Computational study of the structural and electronic properties of dopant ions in microporous AlPOs. 2. Redox catalytic activity of trivalent transition metal ions

Periodic ab initio QM calculations are employed in order to study the structure and redox properties of Cr, Mn, Fe. and Co trivalent transition metal dopants in AlPO-34. Our results show that the local environment of Mn and Co dopants is a distorted tetrahedron due to Jahn-Teller distortions. The bonding between the Me-III dopants and the neighboring oxygens is ionic in nature, and explains the Lewis acidity of the Me-III ions. The replacement energy DeltaE(III) of the 3+ dopant ions in AlPO-34 increases linearly as a function of the Me-O bond distance. Finally, the calculated redox energies of the Me-II/Me-III couples indicate that, among the transition metal ions investigated, Fe is the most stable in the 3+ oxidation state, whereas Mn is the most stable as 2+ ion. Cr and Co, instead, have intermediate behavior and can switch more easily between the two oxidation states. These results contribute to elucidate the mechanistic details of catalytic processes occurring in MeAlPOs.