Direct Probe Fourier Transform Far-Infrared Spectroscopy of Metal Atoms, Metal Ions, and Metal Clusters in Zeolites

Abstract

A class of crystalline aluminosilicates, known as the zeolites, are widely used as effective heterogeneous catalysts in the chemical industry. They possess excellent stability, activity and selectivity patterns for a wide range of hydrocarbon transformations [1–10]. X-ray diffraction studies have shown that zeolites are giant macromolecules formed by AlO₄ and SiO₄ tetrahedra joined by shared oxygens, and the structures of many synthetic and naturally occurring zeolites are now known and well documented [11]. As a simplification we can think of certain zeolites as a three-dimensional channelled network of interconnected cavities of different sizes. The excess negative charge on the framework of the zeolite, due to the AlO₄ tetrahedra, is either balanced by positively charged metal ions or protons which usually reside at well-defined sites in the zeolite lattice. The presence of exchangeable cations makes some zeolitic materials especially attractive in applications involving water treatment, such as domestic softeners and detergents, wastewater clean-up, and disposal and storage of radioactive elements. These subjects have been reviewed recently vis-a-vis the thermodynamics and kinetics of ion exchange in naturally occurring and synthetic zeolites [12]. This review will focus mainly on faujasite-type zeolites, one aspect of which will concern the vibrational spectra of the metal cations. The structure of zeolite Y is shown in Fig. 1, and the sites where cations can reside are indicated by Roman numerals in the accepted convention. These sites are of particular importance catalytically since they are thought to be the active centers for reaction [12–14] or the seeds of metal cluster formation [15]. Of these sites, those that can directly interact with molecules that have entered the large a-cage (supercage) at the center of the figure are more important in terms of hydrocarbon transformations. For example, to illustrate the difference between the α- and β-cavities of faujatites, even molecular oxygen with a kinetic diameter [161 of 3.54 å cannot enter through the oxygen 6–rings of the sodalite (β) cages at room temperature (see Fig. 11, but up to 6 pyridine molecules can simultaneously occupy the α-cages.