Experimental Methods and Simulations

In the frequency response technique the volume is periodically increased and decreased in square waves, and the response of the pressure measured and analysed in terms of Fick s laws under equilibrium conditions. It has the twin advantages of being applicable over a wide frequency range and being able to distinguish between independent kinetic processes. As a result, it appears to show close agreement with microscopic processes, because it can resolve the effects on different length scales. 

Interference optical microscopy is a recent and highly attractive experimental method to study diffusion processes on an intermediate length seale of microns as a single crystal is exposed to molecules in the gas phase 

The evidence from these methods suggests that lower values of diffusion constants are obtained from methods which are sensitive to factors important over longer distances, and so indicate that defects can have a controlling effect in diffusional rates. This is shown clearly in interference optical microscopy studies of methanol adsorbing into crystals of CrAPO-5, where the hexagonal prismatic crystals are found to possess large sectors that are inaccessible to adsorbate molecules. The existence of barriers to molecular adsorbates at the external surfaces of zeolites has also frequently been reported and can be visualised directly by comparative interference microscopy on as-prepared and also surface-etched ZSM-5 crystals.  

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Cation exchanged zeolites are successfully applied as catalysts or selective sorbents in separation technologies. " For both catalytic and sorption processes a concerted action of polarizing cations and basic oxygen atoms is important. In addition, transition metal cation embedded in zeolites exhibit peculiar redox properties because of the lower coordination in zeolite cavities compared to other supports." " Therefore, it is important to establish the strength and properties of active centers and their positions in the zeolite structure. Various experimental methods and simulation techniques have been applied to study the positions of cations in the zeolite framework and the interaction of the cations with guest molecules.Here, some of the most recent theoretical studies of cation exchanged zeolites are summarized. 

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