By using ab initio periodic and molecular cluster models, an investigation was made of the transformations of Bronsted acid sites (substitutional Al compensated by H) during steaming and calcinations; sometimes assisted by γ-irradiation. These treatments could result in the formation of hydroxyl nest (hydrogarnet) defects, O vacancies coupled with Al impurities or over-coordinated substitutional Al. The second of these involved spin or charge separation at the defect site. In the case of zeolites having high Si/Al ratios, it was shown that 2 paramagnetic centres were preferentially formed. These were an electron which was bound to Si that was adjacent to the O-vacancy site, and an electron hole which was localized on O ions at the dehydrogenated Bronsted site.

A Model for the Formation of Point Defects in Zeolites. C.R.A.Catlow, P.E.Sinclair, A.A.Sokol: Radiation Effects and Defects in Solids, 1999, 151[1-4], 1047-53