in zeolite synthesis, which can be easily changed by synthetic conditions. In other words, it is difficult to obtain a general agreement under different zeolite synthetic conditions because the kinetics of zeolite crystallization is affected by synthetic parameters, which are interdependent. To clarify the mechanisms of zeolite formation in the presence of OSDAs, the synthetic conditions should be designed carefully and logically to eliminate contributions of the other synthetic parameters. In this study, we examine the relationships between the characteristics of OSDAs and the crystallization behaviors of zeolites. We selected the synthetic conditions based on two criteria: (1) alkali metals were excluded to eliminate their structure-directing effects; and (2) with the exception of the type of OSDA, the synthetic conditions should be the same to eliminate the effects of other synthetic parameters. For the first time, we synthesize *BEA- and CHA-type zeolites using the same synthetic conditions except for the type of OSDAs. The crystallization behaviors are clarified by various analytical techniques and are found to depend on the structure-directing abilities of TEA+ for *BEA-type zeolite and N,N,N-trimethyl-1adamantylammonium cation (TMAda+) for CHA-type zeolite.

cooled to room temperature, and the required amounts of water and fumed silica (Cab-O-Sil M5, Cabot) were then added. Fumed silica was added slowly under stirring for 15 min at room temperature. Nearly the same weight of the reactant was transferred to a conventional Teflon-lined stainless-steel autoclave (#4749, Parr Instrument Company). The sealed autoclave was heated at 150 °C under static conditions in a preheated oven. After the prescribed hydrothermal treatment period, the autoclave was removed from the oven and quenched in pooled cold water. The solid product was washed with deionized water until the aqueous mixture showed an approximately neutral pH by repeating centrifugation (19 000 rpm, 20 min). The recovered solid product was dried overnight at 80 °C. B. Characterization. Powder X-ray diffraction (XRD) patterns were collected on an Ultima IV diffractometer (Rigaku, Japan) using Cu Kα radiation with a D/Tex Ultra detector. Crystallinity (α) was calculated as the sum of selected peak areas as follows: selected peak areas of the solid product × 100 selected peak areas of the reference sample

α(%) =

The selected peaks were those at 21.5°, 22.1°, 22.5°, and 22.7° for *BEA and those at 20.9°, 25.0°, and 26.3° for CHA. The reference samples for *BEA and CHA were the solid products obtained after 93 and 24 h of hydrothermal treatment, respectively. High-energy X-ray total scattering (HEXTS) measurements were performed at the BL04B2 high-energy XRD beamline (SPring-8, Japan).42 The powder samples were loaded in quartz capillaries. The scattering patterns were measured at room temperature under vacuum conditions. The incident photon energy was 61.4 keV (λ = 0.202 Å). The maximum Q (Qmax, where Q = 4π sin(θ/λ)) observed in this study was 21 Å−1. The obtained data were handled by well-established analytic procedures, including absorption, background, polarization, and Compton scattering corrections, and were then normalized to give the Faber−Ziman total structure factor S(Q).43 These collected data were used to calculate the reduced pair distribution function (PDF; G(r)) as follows:44,45 G(r ) = 4πr[ρ(r ) − ρ0 ] =

2 π

∫Q

Q max

Q [S(Q ) − 1]M(Q )sin(Qr ) dQ

min

where M (Q ) =

sin(Q Δr ) for Q < Q max Q Δr

and

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M(Q ) = 0 for Q > Q max

EXPERIMENTAL SECTION A. Zeolite Synthesis. *BEA- and CHA-type zeolites were synthesized using the same synthetic conditions but different OSDAs (TEAOH for *BEA-type zeolite and TMAdaOH for CHA-type zeolite). The starting mixture was prepared with a molar composition of 1.0SiO2:0.025Al2O3:0.40OSDA:16H2O. Aqueous solutions of TEAOH (35 wt %, Sigma-Aldrich) and TMAdaOH (25 wt %, Mitsubishi Chemical Group) were used. Aluminum hydroxide (Wako Pure Chemical Industries) was dissolved in an aqueous solution of OSDA by heating the mixture overnight at 80 °C. After dissolution, the solution was

Here ρ is the atomic number density, and M(Q) is the window function developed by Lorch.46 1H−29Si dipolardecoupled magic-angle-spinning (MAS) and 27Al MAS nuclearmagnetic-resonance (NMR) spectra were collected on a Bruker AVANCEIII 600. 27Al MAS NMR spectra were recorded at 156.4 MHz for 27Al. 29Si DD MAS NMR spectra were recorded at 119.2 MHz for 29Si and 600.2 MHz for 1H. CHN elemental analyses were performed on a CE-440 elemental analyzer (Exeter Analytical). The ratios of silicon to aluminum in the solid products were quantified by inductively coupled plasmaB


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The Journal of Physical Chemistry C

occurred between 69 and 74 h. Further increase in the synthetic period beyond this time did not yield materials with higher crystallinities. Figure 1b shows the powder XRD patterns of CHA-type zeolite samples synthesized with TMAdaOH. A notable change was observed after 7 h of hydrothermal synthesis, indicated by the appearance of Bragg diffraction peaks that was attributed to a CHA crystalline phase. The peak intensities increased with increasing hydrothermal treatment up to 12 h, indicating the growth of CHA-type zeolite crystals. Figure 2 shows the crystallization curves of the two zeolites.

atomic emission spectrometry (ICP-AES; iCAP-6300, Thermo Scientific) after dissolving samples in 1 M KOH solution. Thermogravimetric and differential thermal analyses (TGADTA) were performed on a PU 4K instrument (Rigaku) from 30 to 800 °C at a heating rate of 5 K min−1 under a flow (200 mL min−1) of a 10% O2/90% He mixed gas. Field-emission scanning electron microscopy (FE-SEM) images were obtained with a JSM-7000F microscope (JEOL). High-resolution transmission electron microscopy (HRTEM) observations were conducted on a TITAN E-TEM microscope (FEI) at an accelerating voltage of 300 kV. C. Simulation. The structures of the amorphous aluminosilicates leading to *BEA- and CHA-type zeolites were modeled using a reverse Monte Carlo (RMC) approach.47,48 Samples were calcined under the condition described in Figure S1 in the Supporting Information (SI), and HEXTS measurements were performed. First, the initial configurations of aluminosilicate structures were prepared by a molecular dynamic method reported by Pedone and co-workers.49 After construction of the initial models, structural refinements to fit the experimental S(Q) values were conducted using the RMC+ + program.50 The conditions of the simulations were adopted from the previously reported paper.51 The ring distribution and partial pair correlation function, gij(r), of each atomic pair were analyzed to validate the models and clarify the structural features.

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Figure 2. Relative crystallinities of a series of solid products obtained after hydrothermal treatment at 150 °C for different times (●, CHA; ▲, *BEA).

RESULTS AND DISCUSSION Two different crystalline products were obtained from the same reactants, except for the type of OSDA. Figure 1a shows the

Both curves show an induction period before nucleation and crystallization events. The length of the induction period differed greatly between the two reaction systems (69 h for *BEA and 6 h for CHA). On the other hand, the length of crystallization periods shows a similar rate in the two synthetic systems (24 h for *BEA and 18 h for CHA). Photographs of reactants taken after different periods of hydrothermal treatment during the crystallization of *BEA and CHA zeolites are shown in Figure 3. Obvious differences were observed after the initial hydrothermal treatment (*BEA, 5 h;

Figure 1. Powder XRD patterns of solid products obtained after different hydrothermal reaction times for (a) *BEA and (b) CHA zeolites. All Bragg diffraction peaks are attributed to *BEA in part a and CHA in part b.

powder XRD patterns of the solid products obtained after different periods of hydrothermal synthesis using TEAOH as an OSDA. The patterns of the solid products obtained after synthetic times shorter than 71 h show halo patterns at around 2θ = 20°−25°, which are characteristic of amorphous (alumino)silicate. Each of these patterns also shows a broad peak at 2θ = 7.6°; this peak is also seen in the pattern of the highly crystalline *BEA-type zeolite, indicating the formation of certain periodic structures. The first trace of crystalline *BEAtype zeolite was detected after 71 h of synthesis. The crystallization of the amorphous aluminosilicate into zeolite

Figure 3. Photographs of reactants obtained after different periods of hydrothermal treatment during the crystallization of (a) *BEA and (b) CHA zeolites. C


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Figure 4. FE-SEM images of the products obtained after hydrothermal treatment for different periods of time during the crystallization of (a) *BEA and (b) CHA zeolites.

CHA, 1 h); the reactant of *BEA zeolite appeared as a clear sol, whereas that of CHA zeolite appeared as a white dense gel. The viscosity of the products for *BEA zeolite synthesis began to increase after 9 h of hydrothermal treatment, and a dense gel was formed after 24 h. By contrast, the products for CHA zeolite synthesis formed a dense gel until the crystallinity of CHA zeolite reached 100% after 12 h of synthesis. In both samples, the product appeared as a white suspended liquid after the crystalline phase had formed. The size and morphology of the solid products of *BEA synthesis were observed by FE-SEM (Figure 4a). The products before hydrothermal synthesis (0 h), which consist of a mixture of starting materials, were spherical particles with diameters 10−30 nm, characteristic of undissolved fumed silica. After 9 h of hydrothermal synthesis, aggregates of tiny particles were observed, indicating the transformation of the silicon source (fumed silica) into amorphous aluminosilicate species. After additional hydrothermal treatment for up to 69 h, further aggregation was observed, and an amorphous material was observed. After XRD peaks of the *BEA phase were observed (71 h of hydrothermal synthesis), isolated particles with diameters of 50−100 nm were observed. Extended hydrothermal treatment promoted the generation of more isolated particles, while the sizes of existing particles did not increase further. Figure 4b shows the FE-SEM images of the products during the crystallization of a CHA-type zeolite. The product before hydrothermal treatment (0 h) consisted of undissolved fumed silica with a spheroidal morphology. After 3 h of

hydrothermal treatment, the solid product consisted of particles with diameters 30−50 nm, slightly smaller than those of the *BEA-type zeolite. When the hydrothermal treatment was increased up to 6 h, the size and morphology of the amorphous particles did not change. The products obtained after 7 h of hydrothermal treatment exhibited two distinct populations of particles: spheroidal CHA crystals and tiny amorphous particles. As the hydrothermal synthesis time increased, the population of amorphous particles decreased, indicating that crystal growth proceeded through the consumption of amorphous particles surrounding the crystals. CHA crystallization was completed after 12 h of hydrothermal treatment, as indicated by the disappearance of amorphous particles and the dominance of spheroidal crystals with diameters 100−1000 nm. The clear difference in the sizes of the final products between *BEA- and CHA-type zeolites suggests that the nucleation frequency of the *BEA-type zeolite was much higher than that of the CHA-type zeolite. Time variation of intermediate range structure was evaluated by HEXTS. The PDFs, G(r), of crystalline and amorphous *BEA- and CHA-type zeolites that were obtained after different hydrothermal treatment times are shown in Figure 5a,b. Figure S2 shows the total structural factor, S(Q), used to derive G(r). The PDF provides quantitative information on the short and intermediate range scale structure in real space.52,53 The peaks at 1.6, 2.6, and 3.1 Å are related to the nearest T−O, O−O, and T−T, respectively (T = Si or Al). These peaks are similar for all possible aluminosilicate ring structures; therefore, these local D


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Figure 6. Comparison between the experimental structure (○) and the results of RMC simulation (red ). Successive curves offset along y axis.

ones. The partial pair correlation functions, gij(r), of the amorphous *BEA and CHA derived from RMC simulation are shown in Figure S5 in SI. The partial pair correlation functions were similar, indicating the structural similarity of two amorphous structures. Figure 7a,b shows relative fractions of

Figure 5. Pair distribution functions, G(r), of the solid products obtained after hydrothermal treatment for different times during the crystallization of (a) *BEA and (b) CHA zeolites (dotted lines, amorphous; solid lines, crystalline). Successive curves offset along the y axis.

regions cannot provide the information needed to identify the type of ring structure in the solid product. The peaks between 3.5 and 6.0 Å correspond to the second-nearest T−O or T−T atomic correlations. Differences in structural evolution between the *BEA- and CHA-type zeolites can be understood by examining these peaks. The initial products (0 h) of both zeolites showed nearly identical PDF shapes (Figure S3) indicative of the siliceous starting materials (fumed silica). After several hours of hydrothermal treatment (*BEA, 9 h; CHA, 4 h), an enhancement in the shoulder peak around 3.8 Å was observed, which is assigned to the second-nearest T−O of smaller rings (e.g., 4-rings). This indicates that a different amorphous structure containing a large amount of 4-rings was formed after the initial hydrothermal treatment.20,52,53 After further hydrothermal treatment, a significant difference in the PDFs of the samples appeared around 4.0 Å, which is assigned to the second-nearest T−O correlations. This peak did not change during *BEA-type zeolite synthesis; by contrast, during the crystallization of CHA, this peak shifted clearly toward longer distances. As indicated by previous studies, the mediumrange order of *BEA samples did not change significantly prior to the nucleation of *BEA-type zeolite.39−41 A comparison of the PDFs of the solid products of *BEA and CHA just before the observation of the long-range order (*BEA, 69 h; CHA, 6 h) revealed similar medium-range structures (see Figure S4), reflecting the similar structures of the amorphous materials. The amorphous CHA is assumed to have a different mediumrange structure from that of crystalline CHA-type zeolite. RMC simulations were carried out to evaluate the structural similarity between the two solid products (*BEA, 69 h; CHA, 6 h) in detail. As shown in Figure 6, the simulated S(Q) values of both amorphous products corresponded well with the experimental

Figure 7. Ring structures for amorphous (a) *BEA and (b) CHA obtained from RMC simulations and ring structures for crystalline (c) BEA and (d) CHA calculated from the crystal structures.

rings present in the amorphous structure derived from the RMC simulations; the results show the structural similarity between the amorphous *BEA and CHA. The fractions of ring structures for crystalline *BEA- and CHA-type zeolites are shown in Figure 7c,d. The ring populations are 5-ring > 4-ring > 6-ring > others in *BEA-type zeolite and 4-ring > 8-ring > 6ring > others in CHA-type zeolite. CHA-type zeolite contains only even-membered rings, although the amorphous CHA contains a ring structure with a large amount of 5-rings. We deduced that more reorganization of the ring structure is necessary during the crystallization of CHA-type zeolite compared to during that of *BEA-type zeolite; this may contribute to the peak shift toward a longer distance in the second-nearest T−O correlation during crystal growth (Figure 5b). The above findings indicate that the amorphous *BEA and E


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enhanced the condensation of silicate, as evidenced by the appearance of signals around −100 and −110 ppm, which correspond to Q3 and Q4 species, respectively. Further hydrothermal treatment induced the nucleation of a *BEAtype zeolite, as confirmed by the XRD data and the appearance of a sharp NMR peak at −110 ppm, indicating the enhanced order in the silicate species.41 For comparison, 29Si DD MAS NMR spectra of CHA samples after different hydrothermal reaction periods are shown in Figure 9b. The presence of condensed aluminosilicate species was confirmed after 4 h of hydrothermal treatment. After full crystallization, three peaks were confirmed at −100 ppm [Q3(0Al) or Q4(2Al)], −105 ppm [Q4(1Al)], and −110 ppm [Q4(0Al)]. These well-defined peaks indicate the formation of a rigid CHA framework, which is distinctly different from the disordered framework of the *BEA phase. The comparison of the 29Si DD MAS NMR spectra indicates that the amount of Q3 species was larger in a *BEA-type zeolite than in a CHA-type zeolite. Moreover, the CHA sample obtained after 4 h of hydrothermal treatment already exhibited a well-polymerized structure; such a structure was observed after 24 h for *BEA. The local periodic structures of the *BEA- and CHA-type zeolites were characterized by HRTEM (Figure 10). A significant difference in structural evolution between *BEA and CHA was confirmed in the samples just before Bragg peaks were detected by XRD. After 69 h of hydrothermal treatment in the synthesis of *BEA-type zeolite, ordered structures (below 50 nm in size) were observed within the aggregated amorphous particles (Figure 10a). As indicated by the XRD (Figure 1a) and HEXTS (Figure 5a) results, the amorphous *BEA exhibited an ordered structure before long-range order was established. After 2 h of additional hydrothermal treatment (71 h total), fully crystalline particles were observed, and no amorphous matter was detected by TEM (Figure 10b). This indicates that the large number of nucleation sites exists within the aggregated amorphous particles, and subsequently, the crystal grows by attachment of crystal domains and dissolution of misoriented domains, as reported by Hould et al.21 By contrast, during the synthesis of CHA, no ordered structure was observed prior to the detection of Bragg peaks (Figure 10c). After 7 h of hydrothermal synthesis, two distinct populations of particles were confirmed (Figure 10d): spheroidal, submicrometer-sized CHA crystals, and amorphous particles of 30−50 nm. The amorphous particles were attached to the surfaces of crystals and exhibited no ordered structure with respect to the CHA crystalline structure. These results indicate that the CHA crystals grew via the consumption of amorphous particles, which directly attached to the CHA crystal surfaces and then rearranged into crystalline structures, as confirmed by the RMC simulation (Figure 7). The HRTEM images (Figure 10) show clear differences in the crystallization of the two zeolites; *BEAtype zeolite crystallized by the ordering of an amorphous structure, resulting in the formation of multiple nanocrystals, whereas CHA-type zeolite formed from a locally ordered region in which few crystal nuclei were generated. The chemical compositions of the solid products synthesized for different periods of hydrothermal treatment are summarized in Table 1. The incorporation of Al atoms and OSDAs into the solid products was observed during the initial stage of hydrothermal treatment (*BEA, 9 h; CHA, 3 h). The Si/Al ratios and the numbers of OSDAs per unit cell did not change until Bragg peaks were detected by XRD measurements. Increases in the Si/Al ratios were confirmed after the

CHA zeolites might have similar ring structures until crystallization begins, even though the ring structures of the final products are significantly different. These results are supported by the fact that the particle size was the similar between amorphous and crystalline *BEA, whereas crystalline CHA exhibited a much larger particle size than amorphous CHA. The local structures around Si and Al atoms were evaluated by solid-state MAS NMR. Figure 8 shows 27Al MAS NMR

Figure 8. 27Al MAS NMR spectra of the products obtained after different hydrothermal synthesis times during the crystallization of (a) *BEA and (b) CHA zeolites. Asterisks mark spinning sidebands at 7 kHz.

spectra of the products of *BEA- or CHA-type zeolite syntheses after different periods of hydrothermal treatment. Six-coordinated Al species, which generally appear at a chemical shift of 0 ppm, was not observed in any sample. On the other hand, a clear peak between −59 and −54 ppm corresponding to four-coordinated Al species was observed throughout the syntheses. These results indicate that most aluminum atoms were incorporated into the silicate structure as four-coordinated Al species to form aluminosilicate species. Figure 9a shows 29Si DD MAS NMR spectra of *BEA-type zeolite after different reaction periods. After 9 h of hydrothermal treatment, the dissolution of silica was confirmed by the appearance of a clear peak at −88 ppm, which corresponds to Q2 species.54 Prolonged hydrothermal treatment up to 69 h

Figure 9. 29Si DD MAS NMR spectra of the products synthesized for different periods of time during the crystallization of (a) *BEA and (b) CHA zeolites. F


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Table 1. Solid Yields and Chemical Compositions of the Products Synthesized for Different Periods of Time During the Crystallization of *BEA and CHA Zeolites *BEA synthesis time (h)

solid yield (wt %)

Si yield (wt %)

0 9 24 45 69 71 74 76 87 93

36 15 49 60 54 53 53 51 38 35

38 15 48 59 54 52 53 50 38 34

Al yield (wt %)

Si/AI TEA+/u.c.

9.4 18 61 74 65 68 57 59 41 36 CHA

62 12 12 12 13 12 14 13 14 15

0.92 5.5 5.7 5.4 5.8 5.8 6.5 6.1 6.1 6.1

TEA+/AI 0.90 1.1 1.2 1.1 1.2 1.2 1.5 1.4 1.4 1.5

synthesis time (h)

solid yield (wt %)

Si yield (wt %)

Al yield (wt %)

Si/ AI

TMAda+/u.c.

TMAda+/ Al

0 3 4 5 6 7 8 9 12 24

63 47 51 49 49 50 44 52 71 69

66 46 51 48 48 49 55 51 71 69

18 65 60 61 61 63 73 61 73 74

55 11 13 12 12 12 13 13 15 14

0.65 3.4 3.4 3.5 3.7 3.8 3.6 3.7 3.5 3.6

1.0 1.1 1.3 1.3 1.4 1.4 1.4 1.4 1.6 1.5

Figure 11a,b shows the TGA and DTA curves of *BEA samples under a stream of mixed gas (10% O2/90% He). In Figure 11a, the exothermic DTA peak observed around 430 °C is assigned to the decomposition of TEA+ balancing negative Al sites ( Si−O−Al).55−57 This peak is sharpened as the crystallization of *BEA-type zeolite proceeds, indicating an enhancement in the interaction between OSDA and the framework and a more unified environment around TEA+. The position of this peak did not change during the crystallization, indicating that the degree of the TEA+−aluminosilicate interaction was similar between the amorphous and crystalline products. This finding is supported by previous studies,39−41 which reported similar amorphous and crystalline structures during the crystallization of *BEA-type zeolite. Another exothermic peak associated with TEA+ balancing negatively charged framework defects (Si− O−···HOSi) was observed around 360 °C;55,56 still, this peak is undefinable, and the weight loss in this region was relatively minor compared to that associated with TEA+ balancing Al sites (peak at 430 °C).55−57 Figure 11c,d shows the TGA and DTA curves of the solid products obtained during the crystallization of the CHA-type zeolite. A clear difference in the decomposition behaviors of TMAda+ was observed between the amorphous and crystalline phases. Figure 11c shows three exothermic peaks that correspond to the combustion of TMAda+ (as shown in Figure S6 in SI) occluded in three different aluminosilicate structures at around (1) 250, (2) 400, and (3) 440 °C. Peaks 1 and 2 disappeared as the CHA-type zeolite grows, whereas peak 3 appears after the CHA crystalline phase is formed. The TGA curves show the two weight-loss regions, the first region below 320 °C and the second one above 320 °C; the contribution of the former

Figure 10. HRTEM images of the solid products obtained after (a) 69 h and (b) 71 h of hydrothermal treatment during the crystallization of *BEA zeolite and after (c) 6 h and (d) 7 h of hydrothermal treatment during the crystallization of CHA zeolite.

appearance of Bragg diffraction (*BEA, 71 h; CHA, 7 h), thus indicating the release of Al atoms after the formation of crystalline phases. On the other hand, the OSDA contents did not change throughout the *BEA and CHA syntheses. Chargecompensating behavior inside aluminosilicate species can be indicated by the OSDA/Al ratio, which exceeded 1.0 in both the *BEA- and CHA-type zeolites. This indicates that TEA+ and TMAda+ were stabilized by both Al sites (Si−O− Al) and framework defects (SiO−···HOSi) in the final products. To confirm the interactions between OSDAs and aluminosilicate species, TGA-DTA measurements were performed. G


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to the crystalline phase (assigned to peak 1), and the second weight-loss region is related to the partially ordered aluminosilicate structure, which is similar to that of the crystalline phase (assigned to peaks 2 and 3). The weight-loss distributions of these two regions (first and second regions) are shown in Figure S7 in SI. On the basis of these results, we deduce the existence of “buffer amorphous species” and “partially ordered amorphous species” before the establishment of the CHA-type zeolite structure. The “buffer amorphous species” did not participate in nucleation, but these species took part in crystal growth through direct attachment or solutionmediated attachment to the surfaces of CHA crystals, as observed by FE-SEM (Figure 4b) and TEM (Figure 10d).58 On the basis of the above results, the crystallization schemes of *BEA- and CHA-type zeolites are proposed in Scheme 1. The initial reactants were fumed silica (10−20 nm in diameter) surrounded by aluminum species and OSDA molecules (Scheme 1a,e); these reactants had no structural similarity to the crystalline phases. During *BEA synthesis, initial hydrothermal treatment led to the dissolution of ingredients, thus resulting in a clear sol (Figure 3a). Amorphous nanoparticles (30−50 nm in diameter) appeared from this clear sol, and the degree of polymerization was relatively low (Scheme 1b). These nanoparticles consisted of aluminosilicate species containing TEA+ molecules homogeneously formed from the homogeneous clear sol via dissolution−reprecipitation. The structural similarity between the amorphous and crystalline structures could be observed in this phase, as indicated by the HEXTS (Figure 5a) and TGA-DTA (Figure 11a) results. Inside the amorphous nanoparticles, amorphous aluminosilicate species gradually polymerized to form a *BEA-like structure (see the 29Si DD MAS NMR spectra in Figure 9a). After 71 h of hydrothermal treatment, the internal structure of each amorphous nanoparticle became crystalline, thus resulting in the nucleation of many *BEA nuclei inside the aggregate particles (Scheme 1c). In the crystal growth step, crystals of *BEA-type zeolite (50−100 nm in diameter) formed through

Figure 11. TGA-DTA curves of solid products obtained after different hydrothermal treatment times during the crystallization of (a, b) *BEA and (c, d) CHA zeolites (dotted lines, amorphous; solid lines, crystalline).

region decreased after crystallization (Figure 11d). These two distinct weight-loss regions can be assigned to the three exothermic peaks as follows: The first weight-loss region is related to the amorphous structure, which has no resemblance

Scheme 1. Proposed Crystallization Mechanisms of *BEA- and CHA-Type Zeolites

H


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transformation of amorphous nanoparticles into nanocrystals inside aggregated particles, and (4) crystal growth through unifying nanocrystals. On the other hand, CHA-type zeolite exhibited a completely different crystallization behavior with the following steps: (1) formation of isolated amorphous nanoparticles without the dissolution of primary particles; (2) formation of two different amorphous structures, partially ordered amorphous species and buffer amorphous species; (3) nucleation from partially ordered amorphous species without the structural evolution of the buffer amorphous species; and (4) crystal growth by direct attachment of buffer amorphous species on the surfaces of crystals. This clear difference in the crystallization behaviors of the two zeolites from the same synthetic conditions can be explained by the structure-directing ability. The findings of this study demonstrate that structure-direction can affect the crystallization behaviors of zeolites and provide novel insights into zeolite crystallization behavior.

attachment and dissolution of crystalline domains (Scheme 1d), as suggested by the HRTEM images (Figure 10a,b). Thus, the crystallization of *BEA-type zeolite proceeded via the aggregation of amorphous nanoparticles, as previously reported by Hould et al.21,25,26 In the case of CHA-type zeolite, the initial mixing of reactants led to the immediate formation of a viscous gel, and no clear sol was obtained during the synthesis (Figure 3b). After the initial hydrothermal treatment, isolated amorphous nanoparticles (30−50 nm in diameter) could be identified. These amorphous nanoparticles had no structural similarity with the crystalline phase, as indicated by the HEXTS (Figure 5b) and TGA-DTA (Figure 11b) results. At this point, TMAda+ was occluded in two different aluminosilicate structures: “buffer amorphous species” and “partially ordered amorphous species”. The buffer amorphous species did not contribute to nucleation, as suggested by the FE-SEM (Figure 4b) and HRTEM (Figure 10d) results. The frequency of nucleation events was much lower during CHA crystallization than during *BEA crystallization, suggesting that CHA-type zeolite did not form through disorder-to-order transformation like *BEA-type zeolite. As reported by Kumar et al., CHA crystals grow through the consumption of amorphous nanoparticles consisting of buffer amorphous species (Scheme 1g).58−60 To introduce amorphous species into a crystal structure, the amorphous phase must be structurally rearranged. In this study, the extension of the ring structure confirmed through HEXTS experiments (Figure 5b) was attributed to the rapid structural rearrangement of the buffer amorphous species, specifically the reduction of five-rings to other rings (e.g., 4, 6, and 8-rings). Finally, the “buffer amorphous species” were completely consumed by this crystal growth mechanism, and large spheroidal crystals were formed (Scheme 1h). Although the precise moments of nucleation of *BEA- and CHA-type zeolites could not be identified in details in this study, we rationalized the difference in crystallization behavior as follows: During the synthesis of *BEA-type zeolite, the aluminosilicate structure must gradually evolve to form stable nuclei because of the weaker structure-directing ability of TEA+. The slow formation of stable *BEA nuclei, as compared to the formation of substantially homogeneous amorphous aluminosilicate species, results in a disorder-to-order transformation into *BEA-type zeolite. By contrast, the formation of CHA-type zeolite occurs so quickly that the dissolution and structural evolution of the amorphous structure cannot be completed; this is attributed to the stronger structure-directing ability of TMAda+ and results in the formation of buffer amorphous species and partially ordered amorphous species. These findings imply that the nucleation of CHA-type zeolite occurs in the partially ordered amorphous species, and relatively unstable buffer amorphous species are consumed by the crystal growth of stable nuclei.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07745. Calcination profiles of products, total structure factors, pair distribution functions, partial pair correlation functions, obtained from the RMC models, and weightloss distribution of TMAda+ derived from TGA-DTA curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.O.). *E-mail: [email protected] (T.W.). ORCID

Kaname Yoshida: 0000-0003-0704-3815 Yoshihiro Kubota: 0000-0001-7495-9984 Toru Wakihara: 0000-0002-3916-3849 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by the New Energy and Industrial Technology Development Organization. Part of this work was conducted at the Center for Nano Lithography & Analysis at the University of Tokyo and was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). HEXTS experiments conducted at SPring-8 were approved by the Japan Synchrotron Radiation Research Institute under proposal numbers 2015A0115, 2015B0115, and 2016A0115.

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CONCLUSIONS In this study, *BEA- and CHA-type zeolites have been synthesized using the same synthetic conditions, with the exception of the type of OSDA. This has allowed us to clarify the relationships between the OSDA characteristics and the crystallization behaviors of the zeolites. The crystallization of *BEA-type zeolite proceeded as follows: (1) dissolution and subsequent condensation of primary particles to form amorphous nanoparticles and aggregative particles, (2) structural evolution inside the amorphous nanoparticles, (3)

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