Full Mechanism Of Zeolite Dealumination In Aqueous Strong Acid Medium: Ab Initio Periodic Study On H-Clinoptilolite

Periodic quantum-chemistry methods as implemented in the CRYSTAL14 code were considered to study the mechanism of the dealumination of H-dinoptilolite in aqueous acid medium. A series of models consisting of the primitive cell of a monoaluminated structure of HEU (heulandite) framework type together with water molecules and HCl as catalyzer were considered. Stable and transition state structures together with their relative Gibbs free energy values were computed at the hybrid density functional theory. (DFT) level of theory with dispersion correction (PBE0-D2) employing atomic basis sets. Four different steps for the mechanism have been considered. Each of them corresponds to the hydrolysis of the bonds that connect the Al atom with the framework providing as final product Al(OH)(3) adsorbed on a silanol nest. Results show that the free reaction energy for the whole acid dealumination is about -49 kJ mol(-1). The crucial way the HCl intervents in the reaction is studied in detail, and the polar nature of most intermediates strongly suggests they are significantly stabilized by a solvation-like effect of the aqueous medium.