Article pubs.acs.org/cm
A General Method for Aluminum Incorporation into High-Silica Zeolites Prepared in Fluoride Media Takahiko Moteki† and Raul F. Lobo* Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, Delaware 19716, United States S Supporting Information *
ABSTRACT: The fluoride method of zeolite synthesis yields materials with unique characteristics such as high Si/Al ratio, large crystal size, and hydrophobic properties, and it has been advantageous for the synthesis of new pure-silica or high-silica zeolites. It is often difficult, however, to incorporate aluminumand thus bring about useful catalytic propertiesin materials prepared through the fluorite method. In this report, we show that FAU-type zeolites are an effective source of aluminum to the growing crystals in the high-silica LTA-type zeolite synthesis (ITQ-29). A key advantage of using aluminosilicate zeolite crystals as aluminum source in fluoride media was the high reproducibility and easy control of the Si/Al ratio of the product. The broad applicability of this methodology was demonstrated in the synthesis of several high-silica zeolites in fluoride media: we report the synthesis of aluminosilicate ITQ-12 (ITW-type zeolite). Other more conventional aluminosilicate zeolites (CHA-, *BEA-, and STT-type) were also synthesized by using this methodology. The Si/Al ratio of the final products was controlled by the amount of aluminosilicate zeolite added to the synthesis gel. All the products obtained had the typical features of a fluoride mediated synthesis, and it was found that the thermochemical stability of the aluminum source and seed crystals was an important factor. This simple methodology could be useful for aluminum incorporation into many novel siliceous zeolites, broadening their potential as catalytic materials.
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final product of these fluoride syntheses would be very helpful to increase their compositional range and their potential for applications. The first pure-silica and high-silica (Si/Al ratio of 47) LTAtype zeolites (ITQ-29) were synthesized in fluoride media.3 However, there is a difficulty in the reproducible synthesis of high-silica ITQ-29. In the synthesis of ITQ-29, a self-assembled organic SDA (structure-directing agent) dimer is occluded in the α-cages; a second SDA, tetramethylammonium (TMA) cation, is occluded in the small cages (sodalite- or β-cage).3 Because two different organic SDAs are involved in the synthesis, the product formed is sensitive to impurities in the SDA and/or minor changes in the synthesis conditions. Although several reports have attempted to develop reproducible syntheses, only a handful have succeeded in the synthesis of pure-silica or aluminosilicate ITQ-29.3,14−18 For example, ultra-high-silica ITQ-29 (Si/Al ratio larger than 110) was obtained by adding aluminosilicate seed crystals in the siliceous reaction gel.14 The reproducible synthesis of pure-silica ITQ-29 has been reported using the crown ether “Kryptofix 222” as an organic SDA.15 Very recently, another organic SDA has developed, with which aluminosilicate LTA-type zeolite with a Si/Al ratio of 12−42 was obtained.19 However, the successful synthesis of aluminosilicate LTA-type zeolite is still a challenge. In preliminary experiments, we succeeded in preparing LTA-
INTRODUCTION Zeolites, crystalline porous tectosilicates, have attracted much interest in academia and industry due to properties arising from their nanoscale and uniform pore structures. Size and shape selectivity in catalysts and separation applications are two of representative and valuable properties of zeolites.1,2 As of this writing, 218 framework types have been catalogued by the International Zeolite Association. Aluminosilicate zeolites have been classified according to the aluminum content from lowsilica to high-silica zeolites, and in the limit, there are zeolites that can be prepared in their “pure-silica” form. In fact, there are a number of zeolite structures that, at this point, have only been prepared in their pure-silica form. One important synthetic challenge is to incorporate catalytically active sites, such as aluminum atoms, into these zeolite frameworks. Use of fluoride anion as mineralizer was an important breakthrough in zeolite synthesis and an effective approach to the synthesis of novel zeolites.3−10 The zeolite materials obtained have a unique set of properties such as a very high Si/Al ratio, larger crystal size, and less framework defects.4,5 These properties originate from the presence of fluoride anion in the reaction gel and the final product.4−6 However, new zeolites synthesized in fluoride media can often be only prepared in their pure-silica form; that is, they are difficult to obtain as aluminosilicates,7−10 reducing their potential as catalysts. There are a few exceptions in the synthesis of IFR-,11 *BEA-,12 and AST-type zeolites,13 which achieved the low Si/Al ratio of 20, 7, and 17, respectively. For these reasons, an effective method for the introduction of aluminum into the © XXXX American Chemical Society
Received: November 17, 2015 Revised: December 14, 2015
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DOI: 10.1021/acs.chemmater.5b04439 Chem. Mater. XXXX, XXX, XXX−XXX
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enhances the potential for a successful use of zeolites as aluminum source.
type zeolites from a siliceous reaction gel containing seed crystals, as was also found in a previous report.14 On the other hand, we were unable to prepare aluminosilicate ITQ-29 from the reaction gel even containing seed crystals (Figure S1, Supporting Information). The selection of the aluminum source is an important factor affecting the synthesis and properties of many zeolite products. Among various kinds of aluminum sources utilized in the zeolite synthesis, aluminosilicate zeolites, especially the FAUtype zeolites, have been used as a source of aluminum in some cases.20−23 This approach has often been used in the hydroxide mediated synthesis of zeolites, but there are very few examples in the fluoride mediated synthesis. In the case of ITQ-29, aluminum isopropoxide, a common aluminum source, was used in the original study.3 Later on, nanosized aluminosilicate LTAtype zeolite crystals were used as seeds that were, at the same time, the source of aluminum of the final crystals.14 It is conceivable then that low silica zeolites, such as faujasite, could be used as aluminum source in the synthesis of high-silica LTAtype and other zeolites in fluoride media. In this report, high-silica LTA-type zeolites with a Si/Al ratio of 63−420 have been successfully and reproducibly synthesized in fluoride media using commercial faujasite crystals as aluminum source. The lowest Si/Al of 63 was synthesized with the addition of 10 wt % of faujasite crystals. The aluminosilicate LTA-type zeolite products possessed nearly defect-free structures, a characteristic often seen in fluoride mediated synthesis as well. It was demonstrated that faujasite crystals operate as aluminum source even in the highly condensed fluoride mediated reaction gels (H2O/SiO2 ∼ 2). To enhance the crystallization, small amounts of quasi-siliceous seed crystalscontaining a very small amount of germanium were added to the reaction gel. The unit cell volumes of the high-silica LTA-type zeolites correspond to the amount of Al present in the framework. The crystallization process was followed by a number of characterization techniques, and it was shown that faujasite crystals were dissolved at the early stages of the hydrothermal treatment, and prior to the crystallization of LTA-type zeolites. This methodology was also applicable to other zeolite synthesis in fluoride media: we have demonstrated the synthesis of aluminosilicate ITW-type zeolite. ITW-type zeolite has 2dimensional small pore channels, which has shown good hydrocarbon separation properties.7 However, only pure-silica form zeolite (ITQ-12) synthesized in fluoride media had been known.7,24−26 Very recently, Jo et. al, suggested the synthesis of aluminosilicate ITW-type zeolite, although no synthetic details are described.27 The new aluminosilicate ITW-type zeolite obtained showed a Si/Al ratio of about 70 and a rod-shaped morphology, and the incorporated aluminum atoms were tetrahedrally coordinated. This methodology would be a strong approach to synthesize novel aluminosilicate zeolites. Aluminosilicate zeolites with the CHA-, *BEA-, and STTtype frameworks were also synthesized in fluoride media using zeolite crystals as aluminum source, showing the wide applicability of the methodology. The Si/Al ratio of the products was simply controlled by the amount of added aluminosilicate zeolite crystals. High-silica CHA- and *BEAtype zeolites were synthesized with faujasite as aluminum source, while high-silica STT-type zeolites were only prepared with mordenite as aluminum source. It seems that a similarity in structure and/or chemical composition with target zeolite
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EXPERIMENTAL SECTION
Synthesis of the Organic SDA for LTA-Type Zeolite. 4Methyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3.2.1-ij] quinolinium (methylated-julolidine) hydroxide was used as the SDA for the synthesis of seed crystals and high-silica LTA-type zeolites. The synthesis procedure was described previously.3 Typically, 10 g of julolidine (Alfa Aesar) was dissolved in 100 mL of chloroform (Fischer Sci.). Next, 23 g of methyl iodide (Alfa Aesar) was added to the solution, and the reaction mixture was stirred at room temperature for 3 days. Then, another 23 g of methyl iodide was added to the solution, and the solution was stirred for 3 more days at room temperature. The same procedure (adding the same amount of methyl iodide and stirring at room temperature for 3 days) was repeated one more time, resulting in the total reaction time for 9 days. The solid was obtained by slowly adding ∼200 mL of diethyl ether (Alfa Aesar) into the solution. The dark orange solid precipitate was filtered and dried in air. Purification was carried out by dissolving the solid product into 100 mL of chloroform again and precipitating by addition of 200 mL of diethyl ether. This purification process was repeated three times, and the product was finally dried at room temperature. Before the zeolite synthesis, the iodide form of the organic SDA was ion-exchanged to the hydroxide form with ion-exchange resin (A-26(OH), Amberlyst). Prior to the addition of the resin, the organic SDA was dissolved in water for 1 h because its solubility is low. About 5 g of resin was used for 80 g of SDA iodide solution (0.08 M). The ion-exchange step was carried out for 12 h at room temperature, and this step was repeated three times to get the conversion of iodide to hydroxide form over 90%. The solution was concentrated up to 0.2 M using a rotary evaporator and titrated with hydrochloric acid to measure the OH− concentration. Synthesis of the Organic SDA for ITW-Type Zeolite. 1,2,3Trimethyl imidazolium hydroxide was used as the organic SDA for the synthesis of seed crystals and high-silica ITW-type zeolites. The synthesis procedure was described in detail previously.24 Typically, 8 g of 1,2-dimethyl imidazole (Aldrich) was dissolved in 100 mL of chloroform. Next, 30 g of methyl iodide was added to the solution, and the reaction mixture was stirred at room temperature for 2 days. A white-orange solid precipitate was filtered, washed with chloroform, and dried at room temperature. Before the zeolite synthesis, the iodide form of the organic SDA was ion-exchanged to the hydroxide form with ion-exchange resin. About 5 g of resin was used for 50 g of SDA iodide solution (0.2 M). The ion-exchange step was carried out for 1 day at room temperature, and this step was repeated three times. It should be noted that the final ion-exchange rate was about 70%; it was not possible to increase the level of ion exchange above this value. The solution was concentrated up to 0.2 M by a rotary evaporator and titrated with hydrochloric acid to measure the OH− concentration. Synthesis of High-Silica LTA-Type Zeolite. High-silica LTAtype zeolites were synthesized by the addition of small amounts of asmade quasi-siliceous seed crystals and as-received NH4-form faujasite crystals (Si/Al = 2.47, CBV500, Zeolyst) to the synthesis gel of puresilica ITQ-29.3 The synthesis procedure of quasi-siliceous seed crystals (Si/Ge = 120) is described in the Supporting Information. Figure 1 illustrates the synthesis protocol. Tetraethylorthosilicate (TEOS,
Figure 1. Schematic illustration of synthesis protocol of high-silica LTA-type zeolites. B
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Chemistry of Materials Table 1. Amount of Faujasite Crystals in the Synthesis of High-Silica LTA-Type Zeolites and Product Properties run
amount of faujasite crystals/wt %
Si/Al ratio in the synthesis gela
product phaseb
Si/Al ratio of productc
unit cell size/Å
unit cell volume/Å3
1 2 3 4 5 6
15 10 5 2 1 0
29 42 81 200 400 ∞
LTA + RUT LTA LTA LTA LTA LTA
63 91 210 420
11.8686(3) 11.867(1) 11.8574(4) 11.855(2) 11.8521(1)
1671.8(1) 1671.3(6) 1667.1(2) 1666.1(8) 1664.9(2)
a Estimated Si/Al ratio of the synthesis gel calculated by the amount of added faujasite crystals. bDetermined by XRD. cConfirmed by ICP measurements.
Table 2. Synthesis Parameters of High-Silica Zeolites and Some Product Properties synthesis conditions
run
SDA
SDA/Si
HF/Si
H2O/Si
aluminum source (Si/Al ratio)
7 8 9 10 11 12 13 14 15
TMAdaOH TMAdaOH TEAOH TEAOH TMAdaOH TMAdaOH TMAdaOH TMAdaOH TMAdaOH
0.5 0.5 0.54 0.54 0.5 0.5 0.5 0.5 0.5
0.5 0.5 0.54 0.54 0.5 0.5 0.5 0.5 0.5
3 3 7.25 7.25 7.5 7.5 7.5 7.5 7.5
faujasite (2.47) faujasite (2.47) faujasite (2.47) faujasite (2.47) mordenite (5) mordenite (5) faujasite (2.47) faujasite (2.47) zeolite A (1)
product properties amount of aluminum source/wt %
Si/Al ratio of the synthesis gela
product phaseb
10 5 10 5 15 10 10 5 5
40 78 40 78 47 68 40 78 47
CHA CHA *BEA *BEA STT STT SST + CHA STT + CHA STT + CHA
Si/Al ratio of the productc
micropore volume/cm3 g−1
55 76 44 65 85 117
0.25 0.26 0.19 0.22 0.17 0.18
a
Estimated Si/Al ratio of the synthesis gel calculated by the amount of added zeolites. bConfirmed by XRD patterns. cConfirmed by EDX measurements. The hydrothermal treatment was carried out at 448 K for 7 days under rotation. The samples were filtered, washed with DI water, and dried in air in an oven at 353 K. The samples were calcined in air to remove the occluded organic SDAs at 823 K for 5 h. Synthesis of the High-Silica CHA-, *BEA-, and STT-Type Zeolites. High-silica CHA- and STT-type zeolites were synthesized with N,N,N-trimethyl-1-adamantanammonium hydroxide (TMAdaOH, 25% aqueous solution, Sachem); high-silica *BEA-type zeolite was synthesized with tetraethylammonium hydroxide (TEAOH, 35% aqueous solution, Sachem) as organic SDA. The as-received Na form of mordenite (Si/Al ratio = 5, TZM 1011, Tricat) and Linde A (Si/Al ratio = 1, zeolite 4A, Aldrich) were ion-exchanged to the NH4 form prior to use as aluminum source. Five grams of the zeolite crystals were stirred in 300 mL of ammonium nitrate solution (0.2 M) at room temperature for 1 day. To prepare the zeolites, first, TEOS was hydrolyzed in a solution of organic SDA hydroxide under stirring at room temperature for 3 h. After hydrolysis of TEOS, a small amount of as-made pure-silica seed crystals (1 wt % of the total silica formed from TEOS) and the NH4-form zeolite crystals (faujasite, mordenite, or Linde-type A; 5−15 wt % of the total silica formed from TEOS) was added to the solution and stirred for 1 h. Then, hydrofluoric acid (HF) was added to the mixture, and the resultant gel was stirred by hand with a spatula. The homogenized gel was placed in an oven at 353 K to adjust the H2O/SiO2 ratio. The final chemical compositions of the gel for CHA-, *BEA-, and STT-type zeolites are summarized in Table 2. The synthesis was carried out with seed and aluminum-source zeolite crystals. The hydrothermal treatment for CHA-type zeolite was carried out at 423 K for 3 days under rotation; for *BEA-type zeolite, it was carried out at 413 K for 3 days under rotation; and for STT-type zeolite, it was carried out at 448 K for 3 days under rotation. The samples were filtered, washed with DI water, and finally dried in air in an oven at 353 K. The samples were calcined in air to remove the occluded organic SDAs at 823 K for 5 h. The samples prepared for this report are listed in Table 2. Characterization. Powder X-ray diffraction (PXRD) patterns were collected on an X’pert powder diffractometer (Philips) using a Cu Kα radiation (45 kV, 40 mA) at a step size of 0.04° and a 1 s per step
Aldrich) was hydrolyzed in an aqueous solution containing both methylated-julolidine hydroxide (ROH) and tetramethylammonium hydroxide (TMAOH, 25% aqueous solution, Alfa Aesar) under stirring at room temperature for 3 h. After the solution became homogeneous, the required amount of faujasite crystals (0−15 wt % of the total silica formed from TEOS) together with the quasi-siliceous seed crystals (5 wt % of the total silica formed from TEOS) was added to the solution and stirred for 1 h. Then, hydrofluoric acid (HF, 48−51% aqueous solution, Acros) was added to the mixture, and the resulting gel was stirred by hand with a spatula. The homogenized gel (in an open container) was placed in an oven at 353 K to adjust the water-to-silica ratio (H2O/SiO2) of 2. The final chemical composition was 1SiO2/ 0.25ROH/0.25TMAOH/0.5HF/2H2O plus the required amount of faujasite crystals and 5 wt % of seed crystals. If the reaction gel was not homogeneous at this stage, the gel was mixed again before transferring into the autoclave. The final synthesis gel was transferred into a 23 mL Teflon-lined autoclave (#4749, Parr) and subjected to a hydrothermal treatment at 408 K for 5 days under rotation. The samples were then filtered, washed with DI water, and dried in air in an oven at 353 K. Prior to characterization, the samples were calcined in air to remove the occluded organic SDAs at 823 K for 5 h with a ramping rate of 2 K/min. A list of the samples synthesized is summarized in Table 1. Synthesis of the High-Silica ITW-Type Zeolite. High-silica ITW-type zeolite was synthesized by adding a small amount of aluminosilicate faujasite crystals into the synthesis procedure of puresilica ITW-type zeolite, ITQ-12.7,24,25 1,2,3-Trimethyl-imidazolium hydroxide was used as the organic SDA. First, TEOS was hydrolyzed in a solution of organic SDA hydroxide under stirring at room temperature for 3 h. After hydrolysis of TEOS, a small amount of asmade pure-silica seed crystals (1 wt % of the total silica formed from TEOS) and the NH4-form faujasite crystals (5 wt % of the total silica formed from TEOS) was added to the solution and stirred for 1 h. Then, hydrofluoric acid was added to the mixture, and the resultant mixture was stirred by hand with a spatula. The homogenized gel was placed in an oven at 353 K to adjust the H2O/SiO2 ratio. The final chemical composition was 1SiO2/0.5TMAdaOH/0.5HF/12H2O plus the required amount of faujasite crystals and 1 wt % of seed crystals. C
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reactive source of T-atoms.38−43 The advantages are (1) a continuous slow feeding of T-atoms into the reaction mixture during the hydrothermal treatment due to their higher stability, and (2) the presence of preformed aluminosilicate networks or small units in the dissolved aluminosilicate species that originated from the mother crystals.20 However, this approach has not been investigated in detail in fluoride mediated synthesis.44 We found that low silica NH4-form faujasite crystals were effective aluminum sources in LTA-type zeolite synthesis without inducing crystal growth of faujasite or other phases. Since an aluminosilicate zeolite is not a common aluminum source in fluoride-mediated synthesis, below we describe salient features of their use. These procedures were used not only for the LTA-type zeolite synthesis but also for other zeolite synthesis. First, to prepare a homogeneous reaction gel, we added the aluminum source zeolite and the seed crystals before the gel became very thick by the addition of hydrofluoric acid. In the case of LTA-type zeolite synthesis, both the final reaction gels and the obtained products were orange in color (to the naked eye) as a result of the concentrated SDA. Second, the Si/ Al ratio of the reaction gel was determined based on the amount of aluminum source zeolite added. The added amount is described in weight % of the total silica formed from TEOS. In the case of LTA-type zeolite synthesis, addition of 1, 2, 5, 10, and 15 wt % of faujasite crystals resulted in gel Si/Al ratios of 400, 200, 81, 42, and 29, respectively (Table 1). Third, the aluminum source zeolite was added in the NH4 form, first, to avoid contamination by alkaline cations and, second, to keep the thermochemical stability low for ease of dissolution. Fourth in the case of LTA-type zeolite synthesis, the quasi-siliceous seed crystals were added without calcination because the organic SDAs should stabilize the framework toward dissolution in the concentrated fluoride media. The seed crystal was synthesized from Si-Ge type ITQ-29 crystals (Supporting Information) and contained a very small amount of germanium (Si/Ge = 120). The XRD pattern and an SEM image of the seed crystals are shown in Figures S3 and S4 (Supporting Information), respectively. The seed crystals were a highly crystalline form of the LTA-type framework, and the particles had cubic morphology with 500−800 nm in size. Figure 2 shows the XRD patterns of the calcined products synthesized with increasing amounts of faujasite crystals (Table
between 5° and 40° (2θ). For the unit cell parameters refinement, the samples were mixed with a silicon standard (10−20 wt %) to correct the peak positions; XRD patterns were collected using a step size of 0.01° and a count time of 3 s per step between 25° and 35° (2θ). Celref software was used to refine the unit cell parameters of highsilica LTA-type zeolite samples.29 The u.c. refinement was carried out using six peaks observed in 2θ from 25° to 35° in the space group Pm3m. Scanning electron microscopy (SEM) images and chemical compositions by energy-dispersive X-ray spectroscopy (EDX) were obtained on a JSM7400F microscope (JEOL) with an accelerating voltage of 3−10 keV. The micropore volume and surface area were measured using N2 at 77 K on a 3Flex surface characterization analyzer (Micrometrics). Solid-state NMR spectra were collected on an Avance III spectrometer (Bruker). UV−vis spectra were collected on a V-550 spectrometer (Jasco) with a diffuse reflectance cell attachment using BaSO4 as a reference. Elemental analyses were conducted using inductively coupled plasma atomic emission spectroscopy (ICP-AES) by Galbraith Laboratories, Tennessee.
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RESULTS AND DISCUSSION Synthesis of High-Silica LTA-Type Zeolites in Fluoride Media. On the basis of our preliminary experiments, the two key requirements for the reproducible synthesis of high-silica LTA-type zeolites in fluoride media were the addition of seed crystals and the use of faujasite crystals as aluminum source. The effect of seeding in the synthesis of the pure-silica sample was investigated first. Seeding is a common method to enhance the crystallization of the target zeolite and has been widely used in the zeolite literature.30−32 Without seed crystals, we obtained a mixture of pure-silica AST and pure-silica LTA phases (Figure S2). The AST phase, which can be synthesized with TMA as an SDA,33 has been often observed as an impurity in the crystallization of pure-silica ITQ-29.14−16,18 The formation of this impurity could be caused by factors such as the presence of impurities in the organic SDA, incompleted ion-exchange of the SDA from I− to OH−, and/or insufficient homogenization of the reaction gel.14,18 As observed previously,14 the addition of seed crystals leads to the reproducible synthesis of the puresilica LTA-type zeolite from siliceous reaction gel. In our case, the addition of only 1 wt % of seed crystals was sufficient and hindered the formation of the AST phase. We conclude that seeding is an effective method for the reproducible LTA-type zeolite synthesis in fluoride media. Second, for the successful synthesis of the aluminosilicate sample, modification with a suitable aluminum source was necessary besides the seeding. Initially, using aluminum isopropoxide as aluminum source, the product was a mixture of LTA, AST, and RUT phases (Figure S1). Note that the seeding was applied in all of these attempts. Although a small amount of LTA phase along with an amorphous phase was observed at the early stage of the hydrothermal treatment (3 days), AST and RUT phases became dominant at the end of the crystallization (8 days). Although the seed crystals were added to the gel, the crystallization of the LTA phase was not promoted sufficiently and other phases were crystallized. A RUT phase can be synthesized with TMA cation as an SDA,34,35 and the presence of these undesired phases could be caused by local inhomogeneities of aluminum, resulting in unintended nucleation of these phases. To provide aluminum into the system more effectively, we utilized zeolites as aluminum source, as this is a well-known protocol in hydroxide media.20−23,36,37 Zeolite crystals can be a unique T-atom source; these materials do not necessarily work as seeds but give different final product compositions by working as a
Figure 2. XRD patterns of obtained samples synthesized with various amounts of faujasite crystals. D
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morphologies of aluminosilicate LTA-type zeolites with different Si/Al ratio were observed, suggesting, indirectly, that the nucleation and crystal growth rates were not affected. The lowest Si/Al ratio of the reaction gel for the synthesis of pure LTA phase was about 42 (10 wt % of faujasite crystals, Table 1), which is similar to the value found previously, a Si/Al ratio of 50.3 This limit may originate from the charge balance needed between SDA cation, fluoride anion, and charged aluminum sites. In this sense, to achieve a much lower Si/Al ratio, the (partial) use of hydroxide medium and/or a new SDA would be required as developed recently.19 Figure 4 shows the solid-state (a) 29Si and (b) 27Al MAS NMR spectra of the pure-silica and aluminosilicate (synthesized
1). Highly crystalline pure-silica and aluminosilicate LTA-type zeolites were synthesized with 0−10 wt % of faujasite crystals, and no other phases, including faujasite, were observed in the XRD patterns of the product (Figure 2). Although the XRD patterns of these samples showed high crystallinity before and after calcination, the aluminosilicate products synthesized with faujasite crystals were slightly gray after calcination at 823 K for 5 h. With 15 wt % of faujasite crystals, a small amount of a RUT phase was also observed. Figure 3 shows the SEM images of the
Figure 4. (a) 29Si and (b) 27Al solid-state MAS NMR spectra of the LTA samples synthesized without or with 10 wt % of faujasite crystals. Figure 3. SEM images of calcined pure-silica and aluminosilicate LTA samples synthesized with (a) 10 and (b) 5 wt % of faujasite crystals. Scale bars in the panels indicate 1 μm.
with 10 wt % of faujasite, Si/Al = 61) LTA-type zeolites. In the case of the pure-silica sample, the 29Si MAS NMR spectrum showed a strong signal at −113 ppm and weak signals at around −103 ppm (magnified spectrum is inserted in Figure 4), which correspond to Q4 and Q3 species, respectively.45 The single Q4 signal is due to a crystallographically single T-site in the asymmetric unit.3 Weak Q3 signals indicate that the product contains almost no internal defect sites (T-vacancies or silanol nests). This feature is different from conventional LTA-type zeolites synthesized in hydroxide media, because the fluoride anion compensates the charge of the SDA cation during the synthesis (as opposed to SiO−, siloxy groups).4−6 In the case of the aluminosilicate sample, two Q4 signals were observed at −107 and −113 ppm, corresponding to Si(1Al) and Si(0Al) species, respectively.45 Again, a very weak Q3 signal indicates that the aluminosilicate sample also has an almost defect-free structure. The Si/Al ratio calculated from the signals is about 87, which is a little higher than the results of ICP measurement (Si/Al ratio = 63, Table 1). The signal from Si(2Al) sites was not observed, because there is little chance for one Si atom to connect two Al atoms. Solid-state 27Al MAS NMR of the aluminosilicate LTA sample showed a single signal at 57 ppm (Figure 4b), indicating the tetrahedrally coordinated aluminum atoms. These results indicate that the added faujasite crystals were completely dissolved during hydrothermal treatment and all aluminum atomsat this level of substitutionwere successfully incorporated in the LTA-type framework. The unit cell dimensions of the samples further confirm the incorporation of the aluminum atoms into the high-silica LTA-
aluminosilicate LTA-type zeolites synthesized with (a) 10 and (b) 5 wt % of faujasite crystals (the SEM images of other LTAtype zeolite samples are shown in Figure S5). Cubic-shaped crystals were observed in every case. The size of the crystals was uniform, about 0.5 μm in size regardless of aluminum content in the reaction gel, but larger crystals (1−2 μm) were sometimes observed, as shown in Figure 3. The differences in crystal size would result from some inhomogeneity of synthesis gel and insufficient stirring during the hydrothermal treatment, because the gel is very thick that might form a “quasi static” region of the autoclave. The Si/Al ratios of the final products determined by ICP correlate linearly with that of reaction gels, ranging from 63 to 420 (Table 1). The lowest Si/Al of 63 was synthesized with the addition of 10 wt % of faujasite crystals. The nitrogen adsorption isotherm of the sample with a Si/Al ratio of 63 showed type I (Figure S6), and the micropore volume was 0.23 cm3 g−1 as determined by the t-plot method. This value is comparable to the micropore volume of ITQ-29 reported previously, 0.24 cm3 g−1.3 As indicated above, the calcined aluminosilicate LTA-type zeolite showed a gray color, which would be caused by the remaining carbon species in the micropore; these adsorption measurements reveal that micropore access was not blocked by the remaining carbon. The addition of faujasite crystals to the synthesis gel seems to have little influence on crystallization as similar crystal sizes and E
DOI: 10.1021/acs.chemmater.5b04439 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials type zeolites. Because of the difference in bond angle and bond length between Si−O−Si and Al−O−Si, unit cell parameters change along with the Si/Al ratio. In Figure 5, calculated unit
Figure 6. XRD patterns of as-made aluminosilicate LTA samples synthesized with 10 wt % of faujasite crystals for different hydrothermal reaction times.
the XRD patterns of as-made samples synthesized with different hydrothermal reaction times. The crystalline LTA phase was clearly observed after 1 day of hydrothermal reaction, and the crystallization was completed after 12 additional hours (total 1.5 days). Up to 18 h, only a small peak at 22.5° with a broad peak at 23° can be recognized, indicating the presence of LTAtype nanocrystals and an amorphous phase, respectively. Interestingly, no FAU phase was recognizable by XRD even at the initial stage of reaction (0 h), although we added as much as 10 wt % of the faujasite crystals. These results indicate that the faujasite crystals dissolvedor at least lost their crystalline structureat the very early stage of the synthesis process. The stability of zeolite crystals was confirmed by 29Si and 27 Al solid-state MAS NMR measurements (Figure 7a,b, respectively). The sample before hydrothermal reaction (with
Figure 5. Relationship between the unit cell volume and the aluminum content of the synthesized high-silica LTA-type zeolites and commercial zeolite 4A. The red line in the panels shows the linear fitted line of the six plots.
cell volumes of the samples were plotted against the aluminum content. We have used the space group Pm3m (recall that the LTA-type framework has cubic symmetry) and refined the unit cell parameter a and the unit cell volume (Table 1). As a reference, the unit cell volume of zeolite A (Linde A) was also calculated and plotted in Figure 5. The zeolite Linde A was a commercial zeolite 4A (Na-form, Aldrich). The red lines shown in the figures represent the linear fitting line of the six points. The points showed good fit to the line, and the unit cell volume was proportional to the aluminum content. This result indirectly indicates that the aluminum atoms supplied from faujasite crystals were successfully incorporated in the LTAtype framework. Moreover, the unit cell information collected by XRD measurement can be useful to estimate the Si/Al ratio of samples of unknown chemical composition. Note that the unit cell volume of pure-silica ITQ-29 was reported to be 1671.2 Å3 previously.3 Although the value is slightly larger than that of our pure-silica sample synthesized without faujasite crystals (1664.9 Å3), the difference may be the result of the offset of the instrument and the refinement method. Faujasite Crystals as Aluminum Source in LTA-Type Zeolite Synthesis. The crystallization of the aluminosilicate LTA sample was investigated in more detail for the sample synthesized with 10 wt % of faujasite crystals. Figure 6 shows
Figure 7. (a) 29Si and (b) 27Al solid-state MAS NMR spectra of the sample synthesized with 10 wt % of faujasite crystals for different hydrothermal treatment times. F
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Chemistry of Materials 0 h of hydrothermal treatment time) showed two main 29Si NMR signals: a broad signal at −109 ppm and a sharp signal at −113 ppm corresponding to amorphous silica and quasisiliceous seed crystals, respectively (Figure 7a, 0 h). No signals corresponding to faujasite crystals were observed in the 29Si MAS NMR spectrum, as was the case with the XRD patterns. With the progress of the hydrothermal treatment, the relative intensity of the NMR signal from seed crystals decreased (Figure 7a, 6 h). Finally, the spectrum shows a strong signal at −113 ppm and a very weak signal at −107 ppm, indicating that all the silicon atoms were incorporated in the LTA structure (Figure 7a, 3 d). The 27Al NMR spectrum shows the presence of both tetrahedrally and octahedrally coordinated aluminum atoms on the sample with 0 h of hydrothermal treatment at 60 and 0 ppm, respectively (Figure 7b, 0 h). This observation is an indication of the partial decomposition of faujasite crystals. After 6 h of hydrothermal treatment, the fraction of octahedrally coordinated aluminum atoms increased slightly (Figure 7b, 6 h), indicating that the decomposition of faujasite crystals has been completed. The chemical shifts of the aluminum atoms in the sample (27Al MAS NMR) were different in each zeolite due to their structural properties: 57 ppm for aluminosilicate LTA-type zeolite and 60 ppm for faujasite. Therefore, the tetrahedrally coordinated aluminum atoms observed in the early stage of hydrothermal treatment (0 and 6 h) are aluminum atoms incorporated in the faujasite framework. In the end, the 27Al MAS NMR spectrum shows only a single signal at 57 ppm, indicating that all the aluminum atoms were tetrahedrally coordinated in the LTA framework and almost no extraframework aluminum is present (Figure 7b, 3 d). It is concluded that the quasi-siliceous seed crystals maintained their structure throughout the synthesis procedure, but the faujasite crystals (aluminum source) were dissolved early during the synthesis protocol. Low-silica zeolites are thermodynamically less stable than high-silica zeolites, in general, and this agrees with expectations. However, the added faujasite crystals already decomposed before the hydrothermal treatment. This might be possible because, in the present gel preparation process, the zeolite crystals were added into a fluoride mediated solution and then the gel was heated in an oven at 80 °C for several hours to adjust the H2O content. Therefore, if the stability of the zeolite was not high enough, the crystalline phase can be dissolved during this heating process. Faujasite crystals were added as the less-stable NH4 form, and their low Si/Al ratio framework leads to a less stable material under severe reaction conditions. The decomposition of faujasite and/or the reconstruction of the aluminosilicate network can proceed during the gel preparation process, and no peaks and no signals were detected in XRD and NMR measurements, respectively. Moreover, the increase of octahedrally coordinated aluminum atoms after 6 h of hydrothermal treatment indicates that the decomposition and/or reconstruction of the aluminosilicate network proceeds under the hydrothermal treatment. LTA seed crystals, on the other hand, were stable enough to resist dissolution by the gel preparation process. During the hydrothermal treatment, seed crystals were also dissolved (to some extent), as shown by the decrease of 29Si NMR signal at −113 ppm, but most of the LTA crystals must retain their structure as seeds. Here, the difference of the hydrothermal stability between faujasite and LTA crystals was used effectively to use one phase as aluminum source and the other as seed.
Finally, formation of the LTA structure was also confirmed in terms of the amount of the incorporated organic SDA. Figure 8
Figure 8. UV−vis spectra of the as-made samples synthesized with 10 wt % of faujasite crystals for different time scales. Inserted figure is a spectrum of concentrated SDA solution (about 0.3 M).
shows UV−vis spectra of the as-made samples synthesized with different hydrothermal reaction times corresponding to Figure 6. Regardless of the crystallization time, all the samples showed electronic transitions at around 310 and 410 nm. The peak at 310 nm corresponds to the formation of the dimers of supramolecular methylated-julolidine molecules.3 The presence of the dimers was confirmed even in the early stage of the hydrothermal treatment at 12 h, at which no crystalline phase was observed (Figure 6). The formation of dimers does not necessarily indicate formation of a cage structure or incorporation of the organic SDA inside the aluminosilicate network. The dimers are easily formed in concentrated aqueous solutions,3 as we confirmed in the inset in Figure 8. Therefore, dimers of SDA had been already formed just after the gel preparation process because of the low water content. The remarkable difference is the absorption intensity, which reflects the concentration of the organics. Before the UV−vis measurements, the samples taken out from autoclaves were filtered and washed with DI water for several times. The organics adsorbed on the sample surface would be easily removed by this washing process because the interaction between the organics and the aluminosilicate frameworks is weak. On the other hand, if the organic dimers were occluded in α-cages, they would be hardly removed by the washing process. After 1 day of hydrothermal treatment, the concentration of the dimers was significantly higher than that before 18 h (Figure 8). This result indicates that a large amount of organic dimers were occluded in α-cages after 1 day and suggests the formation of LTA-based cage structures. This is in good agreement with the XRD result, in which the apparent formation of a small amount of LTA phase was observed after 1 day of hydrothermal treatment (Figure 6). The additional crystal growth (Figure 6) after the large increase in dimer concentration at 1 day suggests that the dimers are encapsulated in α-cages prior to crystal growth, which has been suggested in the LTA zeolite formation via a charge density mismatch approach,46 although further study is necessary to verify this hypothesis. Synthesis of Aluminosilicate ITW-Type Zeolites in Fluoride Media. Since we succeeded in preparing high-silica LTA-type zeolite using aluminosilicate zeolites as aluminum G
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Chemistry of Materials source in fluoride media, we also consider the application of this methodology to the synthesis of other zeolites in fluoride media. To start, we investigated the synthesis of high-silica ITW-type zeolite, previously mainly known in its pure-silica form (ITQ-12).7,24,25 Here, it is demonstrated that aluminosilicate ITW-type zeolite with a Si/Al ratio of 71 was synthesized by using 5 wt % of faujasite crystal as aluminum source. On the basis of the previous report, 1,2,3-trimethyl imidazolium appears to be the strongest structure-directing agent in the ITQ-12 synthesis,24 and the same organic SDA has been used in the present study. Pure-silica ITQ-12 seed crystals were used to facilitate the crystallization, and 1 wt % of the seed crystals were added without calcination as before. Figure 9 shows the XRD patterns of the as-made pure-silica seed and the synthesized aluminosilicate product before and
Figure 10. SEM images of (a) synthesized aluminosilicate and (b) pure-silica seed ITW-type zeolites. Scale bars in the panels indicate 2 μm.
mixture of TON and an unidentified phase became dominant in the product after 21 days. At this point, 5 wt % is the upper limit of faujasite crystal that can be added for a successful synthesis of pure aluminosilicate ITW-type zeolite. TON-type materials are well-known aluminosilicate zeolitessuch as Theta-1 and ZSM-22and sometimes this phase competes in the synthesis of pure-silica ITQ-12.24 It has been shown previously that TON and MTW phases transform, in situ, into ITW phase under certain conditions with specific organic SDAs.26,28 These examples suggest that an ITW phase would be thermodynamically more stable than TON or MTW phases. In our case, it is possible that the TON phase obtained would transform into a pure ITW phase after long hydrothermal treatment. However, the process would be very slow (more than 3 weeks under hydrothermal treatment at 175 °C). Further adjustment of the chemical composition of the gel and the synthesis parameters is needed to obtain aluminosilicate ITW-type zeolite with lower Si/Al ratios. The successful introduction of aluminum atoms into the ITW framework is confirmed by solid-state 29Si and 27Al MAS NMR measurements (Figure 11). The silicon NMR spectrum shows several overlapped signals, and it is difficult to assign these complicated signals to each silicon site (Figure 11a) with certainty. A previous report shows that the 29Si MAS NMR spectrum of calcined ITQ-12 (pure-silica ITW-type zeolite) has five signals between −108 and −118 ppm, corresponding to
Figure 9. XRD patterns of as-made pure-silica seed and synthesized aluminosilicate ITW-type zeolites.
after calcinaton. Highly crystalline aluminosilicate ITW-type zeolite was synthesized with 5 wt % of faujasite crystals, and no other phases, including faujasite, were observed in the XRD pattern (Figure 9). The 2θ angle peak position of the synthesized aluminosilicate crystals was slightly shifted to lower angles from that of pure-silica seed crystals, suggesting the incorporation of aluminum atoms into the framework. The crystalline structure was maintained after the calcination at 823 K for 5 h without decomposition to an amorphous phase, and a change of peak intensities was observed due to the removal of occluded SDAs (Figure 9). The Si/Al ratio of the aluminosilicate crystals was 71 as measured by EDX, almost the same as that of the synthesis gel (Si/Al ratio of 78). The SEM images of pure-silica seed and aluminosilicate ITW-type zeolites are shown in Figure 10. In both cases, small rodlike crystals of about 100−300 nm in length aggregate to form large agglomerates of about 4−5 μm in size. The N2 adsorption isotherms of aluminosilicate and pure-silica seed ITW-type zeolites showed a typical type-I isotherm (Figure S8), the micropore volumes of the samples determined by the t-plot method were 0.17 and 0.19 cm3/g, and BET surface areas were calculated as 373 and 392 m2/g, respectively. These results indicate that the aluminosilicate ITW-type zeolite obtained has similar structural properties to the pure-silica ITW-type zeolite. When we increased the amount of faujasite to 10 wt % to increase the aluminum content, the crystallization of the ITW phase becomes slow and the crystallization was not completed in 7 days of hydrothermal treatment (data not shown). A
Figure 11. Solid-state (a) 29Si and (b) 27Al MAS NMR spectra of the aluminosilicate ITW-type zeolite sample synthesized with 5 wt % of faujasite crystals. H
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Chemistry of Materials crystallographically different Q4 framework sites.7 If aluminum atoms are incorporated in the framework, the signal from the Si(1Al) site should appear in a higher chemical shift region. We observed several overlapped peaks in the −108 to −118 ppm region, and a small shoulder at −105 ppm (Figure 11a), which have not been observed in the pure-silica sample. The shoulder is assigned to an aluminum-connecting Q4 silicon, Si(1Al), or perhaps it could be assigned to Q3 silicon atoms (indicating structural defects). To establish that the sample has structural defects, the cross-polarization 29Si MAS NMR measurement was performed (Figure S9). This spectrum does not show any remarkable signal at −105 ppm, and therefore, we assign the small shoulder in the spectrum to Si(1Al) sites. Although the deconvolution of the complicated signal remains incomplete, the Si/Al ratio was calculated to be about 100 from the peak ratio of Si(1Al) and Si(0Al) signals, a value that is higher than that measured by EDX. The 27Al MAS NMR spectrum showed major signals at around 57 ppm, corresponding to the tetrahedrally coordinated aluminum sites. Note that the chemical shift of the observed signal is different from that of faujasite. A small, minor signal observed at 0 ppm indicates the presence of a very small fraction of extraframework aluminum atoms. On the basis of these results, it is concluded that the obtained aluminosilicate ITW-type zeolite has an almost defectfree structure and most of the aluminum atoms supplied from faujasite crystals are successfully incorporated in the ITW-type framework. Here it is shown that aluminosilicate ITW-type zeolite was synthesized by using faujasite crystals as aluminum source. This methodology could be applicable to the synthesis of novel aluminosilicate zeolites in fluoride media. These zeolites would broaden the application field and be useful for catalytic applications. Aluminosilicate Zeolites as Aluminum Source for Other Fluoride Mediated High-Silica Zeolite Synthesis. Besides ITW-type zeolite, the high-silica CHA-, *BEA-, and STT-type zeolites were also synthesized using low-silica zeolites as an aluminum source via fluoride mediated synthesis. Although these zeolites have been already reported as aluminosilicate and pure-silica zeolites,47−53 the success of the syntheses shows the wide applicability of this methodology. The synthesis procedure was almost the same as that in the case of LTA- and ITW-type zeolites, except for different organic SDAs and chemical compositions in the synthesis gel. Synthesis conditions and some product properties are summarized in Table 2. We added a small amount of as-made pure-silica seed (1 wt %) to hasten the crystallization and a low-silica zeolite (515 %wt) as an aluminum source. Not only faujasite but also mordenite and Linde type A were used as an aluminum source. The Na form of the zeolites was ion-exchanged to the ammonium form because this would lower the materials’ stability under hydrothermal conditions. Moreover, sodium cations could work as an inorganic SDA and could induce undesired nucleation of other zeolites. In the case of STT-type zeolites, mordenite was the only aluminum source that led to the pure STT phase (Table 2). The effect of the framework type and Si/Al ratio of the low-silica zeolite used as an aluminum source was investigated. Figures 12 and 13 show the representative XRD patterns and SEM images of the high-silica CHA-, *BEA-, and STT-type zeolites (Runs 7, 9, and 11, respectively). The XRD patterns and SEM images of other samples are reported in (Figures S10 and S11). All the peaks of the XRD patterns were indexed to
Figure 12. XRD patterns of the high-silica samples synthesized in fluoride media, corresponding to samples 7, 9, and 11 in Table 2.
Figure 13. SEM images of the high-silica samples synthesized in fluoride media, corresponding to samples 7, 9, and 11 in Table 2. Scale bars in the panels indicate 10 μm.
CHA, *BEA, and STT phases (Figure 12), and no other zeolite phasesuch as faujasite and mordenitewere observed in any case. They all showed high crystallinity, and the structures remained stable after the calcination at 823 K for 5 h. The crystal morphologies and sizes of the aluminosilicate products were confirmed by SEM measurements (Figure 13), and they are comparable to the conventional pure-silica products synthesized in fluoride media.47,51,52 Crystal sizes were much larger, in the micron size, than the ones for samples synthesized in hydroxide media. The Si/Al ratios of the final products were estimated by EDX (Table 2), and they were roughly proportional to the amount added as low-silica zeolites. The I
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Chemistry of Materials Si/Al ratios of the CHA- and *BEA-type zeolites were similar to those of reaction gels, but STT-type zeolites showed a remarkably higher Si/Al ratio than their reaction gel (Runs 11 and 12). The micropore volumes determined from the N2 adsorption isotherms (Figure S12) by the t-plot method (Table 2) are comparable to those of the zeolites synthesized in the conventional hydrothermal method.47,51,52 On the basis of these results on the synthesis of a variety of high-silica zeolites, it is likely that not only the zeolites investigated here but also many others could be synthesized via this methodology. Figure 14 shows the solid-state 29Si MAS NMR spectra of the samples 7, 9, and 11, corresponding to Figures 12 and 13. The
14, and 15, Figure S13). It is unclear whether the obtained STT-type zeolite contained aluminum atoms in the framework or not. In general, CHA and STT phases are synthesized under similar synthesis conditions.4 Therefore, even in the presence of STT-type seed crystals, the nucleation of the CHA phase may be induced by other factors such as the changes in local gel composition. Faujasite, for example, has been a common Tatom source used in the synthesis of aluminosilicate CHA-type zeolite.21 We were also unable to avoid CHA-type zeolite impurities with using zeolite A as source of aluminum. Pure STT phase was obtained only when using mordenite as aluminum source (Table 2). The apparent differences between the faujasite, zeolite A, and mordenite crystals were the Si/Al ratios and framework structures. Although the overall reactant gel composition is the same, these small differences would have an effect on the local Si/Al ratio in the gel, relative solubility in fluoride media, and/or the “precursor” formation prior to the crystallization.40,55,56 In this sense, appropriate aluminosilicate zeolites should be selected as aluminum source for the successful synthesis.
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CONCLUSIONS
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ASSOCIATED CONTENT
We have demonstrated that aluminosilicate zeolite crystals can be an effective aluminum source for the synthesis of high-silica zeolites in fluoride media. We have shown that aluminosilicate high-silica LTA-type zeolite was reproducibly synthesized by this approach. The faujasite crystals used as aluminum source were dissolved during the synthesis procedure, and the aluminum atoms were incorporated in the final product with tetrahedral coordination. The added pure-silica crystals maintained their structure and worked as seeds under severe hydrothermal conditions. The unit cell size of the high-silica LTA zeolites was correlated with the aluminum content in the framework, also indicating the successful incorporation of aluminum atoms. We have also demonstrated the synthesis of aluminosilicate ITW-type zeolite, showing that the methodology is applicable to other high-silica zeolite syntheses. The high-silica ITW-type zeolite obtained showed a highly crystalline framework and the unique properties associated with the fluoride synthesis. Lastly, other high-silica zeolites, such as CHA-, *BEA-, and STT-type zeolites, were also synthesized using the same methodology. The choice of aluminum-source zeolite, with the proper thermochemical stability and structural properties under synthesis conditions, was an important factor for a successful synthesis. The methodology reported here can be used as a general method to synthesize high-silica zeolites. By choosing an appropriate zeolite as a T-atom source, new zeolites having larger crystal size, defect-free frameworks, and having hydrophobic properties, could be potentially synthesized.
Figure 14. 29Si solid-state MAS NMR spectra of the high-silica samples synthesized in fluoride media, corresponding to samples 7, 9, and 11 in Table 2.
chemical shifts of the NMR signal depend on each zeolite framework type, and are assigned to CHA-, *BEA-, and STTtype frameworks.48,51,52 In the case of CHA-type zeolite, signals at around −100, −105, and −110 ppm, assigned to Q4 Si(2Al), Si(1Al), and Si(0Al) sites, respectively (Figure 14) are observed.45,48 The defect site (Q3 site) also has a chemical shift at −100 ppm and overlaps with the Si(2Al) signal. The simulated spectrum of ideal high-silica CHA-type zeolite (Si/Al ratio of 50) does not show a signal at −100 ppm,48 suggesting that the observed signal in this region probably originated from defect sites. The concentration of the defect sites, however, is as small as the previous high-silica CHA-type zeolite synthesized in fluoride media.48 *BEA- and STT-type zeolites also show signals from Q4 and Q3 sites (Figure 14). The Q4 signals of *BEA- and STT-type zeolites are observed at around −110 to −118 ppm and around −105 to −120 ppm, respectively.51,52 Small signals due to defect sites are also observed on the shoulder of main signals at around −100 ppm. The concentration of defects sites is small and comparable to that in previous reports.51,52 The results of NMR studies indicate that the products synthesized have fewer defect sites than the product synthesized in hydroxide media. The obtained highsilica aluminosilicate zeolites may show hydrophobic properties and may be useful in a number of applications.54 In the case of STT-type zeolite, the type of zeolite used as aluminum source affected the final product (Table 2). Besides STT-type zeolite, the crystallization of CHA-type zeolite was induced with the use of faujasite or zeolite A crystals (Runs 13,
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04439. XRD patterns, SEM images, 29Si and 27Al solid-state MAS NMR spectra, and detailed synthesis procedures of zeolite samples (PDF) J
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(18) Sun, M.; Hunt, H. K.; Lew, C. M.; Cai, R.; Liu, Y.; Yan, Y. A Dynamic Organic Structuring-Directing Agent for Pure-Silica-Zeolite AST and LTA Syntheses. Chin. J. Catal. 2012, 33, 85−91. (19) Boal, B. W.; Schmidt, J. E.; Deimund, M. A.; Deem, M. W.; Henling, L. M.; Brand, S. K.; Zones, S. I.; Davis, M. E. Facile Synthesis and Catalysis of Pure-Silica and Heteroatom LTA. Chem. Mater. 2015, 27, 7774−7779. (20) Cundy, C. S.; Cox, P. A. The Hydrothermal Synthesis of Zeolites: Precursors, Intermediates and Reaction Mechanism. Microporous Mesoporous Mater. 2005, 82, 1−78. (21) Zones, S. I. Conversion of Faujasites to High-Silica Chabazite SSZ-13 in the Presence of N,N,N-Trimethyl-1-Adamantammonium Iodide. J. Chem. Soc., Faraday Trans. 1991, 87, 3709−3716. (22) Chiyoda, O.; Davis, M. E. Hydrothermal Conversion of YZeolite Using Alkaline-Earth Cations. Microporous Mesoporous Mater. 1999, 32, 257−264. (23) Dwyer, F. G.; Chu, P. ZSM-4 Crystallization via Faujasite Metamorphosis. J. Catal. 1979, 59, 263−271. (24) Rojas, A.; Martínez-Morales, E.; Zicovich-Wilson, C. M.; Camblor, M. A. Zeolite Synthesis in Fluoride Media: Structure Direction Toward ITW by Small Methylimidazolium Cations. J. Am. Chem. Soc. 2012, 134, 2255−2263. (25) Yang, X.; Camblor, M. A.; Lee, Y.; Liu, H.; Olson, D. H. Synthesis and Crystal Structure of As-Synthesized and Calcined Pure Silica Zeolite ITQ-12. J. Am. Chem. Soc. 2004, 126, 10403−10409. (26) Zicovich-Wilson, C. M.; Gándara, F.; Monge, A.; Camblor, M. A. Structure-Direction Overcoming Framework Instability in the Synthesis if SiO2 Zeolties. J. Am. Chem. Soc. 2010, 132, 3461−3471. (27) Jo, D.; Hong, S. B.; Camblor, M. A. Monomolecular Skeletal Isomerization of 1-Butene over Selective Zeolite Catalysts. ACS Catal. 2015, 5, 2270−2274. (28) Rojas, A.; San-Roman, M. L.; Zicovich-Wilson, C. M.; Camblor, M. A. Host-Guest Stabilization of a Zeolite Strained Framework: In Situ Transformation of Zeolite MTW into the Less Dense and More Strained ITW. Chem. Mater. 2013, 25, 729−738. (29) http://www.ccp14.ac.uk/tutorial/lmgp/celref.htm. (30) Meng, X.; Xiao, F.-S. Green Routes for Synthesis of Zeolites. Chem. Rev. 2014, 114, 1521−1543. (31) Iyoki, K.; Itabashi, K.; Okubo, T. Progress in Seed-Assisted Synthesis of Zeolites without Using Organic Structure-Directing Agents. Microporous Mesoporous Mater. 2014, 189, 22−30. (32) Moteki, T.; Okubo, T. From Charge Density Mismatch to a Simplified, More Efficient Seed-Assisted Synthesis of UZM-4. Chem. Mater. 2013, 25, 2603−2609. (33) Caullet, P.; Guth, J. L.; Hazm, J.; Lamblin, J. M.; Gies, H. Synthesis, Characterization and Crystal Structure of the New Clathrasil Phase Octadecasil. Eur. J. Solid State Inorg. Chem. 1991, 28, 345−361. (34) Broach, R. W.; McGuire, N. K.; Chao, C. C.; Kirchner, R. M. Direct Methods Structure Determination from Synchrotron Powder Diffraction Data of a New Clathrasil, TMA Silicate. J. Phys. Chem. Solids 1995, 56, 1363−1368. (35) Bellussi, G.; Millini, R.; Carati, A.; Maddinelli, G.; Gervasini, A. Synthesis and Comparative Characterization of Al, B, Ga, and Fe Containing Nu-1-Type Zeolitic Framework. Zeolites 1990, 10, 642− 649. (36) Zones, S. I.; van Nordstrand, R. A. Novel Zeolite Transformations: The Template-Mediated Conversion of Cubic P Zeolite to SSZ-13. Zeolites 1988, 8, 166−174. (37) Ahedi, R. K.; Kubota, Y.; Sugi, Y. Hydrothermal Synthesis of [Al]-SSZ-31 from [Al]-BEA Precursors. J. Mater. Chem. 2001, 11, 2922−2924. (38) Kubota, Y.; Maekawa, H.; Miyata, S.; Tatsumi, T.; Sugi, Y. Hydrothermal Synthesis of Metallosilicate SSZ-24 from Metallosilicate Beta as Precursors. Microporous Mesoporous Mater. 2007, 101, 115− 126. (39) Zones, S. I.; Nakagawa, Y. Boron-Beta Zeolite Hydrothermal Conversions: The Influence of Template Structure and of Boron Concentrations and Source. Microporous Mater. 1994, 2, 557−562.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
University of Illinois Urbana-Champain, Urbana, IL61801, USA. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Johnson Matthey Inc. for the financial support provided for this investigation. REFERENCES
(1) Davis, M. E. Ordered Porous Materials for Emerging Applications. Nature 2002, 417, 813−821. (2) Sherman, J. D. Synthetic Zeolites and Other Microporous Oxide Molecular Sieves. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3471−3478. (3) Corma, A.; Rey, F.; Rius, J.; Sabater, M. J.; Valencia, S. Supramolecular Self-Assembled Molecules as Organic Directing Agent for Synthesis of Zeolites. Nature 2004, 431, 287−290. (4) Camblor, M. A.; Villaescusa, L. A.; Díaz-Cabañas, M. J. Synthesis of All-Silica and High-Silica Molecular Sieves in Fluoride Media. Top. Catal. 1999, 9, 59−76. (5) Caullet, P.; Paillaud, J.-L.; Simon-Masseron, A.; Soulard, M.; Patarin, J. The Fluoride Route: A Strategy to Crystalline Porous Materials. C. R. Chim. 2005, 8, 245−266. (6) Chézeau, J.-M.; Delmotte, L.; Guth, J. L.; Soulard, M. HighResolution Solid-State 29Si and 13C n.m.r. on Highly Siliceous MFIType Zeolites Synthesized in Nonalkaline Fluoride Medium. Zeolites 1989, 9, 78−80. (7) Barrett, P. A.; Boix, T.; Puche, M.; Olson, D. H.; Jordan, E.; Koller, H.; Camblor, M. A. ITQ-12: A New Microporous Silica Polymorph Potentially Useful for Light Hydrocarbon Separations. Chem. Commun. 2003, 2114−2115. (8) Rojas, A.; Camblor, M. A. A Pure Silica Chiral Polymorph with Helical Pores. Angew. Chem., Int. Ed. 2012, 51, 3854−3856. (9) Smeets, S.; Xie, D.; Baerlocher, C.; McCusker, L. B.; Wan, W.; Zou, X.; Zones, S. I. High-Silica Zeolite SSZ-61 with Dumbbell-Shaped Extra-Large-Pore Channels. Angew. Chem., Int. Ed. 2014, 53, 10398− 10402. (10) Zones, S. I.; Hwang, S.-J.; Elomari, S.; Ogino, I.; Davis, M. E.; Burton, A. W. The Fluoride-Based Route to All-Silica Molecular Sieves; A Strategy for Synthesis of New Materials Based upon ClosePacking of Guest-Host Products. C. R. Chim. 2005, 8, 267−282. (11) Villaescusa, L. A.; Barrett, P. A.; Kalwei, M.; Koller, H.; Camblor, M. A. Synthesis and Physicochemical Characterization of an Aluminosilicate Zeolite with IFR Topology, Prepared by the Fluoride Route. Chem. Mater. 2001, 13, 2332−2341. (12) Camblor, M. A.; Corma, A.; Valencia, S. Synthesis in fluoride media and characterisation of aluminosilicate zeolite beta. J. Mater. Chem. 1998, 8, 2137−2145. (13) Villaescusa, L. A.; Barrett, P. A.; Camblor, M. A. Calcination of Octadecasil: Fluoride Removal and Symmetry of the Pure SiO2 Host. Chem. Mater. 1998, 10, 3966−3973. (14) Bouizi, Y.; Paillaud, J.-L.; Simon, L.; Valtchev, V. Seeded Synthesis of Very High Silica Zeolite A. Chem. Mater. 2007, 19, 652− 654. (15) Huang, A.; Weidenthaler, C.; Caro, J. Facile and Reproducible Synthesis of ITQ-29 Zeolite by Using Kryptofix 222 as the Structure Directing Agent. Microporous Mesoporous Mater. 2010, 130, 352−356. (16) Huang, A.; Caro, J. Hydrothermal Synthesis if Uniform and Well-Shaped ITQ-29 Crystals. Microporous Mesoporous Mater. 2009, 124, 153−161. (17) Hunt, H. K.; Lew, C. M.; Sun, M.; Yan, Y.; Davis, M. E. PureSilica LTA, CHA, STT, ITW, and−SVR Thin Films and Powders for Low-k Applications. Microporous Mesoporous Mater. 2010, 130, 49−55. K
DOI: 10.1021/acs.chemmater.5b04439 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials (40) Sano, T.; Itakura, M.; Sadakane, M. High Potential of Interzeolite Conversion Method for Zeolite Synthesis. J. Jpn. Pet. Inst. 2013, 56, 183−197. (41) Zones, S. I. Direct Hydrothermal Conversion of Cubic P Zeolite to Organozeolite SSZ-13. J. Chem. Soc., Faraday Trans. 1990, 86, 3467−3472. (42) Khodabandeh, S.; Davis, M. E. Zeolites P1 and L as Precursors for the Preparation of Alkaline-Earth Zeolites. Microporous Mater. 1997, 12, 347−359. (43) Jon, H.; Ikawa, N.; Oumi, Y.; Sano, T. An Insight into the Process Involved in Hydrothermal Conversion of FAU to *BEA Zeolite. Chem. Mater. 2008, 20, 4135−4141. (44) Shibata, S.; Itakura, M.; Ide, Y.; Sadakane, M.; Sano, T. FAULEV Interzeolite Conversion in Fluoride Media. Microporous Mesoporous Mater. 2011, 138, 32−39. (45) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; John Wiley & Sons: New York, 1987. (46) Park, M. B.; Lee, Y.; Zheng, A.; Xiao, F.-S.; Nicholas, C. P.; Lewis, G. J.; Hong, S. B. Formation Pathway for LTA Zeolite Crystals Synthesized via a Charge Density Mismatch Approach. J. Am. Chem. Soc. 2013, 135, 2248−2255. (47) Díaz-Cabañas, M.-J.; Barrett, P. A. Synthesis and Structure of Pure SiO2 Chabazite: The SiO2 Polymorph with the Lowest Framework Density. Chem. Commun. 1998, 1881−1882. (48) Eilertsen, E. A.; Arstad, B.; Svelle, S.; Lillerud, K. P. Single Parameter Synthesis of High Silica CHA Zeolites from Fluoride Media. Microporous Mesoporous Mater. 2012, 153, 94−99. (49) Higgins, J. B.; LaPierre, R. B.; Schlenker, J. L.; Rohrman, A. C.; Wood, J. D.; Kerr, G. T.; Rohrbaugh, W. J. The Framework Topology of Zeolite Beta. Zeolites 1988, 8, 446−452. (50) Caullet, P.; Hazm, J.; Guth, J. L.; Joly, J. F.; Lynch, J.; Raatz, F. Synthesis of Zeolite Beta from Nonalkaline Fluoride Aqueous Aluminosilicate Gels. Zeolites 1992, 12, 240−250. (51) Camblor, M. A.; Corma, A.; Valencia, S. Spontaneous Nucleation and Growth of Pure Silica Zeolite-β Free of Connectivity Defects. Chem. Commun. 1996, 2365−2366. (52) Camblor, M. A.; Díaz-Cabañas, M.-J.; Perez-Pariente, J.; Teat, S. J.; Clegg, W.; Shannon, I. J.; Lightfoot, P.; Wright, P. A.; Morris, R. E. SSZ-23: An Odd Zeolites with Pore Openings of Seven and Nine Tetrahedral Atoms. Angew. Chem., Int. Ed. 1998, 37, 2122−2126. (53) Zones, S. I. Preparation of Molecular Sieve SSZ-23. U.S. patent 8,512,674, August 20, 2013. (54) Román-Leshkov, Y.; Davis, M. E. Activation of CarbonylContaining Molecules with Solid Lewis Acids in Aqueous Media. ACS Catal. 2011, 1, 1566−1580. (55) Itakura, M.; Ota, K.; Shibata, S.; Inoue, T.; Ide, Y.; Sadakane, M.; Sano, T. Influence of Starting Zeolite on Synthesis of RUT Type Zeolite by Interzeolite Conversion Method. J. Cryst. Growth 2011, 314, 274−278. (56) Itabashi, K.; Kamimura, Y.; Iyoki, K.; Shimojima, A.; Okubo, T. A Working Hypothesis for Broadening Framework Types of Zeolites in Seed-Assisted Synthesis without Organic Structure-Directing Agent. J. Am. Chem. Soc. 2012, 134, 11542−11549.
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DOI: 10.1021/acs.chemmater.5b04439 Chem. Mater. XXXX, XXX, XXX−XXX
