Characterisation of CoII and CoIII in CoAPO molecular sieves

Abstract

CoAPO molecular sieves of the structure types 5, 11, 16 and 34 have been investigated by means of electron absorption spectroscopy, temperature-programmed reduction and infrared spectroscopy in combination with pyridine adsorption, the purpose being to determine the amounts of framework and non-framework cobalt and to study the properties of incorporated cobalt with respect to stability, redox behaviour and associated acidic properties. It has been shown that framework and non-framework cobalt species exhibit different spectroscopic properties and can be distinguished from each other. The framework cobalt acceptance and the stability of the different structures decrease in the order CoAPO₄-5 > CoAPO₄-11 > CoAPO₄-16 > CoAPO₄-34. In stable structures such as 5 and 11, framework cobalt can perform reversible redox reactions Co^II⇌ Co^III, which are accompanied by changes in the concentration of Brønsted and Lewis acid sites.