ON THE NATURE OF WATER INTERACTING WITH BRONSTED ACIDIC SITES - AB-INITIO MOLECULAR-DYNAMICS STUDY OF HYDRATED HSAPO-34

Recent neutron-diffraction experiments revealed the presence of hydrox onium ions in the hydrated HSAPO-34, an aluminophosphate type zeolitic material, structurally isotypic with chabazite. Ab initio molecular d ynamics (AIMD) simulations were used to decide whether the proton tran sfer from a Bronsted acid site to a single water molecule is possible in this material, or the simultaneous presence of two water molecules in the zeolite cage-is necessary to realize such a transfer. The molec ular dynamics calculations support the view that while the intrinsic a cidity of the Bronsted site in HSAPO-34 is insufficient to protonate a n isolated water molecule, the basicity of a hydrogen-bonded water dim er is high enough to act as proton acceptor at the acid site.