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Black plate (241,1) monotonic dependence of the intrinsic diffusivity with chain length43. A series of Grand Canonical Monte Carlo simulations of water adsorption in the faujasites NaY and NaX and in silicalite-1 has been reported44. It is proposed that the method and forcefield used in this work should be useful for practically important applications such as the adsorption and separation of multi-compo- nent mixtures in various stable zeolites, since it provides a means to predict the effect of the small quantities of water which are virtually present in all such media44. A similar investigation was made of the adsorption of water from the vapour phase into silicalite. The simulations predict that when the micropores are completely hydrophobic, essentially no adsorption of water should occur at low pressures, but as the pressure is increased the degree of filling should follow suit. Using simulations based around ‘‘seeded’’ water molecules to model hydrophilic defects, the effect of silanol defects in real silicalite samples was investigated. Water molecules form hydrogen-bonded clusters around these defects, and also cause the pores to fill at a lower pressure than is the case for the completely hydrophobic material45. Molecular dynamics simulations have been used to investigate cation migration upon adsorption of methanol in NaY and NaX faujasites as a function of loading. According to the calculations for NaY, at low and intermediate loadings, SII cations can migrate toward the centre of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI0 cations from the sodalite cage into the supercage to fill the vacant SII site. An SII cation can also migrate across the double six ring, where it takes a SI0 vacant position. SI cations mainly remain trapped in their initial sites irrespective of the loading. At high loadings, only limited motions are found for SII cations because of steric effects induced by the presence of adsorbates within the supercage. For NaX, the SIII0 cations which occupy the most accessible adsorption sites are found to shift their position through coordination to the methanol molecules, and a maximum displacement is found at a loading of 48 methanol molecules per unit cell. At higher loadings the displacement is ameliorated by steric effects. In contrast, the SI 0 and SII cations remain practically at their initial sites whatever loading is introduced. It is proposed that the cations migrate via SIII0 cation displacements into neighbouring SIII0 sites46. Molecular dynamics simulations were extended by the same research group to elucidate the concentration dependence of the self-diffusion of methanol in the faujasites. Changes in the self- diffusivity were studied as a function of coverage for DAY and NaY systems, with a particular focus on the effect of pore www.scienceprogress.co.uk Properties and applications of zeolites 241PDF Image | Properties and applications of zeolites
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