Comparison of copper and cobalt exchanged zeolites by ultraviolet–visible and infrared spectroscopies. Adsorption of propene and redox properties

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Abstract
In the context of the partial oxidation of propene, this paper describes a comparison of copper and cobalt exchanged Y-type zeolites relative to propene adsorption. The modifications induced in the hydrocarbon molecule, the migration of the cations and their redox properties are studied by electronic and vibrational spectroscopies. Co2+ ions are not reduced and adsorb propene, retaining a tetrahedral coordination. The resulting allylic carbonium ion appears to be a probable precursor in coke formation and consequently to induce deactivation. A mechanism is proposed. Cu2+ ions migrate to accessible sites under propene adsorption and are easily reduced to Cu+ ions which coordinate propene to give a π-propene complex. Extra-framework oxide ions are available to oxidize CO into CO2 and propene into partial oxidation products. Upon drastic reduction, the Cu2+ ions are reduced into atomically dispersed Cu0(CO reduction) or into agglomerated Cu0, i.e. metallic copper (H2 reduction). The catalytic properties of the two sets of zeolites may be more closely related to the redox properties of the ions than to the acidic properties of the exchanged zeolites.

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