Design, construction and commissioning of an apparatus to perform frequency response diffusivity measurements and high time-resolution ftir spectroscopy
The active catalytic sites of zeolites are usually situated within the micropores of the crystals. Since the size of zeolite micropores is of a similar order of magnitude to that of many molecules, the intracrystalline diffusion of reactants and products is often the step that determines both the reaction rate and selectivity. Thus, knowledge of the diffusion rates, and of the diffusional behaviour of molecules within zeolites, is of primary importance in the understanding of these phenomena. While several methods are used to measure diffusivities, most of these have limitations associated with the minimum crystal size that can be used and/or the range of diffusivities that can be measured. Other problems frequently experienced are experimental complexity and the definition of experimental conditions within a range in which the theoretical models are applicable. A powerful method for measuring diffusivities under well-defined conditions is the frequency response method, in which the pressure response to a small volume modulation in a closed system is recorded over a range of frequencies. Models have been developed to determine the diffusivity from such experiments for a variety of circumstances, including non-isothermal conditions and multiple diffusion processes. Fourier transform infra-red (FTIR) spectroscopy can be used to monitor the uptake of sorbate molecules (and hence estimate their diffusivity) in zeolites. In addition, the behaviour of these molecules at the surface, and of the functional groups of the zeolite, can be observed. These observations reveal information about the sorption and diffusional behaviour of the molecules. For a completely reproducible process (e.g. a constant frequency volume modulation), special high time-resolution methods (i.e. rapid- and step-scan) can be used. With these techniques, very rapid molecular processes can be probed.The objective of this study was to design, construct and commission an apparatus capable of measuring diffusivities using the frequency response method, and to integrate it with an FTIR spectrometer to allow the use of standard and high time-resolution spectroscopy.
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