Synthesis and characterization of ultrafine sub-micron Na-LTA zeolite particles prepared via hydrothermal template-free method

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Synthesis and characterization of ultrafine submicron Na-LTA zeolite particles prepared via Hydrothermal template-free method Mostafa Jafari, Toraj Mohammadi, M. Kazemimoghadam

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Cite this article as: Mostafa Jafari, Toraj Mohammadi, M. Kazemimoghadam, Synthesis and characterization of ultrafine sub-micron Na-LTA zeolite particles prepared via Hydrothermal template-free method, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2014.04.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and characterization of ultrafine sub-micron Na-LTA zeolite particles prepared via hydrothermal template-free method *

Mostafa Jafari, Toraj Mohammadi , M. Kazemimoghadam

Research Centre for Membrane Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran Corresponding author: E-mail: [email protected] Tel: +98 21 789 6621; Fax: +98 21 789 6620

Abstract Ultrafine sub-micron Na-LTA zeolite crystals ranging from 150 to 350 nm were synthesized without template via hydrothermal method. Aluminum iso-propoxide and colloidal silica were used as Al and Si sources respectively to synthesize the small and uniform zeolite crystals. Zeolite Na-LTA was synthesized hydrothermally from a gel formula of 1.0 Al2O3: 2.7 SiO2: 5.85 Na2O: 150 H2Oin molar basis. The crystallization conditions including synthesis time, and synthesis temperature were optimized to reduce the zeolite particle size. The final crystals were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results confirmed that synthesis temperature should be lowered for better control of the crystal growth. On the other hand, at low synthesis temperatures, synthesis time should be extended to acquire the high crystallinity. Ultrafine sub-micron Na-LTA zeolite crystals were synthesized at synthesis temperature of 60 Ԩ and synthesis time of 11 h.

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Keywords: Ultrafine Na-LTA zeolite; Sub-micron crystals; Hydrothermal; Template-free

1. Introduction Zeolites as microporous inorganic materials have gained extensive applications in various industries such as ceramic membranes, mixed matrix membranes, adsorbents, catalysts, environmental remediation, chemical sensors, and ion-exchangers. Because of their superior properties including acidity, size or shape selectivity, thermal stability, and high ion-exchange capacity, zeolites are currently being used in chemical and petrochemical industries [1-9]. Much works have been conducted in zeolite synthesis to prepare high-quality zeolites with small particle size and narrow particle size distribution (PSD). It is also evident that reduction of particle size of zeolites leads to considerable changes in their properties. Smaller zeolites provide higher surface area per unit volume. Smaller zeolites also exhibit less mass transfer resistance against transport of components [3,10, 11]. Some synthesis methods have been proposed to control the crystal size of zeolites. Confined-space method [1215], and utilization of tetra methylammonium hydroxide (TMAOH) organic cation as template [11,16, 17] are some novel techniques which have been recently developed. Using template in synthesis of zeolites has several disadvantages. Firstly, template removal from zeolite structure is normally carried out by calcination process which causes irreversible aggregation of the crystals. Second, using template in synthesis solution may change Si/Al ratio in the final product. The latter can drastically affect the zeolites applications (e.g., air separation). For example, zeolite A, synthesized using template (tetra methylammonium), has a Si/Al ratio higher than 1. After Na+ exchange, it has a pore size larger than 4 Å [13, 18]. In contrast, template-free synthesis leads to Si/Al=1 and zeolite 4A after Na+ exchange. This slight change

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in pore size can affect capability of zeolite A in air separation dramatically [19]. Finally, using template in zeolite synthesis increases production cost considerably. Confined-space is a novel method which is recently developed for synthesis of small zeolite crystals. In this method, the crystal growth is stopped by limiting the nutrients in an inert mesoporous matrix. Some results for synthesis of nano size LTA zeolite via confined-space method are listed in Table 1.

Table 1: Studies on synthesis of LTA nanocrystals by confined-space methods

Confined-space method offers some disadvantages compared to other synthesis procedures. The latter method is complicated and designing a proper reaction medium is difficult. It needs exact control of operational parameters. On the other hand, adding confined materials may affect properties of final zeolite crystals. Finally, efficiency of this method is low compared to other synthesis techniques. Hydrothermal template-free method has been known as a simple and effective method for synthesis of zeolites. Synthesis of small zeolite particles using hydrothermal template-free method depends on selection of appropriate sources and control of crystal growth using particular methods. Some research works have been carried out on development of template-free method for preparation of nano crystals of zeolite LTA. Nikolakis [21] performed a review on interactions between zeolite crystals and precursors during hydrothermal synthesis. It was reported that the key parameter is understanding of the interactions between the zeolite nanocrystals. This is helpful for improvement of suspension handling. Another work on synthesis of nano zeolites was conducted by Tosheva and Valtchev [22]. Their results showed that utilization of initially clear solutions is the most effective and widely used approach for

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synthesis of nano zeolite crystals. However, synthesis yield of nano zeolite crystals is low due to low synthesis temperature. Therefore, formation of LTA-type zeolite nanocrystals at room temperature was considered as the first idea. Various synthesis methods have also been suggested for controlling synthesis of zeolite nanocrystals. Casado et al. [23] synthesized microporous titanosilicate ETS-10 using hydrothermal method with various Ti sources. The authors used seeds of ETS-10 crystals for controlling zeolite growth. They also showed that adjustment of synthesis parameters can control crystallization rate. Casado-Coterillo et al. [24] synthesized ITQ-29 zeolite crystals for use in mixed matrix membranes. They could reduce crystal size down to 2.5 microns by introduction of zeolite seeds in synthesis gel. The use of seeds for synthesis of nano zeolites is based on the fact that in nano zeolites, low synthesis temperatures are usually applied to prevent the formation of large zeolite crystals. However, for synthesis of high Si/Al ratio zoelites such as ZSM-5, ITQ-29, and Beta which have high synthesis temperatures, to promote the formation of zeolite crystals at low synthesis temperatures, seeds are used in the initial gel [24-26]. Selection of appropriate sources is of great importance for synthesis of nano crystals of zeolite LTA. The results have shown that aluminum iso-propoxide and colloidal silica are favorable as alumina and silica sources, respectively for synthesis of small LTA zeolite crystals [27]. The main aim of this study is to synthesize ultrafine sub-micron Na-LTA zeolite crystals using the template-free method. The effects of synthesis time and temperature on crystal growth are investigated. The optimum time and temperature for synthesis of ultrafine sub-micron Na-LTA zeolite crystals are then obtained. The synthesized sub-micro crystals are characterized by SEM and XRD analysis.

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2. Experimental 2.1. Materials Sub-microNa-LTA zeolite powder was synthesized via hydrothermal heating method in a PTFE autoclave. The used reagents for synthesis of sub-micro crystals were aluminum isopropoxide (Merck, 97% C9 H21 Al O3) as source of Al and colloidal silica (Ludox, Aldrich 50% SiO2) as source of Si. Sodium hydroxide (Merck, 98% NaOH) and deionized water (resistivity of 13.2 Mȳ·cm) were also used for synthesis of sub-micron powder.

2.1. Synthesis of Na-LTA sub-micron powder In this work, sub-micronn zeoliteNa-LTA powders (Z1-Z6) were synthesized via hydrothermal method with gel composition of 1.0 Al2O3: 2.7 SiO2: 5.85 Na2O: 150 H2O. The synthesis gel for hydrothermal treatment was prepared by mixing precursors of aluminate and silicate. For this purpose, NaOH was completely dissolved in deionized water in a polypropylene beaker. The solution was then divided into two equal parts in two polypropylene beakers. For preparation of aluminate solution, aluminum isopropoxide was added to one part of the NaOH solution. The soultion was mixed at 60 Ԩ until cleared. Silicate solution was prepared by mixing Ludox to the second part of the NaOH solution at 60 Ԩ until cleared. Two solutions were then cooled down to 25 Ԩand afterward, the silicate solution was poured into the aluminate solution and mixed until a thick homogenized gel was formed for synthesis. The formed gel was then aged for 48 h at room temperature. After synthesis of the powders (S1-S6), the sub-micro particles were recovered by washing with deionized water and filtering with filter paper (Whatman Grade 3),

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until pH of the suspensions became close to 8-9. The procedure of zeolite the Na-LTA synthesis is schematically presented in Fig. 1.

Figure 1: Schematic representation of the procedure of zeolite Na-LTA synthesis

3. Characterization of zeolite powders 3.1. SEM and XRD The formation of Na-LTA phase was verified by X-ray diffraction (XRD). XRD measurements were conducted by a Siemens powder diffractometer using Cu KD radiation operating at 40 kV and 30 mA. The size and morphology of the synthesized Na-LTA zeolite powders were observed by SEM. The SEM images were obtained using a Vega Tescan scanning electron microscope.

3.2. Software The crystal size of the synthesized zeolite powders was estimated using Micro Structure Measurement software. This estimates the crystal size from the SEM images. The crystallinity of the synthesized powder was also calculated using X-powder (ver.2004.04.82 PRO) software.

4. Results and discussion 4.1. Effect of synthesis time on particle size and crystallinity Table 2 presents the conditions and the results for synthesis of different zeolits powders. The XRD patterns of the synthesized powders (Z1-Z4) are presented in Fig. 2. The obtained results show that all the samples are A-type zeolite, but peak intensity of the samples increases by

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increasing synthesis time from 6 h to 15 h. It confirms that crystallinity increases by increasing synthesis time [28]. Fig. 3 shows the SEM micrographs of the synthesized powders (Z1-Z4). The SEM results show that increasing synthesis time from 6 h to 15 h does not change the morphology of the synthesized zeolite powders and the spherical shape of the particles remains almost unchanged. This can be attributed to the low synthesis temperature, and it can be concluded that at 60 Ԩ, synthesis time has the modest effect on morphology of the synthesized zeolite powders and also it does not have any significant effect on the average particle size of the synthesized A-type zeolite powders. Moreover, the calculated crystallite size as presented in Table 2, confirms this observation. This is in agreement with the results of Zhan et al. [29].

Table 2: Synthesis conditions and particle size of the zeolite crystals

As it can be seen from the SEM images, the sub-micron zeolitic powders are synthesized at the synthesis temperature of 60 Ԩ. It can be concluded that synthesis time of 6 h is a confident time for synthesis of A-type zeolite powders at synthesis temperature of 60 Ԩ. At this temperature, the pure zeolite A is synthesized even at higher synthesis times (such as 15 h) and no other competing phases such as NaX are formed. Synthesis time of 11 h can be selected as the optimum time for synthesis of A-type zeolite because at this time the complete crystallinity is achieved and then the formed zeolite crystals grow which causes the formation of larger crystals [30].

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Figure 2: XRD patterns of the A-type zeolite powders synthesized at different synthesis times (Z1: 6 h, Z2: 8 h, Z3: 11 h, and Z4: 15 h) at synthesis temperature of 60 Ԩ

Figure 3: SEM images of the synthesized A-type zeolite particles. (Z1: 6 h & 60 Ԩ, Z2: 8 h & 60 Ԩ, Z3: 11 h & 60 Ԩ, Z4: 15 h & 60 Ԩ, Z5: 6 h &70 Ԩ, Z6: 6 h &80 Ԩ)

4.2. Effect of synthesis temperature on particle size and crystallinity Synthesis temperature plays a crucial role in the formation of zeolites. Many researchers in the field of zeolite synthesis have investigated effect of crystallization temperature on zeolite formation. The optimal temperature mainly depends on the Si to Al ratio of zeolites [28, 31-33]. Fig. 4 shows XRD patterns of the zeolite powders synthesized at different temperatures (Z1, Z5, and Z6). The XRD patterns (Fig. 4) and SEM images (Fig. 3) show that for the synthesis time of 6 h and at synthesis temperature of 60 Ԩ, both amorphous and crystalline phases coexist. Upon increasing temperature to 70 Ԩ, significant growth of NaA crystals is observed. It is observed 2ߤ݉

2ߤ݉

that increasing temperature (from 60 to 70 Ԩ) for synthesis time of 6 h has more significant effect on particle size and larger zeolite particles are formed compared with extending (from 6 to 15 h) at synthesis temperature of 60 Ԩ. All these observations reveal that the kinetics of the crystal growth is temperature-dependent [17]. The desired zeolite phase can only be synthesized within a particular temperature range. Increasing temperature can affect the type of synthesized zeolites and both NaA and NaX zeolites are formed with increasing temperature from 70 Ԩ to 80 Ԩ for synthesis time of 6 h. The measured crystal size and the type of synthesized zeolite are presented in Table 2. The results showed that the temperature has significant effect on crystal growth so that by increasing 10 Ԩ on temperature, crystals growth were considerable and with further increasing, another 8 

zeolitic phases were formed. Therefore, synthesis time was adjusted at low temperature (60 Ԩ) due to the slight effect of time on crystal growth.

Synthesis temperature also affects morphology of the zeolite powders. At lower temperatures, the crystals are almost spherical (Fig. 3-Z1-Z4), whereas at higher temperatures, the crystals are almost cubic (Fig. 3-Z5). Based on the autocatalytic nucleation mechanism, high temperature accelerates the gel dissolution rate in the synthesis gel and makes the growth of numerous nuclei faster. Moreover, at higher temperature, the crystal growth rate increases significantly and equilibrium of the crystal growth is reached within shorter time and as a result effect of synthesis time on the size of zeolite particles is negligible [32, 33]. Therefore, only at lower temperatures, adjustment of synthesis time is effective to easier control the size of sub-micron zeolite crystals [33].

Figure 4: XRD patterns of the A-type zeolite powders synthesized at different temperatures (Z1: 60 Ԩ, Z5: 70 Ԩ, and Z6: 80 Ԩ) for synthesis time of 6 h

5. Conclusions Zeolite LTA powder in the range of 150 to 350 nm was synthesized in this work. The synthesis method was hydrothermal treatment without using templates structure directing agent. The synthesis gel formulawas1.0 Al2O3: 2.7 SiO2: 5.85 Na2O: 150 H2O in molar basis. Synthesis parameters including time, and temperature were optimized to prepare ultrafine submicron zeolite LTA particles. The synthesized particles were characterized using solid characterization including X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was indicated that better control of crystal growth is achieved by reduction of synthesis temperature. Moreover, 9 

synthesis time should be lowered to obtain zeolites with higher crystallinity. Finally, synthesis temperature of 60 Ԩ and synthesis time of 11 h was selected for preparation of ultrafine submicron LTA zeolite powders.

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Table captions Table 1: Studies on synthesis of LTA nanocrystals by confined-space methods. Table 2: Synthesis conditions and particle size of the zeolite crystals.

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Figure captions Figure 1: Schematic representation of the procedure of zeolite Na-LTA synthesis. Figure 2: XRD patterns of the A-type zeolite powders synthesized at different synthesis times (Z1: 6 h, Z2: 8 h, Z3: 11 h, and Z4: 15 h) at synthesis temperature of 60 Ԩ. Figure 3: SEM images of the synthesized A-type zeolite particles. (Z1: 6 h & 60 Ԩ, Z2: 8 h & 60 Ԩ, Z3: 11 h & 60 Ԩ, Z4: 15 h & 60 Ԩ, Z5: 6 h &70 Ԩ, Z6: 6 h &80 Ԩ). Figure 4: XRD patterns of the A-type zeolite powders synthesized at different temperatures (Z1: 60 Ԩ, Z5: 70 Ԩ, and Z6: 80 Ԩ) for synthesis time of 6 h.

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Table 1: Molar gel composition

confined-space medium

Synthesis time (min)

synthesis temperature (K)

Ref

1 Al2O3: 2.7 SiO2: 5.85 Na2O: 182 H2O

methylcellulose

180

353

[14]

1 Al2O3: 5 SiO2: 8 (TMA)2O: 0.22 Na2O: 400H2O

CTAB/nbutanol/heptane

378

373

[15]

CTAB/nbutanol

20

348 (MW)

[13]

carbon black

900

453

[20]

1 Al2O3:2.7 SiO2: 5.85 Na2O:182 H2O

1Al2O3: 10SiO2: 4Na2O : 162H2O MW=microwave heating.

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Table 2: No.

Time(h)

temperature (Ԩ)

Zeolite

Crystal size (nm)

Z1

6

60

NaA

100-250

Z2

8

60

NaA

100-350

Z3

11

60

NaA

150-350

Z4

15

60

NaA

300-550

Z5

6

70

NaA

300-600

Z6

6

80

NaA+NaX

300-800

15 

fig 1.doc

Fig 1:

1

Adding aluminum isopropoxi d

Addition of NaOH

2

NaOH solution

NaOH solution

DI water

Heating at 333 K with Stirring (1 h)

Heating at 333 K with Stirring (1 h)

Silicate solution (297 k)

3

Aluminate solution (297 K)

Adding collodial silica

4

Alumina silicate solution

Aging at 297 K with Stirring (48 h)

fig 4.tif

fig2.tif

fig3.tif