Silicalite argon

Figure 6. Argon (36Ar) neutron diffractograms measured at T = 87 K for different loadings in Silicalite-I zeolite.

Figure 11.19. Adsorption isotherm and corresponding microcalorimetric recording for argon at 77 K on Silicalite-I (reproduced courtesy of Y. Grillet and P.L. Llewellyn, personal communication).

Adsorbent-adsorbate potential energy calculations have been made for the adsorption of argon in the channels and intersections of Silicalite-I (Muller et al., 1989). The most favourable sites for localized adsorption are within the straight and sinusoidal channels, which together should be able to accommodate 20 molec uc-1. At a loading of 24 molec uc 1 all the available sites in the channels and intersections are probably occupied by localized molecules. 

As predicted, at low loadings, argon and nitrogen are adsorbed in a very similar manner on pure Silicalite. Thus, in each case the adsorption energy remains almost constant until TV" = 20 molec uc 1. This suggests that localized adsorption is taking place with very little adsorbate-adsorbate interaction. The adsorbed molecules are mainly located in the channels and at a lower concentration in the intersections. 

It is of interest to compare the behaviour of argon and nitrogen with that of other adsorptives on Silicalite-I. Recent work (Llewellyn et al., 1993a,b) has shown that in certain respects krypton and argon behave in a similar way. Thus, up to the loading Na = 20 molec uc 1 the adsorption energies at 77 K are both constant and almost... 

Figure 8. Adsorption isotherms of argon and nitrogen at 77 K on silicalite-I obtained with high-resolution dynamic volumetric sorption equipment. (Reproduced with permission from reference 45. Copyright 1989.)...

Figure 5. Part a Nitrogen (77 K) (O and ), argon (77 K) (U) and water vapor (298 K) (A and A) isotherms for Silicalite I. Part b detailed low relative pressure data. Clear symbols denote adsorption dark symbols denote desorption.

Four main types of porous silica adsorbents have been identified compacts of pyrogenic powders, precipitated silicas, silica gels, and zeolitic silicas. The importance of porosity relative to the adsorptive properties of each group is reviewed, with particular reference to the adsorption of nitrogen, argon, and water vapor. The differences in size and specificity of these adsorptive molecules may be exploited to explore the surface properties of each grade of silica. A notable feature cf Silicalite I, which is the best known of the zeolitic silicas, is its remarkable hydrophobic character. Furthermore, the uniform tubular pore structure of this microporous silica is responsible for other highly distinctive properties. 

This phosphorescence emission was obtained at room temperature and with air equilibrated samples. Oxygen diffusion inside the silicalite channels is reduced into the channels which already have -phenylpropiophenone molecules and many triplet excited molecules are not quenched and are therefore emitting. In argon purged samples the triplet lifetime increases about ten times [83c]. jfl-Phenylpropiophenone included within microcrystalline cellulose chains enabled us to detect the ketyl radical of this species and therefore contrast with its solution photochemical inertia [83a]. 

Early molecular dynamics simulations focused on spherically shaped particles in zeolites. These particles were either noble gases, such as argon, krypton, and xenon, or small molecules like methane. For these simulations, the sorbates were treated as soft spheres interacting with the zeolite lattice via a Lennard-Jones potential. Usually the aluminum and silicon atoms in the framework were considered to be shielded by the surrounding oxygen atoms, and no aluminum and silicon interactions with the sorbates were included. The majority of those studies have concentrated on commercially important zeolites such as zeolites A and Y and silicalite (all-silica ZSM-5), for which there is a wealth of experimental information for comparison with computed properties. 

R. J.-M. Pellenq and D. Nicholson, in Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, M. Suzuki, Ed., Kodansha, Tokyo, 1993, p. 515. Two-Body and Many-Body Interactions for Argon Adsorbed in Silicalite Zeolites. 

D. Douguet, R. J.-M. Pellenq, A. Boutin, A. H. Fuchs, and D. Nicholson, Mol. Simul., 17, 255 (1996). The Adsorption of Argon and Nitrogen in Silicalite-1 Zeolite A Grand Canonical Monte-Carlo Study. 

Figure 14.1 The potential energy Up as a function of distance from the crystal surface / for the interaction of argon with silicalite-1 inside silicalite pores.

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