Mesoporous zeolites by fluoride etching

A critical overview of post-synthetic approaches to prepare hierarchical zeolites is presented. Historical methods, as dealumination by steaming and/or acid leaching and desilication in basic solutions, are briefly outlined and compared to a newly developed approach based on bi-fluoride anions. The latter offers the advantage of indiscriminate extraction of silicon and aluminum from zeolite frameworks, while the formers discriminate either Al or Si. The resulting material retains the framework composition and intrinsic acidity of its parent zeolite. Such a route proceeds by eliminating the interfaces between intergrown crystals and preserves only the highly crystalline domains. In addition, the secondary porosity formation does not lead to a substantial increase of external surface area and deposition of extra framework species.