Computational Study of the Structural and Electronic Properties of Dopant Ions in Microporous AlPOs. 1. Acid Catalytic Activity of Divalent Metal Ions

Periodic ab initio QM calculations are applied to study the structure and acidity of Mg, Ca, Cr, Mn, Fe, Co, Ni, Zn, and Sr divalent metal ions in the AlPO-34 framework, charge compensated by an acid proton on a neighboring oxygen. Our results show that the local environment of the divalent dopants is a distorted tetrahedron, in which the Me−OH bond of the dopant to the protonated oxygen is ∼0.15 Å longer than the other three Me−O bonds. The nature of bonding between the Me2+ dopants and the neighboring oxygens is ionic in nature, explaining the Lewis acidity of the Me2+ ions. However, for Ni2+, the Lewis active orbitals are oriented within the framework, where they are screened from an effective Lewis- type interaction with adsorbed molecules. The attack of Lewis bases is favored from the side of the framework opposite to the Brønsted acid proton. The replacement energy of a framework Al3+ with a Me2+ ion increases linearly as a function of the ionic size of Me2+. Finally, we show that the acid strength is attributable to a complex combination of the structural and electronic features of the dopant ion and does not show appreciable correlation with the local environment or electronic distribution of the Me2+ dopant in the framework.