Y zeolites framework

Y zeolite framework showing oxygen type (0) and non-framework ( ) locations (101). 

Gasoline composition from hexadecane cracking over calcined and steamed AFS and USY zeolites can be represented by general correlations as shown in Figure 5. As these correlations are unique to zeolite Y, they indicate that the Y zeolite framework topology plays an important role in the mechanism of product formation. The method of dealumination and subsequent steam treatment lead to various PONA compositions however, these compositions result from a... 

The fresh RE,NaY samples all displayed spectra similar to a NaY sieve. For the lanthanum-cerium binary exchanges, only two peaks were well resolved at 94-98 and 100-103 ppm. Steam treatment of Hi-REY, similarly to RE,NH.Y, resulted in dealumination characterized by the appearance of a peak at 108-111 ppm. Substantial retention of the Y zeolite framework was observed for 100% La and 75% La/25% Ce exchanges indicating the stabilizing influence of lanthanum over cerium. 

Table 2 Location of different cations in crystallographic sites of the Y zeolite framework...

Each zeolite type has a typical silica/alumina ratio related to the crystal stmcture. It is possible, to increase the silica/alumina ratio however, by removing aluminum atoms from the Y-zeolite framework, with no effect on crystallinity. This, of course, modifies catalyst performance by changing the nature and... 

Zeolites (section C2.13) are unique because they have regular pores as part of their crystalline stmctures. The pores are so small (about 1 nm in diameter) that zeolites are molecular sieves, allowing small molecules to enter the pores, whereas larger ones are sieved out. The stmctures are built up of linked SiO and AlO tetrahedra that share O ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The stmcture of faujasite is represented in figure C2.7.11 and that of ZSM-5 in figure C2.7.12. The points of intersection of the lines represent Si or A1 ions oxygen is present at the centre of each line. This depiction emphasizes the zeolite framework stmcture and shows the presence of the intracrystalline pore stmcture. In the centre of the faujasite stmcture is an open space (supercage) with a diameter of about 1.2 nm. The pore stmcture is three dimensional. 

The elementary building block of the zeolite crystal is a unit cell. The unit cell size (UCS) is the distance between the repeating cells in the zeolite structure. One unit cell in a typical fresh Y-zeolite lathee contains 192 framework atomic positions 55 atoms of aluminum and 1atoms of silicon. This corresponds to a silica (SiOj) to alumina (AI.O,) molal ratio (SAR) of 5. The UCS is an important parameter in characterizing the zeolite structure. 

Fig. 2 Calculated low energy conformation of the protonated dithiane oxide cation (R=H) in zeohte Y (Si/Al = 1). The bottom view shows a view through the twelve ring containing e deprotonated framework oxygen, the top view is perpendicular to this. For clarity the zeolite framework is shown using a stick model and the adsorbed molecule is drawn in space filled form represented by tlie Van der Waals radii for the atoms being in the order S>0>C>H.

Samples of Y faujasites were prepared by sodium exchange of a starting ultrastable Y zeolite (H form, denoted in the following as USY). Global Si/Al ratio is 16 according to X fluorescence measurements framework Si/Al is 21 as measured by 29Si MAS NMR. 

Kerr (7-9) has shown the critical role of the calcination environment and bed geometry in the formation of USY zeolites ("deep bed" vs."shallow bed"calcination). Ward (10) prepared USY zeolites by calcining ammonium Y zeolites in flowing steam. The work done by Kerr and Maher et al. (11) has clearly demonstrated that USY zeolites are formed as a result of aluminum expulsion from the framework at high temperatures in the presence of steam. The nature of the non-framework aluminum species has not been completely clarified. Obviously, their composition will be strongly affected by the preparation procedure of the USY zeolite. Table II shows different oxi-aluminum species assumed to be formed during thermal dealumination of the zeolite framework. 

Reaction with chelating agents. Such reactions have been used primarily for partial dealumination of Y zeolites. In 1968, Kerr (8,21) reported the preparation of aluminum-deficient Y zeolites by extraction of aluminum from the framework with EDTA. Using this method, up to about 50 percent of the aluminum atoms was removed from the zeolite in the form of a water soluble chelate, without any appreciable loss in zeolite crystallinity. Later work (22) has shown that about 80 percent of framework aluminum can be removed with EDTA, while the zeolite maintains about 60 to 70 percent of its initial crystallinity. Beaumont and Barthomeuf (23-25) used acetylacetone and several amino-acid-derived chelating agents for the extraction of aluminum from Y zeolites. Dealumination of Y zeolites with tartaric acid has also been reported (26). A mechanism for the removal of framework aluminum by EDTA has been proposed by Kerr (8). It involves the hydrolysis of Si-O-Al bonds, similar to the scheme in Figure 1A, followed by formation of a soluble chelate between cationic, non-framework aluminum and EDTA. 

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). 

Si-MASNMR spectra of Y zeolites with different Al-content in the framework. 

Aluminum-deficient Y zeolites prepared by reacting Y zeolites with SiCl vapors at 500°C also showed an enrichment of the surface in aluminum (44). The X-ray data show a shift of diffraction peaks to higher 20 values, consistent with a more silicious framework (27). However, the X-ray pattern also indicates some structural differences between this material and the one prepared by the steam/acid treatment. 

X-ray studies carried out by Gallezot et al. (46) on a 53 percent EDTA-dealuminated Y zeolite, have shown that the aluminum extraction does not leave any vacancies in the framework after calcination at 400°C in flowing, dry oxygen and nitrogen (46). It was suggested that a local re-crystall-ization of the framework occurs even in the absence of steam. The silica necessary for the process presumably originates in the destroyed surface layers of the crystallite and diffuses into its interior. 

Aluminum-deficient Y zeolites prepared by partial removal of aluminum with a chelating agent (e.g. EDTA) also show improved thermal and hydrothermal stability compared to the parent zeolite. The optimum stability was found in the range of 25 to 50 percent of framework A1 extraction (8). However, the maximum degree of dealumination is also affected by the SiO /Al O ratio in the parent zeolite a higher ratio appears to allow more advanced dealumination without loss of crystallinity (8,25,45). Above 50 or 60 percent dealumination, significant loss of crystallinity was observed. Calcination of the aluminum-deficient zeolite resulted in a material with a smaller unit cell size and lower ion-exchange capacity compared to the parent zeolite. 

---