Single-Component Diffusion in Zeolites

In this section, single-component diffusion in zeolites [20,87-92], with the help of the case study of the diffusion of p-xylene and o-xylene in H-ZSM-11 and H-SSZ-24 zeolites, is discussed [90], SSZ-24 is a 12-MR zeolite that was first obtained as the silicon counterpart of AlP04-5, and, later, was obtained as a borosilicate (B-SSZ-24), which could be exchanged with A1 to yield the H-SSZ-24 zeolite [111], This zeolite exhibits the AFI framework, which embodies a one-dimensional channel network without cavities, consisting of parallel 12-MR channels with a free-channel diameter, ow = 7.0A [112], Additionally, ZSM-11 encloses an intersecting two-directional 10-MR channel system, where the two-dimensional channel system is characterized by the free-channel diameter, ow = 5.8 A [112], 

FIGURE 5.31 Schematic representation of an experimental facility to perform measurements of theFickean diffusion coefficient by the FTIR method. 

The Physical Chemistry of Materials Energy and Environmental Applications 

The measurements were carried out as follows the compound to be tested was filled in one or both of the stainless steel saturators, which were held thermostatically at 25°C. The flow of the gas carrier was divided in two, and each of these streams were passed through a flow controller [90], One stream went through a tube with a sinterplate in the base into the saturator where the carrier gas bubbles into the liquid adsorbate. This gas flow, that is, the stream which has passed through the saturator and becomes saturated with the test substance, was then mixed with the bypass stream of pure carrier gas at the outlet of the saturator, and the unified stream was then passed through the IR-cell [90], 

This equipment can be used for the study of a single-component diffusion, and the measurement of the corresponding Fickean diffusion coefficient made using a solution of Fick s second law for a geometry appropriate for the experimental setup [87-92], In this case, the flow rates were adjusted to get the desired partial pressure (6.7 Pa, P/Pn = 0.006) [90], 

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