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Novel hydrothermal synthesis of hierarchically-structured zeolite LTA microspheres
Accepted Manuscript Novel hydrothermal synthesis of hierarchically-structured zeolite LTA microspheres Mansoor Anbia, Esmat Koohsaryan, Asieh Borhani PII:
S0254-0584(17)30189-X
DOI:
10.1016/j.matchemphys.2017.02.048
Reference:
MAC 19543
To appear in:
Materials Chemistry and Physics
Received Date: 10 January 2017 Revised Date:
14 February 2017
Accepted Date: 23 February 2017
Please cite this article as: M. Anbia, E. Koohsaryan, A. Borhani, Novel hydrothermal synthesis of hierarchically-structured zeolite LTA microspheres, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.02.048. 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 proof before it is published in its final 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.
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ACCEPTED MANUSCRIPT Novel hydrothermal synthesis of hierarchically-structured zeolite LTA microspheres Mansoor Anbia*, Esmat Koohsaryan, Asieh Borhani Research Laboratory of Nanoporous Materials, Faculty of Chemistry, Iran University of Science and
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Technology, Farjam Street, Narmak, P.O. Box 16846-13114, Tehran, Iran E- mail: [email protected], Phone: +98 21 77240516-17, Fax: +98 21 77491204
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Abstract
A robust hydrothermal preparation of nanozeolite LTA has been made. It is the first report
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of zeolite NaA synthesis with smallest crystal sizes, highest surface area and a hierarchical structure as micron-sized spheres formed by nanocrystallites assemblies in a short time. The effects of time, temperature of crystallization and seeding on the final products properties have been studied. The prepared samples have been characterized using various techniques.
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Moreover, the synthesis procedure has been fulfilled using the initial natural precursors supplied from the mines in Iran. The results ascertain the successful synthesis of the high-
Keywords
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qualified products with potential adsorptive and catalytic applications.
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Nanozeolite LTA, Seeding, Hydrothermal synthesis, Microsphere, Hierarchical zeolite
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ACCEPTED MANUSCRIPT 1. Introduction Zeolites are hydrated crystalline microporous aluminosilicates constructed from 3dimentional frameworks of tetrahedral TO4 (most of the time T is Al or Si) which are connected with each other through sharing corner oxygen atoms. In such arrangement,
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numerous channels and cavities containing water and charge compensating alkali and earth alkali cations are formed [1-3].
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Zeolitic structures have many applications in different fields such as agriculture, water and waste water treatment, petroleum, petrochemical and detergent industries due to their
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prominent properties such as porous system, high surface area, outstanding hydrothermal and chemical stability, being eco-friendly and also not toxic compounds[4-7]. However, commercialized zeolitic materials are provided as micron-sized powder giving rise to some limitations like difficult mass transfer of reactants and product molecules, pore mouth
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blockage and deactivation of catalysts and adsorbents in the chemical reactions [8].
In order to overcome these constraints, two general strategies have been proposed so far: zeolite crystal size reduction (˂ 100 nm) i.e. nanozeolite preparation and secondary
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porosity introduction into zeolite framework leading hierarchical zeolites with larger pores
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besides their intrinsic micropores [9-13]. Substantial advantages such as enhanced surface area, reduced diffusion path length, decreased mass and heat transfer resistance, prohibited side reactions and postponed catalysts and sorbents deactivation are achieved as a result of zeolitic nanocrystallites utilization in the catalytic and adsorptive reactions [14,15]. Owing to their superior features, nanozeolites not only serve as conventional micron-sized zeolites act, but also can be considered as a model system for fundamental studies of crystals growth, preparation of low dielectric constant films, sensors, polymer-zeolite nanocomposite
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ACCEPTED MANUSCRIPT membranes, hierarchically-structured materials, pigments, drug delivery, and magnetic resonance imaging [16-20].
Despite these benefits, there are some obstacles in handling of nanosized zeolites using
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high speed centrifuging. Therefore, it is anticipated that the best strategy is to prepare micronsized hierarchical zeolitic permanent aggregates involving nano-crystals along with
mesopores between them. In this way, the advantages of both structures (nanosized and
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hierarchical zeolites) would be there while there is no need for organic or inorganic binders to shape the zeolite powders and their possible side effects [5,21,22]. It is noteworthy to
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mention that the introduction of an auxiliary porosity in a structure not only could be applied in the case of zeolites, but also is sensible for other compounds [23,24].
One of the potentially useful small pore zeolites, is the LTA (Linde Type A) zeolitic framework with cubic structure, which is usually synthesized in the sodium form (NaA or 4A
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zeolite) and has great applications in various areas specially in laundry detergents as water softening agent [4,25-29]. There have been many efforts to obtain zeolite LTA with crystal size of ca. 500 nm. However, most of the procedures are based on applying
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tetramethylammonium hydroxide (TMAOH) as an appropriate organic structure directing
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agent (SDA) for the LTA framework formation, which are not cost-effective and ecofriendly. In addition, the other drawback is the template removal requiring high temperature calcination process ending in irreversible aggregation of nanocrystallites [30,31]. So, investigation of alternative synthesis routes to get LTA nanozeolites or template-assisted strategies using inexpensive and recoverable agents seems to be a crucial subject.
On the other hand, synthesis processes such as confined-space, milling, micro-reactors and microwaves-mediated methodologies have received attention, but they suffer from some defects e.g. zeolite structure damages due to combustion at elevated temperatures for space3
ACCEPTED MANUSCRIPT limiting matrix elimination, the crystallinity loss of zeolite, high cost and personal safety, respectively [26,5]. It seems that the optimization of synthesis parameters like crystallization time and temperature as well as the type of precursors, could be rationally beneficial to reach defined favorable nanozeolite framework. In this regard, Liu et al. [32] have investigated the
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effects of temperature and time crystallization, system alkalinity and aluminum sources on the zeolite NaA synthesis via dry-gel conversion method. The results have indicated that in the presence of chloride anions (aluminum chloride as Al source), a few zeolite NaX crystals
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were observed along with the end product (zeolite LTA) particles, whereas sodalite crystals formed on using aluminum sulfate as Al source. They have claimed that these impurities
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were present owing to the different steric and electrostatic interactions between anions and zeolites frameworks.
In Alfaro and co-workers study [33], nanocrystals of zeolite LTA (200-300 nm) were
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synthesized at 100 ˚C in the absence of an organic template and the effect of aging time as an important factor on the crystal size was evaluated. It was declared that as-prepared samples during longer aging time (144 h) had the smaller crystal size in comparison with shorter
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aging times (24, 48 and 72 hours). The other work was focused on the preparation of zeolite NaA through hydrothermal method and the examination of various synthetic parameters like
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stirring rate, aging time as well as different Al and Si resources [34]. Almost identical to the previous study [33], increasing the aging time from 1 to 3 hours, resulted in the more fine crystallites (314 nm) due to more crystal nucleation. The stirring rate did not affect the size of particles and Si sources had partial influence on the end product rather than aluminum species.
Contrary to the reported studies investigating a Na2O-SiO2-Al2O3-H2O synthesis system, Edelman et al. [35] have reported that LTA crystals seeding into a synthetic mixture
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ACCEPTED MANUSCRIPT of this zeolite has promoted its crystallization. In fact, by addition of larger seed crystals (ca. 40 µm) to a clear syntheses gel, the nucleation of new population of LTA crystals were promulgated through initial breeding mechanism, while this did not take place in the case of 1-3 µm seed crystals. Moreover, some phase impurities were detected in the absence of seeds
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[36]. In the preparation of high-silica nanozeolite ITQ-29 by Casado-Coterillo et al. [37], seeding influences on the process from two prospects; the crystal size were decreased down to 2.5 µm by increasing the amount of seeds and the second was an elevated crystallinity of
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zeolite ITQ-29 in the presence of seed crystals.
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As mentioned above, nanozeolite LTA is one of the most attractive molecular sieves with unique applications clarifying the importance of assessing its different synthetic procedures and attempts to attain its favorite defined structure via sturdy processes. According to the literatures, there have not been any reports on the preparation of nanozeolite
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LTA with crystal sizes below 100 nm with hierarchical structure involving inter-crystalline mesopores as µ-sized micro-mesoporous LTA spheres at mild ambient reaction conditions and using inexpensive starting materials such as water glass. Therefore, in this study we
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describe the morphology and crystalline phase of the resultant nanozeolite LTA by altering the crystallization time and temperature as well as seed addition. Moreover, the synthesis
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process at the optimal circumstances has been carried out to obtain nanosized zeolite NaA using as-provided Al and Si sources from the natural mines in Iran.
2. Materials and methods 2.1. Chemicals
The used reagents for the synthesis of nanosized zeolite LTA were sodium aluminate (Aldrich, 53% Al2O3 and 42.5% Na2O) as aluminum and water glass (sodium silicate solution, Merck, 27% SiO2 and 8% Na2O) as silicon sources, respectively. Sodium hydroxide 5
ACCEPTED MANUSCRIPT from Ameretat Shimi- Iran was provided to prepare alkali solutions. Distilled water was also used to dissolve precursors and prepare the synthetic mixture. Commercial zeolite 4A and natural sodium and aluminum sources were kindly provided by GaharCeram and BEHDASH
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Chemical Companies (Iran), respectively.
2.2. Synthesis of LTA
Nanozeolite LTA was hydrothermally synthesized pursuant to the syntheses gel
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formula 6Na2O: 2.4SiO2: 1.0Al2O3: 180.0H2O in molar ratio of precursors. At first, alkali solution was prepared using an appropriate amount of NaOH tiny pellets (6.72 g) in 60 mL
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distilled water. Then after, sodium aluminate powder (4.05 g) was added to the solution. After agitation for several minutes, sodium silicate solution (8.35 mL) was also added drop by drop. The obtained milky gel was aged for 30 minutes under continuous stirring. Transferring to a stainless steel autoclave for crystallization treatment in an oven under
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different temperature and time conditions was the next step the synthesis mixture underwent. Overall, three series of experiments were fulfilled as follows: the first one covered the reaction condition with different temperatures i.e. 70, 85, 100 and 115 °C while the
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crystallization time was constant at 8 hours and the end products were named 4A-T1, 4A-T2,
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4A-T3 and 4A-T4, respectively. In order to evaluate the effect of time on the crystallization process, the second series of experiments were carried out under the same temperature (100 °
C) with various times intervals namely 3, 5, 8 and 11 hours and the resulting samples were
labeled as 4A-t1, 4A-t2, 4A-t3 and 4A-t4, respectively.
The addition of a certain quantity of commercial zeolite 4A powder (previously ground using a mortar and pestle) to the zeolite synthetic mixture during aging stage was included in the third step of experiments to specify how seeding could influence the final product properties at different synthesis times pointed earlier. The obtained samples were labeled as 6
ACCEPTED MANUSCRIPT 4A-S1 (t= 3h), 4A-S2 (t= 5h), 4A-S3 (t= 8h) and 4A-S4 (t= 11h). The details of each experiment are listed in Table 1. After hydrothermal process, the slushes were filtered using filter paper and washed several times with distilled water until the liquid reached pH~ 7. The product powders were heated in an oven at ca. 100 °C for some hours to be dried and then
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characterized. Moreover, there were some treatments on the sands in the presence of sodium carbonate to provide solid or liquid sodium silicate with defined SiO2/Na2O ratios in an autoclave or a digester unit under appropriate temperatures and pressures. Likewise, a
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mixture of aluminum hydroxide and liquor containing ca. 22 percent sodium hydroxide
ratio was made.
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endured some thermal process until sodium aluminate solution with a certain Na2O to Al2O3
Table 1. Experimental conditions in hydrothermal synthesis of nanozeolite LTA Crystallization Temperature (˚C)
Crystallization Time (h)
Added commercial zeolite 4A seeds (g)
4A-T1
70
8
na
4A-T2
85
8
na
100
8
na
115
8
na
100
3
na
100
5
na
4A-T3 4A-T4 4A-t1
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4A-t2
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Sample
4A-t3
100
8
na
4A-t4
100
11
na
4A-S1
100
3
0.1
4A-S2
100
5
0.1
4A-S3
100
8
0.1
4A-S4
100
11
0.1
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na: not added
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Nitrogen adsorption–desorption isotherms were performed on a Belsorp-Max equipment (BEL Japan Inc., Japan) at -196 °C while samples had been degassed at 350 °C .
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The pore size distribution and the BET specific surface area were determined by the BJH (Barrett–Joyner–Halenda) and BET (Brunauer, Emmett, and Teller) methods, respectively. In order to investigate the morphological and compositional characteristics of the products, a
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scanning electron microscopy-energy dispersive x-ray equipment (VEGA3-TESCAN) was used. X-ray diffractometer (JEOL-JDX 8030) by Cu-Kα radiation operating at 30 kV and 20
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mA was used to record the XRD patterns. To make a systematic and thorough investigation of the final zeolitic products, the transmission electron microscopy images were observed by a Zeiss-EM10C apparatus operating at 100 kV. FT-IR spectrometer (SHIMADZU-8400-s) with potassium bromide pellets was also employed to detect the absorption bands assigned to
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the zeolites frameworks.
3. Results and discussion
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Fig. 1 represents the x-ray diffraction patterns and FT-IR spectra of the zeolite samples prepared at different temperatures. As can be seen from XRD, there are no observable peaks
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at 70 °C (Fig. 1A (a)), meaning that the amorphous syntheses gel has not been transfigured into a crystalline zeolitic phase. Accordingly, absorbance bands related to the zeolites frameworks are not detectable in the FT-IR spectra (Fig. 1B (a)). Increasing the crystallization temperature to 85 °C leads to partially weak peaks assigned to the zeolite FAU. At 100 °C, the crystalline phase changed into the eligible zeolite LTA framework with its characteristic peaks as reported earlier [21,25]. At higher temperature up to 115 °C, phase impurities i.e. SOD and FAU are formed and their characteristic peaks become apparent. Compatible to the XRD data, in Fourier transform infrared spectra, absorbance bands 8
ACCEPTED MANUSCRIPT correspondent to the zeolitic frameworks are gradually appeared with a rise in the temperature implying the transformation of the initial amorphous gel into crystalline zeolites
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phases.
Fig. 1. (A) X-ray diffraction patterns and (B) FT-IR spectra of the zeolite samples prepared at different temperatures, (a) 70, (b) 85, (c) 100 and (d) 115 °C.
In general, these bands are emerged in the middle and far infrared ranges (1200-400 cm-1) for zeolites frameworks [33,38]. Peaks in the 500-420 cm-1 are relevant to the T-O
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bands in β-cages [39]. In fact, internal bending vibration of (Si or Al)-O at 400 cm-1 suggests the beginning step of crystalline zeolitic phase formation [30]. On the other hand, signals
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which are detectible in just zeolite LTA structure spectra (proportional to the double-4membered rings (D4R)) and not FAU or SOD frameworks are located in the wavenumber
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range of 650-500 cm-1 and as-synthesized sample 4A-T3 shows a band at ~ 517cm-1 [33]. Broad bands at approximately 1000 cm-1 belong to the internal asymmetric stretching vibration of tetrahedra T-O becoming sharper as amorphous materials are being converted to the crystallized zeolitic phases [38].
Contrary not being observed in the XRD pattern of sample 4A-T3, absorbance bands in the range of 800-600 cm-1 attributed to the simple-4-membered rings (S4R) in the zeolite SOD structure, could be seen in all spectra except in the case of product 4A-T1 indicating the
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ACCEPTED MANUSCRIPT presence of a slight impurity in the end products [39]. Peak related to the transfiguring vibration of water molecules bounded in the zeolite channels are exhibited at nearly wavenumber 1650 cm-1 [38].
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Interestingly, the formation of small zeolite LTA crystallites is confirmed from the bands width. Indeed, band broadening in the XRD patterns consistent with the crystal size reduction could be explained by Scherrer equation as Dhkl = Kλ/(β xCosƟhkl) [30].
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SEM micrographs and chemical composition of the products obtained from EDX
analyses are illustrated in Fig. 2. As could be observed, temperature modification has resulted
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in changes of zeolites phases and morphologies. Matched to the XRD data, at low temperature (70 ˚C), there is no clue of a formed crystalized phase. By increase in the temperature, octahedral FAU crystals along with sodalite particles are detected in the pictures. However, sphere aggregates from nano-cubic zeolite LTA assemblies are created at
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100 ˚C and average crystal size of these cubes is estimated to be ca. 80 nm (Fig. 2 C). In addition, the silicon to aluminum ratio of the product is calculated ca. 1, in the range of that value for zeolite LTA synthesized via template-free procedures [30]. As a raise in the
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temperature again up to 115 ˚C, zeolite LTA particles have been broken followed by sodalite
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crystals formation. Moreover, by being provided an appropriate condition for zeolite FAU framework to be made at high temperatures, LTA cubic crystals destroyed and a mixture of both octahedral and yarn-ball crystals of zeolites FUA and SOD, respectively were created (Fig. 2 D).
Based on the auto-catalytic nucleation mechanism, high temperatures accelerate the gel dissolution and make it possible for the numerous nuclei to be present in the synthetic gel influencing the crystallinity degree, morphology as well as phase purity of the final product. The same findings have been reported by Jafari et al. [31], too. 10
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Fig. 2. SEM micrographs and chemical composition of the products synthesized at different temperatures: (A) 70, (B) 85, (C) 100 and (D) 115 °C.
Zeolites are metastable materials in comparison with more dense compounds like quartz from the view point of thermodynamic rules. Moreover, crystallization of zeolites often involves a transition from a metastable structure into more stable one. This commonlyoccurred process in both synthetic and natural structures could be clarified by Ostwald rule of 11
ACCEPTED MANUSCRIPT stages, in which kinetic pathways allow the initial formation of a thermodynamically metastable zeolitic structure which would underwent several crystallization treatments so that a more stable phase is constructed. This conversion is coincident with gradual dissolution of one phase beside nucleation and growth of the second more stable instance. In this regard,
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one of the most probable transformation phenomena is the formation of zeolite FAU from zeolite framework type LTA [40,41]. In our study, according to the predominant phase LTA formed at 100 ˚C as an optimal temperature, other experiments have been carried out at the
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same temperature.
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Fig. 3 represents the FT-IR spectra and x-ray diffraction patterns of as-prepared
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samples as a function of crystallization time intervals.
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Fig. 3. (A) X-ray diffraction patterns and (B) FT-IR spectra of as-prepared samples as a function of time: (a) 3, (b) 5, (c) 8 and (d) 11 hours.
As can be seen, at the short times, zeolite FAU is the dominant zeolite phase whereas by increasing the time of crystallization into 8h, peaks relevant to the cubic LTA crystals are perceived in XRD patterns. Lengthening this period to 11h, LTA framework-associated peaks have been disappeared and those of SOD and FAU structures are explicit.
SEM micrographs and compositional features of the products are shown in Fig. 4. In the beginning of the crystallization process, octahedral zeolite FAU crystals and yarn-ball 12
ACCEPTED MANUSCRIPT SOD units are obvious in the images while in sample 4A-t3 (Fig. 4C), cubic crystals of zeolite LTA are prevailing product. This data is apparently contradicts the results of previous work done by Zhang et al. [34] reported the formation of zeolite NaA at shorter times without any NaX zeolitic phase. Factually, by considering the fact that the more valued pore volume, the
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less framework density, so FAU structure with lower framework density (pore volume is 45.3 cm3/mol) is not as stable as zeolite LTA with pore volume of 42.4 cm3/mol. On the other hand, both types of zeolites (FAU and LTA structures) have 3-dimentional frameworks made
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from similar primary structural units i.e. sodalite cages. In the case of zeolite FAU, these cages are connected with each other through double-6-membered rings (D6R) while the
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linkage between sodalite cages by double-4-membered rings (D4R) ends in zeolite LTA framework. Therefore, according to the Ostwald rule, the less stable zeolite FAU could not be formed at the initial stages.
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However similar observations were reported by Maldonado and co-workers [41] assuming that the first insufficient dissolution of Si precursors in the medium and subsequent silicon enriching was the key reason for the transformation of LTA into FAU phase; it means
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that at the initial times as silicon source is not completely dissolved, sodalite or LTA particles are formed but as reaction is proceeding and more accessible Si species being reached, these
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early-formed zeolitic phases are converted to the FAU framework.
As it is evident from Fig. 4D, synthetic gel was allowed to be crystallized at longer time up to 11h, provided the zeolite NaX which needs more crystallization time due to its intricate large silicate polymer units (D6R) contrary to the NaA structure [31,42]. Concurrently, characteristic peaks of zeolite SOD at 2theta about 14 degree is observed, too.
It is well known that both zeolites SOD and LTA are made from β-cages formed by 24 preliminary tetrahedral SiO4 and AlO4 units. In LTA framework, each β-cage is related to the 13
ACCEPTED MANUSCRIPT six nearest neighbors through D4Rs. However, this is happened by S4Rs in the case of zeolite SOD resulting in more framework density (2.29 g/cm3) and stability compared to the LTA structure with that amount of 2.00 g/cm3 [39]. FT-IR spectra of the samples also indicate the characteristic peaks of zeolitic structures as mentioned earlier. Matched to the data from x-ray
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diffraction patterns, the presence of zeolite LTA cubes are clear at just 8 hours crystallization
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period, so this duration was considered as the optimum crystallization time.
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Fig. 4. SEM images and chemical composition of the samples prepared at different crystallization times: (A) 3, (B) 5, (C) 8 and (D) 11 hours.
With regard to the seeding influence on the formation induction of a desired zeolitic phase with reduced particle size [5], a defined amount of commercial zeolite 4A seeds was added to the starting syntheses mixture (Fig. 5).
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Fig. 5. SEM micrographs of commercial zeolite 4A at different magnifications.
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Fig. 6 shows the XRD patterns and FT-IR spectra of the samples. As it was expected, seed addition diminished the initial crystallization time of zeolite LTA. As a matter of fact, only FAU zeolitic phase is formed at the initial steps of the hydrothermal process, and in the absence of 4A seed crystals, while seeding the synthesis gel leads to
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reaction time.
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the appearance of characteristic peaks pertinent to the LTA framework at 3h of the
Fig. 6. (A) XRD patterns and (B) FT-IR spectra of the samples prepared by addition of zeolite 4A seed crsytals into the synthesis gel at different crystallization times: (a) 3, (b) 5, (c) 8 and (d) 11 hours.
At this time, an efficient mass and heat transfer required for creation of cubic crystallites of zeolite LTA is supplied as the presence of seed crystals with 4A quiddity. It is believed that in this enhanced nucleation situation, reverse mechanism of crystal 16
ACCEPTED MANUSCRIPT growth in which crystallization process is commenced on the surface of nanoparticles so that a crystalline shell with a disordered core would be made and followed by core crystallization, is presumable. Moreover, if the prime particles have a crystalline essence, synchronously crystallization of core and shell may be occurred. However, seeding at
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longer times directed the formation of impurities FAU and SOD phases. Zeolite LTA particles are transformed into the more stable phases FAU and SOD ones due to the more dissolution of as-formed LTA and added zeolite 4A seed crystals at longer crystallization
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time resulting in the supersaturation of the gel with aluminosilicate ions and the growth
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of new zeolitic phases [43].
SEM pictures and elemental analysis unanimously by XRD and FT-IR data affirm
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the morphology and chemical composition of the samples gotten at each stage (Fig. 7).
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Fig. 7. SEM pictures and elemental analysis of the samples prepared by addition of zeolite 4A seed
crsytals into the synthesis gel at different crystallization times: (A) 3, (B) 5, (C) 8 and (D) 11 hours.
By precise investigation of the results, the best phase purity and morphology were obtained for the samples prepared at 100 ˚C and 8h crystallization time without seed addition (4A-T3). In order to deeply comprehend the structural features of this product, 18
ACCEPTED MANUSCRIPT its transmission electron microscopy images have been recorded at different magnifications as shown in Fig. 8. It is evident from the micrographs that the final product structure is made from zeolite LTA microspheres formed by agglomeration of
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nano-cubic crystals with an average size of ca. 40 nm.
This morphology i.e. microspheres containing zeolitic nanocrystallites, donates a hierarchical structures with larger pore sizes rather than its intrinsic microporosity
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system to the zeolite framework structure. It seems that at least three advantages are
attainable with this hierarchical zeolites; the benefits of reduction size of the particles
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down to the nanometer scale and their fantastic properties, obviated the handling and separation difficulties of nanoparticles due to being as micron-sized spheres which could be easily filtered by filter papers, and eventually overcoming the diffusion obstacles owing to the presence of secondary porosity with larger sizes in the range of mesopores
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(2-50 nm) created between nanocrystals or microspheres.
Although, there is not a manifest clue of sodalite phase in the SEM micrographs and x-ray diffraction pattern for sample 4A-T3 (Fig. 1A and Fig. 2C), in consistent with
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associated FT-IR spectrum (wavenumbers at 800-600 cm-1), a low phase impurity of
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zeolite SOD is detected in the TEM images demonstrated by a red circle.
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Fig. 8. Transmission electron microscopy images of sample 4A-T3 at different magnifications, the presence of
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zeolite SOD is noted by the red circle.
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Nitrogen adsorption-desorption analysis for three samples 4A-T3, 4A-t1 and 4A-S1 were performed. The results represent the isotherm type IV for all three samples (Fig. 9).
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The beheld adsorption amount at low relative pressures (P/P0 < 0.1) is related to the micropores filling of zeolite framework. Further, a hysteresis loop is exhibited in the sorption curve of sample 4A-T3 at relative pressure higher than 0.7 while for samples 4A-t1 and 4A-S1, these loops are appeared at the P/P0 > 0.4 due to the intercrystalline mesopores filling present in the microspheres as a result of capillary condensation phenomenon [44]. According to the obtained BJH pore size distribution curves, it is well determined that the cavities between crystallites in the product 4A-T3 are about 2.4 nm in
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ACCEPTED MANUSCRIPT size and simultaneously, a comparable frequency of the pores population of ca. 9 nm are present in this structure. However, the size of about 6 and 7 nm are the average extent of mesopores sizes in the case of samples 4A-t1 and 4A-S1, respectively.
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The BET surface areas of the as-prepared zeolitic structures were calculated 542.88, 454.85 and 397.94 m2/g for 4A-T3, 4A-t1 and 4A-S1, respectively which are
more than those reported for the nanozeolite LTA in the previous studies [1,16,26,30]. In
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addition, the same nitrogen adsorption-desorption analysis was fulfilled in the case of commercial zeolite 4A (used as seeds in the hydrothermal process) like what happened
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for other samples and its BET surface area was calculated less than 1 m2/g. It seems that size reduction of the zeolite crystals down to nanometer scale and appearance a secondary porosity between crystals have led to high BET surface areas of samples synthesized in this study in comparison with that amount of commercial one with
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micron-sized crystals.
It is noteworthy to mention that the existence of the mesoporosity systems along with the innate microporosity of zeolites frameworks would amend their adsorptive and
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catalytic performances, since diffusion path length is reduced and reactants and product
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molecules transferring into and out from the zeolite channels are improved [5].
Remarkably, the monitored N2 sorption behaviors of the samples are in a good
agreement with the TEM images which intelligibly indicate the assemblies of nanocrystals with inter-crystalline spaces forming the individual microspheres.
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Fig. 9. Nitrogen adsorption-desorption analysis of three samples: (A) 4A-T3, (B) 4A-t1 and (C)
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4A-S1. Insets show the BJH pore size distribution curves for each sample.
As mentioned before, in order to investigate the possibility of the proposed method
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to prepare nanozeolite LTA from the natural sources, our study has been carried out
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using silicon and aluminum precursors obtained from domestic mines. The SEM images and compositional characteristics of the final powder produced by slight modified gel formula via hydrothermal process at 100 ˚C and 8 hours are depicted in Fig. 10 which proves the successful synthesis of nanozeolite LTA.
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Fig. 10. SEM images and compositional characteristics of the sample prepared using
4. Conclusions
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domestic silicon and aluminum sources.
In this study, nanozeolite NaA has been synthesized via a hydrothermal process at
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100 ˚C and 8h. The role of effective parameters on the size, morphology and phase purity of the final products such as crystallization time and temperature and seed addition into
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the synthetic mixture has been investigated. The results have indicated that changes in time and temperature leads to the crystalline phase alteration while seeding reduces the time needed to prepare nanozeolites. Also, the same procedure has been used to synthesize nanozeolite LTA from the natural starting materials supplied from domestic mines.
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ACCEPTED MANUSCRIPT Acknowledgments
The authors are thankful to the Research Council of Iran University of Science and Technology (Tehran) for the financial support and to GaharCeram and BEHDASH Chemical
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Companies for providing the commercial and mineral samples.
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ACCEPTED MANUSCRIPT 1. Hierarchical zeolite NaA microspheres have been hydrothermally synthesized 2. The effect of time, temperature and seeding on product properties was investigated
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3. Seeding treatment shortened the crystallization time of nanosized crystallites
S0254-0584(17)30189-X
DOI:
10.1016/j.matchemphys.2017.02.048
Reference:
MAC 19543
To appear in:
Materials Chemistry and Physics
Received Date: 10 January 2017 Revised Date:
14 February 2017
Accepted Date: 23 February 2017
Please cite this article as: M. Anbia, E. Koohsaryan, A. Borhani, Novel hydrothermal synthesis of hierarchically-structured zeolite LTA microspheres, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.02.048. 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 proof before it is published in its final 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.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT Novel hydrothermal synthesis of hierarchically-structured zeolite LTA microspheres Mansoor Anbia*, Esmat Koohsaryan, Asieh Borhani Research Laboratory of Nanoporous Materials, Faculty of Chemistry, Iran University of Science and
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Technology, Farjam Street, Narmak, P.O. Box 16846-13114, Tehran, Iran E- mail: [email protected], Phone: +98 21 77240516-17, Fax: +98 21 77491204
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Abstract
A robust hydrothermal preparation of nanozeolite LTA has been made. It is the first report
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of zeolite NaA synthesis with smallest crystal sizes, highest surface area and a hierarchical structure as micron-sized spheres formed by nanocrystallites assemblies in a short time. The effects of time, temperature of crystallization and seeding on the final products properties have been studied. The prepared samples have been characterized using various techniques.
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Moreover, the synthesis procedure has been fulfilled using the initial natural precursors supplied from the mines in Iran. The results ascertain the successful synthesis of the high-
Keywords
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qualified products with potential adsorptive and catalytic applications.
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Nanozeolite LTA, Seeding, Hydrothermal synthesis, Microsphere, Hierarchical zeolite
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ACCEPTED MANUSCRIPT 1. Introduction Zeolites are hydrated crystalline microporous aluminosilicates constructed from 3dimentional frameworks of tetrahedral TO4 (most of the time T is Al or Si) which are connected with each other through sharing corner oxygen atoms. In such arrangement,
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numerous channels and cavities containing water and charge compensating alkali and earth alkali cations are formed [1-3].
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Zeolitic structures have many applications in different fields such as agriculture, water and waste water treatment, petroleum, petrochemical and detergent industries due to their
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prominent properties such as porous system, high surface area, outstanding hydrothermal and chemical stability, being eco-friendly and also not toxic compounds[4-7]. However, commercialized zeolitic materials are provided as micron-sized powder giving rise to some limitations like difficult mass transfer of reactants and product molecules, pore mouth
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blockage and deactivation of catalysts and adsorbents in the chemical reactions [8].
In order to overcome these constraints, two general strategies have been proposed so far: zeolite crystal size reduction (˂ 100 nm) i.e. nanozeolite preparation and secondary
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porosity introduction into zeolite framework leading hierarchical zeolites with larger pores
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besides their intrinsic micropores [9-13]. Substantial advantages such as enhanced surface area, reduced diffusion path length, decreased mass and heat transfer resistance, prohibited side reactions and postponed catalysts and sorbents deactivation are achieved as a result of zeolitic nanocrystallites utilization in the catalytic and adsorptive reactions [14,15]. Owing to their superior features, nanozeolites not only serve as conventional micron-sized zeolites act, but also can be considered as a model system for fundamental studies of crystals growth, preparation of low dielectric constant films, sensors, polymer-zeolite nanocomposite
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ACCEPTED MANUSCRIPT membranes, hierarchically-structured materials, pigments, drug delivery, and magnetic resonance imaging [16-20].
Despite these benefits, there are some obstacles in handling of nanosized zeolites using
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high speed centrifuging. Therefore, it is anticipated that the best strategy is to prepare micronsized hierarchical zeolitic permanent aggregates involving nano-crystals along with
mesopores between them. In this way, the advantages of both structures (nanosized and
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hierarchical zeolites) would be there while there is no need for organic or inorganic binders to shape the zeolite powders and their possible side effects [5,21,22]. It is noteworthy to
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mention that the introduction of an auxiliary porosity in a structure not only could be applied in the case of zeolites, but also is sensible for other compounds [23,24].
One of the potentially useful small pore zeolites, is the LTA (Linde Type A) zeolitic framework with cubic structure, which is usually synthesized in the sodium form (NaA or 4A
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zeolite) and has great applications in various areas specially in laundry detergents as water softening agent [4,25-29]. There have been many efforts to obtain zeolite LTA with crystal size of ca. 500 nm. However, most of the procedures are based on applying
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tetramethylammonium hydroxide (TMAOH) as an appropriate organic structure directing
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agent (SDA) for the LTA framework formation, which are not cost-effective and ecofriendly. In addition, the other drawback is the template removal requiring high temperature calcination process ending in irreversible aggregation of nanocrystallites [30,31]. So, investigation of alternative synthesis routes to get LTA nanozeolites or template-assisted strategies using inexpensive and recoverable agents seems to be a crucial subject.
On the other hand, synthesis processes such as confined-space, milling, micro-reactors and microwaves-mediated methodologies have received attention, but they suffer from some defects e.g. zeolite structure damages due to combustion at elevated temperatures for space3
ACCEPTED MANUSCRIPT limiting matrix elimination, the crystallinity loss of zeolite, high cost and personal safety, respectively [26,5]. It seems that the optimization of synthesis parameters like crystallization time and temperature as well as the type of precursors, could be rationally beneficial to reach defined favorable nanozeolite framework. In this regard, Liu et al. [32] have investigated the
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effects of temperature and time crystallization, system alkalinity and aluminum sources on the zeolite NaA synthesis via dry-gel conversion method. The results have indicated that in the presence of chloride anions (aluminum chloride as Al source), a few zeolite NaX crystals
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were observed along with the end product (zeolite LTA) particles, whereas sodalite crystals formed on using aluminum sulfate as Al source. They have claimed that these impurities
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were present owing to the different steric and electrostatic interactions between anions and zeolites frameworks.
In Alfaro and co-workers study [33], nanocrystals of zeolite LTA (200-300 nm) were
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synthesized at 100 ˚C in the absence of an organic template and the effect of aging time as an important factor on the crystal size was evaluated. It was declared that as-prepared samples during longer aging time (144 h) had the smaller crystal size in comparison with shorter
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aging times (24, 48 and 72 hours). The other work was focused on the preparation of zeolite NaA through hydrothermal method and the examination of various synthetic parameters like
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stirring rate, aging time as well as different Al and Si resources [34]. Almost identical to the previous study [33], increasing the aging time from 1 to 3 hours, resulted in the more fine crystallites (314 nm) due to more crystal nucleation. The stirring rate did not affect the size of particles and Si sources had partial influence on the end product rather than aluminum species.
Contrary to the reported studies investigating a Na2O-SiO2-Al2O3-H2O synthesis system, Edelman et al. [35] have reported that LTA crystals seeding into a synthetic mixture
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ACCEPTED MANUSCRIPT of this zeolite has promoted its crystallization. In fact, by addition of larger seed crystals (ca. 40 µm) to a clear syntheses gel, the nucleation of new population of LTA crystals were promulgated through initial breeding mechanism, while this did not take place in the case of 1-3 µm seed crystals. Moreover, some phase impurities were detected in the absence of seeds
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[36]. In the preparation of high-silica nanozeolite ITQ-29 by Casado-Coterillo et al. [37], seeding influences on the process from two prospects; the crystal size were decreased down to 2.5 µm by increasing the amount of seeds and the second was an elevated crystallinity of
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zeolite ITQ-29 in the presence of seed crystals.
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As mentioned above, nanozeolite LTA is one of the most attractive molecular sieves with unique applications clarifying the importance of assessing its different synthetic procedures and attempts to attain its favorite defined structure via sturdy processes. According to the literatures, there have not been any reports on the preparation of nanozeolite
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LTA with crystal sizes below 100 nm with hierarchical structure involving inter-crystalline mesopores as µ-sized micro-mesoporous LTA spheres at mild ambient reaction conditions and using inexpensive starting materials such as water glass. Therefore, in this study we
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describe the morphology and crystalline phase of the resultant nanozeolite LTA by altering the crystallization time and temperature as well as seed addition. Moreover, the synthesis
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process at the optimal circumstances has been carried out to obtain nanosized zeolite NaA using as-provided Al and Si sources from the natural mines in Iran.
2. Materials and methods 2.1. Chemicals
The used reagents for the synthesis of nanosized zeolite LTA were sodium aluminate (Aldrich, 53% Al2O3 and 42.5% Na2O) as aluminum and water glass (sodium silicate solution, Merck, 27% SiO2 and 8% Na2O) as silicon sources, respectively. Sodium hydroxide 5
ACCEPTED MANUSCRIPT from Ameretat Shimi- Iran was provided to prepare alkali solutions. Distilled water was also used to dissolve precursors and prepare the synthetic mixture. Commercial zeolite 4A and natural sodium and aluminum sources were kindly provided by GaharCeram and BEHDASH
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Chemical Companies (Iran), respectively.
2.2. Synthesis of LTA
Nanozeolite LTA was hydrothermally synthesized pursuant to the syntheses gel
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formula 6Na2O: 2.4SiO2: 1.0Al2O3: 180.0H2O in molar ratio of precursors. At first, alkali solution was prepared using an appropriate amount of NaOH tiny pellets (6.72 g) in 60 mL
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distilled water. Then after, sodium aluminate powder (4.05 g) was added to the solution. After agitation for several minutes, sodium silicate solution (8.35 mL) was also added drop by drop. The obtained milky gel was aged for 30 minutes under continuous stirring. Transferring to a stainless steel autoclave for crystallization treatment in an oven under
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different temperature and time conditions was the next step the synthesis mixture underwent. Overall, three series of experiments were fulfilled as follows: the first one covered the reaction condition with different temperatures i.e. 70, 85, 100 and 115 °C while the
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crystallization time was constant at 8 hours and the end products were named 4A-T1, 4A-T2,
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4A-T3 and 4A-T4, respectively. In order to evaluate the effect of time on the crystallization process, the second series of experiments were carried out under the same temperature (100 °
C) with various times intervals namely 3, 5, 8 and 11 hours and the resulting samples were
labeled as 4A-t1, 4A-t2, 4A-t3 and 4A-t4, respectively.
The addition of a certain quantity of commercial zeolite 4A powder (previously ground using a mortar and pestle) to the zeolite synthetic mixture during aging stage was included in the third step of experiments to specify how seeding could influence the final product properties at different synthesis times pointed earlier. The obtained samples were labeled as 6
ACCEPTED MANUSCRIPT 4A-S1 (t= 3h), 4A-S2 (t= 5h), 4A-S3 (t= 8h) and 4A-S4 (t= 11h). The details of each experiment are listed in Table 1. After hydrothermal process, the slushes were filtered using filter paper and washed several times with distilled water until the liquid reached pH~ 7. The product powders were heated in an oven at ca. 100 °C for some hours to be dried and then
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characterized. Moreover, there were some treatments on the sands in the presence of sodium carbonate to provide solid or liquid sodium silicate with defined SiO2/Na2O ratios in an autoclave or a digester unit under appropriate temperatures and pressures. Likewise, a
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mixture of aluminum hydroxide and liquor containing ca. 22 percent sodium hydroxide
ratio was made.
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endured some thermal process until sodium aluminate solution with a certain Na2O to Al2O3
Table 1. Experimental conditions in hydrothermal synthesis of nanozeolite LTA Crystallization Temperature (˚C)
Crystallization Time (h)
Added commercial zeolite 4A seeds (g)
4A-T1
70
8
na
4A-T2
85
8
na
100
8
na
115
8
na
100
3
na
100
5
na
4A-T3 4A-T4 4A-t1
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4A-t2
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Sample
4A-t3
100
8
na
4A-t4
100
11
na
4A-S1
100
3
0.1
4A-S2
100
5
0.1
4A-S3
100
8
0.1
4A-S4
100
11
0.1
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na: not added
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Nitrogen adsorption–desorption isotherms were performed on a Belsorp-Max equipment (BEL Japan Inc., Japan) at -196 °C while samples had been degassed at 350 °C .
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The pore size distribution and the BET specific surface area were determined by the BJH (Barrett–Joyner–Halenda) and BET (Brunauer, Emmett, and Teller) methods, respectively. In order to investigate the morphological and compositional characteristics of the products, a
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scanning electron microscopy-energy dispersive x-ray equipment (VEGA3-TESCAN) was used. X-ray diffractometer (JEOL-JDX 8030) by Cu-Kα radiation operating at 30 kV and 20
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mA was used to record the XRD patterns. To make a systematic and thorough investigation of the final zeolitic products, the transmission electron microscopy images were observed by a Zeiss-EM10C apparatus operating at 100 kV. FT-IR spectrometer (SHIMADZU-8400-s) with potassium bromide pellets was also employed to detect the absorption bands assigned to
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the zeolites frameworks.
3. Results and discussion
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Fig. 1 represents the x-ray diffraction patterns and FT-IR spectra of the zeolite samples prepared at different temperatures. As can be seen from XRD, there are no observable peaks
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at 70 °C (Fig. 1A (a)), meaning that the amorphous syntheses gel has not been transfigured into a crystalline zeolitic phase. Accordingly, absorbance bands related to the zeolites frameworks are not detectable in the FT-IR spectra (Fig. 1B (a)). Increasing the crystallization temperature to 85 °C leads to partially weak peaks assigned to the zeolite FAU. At 100 °C, the crystalline phase changed into the eligible zeolite LTA framework with its characteristic peaks as reported earlier [21,25]. At higher temperature up to 115 °C, phase impurities i.e. SOD and FAU are formed and their characteristic peaks become apparent. Compatible to the XRD data, in Fourier transform infrared spectra, absorbance bands 8
ACCEPTED MANUSCRIPT correspondent to the zeolitic frameworks are gradually appeared with a rise in the temperature implying the transformation of the initial amorphous gel into crystalline zeolites
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phases.
Fig. 1. (A) X-ray diffraction patterns and (B) FT-IR spectra of the zeolite samples prepared at different temperatures, (a) 70, (b) 85, (c) 100 and (d) 115 °C.
In general, these bands are emerged in the middle and far infrared ranges (1200-400 cm-1) for zeolites frameworks [33,38]. Peaks in the 500-420 cm-1 are relevant to the T-O
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bands in β-cages [39]. In fact, internal bending vibration of (Si or Al)-O at 400 cm-1 suggests the beginning step of crystalline zeolitic phase formation [30]. On the other hand, signals
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which are detectible in just zeolite LTA structure spectra (proportional to the double-4membered rings (D4R)) and not FAU or SOD frameworks are located in the wavenumber
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range of 650-500 cm-1 and as-synthesized sample 4A-T3 shows a band at ~ 517cm-1 [33]. Broad bands at approximately 1000 cm-1 belong to the internal asymmetric stretching vibration of tetrahedra T-O becoming sharper as amorphous materials are being converted to the crystallized zeolitic phases [38].
Contrary not being observed in the XRD pattern of sample 4A-T3, absorbance bands in the range of 800-600 cm-1 attributed to the simple-4-membered rings (S4R) in the zeolite SOD structure, could be seen in all spectra except in the case of product 4A-T1 indicating the
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ACCEPTED MANUSCRIPT presence of a slight impurity in the end products [39]. Peak related to the transfiguring vibration of water molecules bounded in the zeolite channels are exhibited at nearly wavenumber 1650 cm-1 [38].
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Interestingly, the formation of small zeolite LTA crystallites is confirmed from the bands width. Indeed, band broadening in the XRD patterns consistent with the crystal size reduction could be explained by Scherrer equation as Dhkl = Kλ/(β xCosƟhkl) [30].
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SEM micrographs and chemical composition of the products obtained from EDX
analyses are illustrated in Fig. 2. As could be observed, temperature modification has resulted
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in changes of zeolites phases and morphologies. Matched to the XRD data, at low temperature (70 ˚C), there is no clue of a formed crystalized phase. By increase in the temperature, octahedral FAU crystals along with sodalite particles are detected in the pictures. However, sphere aggregates from nano-cubic zeolite LTA assemblies are created at
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100 ˚C and average crystal size of these cubes is estimated to be ca. 80 nm (Fig. 2 C). In addition, the silicon to aluminum ratio of the product is calculated ca. 1, in the range of that value for zeolite LTA synthesized via template-free procedures [30]. As a raise in the
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temperature again up to 115 ˚C, zeolite LTA particles have been broken followed by sodalite
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crystals formation. Moreover, by being provided an appropriate condition for zeolite FAU framework to be made at high temperatures, LTA cubic crystals destroyed and a mixture of both octahedral and yarn-ball crystals of zeolites FUA and SOD, respectively were created (Fig. 2 D).
Based on the auto-catalytic nucleation mechanism, high temperatures accelerate the gel dissolution and make it possible for the numerous nuclei to be present in the synthetic gel influencing the crystallinity degree, morphology as well as phase purity of the final product. The same findings have been reported by Jafari et al. [31], too. 10
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Fig. 2. SEM micrographs and chemical composition of the products synthesized at different temperatures: (A) 70, (B) 85, (C) 100 and (D) 115 °C.
Zeolites are metastable materials in comparison with more dense compounds like quartz from the view point of thermodynamic rules. Moreover, crystallization of zeolites often involves a transition from a metastable structure into more stable one. This commonlyoccurred process in both synthetic and natural structures could be clarified by Ostwald rule of 11
ACCEPTED MANUSCRIPT stages, in which kinetic pathways allow the initial formation of a thermodynamically metastable zeolitic structure which would underwent several crystallization treatments so that a more stable phase is constructed. This conversion is coincident with gradual dissolution of one phase beside nucleation and growth of the second more stable instance. In this regard,
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one of the most probable transformation phenomena is the formation of zeolite FAU from zeolite framework type LTA [40,41]. In our study, according to the predominant phase LTA formed at 100 ˚C as an optimal temperature, other experiments have been carried out at the
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same temperature.
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Fig. 3 represents the FT-IR spectra and x-ray diffraction patterns of as-prepared
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samples as a function of crystallization time intervals.
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Fig. 3. (A) X-ray diffraction patterns and (B) FT-IR spectra of as-prepared samples as a function of time: (a) 3, (b) 5, (c) 8 and (d) 11 hours.
As can be seen, at the short times, zeolite FAU is the dominant zeolite phase whereas by increasing the time of crystallization into 8h, peaks relevant to the cubic LTA crystals are perceived in XRD patterns. Lengthening this period to 11h, LTA framework-associated peaks have been disappeared and those of SOD and FAU structures are explicit.
SEM micrographs and compositional features of the products are shown in Fig. 4. In the beginning of the crystallization process, octahedral zeolite FAU crystals and yarn-ball 12
ACCEPTED MANUSCRIPT SOD units are obvious in the images while in sample 4A-t3 (Fig. 4C), cubic crystals of zeolite LTA are prevailing product. This data is apparently contradicts the results of previous work done by Zhang et al. [34] reported the formation of zeolite NaA at shorter times without any NaX zeolitic phase. Factually, by considering the fact that the more valued pore volume, the
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less framework density, so FAU structure with lower framework density (pore volume is 45.3 cm3/mol) is not as stable as zeolite LTA with pore volume of 42.4 cm3/mol. On the other hand, both types of zeolites (FAU and LTA structures) have 3-dimentional frameworks made
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from similar primary structural units i.e. sodalite cages. In the case of zeolite FAU, these cages are connected with each other through double-6-membered rings (D6R) while the
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linkage between sodalite cages by double-4-membered rings (D4R) ends in zeolite LTA framework. Therefore, according to the Ostwald rule, the less stable zeolite FAU could not be formed at the initial stages.
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However similar observations were reported by Maldonado and co-workers [41] assuming that the first insufficient dissolution of Si precursors in the medium and subsequent silicon enriching was the key reason for the transformation of LTA into FAU phase; it means
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that at the initial times as silicon source is not completely dissolved, sodalite or LTA particles are formed but as reaction is proceeding and more accessible Si species being reached, these
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early-formed zeolitic phases are converted to the FAU framework.
As it is evident from Fig. 4D, synthetic gel was allowed to be crystallized at longer time up to 11h, provided the zeolite NaX which needs more crystallization time due to its intricate large silicate polymer units (D6R) contrary to the NaA structure [31,42]. Concurrently, characteristic peaks of zeolite SOD at 2theta about 14 degree is observed, too.
It is well known that both zeolites SOD and LTA are made from β-cages formed by 24 preliminary tetrahedral SiO4 and AlO4 units. In LTA framework, each β-cage is related to the 13
ACCEPTED MANUSCRIPT six nearest neighbors through D4Rs. However, this is happened by S4Rs in the case of zeolite SOD resulting in more framework density (2.29 g/cm3) and stability compared to the LTA structure with that amount of 2.00 g/cm3 [39]. FT-IR spectra of the samples also indicate the characteristic peaks of zeolitic structures as mentioned earlier. Matched to the data from x-ray
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diffraction patterns, the presence of zeolite LTA cubes are clear at just 8 hours crystallization
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period, so this duration was considered as the optimum crystallization time.
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Fig. 4. SEM images and chemical composition of the samples prepared at different crystallization times: (A) 3, (B) 5, (C) 8 and (D) 11 hours.
With regard to the seeding influence on the formation induction of a desired zeolitic phase with reduced particle size [5], a defined amount of commercial zeolite 4A seeds was added to the starting syntheses mixture (Fig. 5).
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Fig. 5. SEM micrographs of commercial zeolite 4A at different magnifications.
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Fig. 6 shows the XRD patterns and FT-IR spectra of the samples. As it was expected, seed addition diminished the initial crystallization time of zeolite LTA. As a matter of fact, only FAU zeolitic phase is formed at the initial steps of the hydrothermal process, and in the absence of 4A seed crystals, while seeding the synthesis gel leads to
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reaction time.
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the appearance of characteristic peaks pertinent to the LTA framework at 3h of the
Fig. 6. (A) XRD patterns and (B) FT-IR spectra of the samples prepared by addition of zeolite 4A seed crsytals into the synthesis gel at different crystallization times: (a) 3, (b) 5, (c) 8 and (d) 11 hours.
At this time, an efficient mass and heat transfer required for creation of cubic crystallites of zeolite LTA is supplied as the presence of seed crystals with 4A quiddity. It is believed that in this enhanced nucleation situation, reverse mechanism of crystal 16
ACCEPTED MANUSCRIPT growth in which crystallization process is commenced on the surface of nanoparticles so that a crystalline shell with a disordered core would be made and followed by core crystallization, is presumable. Moreover, if the prime particles have a crystalline essence, synchronously crystallization of core and shell may be occurred. However, seeding at
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longer times directed the formation of impurities FAU and SOD phases. Zeolite LTA particles are transformed into the more stable phases FAU and SOD ones due to the more dissolution of as-formed LTA and added zeolite 4A seed crystals at longer crystallization
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time resulting in the supersaturation of the gel with aluminosilicate ions and the growth
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of new zeolitic phases [43].
SEM pictures and elemental analysis unanimously by XRD and FT-IR data affirm
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the morphology and chemical composition of the samples gotten at each stage (Fig. 7).
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Fig. 7. SEM pictures and elemental analysis of the samples prepared by addition of zeolite 4A seed
crsytals into the synthesis gel at different crystallization times: (A) 3, (B) 5, (C) 8 and (D) 11 hours.
By precise investigation of the results, the best phase purity and morphology were obtained for the samples prepared at 100 ˚C and 8h crystallization time without seed addition (4A-T3). In order to deeply comprehend the structural features of this product, 18
ACCEPTED MANUSCRIPT its transmission electron microscopy images have been recorded at different magnifications as shown in Fig. 8. It is evident from the micrographs that the final product structure is made from zeolite LTA microspheres formed by agglomeration of
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nano-cubic crystals with an average size of ca. 40 nm.
This morphology i.e. microspheres containing zeolitic nanocrystallites, donates a hierarchical structures with larger pore sizes rather than its intrinsic microporosity
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system to the zeolite framework structure. It seems that at least three advantages are
attainable with this hierarchical zeolites; the benefits of reduction size of the particles
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down to the nanometer scale and their fantastic properties, obviated the handling and separation difficulties of nanoparticles due to being as micron-sized spheres which could be easily filtered by filter papers, and eventually overcoming the diffusion obstacles owing to the presence of secondary porosity with larger sizes in the range of mesopores
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(2-50 nm) created between nanocrystals or microspheres.
Although, there is not a manifest clue of sodalite phase in the SEM micrographs and x-ray diffraction pattern for sample 4A-T3 (Fig. 1A and Fig. 2C), in consistent with
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associated FT-IR spectrum (wavenumbers at 800-600 cm-1), a low phase impurity of
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zeolite SOD is detected in the TEM images demonstrated by a red circle.
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Fig. 8. Transmission electron microscopy images of sample 4A-T3 at different magnifications, the presence of
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zeolite SOD is noted by the red circle.
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Nitrogen adsorption-desorption analysis for three samples 4A-T3, 4A-t1 and 4A-S1 were performed. The results represent the isotherm type IV for all three samples (Fig. 9).
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The beheld adsorption amount at low relative pressures (P/P0 < 0.1) is related to the micropores filling of zeolite framework. Further, a hysteresis loop is exhibited in the sorption curve of sample 4A-T3 at relative pressure higher than 0.7 while for samples 4A-t1 and 4A-S1, these loops are appeared at the P/P0 > 0.4 due to the intercrystalline mesopores filling present in the microspheres as a result of capillary condensation phenomenon [44]. According to the obtained BJH pore size distribution curves, it is well determined that the cavities between crystallites in the product 4A-T3 are about 2.4 nm in
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The BET surface areas of the as-prepared zeolitic structures were calculated 542.88, 454.85 and 397.94 m2/g for 4A-T3, 4A-t1 and 4A-S1, respectively which are
more than those reported for the nanozeolite LTA in the previous studies [1,16,26,30]. In
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addition, the same nitrogen adsorption-desorption analysis was fulfilled in the case of commercial zeolite 4A (used as seeds in the hydrothermal process) like what happened
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for other samples and its BET surface area was calculated less than 1 m2/g. It seems that size reduction of the zeolite crystals down to nanometer scale and appearance a secondary porosity between crystals have led to high BET surface areas of samples synthesized in this study in comparison with that amount of commercial one with
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micron-sized crystals.
It is noteworthy to mention that the existence of the mesoporosity systems along with the innate microporosity of zeolites frameworks would amend their adsorptive and
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catalytic performances, since diffusion path length is reduced and reactants and product
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molecules transferring into and out from the zeolite channels are improved [5].
Remarkably, the monitored N2 sorption behaviors of the samples are in a good
agreement with the TEM images which intelligibly indicate the assemblies of nanocrystals with inter-crystalline spaces forming the individual microspheres.
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Fig. 9. Nitrogen adsorption-desorption analysis of three samples: (A) 4A-T3, (B) 4A-t1 and (C)
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4A-S1. Insets show the BJH pore size distribution curves for each sample.
As mentioned before, in order to investigate the possibility of the proposed method
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to prepare nanozeolite LTA from the natural sources, our study has been carried out
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using silicon and aluminum precursors obtained from domestic mines. The SEM images and compositional characteristics of the final powder produced by slight modified gel formula via hydrothermal process at 100 ˚C and 8 hours are depicted in Fig. 10 which proves the successful synthesis of nanozeolite LTA.
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Fig. 10. SEM images and compositional characteristics of the sample prepared using
4. Conclusions
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domestic silicon and aluminum sources.
In this study, nanozeolite NaA has been synthesized via a hydrothermal process at
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100 ˚C and 8h. The role of effective parameters on the size, morphology and phase purity of the final products such as crystallization time and temperature and seed addition into
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the synthetic mixture has been investigated. The results have indicated that changes in time and temperature leads to the crystalline phase alteration while seeding reduces the time needed to prepare nanozeolites. Also, the same procedure has been used to synthesize nanozeolite LTA from the natural starting materials supplied from domestic mines.
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ACCEPTED MANUSCRIPT Acknowledgments
The authors are thankful to the Research Council of Iran University of Science and Technology (Tehran) for the financial support and to GaharCeram and BEHDASH Chemical
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Companies for providing the commercial and mineral samples.
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ACCEPTED MANUSCRIPT 1. Hierarchical zeolite NaA microspheres have been hydrothermally synthesized 2. The effect of time, temperature and seeding on product properties was investigated
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3. Seeding treatment shortened the crystallization time of nanosized crystallites