Combined DFT and experimental investigation on ion isomorphic substitution of aluminophosphate microporous crystals

Isomorphic substitution of zeolite and zeotype materials can affect significantly their application performance. The work presented herein assesses the impact of cobalt and silicon substitution at low and high H₂O/Al₂O₃ of the reaction mixture by a combination of experimental and density functional theory (DFT) calculations. The presence of cobalt in the synthesis mixture promotes the formation of large single columnar crystals exhibiting dominant growth along the c- (out-of-plane) axis. In dense reaction mixtures (H₂O/Al₂O₃ molar ratio of 100), the promotional effect of cobalt is counteracted by the effect of the low water content which alters crystal morphology. For cobalt substituted AlPO₄-5, the unit cell experiences contraction or expansion when Co substitutes Al or P, respectively. The lack of stability at higher cobalt content is attributed to the introduction of structural defects upon Co⁺² incorporation. The presence of silicon promotes the formation of intergrown crystals at all conditions. At dilute reaction mixtures (H₂O/Al₂O₃ ratio of 400), the presence of silicon promotes growth in all directions especially along the out-of-plane axis. At H₂O/Al₂O₃ of 100, the presence of silicon promotes in-plane growth. Lattice refinement results do not show a clear correlation between Si/Al₂O₃, H₂O/Al₂O₃ and the lattice parameters. DFT analysis suggest that the unit cell experiences a contraction when: 1) Si substitutes Al-P pairs and 2) Si substitutes one P in two unit cells. An expansion in the unit cell occurs when Si replaces P in one unit cell. The results also suggest the absence of preferred substitution mode for Si (i.e. either P or Al-P pairs).