Crystallization omega zeolite

The omega zeolite crystals synthesized with use of nucleation gel are oval agglomerates (see Figure 9), about 3 X 4 p,m in size. They are smaller than those synthesized without use of nucleation gel, about 30 to 40 p.m in size " ). This indicates that more nuclei of omega crystals are formed when nucleation gel is used. 

In the synthesis of faujasite type zeolite, the sequence of the phase evolution amorphous faujasite "P", is well known. Dwyer and Chu( l demonstrated that when TMA ions was added to the initial synthesis mixture a new sequence amorphous faujasite —> ZSM-4, prevailed. In the present work, we utilised a nucleation gel, usually used for directing faujasite phase( ), to the new synthesis system for omega zeolite. Here, we report the effects of nucleation gel on the crystallization processes. 

Figure 9. Scanning electron micrograph of omega zeolite. Ageing of ucleation gel >40 days, 35°C crystallization 14 days, 120 C.

N2 02, neopentane) in the zeolites A, X, L, mordenite, omega, and a synthetic offretite type have been determined from isotherms. These have been compared with the void volumes calculated from the known crystal structures. For most adsorbates the measured and calculated void volumes are in good agreement. However, helium and nitrogen exhibit anomalous behavior. A void volume-framework density relation for zeolites is given. 

Unlike the usual amorphous, microporous adsorbents, it is possible to calculate the theoretical micropore volume of a dehydrated zeolite from the known crystal structure. We have performed these calculations here for several of the better known zeolites including zeolite A, zeolite X, zeolite L, mordenite (Zeolon), (8) zeolite omega, (4) and the zeolite 0 (offretite... 

Influence of Agitation upon the Formation of Sieve Omega. When the preferred reaction mixture was not stirred throughout the reaction period, mixtures of zeolite to and erionite ( 60/40) were obtained. Thus, at least a mild agitation is necessary when crystallizing pure specimens according to this recipe. 

Kinetics and Mechanism of Crystal Growth of Zeolite Omega... 

The change in morphology of crystals of zeolite omega has been analyzed with respect to the evolution of the concentration of aluminium in the parent liquor. In agreement with the overlapping principle, crystal habit is determined by the faces with the lowest growing rate. 

Methods. The crystallization of silicoaluminate mixtures into zeolite omega in the temperature range 105-130°C was performed in the presence of a structure-directing mixture (SDM) (10,11). The method gives minimum overlap between the nucleation and growth steps as indicated by the very homogeneous distribution of size of the crystals in the final product. The use of kaolinite as the aluminium source presents two main advantages (10). First, the low solubility of the clay under the crystallization conditions prevents the formation of a gel. Second, under the low supersaturation levels achieved, secondary nucleation is hindered. 

A better picture of the crystallization phenomena was obtained from the microscope examination of the crystals at different stages of their development. The first crystals of zeolite omega appeared as spheres with diameters around 0.5-1 pm (Fig.2). With the appearance of (001) faces, this form evolved into barrel-shaped particles (Fig.3). At a later stage of the syntheses, the habit evolved to euhedral hexagonal with the development of (100) faces (Fig.4). 

Crystal habit modification. Several crystal habits have been reported in the open and patent literature for zeolite omega. Elongated hexagonal rods (15,16) or fibres (17) have been reported when the zeolite resulted from the recrystallization of another zeolite, Y (15) or S (17), or of clays (16). Moreover the natural counterpart of zeolite omega, mazzite, appears as bundles of needle-shaped particles (18). All these solids have been grown at low supersaturation levels, hence under conditions in which our results show that the growth in the direction <001> prevails on the growth normal to the c-axis (Fig.8). 

Figure X. Crystallization curves for zeolite omega at 105°C, A115°C, + 130°C.

Figure 2. Mixture of unreacted kaolinite and of spheroids (high index faces exposed) of zeolite omega obtained in the first stages of the crystallization. Scale bar 5 urn.

Figure 3. Kaolinite and barrel-shaped ((001) and (hkO) faces exposed) crystals of zeolite omega. (See text). Scale bar ...

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