From Charge Density Mismatch to a Simplified, More Efficient Seed

Jun 18, 2013 - The charge density mismatch (CDM) approach has been a strategy to apply multiple templates in the zeolite synthesis. In the synthesis o...
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From Charge Density Mismatch to a Simplified, More Efficient SeedAssisted Synthesis of UZM‑4 Takahiko Moteki and Tatsuya Okubo* The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ABSTRACT: The charge density mismatch (CDM) approach has been a strategy to apply multiple templates in the zeolite synthesis. In the synthesis of the UZM-4 zeolite, tetraethylammonium (TEA) aluminosilicate solution is used as CDM solution although the TEA cation is not incorporated in the final product. We focused on the aging process to prepare the CDM solution, and a more simplified pathway for the synthesis of UZM-4 is explored. The preparation of homogeneous aluminosilicate solution prior to the addition of crystallization-inducing templates, tetramethylammonium, and lithium cations in this work, is essential to the successful synthesis of UZM-4 crystals. We confirmed that small aluminosilicate species were formed under the preparation of CDM solution and stabilized in the liquid phase. On the basis of these results, the efficient, economical synthesis of the UZM-4 zeolite, eliminating the TEA aluminosilicate solution, was achieved for the first time by a seed-assisted approach. KEYWORDS: zeolite, UZM-4, charge density mismatch, seed-assisted synthesis

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INTRODUCTION The synthesis of new zeolites has been a challenging target for materials chemists to widen their applications. The introduction of new organic/inorganic structure-directing agents (SDAs) has been widely employed to synthesize new zeolites.1−3 Recent remarkable progresses in the zeolite synthesis have recently been achieved by the introduction of large and complicated organic SDAs (OSDAs) into the reactant mixture.4 However, higher cost of OSDAs has prevented the new zeolite from being used in an industrial application. Therefore, an alternative approach, free from the use of complicated and expensive OSDAs, has been strongly desired to control the framework and crystal properties.5,6 Combining simple SDAs is one of strategic answers to this problem. Suitable combinations of multiple inorganic SDAs have effectively worked in the production of synthetic zeolites.7,8 Many combinations of OSDAs may be worth consideration, therefore, many trials have been carried out.9−11 A new strategy, based on the charge density mismatch (CDM) concept, has been recently developed, in which plural SDAs are used.12−22 This strategy is designed to foster cooperation between multiple SDAs. The mismatch of a charge density associated with the SDA cations and the aluminosilicate framework species expected to be formed from the reactant mixture is the key.12−16 Negative charge on the zeolite framework introduced by the [AlO4/2]− moieties is compensated by the charge on the SDA cation. The CDM reactant mixture has been prepared at a lower Si/Al ratio (high charge density) with a template cation possessing lower charge density, such as a larger tetraalkylammonium (TAA) cation.15,16 Therefore, zeolite crystallization has been hindered because of © 2013 American Chemical Society

the mismatch between the lower charge density of the larger template and the higher charge density of the expected final aluminosilicate framework. To induce crystallization, other SDAs with suitable charge densities for the final aluminosilicate framework, such as a smaller TAA cation, should be added.15,16 An aluminosilicate zeolite, UZM-4, has been synthesized using the CDM approach,12−15 which has the BPH-type framework with a three-dimensional channel system: 8membered ring (MR) pores running parallel to the ab-plane and 12MR pores running along the c-axis. Linde type Q is the other aluminosilicate zeolite having the BPH-type framework and a low Si/Al ratio, which has been synthesized using the potassium cation as an SDA by a hydrothermal treatment.23,24 In the synthesis of UZM-4, multiple organic, and inorganic cations have been effectively employed as SDAs. Each SDA is added at an appropriate stage in the crystallization process to control the crystallization path.12 In the synthesis of zeolites, aging is a process for preparing suitable aluminosilicates by keeping the reactant mixture at a constant temperature (usually between RT and 373 K) for a specific time before the crystallization. The aging process has been used to synthesize larger crystals or to prevent byproducts. Moreover, aging is sometimes an essential process for obtaining a specific zeolite. The aging process is usually regarded as an operation process. However, we have regarded the aging process as a series of chemical reactions, and shown that the Received: March 5, 2013 Revised: May 31, 2013 Published: June 18, 2013 2603

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solution was then added to the aged mixture to obtain the precursor mixture with the chemical composition 2X2O/2.5SiO2/0.5Al2O3/ TMACl/0.5LiCl/100H2O. After homogenization of the precursor mixture by stirring for 2 h, it was transferred into a 23 mL Teflon-lined autoclave (#4749, Parr), and subjected to a hydrothermal treatment at 393 K for a specific period of time between 0 and 72 h under rotation at 20 rpm. The samples were collected by centrifugation, washed with distilled water, and then dried in an oven at 333 K. Seed-Assisted Synthesis of UZM-4. UZM-4 seed crystals were synthesized by aging the reactant mixture for 24 h, and then subjected to the hydrothermal treatment at 393 K for 24 h with the final chemical composition 2TEA2O/2.5SiO2/0.5Al2O3/TMACl/0.5LiCl/ 100H2O. The seed-assisted synthesis was carried out by adding assynthesized UZM-4 seed crystals into the aluminosilicate mixture before the hydrothermal treatment. The chemical composition of the reactant mixture was adjusted to 2.5SiO2/0.25Al2O3/2.5TMA2O/ 0.5LiCl/150H2O. Aluminum hydroxide was dissolved in an aqueous TMAOH solution, and then lithium chloride was added. After the mixture was homogenized, fumed silica was added to the mixture and homogenized by stirring for 2 h; the UZM-4 seed crystals (5 wt. % relative to the added fumed silica) was added. The mixture was transferred into a 23 mL Teflon-lined autoclave and subjected to a hydrothermal treatment at 393 K for 24 h under rotating at 20 rpm. The samples were collected by centrifugation, washed with distilled water, and dried in an oven at 333 K. Characterization. Powder X-ray diffraction (XRD) patterns were collected on an M03X-HF (Bruker AXS) instrument using CuKα radiation (40 kV, 30 mA) at a scanning rate of 4 deg/min between 5° and 45° (2θ). The crystallinity of the solid product was calculated by comparing the intensity of the diffraction peak at 2θ = 7.7° (hkl = 100) to that of the fully crystallized UZM-4 zeolite. Liquid-state 29silicon and 27aluminum NMR spectra were obtained on an EX270 spectrometer (JEOL). Elemental analyses of the products were performed using an inductively coupled plasma−atomic emission spectrometer iCAP-6300 (ICP-AES, Thermo) after dissolution in a potassium hydroxide solution. The crystal size and morphology of the products were observed with a field emission scanning electron microscope S-4800 (FE-SEM, Hitachi). Thermogravimetric (TG) analyses were performed on a Thermo plus (Rigaku) with a heating rate of 5 K/min using a mixture of 10% O2 and 90% He as the carrier gas.

formation of specific aluminosilicate species proceeds during the aging process.7,8,25,26 The CDM approach enables cooperation of multiple SDAs by using aluminosilicate solutions so that all of the templates have access to all of the aluminosilicate species. The UZM-4 synthesis is a two-step process in which the first step is to make the tetraethylammonium (TEA) aluminosilicate solution, with TEA functioning as the CDM template.12 The CDM template is used in the hydroxide form to make the aluminosilicate solution and hinders crystallization.16 If a solid silica source is used in the synthesis, an aging process is required to form the CDM aluminosilicate solution.12 After the CDM solution was prepared, crystallization is induced via the addition of other SDAs, in this case, small amounts of tetramethylammonium (TMA) and lithium cations.12,16 The SDAs added into the CDM solution (before the hydrothermal treatment) are designated as “crystallization-inducing templates (CTs)”.15,16 Although the new BPH-type zeolite was synthesized using simple and conventional SDAs by the CDM approach,12 the fact that only Li and TMA are incorporated in the final UZM-4 product raises the question of whether the TEA aluminosilicate solution is necessary or not. This is quite a characteristic feature of UZM-4. It is worth elucidating the nature of the aging process to prepare the TEA aluminosilicate solution (CDM solution) and its role in the formation of UZM-4. Moreover, it is important to simplify the UZM-4 synthesis toward an economically and environmentally friendly single-step reaction by either reducing or eliminating the aging step that gives the TEA aluminosilicate solution. In this study, we investigate the aging process of UZM-4 synthesis for the preparation of CDM solution and confirmed that TEA aluminosilicate solution is a critical requirement in the synthesis of UZM-4. We focus on the aluminosilicate precursor that is speculated to be formed during the aging process to prepare the CDM solution. The formation of aluminosilicate species became clear from the characterization of the liquid and solid phases of the reactant mixture. In the CDM solution, the stabilized small aluminosilicate species for the subsequent nucleation of UZM-4 crystals are formed in the liquid phase. On the basis of the results, we demonstrate an efficient, single-step synthesis of UZM-4 crystals by seedassisted approach for the first time, which totally eliminated the both CDM template and aging process to prepare CDM aluminosilicate solution.

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RESULTS AND DISCUSSIONS Effect of Aging Time and Aging Agents. Table 1 shows the products synthesized by hydrothermal treatment of the Table 1. State of the Reaction Mixture after Various Aging Time Using Homologous Tetraalkylammonium Hydroxides and Phases of the Obtained Productsa

EXPERIMENTAL SECTION

aging agent

Synthesis of UZM-4. The UZM-4 zeolite was synthesized by the hydrothermal treatment of an aged reactant mixture on the basis of a previous report.12 To avoid unintended impurities, aluminum hydroxide and fumed silica were used as aluminum and silicon sources instead of the original aluminum-sec-butoxide and colloidal silica, respectively.12 Aluminum hydroxide (Wako) was dissolved in a solution of one of homologous TAA hydroxides, which are tetramethylammonium hydroxide (TMAOH, Wako), tetraethylammonium hydroxide (TEAOH, Aldrich), tetrapropylammonium hydroxide (TPAOH, Aldrich), and tetrabutylammonium hydroxide (TBAOH, Aldrich) followed by the addition of fumed silica (Cab-O-sil M5, Cabot). The chemical composition was fixed at 2X2O/2.5SiO2/ 0.5Al2O3/90H2O (X represents the quaternary ammonium cation). After the mixture was homogenized, it was aged at 373 K in an oven for a specific period of time, between 0 and 24 h, under static conditions. Tetramethylammonium chloride (TMACl, Aldrich) was dissolved in a lithium chloride (LiCl, Wako) solution to obtain a CTs solution with a chemical composition TMACl/0.5LiCl/10H2O. This CTs

aging time (h)

TMAOH

TEAOH

TPAOH

TBAOH

1 3 6 12 24

H/Am H/Li-ABW H/Li-ABW H/Li-ABW H/Li-ABW

H/Am H/Am H/Li-ABW S/UZM-4 S/UZM-4

H/Am H/Am S/UZM-4 S/UZM-4 S/UZM-4

H/Am + D H/Am S/UZM-4 S/UZM-4 S/UZM-4

a

H: Heterogeneous. S: Solution. Am: Amorphous phase. D: Dense phase.

reactant mixtures for 72 h after they were aged with one of homologous TAA hydroxides as a CDM template. For TMAOH, the reactant mixture was heterogeneous and did not become a solution after any of the aging times and no UZM-4 crystal but ABW-type zeolite, which is rich in Li, was obtained. Using the larger TAA hydroxides (TEAOH, TPAOH, and TBAOH), UZM-4 crystals were obtained after the specific 2604

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mixture after aged with TEAOH for 24 h. After 3 h of the hydrothermal treatment, hexagonal crystals, 50−200 nm in size, were observed. The crystals grew larger, and, after 6 h of the hydrothermal treatment, crystals, 200−400 nm in size, were observed. Other products, such as amorphous matter, were hardly observed by SEM measurements. These results corresponded well with the changes in the crystallinity and the yield shown in Figure 1. The rapid crystallization was due to the spontaneous nucleation of UZM-4 crystals, and the gradual increase of the yield was because of the growth of highly crystalline UZM-4 crystals. Aging the reactant mixture containing the CDM template long enough to form a solution is essential for the synthesis of UZM-4 crystals (Table 1). Moreover, the reactant mixture did not become a solution and UZM-4 crystals were not obtained from the mixture that initially contains both the CDM template and CTs even if it was aged for 24 h (data not shown). Thus, the controlled step-by-step addition of SDAs (CDM template and CTs) before and after the formation of CDM solution, which contains soluble aluminosilicate species, was shown to be essential for the successful synthesis of UZM-4 crystal. In this study, to understand the UZM-4 aging process more deeply, the formation and reaction of aluminosilicate species during the aging process are investigated in the following section. Details of the Aging Process. Changes in the appearance of the reactant mixture during the aging process could be observed with the naked eye. The initially heterogeneous reactant mixture gradually changed morphology, turning into a solution during the aging process (Table 1). The solid phase of the reactant mixture was gradually dissolved into the liquid phase. The changes in the appearance of the reactant mixture reflected the progress of the reaction during the aging process. The final form of the reactant mixture aged for 24 h was an apparently clear solution, but the Tyndall effect was detected with laser light, which indicated that colloidal particulates were dispersed in the solution. On the other hand, a previous study showed that a well-aged solution was free of the Tyndall effect.15 This difference would be caused by the different silicon sources used, fumed silica in this study and colloidal silica in the previous study. Moreover, if the mixing was poor or the addition of CT’s was too fast, it can cause the formation of other gel-like precipitates (data not shown). As the CTs have higher charge densities, the interaction with aluminosilicate species are stronger than with the CDM templates. The reactant mixture was homogenized before it was finally subjected to the hydrothermal treatment. Therefore, both

aging times at which the state of the reactant mixture changed from gel to solution. This is the first report for using TBAOH as a CDM template in the synthesis of UZM-4.13,15 The minimum aging time for the UZM-4 synthesis was dependent on the CDM templates. Amorphous or other phases were generated when the aging was not sufficient to form a CDM solution. It should be noted that, any CDM templates were rarely contained in the obtained UZM-4 crystals, which was confirmed by TG-DTA and elemental analysis in this study (data not shown, unit cell composition of UZM-4: Li6.1TMA3.3TEA0.1Al10Si18O56).12 The TEAOH is employed as the representative CDM template hereafter. Figure 1 is the crystallization curve, and the

Figure 1. Crystallization curve and yields of UZM-4 crystals synthesized from the reactant mixture aged with TEAOH for 24 h.

yield of UZM-4 crystals synthesized from the reactant mixture aged for 24 h is also shown. The solid yield of the obtained zeolite was defined as the weight ratio percentage (g/g × 100) of the calcined solid product to the sum of the dry SiO2 and Al(OH)3 in the starting aluminosilicate mixture. Crystallization of UZM-4 rapidly proceeded after 2 h of hydrothermal treatment, and the crystallinity reached to 100% in another few hours of the hydrothermal treatment. On the other hand, the yield of UZM-4 crystals increased gradually, and finally reached to ∼26% after 9 h of the hydrothermal treatment. Solid products were not recovered before 2 h of the hydrothermal treatment. A few hours of the induction time before the rapid crystallization would be mainly caused by the heating time lag of the reactant mixture in the autoclave reaching to the preset oven temperature. In other words, crystallization would start as soon as the reactant mixture reaches the preset crystallization temperature. Other CDM templates showed the similar crystallization curves, with a rapid increase after a few hours of an induction time (data not shown). Figure 2 shows SEM images of UZM-4 crystals synthesized by the different times of hydrothermal treatment of the reactant

Figure 2. SEM images of UZM-4 crystals synthesized for different hydrothermal treatment time from the reactant mixture aged with TEAOH for 24 h, corresponding to Figure 1. 2605

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species appeared around −75, −80, and −90 ppm, respectively (Figure 3a).27 In particular, characteristic strong signals for the Q2 and Q3 states are due to small clusters, a three-membered ring (3R) and a double three-membered ring (D3R) at ca. −81 ppm and ca. −88 ppm, respectively (Figure 3a).27 Additional small signals near these characteristic signals indicated that Al atoms were incorporated into the clusters.27 When the aging time was long enough (more than 12 h), these species were still observed even after the addition of CTs, and an additional characteristic signal originating from a double four-membered ring (D4R) appeared at ca. −99 ppm (Figure 3b). The corresponding 27Al NMR spectra are shown in Figure 3c and 3d. Initially, aluminum species existed in the liquid phase as tetrahydroxoaluminate ions, and the corresponding signal was observed around 80 ppm (Figure 3c).29−31 As aging proceeded, NMR signals shifted to 73 and 69 ppm, corresponding to a change in the aluminum state to q1 ((SiO)Al(O−)3) and q2 ((SiO)2Al(O−)2) states, respectively.29−31 These q1 and q2 signals were observed when the aging time was longer than 12 h, and the aluminosilicate species were maintained after the addition of CTs (Figure 3d), which was in a good agreement with 29Si NMR results (Figure 3b). These NMR results indicate the dissolution of silica particulates and the formation of silicate/aluminosilicate species in the liquid phase under the aging process, which are in good agreement with the change in appearance of the reactant mixture as stated above. It is well-known that the characteristic small silicate/aluminosilicate clusters, D3R and D4R, are stabilized by TAA cations.28,29 However, we are not going to emphasize the existence of the specified clusters before the crystallization of UZM-4, because the NMR spectra were obtained under the condition being different from the hydrothermal treatment. Instead, we simply note that the NMR results showed the formation of small silicate/ aluminosilicate species during the aging process. Moreover, UZM-4 crystals were obtained by the hydrothermal treatment of reactant gel aged for at least 12 h (Table 1, TEAOH). And, when the reactant gel was aged for at least 12 h, the small silicate/aluminosilicate species were maintained after the addition of CTs (Figures 3b and 3d). Therefore, the formation of small silicate/aluminosilicate species is essential for the UZM-4 nucleation. The amount of the solid phase decreased with the progress of the aging process. When the aging process was less than 12 h and not enough to prepare the CDM solution, the silica source was still dissolving into the liquid phase. The Si/Al ratio in the solution was low and excess Al was present as Al(OH)4−. The addition of CTs forced the precipitation of the low Si/Al ratio aluminosilicate species and these never dissolve into a clear solution and crystallize into UZM-4. The solid phases, including both undissolved silica and precipitate formed by the addition of CTs, were characterized for further understanding of the aging process. Figure 4 shows the amount of precipitated solid phase collected by centrifugation after the aging process (Figure 4a) and after the addition of CTs (Figure 4b). The precipitates contained both organics and aluminosilicate species. The total amount of the as-obtained precipitates is shown as closed circles, and the amount of aluminosilicates, calculated from measurements of weight loss after calcination, is shown as open circles (Figure 4). The amounts are shown relative to the total amount of added silica and aluminum hydroxide. In the early stages of the aging process (less than 4 h), the total amount of

CDM template and CTs would form the precursor aluminosilicate species for UZM-4 crystals. After the aging process or the addition of CTs, the liquid phase of the reactant mixture was separated from the solid phase by centrifugation and characterized using liquid-state 29Si and 27Al NMR. The sample prepared with fumed silica (Cab-Osil, M5) was not suitable for 29Si NMR measurements because no signal was detected. To obtain useful data, another raw material, colloidal silica (Ludox, AS-30), was used only for this measurement, and this gave clear NMR signals. It should be noted that the crystallization behavior was almost the same for both of the silicates, evaluated by the crystallization curve and changes in appearance of the reactant mixture during the aging process (data not shown). Therefore, the results can be used for understanding the UZM-4 aging process. Figure 3 shows the 29Si (Figure 3a and 3b) and 27Al (Figure 3c and 3d) NMR spectra of the liquid phases of the reactant

Figure 3. 29Si NMR spectra of the sample liquid phase collected (a) after the aging process for specific period of time and (b) after the addition of CTs into the reactant mixture aged for specific period of time. Corresponding 29Al NMR spectra of the sample liquid phase collected (c) after aging process and (d) after the addition of CTs.

mixtures after the aging process (Figure 3a and 3c) and after the addition of CTs (Figure 3b and 3b). At the early stage of aging process, less than 3 h, the 29Si NMR spectra showed a broad signal for Q4 ((SiO)4Si) species around −110 ppm (Figure 3a).27 As aging proceeded, characteristic signals for Q1 ((SiO)Si(O−)3), Q2 ((SiO)2Si(O−)2, (SiO)2Si(O−)(OH) or (SiO)2Si(OH)2), and Q3 ((SiO)3Si(O−) or (SiO)3Si(OH)) 2606

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Figure 4. Amount of precipitated solid phase collected (a) after the aging process for specific period of time and (b) after the addition of CTs into the reactant mixture aged for specific period of time. The amounts are shown relative to the total amount of added silica and aluminum hydroxide. Closed circles show the total amount of precipitate (including aluminosilicates and organics), and open circles show the amount of aluminosilicates (calculated from the weight loss after calcination).

precipitate was 70% (Figure 4a, closed circles), and the amount of aluminosilicates was 55−60% (Figure 4a, open circles). After 4 h, the amount of precipitate gradually started to decease, and finally almost no solid phase was collected by centrifugation after the aging process for 24 h (Figure 4a). Addition of CTs caused increases in the total amount of precipitate and the amount of aluminosilicates (Figure 4b). In particular, in the early stages of aging (less than 4 h), the addition of CTs caused the total amount of isolated precipitate to exceed 100%. The increased amounts of precipitates were correlated well with the amounts of initial precipitates. Therefore, almost no solid was collected by the addition of CTs into the reactant mixture aged for 24 h (Figure 4b). The increase of the amount of precipitates by the addition of CTs indicated the interaction between CTs and aluminosilicate species in liquid phase. If aging time was not long enough (less than 4 h), larger amounts of organics and aluminosilicates were precipitated by the addition of CTs (Figure 4b). In the early stages of aging, there is an excess of Al over Si in the solution phase forming very highly charged aluminosilicate species (Si/ Al = 1). These species are very sensitive to the highly charged CT’s and are readily precipitated from solution, resulting in the observed increases. As the aging continues, more Si dissolves and associates with the Al, leading to lower charge density aluuminosilicates which are not as susceptible to precipitation by the CTs. This is the reason why aging process to prepare CDM solution was required in our study. After a 6 h aging process, the presence of small aluminosilicate species in the liquid phase was confirmed by NMR measurement (Figure 3), and the increased amount of precipitates were less significant (Figure 4). Therefore, it was considered that these small aluminosilicate species generated during the aging process would be stabilized against the addition of CTs. The chemical composition of the aluminosilicate precipitates corresponding to Figure 4 was confirmed by ICP measurements. Figure 5 shows the Si/Al ratios of the precipitates collected after the aging process (Figure 5a) and after the addition of CTs (Figure 5b). It should be noted that the initial Si/Al ratio in the reactant mixture was 2.5. After the aging process, regardless of the aging time, the precipitates Si/Al ratios were almost constant at about 8 (Figure 5a). It is clear that the precipitate included less aluminum than expected from the initial chemical composition. After the addition of CTs, the precipitate Si/Al ratios decreased. Especially, when the aging was less than 4 h, the precipitate Si/Al ratio was almost the same as the initial ratio in the reactant mixture, about 2.4. After aging for 6 h, the precipitate Si/Al ratio did not show such a significant decrease after the addition of CTs, decreasing to

Figure 5. Silicon to aluminum (Si/Al) ratios of the solid phase collected (a) after the aging process for specific period of time and (b) after the addition of CTs into the reactant mixture aged for specific period of time.

6−7. Therefore, when the aging process was not sufficient, low Si/Al ratio aluminosilicate species were precipitated by the addition of CTs. This result supports the discussion in Figure 4. Figure 6 shows a schematic illustration of the aging process of UZM-4 based on the above results and discussion. Initially, the aluminum species were in the liquid phase as tetrahydroxoaluminate ions, and the silicon species were in the solid phase as colloidal silica (Figure 6a). During the aging process with the CDM template, silica particulates gradually dissolved into the liquid phase and reacted with tetrahydroxoaluminate ions to form stabilized small silicate/aluminosilicate species (Figure 6b). If the aging process was insufficient, aluminosilicate species were not stabilized, and large amount of precipitates including organics and low Si/Al ratio aluminosilicate species were formed by the addition of CTs (Figure 6c). After the hydrothermal treatment at 393 K, no crystalline phase was obtained from these precipitated aluminosilicate species. It is therefore reasonably concluded that the small aluminosilicate species have important roles in the nucleation of UZM-4 crystals. Moreover, as shown in Table 1, aging for at least 6 h was necessary to obtain zeolitic materials, and this corresponded well with the formation of stabilized small aluminosilicate species as shown in Figures 3−5. Redissolution of the precipitate into the liquid phase could hardly proceed because the low Si/Al ratio aluminosilicate are very insoluble in the presence of the CTs, and therefore zeolitic materials were not obtained with the aging process less than 6 h. However, when aging was long enough to form and stabilize small aluminosilicate species in the liquid phase, as required in traditional CDM preparations, no precipitates were formed after the addition of CTs (Figure 6b′), and UZM-4 was successfully synthesized by hydrothermal treatment. We therefore speculate that the nucleation of UZM-4 is originated 2607

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Figure 6. Schematic illustration of the crystallization scheme of UZM-4.

from the small aluminosilicate species. In addition, it is speculated that the CDM templates do not function as the conventional “structure-directing agents (SDA)” in the synthesis of the UZM-4 zeolite, because they are not incorporated into the zeolite. The CDM templates worked to form the small soluble precursors of the UZM-4 nucleation and hinder crystallization so that Li and TMA could cooperate to make UZM-4 rather than the Li-ABW they made by themselves (Table 1). Seed-Assisted Single-Step Synthesis of UZM-4 without an Aging Process. The final UZM-4 structure was directed by two SDAs: TMA+ and Li+. So we considered that we might synthesize UZM-4 with only TMA and Li cations when the nuclei of the UZM-4 crystals were previously provided. In other words, the seed-assisted synthesis of the UZM-4 without aging process or addition of a CDM template would be achieved. Recently, several important zeolites have been synthesized by the seed-assisted, organic SDA-free approach.32−34 These are highly economical process because of the elimination of the expensive organic SDA that was essential for the specific zeolite synthesis. After adjusting the gel composition, we achieved, for the first time, the single-step seed-assisted synthesis of UZM-4 using just TMAOH, LiCl, and silicon and aluminum sources, in addition to UZM-4 seeds. It is important to note that we not only simplified the synthesis process of UZM-4 but also eliminated the most expensive reagent, CDM template, which is not incorporated into the final product but needed for the preparation of CDM solution. Figures 7 and 8 show the XRD patterns and SEM images, respectively, of the sample synthesized by the seed-assisted approach. High quality UZM-4 crystals were obtained by the seed-assisted approach. It should be noted that, without adding the seed, only amorphous matter was obtained from the reactant gel with the same hydrothermal reaction condition. The hexagonal UZM-4 seed crystals grow from 100−300 to 300−600 nm in size (Figure 8), showing the success of the seed-assisted approach. Approximately 0.26 g of the assynthesized product was obtained from the reactant mixture

Figure 7. XRD patterns of (a) the obtained product synthesized using a seed-assisted approach and (b) seed crystals.

containing 0.02 g of seed crystals. The Si/Al ratio of the product obtained (Si/Al = 1.9) was slightly higher than that of the seed crystals (1.6) because the Si/Al ratio of the initial seedassisted synthesis reactant mixture (5.0) was higher than that of the conventional CDM reactant mixture (2.5). Tuning the chemical composition of the reactant mixture was important for the seed-assisted approach.34 Moreover, the higher Si/Al ratio of the reaction mixture was more suitable for the preparation of soluble aluminosilicate species. On the basis of the above discussions that the crystallization of UZM-4 required soluble aluminosilicate species, we speculated that the precursor for the seed-assisted growth would be also supplied from solution phase. Using a seed-assisted approach, it would also be possible to obtain BPH-type zeolites with different Si/Al ratios.

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CONCLUSIONS The role of the aging process in the synthesis of the UZM-4 zeolite using typical CDM compositions was investigated to see if it is necessary to start with aluminosilicate solutions as is done in the CDM approach. It was confirmed that the formation of stabilized small aluminosilicates in the liquid phase during the aging process was the key for the preparation of active CDM solutions and the ensuing UZM-4 zeolite nucleation, for which the crystallization scheme is proposed. On the basis of this investigation, the efficient, single-step, seed-assisted synthesis of 2608

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(2) Cundy, C. S.; Cox, P. A. Chem. Rev. 2003, 103, 663. (3) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 756. (4) Wang, Z.; Yu, J.; Xu, R. Chem. Soc. Rev. 2012, 41, 1729. (5) Ng, E.-P.; Chateigner, D.; Bein, T.; Valtchev, V.; Mintova, S. Science 2012, 70, 335. (6) Itani, L.; Bozhilov, K. N.; Clet, G.; Delmotte, L.; Valtchev, V. Chem.Eur. J. 2011, 17, 2199. (7) Suzuki, Y.; Wakihara, T.; Itabashi, K.; Ogura, M.; Okubo, T. Top. Catal. 2009, 52, 67. (8) Isobe, M.; Takahiko, M.; Tanahashi, S.; Kimura, R.; Kamimura, Y.; Itabashi, K.; Okubo, T. Microporous Mesoporous Mater. 2012, 158, 204. (9) Corma, A.; Rey, F.; Rius, J.; Sabater, M. J.; Valencia, S. Nature 2004, 31, 287. (10) Lee, H.; Zones, S. I.; Davis, M. E. Nature 2003, 425, 385. (11) Marcus, B. K.; Lok, B. M. U. S. Patent 4,857,288, 1989. (12) Blackwell, C. S.; Broach, R. W.; Gatter, M. G.; Holmgren, J. S.; Jan, D.-Y.; Lewis, G. J.; Mezza, B. J.; Mezza, T. M.; Miller, M. A.; Moscoso, J. G.; Patton, R. L.; Rohde, L. M.; Schoonover, M. W.; Sinkler, W.; Wilson, B. A.; Wilson, S. T. Angew. Chem., Int. Ed. 2003, 42, 1737. (13) Lewis, G. J.; Moscoso, J. G.; Miller M. A.; Wilson, B. A. U.S. Patent 6,419,895, 2002. (14) Lewis, G. J.; Jan, D. Y.; Mezza, B. J.; Moscoso, J. G.; Miller, M. A.; Wilson, B. A.; Wilson, S. T. Stud. Surf. Sci. Catal. 2004, 154, 118. (15) Lewis, G. J.; Miller, M. A.; Moscoso, J. G.; Wilson, B. A.; Knight, L. M.; Wilson, S. T. Stud. Surf. Sci. Catal. 2004, 154, 364. (16) Lewis, G. J.; Miller, M. A.; Moscoso, J. G.; Wilson, B. A. U.S. Patent 7,578,993, 2009. (17) Park, M. B.; Cho, S. J.; Hong, S. B. J. Am. Chem. Soc. 2011, 133, 1917. (18) Miller, M. A.; Lewis, G. J.; Moscoso, J. G.; Koster, S. C.; Modica, F.; Gatter, M. G.; Nemeth, L. T. Stud. Surf. Sci. Catal. 2007, 170, 487. (19) Miller, M. A.; Moscoso, J. G.; Koster, S. C.; Gatter, M. G.; Lewis, G. J. Stud. Surf. Sci. Catal. 2007, 170, 347. (20) Kim, S. H.; Park, M. B.; Min, H.-K.; Hong, S. B. Microporous Mesoporous Mater. 2009, 123, 160. (21) Lee, J. H.; Park, M. B.; Lee, J. K.; Min, H.-L.; Song, M. K.; Hong, S. B. J. Am. Chem. Soc. 2010, 132, 12971. (22) Park, M. B.; Lee, Y.; Zheng, A.; Xiao, F.-S.; Nicholas, C. P.; Lewis, G. J.; Hong, S. B. J. Am. Chem. Soc. 2013, 135, 2248. (23) Breck, D. W.; Acara, N. A. U.S. Patent 2991151, 1961. (24) Andries, K. J.; De Wit, B.; Grobet, P. J.; Bosmans, H. J. Zeolites 1991, 11, 116. (25) Ogura, M.; Kawazu, Y.; Takahashi, H.; Okubo, T. Chem. Mater. 2003, 15, 2661. (26) Iwama, M.; Suzuki, Y.; Plvert, J.; Itabashi, K.; Ogura, M.; Okubo, T. Cryst. Growth. Des. 2010, 10, 3471. (27) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; John Wiley & Sons: New York, 1987. (28) Knight, C. T. J. Zeolites 1989, 9, 448. (29) Mortlock, R.-F.; Bell, A.-T.; Radke, C.-J. J. Phys. Chem. 1991, 95, 7847. (30) Mortlock, R.-F.; Bell, A.-T.; Radke, C.-J. J. Phys. Chem. 1991, 95, 372. (31) Muller, D.; Gessner, W.; Samoson, A.; Lippmaa, E.; Scheler, G. J. Chem. Soc., Dalton Trans. 1986, 1277. (32) Majano, G.; Delmotte, L.; Valtchev, V.; Mintova, S. Chem. Mater. 2009, 21, 4181. (33) Xie, B.; Zhang, H. Y.; Yang, C. G.; Liu, S. Y.; Ren, L. M.; Zhang, L.; Meng, X. J.; Yilmaz, B.; Muller, U.; Xiao, F. S. Chem. Commun. 2011, 47, 3945. (34) Itabashi, K.; Kamimura, Y.; Iyoki, K.; Shimojima, A.; Okubo, T. J. Am. Chem. Soc. 2012, 134, 11542.

Figure 8. SEM images of (a) the obtained product synthesized using a seed-assisted approach and (b) seed crystals.

the UZM-4 zeolite, eliminating the need for the CDM template and an aging process, was achieved for the first time by tuning the chemical composition of the reactant mixture.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

Takahiko Moteki: Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, U.S.A. E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported in part by Tosoh Co. Ltd., and a Grant-in-Aid for Young Scientists from the Japan Society for the Promotion of Science (JSPS). T.M. is a JSPS research fellow and is grateful to JSPS for Research Fellowships for Young Scientists. The authors thank Dr. Keiji Itabashi (The University of Tokyo) for helpful discussions and the comment on this work. Also the authors are sincerely grateful to the reviewer of this manuscript for many useful comments and advices to further strengthen this manuscript.

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REFERENCES

(1) Wang, Z.; Yu, J.; Xu, R. Chem. Soc. Rev. 2012, 41, 1729. 2609

dx.doi.org/10.1021/cm400727r | Chem. Mater. 2013, 25, 2603−2609