Preparation of a New Micro-mesoporous Omega Zeolite by Hydrothermal Route: Effect of Crystallization Time

This work concerns the preparation of a new micro-mesoporous omega zeolite by hydrothermal route. This method consists of the self-assembly of the omega zeolite precursors with a cethyltrimethyl-ammonium bromide surfactant. Different stages of crystallization were studied in order to determine their impact on the textural and structural properties of the resulting materials. For this, several characterization methods were used such as X-ray diffraction, N2 adsorption/desorption, scanning electron microscopy, and Energy-Dispersive X-ray. The results showed that the processing time significantly influences on the crystallinity, porosity and Si/Al ratio of the resulting materials. A specific surface area almost three times greater than the parent zeolite was obtained when using non-hydrothermally treated zeolite precursors. While the precursors prepared hydrothermally for 48 or 72 h essentially lead to the formation of the microporous phase corresponding to the Omega zeolite. These new properties can open new applications of these solids notably in the catalysis for the conversion of bulky molecules and also in the adsorption of organic and inorganic pollutants.


Introduction
Zeolites have been widely used as adsorbents and catalysts in many chemical and petro-chemical processes due to their excellent properties such as higher acidity, hydrothermal stability, and form selectivity [1,2]. However, the own presence of micropores in zeolites often imposes diffusion restrictions related to the intracrystalline transport of the reactants to reach the active sites and of products to move away from these sites. In addition, the large compound cannot access in many cases the internal surface area of zeolite, which limits their use as catalysts in important chemical processes [3,4].
An alternative solution to deal with bulky substrates has been the synthesis of ordered mesoporous materials which possess uniform mesopores that can be varied in a wide range [5,6], the synthesis of nanosized zeolites [7] and ultra largepore zeolites [8].
Nevertheless, the application of these materials is fairly limited because of the difficult separation of nanosized zeolite crystals from the synthesis medium, the low acidity and hydrothermal stability of mesoporous molecular sieve materials and the need of employing Geranium, as well as the addition of special template in the synthesis of ultra-large-pore zeolite [9,10].
One of the most successful strategies to overcome the above disadvantages is to prepare micro/mesoporous material. By this way, both advantages of large pore size of mesoporous materials and strong acidity and high hydrothermal stability of zeolites would be combined. Several successful routes for synthesizing composites with excellent catalytic performances have been reported in literatures [11,12].
Omega zeolite or ZSM-4, the synthetic analog of mazzite is a large-pore (7.5Å) zeolite having hexagonal symmetry with a silica-alumina ratio (Si/Al) in the intermediate range (3)(4)(5). Omega framework contains gmelinite cages bridged by oxygen atoms to give a 12-membered cylindrical channel system along the crystallographic c-axis. Due to its high acidity and unique structural features, omega zeolite received much attention as catalyst and adsorbent [13][14][15][16][17][18]. To date, to improve the textural properties of the omega zeolite, researchers have turned to dealumination of this zeolite. However, it has been shown in several studies that this method leads to a decrease in the acidity strength, which subsequently influences on its catalytic performance.
In this work, and for the first time, the micro-mesoporous omega zeolite was prepared by assembling the precursors of the omega zeolite obtained at different stages of crystallization and then modified with a CTABr surfactant to form the mesoporous phase. The textural and structural properties of the materials obtained have been correlated and discussed.

Reagents
Reactants used in the synthesis were: Sodium aluminate (54 %Al 2

Synthesis Micro-mesoporous Omega Zeolite
The micro-mesoporous omega zeolite was synthesized as follows: 0.48 mol of NaOH, 0.77 mol of TMAOH and 0.32 mol of NaAlO 2 were dissolved in 18.67 mol of H 2 O. To this mixture 0.17 mol of silica was added. The resulting solution was stirred at room temperature for 17 h and then hydrothermally treated at 383 K for various periods of time (0 h, 48 and 72 h). After cooling at room temperature, 10 g of the resulting precursors were added drop wise to 27 g solution of cetyltrimethylammonium bromide (CTAB, 4wt.% aqueous solution) and then stirred vigorously. Subsequently, the pH value of the synthetic mixture was adjusted to 9 with HCl. The obtained mixture was stirred at room temperature for 15 min and then heated for 24 h at 383 K. The obtained materials were filtered, washed extensively, dried and calcined at 823 K for 8 h to remove organic template.

Synthesis of Omega Zeolite
The method of preparing the omega zeolite has been well detailed in the work published previously [15][16][17][18]. Briefly a solution containing 0.48 mol of NaOH, 0.77 mol of TMAOH, 0.32 mol of NaAlO 2 and 18.67 mol of H 2 O. Then 0.17 mol of silica (SiO 2 ) was added and stirred for 17 h at room temperature. The mixture was transferred into an autoclave for crystallization at 383 K for 72 h. The obtained zeolite was recovered by filtration, dried and then calcined at 823 k for 6 h to remove the organic template.

Characterization
The XRD powder diffraction patterns of the obtained samples were obtained by a Bruker AXS D-8 diffractometer using Cu-Kα radiation. Both low-and wide-angle X-ray patterns were recovered to assess the order of the mesopores and the crystallinity of the microporous phase in the different samples. Nitrogen adsorption-desorption isotherms at 77 K of the calcined samples were measured by a Micromeritics TRISTAR 3000 V6.04 A instrument. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method [19]. The pore size distributions were determined by the Barrett-Joyner-Halenda (BJH) method using the adsorption branch of the isotherms [20]. The microporous volume was obtained from the t-plot analysis [21,22]. The energy dispersive X-ray analysis (EDX) jointed to a XL-30 scanning electron microscope was used to calculate the Si/Al ratio for each sample. The surface topography of the different materials was observed using SEM on a Hitachi 4800 S microscope.
obtained at different processing times of omega zeolite precursors (0 h, 48 and 72 h) using the assembly of preformed zeolite omega precursors with CTABr surfactant.
The powder XRD patterns of the calcined micro/ mesoporous composites and omega zeolite are presented in Fig. 1. In the high 2θ region, the diffraction peaks of the sample omega are corresponding to typical mazzite structure [23]. When the precursor zeolite species was used without a previous crystallization, the XRD pattern for the resulting material (sample C (0 h)) indicates that a structurally well ordered hexagonal aluminosilicate was successfully assembled from omega zeolite seeds. The diffraction peak corresponding to (1 0 0) reflections is shifted towards lower angles, i.e. larger dspacing (see Table 1), compared with Al-MCM-41 indicating a higher content of aluminum in the framework [24].
Extending the crystallization time of the precursor zeolite species resulted in the formation of composite materials (samples C(48 h) and C(72 h)) composed of a hexagonal mesoporous phase and omega zeolite. It is noted from the XRD patterns (see Fig. 1) that the contribution of the microporous phase in the composite has become greater with the increase in the crystallization time. Thus, we can deduce that an extension of the crystallization time promotes the formation of the omega zeolite.
It should be noted that the addition of CTAB plays a role as structuring agent but also it helped the formation of the omega zeolite as a pure zeolitic phase. The Use the same conditions without the addition of CTAB leads to the formation of a mixed zeolite phase consisting mainly of Y and Omega zeolite (see Fig. 2). XRD patterns of the untreated sample (0 h) clearly shows that no zeolitic phase has been formed. The hydrothermal treatment of the mixture without CTAB for 24 h gave rise to the formation of pure zeolite Y while extending the crystallization time to 48 h generates the formation of a mixed phase consisting of the zeolite omega and Y. The pure omega zeolite was obtained only during three-day of hydrothermal treatment. This clearly shows that the zeolite Y was formed as an intermediate phase during the crystallization of the zeolite omega.
The FTIR spectra of all samples are shown in Fig. 3. As shown in this figure the infrared spectra of all samples are almost identical. The only difference is located at 3356 cm − 1 and 1007 cm −1 .
It is known that omega zeolite is characterized by the following bands: a broad band at 3356 cm − 1 which is assigned to stretches of associated and free OH [15]. The band at 1632 cm −1 also corresponds to the vibrations of the OH group of physisorbed water molecules. Strong bands at 1007 cm −1 which is assigned to the asymmetric stretching vibrations of the T-O bond (T = Si or Al) [17,18]. Weak bands observed at 817 and 716 cm −1 which are attributed to symmetrical elongations of the T-O bond. The weak band at 614 cm −1 is assigned to T-O vibrations with 6 tetrahedra [17,18]. For micro-mesoporous composite materials had almost the same bands as the omega zeolite the only difference took place in the T-O band which was shifted to the highest wave numbers 1089 cm − 1 probably due to the low aluminum content which causes this displacement.

Textural Properties
The N 2 adsorption-desorption isotherms and the corresponding BJH pore size distribution curves for the calcined micromesoporous composites are depicted in Fig. 4. The C(0 h) and C(48 h) samples isotherms are of type-IV [25] and exhibit three stages. The first step relating to low pressures (P/P0 < 0.25) reflects the monolayer adsorption of nitrogen on the walls of the mesopores. The second stage is characterized by a steep increase in the adsorbed amount at a relative pressure (0.25 < P/ P 0 < 0.4), caused by the mesopores filling. The third stage (P/P 0 > 0.4) in the adsorption isotherm is the gradual increase in volume with P/P 0 , due to multilayer adsorption on the outer surface of the particles. The BJH pore size distribution confirms that both composites possess well defined mesopores. The sample C (72 h) shows an adsorption isotherm of type I characteristic of microporous materials and a hysteresis loop in the desorption branch which reveals the existence of mesopores. The pore size distribution profile also shows a wide distribution which confirms the existence of several and heterogeneous pores.
The textural data are presented in Table 2, it is clearly indicated that the textural properties of micro-mesoporous materials decrease during the increase in the duration of hydrothermal treatment of the precursors due to the domination of the miroporouse phase.
These changes in textural properties are consistent with the gradual transformation of zeolite seed containing mesostructured phase in the composite materials. On the other hand, There was a slight increase of the wall thickness in the C(0 h) composite (12.07Å). These results could suggest the existence of secondary building units' characteristic of omega zeolite into the mesopore wall of the composite [26].

Electron Microscopy
The morphology of obtained samples was investigated by scanning electron microscopy (SEM). The sample C(0 h) is essentially made up of aggregated sphere shaped particles with size of smaller than 1 μm (see Fig. 5). This morphology is typical of silicate mesoporous materials [27].
The SEM images of the samples C(48 h) and C(72 h) show the presence of hexagonal prisms shaped particles which can be assigned to omega zeolite crystals. It is found that every omega crystal is evenly embedded in the loose MCM-41 aggregates. However, the proportion of omega crystals increases with the gradual increase in the crystallization time. These  Wall thikness D w is calculated from D w = a 0 -D p results are in good agreement with nitrogen sorption measurements and XRD analysis.

Conclusions
Composite micro/mesoporous materials are interesting because they combine the advantages of mesoporous materials with those of zeolites. Using this way, a new type of molecular sieve with micro-mesoporous structures, was successfully prepared. The synthesis process involved the assembly of omega zeolite precursor species obtained at various stages of crystallization into a mesostructured phase. By varying the time allowed for the crystallization of the precursor zeolite species we were able to modify the composition of the composite materials. The adsorption data of nitrogen suggest the presence of micropores and mesopores in the composite materials. The mesopores diameter is slightly higher in the composite materials compared to the parent material. The results indicate a higher content of aluminum in the composites framework which can improve substantially their acidity and hydrothermal stability. The hydrothermal treatment of the omega gel without the addition of CTAB showed that the pure zeolite Y was obtained in a reaction time not exceed then 24 h. The extension of the hydrothermal treatment for 48 h generates the formation of a mixture between the zeolite Y and Omega, and the pure phase of the zeolite omega cannot be obtained only if the treatment time is carried out for 72 h. With these properties the composite material (micromesoporous) can be used as a very good acid catalyst for the conversion of large molecules and also as an adsorbent for the retention of large organic molecules.