A computational investigation on substitution of magnesium for aluminium in AlPO4-5 microporous material

Magnesium incorporation in the AlPO4-5 microporous material was studied via energy minimization techniques. This process was simulated through two models. In the first one, aluminium tetrahedral sites were occupied by two Mg2+ cations and another Mg2+ cation was found in an extra-framework position. In the second model, one Mg2+ cation was located in an aluminium tetrahedral site and one H+ was bonded to one of the oxygen of the tetrahedron. The last model mimics the existence of Brønsted acid centers in the material. The results of simulations for the first model suggested that both Mg2+ framework cations and the Mg2+ extra-framework cation were preferentially located in the same 6-membered ring. In contrast, the least stable configurations were those in which Mg2+ framework cations and the Mg2+ extra-framework cation were located in the same 4-membered ring. Simulations in the second model yielded that the most stable configuration presented the H+ confined into the 6-membered ring. Structural data such as Mg–O bond distances and bond angles supported these results. For both models, a direct relation between local structural disorder and structural stability was found. The deformations of the unit cell obtained for the second model would explain the relative low amount of acid sites experimentally reported in the MgAPO-5 structure. Simulated IR spectrum of MgAPO-5 showed a band (centered at 860 cm−1), which was assigned to isolated Mg2+ cations in tetrahedral positions.