3.1 Effect of Crystallization Temperature and Crystallization Time
The effect of hydrothermal temperature on the crystallization of zeolite Y was evaluated by synthesis from 70 to 95 °C without using any template. The mole oxide compositions of the gels were 7 Na2O: 1 Al2O3: 10 SiO2: 300 H2O. The prepared gels were aged at temperatures of 30 °C for 24 h and then crystallized for 24 h. Figure 1 shows the crystallization rate curves of the products synthesized at different temperatures.It can be seen from the crystallization curve that different temperatures have a great influence on the nucleation stage of crystals.With the increase of crystallization temperature, the induction period of nucleation is shortened.However, it can be seen from the slope of the crystallization curve that the slopes of several curves are basically the same, indicating that different crystallization temperatures have little effect on the growth rate of crystals. With the increase of crystallization temperature, the crystal growth time is shortened. The zeolite Y in the figure is a sample that crystallizes for 32 hours at a crystallization temperature of 85° C.
Noticeably, at a low temperature of 70 °C, highly crystallized Y zeolite could be obtained with a longer crystallization time 80 hours. Early efforts under hydrothermal conditions revealed that Y zeolite was created mostly at 100 °C, and impurities like P zeolite were observed at a lower temperature 80°C[18]. The present result convinced that the synthesis method by acid-catalyzed hydrolysis of tetraethylorthosilicate allows the generation of pure Y zeolite at a very low crystallization temperature 70 °C, though needing a longer crystallization time.
The SEM images of Y products at different crystallization stages are shown in Figure 2. It can be seen that the particle size of Y zeolite synthesized at different crystallization temperatures is also different.At the same crystallization temperature, the particle size of the product increases with the extension of crystallization time.When the temperature increases from 80℃ to 85℃ , the product size increases from 1.6 μm to 2.4 μm with crystallization time 32 hours while from 3.6 μm to 4 μm with crystallization time 48 hours. This is because the particle size of zeolite depends on the nucleation rate and growth rate. At low temperature, it is beneficial to promote the formation of crystal nuclei and control the growth rate of grains. With the increase of temperature, the growth process of crystal nucleus is obviously faster, and the nucleation process is inhibited, and the particle size of the product is increased [19-20].
3.2 Effect of Initial SiO2/Al2O3 ratio Amount
The effect of SiO2/Al2O3 ratio on the crystallization of Y zeolite was tested, when the mole oxide com-positions of the gels were 7 Na2O: 1 Al2O3: X SiO2: 300 H2O, in which X varied from 6.0 to 17.0. Figure 3 shows the XRD pattern and crystallinity of zeolite prepared with different SiO2/Al2O3 ratios of initial gels. As shown in Figure 3, pure Y zeolites were obtained at the SiO2/Al2O3 ratios from 6 to 15, in which the sample with SiO2/Al2O3 12 exhibited the highest crystallinity.
Table 1 shows the physical and chemical properties of Y zeolite. According to the ICP analysis, SiO2/Al2O3 ratios for final Y solids obtained with initial gel SiO2/Al2O3 ratios of 6, 9, 12 and 15 were 2.47, 3.89, 5.55 and 4.93, respectively. The BET surface areas were 494, 641, 743 and 567 m2/g, respectively. The lower surface areas at SiO2/ Al2O3 ratio of 6 correspond to its lower crystallinity as shown in Fig.3.When the SiO2/Al2O3 ratio was as low as 6 and as high as 15, the XRD Patterns in Fig. 3 both showed much lowered crystallinities of Y phase. On the other hand, the much higher SiO2/Al2O3 ratio of 12 cause a comparatively high crystallinity of Y zeolite.When the SiO2/Al2O3 ratio was up to 17, the XRD showed that the synthesized zeolite Y is an amorphous product. This is because the excessive addition of Si in the feed ratio leads to the decrease of the relative content of sodium in the mother liquor, which reduces the participation of silicon and aluminum in the structure of zeolite and eventually generates an amorphous product.
Table 1 Textural parameters and of the samples obtained at different Si/Al ratio.
Samples
|
SiO2/Al2O3
Radio
|
SBET,
m2/gc
|
Vmicro,
cm3/gd
|
VTot,
Cm3/ge
|
NaY-1
|
6a(2.47)b
|
494
|
0.18
|
0.22
|
NaY-2
|
9(3.89)
|
641
|
0.24
|
0.27
|
NaY-3
|
12 (5.55)
|
743
|
0.27
|
0.31
|
NaY-4
|
15 (4.93)
|
567
|
0.21
|
0.24
|
a SiO2/Al2O3 ratio in the initial gel.
b SiO2/Al2O3 ratio calculated from ICP-AES analysis.
c BET surface area.
d Microporous surface area evaluated by the t-plot method.
e Total pore volume calculated by the single-point method at P/P0=0.95.
3.3 Effect of Initial Na2O Amount
It is well known that the alkalinity of the initial gel is an important parameter affecting the crystallization of zeolite.Thus, the mole oxide compositions of the gels were X Na2O: 1 Al2O3: 10 SiO2: 300 H2O, in which X varied from 3.0 to 10.0 to evaluate the influence of gel’s alkalinities. Fig. 5 shows the XRD patterns of the resultant as-calcined samples. When the Na2O content was 3 ~ 8.5, pure Y products can be obtained. In this range, crystallinities increased with the increase of Na2O content due to rapid dissolution of silicon in high alkalinity media. At low alkalinity ( Na2O content was 3), a poorly crystallized Y phase was observed because the low concentration of hydroxyl ions could not depolymerize the silica source to provide sufficient solubilized aluminosilicate species for nucleation[21-24]. In contrast, high alkalinity (Na2O content was 8.5) reduces crystallinity, which may be due to the presence of excessive hydroxyl ions leading to dissolution of the already formed zeolite nuclei. A higher Na2O content up to 10 caused occurrence of P zeolite crystals. Accordingly, the Na2O content range from 4.5 to 8.5 is optimal for producing the pure phase of Y zeolite.
3.4 Effect of Initial H2O Amount
The water content in the synthetic system will directly affect the concentration of each substance in the gel, thus affecting the crystallization process of zeolite. Fig. 6 shows the XRD patterns for the samples synthesized with various H2O content with the mole oxide compositions of the gels were 7 Na2O: 1 Al2O3: 10 SiO2: X H2O, in which X varied from 200 to 640. At the very low H2O content of 200, the very concentrated gel created only p zeolite crystals. Because the low water content in the synthetic gel was, the induction period can be shortened, the nucleation rate and crystal growth rate can be accelerated, which was conducive to rapid crystallization, but too little water content in the synthetic system cannot form a uniform gel, resulting in difficult to form Y zeolite products and easy to appear impurity crystals. Also, the much diluted gel mixture (H2O content 600) created only p zeolite crystals, which is ascribed to the very slow nucleation rate arising for the longer distance between nutrients in this diluted solution. When H2O contents were changed from 300 to 480, pure and highly crystallized Y solids were produced. However, the slightly diluted gel mixture (H2O content 480) resulted in a lower crystallinity of Y which due to the high-water content generated the low concentration of reactants in the synthetic system leading to slowing crystallization rate. Consequently, the moderate range of H2O content from 300 to 480 is suitable for synthesizing zeolite Y[25].