ADSORPTION AND DIFFUSION OF AROMATIC-HYDROCARBONS IN ZEOLITE-Y BY MOLECULAR MECHANICS CALCULATION AND X-RAY-POWDER DIFFRACTION

The sorptive behavior of aromatic hydrocarbons in zeolite Y was studied by molecular mechanics calculations of the host/guest interaction. Investigated were benzene, toluene, the xylenes, mesitylene, aniline, m-nitroaniline, and m-dinitrobenzene. Good agreement was obtained between the calculated and the experimental guest-molecule structures. Molecular mechanics calculations are therefore well suited to analyze the preferred adsorption sites of guest-molecules in microporous materials, especially in direct combination with X-ray or neutron diffraction techniques. Structural data of the nonpolar aromatic hydrocarbons, needed for the comparison with the results of the molecular mechanics calculation, were taken from Literature. The positions of the nitro-substituted compounds are reported here, based on X-ray powder diffraction. The calculations revealed a stabilization of the nonpolar aromatic molecules in front of the Na-SII cation by electrostatic interaction. The steric requirements for the stability of this adsorption site are scarce, but the preference of orientations on this site is due to the methyl groups. Aromatic molecules with electron-pulling substituents like m-nitroaniline and m-dinitrobenzene showed different minima of potential energy. The preferred adsorption sites of these molecules are unique, depending on the position of cations and the formation of H-bonds to framework oxygen. The theoretical adsorption and activation energies for diffusion are in reasonable agreement with experimental data. The calculated diffusion pathways are consistent with the hypothesis of surface-mediated diffusion at low loading and low temperature. Minimum energy paths for polar and nonpolar aromatic molecules differed significantly.