Zeolites interaction energies between molecules

In the framework of the present model s description of diffusion, the zeolite is considered to be a three-dimensional array of N identical cells, i, centered at Rh each containing N0 identical sites localized at Ria = R, + Ua, where the potential energy is a minimum (see Figure 5.30 [104]). If a molecule is localized at Ria, its energy would be -8 and the interaction energy between molecules... 

The acid-base properties of zeolites are further eomplicated by the confinement of host molecules in narrow mieropores. Several eomponents of the interaction energy between (basic) host molecules and the walls or acidic sites of zeolite cages may be strongly enhaneed in micropores with molecular dimensions. 

In sorption calculations, the energy is determined by summing the interaction energy between all atoms using defined potentials. The sorbed molecule is positioned into the zeolite cavity surrounded by framework atoms which extend to infinity. The zeolite and molecule are then allowed to relax to their lowest energy configuration. The calculated heats of adsorption (q j) of the molecule in the zeolite is obtained from - r(Z+mol) +... 

Two methods for including explicit electrostatic interactions are proposed. In the first, and more difficult approach, one would need to conduct extensive quantum mechanical calculations of the potential energy variation between a model surface and one adjacent water molecule using thousands of different geometrical orientations. This approach has been used in a limited fashion to study the interaction potential between water and surface Si-OH groups on aluminosilicates, silicates and zeolites (37-39). 

Interaction energies are calculated between one guest molecule and the zeolite lattice. Considering only a single guest molecule in the zeolite models the case of infinite dilution. 

When two or more molecular species involved in a separation are both adsorbed, selectivity effects beeome important beeause of interaction between lha zeolite and the ndsorbate molecules. These interaction energies include dispersion and short-range repulsion energies, polarization energy, and components attributed to electrostatic interactions. 

When a molecule adsorbs on the siliceous part of the micropore of a zeolite, the main interaction it experiences is a dispersive van der Waals-type interaction. This is due to the dominant interaction with the large polarizable oxygen atoms that make up the zeolite framework. For example, the interaction between a hydrocarbon CH3 or CH2 group and the siliceous framework typically results in an interaction energy that is on the order of 5-10 kJ/mol. These electrostatic interactions are small. 

When an organic molecule approaches the zeolitic proton, in addition to the van der Waals dispersion forces, there is a weak additional interaction which is on the order of 5 kJ/mol. The large interaction energy of hydrocarbons with the sUiceous zeolite chaimel [e.g. hexane in silicalite (ZSM-5), 60 kJ/mol] is due to the fact that there are multiple contacts between the hydrocarbon and the zeolite channel which are additive in nature. This helps to illustrates the fact that the siliceous zeolite channel is quite hydrophobicl 1. 

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