Local environment and acidity in alkaline and alkaline-earth exchanged β zeolite: Structural analysis and catalytic properties

Several materials consisting of β zeolite exchanged under similar conditions with several alkaline and alkaline-earth cations (i.e., Na, K, Mg and Ba) have been thoroughly characterized by diverse structural and surface techniques. Even though these metal cations are exchanged in different extensions, generally the zeolite is hardly affected in its Si/Al ratio or crystallinity, as shown by elemental and XRD analyses. However, different environments for aluminum and exchanged metal cations have been found by DRIFT, 27Al MAS NMR and XPS. Exchanged cations also influence the textural properties of the final materials, particularly the micropore volume, whereas pyridine and acetonitrile TPD have revealed that they also modify both Brönsted and Lewis acidity. Some relationships between these properties and the catalytic activity in the transformation of methanol or acetone, which have been used as model reactions, have been proposed.

Graphical abstractThe local environments of the aluminum atoms and the exchanged alkaline and alkaline-earth cations in a β zeolite have been investigated by different techniques, including a model reaction in which the activity and the selectivity have been related to their structural and surface properties.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights►The exchange affects the local environment of the aluminum atoms in the framework. ► The exchanged metals can be found as such or as metal oxide-like structures. ► Acidity and catalytic activity are considerably altered upon ion exchange. ► Some exchanged zeolites can give rise to shape-selective transformations.