Relative Importance of Electrostatic and Dispersion Energies

The following procedure was suggested by Barrer as a means of determining the electrostatic contribution to the adsorption potential directly from experimental data. For nonpolar molecules such as the inert gases or paraffin hydrocarbons, only the nonspecific terms can contribute to the 

FIGURE 2J(A), Theoretical potential profiles from center of cage to center of eight-ring, calculated according to Eqs. (2.7)- 2.9) for rare gases in 5A zeolite. (From ref. 14 reproduced by permission of the National Research Council of Canada from the Canadian Journal of Chemistry, Volume 53, 1975.) 

A somewhat similar approach was followed by Kiselev for MaX zeolite and by Schirmer el al. for CaA zeolite. Representative data from these studies are summarized in Tables 2.6 and 2.7. The effect of specific interactions is clearly evident and for small polar molecules such as HjO, NHj, and CH3OH the electrostatic contribution is in fact dominant. Even for N2, which has only a relatively small quadrupole moment and no dipole, the specific contribution to the adsorption energy amounts to between 25 and 50%. 

A theoretical calculation of the heat of sorption for CO2 in various cationic forms of zeolite X and has been carried out by Barrer and Gibbons. The 

TABLE 2,5, Division of Initial Heats of Sorption into Specific (Dipole + Quadrupole) and Nonspecific (Dispersion f Repulsion + Polarization) Contributions 

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