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

• Published in: Microporous and mesoporous materials 2020-03, Vol.294, p.109859, Article 109859
• Main Authors: Gaber, Safa Ali, Dawaymeh, Fadi, Gaber, Dina Ali, Basina, Georgia, Ismail, Issam, Tharakshmy, Anjana, AlYafeai, Abeer, Subrati, Ahmed, Karanikolos, Georgios N., Al Hashimi, Saleh, Al Wahedi, Yasser F.
• Format: Article
• Language: English
• Subjects: AlPO4-5; Aluminophosphates; Crystals; Density functional theory; Morphology; Substitution
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/j.micromeso.2019.109859

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). [Display omitted] •Isomorphic substitution of zeolite and zeotype materials can significantly affect their application potential.•Co- and Si- substituted AlPO4-5 were synthesized under dense and diluted reaction conditions.•Co incorporation promotes growth along the out-of-plane axis.•AlPO4-5 unit cell expands or contracts if Co substitutes Al or P, respectively.•Si incorporation promotes formation of intergrown crystals.