Microporous materials diffusion within

Molecular dynamics has been successful in revealing preferred adsorption sites within microporous materials diffusion paths in microporous materials and on single crystals, e.g., Refs. calculation of sticking coefficients of small adsorbates, e.g., Ref etc. It is in the calculation of dynamics though, such as sticking coefficients and diffusion paths, where the real merit of MD lies. It works well when there is no large activation barrier compared to the thermal energy, i.e., a fluid-like behavior. The small reachable scales have limited direct comparison of MD simulation results with data from most-far-from-equilibrium systems. 

The mechanisms of diffusion in these two systems (gas and liquid) are different and unrelated diffusion in gases is the result of the collision process, whereas that in liquids is an activated process (Bird et al., 1960). Diffusion in microp-orous materials is neither gaseous nor liquid diffusion. The closest case for such diffusion is surface diffusion, where molecules hop within the surface force field (see review by Kapoor et al., 1989b). Pick s law is used for both application (in modeling of adsorption processes) and experimental measurement of diffusion. Extensive reviews are available on diffusion in microporous materials and zeolites (Karger and Ruthven, 1992 Do, 1998). A lucid discussion on the nonlinear, and in some cases peculiar, phenomena in zeolite diffusion was given... 

Studies of metal compound diffusion in porous media have consistently demonstrated that the rate of diffusion within the microporous material is less than would be observed in an unrestricted medium. This discrepancy, observed for all liquid diffusion processes in pores of small diameter is related to hydrodynamic phenomena. The proximity of the molecule to the pore wall increases the frictional drag on the diffusing species when the... 

Inelastic and quasielastic neutron scattering have special advantages for studying certain of the motional properties of protonated or organic species within zeolites and related microporous materials. These advantages and various experimental methods are outlined, and illustrated by measurements of torsional vibrations and rotational diffusion of tetramethylammonium (TMA) cations occluded within zeolites TMA-sodalite, omega, ZK-4 and S APO-20. 

More recently, continuous flow or open FR systems have been developed to measure the adsorption and diffusion properties of sorbate molecules in microporous materials [13-15]. hi these systems, either the concentration of the sorbate feed [13] or the pressure within the reactor [14,15] is oscillated and the resulting changes at the exit stream are measured by using mass spectrometry or a mass flow meter. For a flow FR system, the effect of adsorption heat is reduced as the flowing gases attenuates the temperature change. 

PORE. I A minute cavity in epidermal tissue as in skin, leaves, or leather, having a capillary channel to the surface that permits transport of water vapor from within outward but not the reverse. 2. A void of interstice between particles of a solid such as sand minerals or powdered metals, that permits passage of liquids or gases through the material in either direction. I11 some structures, such as gaseous diffusion barriers and molecular sieves, the pores ate of molecular dimensions, i.e 4-10 A units. Such microporous structures are useful for filtration and molecular separation purposes in various industrial operations. 3. A cell in a spongy structure made by gas formation (foamed plastic) that absorbs water on immersion but releases it when stressed. 

Consider now real materials with model micropores, that is to say with regular dimensions and whose pore walls consist of well-defined crystalline adsorption sites (including possible cationic sites). Such solids can be found within the realm of zeolites and associated materials such as the aluminophosphates. One can imagine the probability that a fluid adsorbed within such micropores may be influenced by the well-defined porosity and thus become itself "ordered. Such phenomena have already been highlighted with the aid of powerful but heavy techniques such as neutron diffraction and quasi-elastic incoherent neutron diffusion. The structural characterisation of several of the following systems was carried out with the aid of such techniques in collaboration with the mixed CNRS-CEA Leon Brillouin Laboratory at Saclay (France). 

Diffusion Mechanisms. Since minerals and soils are porous materials containing both macropores (>2nm) and micropores (<2nm) diffusion is a mechanism that can control the slow rate of metal sorption (Figure 6). These pores can be inter-particle (between aggregates) or intra-particle (within an individual particle). 

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