Crucial Factors on Seed-Directed Synthesis of CON-type

Aug 8, 2019 - Designing efficient synthesis methodologies, exempt from using expensive organic structure-directing agents (OSDAs), for industrially ...
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Crucial Factors on Seed-Directed Synthesis of CON-type Aluminoborosilicate Zeolites using Tetraethylammonium Sibel Sogukkanli, Koki Muraoka, Kenta Iyoki, Shanmugam P. Elangovan, Yutaka Yanaba, Watcharop Chaikittisilp, Toru Wakihara, and Tatsuya Okubo Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00724 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Crystal Growth & Design

Crucial Factors on Seed-Directed Synthesis of CONtype Aluminoborosilicate Zeolites using Tetraethylammonium Sibel Sogukkanli,§[a] Koki Muraoka,[a] Kenta Iyoki,[a] Shanmugam P. Elangovan,[a] Yutaka Yanaba,[b] Watcharop Chaikittisilp,‡[a] Toru Wakihara,[a] and Tatsuya Okubo*[a]

aDepartment

of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

bInstitute

of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan

KEYWORDS

Seed-directed synthesis, OSDA, TEAOH, zeolites, multipore zeolites

ABSTRACT

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Designing efficient synthesis methodologies, exempt from using expensive organic structure-directing agents (OSDAs), for industrially important zeolites is of high significance not only to realize their commercialization but also to control their structural properties. Herein, we present a systematic study on seed-directed synthesis for aluminoborosilicate CON-type zeolites using TEAOH as a simple OSDA, by particularly focusing on the precise size control of CIT-1 seeds, that allowed to improve the overall production efficiency and to reach the highest solid yield of 38 wt.%. It is revealed that the use of a certain amount of seeds jointly with TEAOH is key factor governing the synthesis, where seeds act as growth centers to promote the crystallization, while TEAOH stabilizes the structure by charge-compensating with framework atoms. These findings provide an insight into the synthesis of multipore structures from simple OSDA, which can be further extended to many other systems currently requiring complex OSDAs.

INTRODUCTION Zeolites, a class of crystalline microporous materials with their well-defined pore architectures and tunable chemical compositions, have found a wide variety of

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applications as heterogeneous catalysts in chemical and petrochemical processes.[1–3] To date, 248 frameworks have been identified by International Zeolite Association,[4] among them, the ones containing either three-dimensional medium (10-ring) or large (12ring) pores have been the most successful and most utilized zeolite catalysts, such as zeolite beta (*BEA, 12×12×12-ring) and ZSM-5 (MFI, 10×10×10-ring), because of their ability to induce different product selectivities depending on their pores.[5,6] Over the last decades, great efforts have been therefore made in the synthesis of new materials with intersected, three-dimensional medium and large pores as multipore zeolites to combine some of the properties of those medium and large pore zeolites within a same structure.[7,8] In this sense, the use of specially-designed organic structure-directing agents (OSDAs) have been greatly benefited for the discoveries of a handful of frameworks having either 12×12×10-ring[9–15] or 12×10×10-ring[16] channel systems, that offer exceptional shape-selectivity and stability not previously observed. Nevertheless, from a manufacturing perspective, higher cost of applied OSDAs has strictly prevented the commercial feasibility of these materials. This has consequently stimulated the recent research studies towards simplifying the synthesis for multipore zeolites.

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Particular interest in this regard has focused on CIT-1, which is the first reported pure CON-type borosilicate multipore zeolite synthesized by N,N,N-trimethyl-(–)-cismyrtanylammonium hydroxide (TMMAOH) as an OSDA, which is, however, complex in structure and thereby expensive.[10,17] Since its unique CON structure consisting of the connected 12×10×10-ring channels and a large void at 12×10-ring intersections provides high accessibility and diffusibility to crystal interior,[4] CIT-1 has become a prominent catalyst for several applications relevant to energy and environment.[18] Especially, after the isomorphic substitution of framework B with Al via post-synthetic treatments, the zeolite is found to successfully catalyze the hydrocarbon cracking[18,19] and m-xylene isomerization[20,21] reactions, in which it exhibited excellent catalytic activities that cannot be expected from its borosilicate nature. Because of this reason, Yoshioka et al. have lately investigated a direct synthesis route for CON-type aluminoborosilicate zeolites using TMMAOH and studied the performance of resultant materials as methanol-to-olefin (MTO) catalysts.[22] Their results marked that directly-synthesized aluminoborosilicate CON-type zeolites with significantly improved acidic properties have shown superior catalytic performance compared to even post-synthetic CIT-1, zeolite beta, and ZSM-5 in

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terms of the lifetime and selectivity. Considering this great potential of Al inserted CONtype materials, developing an efficient direct synthesis, exempt from the use of expensive and complex OSDA, is of high significance to boost their larger-scale applications. Replacement of conventional OSDAs by commercially available, simple organic substances is one of the strategic ways to circumvent the OSDA cost problem in the production of zeolites having unique frameworks or highly siliceous compositions.[5,23–26] On the basis of this, very recently, we have designed a novel, simplified, seed-directed and high-yield (ca. 54 wt.%) synthesis route for aluminosilicate MSE-type multipore zeolites (MSE-TEA, 12×10×10-ring) by solely using tetraethylammonium hydroxide (TEAOH) as a low-cost OSDA, the selection of which was determined with the help of extended composite building unit (CBU) hypothesis.[27] Moreover, this approach was further employed to advance MSE structure beyond aluminosilicates by partial substitution of framework Al with Zn as a divalent heteroatom, allowing to open up new applications for the zeolite.[28] Collectively, these results suggest the simplified method requiring the combined use of zeolite seed crystals and a simple OSDA as an alternative route to be applied not only commercialization of MSE-type zeolites but also for

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controlling/tailoring its structural properties. Based on these findings, the simplified seeddirected method was introduced for the synthesis of CON-type zeolites. After proper adjustments on the proposed synthesis composition and conditions of MSE-TEA, a microporous zeolite with CON topology (referred to as CON-TEA-1) was successfully synthesized for the first time from the aluminoborosilicate reaction mixture containing TEAOH as an OSDA.[29] Even though the product achieves aluminum-rich CON structure (Si/Al=125) similar to those of previously reported ones by conventional OSDA (Si/Al=109–196),[22] the synthesis of CON-TEA-1 was conducted under narrow and delicate conditions resulting in 28 wt.% of solid yield, that may delimitate the actual use of simplified seed-directed method. In the present work, we hence provide a systematic study on the synthesis of CON-type zeolites using TEAOH to answer the question whether its overall production efficiency could be further improved or not. We first pay attention to the precise size control of CIT-1 seed crystals, as the particle size of the seeds is an important factor that significantly influences the crystallization in a seed-directed synthesis. Generally, seeds having smaller crystals are more effective since smaller crystals can supply a larger surface area

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which can receive more nutrient species to prompt more crystal growth.[30,31] Therefore, we considered that the use of CIT-1 seeds in much smaller sizes is more preferable for simplified synthesis, as typical CIT-1 crystals are a few micrometer in size. After successful downsizing the seeds with high crystallinity, the simplified synthesis method could be expanded for further aluminum-rich CON-TEA (Si/Al= 130–132) zeolites, which led us to reach the maximum solid yield of 38 wt.%: the highest solid yield obtained from an alternative route, so far. Besides, we present a detailed investigation on the effect of each synthesis parameters, especially applied simple OSDA and its possible role in the formation of multipore structures (MSE and CON) by combining experimental outputs with the computational modeling. EXPERIMENTAL SECTION Materials. The following commercially available raw materials for the synthesis of borosilicate CIT-1 seed crystals in different sizes and aluminoborosilicate CON-type zeolites were used as provided: fumed silica (Cab-O-Sil® M5, Cabot) and colloidal silica (Ludox® AS-40, 40 wt.% in water, DuPont) as silica sources; sodium tetraborate decahydrate (Na2B4O7.10H2O, Wako) and boric acid (B(OH)3, Wako) as boron sources;

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aluminum hydroxide (Al(OH)3, Sigma Aldrich) as an aluminum source; sodium hydroxide (NaOH, 50 w/v% in water, Wako) and potassium hydroxide (KOH, 50 wt.% in water, Wako) solutions as alkali cation sources; tetramethylammonium hydroxide (TMAOH, 25 wt.% in water, Wako); tetraethylammonium hydroxide (TEAOH, 35 wt.% in water, Aldrich); tetrapropylammonium hydroxide (40 wt.% in water, Merck) solutions as OSDAs; and deionized water. Moreoever, TMMAOH was prepared to as a conventional OSDA for borosilicate CIT-1 seed synthesis as shown in Supporting Information (SI) from the following chemicals: (−)-cis-myrtanylamine (98 wt.% in water, Sigma Aldrich), iodomethane (JIS Special Grade, Wako), potassium carbonate (K2CO3, Wako), tetrahydrofuran (JIS Special Grade, Wako), chloroform (JIS Special Grade, Wako), and methanol (JIS Special Grade, Wako). Synthesis of CIT-1 seed crystals. CIT-1 seed crystals were performed in a procedure similar to that described elsewhere

[10]

from a reaction mixture with following molar

composition: 1.0SiO2: 0.02B2O3: 0.12NaOH: 0.24TMMAOH: 60H2O. In a typical preparation of CIT-1, sodium tetraborate decahydrate, NaOH, and TMMAOH were first dissolved in distillated water under stirring for 30 min. To this solution, fumed silica was

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slowly added under vigorous stirring, which was finally homogenized for 6 h. After that, as-synthesized either borosilicate or aluminosilicate beta seed crystals (see Supporting Information (SI) for detail synthesis procedure) using 1 wt.% respect to the initial silica weight were added followed by stirring for additional 15 min. Then, the resultant mixture was placed in a conventional autoclave (#4749, Parr Instrument) and hydrothermally treated at 160 °C under static conditions for 12 days. After crystallization, the solid products were collected by centrifugation, thoroughly rinsed with water, and evaporated overnight at 80 °C. Lastly, the obtained as-synthesized CIT-1 samples were calcined at 650 °C with a temperature ramp of 5 °C/min for 4 h. Synthesis of aluminoborosilicate CON-type zeolites using TEAOH. The reaction mixtures of CON-type zeolites (i.e., CON-TEA) were prepared by using B(OH)3, Al(OH)3 and fumed silica together with NaOH, KOH, and TEAOH solutions as previously reported by our group.[29] Typically, aluminum hydroxide and boric acid powders were first dissolved in a aqueous mixture of organic and alkaline solutions under vigorous stirring of 30 min, then fumed silica was slowly added into this solution. The resultant mixture with the following molar ratios: 1.0SiO2: xAl(OH)3: yB(OH)3: z(KOH+NaOH): 0.22TEAOH:

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60H2O (on which x = 0.0040–0.075, y = 0.040–0.060, z = 0.35–0.55, and the KOH/(KOH+NaOH) ratio ranged from 0 to 0.50) was homogenized under stirring for 6 h. After proper homogenization, as-synthesized borosilicate CIT-1 seeds, different in size, (from 0 to 30 wt.% relative to the silica source) were added into this mixture, and continued stirring for extra 30 min. Then, the reaction mixture was placed in a conventional autoclave (# 4749, Parr Instrument), and hydrothermally treated at 160 °C under static conditions for 168 h. After recovering the products as described above, calcination has been applied to solid samples at 550 °C with a temperature ramp of 5 °C/ min for 10 h. The yield of final materials was presented as the weight ratio percentage (g/g × 100) of the calcined solids to total sum of weights of SiO2, Al2O3, B2O3, Na2O, K2O, and added seeds in the reactant mixture. The relative crystallinity of each samples were calculated as ratio of total area of the five peaks in the XRD patterns (2 theta ca. 20.52, 21.54, 22.12, 23.02, and 23.18) for solids to those of seeds, where seeds are settled as 100%. For comparison, the synthesis of CON-type zeolites was performed by employing different simple quaternary ammonium hydroxide such as TMAOH and TPAOH in a

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similar procedure described in the above-mentioned case. Later, the structural properties of the obtained products were compared with that prepared using TEAOH. Characterization. The crystallinity of products were determined by X-ray powder diffraction (XRD) with a Rigaku Ultima IV diffractometer equipped with CuKα radiation (λ = 0.015406 nm, 40 kV, 40 mA). The sizes and morphologies of zeolite crystals were determined by a field emission scanning electron microscope (FE-SEM, JSM-7500FA, JEOL).

Nitrogen

adsorption–desorption

measurements

were

perfomed

on

a

Quantachrome Autosorb-iQ2-MP instrument at liquid nitrogen temperature after an autogas pretreatment at 400 °C for 6h under vacuum. The contents of silicon, boron and aluminum of products were analyzed by a Thermo Scientific iCAP-6300 inductively coupled plasma-atomic emission spectrometer (ICP-AES) system after dissolving in a KOH solution. Moreover, a Hitachi Z-2000 atomic absorption spectrometer (AAS) was carried out to determine the sodium and potassium contents of each products after dissolving in KOH and NaOH solutions, respectively. CHN elemental analyses were conducted on an Exeter CE-440 elemental analyzer to determine occluded organic substances in zeolite pores. Thermogravimetric and differential thermal analysis (TG-

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DTA) was performed by a Rigaku TG 8120 from 30 to 800 °C at a heating rate of 10 °C/min with a flow (200 mL/min) of 10% O2/90% He mixed gas. Solid-state Magic-AngleSpinning Nuclear Magnetic Resonance (MAS-NMR) spectra were acquired on a JOEL JNM-ECA 500 spectrometer. The 27Al spectra were recorded at 130.33 MHz using a 3.2 μs π/2 pulse length, 14 kHz spinning speed, and a recycle delay of 5 s. The 29Si spectra were recorded at 99.37 MHz using a 5.0 μs π/2 pulse length, 10 kHz spinning speed, and a recycle delay of 60 s. The

11B

spectra were recorded at 128.33 MHz using a 3.2 μs

π/2pulse length, 19.2 kHz spinning speed. Computational Methods. The computational protocol generally followed the method employed in a previous study using Dreiding force field and GULP software.[33] TEA+ cations with either tt.tt configuration or tg.tg configuration were introduced in the pure silica zeolite models. In order to fully occupy voids in zeolites with TEA+ without considering alkali metal cations, 5 cations and 10 TEA+ cations were modeled in the unit cell of CON and MSE, respectively. The molecular dynamics run was performed at 343 K for 30 ps and the last 5 ps was used to calculate average energies of the zeolite–OSDA complexes.

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3.

RESULT AND DISCUSSION

3.1. Characterization of CIT-1 seed crystals In order to reduce the crystal size of CIT-1 seeds, we have attempted to modify the previous synthesis procedure[10] by applying different as-synthesized zeolite beta crystals. Even though the exact behavior of zeolite beta in the crystallization of CIT-1 could not be determined in the previous studies,[8,15] it is apparent that addition of small amount of zeolite beta in the synthesis of CIT-1 dramatically accelerates the crystallization kinetic even more than the addition of its own seeds, indicating a significant effect of zeolite beta on the structural formation of CIT-1 crystals. Additionally, it is known that CON and *BEA possess similar topologies, in which 12-ring channels parallel to c axis are topologically equivalent to each other as shown in Figure S1 in the SI (here it is just used for convenience to present the similarities between those frameworks).[4] Taking these facts into account, one might surmise that zeolite beta crystals act as seeds, which induce the crystal growth of CON-type materials by likely providing a specific surface for that, although zeolite beta was existing in small amounts in the reaction mixtures. This made us to consider decreasing the crystal size of applied zeolite beta crystals in the

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synthesis of CIT-1 synthesis, that may in turn yield smaller CIT-1 crystals than that of conventional ones. Since borosilicate zeolites are known with their low hydrothermal stabilities compared to their aluminosilicate analogues,[29] small borosilicate beta zeolites can easily dissolve under harsh synthesis conditions, and not work as seeds. Therefore, aluminosilicate zeolite beta with its small sized crystals is used for CIT-1 synthesis. Figure 1 depicts XRD patterns and SEM images of as-synthesized zeolite beta materials in aluminosilicate and borosilicate form for comparison (see SI for detail synthesis procedures), and the CIT-1 seeds crystallized after the use of those zeolite beta crystals in the reaction mixtures. The obtained products from both synthesis media resulted in CIT-1 phase with good crystallinity (see Figure 1(a)). Also, their SEM images displayed that these CIT-1 products consist of crystals having acuate-square prism morphology with fairly different in size (see Figure 1(b)). The size of individual crystals became smaller (ca. 500 nm–2 μm) as the small aluminosilicate zeolite beta crystals (ca. 20–40 nm) were applied to synthesis rather than those of conventionally synthesized ones (ca. 4 μm) from borosilicate zeolite beta (ca. 200 nm–1 μm). Regarding these results, it can be said that the synthesis of CIT-1 is more likely seed-induced, and the use of small aluminosilicate

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zeolite beta crystals was not only efficient to promote the CIT-1 synthesis but also successful to reduce its crystal size.

Figure 1. (a) Powder XRD patterns and (b) FE-SEM micrographs of borosilicate zeolite beta (B-beta), aluminosilicate zeolite beta (Al-beta), and CIT-1 products, conventional (CIT-1) and small sized (small CIT-1) ones, which are crystallized by using borosilicate zeolite beta and aluminosilicate zeolite beta, respectively.

3.2. Characterization of aluminoborosilicate CON-type zeolites synthesized by simple OSDA After successful downsizing of the CIT-1 crystals, the obtained materials were applied as seeds (20 wt.% relative to silica source) in the formerly proposed simplified seeddirected synthesis[29] with a reactant mixture comprising of: 1.0SiO2: 0.04B(OH)3:

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0.005Al(OH)3: 0.138KOH: 0.413NaOH: 0.22TEAOH: 60H2O at 160 °C for 7 days. The obtained product (hereafter referred to as CON-TEA-2 in Figure 2(a) and shown in Table 1 as Sample 9) revealed a pure CON phase with enhanced solid yield of 36 wt.%. This result implies that carrying out the synthesis with small-sized seed crystals was favorable because their large external surfaces bring about an increase in contact with the reaction liquid, which in turn provide an improvement in the crystal growth of CON-type material. Moreover, the SEM images displayed that CON-TEA-2 is composed of highly-crystalline, aggregated wide-width particles with a size ca. 2 μm (Figure 2(c)), which lays out a clear alteration in the crystal shapes compared with seeds (Figure 2(b)). The crystal size of the products larger than that of seed crystals is one of the evidences of the crystal growth that proceeded on the external surface of seed crystals, as it is commonly observed phenomenon in the seed-directed method.[30,31] Meanwhile, in order to reduce the crystal size of CIT-1 seeds, the mechanical treatment such as bead milling was performed to conventional synthesized CIT-1 zeolites for 30 min on a bead milling apparatus using ZrO2 as beads (for details see the SI). Mechanical force crushed micrometer-sized CIT-1 seeds into irregularly nanosized-pieces (Figure 2(d)); however, when they applied into

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simplified synthesis, a totally different phase, zeolite beta, was observed (Figure 2(e)). This is probably because the strong mechanical treatment brings severe amorphization of the zeolites with a loss of crystallinity,[32] which can nullify the residue crystals, and in contrast, the amorphous matters along with reaction mixtures turn out the nucleation of another structure. Considering these results, it can be suggested that the use of highly crystalline small-sized seed crystals is substantial to conduct a successful high yield, simplified synthesis for CON-type zeolites.

Figure 2 (a) Powder XRD patterns of as-synthesized small and 30 min milled borosilicate CIT-1 seeds, and the products synthesized by using those seeds in described simplified seed-directed synthesis mixture. FE-SEM images of: as-synthesized (b) small CIT-1

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seeds, (c) CON-TEA-2 product, (d) 30 min milled borosilicate CIT-1 seeds, (e) the sample prepared by using milled CIT-1 seeds.

Several synthetic parameters were also examined with the purpose of expanding the synthesis conditions for CON-type zeolites by using TEAOH as an OSDA. In Table 1, the applied chemical compositions of the initial synthesis mixture, the crystalline phase, the solid yield, and pore characteristics of final products are tabulated. As seen in Table 1 and Figure S2, when the Si/(Al+B) ratio was reduced to 20, the formation of CON-type zeolite was confirmed with a solid yield of ca. 38 wt.% (Sample 8 in Table 1 referred as CON-TEA-3 in Figure S2), which is the highest solid yield for CON-type zeolites obtained from a simplified synthesis using TEAOH, so far. It is a general idea in the zeolite synthesis that the yield of products tends to further increase as the Si/Al ratio and alkaline content in the reaction mixture decrease, or the concentration of initial mixture increase. However, further decrement in the Si/(Al+B) along with (KOH+MOH)/Si ratios caused cocrystallization of other phases such as *BEA, MFI and MOR, suggesting a strong structure-directing effect of TEAOH for those frameworks.[33–35] On the other hand,

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increasing Si/(Al+B) ratio (Sample 12) or decreasing the water content to H2O/Si=15 (Sample 13) triggered the formation of kenyaite (a layered silicate) from amorphous matter with a trace of CIT-1 seed crystals due to mainly adopting highly alkaline reaction mixture (OH/Si=0.77), indicating unfavorable synthetic environment for the crystal growth of CON-type materials. It is also worth noting that the balance between K+ and Na+ cations play one of the critical roles in the crystallization of CON-type materials from a seeded system. Even though the original synthesis of CIT-1 strongly requires the use of Na+ rather than K+ cations, applying only Na+ as an alkali cation in a TEA+ containing reaction mixture promoted mainly *BEA-type material together with remaining CIT-1 seed crystals (Sample 10). The use of K+ together with Na+ cations seems to hinder the appearance of *BEA phase up to some extent since the addition of excess amount of potassium restrained the overall crystallization process and eventually resulted in crystallization of MFI as another dominant phase (Sample 11). It is clear that highly crystalline, pure CONTEA products could be obtained from the reaction mixtures containing K+ and Na+ in an exact ratio of 0.25. On the basis of these results, it might be assumed that the applied alkali cations have shown a cooperative effect by preventing the formation of *BEA and

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MFI phases as by-products, and also efficiently promoting the crystal growth of CON-type zeolites. Therefore, it can be claimed that the use of binary alkali cation system for the successful crystallization of CON-type zeolites from simplified seed-directed synthesis is indispensable, as it was also the case for previously reported MSE-TEA synthesis.[27] Additionally, nitrogen physisorption analysis confirms that all of the obtained CON-TEA products show micropore characteristics (0.20−0.21 cm3g−1) similar to those of the previously reported aluminoborosilicate CON-type zeolite[22] and synthesized borosilicate CIT-1 seed crystals by using conventional OSDA. Table 1 Summary for the seed-directed synthesis of CON-type zeolites by using TEAOH as a simple OSDA.

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The effect of the additive amount of seeds was investigated to clarity its role in the synthesis. Figure S3 in the SI exhibits change in crystallinity and the solid yield of final products which were obtained from a reaction mixture with the same composition as that used for CON-TEA-2 after the addition of a various amount of seeds. As seen here, when the applied seed amount was more than 20 wt.%, CON-type zeolite was obtained with a similar solid yield of CON-TEA-3 sample, and the product showed comparable crystallinity to that in the typical synthesis using 20 wt.%. On the other hand, seeding at lower amounts resulted in low crystalline CON-TEA products with amorphous matter which

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increased by decreasing the amount of seeds, and no crystalline product could be observed without seeds addition. This simply demonstrates that the addition of more seeds provided more growth surfaces during the crystallization process and the applied seeds acted as growth centers, as reported earlier.[36] The effect of type and amount of applied simple OSDA were also explored by preparing some comparison samples using TMAOH and TPAOH instead of TEAOH in the synthesis mixture of CON-TEA-2. As seen in Figure S4 (a) in the SI, the XRD patterns exhibit that those OSDAs resulted dominant structure-directing effect for RUT[37] and MFI-type[38] zeolites, respectively. Regarding these results, one may question either TEAOH can also direct CON structure due to its relatively intermediate charge density (C/N ratio) or not (see Figure S4 (b)). However, when different amount of TEAOH was applied to the synthesis as shown in Figure S4 (c), it is found that only a certain amount of TEAOH can favor CON growth in the seed-directed synthesis. Also, it can be seen that TEAOH is one of the indispensable factors to obtain pure CON product since the sample prepared in OSDA-free manner yielded dense phase with a small amount of residual seed crystals. These results as a whole point out that a certain amount of seed crystals along with

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TEAOH as a simple OSDA are having a collaborative growth effect in the simplified synthesis of CON-type zeolites. To give some insights into the formation mechanism, the morphological evolution of the products by different period of hydrothermal treatment was followed. As shown in Figure 3 (a), the crystallinity curve presented that the as-synthesized product had approximately 15% relative crystallinity after 6 h of heating which is possibly due to the residual CIT-1 seed crystals. After prolonging the synthesis time up to 24, 72, and 120 h, the crystallinity of the as-synthesized CON-TEA product gradually increased, and the maximum crystallinity could be attained after ca. 168 h of heating. Time-evolved FE-SEM images of those samples in Figure 3 (b)–(e) in the SI also exhibited that seed crystals were embedded with amorphous matter which induces the crystallization of CON-type zeolite by extending the hydrothermal treatment after 24, 72, 120 h, and eventually completes the crystallization at 168 h. Prolonging the heating time over 168 h caused the gradual appearance of some layered silicate as an impurity, which eventually became dominant after 2 weeks of heating (data not shown here). These results confirm the formation of

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CON-type zeolites on the partially dissolved CIT-1 seed crystals from the synthesis mixture containing TEAOH after a certain heating period of time.

Figure 3 (a) The crystallinity curve, and FE-SEM images of as-synthesized products synthesized from the simplified seed-directed method at 160 °C for (b) 12 h, (b) 24 h, (c) 72 h (d) 120 h, and (e) 168 h.

Solid-state 27Al and 11B MAS NMR measurements were conducted to disclose the local environments of Al and B species in CON-TEA-2 sample, which is selected as a representative sample, and borosilicate CIT-1 seeds as a reference. As shown in Figure 4 (a), the

11B

spectra of the as-synthesized CON-TEA-2 product and borosilicate CIT-1

seeds consisted of a strong resonance at the chemical shift (δ) around –3 ppm,

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corresponding to tetrahedrally coordinated framework B. Additionally, the

27Al

spectrum

of the as-synthesized CON-TEA-2 product in Figure 4 (b) revealed no octahedrally coordinated Al species appearing at δ of ca. 0 ppm, but one resonance centered at δ = 57 ppm, which can be assigned to the tetrahedrally coordinated framework Al. These results clearly confirm the incorporation of Al and B species into the silicate structures in tetrahedral coordination to form aluminoborosilicate CON frameworks. Figure 5 displays solid-state

29Si

MAS NMR spectra of aluminoborosilicate CON-TEA zeolites (i.e., CON-

TEA-1 and CON-TEA-2) and borosilicate CIT-1 seeds as a reference. The deconvolution of the spectra revealed three signals ascribed to Q4(0T) silicon species (Si(OSi)4) at δ around –120 ppm and –115 ppm, and Q4(1T) silicon species (Si(OT)(OSi)3) centred at δ around –105 ppm,[39,40] where T represents substituted Al and B atoms. The Si/(Al+B) ratios calculated from the

29Si

MAS NMR spectra of the borosilicate CIT-1, and

aluminoborosilicate CON-TEA-1 and CON-TEA-2 were 21.3, 13.9, and 15.2, respectively, which are close to the values obtained by ICP-AES (see Table 2). This concurrence in the calculated chemical compositions by 29Si MAS NMR and ICP-AES results assert the successful incorporation of both Al and B atoms into newly grown CON-TEA frameworks.

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Figure 4 Solid-state (a) 11B MAS NMR spectra of the as-synthesized aluminoborosilicate CON-TEA-2 product and borosilicate CIT-1 seeds as a reference, and (b) 27Al MAS NMR spectrum of the as-synthesized aluminoborosilicate CON-TEA-2 product.

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Figure 5 Solid-state

29Si

MAS NMR spectra and their peak deconvolution of

aluminoborosilicate CON-TEA zeolites obtained by differently sized seed crystals (i.e., CON-TEA-1 and CON-TEA-2), and borosilicate CIT-1 seeds as a reference.

Chemical compositions of the obtained aluminoborosilicate CON-TEA products along with aluminoborosilicate CON ([Al,B]-CON) and borosilicate CIT-1 seeds for comparison analyzed by ICP-AES, AAS, CHN, and TGA are summarized in Table 2. ICP-AES results revealed that CON-type zeolites with different molar Si/B and Si/Al ratios were successfully synthesized by varying the size of applied borosilicate CIT-1 seeds. Moreover, it is found that CON-TEA products consist of an aluminum-rich structure with their Al/u.c. values of 0.44 and 0.43, which lying in the range of reported aluminoborosilicate CON-type zeolites having Al/u.c. values of 0.28–0.51, and unlike conventional borosilicate CIT-1. These results indicate the incorporation of a certain amount of Al into the CON structure, that can be another proof for newly grown CON-type zeolites from the synthesis mixture with TEAOH. Also, it can be seen that the sample prepared by using smaller seed crystals showed Si yield higher than that of one prepared

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from larger seed crystals, suggesting the significant effect of large surfaces of seeds on the structural growth. Moreover, AAS results confirmed that K+ is occluded together with Na+ cations in the cavities of CON-TEA structure, indicating once again a cooperative effect of these cations and the necessity of using binary alkali cation system for successful crystallization of CON-type zeolites from the synthesis mixtures consisting of simple OSDA. The overall results of chemical analysis showed that TEA+, Na+, and K+ cations were perfectly compensated with the negatively charged framework, and almost 3 TEA+ cations are occluded per unit cell regardless of Si/B and Si/Al ratios suggesting the spacefiller role of OSDA for TEAOH during the formation of a multipore structure. Table 2 Chemical compositions of aluminoborosilicate CON-TEA products, [Al, B]-CON, and borosilicate CIT-1 seeds.

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3.3. Molecular modeling To further elucidate this space-filler role of TEA+ cations in the formation of a multipore structure, a computational study based on molecular modeling was carried out for MSETEA and CON-TEA.[33] The energy difference between the occluded TEA+ cations in the

tt.tt and/or tg.tg conformers and the target structure were calculated, and then the results along with reference data of LTA, *BEA, MOR, CHA, and MFI topologies[33] were plotted versus the fraction of occluded tg.tg conformers of TEA+ as shown in Figure 6. It is wellknown that TEAOH as a common and versatile OSDA exists in two different conformations, referred as tt.tt and tg.tg, which can be determined by Raman spectroscopy.[41] In our previous reports,[27-29] it is shown that the conformer distribution of TEA+ cations are different when they occluded in different multipore frameworks, as 90 % and 60 % of the occluded TEA+ cations were in tt.tt conformation for MSE-TEA and CON-TEA products, respectively (see Figure S5 in the SI, here it is just used for convenience to present experimental data). The molecular modeling of these structures calculated the tt.tt conformer to be favored by 4.29 kJ/mol TEA+ in MSE-TEA, and 1.42 kJ/mol TEA+ in CON-TEA. The energy difference of CON-TEA showed smaller value than

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that of the interconversion of two distinct conformers in solution (ΔEtg.tg–tt.tt = 4.1 kJ/mol TEA+),[33] whereas MSE-TEA one was similar. Since both conformers are detected in aqueous solution of TEAOH, it is also expectable to find both conformers embedded in the materials since MSE and CON frameworks contain 12-ring channels together with supercage or cages which present voids large enough to fit both. Considering these results, the high tendency towards tt.tt conformer for both structures can be attributable to the electrostatic interactions between positively charged TEA+ cations and negatively charged framework elements, by which conformational changes occurred to fit TEA+ cations inside the voids, akin to the previous report.[33] These findings manifest the significant contribution of our research studies to broaden the applicable scope of TEA synthesis zeolites to even multipore zeolites with MSE and CON topologies. 4.

CONCLUSION

In summary, the synthesis window of aluminoborosilicate CON-type zeolites from the synthesis mixture containing TEAOH as a simple OSDA could be widen after successful downsizing of the borosilicate CIT-1 seed crystals. The conventional synthesis of borosilicate CIT-1 was slightly modified by introducing much smaller aluminosilicate

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zeolite beta crystals instead of large borosilicate ones, resulting in the highly crystalline small-sized seed crystals. This precise control on seed crystals was favorable and led us to reach the solid yield of maximum 38 wt.%. To the best of our knowledge, this is the highest solid yield for CON-type zeolites obtained from a simplified synthesis, so far. It was also shown that synthesis conditions such as Al-B and Na-K balance should be carefully optimized to prevent the formation of other phases prior to the completion of CON-TEA crystallization. The obtained results revealed that the use of a certain amount of seed-crystals jointly with TEAOH is a key factor governing the efficient synthesis, in which seeds act as growth centers by providing surfaces to promote the crystallization, while TEAOH stabilizes the structure by compensating with the negatively charged framework atoms. Moreover, the experimental results together with computational modeling outcomes conceived the space-filler role for TEAOH during the formation of a multipore structure. These findings manifest the significant contribution of our research studies to broaden the applicable scope of the synthesis zeolites involving TEAOH to even multipore zeolites with MSE and CON topologies. The concepts demonstrated herein are believed to provide an insight into the synthesis of multipore structures using

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simple OSDA, which can be further extended to many other systems currently requiring complex and expensive OSDAs. ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:

Preparation methods of TMMAOH as organic structure directing agent and milled borosilicate CIT-1 seeds. Synthesis of borosilicate beta seed crystals, aluminosilicate beta seed crystals. Also, Comparison figure of CON and BEA structures; XRD patterns of products synthesized at different Si/(Si+B) ratios in the simplified synthesis mixture; changes in crystallinity and the final solid yield of the products obtained after various amount of seed additions; XRD patterns of the samples prepared by using different type and amount of simple OSDAs; representative Raman spectra for as-synthesized samples (PDF).

AUTHOR INFORMATION

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Corresponding Author

* [email protected]

Present Addresses § Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan

‡ Research and Services Division of Materials Data and Integrated System (MaDIS), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT

This work was supported in part by the Japan Society for the Promotion of Science (JSPS) through Grant-in-Aid for Scientific Research (A) and Grant-in-Aid for Young Scientists. We thank Dr. K. Itabashi (The University of Tokyo) for fruitful discussion and

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insightful advice, and also Mr. S. Ohtsuka for the assistance with SEM observations. A part of this work was conducted at the Center for Nano Lithography & Analysis at The University of Tokyo, which is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). S.S. is grateful to the Ministry of Education, Culture, Sports, Science and Technology, Japan, for a MonbuKagakusho Scholarship.

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For Table of Content Use Only

Crucial Factors on Seed-Directed Synthesis of CON-type Aluminoborosilicate Zeolites using Tetraethylammonium

Sibel Sogukkanli,§[a] Koki Muraoka,[a] Kenta Iyoki,[a] Shanmugam P. Elangovan,[a] Yutaka Yanaba,[b] Watcharop Chaikittisilp,‡[a] Toru Wakihara,a and Tatsuya Okubo*[a]

The synthesis window of aluminoborosilicate CON-type zeolites by using TEAOH as a simple OSDA could be widen after successful downsizing of the borosilicate CIT-1 seeds. Experimental and computational results revealed that the use of seeds jointly with TEAOH is a key factor governing the efficient synthesis.

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