Structure of aluminum, iron, and other heteroatoms in zeolites by X-ray absorption spectroscopy

Abstract

Heteroatoms in zeolites provide them their catalytic activity. The silicon atom is always tetrahedrally coordinated and its valence IV, those of the heteroatoms much more varied. This is further complicated by the easy removal of heteroatoms out of the zeolite framework forming extra-framework phases. The change in coordination and removal of the framework atoms is often paralleled with enhanced catalytic performance and thus the precise characterization of heteroatoms in zeolites, as a function of synthesis and post-synthesis treatments, is topic of much research. In this review we summarize structural characterization of various heteroatoms using X-ray absorption spectroscopy. This method is in particular suitable, because it is element-specific, can be used to determine structure of amorphous samples, and can be applied under pretreatment and catalytic conditions. Aluminum shows a rich variation in coordination and, depending on the conditions, such as temperature and, notably, water content, zeolitic aluminum can be three, four, five and six-coordinate. The Brønsted acid site is associated with a strongly distorted tetrahedral coordination of the framework aluminum. During activation and along catalytic reactions, there is a large variation in the distortion of the local environment of aluminum, and the flexibility of the zeolite framework to accommodate such structural differences is essential to guarantee a long life time of the material as working catalyst.Iron in the zeolite framework is easily removed. The extra-framework iron species are extensively characterized, however, no consensus about their structure has been reached. Framework iron is Fe3+, extra-framework iron mostly Fe2+. The coordination of other tri- and tetravalent heteroatoms, such as gallium, boron, titanium, germanium, and tin are less investigated. It is however obvious that their structure is a function of the conditions the zeolite is exposed to. Like aluminum, the titanium atom easily switches coordination in a reversible way. All hetero-atoms often form extra-framework species. This review identifies the importance of changes in local structure of heteroatoms and the easy removal of these hetero-atoms from the framework. Often it is the extra-framework species that is responsible for catalytic activity.