Silica faujasite

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

McDaniel, C.V. and Duecker, H.C. (1971) Process for preparing high silica faujasite. US Patent 3,574,538. 

Arika,)., Aimoto, M., and Miyazaki, H. (1986) Process for preparation of high-silica faujasite type zeolite. US Patent 4,587,115. 

ROBSON High-Silica Faujasite by Direct Synthesis... 

Blatter, F. Schumacher, E. The Preparation of Pure Zeolite NaY and its Conversion to High-Silica Faujasite, J. Chem. Educ. 1990, 67, 519-521. 

Figure 11 Time history of the kinetic and total energies in the NVE molecular dynamics simulation of silica faujasite.

Figure 12 Power and infrared spectra of silica faujasite structure.

Figure 13 (a) Mean-square displacement (Eq. [63]) and (b) CoM velocity autocorrelation function (Eq. [65]) for methanol molecules adsorbed in silica faujasite... 

Figure 15 Power spectrum of opening motion of four-membered rings in silica faujasite compared to the infrared and Raman spectra of the structure. SOD and D6-R notations correspond to the rings situated in the sodalite blocks and hexagonal prisms, respectively.

Figure 17 Fluctuations of the window area (inset) and spectrum of the fluctuations of the 12-R windows in silica faujasite structure.

The stabilizing effect of the aluminosilicate layer of DAY-Saim and DAY-T can be explained by the elimination of the terminal silanol groups and the blocking of the energy-rich Si-O-Si bonds at the crystal surface, where the water molecules attack the framework. In this case, the question of stability of high-silica faujasites is transferred to the question of stability of the aluminosilicate structures and, accordingly, transferred from the alkaline to the acid mechanism of decomposition which is rate-controlling. 

The rate of decomposition of high-silica faujasite is controlled by the size of the shrinking surface 5 r of the crystals, the related undestroyed crystal itself cf eq. (1), and a function gCw) of the available amount of water w. These assumptions lead to a kinetic model of the form... 

Compared with the high-alumina zeolites NaCaA or NaY, the relatively low hydrothermal stability of high-silica faujasites DAY-S and DAY-Tg result from the attack of water molecules at the silanol groups and the energy-rich Si-O-Si bonds of the crystal surface. The kinetics of this dissolution is significantly more rapid than that of the hydrolysis of aluminosilicates. 

An aluminosilicate layer at the crystal surface generated subsequently by an alumination of DAY-S zeolite or directly by the steaming of NaY in order to get DAY-T zeolite stabilizes the high-silica faujasites. The decomposition follows the acid hydrolysis of usual aluminosilicates. Consequently, in this case the kinetic model described in ref [5] is valid at least for the surface layer. 

In this paper a mathematical model is applied which allows for high-silica faujasites the prediction of their crystal destruction for different hydrothermal conditions and treatment periods. The model is based on the assumption that the dissolution of the framework proceeds with a water attack at surface Si-O-Si bonds. It is shown that the rate of decomposition of such high-silica faujasites is mainly controlled by the size of the shrinking surface of the crystals. 

G.H. Kuhl, Crystallization of Low-Silica Faujasite (SiO2/Al2O3-Approximately-2.0). Zeolites, 1987, 7, 451 457. 

H.K. Beyer, I.M. Belenykaja, F. Hange, M. Tielen, PJ. Grobet, and P.A. Jacobs, Preparation of High-Silica Faujasite by Treatment with Silicon Tetrachloride. J. Chem. Soc., Faraday Trans. 1, 1985, 81, 2889-2901. 

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