Synthesis of tris(2,2′-bipyridine)iron(II) complexes in zeolite Y cages:: Influence of exchanged alkali metal cations on physicochemical properties and catalytic activity

A series of catalysts containing tris(2,2'-bipyridine)iron(II) (Fe(bpy)(3)(2+)) complexes inside zeolite Y cages with various extraframework alkali metal cations (Li+, Na+, K+, Rb+, and Cs+) have been synthesized via a "ship-in-a-bottle" method. In this study, the influences of the alkali metal cations on the physicochemical properties of the catalysts as well as their catalytic performances were investigated. Formation of the Fe(bpy)(3)(2+) complexes was ascertained by XRD, diffuse-reflectance UV-vis spectroscopy, and Fe K-edge XAFS measurement. The steric constraint induced by increasing the size of the alkali metal cations resulted in a decrease in both Fe content and BET surface area. The intensity of the MLCT absorption band of Fe(bpY)(3)(2+) complexes associated with zeolites increased in the presence of heavier alkali metal cations, while the electron density of the Fe atoms decreased as the ionic radius of the alkali metal cations increased. The encapsulation of Fe(bpy)(3)(2+) within zeolite Y cages resulted in the creation of a new photocatalytic system, enabling efficient oxidation of styrene to benzaldehyde and styrene oxide under visible-light irradiation (lambda > 430 nm) in the presence of molecular oxygen. The TON was found to correlate with the increased intensity of the MLCT band and the decreased electron density of the Fe atoms. Additionally, Fe(bpy)(3)(2+) complexes encapsulated within zeolite cages have been shown to act as heterogeneous catalysts for the oxidation of benzene to phenol using hydrogen peroxide as an oxidant. In contrast to the photooxidation of styrene, catalytic activity was enhanced in the presence of lighter alkali metal cations. It is suggested that the electron density of the Fe atom predominantly determines the oxidation rate; this may be rationalized in terms of mechanistic considerations.