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 studie d by molecular mechanics calculations of the host/guest interaction. I nvestigated were benzene, toluene, the xylenes, mesitylene, aniline, m -nitroaniline, and m-dinitrobenzene. Good agreement was obtained betwe en the calculated and the experimental guest-molecule structures. Mole cular 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 t echniques. Structural data of the nonpolar aromatic hydrocarbons, need ed for the comparison with the results of the molecular mechanics calc ulation, were taken from Literature. The positions of the nitro-substi tuted compounds are reported here, based on X-ray powder diffraction. The calculations revealed a stabilization of the nonpolar aromatic mol ecules in front of the Na-SII cation by electrostatic interaction. The steric requirements for the stability of this adsorption site are sca rce, but the preference of orientations on this site is due to the met hyl groups. Aromatic molecules with electron-pulling substituents like m-nitroaniline and m-dinitrobenzene showed different minima of potent ial energy. The preferred adsorption sites of these molecules are uniq ue, depending on the position of cations and the formation of H-bonds to framework oxygen. The theoretical adsorption and activation energie s for diffusion are in reasonable agreement with experimental data. Th e 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 signi ficantly.