Defect structures in zeolite crystals

Zeolites are known to be chemically active as both Bronsted and Lewis catalysts. Characterisation of the defect sites associated with zeolite frameworks, which can be responsible for both kinds of activity, has been the focus of this study. In particular, we consider the defects introduced into materials during synthesis and various post-synthetic treatments along with the defect transformations. Aluminium framework substitutionals (the Bronsted acid sites), hydroxyl nests and vicinal disilanols are the major defect species present in these materials. The paths for their transformation include dehydroxylation, deprotonation, direct dehydration and interaction with guest molecules present in the zeolite pores. Three processes in which both spin and charge polarised species can be formed have been investigated: (i) dehydration of a pair of adjacent Bronsted acid sites has led to the formation of oxygen vacancies, trigonal Al and counterpart electron and hole centres, (ii) nest annealing that, most interestingly, has produced the hydrogen and peroxide bridge defects at the tetrahedral site vacancies, (iii) reaction of the vicinal disilanols with molecular oxygen that has given rise to a number of low-energy peroxide defects. On the basis of the calculated defect formation energies, we conclude that The peroxide containing defects are the major Lewis acid sites in the zeolites. The annealing of the hydroxyl nests most probably proceeds through the migration of the defect and not by forming "non-intact" silane bridges. The vicinal disilanols form a ring structure in the ground state or decompose into isolated silanols, but do not exhibit internal hydrogen bonding. Fluoride ions are incorporated into the zeolite framework forming a ring structure of the disilanol type.