Stepwise Gel Preparation for High-Quality CHA Zeolite Synthesis: A

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Stepwise Gel Preparation for High-Quality CHA Zeolite Synthesis: A Common Tool for Synthesis Diversification Yoko Joichi, Daigo Shimono, Nao Tsunoji, Yasuyuki Takamitsu, Masahiro Sadakane, and Tsuneji Sano Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00963 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Crystal Growth & Design

Stepwise Gel Preparation for High-Quality CHA Zeolite Synthesis: A Common Tool for Synthesis Diversification Yoko Joichi,a Daigo Shimono,a Nao Tsunoji,*,a Yasuyuki Takamitsub, Masahiro Sadakane,a and Tsuneji Sano*,a a

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan b Inorganic Materials Research Laboratory, Tosoh Corporation, Shunan, Yamaguchi 746-8501, Japan ABSTRACT: Aluminosilicate molecular-sieve zeolites are widely used in industrial processes, mostly as catalysts or adsorbents and advances in the synthesis are still required in order to progress further applications. To further expand the zeolite synthesis system, herein we report the stepwise preparations of synthesis gels through divided compositional control; this strategy is applied to the effective synthesis of a CHA zeolite in the presence of an inexpensive organic structure directing agent (OSDA), benzyltrimethylammonium. Highly crystalline nanosized CHA zeolites were prepared at higher crystallization rates compared to those prepared using the conventional one-step gel-preparation method. The stepwise method also provided CHA zeolite in wide range of starting gel composition, and aluminium content within the formed CHA could be tuned. The aluminosilicate-formation process, investigated by the combination of analytical methods including X-ray diffractometry, NMR and Raman spectroscopy, and electrosprayionization mass spectrometry (ESI-MS) and several synthetic experiments, revealed that the aluminosilicate cluster that forms in the highly alkaline aluminum-rich intermediate gel during the stepwise method helps to effectively form the CHA zeolite in the final synthesis step. We also demonstrate that this strategy is applicable to other synthesis systems with different OSDAs and target zeolites. Furthermore, the stepwise method provides efficient zeolite catalysts with high activities and durabilities for emission-gas purification applications. This sharable concept is expected to become a common tool that brings additional synthetic diversity to a variety of zeolite-synthesis system.

INTRODUCTION Considerable interest has focused on the synthesis of crystalline porous material, zeolite, owing to their wide range of industrial applications, including catalysis, ion exchange, and molecular sieving.1-3 Since the pioneering work of Barrer on the artificial synthesis of zeolites,4-5 hydrothermal synthesis techniques have been extensively investigated in order to satisfy the high demand. Unlike typical organic syntheses, direct control over a zeolite-synthesis system during hydrothermal heating is difficult. Consequently, until recently, progress in the syntheses of these materials has been achieved through the introduction of a variety of techniques at the initial stage of the synthesis, such as the inclusion of an organic structure directing agent (OSDA),68 the use of fluoride media,9-10 other framework elements,11-13 and the seed-assisted technique14-15, and semiempirical experiments have been conducted based on product and startingcomponent information. In contrast, the syntheses of zeolites from another zeolites as a starting material have recently gained attention since the intermediates state during zeolite formation can potentially be controlled in an intentional manner.16-19 The starting zeolite was regarded not only as the simple silica/alumina source in these reports, but also as the specific starting material that provides hydrothermally decomposed aluminosilicate species that partially retain the properties of the starting zeolite.18-22 A

starting zeolite is useful and sometimes essential for generating a specific zeolite framework, such as MFI,23 BEA,24-25 CHA,17, 22, 26-30 LEV, 31 AEI,32-37 AFX,38-39 MSE,40 and MWW.20 Especially recently, the synthesis of the novel zeolite, YNU-5, the first zeolite containing interconnected 12-, 12-, and 8-ring pores, as well as independent straight 8-ring channels, was synthesized from an FAU-type zeolite.41 In addition, when the crystallization behavior of zeolites and conventional amorphous gels are compared, zeolite precursors provide higher crystallization rates,24, 29, 31, 40 crystallinities,29, 31 product yields,22, 27 specific Al-distributions,42 and unusual crystal shapes,23, 29, 31 even with the same starting-gel compositions. Zeolite crystallization processes from zeolite are complicated compared to other typical zeolite synthesis systems, however, several analytical studies, including detailed XRD,40 TEM,20, 25 solid-state NMR,20, 25 FT-IR,20, 25 and mass spectrometric18 analyses indicate that non-crystalline aluminosilicate species that retain the local ordered structures and/or morphologies of the starting zeolite facilitate unique crystallization behavior and endow the product with interesting physicochemical properties. Furthermore, this synthesis system is applicable to a wide range of syntheses with a variety of OSDAs,18, 26-27 others containing only inorganics,21 incorporated heteroelements,23, 28, 30, 37 and specific heating systems,22 through the use of a zeolite as the starting materials instead of a more-generally used amorphous gel.

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With this in mind, the creation of a suitable starting system that provides aluminosilicates with the desired structural order and distribution will lead to an additional sharable key strategy for the further development of zeolite syntheses. The chargedensity mismatch (CDM) approach,43-45 the use of twodimensional zeolite precursors,46-49 and the carefully optimization of classic inorganic-synthesis systems50-53 are also useful for the rational construction of zeolite frameworks. However, existing intentional synthesis routes focusing on the buildingblock intermediate, including the above-mentioned “zeolite synthesis from zeolite” method, still requires special additives. In contrast, such specific synthetic diversity in the intentional routes have not been attained in general synthesis gels prepared by typical “aging” due to the limited availabilities of tunable parameters (normally only temperature and time).

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dows and large cavities, is among the most promising zeolite catalysts and separation media. In particular, its catalytic ability to purify diesel emissions55-57 and to produce light olefins5859 has attracted significant recent attention. However, despite the demands of industry, the exiting CHA-synthesis method is costly and inefficient due to the inevitable use of an expensive OSDA and long synthesis times that typically exceed 6 d.29, 42 Therefore, to construct a facile synthesis system, herein, we rapidly synthesized the CHA zeolite using benzyltrimethylammonium (BTMA) as an inexpensive OSDA; we previously reported that this OSDA provides highly crystalline CHA zeolites without structural defects, but still requires an FAU zeolite as the starting material and long synthesis times (more than 1 week).26 During the optimization of various synthesis parameters, we found that our stepwise method was a suitable crystal-growth system for CHA zeolites with high crystallinities, enhanced crystallization rates, and wider synthesis windows compared to the conventional synthesis method involving the one-step preparation of the synthesis-gel. The local structure and distribution of the aluminosilicate species in the stepwise synthesis gel was clarified through characterization of the liquid and solid phases. We also found that the CHA zeolite from the stepwise gel-preparation method exhibited good catalytic activity and stability for the selective catalytic reduction (SCR) of NOx by ammonia.

EXPERIMENTAL Zeolite syntheses All chemicals were reagent grade and used as supplied. Laboratory-prepared deionized water (Millipore, Elix) was used in all experiment.

Scheme 1. Schemes for the syntheses of zeolites through (A) conventional gel preparation and (B) stepwise gel preparation.

Herein, in order to diversify the general synthesis system, we divided the procedure for the preparation of the synthesis gel. During the conventional preparation of a gel (Scheme 1A), all components required for the starting gel (silica, alumina, alkali media (NaOH), OSDA, etc.) are mixed in onestep, with the composition adjusted to obtain the target zeolite. We assumed that dramatically varying the composition during synthesis-gel preparation helps to form a specific locally ordered or distributed aluminosilicate. We chose to separate the parts of the procedure that adjust the “Si/Al” and “NaOH/Si” ratios during synthesis-gel preparation because these factors strongly affected the obtained zeolite phase in previous systems that use zeolite precursors,21, 32 and classic systems that use only inorganic components (such as faujasite);51-52 these components are commonly controlled in almost syntheses. During the “stepwise gel preparation”, we first prepared a precursor gel with a relatively high aluminum content (low Si/Al ratio) and high alkalinity (high NaOH/Si ratio), which we refer to as the “first gel”. Following aging (stirring) of the first gel, the composition was adjusted to that suitable for zeolite formation by the addition of a silica source. Finally, the resultant synthesis gel, referred to as the “second gel”, was subjected to hydrothermal treatment. As a first example of the use of this method, in this study, we present an effective synthesis route to the CHA zeolite. The CHA zeolite,54 with small-pore (8-membered ring) win-

CHA-zeolite synthesis by the conventional gel-preparation method: Colloidal silica (1.59 g, Cataloid SI-30, SiO2 = 30.5 wt%, Na2O = 0.4 wt%, H2O = 69.1 wt%, JGC Catalysts and Chemicals Ltd., Japan) was added to an aqueous solution containing sodium aluminate (0.033 g, NaAlO2, >99%, Kanto Chemical Co., Inc., Japan), sodium hydroxide (0.112 g, NaOH, >99%, Kojundo Chemical Laboratory, Japan), benzyltrimethylammonium hydroxide (0.67 g, BTMAOH, 40 wt%, Tokyo Chemical Ind. Co. Ltd. (TCI), Japan) and water (1.04 g). After the mixture had been stirred at room temperature for 24 h (aging), a seed crystal of the CHA zeolite (0.01 g, 2 wt% of silica sourse, prepared by the literature procedure29) was added. The resultant gel (Si/Al = 20, BTMAOH/Si = 0.20, NaOH/Si = 0.40, and H2O/SiO2 = 10), was placed into a 30 cm3 Teflon-lined stainless steel autoclave, and hydrothermally treated at 125 °C for 4-96 h in a dry convection oven under static conditions. The solid product was collected by centrifugation and washed thoroughly with deionized water until the pH of the wash water was almost neutral, after which it was dried overnight at 70 °C to yield the as-synthesized CHA zeolite. To remove the organic molecules present in the zeolitic pores, the as-synthesized sample was calcined in air at 600 °C for 10 h, after which the Na cations in the calcined CHA zeolite (1.0 g) were removed by ion exchange with 20 ml of 1.0 M aqueous ammonium nitrate (NH4NO3, >99.0%, Wako Pure Chemical Industries, Ltd., Japan) at 60 °C for 2 h; the ion-exchange treatment was repeated three times. The NH4form of the zeolite obtained in this manner was calcined at 450 °C for 6 h to yield the H-form of the zeolite. CHA-zeolite synthesis by the stepwise gel-preparation method: Colloidal silica (0.40 g) was thoroughly mixed into an aqueous solution containing sodium aluminate (0.033 g), sodium hydroxide (0.112 g) and water (0.27 g) to yield the first gel. The composition of the first gel was set to: Si/Al = 5.0, NaOH/Si = 1.6, and H2O/SiO2 = 15. After the first gel was stirred at room temperature for 24 h, BTMAOH (0.67 g, as an aqueous solution), a CHA seed crystal (0.01 g) , water (0.77


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g) and additional colloidal silica (1.2 g) were added to adjust the gel composition to that used during conventional gel preparation (Si/Al = 20, BTMAOH/Si = 0.20, NaOH/Si = 0.40, and H2O/SiO2 = 10) to yield the second gel, which was then hydrothermally treated at 125 °C for 4-96 h. The solid product obtained in this manner was washed and dried as described above, to yield the as-synthesized CHA zeolite, which was then calcined and protonated as described in the conventional method to yield the H-form of the zeolite. Preparation of Cu-loaded CHA-zeolite catalysts Cu-loaded zeolite catalysts were prepared using an impregnation method. 5 ml of an aqueous solution containing the required amount of Cu(NO3)2 (99.999%, Kishida Chemical Co. Ltd., Japan) was added to the NH4-form of the zeolite (1.0 g) and mixed thoroughly. The resultant wet powder was dried at 110 °C for overnight and then calcined at 550 °C for 1 h. The Cu loading was 1.5 wt% in all samples. Characterization X-ray diffraction (XRD) patterns of the solid products were acquired using a powder X-ray diffractometer (Bruker, D8 Advance) with graphite-monochromatized Cu Kα radiation at 40 kV and a tube current of 40 mA. The Si/Al, Na/Al, and Cu/Al ratios of the zeolites were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Seiko SPS7000). Zeolite crystal morphology was determined by scanning electron microscopy (SEM, Hitachi S4800) coupled with an energy dispersive X-ray (EDX) analyzer. 13 C, 27Al, and 29Si magic angle spinning (MAS) NMR spectra were recorded at 150.88, 156.33, and 119.17 MHz, respectively, on a Varian 600PS solid-state NMR spectrometer. A 3.2-mm-diameter zirconia rotor, rotating at 15 kHz, was selected for 27Al, while a 6-mmdiameter zirconia rotor at 7 kHz was used for 13C and 29Si MAS NMR spectroscopy. 27Al MAS NMR spectra were acquired using 2.8 µs pulses, 1 s recycle delays, and 256 scans. 29Si MAS NMR spectra were acquired using 6.2 µ pulses, 100 s recycle delays, and 100 scans. 1 H-13C cross polarized (CP) MAS NMR spectra were acquired at a spinning frequency of 7 kHz, a 90° pulse length of 5.6 µs, and a delay time of 5 s. Hexamethylbenzene, AlK(SO4)2•12H2O, and 3(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt, were used as the 13 C, 27Al, and 29Si MAS NMR chemical-shift references, respectively. Samples were moisture-equilibrated over a saturated solution of NH4Cl for 24 h prior to 27Al MAS NMR spectroscopy. Nitrogen-adsorption isotherms were obtained at −196 °C using a conventional volumetric apparatus (BELSORPmini, Bel Japan). The calcined sample (~50 mg) was heated at 400 °C for 10 h under a flow of N2 prior the adsorption experiment. IR spectra were recorded at room temperature using an FT-IR spectrometer (NICOLET 6700) at a resolution of 4 cm−1. Samples was pressed into self-supporting thin wafers (~6.4 mg cm−2) and placed into quartz IR cells fitted with CaF2 windows during the acquisition of spectra in the OH-stretching region. Prior to spectral acquisition, each sample was dehydrated under vacuum at 400 °C for 3 h. Raman spectra of the solids were recorded on a T64000 (HORIBA-JY) instrument at the Natural Science Center for Basic Research and Development (N-BARD) of Hiroshima University. The solids were analyzed using an 1800-lines/mm grating. ESI-MS was performed using an LCMS-2020 instrument (Shimadzu Co., Japan). Spectra were recorded at an applied voltage of −3.5 kV, a flow rate of 15 µL s-1, and a desolvation temperature of 200 °C. Thermal stabilities The thermal stabilities of the CHA zeolites were evaluated by XRD by comparing the changes in the sums of the peak intensities of the samples in the 8–37° 2θ range before and after calcination in air at 600–1050 °C for 1 h. Selective catalytic reduction (SCR) of NOx with ammonia NOx-SCR reactions were carried out in a fixed-bed flow reactor under atmospheric pressure. The Cu-loaded CHA-zeolite catalyst was pressed and sieved to a diameter of ~1 mm. The required amount of catalyst (1.5 mL, ~0.8 g) was positioned with quartz wool at the center of a quartz reactor. A thermocouple inserted at the center of the catalyst bed was used to measure temperature during the reaction. The

reactant gas was composed of 200 ppm NO, 200 ppm NH3, 10% O2, 3% H2O, with N2 as the balance (NO + NH3 + O2 + H2O + N2 = 100%). The gas composition is based on that emitted from diesel engines. The total flow rate was fixed at 1.5 L min−1, and the gas hourly space velocity (GHSV) value was 60,000 h−1. The temperature was varied from 500 to 150 °C in steps of approximately 50 °C during these experiments. NO conversion was defined as:

where NOin represents the NO concentration at the inlet (200 ppm), and NOout and NO2out represent the NO and NO2 concentrations at the outlet, respectively. To evaluate steady-state catalytic activity, the concentrations of NH3, NO, and NO2 in the outlet gas were analyzed by an FT-IR spectrometer (FT/IR-6100, JASCO, Japan) equipped with a gas cell (LPC-12M-S, 12m) and a mercury cadmium telluride detector cooled by liquid N2, after 10 min of time-on-stream at each reaction temperature. The concentrations were determined from the intensities of the peaks at 1033, 1875, and 2917 cm-1 that correspond to NH3, NO, and NO2, respectively. Thirty scans were averaged for each normalized spectrum.

RESULTS AND DISCUSSION Syntheses of CHA zeolites using benzyltrimethylammonium as the OSDA

Figure 1. (A) Crystallization curves for CHA zeolites prepared through stepwise gel preparations with first-gel with different aging time. (B) SEM images of CHA zeolites crystallized for 4 d following different first-gel aging times.

In order to assess the influence of first-gel aging during stepwise gel preparation, we prepared reactant gels (second gels) from first gels aged for various times. Figure 1A displays the relative XRD-determined crystallinities of the CHA zeolites as functions of crystallization time (crystallization curves). The relative crystallinity of the unaged first-gel sample (0 h) hardly increased with time; the crystallinity value was about 10 % based on the pure CHA-zeolite phase, even after 24 h of crystallization time. However, the rate of crystallization was dramatically higher when the first-gel aging time was increased to 4 h, with relative crystallinities in excess of 90% after 24 h of crystallization. The SEM images of the products fully crystallized after 4 d (Figure 1B) reveal that first-gel aging affects the CHA-zeolite crystal size, with the crystal size decreasing with increasing aging time. In studies into the formation of nanosized zeolites,60 the key parameter responsible for the formation of the nanosized zeolite crystals was determined to be the abundant and uniform nucleation resulting from control of the hydrothermal-synthesis conditions.



Figure 2. (A) Crystallization curves and (B) XRD patterns of CHA zeolites obtained through conventional and stepwise gelpreparation.

Figure 3. SEM images of CHA zeolites obtained through conventional and stepwise gel-preparation.

Therefore, our stepwise synthesis results reveal that the formation of the nanosized CHA zeolite are associated with a suitable crystal-growth system constructed from first-gel aging. The crystallization behavior of the zeolite formed through the stepwise method with the optimum first gel (aged for 24 h) was compared to that formed using the conventionalgel preparation method. In the conventional method, the starting gel was aged for 24 h after mixing the all components. Figure 2A displays the crystallization curves for CHA zeolites obtained through the different gel-preparation routes. The crystallization rate in the stepwise method was considerably higher, whereas the conventional method required 96 h of crystallization time for CHA-zeolite formation. The XRD patterns of samples crystallized for 96 h (Figure 2B) reveal that both synthesis routes provide CHA zeolites of good crystallinity. The yields based on Al, Si and the Si/Al ratios of the obtained zeolites were 47% (Al), 29% (Si) and 11 for the stepwise method, and 58% (Al), 32% (Si) and 10 for the conventional method, respectively. We also found intact OSDA (BTMA) to be present in the zeolitic pores of the assynthesized CHA zeolites by 13C cross-polarization (CP) MAS NMR spectroscopy (Figure S1), which confirms the successful synthesis of the CHA zeolite over a short crystallization time in the presence of an inexpensive OSDA. Although there were no significant differences in crystallinity, the yields and compositions of the CHA zeolites obtained

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through the two synthesis-gel preparation methods, the crystal sizes were very different. The SEM image of the CHA zeolite obtained conventionally (Figure 3, left) exhibits cubic crystals with crystal sizes of about 0.5–1.0 µm, whereas the CHA zeolite synthesized using the stepwise gel-preparation method exhibits smaller cubic crystals in the 80–150 nm range. The nitrogen-adsorption isotherms of the CHA zeolites prepared conventionally display typical type I behavior (Figure 4A), suggesting the existence of only micropores. In contrast, the sample obtained using the stepwise method exhibits a steep increase in the amount of adsorption at high relative pressures that correspond to macropores between nanosized crystals. The BET surface areas and micropore volumes of the CHA samples were 762 m2 g−1 and 0.31 cm3 g−1, and 821 m2 g−1 and 0.32 cm3 g−1 for the CHAs prepared using the conventional and stepwise method, respectively. The values obtained using the stepwise method were comparable or superior to those of existing CHA zeolites synthesized using conventional OSDAs and amorphous starting materials or starting zeolites22, 27, 42 (micropore volumes of 0.26-0.30 cm3 g−1), suggesting good porosity derived from its high crystallinity. The 27Al MAS NMR spectra (Figure 4B) exhibit remarkably high peak intensities centered at approximately 57 ppm, which correspond to tetrahedrally coordinated aluminum, compared to that of the peak at 0 ppm attributed to octahedrally coordinated aluminum species; this shows that most of the aluminum species exist within the zeolite framework. In the FT-IR spectra of both samples (Figure S2A), the peaks at around 3500 cm−1 assigned to internal silanol groups of hydroxy nests were hardly detected. In addition, enhancements in the levels of Q2 and Q3 species in the 1H–29Si CP MAS NMR spectra (Figure S2B) were the same for both samples, suggesting that there is no difference in the number of silanol group. The size of the CHA-zeolite crystal produced through the stepwise method was much smaller than that obtained conventionally; nanosized crystals are generally less stable and less crystalline than typical crystals of larger crystal size.29 However, the FTIR and NMR results strongly indicate that the nanosized CHA zeolite was formed in high crystallinity and without structural defects using the stepwise gel-preparation method.

Figure 4. (A) N2-adsorption isotherms and (B) 27Al MAS NMR spectra of CHA zeolites (H-forms) obtained through conventional and stepwise gel-preparation.


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Crystal Growth & Design Table 1 Starting-gel preparation conditions and elemental analyses of the separated solid and liquid phases Starting gel

Separated solid phase

Separated liquid phase Si/Ala

Si/Ala

NaOH/SiO2

Aging time [h]

Si/Ala

Al contentb [%]

Al contentb [%]

First gel-1

5.0

1.6

-

14

77

0.4

23

First gel-2

5.0

1.6

24

-

-

5.0

100

Second gelc

20

0.4

-

39

28

14

72

20

0.4

24

28

34

17

66

Conventional gel a

b

Determined by ICP. Calculated from the silica and alumina yields and the Si/Al ratio, compared to those of starting gel. c Extra silica source and OSDA were added to First gel-1.

The adsorption and catalytic properties of a zeolite are strongly affected by its Si/Al ratio, and establishing a tunable Si/Al ratio range is an important synthetic feature.10, 29, 33 Therefore, we optimized the Si/Al and NaOH/Si ratios of the starting gel in order to obtain CHA zeolites with higher Si/Al ratios; the crystallization behavior of the samples prepared using the different gel-preparation methods were compared by XRD (Figure 5). CHA zeolites were produced from both synthesis routes using starting gels with Si/Al and NaOH/Si ratios of 30 and 0.27, respectively. However, when the starting Si/Al ratio was further increased (Si/Al = 40 and NaOH/Si = 0.2), low product crystallinity consistent with the presence of an impurity (amorphous phase) was observed for the product prepared conventionally. In contrast, increasing the starting Si/Al ratio did not affect CHA-zeolite crystallinity using the stepwise method, and the obtained CHA zeolite exhibited an improved Si/Al ratio of 17. This promising synthetic feature was also observed over a wider range of synthesis conditions (Figure S3), in which the stepwise method provided CHA zeolites with even higher Si/Al and lower NaOH/Si ratios compared to the conventional method. These results show that the stepwise gel-preparation method provides promising crystallization results over a substantially varied total gel-composition. In addition, we also observed that the product yield, crystal size, and crystallinity could be controlled by adjusting the composition of the first gel and by altering the aging time, while maintaining the (total) composition of the second gel. In existing zeolite synthesis system, synthetic benefits of zeolite, such as high crystallization rate, high crystallinity of nanosized zeolite, and wide synthesis window, have been generally attained in the synthesis system by varying gel composition and/or with additional components.

Figure 5. XRD patterns of CHA zeolites obtained through conventional and stepwise gel-preparation using gels with higher Si/Al ratios: (A) Si/Al = 30, NaOH/Si = 0.27 and (B) Si/Al = 40, NaOH/Si = 0.2.

In contrast, the stepwise method brings such benefits in CHA zeolite synthesis by only dividing the gel preparation section maintaining existing control factor. Therefore, this stepwise method is expected to be useful to give additional advantage for a variety of zeolite synthesis system. In addition, to investigate the role of aluminosilicate species constructed during this stepwise method is also important. Aluminosilicate formation during the aging process. To obtain insight into the nature of the aluminosilicate present in the starting gel, we characterized its solid and liquid phases. Table 1 lists the chemical compositions of the solid and liquid phase separated from aluminosilicate gels prepared under different aging conditions, which included “First gel-1” (aged for 0 h), “First gel-2” (aged for 24 h), “Second gel” from the stepwise method (prepared from First gel-2), and “Conventional gel” aged for 24 h aging using the conventional onestep method. In the absence of aging, the first gel (First gel-1) contained a silica-rich solid phase with a Si/Al ratio of 14, and an aluminum-rich liquid phase with a Si/Al ratio of 0.4. After aging for 24 h, the solid phase had completely dissolved into liquid phase to produce a homogeneous clear solution (First gel-2). During the preparation of the second gel with a target Si/Al ratio of 20 (Second gel), a solid phase was again observed upon addition of the extra silica source. The Si/Al ratios in the solid and liquid phases of final resultant gel (Second gel) were 39 and 14. In contrast, the Si/Al ratios in the conventional gel were 28 and 17 in the solid and liquid phases, respectively. The liquid phase of the gel prepared by the stepwise method has a relatively large aluminum content (72 %) as shown in Table 1, even though the final (total) compositions of the gels prepared by the conventional and stepwise methods were identical. The local structure of the liquid-phase aluminosilicate species was examined by NMR spectroscopy (Figure 6). The 27Al NMR spectrum of the conventional gel exhibited a peak at 59 ppm assigned to tetrahedrally aluminum surrounded by four silicone oxides, q4 ((SiO)4Al).61-62 The 29Si NMR spectrum exhibited relatively large corresponding resonances peaks at – 88 and –97 ppm, which are assignable to Q2 ((SiO)2Si(O−)2), Q3 ((SiO)3Si(O−)), and Q4 ((SiO)3(AlO)Si) structures,61-63 and suggests a relatively high degree of oligomerization of the silicon-rich aluminosilicate species. In the initial unaged first



gel (First gel-1), aluminum species exist as tetrahydroxoaluminate ions, with a corresponding 27Al NMR signal at around 80 ppm (Figure 6A),61 whereas no signal was detected in the 29 Si NMR due to the low Si content of this gel, derived from its low liquid-phase Si/Al ratio (0.4). The aluminate had reacted with the dissolved silicate upon aging for 24 h, with 27Al NMR signals observed at 73, 69, and 62 ppm, consistent with a change in the aluminum state to the q1 ((SiO)Al(O−)3), q2 ((SiO)2Al(O−)2), and q3 ((SiO)3Al(O−)) environments,61 respectively. Even after additional silica had been added to the first gel, these characteristic signals were still observed in the 27Al NMR spectra, while no significant differences were observed between the 29Si NMR spectrum of First gel-2 and that of the second gel. In contrast, the aluminosilicate structure formed through the stepwise method was totally different to that formed in the conventional gel.

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ber of water molecules lost through dehydration and condensation, respectively. Several aluminosilicate species with 5–7 T atoms and relatively high dehydration values (above 2) are clearly observed. These particularly prominent peaks correspond to structures containing three and four-membered ring resulting from high levels of dehydration. The coordination environment of the T atoms in these proposed structures are in good agreement with the 27Al and 29Si NMR results. The solid phases of the prepared samples were amorphous with no significant morphological differences (Figure S4). In addition, the 27Al MAS NMR of samples exhibited only a single resonance peak at 58 ppm attributed to tetrahedrally coordinated aluminum (Figure S5). However, differences in the shortrange orders of the aluminosilicate species were confirmed by Raman (Figure 8A) and 29Si MAS NMR spectroscopies (Figure 8B). The Raman spectrum of the reference sample (dried colloidal silica (SI-30)) and the second gel prepared from the unaged first gel exhibited only single broad peaks centered at 450 cm−1 that correspond to the typical disordered structure of an amorphous phase.65-66 In contrast, the second gel prepared from the aged first gel displayed several sharp peaks in the 250–600 cm−1 region that are attributed to various ordered aluminosilicate species with 4-, 5-, and 6-membered-ring structures. The characteristic peaks observed in the gel prepared by the stepwise method were also observed during the previously reported syntheses system of the FAU,67 FER,66 and SOD65 zeolites. The 29Si MAS NMR spectra of the second ge l exhibited a relatively high-intensity peak at ca. −100 and −110 ppm corresponding to Q3 ((SiO)3Si(O− or OH)) and Q4 ((SiO)4Si) structures, respectively.

Figure 6. (A) 27Al and (B) 29Si NMR spectra of the liquid phases from the conventional gel, and the unaged first gel (0 h) (First gel1), the first gel aged for 24 h (First gel-2), and the second gel obtained through the stepwise gel-preparation.

In the 29Si NMR spectrum of first gel-2 and the second gel, signals for Q0 (Si(O−)4) and Q1 ((SiO)Si(O−)3) or Q2 ((AlO)2Si(O−)2) species appeared at around −71 and −79 ppm, respectively, and signals at −81, −87 and −89 ppm are attributed to Q2 ((SiO)2Si(O−)2), Q3 ((SiO)3Si(O−)) or Q3 ((AlO)2(SiO)Si(O−)) species.61-63 Among them, the signals for the Q2 and Q3 states at about −81 and −89 ppm are also assignable to small clusters with three- or four-membered-ring structures (3R or 4R).61-63 These results indicate that a specific aluminosilicate cluster with high aluminum content and/or an intramolecularly condensed structure formed during first-gel aging, which remained in the second gel. The specific aluminosilicate species were also detected by electrosprayionization mass spectrometry (ESI-MS)64 (Figure 7). Characteristic signals at specific m/z values were not detected in the ESI-MS spectrum of the aqueous solution obtained during conventional-gel preparation, due to the low concentrations of aluminosilicate species. Considering from the determined Si/Al ratio (17), Al content (66) and NMR results of the liquid phase, the low sensitivity in ESI-MS is ascribable to the formation of oligomeric aluminosilicates with large m/z values. In contrast, the stepwise samples exhibited high-intensity signals that include those corresponding to BTMA+ or Na+ as cationic adducts. The proposed chemical structures of the aluminosilicates based on the observed m/z values are also shown in Figure 7, in which the aluminosilicate structures are labeled as “TX-WY”, where X and Y refer to the number of T (Si and Al) atoms in the aluminosilicate species and the num-

Figure 7. ESI-MS spectra of the liquid phase from (A) the conventional and (B) stepwise gel-preparation.

Figure 8. (A) Raman spectra of solid samples: (a) colloidal silica (SI-30) and second gels (b) with and (c) without first-gel aging. (B) 29Si MAS NMR spectra of the solid products from conventional gel, the unaged first gel aged (First gel-1), the first gel aged


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for 24 h (First gel-2), the second gel during stepwise gel preparation, and reference SI-30.

increase in framework density indicate the formation of a dense aluminosilicate during aging of the first gel. Taking into

Figure 9. Illustrating aluminosilicate formation during stepwise gel preparation, and the zeolite structures hydrothermally obtained from each synthesis gel.

This is derived from differences in the local distributions of Si sites with fewer aluminum and hydroxyl groups. These solidphase analyses also revealed that other specific aluminosilicate species, with ordered ring structures and lower aluminum concentrations, formed in the solid phase. Taking into account the SEM observation that the prepared solid gels and the starting SI-30 are materials with large micrometer-scale particles (Figure S4), the specific species may be created at the solid-liquid interfaces. Figure 9 provides a schematic illustration of the aluminosilicate-forming process during the stepwise method based on the results and discussion presented above. Initially, the aluminum species in the first gel exist in the liquid phase as tetrahydroxoaluminate ions that are separated from the solid phase (colloidal silica,). The silica source gradually dissolve and react with the aluminate ions during the aging of the first gel to form highly condensed aluminosilicate clusters in the liquid phase. Even after the additional components (silica and OSDA) had been added during the preparation of the second gel, these aluminosilicate species were present in the liquid phase where they, once again, reacted with the additional silica source to provide other specific aluminosilicate species with ordered ring structures at the solid-liquid interphase. With this in mind, we speculate that the promising synthetic features of the stepwise method are associated with the specific aluminosilicate species formed in the first gel. To gain further insight into the roles of the specific aluminosilicate species during the formation of the CHA zeolite, we subjected the first gel itself to hydrothermal treatment (Table S1). To carefully characterize the influence of first-gel-aging time, we performed the hydrothermal treatment at a relatively low temperature (80 °C) and over a long time (7 d). The unaged first gel or that aged for a short time (4 h) yielded an FAU zeolite with a framework density of 13.3 T/1000 Å3, whereas a GIS zeolite with a higher framework density (16.4 T/1000 Å3) was formed from the first gel aged for 24 h. This

account that (1) the first-gel preparation conditions are more severe than those of the second gel because of the high alkalinity, (2) the NMR results for the second gel prepared from the aged first gel that reveal the retention of specific aluminosilicate structures following addition of the silica source, and (3) the second gel prepared from the unaged first gel exhibited almost identical crystallization behavior to that of a conventional gel, we conclude that the first gel, prepared at a high alkali concentration, stabilizes the specific high-aluminumcontent aluminosilicate clusters that are not fully decomposed in second-gel system, to provide a system with inhomogeneously distributed aluminium between the solid and liquid phases. In contrast, when we subjected the second gel to hydrothermal treatment without the OSDA or the seed crystal to promote crystallization, the highly crystalline CHA zeolite did not form or the rate of crystallization was dramatically lower, confirming that these two factors (OSDA and seed crystal) are essential (or vitally important) for constructing the CHA zeolite framework. In our previous work in which we used a FAU zeolite as the starting material in the presence of an OSDA and a seed crystal, we concluded that a suitable crystal-growth system arises from locally ordered aluminosilicate species formed by the decomposition of starting materials that are structurally similar to the target zeolite, in which locally ordered species were assembled by the OSDA and the seed crystals facilitated the formation of the target zeolite framework. In addition, during an OSDA-free zeolite synthesis, Okubo et al., detailed the “CBU hypothesis” as a guidelines for exploring synthesis conditions, where when the framework structure of the target and the seed zeolites is the same, the seeds should have at least one common composite building unit with the zeolite to be synthesized from the gel without seeds.15, 68 In this study, although the CHA zeolite could not be obtained from the stepwise gel in the absence of the OSDA, the system



exhibited common or similar structural building units, which are mainly composed of 4-membered rings, between the zeolites obtained from the first (FAU and GIS) and second (CHA) gels. As a consequence, we speculate that specific aluminosilicates in the second gel are assembled and oligomerized by OSDA and the seed crystals to produce an aluminosilicate intermediate suitable for the formation of the CHA zeolite during hydrothermal heating. As a result, we propose that the specific aluminosilicate species in first gel that are suitable for constructing the 4-membered-ring-based GIS zeolite also help to crystallize the CHA zeolite in the second gel. Synthetic diversity of the stepwise synthesis-gelpreparation method.

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conventionally. These results strongly indicate that the stepwise gel-preparation method is a suitable sharable crystalgrowth system for CHA zeolites derived from specifically formed aluminosilicates, which also indicate that these additional synthetic concepts can be used in a variety of synthesis systems. We also found that the stepwise method is useful for the specific synthesis of the AEI zeolite framework, which is another promising small-pore zeolite, but for its synthesis the choice of starting zeolite as the silica/alumina source is essential.35 This observation also highlights the high potential of this method as an alternative synthesis concept for the preparation of novel zeolites that require specific starting materials. Selective catalytic reduction (SCR) of NOx with ammonia over the Cu-loaded CHA-zeolite catalyst

Figure 11. (A) Relationship between reaction temperature and NO conversion, and (B) NO conversion at 300 °C over Cu-loaded CHA-zeolite catalysts prepared by conventional and stepwise methods after hydrothermal treatment at 900 °C.

Figure 10. (A) Crystallization curves, (B) FT-IR spectra, and (C) SEM images of CHA zeolites obtained through conventional and stepwise gel-preparation in the presence of TMAdaOH as the OSDA.

The synthetic features of the stepwise method are expected to be based on the specific aluminosilicate formed by the Si, Al, and NaOH components. Consequently, we speculated that if we created a specific aluminosilicate using only the abovementioned components, the CHA zeolite would effectively be formed even if other types of OSDAs were used. Therefore, we used N,N,N-trimethyladamantylammonium (TMAda), a generally used OSDA for CHA-zeolite synthesis.16-17 Figure 10 displays the CHA-zeolite synthesis results using the stepwise and conventional gel-preparation methods. Even when a different OSDA was used, promising results were obtained using the stepwise method. Crystallization of the CHA zeolite from the conventional gel required 48 h, whereas crystallization time was dramatically shorter in the stepwise method; highly crystalline CHA zeolite was obtained after only 4 h of crystallization. While different amounts of defects were not observed by comparing the FT-IR spectra of both samples, the CHA zeolite prepared using the stepwise method exhibited smaller crystals 100–300 nm in size, compared those obtained

As detailed above, highly crystalline CHA zeolites were synthesized by a stepwise gel-preparation procedure in the presence of BTMA, an inexpensive OSDA. We applied it to the NH3-SCR reaction, which is one of the most common commercial applications of small-pore zeolites. The CHA zeolite obtained conventionally in the presence of BTMA and one-step gel preparation was used as a reference. We selected Cu as the catalytically active component in this study; the Cu loading was maintained at 1.5 wt% in each sample. Figure 11A displays NO conversions over the Cu-loaded CHA catalysts as functions of temperature. A NO conversion in excess of 90% was obtained at 200 °C for both samples, confirming that the CHA topology is endowed with high purifying ability over wide a temperature range.55, 57 Because diesel-engine emissions contains up to 10 vol% H2O, resistance against steam is also important for automotive applications. Therefore, in order to assess the long-term hydrothermal durabilities of the catalysts, we also investigated their catalytic performance following hydrothermal treatment at 900 °C under an atmosphere containing 10 vol% H2O and 90 vol% air; Figure 11B displays NO conversions at 300 °C under these conditions. Although the fresh catalysts exhibited no difference in NO conversion, large reductions are observed following hydrothermal treatment. However, the levels of reduction observed for both samples are lower than that observed for the conventional CHA-type catalyst prepared from an amorphous gel and a general OSDA (TMAda),29 which illustrates the high durabilities of zeolite catalysts prepared using BTMA. Furthermore, among the two catalysts prepared using BTMA, greater


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NO conversions following hydrothermal treatment were observed for the CHA-zeolite catalyst prepared using the stepwise method, which is ascribable to the superior thermal stability of the stepwise -synthesized CHA zeolite (Figure S6), which exhibited higher crystallinity after heating at various temperatures, compared to the conventional zeolite. Generally, nanosized zeolites are less crystalline than micrometer-sized zeolites due to large amount of defects. Although several reports have focused on solutions to this problem, post-synthesis treatments39 are still required. The catalysis results in the current study reveal the potential of our facile method for the synthesis of nanosized CHA zeolites with high catalytic stabilities, and their practical applications. These results also demonstrate that synthesis concept of the stepwise gelpreparation provides catalytic diversity of zeolite, which provides physicochemical property suitable for each application.

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CONCLUSIONS In order to develop additional synthetic diversity, a stepwise synthesis-gel preparation method, with controlled (stepwise) adjustment of composition, was investigated for the synthesis of CHA zeolites. On the basis of this investigation, the rapid synthesis of highly crystalline nanosized CHA zeolites that eliminates the need for expensive OSDAs, was achieved. We confirmed that the formation of highly condensed aluminosilicates stabilized in the liquid phase during the separate aging process is the key to the creation of a suitable crystal-growth system for CHA zeolites. The wide synthesis-application range of this stepwise method was also confirmed through syntheses with various compositions and in the presence of different OSDAs. We also found that the CHA-zeolite catalyst prepared through the stepwise method exhibited good catalytic stability compared to that prepared conventionally. On the basis of this investigation, we revealed for the first time the possibility of zeolite-synthesis diversity through a stepwise synthesis-gel preparation approach even in general synthesis system. We believe that wide scope of this synthetic approach will be important for the future progress of zeolite chemistry.

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ASSOCIATED CONTENT Supporting Information

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Properties of the prepared CHA zeolites; Table S1 and Figures S1–S5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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*Corresponding authors: Nao Tsunoji, tel.: +81-82-424-7606, email: [email protected]; Tsuneji Sano tel.: +81-82424-7607, e-mail: [email protected]

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ACKNOWLEDGMENT

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We thank Dr D. Kajiya of the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University for Raman spectroscopy. This research was partially supported by JSPS KAKENHI Grant Numbers JP16H04218 and 16K14481, and the Center for Functional Nano Oxide at Hiroshima University. We also sincerely thank Prof. M. Ogura at the University of Tokyo and the Reference Zeolite Symposium of the Japan Zeolite Association for useful discussions.

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For Table of Contents Use Only Stepwise Gel Preparation for High-Quality CHA Zeolite Synthesis: A Common Tool for Synthesis Diversification Yoko Joichi,a Daigo Shimono,a Nao Tsunoji,*,a Yasuyuki Takamitsub, Masahiro Sadakane,a and Tsuneji Sano*,a a

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan b

Inorganic Materials Research Laboratory, Tosoh Corporation, Shunan, Yamaguchi 746-8501, Japan

Stepwise preparations of synthesis gels through divided compositional control brings beneficial synthesis features, such as high crystallization rates, wide synthesis ranges, improved obtained catalyst durability, and tunable crystal size.


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Stepwise Gel Preparation for High-Quality CHA Zeolite Synthesis: A

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