Synthesis, characterization and application of nanozeolite NaX from Vietnamese kaolin

This paper presents the results of synthesis of nanozeolite NaX from Vietnamese kaolin. Influence factors on the control of crystal sizes and application of synthesized materials as adsorbent for organic compound are discussed. The results show that there are several factors that influence the synthesis. When water content in gel increases, crystal size of NaX increases sharply. The increase of alkaline and silica contents increases the crystallinity and decreases the particle size of nano NaX, and the particle size reaches the minimum at Na2O/Al2O3 = 5.0 and SiO2/Al2O3 = 4.0. Crystal sizes formed at low crystallizing temperature are smaller than those formed at higher temperature. Ageing time and crystallizing time strongly influence the crystallinity and crystal size, which is related to the number of crystal seeds formed during ageing period, the growth of seed and the partial solubility of crystal at maximum formation. Nano NaX was characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) adsorption, Fourier transform infrared (FTIR) and simultaneous thermogravimetry–differential scanning calorimetry (TG/DSC). At optimized synthesis conditions, nano NaX has surface area of 573 m2 g−1with external surface area of 92 m2 g−1, pore size distribution at 0.81 and 10.8 nm, average crystal size of 25 nm and thermal stability up to 789 °C. This material can adsorb cumene rapidly with high adsorption capacity and stability.


Introduction
Nanoscience and nanoporous materials are currently attracting attention from many scientists. Microporous materials with nanometer particle size (nanozeolites) are being studied because of their outstanding properties that could not be found in the micrometer zeolites.
Reducing the particle size from micrometer to nanometer scale leads to a significant change of material characteristics Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. and their applications in catalysis and adsorption. The number of atoms in the unit cell increases when particle sizes decrease and nanozeolites have large external surface area. The diffusion path length in nanozeolites is shortened as compared to that in the conventional micrometer zeolites.
Most recent research papers about nanozeolite NaX are from pure, expensive chemicals containing organic aluminum and silicium (such as aluminum isopropoxide, tetraethyl orthosilicate-TEOS) and organic templates (tetramethylammonium hydroxide TMAOH, tetramethylammonium bromide TMABr) in alkaline media in certain synthetic conditions [1][2][3][4]. Nanozeolites with crystal  [3]. Mesoporous NaX zeolites with sizes of 4-50 nm were synthesized by using organic templates of cationic polymer (polydiallyldimethylammonium chloride, PDADMAC) at 100 • C in 16 h [5]. Limited studies have reported the synthesis of nanozeolite ZSM-5 from kaolin [6], and to our knowledge nanozeolite NaX synthesized from kaolin has not been reported in literature. This paper presents the results of synthesis of nanozeolite NaX from Vietnamese kaolin. The factors influencing the control of particle sizes and application of synthesized materials as adsorbent for organic compound are discussed.
Micrometer zeolites NaX (noted as micro-NaX) were synthesized with procedure in [8] for the comparison. Molar composition, crystallizing condition and sample codes are presented in table 1.
Obtained nanozeolites were repeatedly washed with distilled water until pH of supernatant was 9. Nanoparticles were dried at 110 • C and calcined at 550 • C.

Characterization
XRD analyses were carried out at room temperature in θ-2θ reflection mode using a SIEMENS D5005 diffractometer. SEM and TEM images were obtained on JSM 5410 LV and JEM 1010. Specific surface area determinations (BET) and pore size distributions were measured on a COULTER SA3100 apparatus. FTIR was performed on Nicolet impact FTIR 410 Spectrometer. TG/DSC was performed on NETZSCH STA 409 PC/PG. Cumene adsorption was performed at 40 • C in dynamic mode. Nitrogen, with a purity of 99.99%, was used as the carrier gas. N 2 was conducted to the pre-saturator containing pure liquid cumene before sending through adsorbent bed. The concentration of xylene vapor in the inlet was 5200 ppm. Before adsorption measurements, adsorbents were out-gassed at 423 K for 1-3 h. The cumene in gas flow was analyzed by GC-14B Shimadzu online.  The XRD patterns of all nanozeolites NaX exhibit diffraction peaks which are characteristic of zeolite NaX. Beside that, there is also the presence of the crystalline phase of α-quartz. There is no kaolinite crystalline phase, that has confirmed that the kaolin in the raw materials has been converted completely.    The rise of XRD baselines and SEM and TEM images of samples crystallized at 60 and 100 • C prove that there are more amorphous phases on these samples than on a sample crystallized at 80 • C and the crystallinity of these samples is low. Crystal sizes of nanozeolite NaX crystallized at 80 • C are the smallest (about 32 nm by TEM).

Results and discussion
At low crystallizing temperature (60 • C), the crystallizing rate is slow that leads to the low crystallinity of the samples and vice versa, at high crystallizing temperature (100 • C), the crystallizing rate is large, so obtained samples have larger crystal size.

Influence of alkaline content
XRD patterns, SEM and TEM images of nanozeolites NaX synthesized with different ratios of Na 2 O/SiO 2 in gel are shown in figures 4, 5 and 6, respectively. Other results are listed in table 3.
The XRD patterns of all nanozeolites NaX show the crystalline phase of zeolite NaX and α-quartz with different intensities. When the ratio of Na 2 O/SiO 2 in gel increases, the peak intensity of NaX increases and reaches the maximum result at ratio of Na 2 O/SiO 2 in gel of 5 (sample X96-12-5N-figure 4(d)), while the peak intensity of quartz decreases and gets the minimum result at this ratio. Sample X96-12-5N also has the largest FWHM.
All the results show that sample X96-12-5N has the highest crystallinity (92%) and smallest crystal size (25 nm  calculated from XRD result, see table 3).
SEM and TEM images of sample X96-12-5N show the average crystal sizes of 54 nm and 32 nm, respectively.
OH − is mineralizing agent that stimulates the formation of AlO − 4 and SiO 4 tetrahedral that are suitable for the crystallization of zeolite NaX. Increasing ratio of Na 2 O/Al 2 O 3 in gel from 2 to 5 will increase OH − contents, but if the OH − content is too high (Na 2 O/Al 2 O 3 ratio in gel > 5), AlO − 4 tetrahedral are partly dissolved, so the crystallinity decreases and quartz contents increase. The highest crystallinity and smallest crystal size of sample X96-12-5N prove that ratio of Na 2 O/Al 2 O 3 in gel of 5 is the most suitable to synthesize nanozeolite NaX.
SEM and TEM images are totally corresponding to the results obtained from XRD patterns. Figure 7 shows XRD patterns of nanozeolites NaX with different SiO 2 /Al 2 O 3 ratios in gel. Sample X96-12-4.0S shows that the intensity of zeolite NaX phase at 2θ = 6.2 is the highest and that of quartz at 2θ = 26.7 o is the lowest. This sample has the highest crystallinity (92%). Full-width at half-maximum (FWHM) of zeolite NaX phase at 2θ = 6.2 is the largest. The crystal size calculated by Scherrer equation is the smallest (25 nm). The framework SiO 2 /Al 2 O 3 ratio of NaX is 2.5, whereas the SiO 2 /Al 2 O 3 ratio in gel to crystallize nanozeolite with high crystallinity is 4, so excess silica content in gel is needed in the preparation of nanozeolite NaX (table 4). SEM and TEM images were shown in figures 8 and 9. Sample X96-12-4.0S has the smallest average crystal size of 54 nm (by SEM) and 32 nm (by TEM).

Influence of H 2 O/Al 2 O 3 ratio in gel
XRD patterns of nanozeolites NaX with different water contents in gel are presented in figure 10. Other experiments were also carried out with H 2 O/Al 2 O 3 ratio in gel < 70. However, the gels were very viscous so that   When water content is low, the concentration of TO 4 tetrahedra in reaction solution is high, crystals are formed with small crystal size and vice versa. When H 2 O/Al 2 O 3 ratio in gel 110, the concentration of TO 4 tetrahedra in reaction solution decreases sharply, zeolite NaX formed have large crystal size (to micrometer scale).

Influence of aging time
XRD patterns, SEM and TEM images of nanozeolites NaX synthesized with different aging times are shown in figures 11, 12 and 13, respectively. Other results are listed in table 6.
value at 96 h. Flusston phase (SiO 2 -Al 2 O 3 -Na-OH, JPCDS 003-0413) appeared in sample X00-12 (without ageing). Sample X96-12 (aging time of 96 h) shows the best result with high crystallinity and smallest crystal size. When the aging time is short, there are few crystal seeds formed, so the crystallinity is low. When the aging time is too long (>96 h), many crystal seeds are formed but the phase transformation could occur to form α-quartz and amorphous alumina that leads to the low crystallinity.

Influence of crystallizing time
In these experiments the optimal crystallizing temperature of 80 • C and aging time of 96 h were used. Influences of crystallizing time have been shown in figures 14, 15 and 16 and table 7.
The crystallinity (by XRD) increases sharply when increasing crystallizing time from 6 to 12 h and then decreases gradually with time on stream.
When crystallizing time is short (<12 h), the crystals are not formed completely so the crystallinity is low. If the crystallizing time is long, the crystal phase can be transformed to stable α-quartz, so the crystallinity is also decreased. The increase of α-quartz phase can be observed at 2θ = 16.7 o .
Average crystal sizes obtained from XRD, SEM and TEM are similar (table 7). Sample X96-12 has the smallest crystal size. The crystallizing time of 12 h is the most suitable for synthesizing nanozeolite NaX.

Comparison of nanozeolite NaX (nano-NaX) and conventional micro-zeolite NaX (micro-NaX)
XRD patterns of nano-and micro-NaX are shown in figure 17. Zeolite NaX phase (Na 2 O · Al 2 O 3 · 2.5SiO 2 · 6.2H 2 O, JCPDS 38-0237) appears in both samples. However, micro-NaX shows a very sharp and high intensity peak. Line broadening in nano-NaX is due to the fact that the crystal sizes of nano-NaX are smaller than that of conventional micro-NaX. Analysis of XRD line broadening using the Scherrer equation gives crystal size of nano-NaX of about 19 nm and that of micro-NaX of 0.4 µm.
SEM and TEM images of nano-and micro-NaX are shown in figure 18. Nano-and micro-NaX are in cubic form. Average crystal size of nano-NaX measured from TEM image ( figure 18(c)) is 32 nm.
FTIR spectra of micro-and nano-NaX are shown in figure 19. All key bands of nano-NaX resemble those exhibited by micro-NaX, however, the intensities of absorption bands of micro-NaX are stronger. The weak absorption band observed around 608 cm −1 in curve (b) of figure 19 is similar to that of [1], this absorption band only appears on nanozeolite NaX.
TG/DSC curves of nano-NaX and micro-NaX are shown in figure 20. The endothermic peaks in DSC curve of micro-NaX at 120 • C can be attributed to the removal of physical adsorbed water on the surface of the materials and that at 332 • C is because of the removal of chemical adsorbed water in the pores. The total weight loss determined by TG curve is about 18.3%. The exothermic peak at 869 • C without weight loss might be assigned to the phase transformation and micro-NaX.
Two similar endothermic peaks at 102 and 345 • C and an exothermic peak at 789 • C are observed in DSC curve of nano-NaX. The total weight loss of nano-NaX is higher than that of micro-NaX (24.1% compared to 18.3%) due to the porosity of nano-NaX being higher than that of micro-NaX. N 2 adsorption/desorption isotherms at 77 K and pore sizes distribution of nano-NaX and micro-NaX are shown in figure 21. The isotherm of micro-NaX is type I (defined by IUPAC) [9] which is the characteristic of microporous material. However, the loop started at p/ p o ≈ 0.45 can be observed in the isotherm of nano-NaX, which belongs to type III [9] because of the condensation of nitrogen in mesopores of the materials. These mesopores could be formed between the nanometer crystals.
Pore distribution analyzed following the method in [10] are shown in figure 22. The average pore sizes of both microand nano-NaX are 0.81 nm. However, small pore distribution in ∼10.8 nm region can be observed in nano-NaX, it might be the secondary porous system formed between the nanometer crystal and causes the loop in the isotherm as mentioned above.
The BET surface areas of nano-NaX and micro-NaX are 573 and 520 m 2 g −1 , respectively. The external surface of nano-NaX is 92 m 2 g −1 and that of micro-NaX is 39 m 2 g −1 .
The increase of the external surface is due to the reduction of  crystal sizes (table 8). The data in table 8 show the outstanding characteristics of nano-NaX compared to those of micro-NaX that can affect the adsorption ability of nano-NaX. Figure 23 shows the cumene adsorption capacity of nano-NaX and micro-NaX. Both samples adsorb cumene vapor well.

Dynamic cumene adsorption on nano-NaX and micro-NaX.
However, the adsorption capacity of nano-NaX is better. During the first 15 min, nano-NaX adsorbs nearly 90% cumene vapor in the flow; the cumene content in the outlet is less than 800 ppm.
The breakthrough curve obtained with nano-NaX is steeper than that obtained with micro-NaX. The steeper the breakthrough curve, the higher this adsorption rate constant. This is likely due to the smaller particle size of nano-NaX (25 nm by XRD instead of 400 nm for micro-NaX). The breakthough time is shorter for nano-NaX (35 min) than for micro-NaX (40 min).
Nano-NaX has smaller crystal size, higher external surface area and higher pore volume (table 8), so the adsorption capacity is higher. Moreover, nano-NaX has    Figure 19. FTIR spectra of micro-NaX (a) and nano-NaX1 (b).
secondary porous system which is much larger than the diameter of cumene, so it can adsorb cumene vapour easily leading to the fast decrease of cumene concentration in the outlet. When the secondary porous system is filled up with cumene vapor, the adsorption will take place in the   micropores. The increase of crystal size of micro-NaX makes both the external and internal transport of the adsorbate more difficult [11].  After each adsorption round, the adsorbent was desorbed and then used again. The adsorption capacity of nano-NaX after 6th round adsorption is shown in figure 24. The breakthrough curve after the 6th round is the same as that after the 1st round. This means that synthesized nano-NaX is very stable and has high adsorption capacity.