A series of high silicon zeolites Y were prepared through direct synthetic method by using silica sol as the silicon source and sodium aluminate as the aluminum source. The effects of alkalinity and crystallization time of the process of synthesis were investigated. To separately reveal the crystalline structure, element content, morphology, and surface areas, the as-synthesized zeolite Y was characterized by powder X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and N2 adsorption-desorption isotherms (BET). The results show the as-synthesized zeolite Y with high relative crystallization and uniform morphology; the SiO2/Al2O3 ratio was about 4.54~6.46. For an application, the zeolite cracking activity was studied with cumene as the probe molecules.
Zeolites are an important class of crystalline aluminosilicates materials with open-framework structures, and they have been widely used as separations, ion exchange, and acidic catalysts for size and shape selective catalytic reactions by molecular-sized microporosity [
Faujasite zeolites are commonly separated into two classes: (1) zeolite X, which has a silicon to aluminum Si/Al ratio between 1 and 1.5, and (2) zeolite Y which has a silicon to aluminum Si/Al ratio above 1.5. The large pore openings and large cavities of zeolite X make it an attractive material for ion exchange, adsorption, and drying processes [
However, conventional zeolite Y (Si/Al molar ratio below 2.5) application to petrochemical industry is limited due to the weak acidity, as well as poor hydrothermal stability. For the sake of better industrial realm application, conventional Y zeolite is replaced by a high silica ultrastable Y zeolite (USY, with Si/Al > 4) [
Compared with these posttreatments, the direct synthesis method is a more convenient way to obtain the high silica Y zeolite. The direct synthesis method includes structure-directing agent (SDA) method [
In the present work, we reduce the basicity of the synthesis system by adding sulfuric acid to improve the SiO2/Al2O3 ratio, and the effect of alkalinity and crystallization time is investigated to obtain a reasonable basicity. Under this basicity, we synthesized high silica zeolite Y without structure-directing agent or organic template and the crystallization time is also acceptable. Different samples are applied as a catalyst for catalytic cracking of cumene to clarify the effect of the SiO2/Al2O3 ratio on the catalytic performance, and, encouragingly, it demonstrates a strong potential in petroleum processing.
All the chemicals were directly used as received with no further purification. The reactants used were aqueous colloidal silica (
Sodium aluminate, sodium hydroxide, and deionized water were placed in a beaker to ensure good mixing on a magnetic stirrer at room temperature. After the solution became transparent, the aqueous colloidal silica was added under agitation for 2 h to yield the sodium aluminate solution. At last, the sulfuric acid was added to the gel to produce a gel with the overall stoichiometry Na2O : Al2O3 : SiO2 : H2O : H2SO4 = 4.3 : 1 : 10 : 180 :
The sodium form of zeolite was converted to the hydrogen form by ammonium ion exchange method. The zeolite was treated with 1 M ammonium chloride solution at 80°C for 2 h. Note that the zeolite to ammonium chloride solution was 1 g : 100 mL. The residue was filtered and washed with distilled water; then the sample was dried at a temperature of 120°C for 4 h. The above cycle was repeated three times to get complete exchange of sodium. As a final step, the samples were heated at 550°C for 5 h to decompose to H-type zeolite. The zeolite HY was denoted as HY-
The zeolite cracking activity was determined with cumene as the probe molecules, which used a flow-type apparatus equipped with a fixed-bed reactor. Nitrogen was used as carrier gas at flows of 3.0 L/h. The catalysts were pressed binder-free and crushed to a particle size of 60–80 meshes, and the catalyst amount was 0.26 g. In the case of cumene cracking, nitrogen saturated with vaporized cumene at 300°C was passed through the reactor (the flow rate of liquid state cumene = 6.0 mL/h), and the reaction temperature was 300°C as well. Reaction products were analyzed by an online gas chromatograph with flame ionization detector.
The powder XRD patterns were collected on the XD-3 of the Beijing Purkinje General Instrument Co., with graphite monochromatized Cu-K
The XRD patterns with different H2SO4/Al2O3 and different crystallization time are shown in Figure
XRD patterns for NaY zeolite samples with different H2SO4/Al2O3 and different crystallization time. (A) NaY-0-12, (B) NaY-0.67-48, (C) NaY-1.34-312, and (D) NaY-2.01-720.
Figure
The relation of relative crystallinity and crystallization time with different H2SO4/Al2O3.
The detailed SiO2/Al2O3 ratio data are listed in Table
The element content and SiO2/Al2O3 measured by XRF.
Sample | SiO2 |
Al2O3 |
Na2O |
SiO2/Al2O3 |
---|---|---|---|---|
NaY-0-12 | 62.8 | 23.5 | 13.5 | 4.54 |
NaY-0.67-48 | 66.2 | 21.4 | 12.3 | 5.26 |
NaY-0.67-144 | 66.1 | 21.4 | 12.3 | 5.25 |
NaY-1.34-312 | 70.3 | 18.5 | 10.9 | 6.46 |
SEM images of the samples collected at different H2SO4/Al2O3 and different crystallization time are presented in Figure
The SEM images of the samples with different H2SO4/Al2O3 and different crystallization time. (a) NaY-0-12; (b) NaY-0.67-24; (c) NaY-0.67-48; (d) NaY-0.67-144; (e) NaY-1.34-312.
Figure
Textural parameters of the samples obtained at different H2SO4/Al2O3.
Sample | Relative crystallinity % | BET surface area [m2g−1] | Micropore surface area [m2g−1] | Micropore volume [cm3g−1] | |
---|---|---|---|---|---|
(A) | NaY-0-12 | 100 | 986.6 | 944.1 | 0.37 |
(B) | NaY-0.67-48 | 91 | 911.4 | 845.6 | 0.33 |
(C) | NaY-1.34-312 | 36 | 401.8 | 357.8 | 0.14 |
N2 adsorption-desorption isotherms of the samples obtained at different H2SO4/Al2O3. (A) NaY-0-12; (B) NaY-0.67-48; (C) NaY-1.34-312.
The XRD patterns of the samples transformed into H-type zeolite are illustrated in Figure
XRD patterns of the H-type zeolite samples. (A) HY-0-12; (B) HY-0.67-24; (C) HY-0.67-48; (D) HY-0.67-144; (E) HY-1.34-312.
The catalytic activity of the samples is shown in Figure
The cumene catalytic reaction performance of the samples and the thermal cracking without catalyst (insert). (A) HY-0-12, (B) HY-0.67-48, and (C) HY-1.34-312.
In addition, we found the activity reduces with the increasing reaction time. This is mainly caused by the coke deposit on catalysts; the coke deposition can block the zeolite pores and prevent active sites contact with cumene.
We have demonstrated that high silica Y zeolite can be synthesized using direct synthesis method without adding any organic additive or SDA. The relative crystallinity and crystallization time of zeolite NaY particles can be controlled by the H2SO4/Al2O3 ratio of the synthesis gel. The crystallization time was also acceptable when the synthesis gel Na2O : Al2O3 : SiO2 : H2O : H2SO4 = 4.3 : 1 : 10 : 180 : 0.67. XRD and XRF show that the zeolite Y nanocrystals obtained are highly crystalline and high SiO2/Al2O3 ratio. The as-synthesized zeolite Y nanocrystals show high N2 adsorption and BET surface area and micropore volume are determined to be 911.4 m2/g and 0.33 cm3/g, respectively. In addition, compared with lower silica Y zeolite, the as-synthesized high silica Y zeolite shows excellent cracking performance.
The authors declare that they have no competing interests.
The research was supported by the project supported by Hebei Provincial Natural Science Foundation of China, Shijiazhuang Pharmaceutical Group (CSPC) Foundation (H2013209192), and the Funding Project for University Students’ Innovation and Venture Education of the North China University of Science and Technology (X2015063).