Lamellar zeolites: an oxymoron?

Abstract

“Lamellar zeolites” can be considered as an oxymoron. Zeolites, by definition, are three-dimensional structures displaying pores of a molecular dimension. Lamellar materials, contrarily, are bidimensional materials that, in some cases, can be delaminated, that is, have their lamella separated. It is exactly the combination of these properties that is so attractive in lamellar zeolites. They are zeolites whose precursors are/can be prepared in such a way as to display bidimensional organization where parts of the structure, the lamella, interact with one another by weak forces through the interactions of organic/inorganic ions and/or molecules housed in the “interlayer” spaces. From these precursors, calcination procedures lead to the 3D zeolitic structures, delamination in solutions provides individual or groups of lamella, pillarization processes conduct to mesoporous molecular sieves. Naturally, all these procedures eliminate the shape selectivity typical of zeolites in all cases of lamellar zeolites known so far, except one particular structure. However, providing that the resulting layers keep the original structure of the precursor, most of the zeolite acidity remains in the final material. In this way, pore sizes that limit the access of larger molecules to the active sites in the zeolites are not formed, the surface area of the material is increased and large molecular weight molecules are free to react in surface sites made available by the delamination or pillaring of layered zeolites. Finally, 3D fully connected zeolites can be deconstructed into layered zeolites from which all the above procedures are foreseen to provide modified solids. This recent discovery opens up a wealth of interesting structures from which to extract lamella with surface exposed active sites. Lamellar zeolites may be considered a true link between 2D and 3D solids and allow us to really determine which properties derive from one or the other structural dimensions, thus allowing us to precisely design structure-based properties for use in a range of technological applications.