Hydrothermal synthesis of Co-rich CoAPO-5 molecular sieves

Abstract

A series of CoAPO-5 materials have been hydrothermally synthesized to maximize the degree of isomorphous substitution of Co²⁺ ions in the AFI framework. The as-synthesized as well as the calcined CoAPO-5 materials have been characterized with XRD, SEM, ICP, DRS (diffuse reflectance spectroscopy) and FT-IR techniques. Optimum conditions for the synthesis of Co-rich CoAPO-5 molecular sieves have been obtained by evaluating the influence of the amount and type of cobalt source, the type of aluminium source, the type of template molecule, the [template]: [P₂O₅] ratio and the amount of different monovalent cations on the isomorphous substitution of Co²⁺ in the lattice. It will be shown that highly-crystalline hexagonal CoAPO-5 crystals with a substitution degree of 12% can be obtained from a CsCl·Co(CH₃COO)₂·4H₂O·pseudo-γ-AlO(OH)·H₃PO₄·(C₂H₅)₃N·H₂O gel (with [(C₂H₅)₃N]: [P₂O₅] and [CsCl]:[Co(CH₃COO)₂] ratios of respectively 1.0 and 0.5) autoclaved for 41 h at 190°C. In addition, the effect of monovalent cations on the redox properties of Co²⁺ in CoAPO-5 molecular sieves will be discussed. The degree of oxidation of framework Co²⁺ to Co³⁺ is always relatively low and increases in the order Li⁺<K⁺≈Rb⁺≈Cs⁺<NH₄⁺≈Na⁺. Thus, the addition of Li⁺ results in a stabilization of Co²⁺ in the framework of CoAPO-5 molecular sieves upon calcination.