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Fig. 9. Diffusion processes in zeolites. http://www.co2crc.com.au/images/imagelibrary/cap_diag/membrane_separationtype_media.jpg Reproduced with permission from CO2CRC following section, there are a range and a diversity of experi- mental and theoretical approaches adopted in this aim. A combi- nation of pulsed field gradient (PFG) NMR and quasi-elastic neutron scattering (QENS) was employed to measure the diffusion of an homologous series of n-alkane molecules, up to n-tetra- decane, in MFI-type zeolites. The diffusivities are of an inter- mediate magnitude between those measured for NaCaA and NaX zeolites, and in order of their respective pore diameters. As for the zeolite NaA, the diffusion rates decrease in a monotonic fashion as the carbon chain length increases, as might be expected. However, this does contrast the behaviour of n-alkanes adsorbed into NaCaA which shows a striking deviation from this trend for chain lengths of six carbon atoms and above. Accepting that a general agreement is found between the two methods, the diffusivities measured by PFG NMR are smaller than those determined using QENS by one order of magnitude. This apparent contradiction may be explained in terms of the different motional ‘‘distance’’ that is accessed by these methods, as is fairly common when using different methods with different measurement time- scales. Thus, PFG NMR monitors molecular scale displacements typically of micron (10 6 m) distance, and hence may reflect additional transport resistances and diffusion barriers imposed throughout the entire zeolite grain. These are inconsequential for QENS, which instead measures excursions of molecules over far lesser distances31. www.scienceprogress.co.uk Properties and applications of zeolites 237 Black plate (237,1)PDF Image | Properties and applications of zeolites
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