Effect of the Al content in the precursor on the crystallization of OSDA-free Beta zeolite

Microporous and Mesoporous Materials 224 (2016) 155e162

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Effect of the Al content in the precursor on the crystallization of OSDA-free Beta zeolite Ryoichi Otomo, Toshiyuki Yokoi* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 June 2015 Received in revised form 17 November 2015 Accepted 21 November 2015 Available online 2 December 2015

To date, many types of zeolites have been successfully synthesized without using any organic structuredirecting agent (OSDA). However, so-called “OSDA-free synthesis of zeolites” including Beta often gives a low product yield. The present study shows that hydrothermal synthesis using precursor suspension with a high-aluminum content substantially improved the yield in the OSDA-free synthesis of Beta zeolite. By means of monitoring time course changes of solid products in detail, it was found that the crystallization proceeded through the “solution-mediated mechanism” in which the amount of aluminosilicate species (not silicate species alone) in the mother solution determined the product yield. Synthesis parameters such as silica raw material, alkalinity, and seed crystal were optimized, achieving ~80% yield based on SiO2. © 2015 Elsevier Inc. All rights reserved.

Keywords: Beta High yield Mechanism OSDA-free Synthesis

1. Introduction So-called “OSDA-free synthesis of zeolites”, which is based on the synthesis of zeolites without using any organic structuredirecting agent (OSDA), has several advantages over conventional ones using OSDAs [1,2]. For example, it saves cost, CO2 and NOx emission, and energy consumption arising from the use of OSDAs. Thus, OSDA-free synthesis of zeolites has attracted much research interest and the increasing types of zeolites that were conventionally synthesized with particular OSDAs have been successfully synthesized without using OSDAs; for example, ECR-1 [3], ZSM-34 [4], Beta [5e12], ZSM-5 small crystal [13,14], RUB-13 [15,16], ZSM12 [17,18], high-silica ferrierite [19], Levyne [20,21], MAZ-tyep aluminosilicate [22], CHA-type aluminosilicate [23], MFI-type metallosilicates [24], Fe-Beta [25], and MCM-68 [26]. Among these successful examples, the synthesis of Beta zeolite without using any OSDAs (“OSDA-free Beta”) has a significant impact because Beta zeolite, with three-dimensional 12-membered ring pore system, has been widely investigated and used in scientific and industrial fields. Recently, the increasing number of catalytic applications of OSDA-free Beta have been reported [8,26e33].

* Corresponding author. Tel.: þ81 45 924 5265; fax: þ81 45 924 5282. E-mail address: [email protected] (T. Yokoi). http://dx.doi.org/10.1016/j.micromeso.2015.11.037 1387-1811/© 2015 Elsevier Inc. All rights reserved.

In 2007, Xiao et al. first reported the synthesis of OSDA-free Beta with seed crystal added [5]. Following this report, several research groups have investigated on OSDA-free Beta [6e12]. Mintova et al. reported that the hydrothermal treatment at a relatively low temperature was favorable because the treatments at higher temperatures easily induced the formation of the MOR phase [6]. Okubo et al. reported the critical factors for the OSDA-free synthesis and proposed the crystallization mechanism [7,9]. Despite the numerous efforts devoted for OSDA-free Beta, the zeolite yield in this synthesis method remained below 30%. Mintova et al. pointed out that the precursors and the synthesis conditions needed further optimization to improve the crystalline yield [6]. Such low yields were occasionally found for other types of OSDA-free zeolites. Although the OSDA-free synthesis of zeolites has many advantages, the low product yield would tone down its importance. One of the reasons for the low yields would be ascribed to the difference in the composition between an initial precursor and a final product; the Al content of OSDA-free Beta is generally uncontrollable and relatively high (Si/Al ¼ 4e6) compared to the conventional Beta zeolites synthesized with tetraethylammonium (TEA) cation (“TEA-Beta”). Unfortunately, synthesis using a precursor with the Si/Al ratio similar to OSDA-free Beta has not been reported to our best knowledge; The Si/Al ratios in the precursors of the reported successful syntheses was 20 or higher, which was similar to the ratio of TEA-Beta. Recently, Xiao et al. reported the synthesis of Beta zeolite having the relatively high Si/Al ratio

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neither OSDAs nor solvents added with the solid-like precursor, achieving a high yield [12]. Xiao et al. claimed that the high yield of OSDA-free Beta was attributed to the no use of water solvent. In the present study, Beta zeolite was synthesized without using any OSDAs from high-aluminum precursor suspension (Si/ Al ¼ 5.5e10), which was similar to the composition of OSDA-free Beta. We thoroughly optimized synthesis parameters such as a kind of silica raw material, alkalinity, and the amount of seed crystal, achieving a high yield up to ~80%. 2. Experimental 2.1. Materials Raw materials in all the syntheses were used as received. Fumed silica (Cab-O-Sil M5, Cabot) or colloidal silica (Ludox HS-40, Aldrich) was used as silica source. Aluminum hydroxide (55 wt% as Al2O3, Aldrich) was used as Al source in OSDA-free syntheses. Al(NO3)3$9H2O (98%, Wako) and tetraethylammonium hydroxide (TEAOH) solution (35 wt%, Alfa aesar) were used as Al source and OSDA, respectively, in the synthesis of TEA-Beta as seed crystal. NaOH (97%, Wako) was used as alkali source in all the syntheses. 2.2. Synthesis of TEA-Beta as seed crystal TEA-Beta as seed crystal was prepared by the conventional hydrothermal synthesis using TEAOH as OSDA [34]. A mixture with the composition of 1.0 SiO2: 0.067 Al(NO3)3: 0.45 TEAOH: 0.1 NaOH: 20 H2O was hydrothermally treated at 140  C for 3 d. Solid product was recovered by centrifugation, dried at 100  C overnight and calcined at 580  C for 5 h. The calcined Beta zeolite was soaked into 1 M NaCl solution (100 ml per 1 g of zeolite) and the mixture was stirred at 80  C for 2 h to obtain Na-form sample. Finally the recovered Na-form sample was calcined at 580  C for 5 h. Thus obtained Na-form Beta zeolite free of TEA cation was used as seed crystal in OSDA-free syntheses. The Si/Al and Na/Al ratios of the seed zeolite were 13 and 1.17, respectively. More detailed information on the seed crystal is shown in Figs. S1 and S2 (See supporting information). 2.3. Synthesis of OSDA-free Beta OSDA-free synthesis of Beta zeolite was performed with the composition varied. Colloidal silica was used as silica raw material unless specially notified. The molar composition was in the following range; 1.0 SiO2: 0.1e0.18 Al source: 0.4e0.8 NaOH: 25 H2O: 0e20 wt% seed crystal. All the syntheses were repeated at least twice for attaining reproducible results. Typically, a precursor suspension was prepared as follows. Aluminum hydroxide (0.74 g) was added into aqueous solution containing 1.86 g of NaOH and the mixture was magnetically stirred for 30 min at ambient temperature. After colloidal silica sol (12.0 g) was added, the suspension was stirred for 3 h. To the suspension was added 0.48 g of Beta seed crystal and the final mixture was stirred for 30 min. Thus prepared suspension with the composition of 1.0 SiO2: 0.1 Al(OH)3: 0.6 NaOH: 25 H2O: 10 wt% seed was transferred to a Teflon-lined stainless-steel autoclave (150 ml) and heated at 140  C for a set time under static conditions. The hydrothermal treatment was quenched by cooling the autoclave in an ice bath. The solid product was recovered by filtration, thoroughly washed with water and dried at 100  C overnight to obtain ~3.6 g of white powder after 4 d of the hydrothermal treatment. Yield of a product is based on the recovery of SiO2 and Al2O3 contained in the solid (See Supporting Information).

2.4. Characterization of samples Powder X-ray diffraction (XRD) patterns of samples were collected on a Rigaku Ultima III diffractometer using a Cu Ka radiation (40 kV, 40 mA). Chemical compositions of solid samples were analyzed by a Shimadzu ICPE-9000 analyzer and thus calculated Si/ Al ratios of the solid samples are shown expressed as “(Si/Al)solid”. Na content of the samples was determined by atomic absorbance using a Shimadzu AA-6200 spectrometer. Solid-state 29Si MAS NMR spectra were measured on a JEOL ECA-400 spectrometer at a resonance frequency of 79.5 MHz using a 6 mm sample rotor with a spinning rate of 5.5 kHz. Solid-state 27Al MAS NMR spectra were measured on a JEOL ECA-600 spectrometer at a resonance frequency of 156.4 MHz using a 4 mm sample rotor with a spinning rate of 15.0 kHz. Morphology of solid products was observed by a field-emission scanning electron microscope (FE-SEM) using a Hitachi S-5200 microscope. 3. Results and discussion 3.1. Influence of silica raw material First, the influence of silica raw material on the crystallization of OSDA-free Beta was investigated using colloidal silica and fumed silica as silica source. For both silica source, the precursors with the same composition of 1.0 SiO2: 0.1 Al(OH)3: 0.6 NaOH: 25 H2O: 10 wt % seed were prepared in several batches and hydrothermally treated at 140  C. The hydrothermal treatment was quenched at different periods in order to monitor the crystallization process. Note that the precursors had the Si/Al ratio of 10, which was a significantly high-aluminum composition compared to the reported successful syntheses (typically the ratio  20) [6e9,11,26e30]. Fig. 1 shows XRD patterns of the solid products synthesized with using colloidal silica. Only a halo peak was observed at 1 d and a very small peak appeared at 23.2 on the halo peak at 2 d. Thereafter the peaks assigned to the *BEA phase were intensified. The halo peak almost disappeared at 3.5 d and perfectly crystallized material was obtained at 4 d. Further crystallization did not lead to the growth of the *BEA phase but resulted in the formation of the MOR and GIS phases. The (Si/Al)solid and Na/Al atomic ratios of the product at 4 d were 5.4 and 1.08, respectively (Table 1, Entry 6). Note that the Na/Al ratios of crystalline samples obtained in this study were in the range of 1e1.1, demonstrating that defect sites such as SiO species as well as the negative charges on the framework Al atoms were compensated by Na cation. TG-DTA profile of the sample showed a large weight loss (14 wt%) accompanied with an endothermic peak below 200  C, compared to the seed crystal (9%), indicating that a large amount of water adsorbed on the zeolite sample due to its high-aluminum composition (Fig S3). No exothermic weight loss above 200  C confirmed the absence of organic moiety. Subtracting the weight of adsorbed water, Al2O3, and Na2O moieties, SiO2 yield was estimated as 53%, which was high compared to the results reported for the OSDA-free syntheses with H2O-rich precursors except for the report using the solid-like precursor by Xiao et al. [12]. Silica raw material had a significant influence on the synthesis. Fig. 2 shows XRD patterns for the syntheses using fumed silica. At 1 d of the crystallization, the recovered solid mainly showed a broad halo peak. Several peaks assigned to the *BEA phase appeared on the halo peak at 2 d. These peaks were more intensely observed at 3 d, while slight peaks derived from the MOR phase were also observed. Intensified diffraction peaks of not only the * BEA phase but also the MOR and GIS phases were observed after 4 d. In conclusion, although the use of fumed silica led to a faster

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Table 1 Structural and compositional changes through crystallization of Beta zeolite using colloidal silica.a Entry

Time (d)

1 2 3 4 5 6 7

1 2 2.5 3 3.5 4 5

Product phase

(Si/Al)solid ()

SiO2 yield (%)

Al2O3 yield (%)

amorphous amorphous BEA þ amorphous * BEA þ amorphous * BEA * BEA * BEA þ GIS þ MOR

6.0 6.1 5.7 5.7 5.7 5.4 5.2

54 54 52 54 53 53 49

89 88 91 95 92 98 94

*

a Synthesis conditions: 1.0 SiO2 (colloidal silica): 0.1 Al(OH)3: 0.6 NaOH: 25 H2O: 10 wt% seed, 140  C, static, 1e5 d.

species. Because Q3 species would appear at around 103 ppm, these two peaks probably overlap each other. The peak at 99.3 ppm is assigned to (OAl)2Si(OSi)2 species. The continuous sharpening in the NMR spectra implies that as the crystallization proceeded, Si atoms were settled in geometrically confined locations (T sites) and their coordination environment was defined. Such behavior was also observed for Al atoms (Fig. 3B); at first, freely or loosely confined Al species were observed as a broad band, but after the crystallization completed, two sharp peaks overlapping each other were observed at 54 and 57 ppm, which are assigned to tetrahedral Al species at T1, T2 sites and T3-T9 sites, respectively [10,36,37]. 3.2.2. Scanning electron microscopy (SEM) SEM observations showed a clear change in the particle morphology of the recovered solid products (Fig. 4). After 2 d of the crystallization, small crystalline particle (~100 nm) with a specific morphology appeared on the amorphous solids. The mean size of the crystalline particles was increased up to 1e2 mm along with the crystallization period. Simultaneously, the amorphous solids diminished. Truncated square-bipyramidal shape of the particle Fig. 1. Evolution of Beta zeolite by using colloidal silica at (a) 1 d, (b) 2 d, (c) 2.5 d, (d) 3 d, (e) 3.5 d, (f) 4 d, and (g) 5 d.a Circle and triangle indicate diffractions of the MOR and GIS phases, respectively. aSynthesis conditions: 1.0 SiO2: 0.1 Al(OH)3: 0.6 NaOH: 25 H2O: 10 wt% seed, 140  C, static, 1e5 d.

crystallization than that of colloidal silica, the contamination by other crystal phases was inevitable. Importantly, the SiO2 yield for fumed silica was slightly lower than that for colloidal silica. 3.2. Crystallization mechanism When using colloidal silica, the crystallization process was monitored by several characterization techniques to obtain information regarding the mechanism of the formation of Beta zeolite. 3.2.1. Solid-state NMR First, the solid-state 29Si MAS NMR technique was applied to investigate the crystallization process. The solid products obtained before 2 d gave a very broad band at 102 ppm derived from the amorphous material (Fig. 3A). When the crystallization time was extended to 2.5 d, the two peaks overlapping the broad band appeared at 105 and 110 ppm. As the crystallization further proceeded, the broad band gradually disappeared. After 4 d of the crystallization, four definite peaks appeared at 114.9, 110.2, 104.8 and 99.3 ppm. The peaks at 114.9 and 110.2 ppm are assigned to Q4 species at T1, T2 sites and T3 e T9 sites, respectively, where Qn ¼ (OH)4nSi(OM)n, M ¼ Si or Al [11,35,36]. The peak at 104.8 ppm corresponds to (OAl)Si(OSi)3

Fig. 2. Evolution of Beta zeolite by using fumed silica at (a) 1 d, (b) 2 d, (c) 3 d, and (d) 4 d.a Circle and triangle indicate diffractions of the MOR and GIS phases, respectively. a Synthesis conditions: 1.0 SiO2 (fumed silica): 0.1 Al(OH)3: 0.6 NaOH: 25 H2O: 10 wt% seed, 140  C, static, 1e4 d.

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Fig. 3. Monitoring the evolution of Beta zeolite by (A) 29Si MAS NMR and (B) 27Al MAS NMR spectroscopies at (a) 1 d, (b) 2 d, (c) 2.5 d, (d) 3 d, (e) 3.5 d, and (f) 4 d.a aSynthesis conditions: 1.0 SiO2 (colloidal silica): 0.1 Al(OH)3: 0.6 NaOH: 25 H2O: 10 wt% seed, 140  C, static, 1e4 d.

was clearly observed at 4 d. Thus, crystalline particles were grown with the amorphous solids consumed. It is noteworthy that the SiO2 yield of the products was constant at ~53% throughout the crystallization (Table 1), though the amorphous solids were consumed. This fact implies that the amorphous solids partially dissolved into the solution, and simultaneously a part of dissolved species in the solution deposited onto the remaining solid surface. In general, amorphous aluminosilicate materials are more easily dissolved into alkaline solution than crystalline ones. Therefore, SiO2 raw material preferentially dissolved and the deposition could occur on the seed crystals. Consequently, the proportion of the crystalline particles in the solid phase was gradually increased, being consistent with the SEM observation. When the crystallization period was increased from 1 to 4 d, the (Si/Al)solid ratios were gradually decreased from 6.0 to 5.4, even though the SiO2 yield was little changed. This slight decrease in the (Si/Al)solid ratio almost corresponded to the increase in the Al2O3 yield from 89% to 98%. The Al species dissolved in the solution were incorporated into the solids again in the course of crystallization and finally almost completely consumed at 4 d. Mintova et al. reported that after the complete crystallization, the Si/Al ratio of the mother solution becomes infinite, proposing that Al content in the solution is the limiting factor for the crystallization of Beta zeolite [6]. 3.2.3. 29Si and 27Al solution NMR The silicate species dissolved in the solution were analyzed by 29 Si solution NMR (Fig. S4). The spectra of the filtrates obtained at 2, 3 and 4 d showed that many kinds of (alumino)silicate species including Q0, Q1, Q2, and Q3 species were present in the solution [38,39] and that the concentration of these dissolved species did not change through the crystallization; the dissolution and the deposition of the silicate species are in equilibrium. Such dissolution equilibrium was first proposed by Zhdanov in the early days for the synthesis of zeolite A; the concentration of Si atoms in the solution remained constant during the crystallization, but that for Al atoms was changed [40]. 27Al solution NMR spectra of the

filtrates in Fig. S5 showed a broad peak at around 60 ppm, indicating that Al atoms were present not in the form of monomer but in the form of aluminosilicate species in tetrahedral coordination [39,41]. Thus, we assume that aluminosilicate species (not silicate species alone) in the solution deposited onto the surface of the seed crystals. 3.2.4. Discussion on the crystallization mechanism Possible crystallization mechanism is shown in Scheme 1. The behavior observed in the present study were similar to that for the synthesis of zeolite A through the “solution-mediated crystallization mechanism” reported by Zhdanov [40]. Okubo et al. also reported a similar solution-mediated crystallization mechanism for the OSDA-free synthesis of Beta zeolite [9]. Here, we considered the mechanism with reference to these reports as follows. (i) Amorphous SiO2 raw material was hydrolyzed and the precursor suspension is separated into the two phases that are connected by the solubility equilibrium; one is a solid phase containing the seed crystals and the other is a liquid phase. (ii) After an induction period for 1e2 d, the crystal growth starts on surface of the seed crystal exposed to the liquid. The growth of the seed crystal proceeds with the equilibrium kept. Because the produced zeolite crystal is less likely to be dissolved than amorphous materials, the total reaction moves forward driven by aluminosilicate species in the solution. Thus produced zeolite can act itself as “seed” later and crystal growth further proceeds. (iii) Finally the aluminosilicate species in the solution was almost consumed and the crystallization was completed. Then a part of silicate species (in some case, also a small amount of aluminosilicate species) remains in the solution, resulting in the formation of Beta zeolite with different composition from the precursor suspension; the Beta zeolite with a low Si/Al ratio was formed. In the synthesis using fumed silica, dissolved species in the filtrates were also analyzed by 29Si solution NMR technique (Fig. S6).

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Fig. 4. Monitoring the evolution of Beta zeolite observed by SEM at (a) 1 d, (b) 2 d, (c) 2.5 d, (d) 3 d, and (e) 4 d. Scale bar in each corresponds to 1 mm.a

Compared with the spectra for the syntheses using colloidal silica, the spectra for fumed silica showed sharp peaks in high intensities on broad peaks in the Q1, Q2 and Q3 regions, indicating a high concentration of small cluster species with a lower polymerization degree. High reactivity of fumed silica, as observed in the shorter induction period, would lead to a high degree of hydrolysis and the high concentration. Similar high reactivity of freeze-dried silica under the OSDA-free synthesis conditions was reported by Mintova

et al. [6]. We assume that in the syntheses using fumed silica, the rate for dissolution of amorphous solid would not match with the rate for consumption (deposition onto the seed) of dissolved species. When using fumed silica, the Al2O3 yield was decreased as the crystallization time prolonged, while the yield was increased for colloidal silica (Tables 1 and 2). Although reasons for this difference have not been elucidated, it might be related to the different kinetics in the dissolution/deposition.

Scheme 1. Solution-mediated mechanism for evolution of Beta zeolite in the absence of OSDA.

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Table 2 Structural and compositional changes through crystallization of Beta zeolite using fumed silica.a Entry

Time (d)

1 2 3 4

1 2 3 4

Product phase

(Si/Al)solid ()

SiO2 yield (%)

Al2O3 yield (%)

amorphous BEA þ amorphous BEA þ trace MOR * BEA þ MOR

5.6 5.5 5.0 5.1

48 45 42 42

87 82 84 81

* *

a Synthesis conditions: 1.0 SiO2 (fumed silica): 0.1 Al(OH)3: 0.6 NaOH: 25 H2O: 10 wt% seed, 140  C, static, 1e4 d.

3.3. Optimization of synthesis parameters 3.3.1. Amount of NaOH Further study on the influence of the precursor composition was conducted by using the colloidal silica. The influence of alkalinity was investigated by performing hydrothermal syntheses with the amounts of sodium hydroxide varied (Table 3, Entries 1e5); the suspension with the composition of 1.0 SiO2: 0.1 Al(OH)3: 0.4e0.8 NaOH: 25 H2O: 10 wt% seed was crystallized at 140  C for 4 d Fig. 5 shows the XRD patterns of the recovered solids. Synthesis with the NaOH/Si ratio at 0.4 gave an amorphous material. Upon increasing the ratio to 0.5, a mixture of the *BEA-type crystal and amorphous material was obtained. The synthesis at the ratio of 0.6 provided a highly crystalline Beta zeolite. Further increase resulted in the formation of the GIS phase. At low alkalinity, the crystallization did not initiate or very slowly proceeded probably because the hydrolysis of raw materials was slow [42,43] and seed particles were not exposed to the solid surface where the crystal growth would proceed. Undesired crystal phases (e.g. the GIS phase) were dominant at high alkalinity because not only amorphous aluminosilicate but also seed particles were hydrolyzed and dissolved into the solution, resulting in the crystallization without any structure-direction. At moderate alkalinity, amorphous aluminosilicate preferentially were dissolved into the solution and seed particles selectively grew. 3.3.2. Amount of seed crystal In the seed-directed OSDA-free synthesis of zeolites, the amount of seed crystal is a critical factor determining the synthesis results [7,10,14,18,21]. Precursor suspensions containing 0e15 wt% of seed crystal were heated at 140  C for 4 d (Table 3, Entries 3 and 6e9). The synthesis in the absence of seed crystal resulted in formation of amorphous materials. As the amount of seed crystal was increased, the intensities of XRD peaks assigned to the *BEA phase were increased and 10 wt% of the seed crystal was found to be enough for obtaining highly crystalline Beta zeolites (Fig. S7). 3.3.3. Effect of the Al content in the precursor on the crystallization Based on the crystallization mechanism stated above, it is considered that the concentration of Al species in the liquid phase

Fig. 5. XRD patterns of samples synthesized with NaOH/SiO2 ratio at (a) 0.4, (b) 0.5, (c) 0.6, (d) 0.7, and (e) 0.8.a aSynthesis conditions: 1.0 SiO2 (colloidal silica): 0.1 Al(OH)3: 0.4e0.8 NaOH: 25 H2O: 10 wt% seed, 140  C, static, 4 d.

is crucial for the SiO2 and zeolite yields. The synthesis of OSDA-free Beta was performed with the Al content in the precursor increased. It would be predicted that upon increasing the Al content, the formation of undesired crystal phase is enhanced. However, the formation of such sub-phases was successfully avoided by increasing the seed amount as follows. In the syntheses performed at the Si/Al ratio of 8, 15 wt% or more of seed crystal enabled the selective formation of the *BEA phase (Table 4, Entries 1e3). A large amount of seed crystal provided a large surface area on which crystal growth occurred, leading to the selective formation of the desired crystal phase. When the Al content of the precursor was further increased, Beta zeolites were also successfully obtained. XRD patterns of these samples are shown in Fig. S8. Finally when the (Si/Al)solid ratio of the precursor was 5.5, 84% of the SiO2 yield was achieved. As far as we know, this

Table 3 Summary of OSDA-free syntheses with the Si/Al ratios in the precursors at 10.a Entry

NaOH/SiO2

Seed (wt%)

Time (d)

Product phase

(Si/Al)solid ()

SiO2 yield (%)

Al2O3 yield (%)

1 2 3 4 5 6 7 8 9

0.4 0.5 0.6 0.7 0.8 0.6 0.6 0.6 0.6

10 10 10 10 10 0 2.5 5 15

4 4 4 4 4 4 4 4 4

amorphous * BEA þ amorphous * BEA GIS þ unknown GIS amorphous * BEA þ amorphous * BEA þ amorphous * BEA þ trace GIS

7.1 6.0 5.4 3.5 3.9 6.0 6.2 5.8 5.9

67 57 53 34 32 52 55 53 57

94 96 98 97 81 87 89 91 97

a

Synthesis conditions: 1.0 SiO2 (colloidal silica): 0.1 Al(OH)3: 0.4e0.8 NaOH: 25 H2O: 0e15 wt% seed, 140  C, static, 4 d.

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Table 4 Summary of OSDA-free syntheses with high-aluminum compositions.a Entry

Si/Al

NaOH/SiO2

Seed (wt%)

Time (d)

Product phase

(Si/Al)solid ()

SiO2 yield (%)

Al2O3 yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

8 8 8 7 7 7 6 6 6 6 5.5 5.5 5.5 5.5

0.6 0.6 0.6 0.6 0.65 0.7 0.55 0.6 0.6 0.65 0.6 0.6 0.6 0.65

10 15 20 15 15 15 15 15 15 15 15 15 15 15

4 4 4 4 4 4 6 4 5 5 1 5 6 5

*

5.5 5.6 6.1 5.5 5.3 4.8 5.5 5.4 5.2 4.8 5.3 5.1 5.2 4.5

60 63 68 71 69 62 80 79 77 69 83 81 84 72

87 89 89 90 91 90 88 88 89 87 87 87 89 88

a

BEA þ GIS þ MOR * BEA * BEA * BEA * BEA * BEA þ trace MOR * BEA * BEA þ amorphous * BEA * BEA þ GIS amorphous * BEA þ amorphous * BEA * BEA þ GIS

Synthesis conditions: 1.0 SiO2 (colloidal silica): 0.125e0.182 Al(OH)3: 0.6e0.7 NaOH: 25 H2O: 10e20 wt% seed, 140  C, static, 1e6 d.

is a significantly high yield compared to the results in the early reports. Note that in our experimental settings, the crystallization evolved from the precursors rich in water, demonstrating that a high zeolite yield can be achieved in the solution-mediated synthesis as well as solvent-less synthesis reported by Xiao et al. [12]. The zeolite yield based on the recovery of SiO2 was plotted against the Si/Al ratio in the precursor in Fig. 6. Obviously, the SiO2 yield was improved by increasing the Al content in the precursor suspension. This correlation implies a mechanism in which dissolved species in the form of particular aluminosilicate units deposited onto the seed crystal. The (Si/Al)solid ratios of the obtained products were in the narrow range of 5e5.5, even though these samples were synthesized from the different starting Al contents (Si/Al ¼ 5.5e10). Moreover, these samples showed similar distributions of framework Al atoms in 27Al MAS NMR spectra (Fig. S9). These results also suggest the presence of particular aluminosilicate species with Si/Al ratios similar to those of the product that deposit onto the surface of seed crystals following particular manners, leading to the similar aluminum distribution in the products. Although several species (secondary and composite building units) have been proposed, we

have not determined the exact structure(s) of the crucial aluminosilicate species [12,44,45]. For the syntheses with the high aluminum contents, 4e6 days were required to complete the crystallization, although the rapid crystallization was reported for the syntheses with low-aluminum contents at the same temperature [5,7]. Such a long crystallization time would be explained from the two reasons. (i) A large amount of the amorphous material covers the seed crystal in a synthesis with a high aluminum content and so it takes a longer induction time. (ii) With the high aluminum content, a large amount of deposition occurs until crystallization completes. Moreover, the concentration of silicate species was decreased at a late stage. Therefore, it takes longer for the crystal growth. The decrease in the concentration of silicate species led to the increase in the basicity of the solution at the late stage. The pH of the filtrate for Entry 13 in Table 4 was 12.5, while the pH value for Entry 6 in Table 1 was 12.1. Under strong basic conditions, a part of aluminosilicate species remained in the solution, which was reflected in the decrease of Al2O3 yield to ~90%. Moreover, Beta zeolite product would be partly dissolved into the solution under highly basic conditions. Consequently, there is an upper limitation of SiO2 and Al2O3 yields. 4. Conclusions OSDA-free Beta zeolite was successfully synthesized from highaluminum precursor suspension, achieving a high zeolite yield. Under the hydrothermal conditions, crystallization proceeded through the solution-mediated mechanism in which aluminosilicate species (not silicate species alone) in the solution deposited onto the surface of seed crystals. Thus, the Al content of the precursor determined the SiO2 and zeolite yields. Upon increasing the Al content of the precursor, the SiO2 yield was improved up to ~80%, which was a significantly high yield compared to the yields in the early reports. The kind of silica raw material strongly affected the crystallization behavior; colloidal silica, which showed moderate reactivity, was more suitable than fumed silica. A moderate alkalinity facilitated the hydrolysis of amorphous precursor and the selective crystal growth. The crystallization was initiated by the exposure of seed crystals at the solid/liquid interface and the crystallization rate was increased by a large amount of seed crystal. Appendix A. Supplementary data

Fig. 6. The dependence of zeolite yield based on SiO2 on the Al content in precursor suspension.

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.11.037.

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