Zeolites site inhomogeneity

In the past two decades, 129Xe NMR has been employed as a useful technique for the characterization of the internal void space of nanoporous materials. In particular, the xenon chemical shift has been demonstrated to be very sensitive to the local environment of the nuclei and to depend strongly on the pore size and also on the pressure [4—6], Assuming a macroscopic inhomogeneity resulting from a distribution of adsorption site concentrations, 129Xe NMR spectra of xenon in zeolites have been calculated, and properties such as line widths, shapes as well as their dependence on xenon pressure can be reproduced qualitatively. A fully quantitative analysis, however, remains difficult due to the different contributions to the xenon line shift. (See Chapter 5.3 for a more detailed description of Xe spectroscopy for the characterization of porous media.)... 

Zeolites. Unlike silica and clay, zeolites possess interior structures that are uniform and well defined in shape and size. In spite of this, inhomogeneity in the microenvironment around a guest included in a faujasite zeolite may arise for two reasons variation in the occupancy number within a cage and the presence of sites of varying microenvironment. Even at low loading levels, the... 

It is known that Silicalite in particular, and zeolites in general contain several different sites (24) the inhomogeneity of adsorption sites is largely responsible for the non-exponential decay law. Figure 2 illustrates the decay observed in the case of 8-phenylpropiophenone in Silicalite. 

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