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Black plate (240,1) neutron spin echo study was not apparent. The difference between the infinite dilution ZLC data and the microscopic techniques cannot be explained by the intrusion of surface resistances since it is clear from partial loading measurements that internal diffusion is the controlling factor. The activation energies for the diffusion of linear alkanes in NaCaA all lie in the range 20–26kJ mol1.37 QENS data have been measured for the self-diffusion of n-alkanes (up to hexadecane) in silicalite. At 300 K, the measured diffusivities are greater in silicalite than in Na-ZSM-5, in agreement with molecular dynamics calculations and with hierarchical transition- state theoryyBrownian dynamics calculations. Activation energies for diffusion are close to 5 kJ mol 1 for a carbon chain of one to six units, but rise as the carbon number increases through the series C8 – C16 to approximately 15 kJ mol 1 for C16. A possible explana- tion emerges from the hierarchical simulation, which indicates that C1 – C6 alkanes tend to be located entirely within the voids of single channel segments and diffuse by jumps across energetically less favourable channel intersections. Molecules of greater length are forced to straddle channel intersections and in order for diffusion to take place, they must first undergo conformational changes. Since the QENS measurements and the simulations both probe motion over the same small (nm) dimensions, good agreement is found between them. When PFG-NMR is used to measure self-diffusiv- ities in the same systems, appreciably lower values are found, because (as noted earlier) the technique measures motion over significantly longer (micron; 106m) distances and is therefore more sensitive to defects in the silicalite crystals38. The force-field used to describe the interactions between linear alkanes and Naþ cations in faujasite type zeolites (Figure 8), has been enlarged so as to include Ca2 þ cations. The force field was fitted to reproduce the Na þ and Ca2 þ cation positions in zeolite LTA along with the adsorption properties of n-alkanes taken over all available tempera- tures and pressures from the enormous literature prevailing on the topic39. There has been made a critical re-examination of experimental data, which was reported originally in 197340 and showed an unusual oscillatory variation of the apparent microcrystalline diffu- sivity of n-alkanes in zeolite-T with carbon number41. This phenomenon was termed the ‘‘window effect’’. In contrast, the recent evaluation shows that the source of the variation was in fact an artefact of the original analysis42. The observed pattern of uptake rates can be explained in terms of the intrusion of finite heat transfer resistance and isotherm non-linearity, taking only the expected 240 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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