Porous materials shape-selective properties

SCHEME 10.1 Shape-selective properties in porous materials according to Csicsery. (Reproduced from Ref. [22] with permission of Elsevier. Copyright 1984.)... 

The well-defined porous structure of zeolitic materials makes these materials true shape-selective molecular sieves. The presence of charge-compensating cations such as alkaline and alkaline earth cations, protons, etc. within the inorganic frameworks adds ion exchange and catalytic properties. Moreover, the hydrophobic nature of pure silica zeolites or the hydrophilic nature of aluminosilicates makes these solids useful as specific adsorbents of organic molecules or water in the gas or liquid phase. 

Diffusion, adsorption, and reactivity of molecules within micro- or mesoporous materials are specifically related with the physico-chemical properties of the material structures. It was originally underlined by Derouane et al. (1988) that the interactions of the host molecules with the material surfaces depend on the volume, shape, and topology of the cavities, which generate particular organizations of these molecules. The inter-relationship between the porous materials and the host molecules has been referred to as confinement and attributed a large role in the selectivity and catalytic activity of zeolite materials, in particular, in acid-catalyzed reactions (Anquetil et al. 1999 Smirnov and Thibault-Starzyk 1999 Thibault-Starzyk et al. 1998). 

Intercalation of smectite clays with polyoxycations provides a new class of porous materials. Intercalated clays are called pillared clays and they have high thermal stability and large surface area. Vaughan and Lussier(ref. 1) were the first to point out the shape selective sorption property of pillared montmorillonite using various probe hydrocarbons. The shape selective catalysis in cracking of alkylbenzenes was demonstrated by Shabtai and co-workers(ref. 2). 

The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the lUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type FIl (lUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. 

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