(30f) Extraframework Cations In Faujasite Adsorbents : Analytical Methods and General Distribution Trends

Understanding the factors that control separation or catalytic properties in zeolites requires a detailed knowledge of the structure of the material. In aluminosilicate zeolites, the presence of aluminum atoms introduces negative charges, which are compensated with nonframework cations. Faujasites are characterized by the fact that the number of potential cation sites in the structure exceeds the number of cations needed to ensure electroneutrality of the material.

Knowledge concerning the positions of the extraframework cations is critical to the understanding of the adsorption, separation, and catalytic properties of cation-exchanged zeolitic materials. The nature and positions of the cations will control the electrostatic field present within the zeolite pores, which strongly influences the adsorption and reactivity of the sorbed molecules.

However, the precise location of cations by conventional diffraction (XRD) techniques is rendered difficult because of partial occupancies affecting low symmetry sites. Moreover, if the system is complex, i.e. if there are different extraframework cations or if an adsorbed phase is present, XRD determination is no longer unambiguous.

The use of more sophisticated techniques such as anomalous X-ray diffraction, neutron diffraction and Solid State Magic Angle Spinning (MAS) and Multi Quanta MAS (MQMAS) Nuclear Magnetic Resonance may then be required to bring additional information for complete understanding.

The interest and drawbacks of such methods will be illustrated and discussed.