Structure of defects in γ-irradiated ZSM-5 and Y zeolites: an e.s.r. study

Abstract

The structure of defects formed in γ-irradiated zeolites containing Al in the skeleton (HZSM-5) and in both the skeleton and extralattice positions (Al x O y ),HZSM-5) has been investigated by e.s.r. and compared with HY and AlHY zeolites. It has been found that the type of defect depends on the Al localization in the zeolite structure as well as on the temperature at which γ-irradiation is carried out. At a temperature of 298 K, HZSM-5 exhibits skeletal Al--Si defects, while for Al x O y HZSM-5, Al--Al defects are found. At 77 K, the γ-irradiation of both these zeolites results in the appearance of skeletal Al--Si defects alone. The e.s.r. spectra indicate that the structure of the individual defects in HZSM-5 is very similar to that of HY defects; however, for high-silica ZSM-5 zeolites, the defects are more localized and more stable. The structure of the Al--Si defects is not changed by their interaction with hydrogen at 77 K, while at 298 K, considerable reorganization of the unpaired electron density takes place.