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Black plate (239,1) diffusivities were measured for benzene loadings of 1, 2, 3 and 4.5 molecules per NaX supercage, and at temperatures of 300, 350 and 400K. Both corrected and transport diffusivities were seen to increase as the loading rose from 1 to 3 benzene molecules per cage, but then to decrease sharply for an average of 4.5 molecules in each cage. An excellent accord was found between the experi- mental and simulated results for both the loading dependence and the overall values for the corrected diffusivities. The latter are mostly in the range 10 12 –10 11 m2 s 1.34 The heat capacity behaviour of benzene and hexafluorobenzene confined in the pores of NaY zeolite has been determined, both for the individual compounds and as a 1:1 binary mixture35. Both molecules are planar and non-polar, with molecular electric quadru- pole moments that are of similar magnitude36 but opposite sign (29C m2 for benzene and þ31.7C m2 for hexafluorobenzene). As solids, both melt at the same temperature to within half a degree C, and have precisely the same boiling point (80.2C). The heat of vapourisation is the same for both too, while the heat of fusion for C6H6 is only 86% that of C6F6. Strong loading dependencies were found, and the heat capacities determined at intermediate loadings were very large, indicating that more energy levels become rapidly accessible after a first energy gap has been overcome. This is reasonable if excitation of shallow (‘‘soft’’) highly anharmonic surface-vibrational modes of the adsorbate species occurs. In an adsorbed 1 : 1 mixture of C6H6 and C6F6 the heat capacity is dominated largely by intramolecular interactions between separate C6H6 and C6F6 molecules, rather than by their interactions with the zeolite. Notably, there are no phase transitions observed over a very large range of temperature and loading-levels, as is reminiscent of the behaviour of supercritical fluids35. In another study, the ostensibly different diffusion behaviour of alkanes adsorbed in zeolites that may be deduced depending on the particular technique employed to measure it, was explained. The results show that when various data-sets reported in the literature are corrected for the loading of the alkane used in the particular kind of measurement, a much closer agreement is obtained between them37. Using the zero length column (ZLC) method, the diffusion of linear alkanes (n-hexane to n-tetradecane) in zeolite NaCaA was measured. There was a monotonic decrease found with increasing chain length from n-hexane to n-undecane, a slight increase from n- undecane to n-tridecane, and a final plateau at n-tetradecane. While the agreement with results obtained from PFG-NMR measurements is reasonable, the pronounced maximum that was found in a recent www.scienceprogress.co.uk Properties and applications of zeolites 239PDF Image | Properties and applications of zeolites
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