A simple synthesis procedure to prepare nanosized faujasite crystals

Microporous and Mesoporous Materials 161 (2012) 67–75

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A simple synthesis procedure to prepare nanosized faujasite crystals Thiago F. Chaves a, Heloise O. Pastore b, Dilson Cardoso a,⇑ a b

Catalysis Laboratory, Chemical Engineering Department, Federal University of São Carlos, P.O. Box 676, 13565-905 São Carlos, Brazil Micro and Mesoporous Molecular Sieves Group, Institute of Chemistry, University of Campinas, UNICAMP, 13084-861 Campinas, SP, Brazil

a r t i c l e

i n f o

Article history: Received 22 November 2011 Received in revised form 9 May 2012 Accepted 16 May 2012 Available online 24 May 2012 Keywords: Nanocrystals Faujasite Aging Alkalinity

a b s t r a c t The interest in synthesizing zeolites with crystals in the nanometric size range has increased greatly in recent years; however most of the works depend on using organic structure directing agents. In this work nanosized crystals of zeolite faujasite were successfully synthesized without the use of organics. Among the synthesis parameters studied, the increased alkalinity of the synthesis gel was found to be the one that caused a decrease in the final particle size. The high alkalinity coupled with an aging step proved to be very effective in affording faujasite nanocrystals with high microporosity and external area. Thus it is possible to obtain nanocrystals of this zeolite using environmentally more friendly procedures, since they avoid the use of organic structure directing agents and their subsequent calcination. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Synthetic zeolites are widely used as catalysts or adsorbents in petroleum refining, petrochemicals production, and fine chemistry. For example, FAU (Y), MFI (ZSM-5), and LTA (A) zeolites are used in fluid catalytic cracking (FCC), aromatic alkylation and adsorbents, respectively. The ultrastable zeolite Y, USY, is the principal component of FCC catalysts [1–3]. This zeolite has a three-dimensional pore structure with a window diameter of approximately 0.74 nm, inherent surface acidity, and high structural resistance to withstand severe hydrothermal conditions during regeneration in the FCC process [1]. Studies using nanosized zeolite crystals for polyolefin cracking showed for instance that the catalytic activity depends directly on the external surface area present in the nanocrystalline HZSM-5 samples [4]. Particle size reduction from the micrometer to the nanometer scale leads to substantial changes in the properties of the materials. The use of zeolites formed by nanosized crystals, for example, had an important impact in traditional application areas such as catalysis [5–7] and separation [8]. The ratio of external to internal number of atoms increases rapidly as the particle size decreases, therefore, zeolite nanoparticles present large external surface areas and higher activity [6]. The external acidity is important when the zeolite is meant to be used as a catalyst in reactions involving bulky molecules, such as 1,3,5-tri-isopropylbenzene [9]. In addi-

⇑ Corresponding author. Fax: +55 16 3351 8266. E-mail addresses: [email protected] (T.F. Chaves), [email protected], [email protected] (D. Cardoso). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.05.022

tion, smaller zeolite crystals will have more pore mouths exposed in the external surface. Consequently, this material presents less diffusion limitations when compared to micrometer-sized zeolite crystals [5,11]. Zeolite nanocrystals can also be used in other catalysis and adsorption systems, such as thin films, fibers, and self-standing zeolite membranes while offer advantages in photochemistry, electrochemistry and optoelectronics [5,12–15]. Synthesis of nanocrystals of A, Y, silicalite, sodalite, ZSM-5 and Beta zeolites have been previously reported [16–24]. Colloidal zeolite suspensions of LTA- and FAU-type zeolites have been prepared from diluted clear solutions containing a large amount of tetramethylammonium cations at temperatures ranging from room temperature to 100 °C [18,23–25]. The synthesis of FAU nanocrystals in the absence of organic template is less common. Zhan et al. studied the influence of temperature and stirring on the synthesis of FAU nanocrystals from systems using tetraethyl orthosilicate as the Si source. The authors observed that the best conditions to obtain faujasite nanosized crystals were by stirring the reaction mixture at 250 rpm for 2 days at 60 °C [26]. More recently, Huang et al. synthesized FAU nanocrystals from gel systems containing aluminum isopropoxide by using a ‘‘three-stage’’ heating scheme with temperatures between 25 and 60 °C. The first stage at 25 °C was performed under agitation at 650 rpm. Secondly, the precursor was moved into a pre-heated oven set at 38 °C and aged for 24 h. At the final-stage, the precursor gel was hydrothermally treated at 60 °C for 48 h. This system yielded 100–300 nm crystal aggregates built of 10–20 nm nanocrystals [27]. Considering the increasing importance of nanosized zeolite in catalysis [6,7,9], in this work the synthesis conditions for obtaining zeolite FAU nanocrystals using a static system without organic

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directing agent and organic precursors of Si and Al were investigated. The influence of hydrothermal treatment time, aging time and the alkalinity of the synthesis gel on the crystal size was investigated. 2. Experimental 2.1. Zeolite synthesis The reactants used were sodium hydroxide pellets (Quimis, 99%), fumed silica (Aerosil 380, Evonik/Degussa), sodium aluminate (Riedel de Haën, 54% Al2O3:41% Na2O:5% H2O), sodium chloride (Merck, 99%) and deionized water. The zeolite was initially prepared according to the traditional composition, similar to that previously reported by Hu et al. [28]. Appropriate amounts of sodium hydroxide and water were mixed under stirring until the solution became clear. The aluminum source (sodium aluminate) was added to the aqueous solution of sodium hydroxide and stirred until completely dissolved. The silica source (Aerosil 380) was added under manual stirring until a homogeneous gel was obtained. Typically, the final molar composition of aluminosilicate gel obtained was: 5.5 Na2O:Al2O3:10 SiO2:180 H2O:x NaCl. The mixture was transferred to Teflon-lined stainless steel autoclaves and maintained at 100 °C for different crystallization times. The solid product was separated by centrifugation, washed several times with distilled water and dried at 60 °C. Table 1 shows the conditions and composition of the samples that were synthesized in this work.

this work. Scherrer’s equation was used to estimate the primary size of FAU crystals. The full-width at half maximum (FWHM) was determined at the peaks 2h of 15° (3 3 1), 23° (5 3 3) and 26° (6 4 2), using metallic Si as the reference to correct peak width. The unit cell parameter (a0) of the faujasites was calculated using an internal silicon standard (10 wt.%); the samples containing the standard were previously humidified for 24 h in a balanced atmosphere chamber with a saturated solution of CaCl2. The (3 1 1), (5 3 3) and (6 4 2) peaks were used to determine a0. Nitrogen sorption experiments were carried out at 196 °C on a Micrometrics ASAP 2020 after degassing the sample at 200 °C for 2 h. The micropore volume (VMicro) and external area (SExt) were determined by the t-plot method. The morphology and size of zeolite crystals or particles were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), using a Phillips XL 30 FEG and Philips CM 120, respectively. After sonication to break up large aggregates a droplet of the methanol suspension was deposited onto Cu grids and examined by TEM. The chemical composition of the synthesized zeolites was determined by several methods. To calculate the average composition of the sample through Energy Dispersive X-ray Spectroscopy (EDS), the spectra were measured in different regions of the samples using a Philips microscope XL 30 FEG. 27Al and 29 Si Solid-state Nuclear Magnetic Resonance high-power decoupling (HPDEC) and 1H/29Si cross-polarization/magic angle spinning (CP/MAS) spectra were obtained on a broad band Bruker 400 Avance + (400 MHz) spectrometer using 4 mm rotors. The spinning frequency used for 29Si was 10 kHz, while the spinning frequency for 27Al NMR was 15 kHz.

2.2. Characterization

3. Results and discussion

The zeolite samples were characterized by means of conventional techniques: powder X-ray diffraction (XRD), N2 physisorption isotherm, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystallinity and phase purity of the solids were determined by powder X-ray diffraction (XRD) using a Rigaku diffractometer (Ni-filtered CuKa radiation, 40 kV, 40 mA) at a scan rate of 2°/min from 5° to 35° with a step size of 0.02°. After background subtraction, the intensity under the peaks in the 23–24.5° 2h (h k l = 5 3 3) range was taken as a measure of the crystallinity, by comparison to a highly crystalline sample prepared in

3.1. Alkalinity influence

Table 1 Composition of the reaction mixture and synthesis conditions for the different samples synthesized at H2O/Al2O3 molar ratio of 180 and SiO2/Al2O3 of 10.

a b

Samplea

Na2O/Al2O3

HTb (h)

Aging (h)

x NaCl

B5.5-24 h B5.5 B6.5 B7.5 B8.5 B8.5-2 h B8.5-4 h B8.5-6 h B8.5-8 h B8.5-10 h B8.5-12 h B8.5-24 h B8.5-48 h B8.5-A24 B8.5-A48 B8.5-A72 B5.5-A72 B7.5NaCl B8.5NaCl

5.5 5.5 6.5 7.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 5.5 5.5 5.5

24 6 6 6 6 2 4 6 8 10 12 24 48 6 6 6 6 6 6

0 0 0 0 0 0 0 0 0 0 0 0 0 24 48 72 72 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 6

The sample code is the following: B = Na2O/Al2O3, A = aging time. Hydrothermal treatment time.

The effect of alkalinity on the properties of the product was verified by changing the number of moles of Na2O in the gel synthesis. This transformation was achieved by adding sodium hydroxide amounts for Na2O/Al2O3 molar ratios of 5.5, 6.5, 7.5 and 8.5. The hydrothermal treatment time was of 6 h at 100 °C. Fig. 1 shows the diffraction patterns for different samples. The increased amount of Na2O in the gel synthesis caused an intensity decrease in the diffraction peaks. For sample B8.5, synthesized with the highest Na2O/Al2O3 content, early contamination is indicated by an asterisk in Fig. 1d, which probably belongs to the zeolite phase NaP1 (GIS type). Thus, the limit of the Na2O/Al2O3 ratio to form the faujasite phase, under these synthesis conditions, is of 8.5. As noted by some authors, the intensity decrease in the diffraction peaks, as well the increase of its width is related to the decrease in the size of crystals in this sample [10]. These samples were characterized by scanning electron microscopy; the micrographs obtained are shown in Fig. 2. Fig. 2 demonstrates that the increase in Na2O content in the reaction mixture, and consequently in the basicity, caused a decrease in particle size. For samples B5.5 and B6.5 (Fig. 2a and b) well defined particles with octahedral morphology are observed, characteristic of the faujasite phase. However, the crystal dimensions obtained by diffraction (Dhkl) and physisorption measures (DExt) presented in Table 2, suggest that these particles are polycrystalline, composed of nanosized crystals. With increasing basicity of the reaction mixture, the particles become more irregular, and no defined morphology was observed for sample B8.5 (Fig. 2d). The change in morphology can be best seen in Fig. S1 (Supplementary information), which shows higher magnification of the sample micrographs with the highest and lowest Na2O/Al2O3 content in the reaction mixture. It can be observed that the increasing

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6000

6000

(b)

5000

5000

4000

4000

Intensity (a.u.)

Intensity (a.u.)

(a)

3000 2000

0

5

6000

10

15

20

2θ ( o )

25

30

35

5000

5000

4000

4000

3000 2000

0

0

15

20

2θ ( o )

25

30

35

15

20

25

30

35

20

25

30

35

2θ ( o )

(d)

2000 1000

10

10

3000

1000

5

5

6000

(c)

Intensity (a.u.)

Intensity (a.u.)

2000 1000

1000 0

3000

* 5

10

15

2θ ( o )

Fig. 1. XRD for samples synthesized at 100 °C for 6 h with different values of Na2O/Al2O3 molar ratios (a) 5.5, (b) 6.5, (c) 7.5 and (d) 8.5.

Fig. 2. SEM for samples synthesized at 100 °C for 6 h with different values of Na2O/Al2O3 molar ratios (a) 5.5, (b) 6.5, (c) 7.5 and (d) 8.5.

Na2O content in the reaction mixture caused a pronounced particle size decrease and a morphology change.

Because of the decrease of the crystal size with increasing Na2O/ Al2O3 molar ratio, the nitrogen physisorption show that micropore

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70

According to Larsen [10] and Song et al. [29], assuming a defined geometry for the zeolite crystals, there is a correlation between the external area SExt and the crystal dimensions, DExt. In the case of cubic crystals, applicable to faujasite, the authors suggest that the relationship is given by DExt = 4061/SExt. Table 2 shows the results of DExt, using this expression for the samples obtained with different Na2O/Al2O3 in the reaction mixture, indicating that there is the same tendency between DExt and the Dhkl size of the crystallites. The exception is the sample B8.5, which, as already shown, is not well structured. Table 2 also displays the micropore volume of the synthesized zeolites, showing that sample B5.5-24 h, which has the highest crystallinity estimated by X-ray diffraction, as expected also has the largest volume of micropores. The value obtained with this sample (0.33 cm3/g) is very close to the maximum value obtained by other authors [27,30,31]. Table 2 also shows that the sample with the highest alkalinity (B8.5) has the lowest micropore volume. Therefore this indicates that the sample is not fully crystallized or has a large concentration of defects, possibly due to the short hydrothermal treatment time. To verify the possibility of improving the organization of this material, a crystallization curve was performed, keeping the Na2O/Al2O3 content of 8.5 in the reaction mixture and varying the hydrothermal treatment time at (2, 4, 6, 8, 10 and 12, 24 and 48 h) at 100 °C. Fig. S2 shows the results. Fig. S2 shows the XRD profiles of the materials obtained under the aforementioned conditions. It is observed that between 2 and 4 h of synthesis there is a rapid transformation of the amorphous material into faujasite. Starting at 6 h of hydrothermal treatment, together with the characteristic peaks of faujasite at around 2h  12°, the appearance of the second most intense peak of the type GIS structure (zeolite NaP1) begins. The contamination becomes more intense with increasing crystallization time, a tendency also observed by Zhu et al. [32]. Fig. S3 shows the variation of relative crystallinity of the samples formed under the above conditions as a function of time, with the following greater purity zeolites as standards: faujasite B5.524 h and gismondine B8.5-48 h. The marked growth of faujasite between 2 and 4 h of hydrothermal treatment can be observed, followed by its crystallinity stabilization in the following hours. Between 6 and 12 h of synthesis, there is the growth of GIS with no crystallinity changes in the FAU phase. This may indicate that the GIS structure is probably being formed in a parallel nucleation from amorphous precursors. The increased crystallization time favors the formation of the GIS phase, a drop in crystallinity in the FAU phase after 12 h of synthesis can be seen, possibly related to the dissolution of FAU zeolite, which is serving as a precursor for the formation of the GIS zeolite. In hydrothermal treatment times higher than 24 h, only the GIS phase is formed.

60

3.3. Effects of aging

50

Some authors observed a decrease in crystal size by the introduction of an aging step of the reaction mixture prior to crystallization, caused by the increase in the number of nuclei [27,34]. In addition, the aging step can ensure the formation of the faujasite phase without contaminating the GIS phase [33]. Thus, the reaction mixture with Na2O/Al2O3 content of 8.5, and that gave rise to the B8.5 sample (Fig. 2d), was aged at a temperature of 25 °C for 24, 48 or 72 h. After this step, the reaction mixture was transferred to an autoclave and crystallized at 100 °C for 6 h. The diffraction patterns (Fig. 4) of the three samples show the characteristic peaks of FAU, with intensity well below the peaks of the standard sample (B5.5-24 h, Table 2) and without the presence of contamination of the GIS phase, observed when crystallization is performed without aging (Fig. S3). This shows that the introduction of the aging step ensured the purity of the faujasite zeolitic phase. By increasing

Table 2 Crystallinity and microporosity values and crystal diameter for different samples. Sample a

Crystallinity (%) Dhkl (nm)b SExt (m2/g)c DExt (nm)d VMicro (cm3/g)c Relative microporosity (%)e VMeso (cm3/g)f a b c d e f

B5.5-24 h

B5.5

B7.5

B8.5

100 82 34 119 0.330 100 0.028

84.8 64 39 103 0.272 82.4 0.031

30.7 50 44 91 0.248 75.1 0.042

15.9 23 27 146 0.063 19.1 0.062

Calculated by XRD. Obtained using the Scherrer equation. t-Plot method. Obtained using the equation DExt = 4061/SExt [27]. Calculated using the sample Y-B5.5-24 h as standard. VMeso = (V(P/Po  0.98) VMicro).

volume also decreases, probably a consequence of more crystal defects. Therefore, the external area of the samples increases, with the exception of sample B8.5. For example, sample B5.5, which has the second highest crystallinity value, had an external area of 39 m2/g and a micropore volume of 0.27 cm3/g. In comparison, sample B8.5 showed 27 m2/g of external area and 0.06 cm3/g of micropore volume, showing that the material is still not well structured. This may be related to increased alkalinity in the reaction mixture for the samples with high Na2O contents, thus favoring a higher dissolution of the zeolite during synthesis. As shown in Fig. 3, the increased basicity is also responsible for the decrease in solid yield, calculated by the ratio between the mass of solid product and mass of solid in reaction mixture. The diminished yields in higher basicities are due to the higher solubility of silicates and aluminates in the reaction medium and their lower incorporation into the solid product.

3.2. Crystallization behavior

Yield (%)

According to Fig. 2, by increasing the Na2O/Al2O3 content in the reaction mixture, there was a decrease in the polycrystalline clusters of the faujasite zeolite. A similar behavior was observed by analyzing the diffractograms of these samples using the Scherrer’s equation (Table 2). However, the dimension values obtained by diffraction are lower and this is because this technique determines the size of crystals or crystalline environments that form the clusters.

40 30 20 10 0 5.5

6.5

7.5

8.5

Na2O/Al2O3 Fig. 3. Yield of faujasite as a function of Na2O/Al2O3 molar ratio in the crystallized reaction mixture after 6 h at 100 °C.

71

2500 2000 1500 1000 500 0 2500 2000 1500 1000 500 0 2500 2000 1500 1000 500 0

c

b

a 5

10

15

20

25

30

35

2θ (Degree) Fig. 4. XRD for samples aged for 24 (a), 48 (b) and 72 h (c) with Na2O/Al2O3 of 8.5 in the synthesis gel and submitted to 6 h of hydrothermal treatment.

the aging time to 48 and 72 h, a broadening of the diffraction peaks is observed (Fig. 4), which, according to the Scherrer equation, is related to a reduction in the crystallite diameter (Table 3). As seen in Fig. 2, the increase in alkalinity caused a significant reduction in particle size. However, a marked decrease in crystallinity and also in micropore volume was observed in these samples (Table 2). The introduction of an aging step in the reaction mixture with higher alkalinity, in addition to promoting the reduction of particle size, contributed to a better structuring of the material, causing an increase in the micropore volume (Table 3). The micropore volume underwent small variations by increasing the aging time, showing that the porous structure of the material is maintained. X-ray diffraction is commonly used to calculate the crystallinity of materials synthesized under different conditions [21,26]. However, due to the low intensity of the diffraction peaks that the nanoscale materials present, relative microporosity was also used as a crystallinity measure. Among the synthesized samples, the one with the highest micropore volume was used as standard (B5.5-24 h). The results are presented in Tables 2 and 3, confirming that the measure of organization of the solids by diffraction shows values much lower than the one performed by the volume of micropores. It can thus be observed that although the aged samples have low intensity diffraction peaks (Fig. 4) they are highly microporous, which shows an elevated organization of the synthesized material. In addition to the higher amount of micropores obtained in the samples with aging, a significant increase was also observed in the amount of mesopores (Tables 2 and 3), reaching values of up to 55% of the total pore volume. This shows that there is a large porosity formed between the crystals. This is a very important property to minimize the diffusional limitations in the

Table 3 Crystallinity values, microporosity volumes and crystal diameters for different samples. Sample a

Crystallinity (%) Dhkl (nm)b SExt (m2/g)c DExt (nm)d VMicro (cm3/g)c Relative microporosity (%)e VMeso (cm3/g)f a–f

See Table 2.

B8.5

B8.5-A24

B8.5-A48

B8.5-A72

15.9 23 27 146 0.063 19.1 0.062

25.5 22 87 46 0.268 81.2 0.107

20.4 19 106 38 0.211 63.9 0.268

13.5 14 115 35 0.235 71.2 0.285

mass transport that leave or enter the pores of the material, as for instance, during catalytic reactions. Fig. 5 presents the physisorption isotherms of nitrogen of samples B5.5, B8.5 and B8.5-A72. The isotherm of the sample B5.5 has a typical profile of a microporous material (Type I) with high adsorption capacity in the region of low pressure. On the contrary, sample B8.5 has a low adsorption capacity, suggesting a low microporosity, and, consequently, a low crystallinity. After aging of sample B8.5, the new material (B8.5-A72) has a similar microporous volume as sample B5.5. It has also an unusual behavior: the amount of nitrogen adsorbed increases gradually with pressure. This is probably related to the adsorption of nitrogen in spaces formed by the aggregation of nanocrystals. The samples synthesized with different aging times were also characterized by SEM, Fig. 6 shows the micrographs. As observed by physisorption and XRD, the micrographs of these samples show a reduction in particle size with increasing aging time. The samples submitted to 24 and 48 h of aging (Fig. 6a and b) show aggregates consisting of particles with highly heterogeneous size and morphology. The sample submitted to 72 h of aging (Fig. 6c) is composed of aggregates of approximately 100 nm in size. TEM analysis of about 120 particles shows that they are formed by crystals that have an average size of around 31 nm. The size of these crystals, calculated by the Scherrer equation, and of the particles by nitrogen physisorption, estimate sizes in the order of 14 and 35 nm, respectively (Table 3). The smallest size obtained by XRD suggests that the particles observed by the other methods are made up of several crystals. The size obtained by physisorption and that observed by transmission microscopy are similar, showing that the particle surface is accessible to nitrogen molecules, which is crucial for the applications of this material in adsorption and catalysis. Fig. 7 shows the TEM micrographs of samples B8.5-A24 and B8.5-A72. Sample B8.5-A24 (Fig. 7a) consists of polycrystalline aggregates with sizes above 200 nm, while in B8.5-A72 the aggregate size is of 80 nm. In the micrograph of the sample submitted to 72 h of aging, with higher magnification (Fig. 7c), the crystal planes corresponding to the plane h k l (1 1 1) are observed, showing that the zeolite is well crystallized. In order to verify the effect of a lower alkalinity on the particle size of FAU zeolite, the synthesis was conducted with Na2O/Al2O3 molar ratio equal to 5.5 while maintaining the aging time at 72 h (sample B5.5-A72). Fig. 7d shows its micrograph. It can be observed that the particles have similar sizes of the sample B5.5, obtained with the same gel composition, but without being subjected to aging (Fig. 2a). This shows that when the basicity is low (Na2O/ Al2O3 = 5.5) the aging step alone is not able to reduce the final size

400

Vol. of Adsorbed (cm3 /g)

Intensity (Arb. Unit)

T.F. Chaves et al. / Microporous and Mesoporous Materials 161 (2012) 67–75

350

B8.5-A72

300 250 200

B5.5

150 100

B8.5 50 0

0.0

0.2

0.4

0.6

0.8

1.0

P/Po Fig. 5. N2 physisorption isotherm for samples B5.5, B8.5 and B8.5-A72.

T.F. Chaves et al. / Microporous and Mesoporous Materials 161 (2012) 67–75

a

Frequency (%)

30 25 20 15 10 5

b

30

Frequency (%)

72

25 20 15 10 5 0

50 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0

50 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0

0

Diameter (nm)

Diameter (nm)

c

Frequency (%)

30 25 20 15 10 5

50 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0

0

Diameter (nm)

Fig. 6. Scanning electron microscopy for samples B8.5-A24 (a), B8.5-A48 (b) and B8.5-A72 (c).

Fig. 7. Transmission electron microscopy for samples obtained from a gel with composition 8.5 Na2O:Al2O3:10 SiO2:180 H2O and aged for 24 (a) and 72 h (b and c). Scanning electron microscopy of sample B5.5-A72 (d).

of the particles. As observed in Table 3, aging has a pronounced effect on particle size if a higher alkalinity is provided to promote the depolymerization of the silica source. Li et al. [34] observed a similar behavior: the smaller the silicate precursor species, the smaller the final size of the particles synthesized. It is possible that with the increased solubility promoted by the greater amount of hydroxyl anions, the highest concentration of Na+ in these syntheses may have stabilized the nuclei, forming a barrier that prevents the growth or coalescence between the nuclei. Another possibility could be that, with a longer aging time, the equilibrium of size of the precursors in this gel has been achieved and that at such high alkalinity it favors smaller precursors.

To investigate the effect of Na+ cations on crystal size and on the particles, two samples with different amounts of sodium cations were synthesized. In these syntheses the same basicity was used as in the synthesis for the B5.5 sample, and adding the appropriate amounts of NaCl to yield the Na2O/Al2O3 contents of 7.5 and 8.5 in the reaction mixture. These samples (B7.5NaCl and B8.5NaCl) showed a diffraction pattern that is characteristic of the FAU structure without the presence of contamination (not shown). Fig. 8 shows the micrographs of these samples. They are composed of spherical-shaped polycrystalline particles. For the first sample (Fig. 8a and c) the diameters are between 0.41 and 1.91 lm, with a mean value, considering 100 particles, of around 1.13 lm. For

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b

a

Average = 1.13 μm

c

20

25

Frequency (%)

Frequency (%)

25

15 10 5

d

Average = 1.58 μm

20 15 10 5 0

0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

0.4 0.8 1.2 1.6 2.0 2.4 2.8

Diameter (μm)

Diameter (μm)

Fig. 8. Scanning electron microscopy of samples (a) B7.5NaCl and (b) B8.5NaCl. Size distribution for samples (c) B7.5NaCl and (d) B8.5NaCl.

the second sample (Fig. 8b and d), the diameters were between 0.60 and 2.53 lm, with an approximate mean value of 1.58 lm. Comparing these values with those of sample B5.5 (mean diameter equal to 1.17 lm), it is observed that their diameters are greater and that there is an increase in size with increasing NaCl content in the reaction mixture. The intensity of diffraction peaks increase for samples B7.5NaCl and B8.5NaCl, following that order. The yield for the two samples was 33% and 30%, respectively, this shows that the yield did not increase significantly with the addition of NaCl. Subotic´ and co-workers [35–38] extensively studied the zeolite crystallization processes of NaA and observed that by increasing the Na+ content in the reaction mixture, increases in the kinetic constants of growth are observed for the zeolite particles. According to the authors, the highest sodium concentration in the liquid phase favors condensation reactions on the surface of the particles, which contributes to their growth. This can also be the cause of the effect observed in Fig. 8a and b, for the FAU structure and rules out the hypothesis that the Na+ ions block the growth of particles. The increase in condensation speed could explain the irregularity of the particles observed in the micrographs, resulting from an intense incorporation of silicate and aluminate species on the surface of the material [36–38]. In the 27Al MAS-NMR spectra of the samples synthesized with different Na2O/Al2O3 contents, only the presence of signals with chemical shift between 60 and 61 ppm were observed, resulting from tetrahedrally coordinated Al species [39], which shows that all the aluminum in the sample is probably embedded in the zeolite structure. The same behavior is seen for the samples submitted to different aging times, with the spectra showing only one peak, at around 60 ppm, for the aluminum species also tetrahedrally coordinated. The high-power decoupling (HPDEC) 29Si MAS-NMR spectra of the samples prepared with or without an aging step (Fig. 9) show the different chemical environments of the silicon atom. The peaks at 84, 88, 93, 98 and 103 ppm, correspond to the chemical environments Si(4Al), Si(3Al,1Si), Si(2Al,2Si), Si(1Al,3Si) and Si(4Si), respectively, in nanometric Y zeolite [39]. The spectra in Fig. 9 are quite similar, with only the increase observed in the area of the peak at 84 ppm accompanied by a decrease in the peak at

- 88 - 84

- 93 - 98 - 103

a b c d

e -70

-80

-90

-100

-110

Chemical Shift [ppm] Fig. 9. Solid-state HPDEC 29Si NMR spectra of samples B5.5 (a), B8.5 (b), B8.5-A24 (c) B8.5-A48 (d) and B8.5-A72 (e).

93 ppm for the three samples of the series A, subjected to aging (Table 4). Fig. 10 shows the spectra of 1H/29Si NMR CP/MAS for the samples and the relative peak areas are listed in Table 4. As aging in the reaction mixture is performed, it can be seen a change in the profile of these spectra in comparison to 29Si NMR HPDEC. As the aging time increases (samples B8.5 to A72), the CP/MAS peaks with chemical shifts at 88 ppm becomes more intense than the one obtained by HPDEC and the area of the peaks at 84 and 93 ppm decreases. This may be related to a larger amount of silanol groups of the Si(3Al,1OH) species on the surface of the crystals [26]. Table 3 and the images in Fig. 6 show that the increased aging time favors the increase of the external area and the reduction of the crystal sizes, with a greater exposed surface there is an increase in the amount of silanol groups. With increasing aging time, an increase in peak area of the signal at 84 ppm was observed in the spectrum HPDEC 29Si NMR. This

74

T.F. Chaves et al. / Microporous and Mesoporous Materials 161 (2012) 67–75

Table 4 Areas under the peaks of the solid-state high-power decoupling and cross polarization 29

29

Si NMR spectra of selected samples. 1

H/29Si NMR CP-MAS (%)

Si NMR HPDEC (%)

Si vicinity Sample

4Al 84

3Al 88

2Al 93

1Al 98

0Al 103

4Al 84

3Al 88

2Al 93

1Al 98

B5.5 B8.5 B8.5-A24 B8.5-A48 B8.5-A72

14.01 25.66 22.62 23.62 26.15

33.13 30.27 35.43 35.44 35.84

34.83 23.92 26.55 25.82 22.89

14.37 13.28 10.27 11.99 11.21

3.66 6.87 5.13 3.13 3.91

15.00 16.83 22.93 21.50 19.29

33.12 40.55 43.40 42.38 49.34

32.59 24.90 25.22 24.66 22.42

19.29 17.72 8.45 11.46 8.65

a b c d e -60

-70

-80

-90

-100

-110

-120

Chemical Shift [ppm] Fig. 10. Solid-state 1H/29Si NMR CP/MAS spectra of samples B5.5 (a), B8.5 (b), B8.5A24 (c) B8.5-A48 (d) and B8.5-A72 (e).

indicates a larger concentration of Si(4Al) groups and can be attributed to a higher solubility of silicate species, remaining in the liquid phase, due to the increased alkalinity of the reaction mixture [40]. Table 5 shows that this hypothesis is confirmed by the chemical analysis results. The values of Si/Al content, calculated by different methods, show that with increasing aging time, a smaller amount of Si was observed in the samples resulting in a decrease in its total content. As only the peak corresponding to the tetrahedrically coordinated aluminum was observed in the spectra of 27Al MASNMR for the samples (not shown), no amorphous aluminosilicate is present and all aluminum atoms are incorporated in the zeolite framework. Table 5 shows that the chemical composition results obtained by EDS and 29Si-HPDEC-MAS-NMR of the samples prepared with aging showed very similar values (standard deviation about 5%). This reaffirms the idea that there are no extraframework Si or Al atoms in these zeolites. In addition to these two techniques, Xray diffraction was used to determine the Si/Al content using different equations. However, there is a divergence between the

results obtained by XRD and NMR (Table 5). It is possible that the equation proposed by Breck and Flanigen [41] and Rüscher et al. [42] are not applicable to faujasite crystals with nanometric dimensions. This does not seem so inconsistent due to the effect caused on the diffraction pattern by the decreased crystal size which may contribute to the inaccuracy of the method for calculating the unit cell parameter a0. The equation that best fit the results was proposed by Fichtner-Schmittler et al. [43], in which values closer to those obtained by NMR and EDS can be observed. 4. Conclusions The synthesis of faujasite zeolite formed by nanosized crystals was successfully accomplished through the usual precursor sources and in the absence of organic templates. The increased basicity in the reaction medium caused the decrease of the crystals; the insertion of an aging step favored a better structuring of the reaction mixture, resulting in a solid with high microporosity after the hydrothermal treatment. The solid obtained after 3 days of aging exhibits high external area and mesopore volume, which are necessary conditions to be applied in catalytic reactions that involve voluminous molecules or as adsorbent material with less diffusional limitation influences. The samples exhibit only tetrahedral aluminum sites and the chemical analysis shows that increasing the aging time favors the decrease in the Si/Al content. Possibly this is due to the dissolution of part of the silica while maintaining the structural aluminum. Acknowledgements The authors are grateful to FAPESP [2008/07179-8 (D.C.) and 2008/00132-6 (H.O.P.)] and CNPq (149616/2010-4) for the financial support to this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2012. 05.022.

Table 5 Chemical composition of samples, obtained by different techniques and calculations.

a b c

Sample

Si/Al (EDS)

Si/Al (NMR)

Si/Al (XRD)a

Si/Al (XRD)b

Si/Al (XRD)c

B5.5 B6.5 B7.5 B8.5 B8.5-A24 B8.5-A48 B8.5-A72

1.72 1.81 1.83 1.86 1.53 1.56 1.48

1.67 1.73 1.65 1.57 1.53 1.51 1.48

1.54 1.58 1.59 1.29 1.46 1.41 1.30

1.56 1.60 1.62 1.33 1.48 1.44 1.34

1.78 1.83 1.84 1.49 1.68 1.63 1.50

Determined using the equation proposed by Breck and Flanigen [41]. Determined using the equation proposed by Rüscher et al. [42]. Determined using the equation proposed by Fichtner-Schmittler et al. [43].

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