Hydrothermal synthesis of Co-rich CoAPO-5 molecular sieves

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+