Dealuminated zeolites unit cell parameters

In figure 2, it is related the cell parameter with the REO content of fresh and deactivated Y-Zeolite. It is shown that the cell parameter expanded linearly with amount of REO, this behavior is well known as the Vegar s law (6) which is interpreted as an evidence that the REO was incorporated into sodalite cage of Y-zeolite. This behavior was the same for both fresh and deactivated materials but in case of the deactivated samples the cell parameter shrank relative to the corresponding fresh samples. It is important to notice that the shrinkage is lower as the REO increases. This suggested that at similar deactivation condition the REO content prevented the dealumination. However, it is important to mention that dealumination of REO-Y-Zeolite does not necessarily mean a decrease in the unit cell parameters. 

The cracking activity and selectivity for n-heptane and gas-oil, of a series of dealuminated HY zeolites (SiCl4 or steam dealuminated) have been measured. The results have been related with the different types of aluminium and acid groups present and to the pore volume distribution. It is clearly shown that the unit cell parameter (framework Si/Al ratio) can not explain, by itself, the activity and selectivity of dealuminated zeolites. 

The BET surface area of the dealuminated samples (given in Table 1) also shows that the observed loss of zeolite structure was maximum in case of DAY-6 sample. The unit cell parameter (a,) showed continuous decrease in the unit cell size with the increase in dealumination. 

Adsorption and desorption isotherms of water vapor on HY zeolites change in shape with increasing dealumination. Thus, for only steamed samples the isotherm is close to the type I whereas for zeolites submitted to an additional acid leaching, it is close to the type IV with a progressive lowering of the point B towards the pressure axis, what emphasizes a more and more hydrophobic character of adsorption. The micropore volume accessible to water and the unit cell parameter are simply correlated with the framework Si/Al ratio. The water molecule does not allow to determine quantitatively structural and secondary pore volumes, but appears to be a selective molecular probe of the structural aluminium ions. For HY zeolites and without any restraint, 8H2O are associated with such a framework aluminic site. 

Fichtner-Schmittler, H., Lohse, U., Engelhardt, G. et al. (1984) Unit-cell constants of zeolites stabilized by dealumination - determination of al-content from lattice-parameters, Cryst. Res. Technol., 19, Kl. 

Parameters in Addition to the Unit Cell That Determine the Cracking Activity and Selectivity of Dealuminated HY Zeolites... 

Parameters in addition to the unit cell that determine the cracking activity and selectivity of dealuminated HY zeolite, in ACS Symposium Series, Vol. 368, eds W.H. Flank and T.E. Whyte (ACS, Washington, DC) pp. 542—554. 

---