Aluminum Complexes as Models for Broensted Acid Sites in Zeolites:  Structure and Energetics of [Al(OH)₄]⁻, [Al(H₂O)₆]³⁺, and Intermediate Monomeric Species [Al(OH)x(H₂O)n₋x·mH₂O]³⁻x Obtained by Hydrolysis

Using ab initio quantum mechanical methods, we examined cluster models for the transformation of aluminum sites in zeolites from tetrahedral to octahedral coordination. We investigated the relative stability of tetracoordinated, pentacoordinated, and hexacoordinated aluminum at different degrees of ligand protonation using monomeric aluminum hydroxy−aquo complexes of the form [Al(OH) x (H 2 O) n - x ] 3- x . For n = 4 and n = 5, we also investigated complexes having water in the second coordination sphere, i.e., [Al(OH) x (H 2 O) n - x · m H 2 O] 3- x, n + m = 6. A shift in preference from tetra- to hexacoordination occurred when the net charge on a complex was equal to or greater than +1. Hydrogen bonds were found to be very important in stabilizing the pentacoordinated and hexacoordinated species, especially for the highly protonated complexes. Trends in bond lengths, angles, and ligand orientations were identified as functions of coordination number and complex charge.