Molecular concentration, zeolite shape

Shape selectivity related to molecular concentration in zeolite micropores... 

The Brunauer type I is the characteristic shape that arises from uniform micro-porous sorbents such as zeolite molecular sieves. It must be admitted though that there are indeed some deviations from pure Brunauer type I behavior in zeoHtes. From this we derive the concept of the favorable versus an unfavorable isotherm for adsorption. The computation of mass transfer coefficients can be accompHshed through the construction of a multiple mass transfer resistance model. Resistance modehng utilizes the analogy between electrical current flow and transport of molecular species. In electrical current flow voltage difference represents the driving force and current flow represents the transport In mass transport the driving force is typically concentration difference and the flux of the species into the sorbent is resisted by various mechanisms. 

As shown in Figure 1, the equilibrium concentration is affected slightly by temperature (11). The actual concentration is affected by the reaction rate and the initial concentration of each isomer. Deviations beyond equilibrium can be achieved when zeolites are used, owing to shape selectivity (see Molecular sieves). The thermal isomerization of the three xylenes has been studied at 1000°C (12). Side reactions predominated, and only a small percentage of xylenes was interconverted. 

The unique properties of zeolites and other micro- or mesoporous solids that may favour their application to fine chemical synthesis are (1) the compatibility between the size and shape of their channels or cavities with the size of the reactants and/or products (generally referred to as molecular shape selectivity) that may direct the reaction away from the thermodynamically favoured route (2) the occurrence of confinement effects increasing the concentration of reactants near the catalytic sites and (3) the ability to tune their catalytic properties (acidic, basic, or other) via various treatments as described in this Volume. 

As stated above, shape selectivity due to molecular sieving depends on the relative rates of diffusion and reaction, hence on the respective sizes and shapes of molecules and pores and on the characteristics of active sites (e.g. concentration, nature and strength of acid sites). Obviously the diffusion rate, hence the selectivity depend also on the length of the diffusion path (i.e., on the size of the zeolite crystallites). The selectivity of a zeolite catalyst can be optimized by an adequate... 

The bonding within these polyatomic cations is weak. One can readily calculate bond orders which are small fiactions. There is no suggestion that any of these cations would be stable outside the zeolite, nor that their geometries are as rigid as those of molecules or molecular ions. All of these polyatomic cations conform to the electrostatic requirements of the zeolite framework, somewhat as liquids adopt the shapes of their containers. Perhaps it is reasonable, because the bonding is so weak, not to refer to these as polyatomic cations at all, but rather as electron traps. This more physical description is consistent with the observation that some of these clusters can be prepared (in low concentration) by y-... 

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. 

In zeolites, diffusion constants will depend strongly on molecular shape. For example, in silicalite branched alkanes prefer to absorb in channel cross-sections, but linear alkanes prefer adsorption in the channels themselves. This has important consequences for differences in diffusion rates within mixtures. At high concentration, the rate of the diffusion of the branched alkane will control the rate of diffusion of the other alkanesl l... 

---