Lamellar solid, pillaring

MOLECULAR ENGINEERING OF LAMELLAR SOLIDS. I. PRINCIPLES DERIVED FROM THE PILLARING OF SMECTITE CLAYS... 

Figure 1. Schematic illustration of a pillared lamellar solid in which the pore de ed by the gallery height d, and lateral free separation between pillars (d2) is sufficient to accommodate molecules with a kinetic diameter equ to or larger than nitrogen. Intercalated solids in which the pillars are nearly in lateral van der Waals contact are "expanded", but not "pillared".

Layer rigidity is an essential requirement for the pillaring of any lamellar solid. In the absence of such rigidity, attractive interactions between layers would result in distortion in the region between pillars, and such layer distortions would lead to the collapse of the gallery pores. 

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

Lamellar double hydroxides are part of, or are precursors of, a more general family of compounds designated as pillared layered structures (PLS). We quote some terms used to describe them (Mitchell 1990 Van Damme 1990). Pillared Layered Structures are nanocomposite materials prepared by linking molecules or colloids to a layered host. They exhibit a remarkably broad spectrum of structural, chemical, electronic, ionic, optical and magnetic properties, and provide supermesh host structures in which chemical reactions or physical processes can proceed under gas-phase conditions, but at liquid/solid state densities. They can be shaped as powders, pellets, or supported or self-standing Aims, and can be dispersed in solid or liquid matrices. 

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