Dealumination advanced

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. 

Using the n-buthylamine titration method, Scherzer and Humphries (18) have shown that USY-B zeolites have considerably less acidity than USY-A zeolites. This is due to the more advanced thermal dealumination of USY-B, which reduces both Bronsted and Lewis type acidity. 

Degree of dealumination. Moderate dealumination generally increases the catalytic activity or leaves it unchanged, while advanced dealumination leads to a decrease in activity. Such a decrease is due to a loss of active sites with advanced framework dealumination. 

The NMR data on clean dealuminated samples of zeolite Y indicate that with good sample preparation and the advanced techniques for structural studies, it will be possible to elucidate further the relationship between structural features and catalytic properties. ... 

In the original method Trj was 650 K, Tr2 was varied from 730 K to 830 K and r was 4 K min l (ref.26). NaY zeolites were thus dealuminated to contain between 4 and 9 Al atoms per unit cell. In another study (ref.27), Trj was about 520 K. A temporary temperature rise between 30 and 70 K occurred during the first minutes of the contact of NaY zeolite with SiCl4 vapor. Only when the reactor temperature had reached its original value of 520 K, the zeolite bed was heated at a rate of 10 K min l to Tr2 values between 600 K and 745 K. This way samples with Al contents between 16 and 2 Al/UC were produced (Table 2, Nos. 24-29). In the same work (ref.27) it was stated that in order to avoid structural damage to the zeolite, the heating to Tr2 should only be started when the exothermicity is over. In view of the reaction mechanism advanced earlier, this means that a simultaneous attack of aluminium atoms associated with hidden and accessible cations causes a too violent reaction and, consequently, lattice destruction and has to be avoided. 

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