Abstract: The N_2 adsorption and desorption isotherms of Y zeolites modified by hydrothermal treatment (HUSY), H_4EDTA dealumination (HEY) and (NH_4)_2SiF_6 isomorphous substitution (HSY) were measured and compared with that of the original NaY and HY zeolites. The size and the shape of the hysteresis loops on the isotherms are different, which shows that the secondary pore systems of these zeolites vary with the modification procedures. The sequence of the secondary pore volumes of the samples is as follows
HY (2.6)The pore distributions of the samples deduced from the theoretical treatment of the N_2 desorption isotherms according to non interconnected cylindrical pore model (BJH model) show that the maximum of secondary porosity of all the modified zeolite Y samples is due to pores of ~2.0 nm radius. However, for HUSY and HEY there are pores with radii >2.0 nm and <2.0 nm as well.
Reactant with various molecular sizes such as propanol, cumene and 1,3,5-tritert-butylbenzene were used as probes to study the catalytic behavior of the modified zeolite samples. The catalytic activities of the probe molecules on the zeolite were correlated with the data of the acidity and the secondary pore volume. It is clear that for larger molecules as cumene and 1,3,5-tri-tert-butylbenzene the presence of secondary mesopores in the zeolite structure promotes the cracking reactions considerably.

Key words: Y zeolite, Zeolite modifieation, Pore structure, Secondary mesopore, Acid catalyzed reaction