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

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.