Hydrophobic zeolites and adsorption

In the ensuing discussion we deal exclusively with hydrophobic (de-aluminated) zeolites. In these zeolites nearly all Al3+ ions have been substituted by Si ions, so that the stoichiometries of these solids are close to those of pure silica, Si02. The diameters of the pores in the framework are typically of the order of 5A, up to 7.sA (zeolite Y). (Recently, wider pore hydrophobic zeolites have been synthesised. These are discussed later in this chapter for now we restrict our attention to the classical zeolites.) 

When these compounds absorb molecules, the heat developed is dependent on the pore size. This heat comes mainly from the non-bonding interaction between the silica framework and the absorbed molecules. 

What is it that makes these weak forces strong enough to crack hydrocarbons, breaking the carbon-carbon bonds It is clear that it must be a collective effect. The (interior) surface of the zeolite imparts a cooperative effect to the adsorbate. Some conclusions can be drawn from a simple analysis of adsorption phenomena as a function of the substrate curvature. 

It is easy to see that adsorption energies are dependent on the curvature of the interface. Consider first adsorption on a planar interface. At low pressures, p, a sub-monolayer, gas-like, and eventually a two-dimensional liquid described by a Langmuir isotherm (or decorations thereof) forms. At higher pressures still (p/ps 0.35, where ps is the saturated vapour pressure) multilayer adsorption isotherms can occur depending on adsorbate, molecular size and adsorbate-substrate interactions. This regime is usually described by the theory of Brxmauer-Emmet-Teller (BET). In this domain, ln(p/pg) = 1/t, where t is the thickness of the film. 

Adsorption within a zeolite can occur everywhere in the crystal. The alumino-silicate framework is a convoluted curved sheet, everywhere exposed to the exterior of the crystal by way of the channels (cf. Fig. 2.9). The process described above holds for zeolites (although the adsorbed molecules hover in the tunnels, and are not boimd to the zeolite), and the local geometry of the continuous alumino-silicate sheet determines the adsorption energy. This geometry varies throughout the sheet continuously, since the Gaussian curvature is not constant. It is certain then that the adsorption 

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