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The interaction of pyrrole with alkali cation exchanged FAU-type zeolites is studied using a combination of the electronegativity equalization method (EEM) and the Monte Carlo technique. Pyrrole adsorbs at S II cations, the NH-group pointing to the most basic oxygen O4. The NH-bond polarization and the pyrrole framework interaction energy reflect the fundamental and well established trends of the intrinsic framework basicity: an increasing basicity with an increasing Al-content and, at a given Si:Al-ratio, an increasing basicity with increasing size of the exchangeable cations. In contrast to a recent interpretation of the IR-spectra of the NH-stretching vibration of pyrrole (D. Murphy, P. Massiani, R. Franck, D. Barthomeuf, J. Phys. Chem., 100 (1996) 6731; D. Murphy, P. Massiani, R. Franck, D. Barthomeuf, Stud. Surf. Sci. Catal., 105 (1997) 639). our Monte Carlo calculations show that there is only one adsorption site for pyrrole in the FAU-structure at low loadings. An alternative explanation for the observed heterogeneity of the IR-spectra of pyrrole is discussed on the basis of our calculations: the orientation of the pyrrole molecule with respect to the six-ring depends on the Si:Al-ratio of the tetrahedra forming the six-rings. |
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