A model for the formation of point defects in zeolites

Using ab initio DFT periodic and molecular cluster models, we investigate transformations of Bronsted acid sites (Al substitutionals compensated by H) in aluminium silicates upon steaming and calcination, possibly assisted by gamma-irradiation. These treatments can result in formation of (i) hydroxyl nest (hydrogarnet) defects; (ii) oxygen vacancies coupled with Al impurities; or (iii) over-coordinated Al substitutionals. We concentrate on process (ii) which involves either spin or charge separation at the defect site. For zeolites with high Si/Al ratios, we show that two paramagnetic centres are formed preferentially: an electron bound to Si adjacent to the O vacancy site, the E' centre (trigonal Si+ being a counterpart), and an electron hole localised on oxygens at the dehydrogenated Bronsted site (the V(1,2), or X(1,2) centre). Energetics, geometries and electron and hole distributions are calculated.