Helge Toufar

Shell structure of atoms

Abstract The average local electrostatic potential function, V(r) ϱ(r) , is calculated for 87 atoms and the corresponding monopositive and mononegative ions in the ground state, using the nonrelativistic average-over-configuration numerical Hartree-Fock density. It is found in general that the shell boundaries are expressed as the successively increasing maxima in V(r) ϱ(r) and the outermost maximum presents good approximate estimates of the core-valence separation in atoms. The reason for such behaviour is explained analytically. The proposed function is identified as the inverse of charge capacity per unit volume which attains a maximum at the shell boundary.

The electronegativity equalization method (EEM) as a promising tool for the analysis of zeolite catalyzed reactions

The H-exchange of methane in FAU and MFI is analyzed with the Electronegativity Equalization Method (EEM). The polarization channels and their hardnesses are presented as promising criteria for estimating the reactivity of acid zeolite catalyzed reactions. The reaction mode hardness is shown to depend on structural parameters (topology) as well as chemical composition (Si/Al ratio and the Al distribution), the latter being dominant. The results theoretically confirm the Next Nearest Neighbors (NNN) principles, which stresses the importance of the number of Al atoms in the second coordination sphere as a key reactivity parameter.

Location and transport properties of ammonia molecules in a series of faujasite zeolite structures as studied by FT-IR and 2H-NMR spectroscopies

The interaction, the location and the transport properties of ammonia molecules in a series of cationic faujasite zeolites have been investigated by means of infrared and 2H-NMR spectroscopies. The results have been correlated with the nature of counter-ions, the Si/Al ratio, the negative charge of oxygen atoms of the zeolite framework and the acido-basicity of the studied zeolites. The present work evidenced experimentally for the first time two kinds of interaction between ammonia and cationic zeolites. The ammonia can interact not only with alkali cations via the lone pair on nitrogen atoms but also with the negatively charged oxygen atoms of the framework via its hydrogen atoms. However, the Lewis acidity of counter-ions is the dominating factor for the ammonia interaction. The correlation times and the activation energies of ammonia in Na+-exchanged faujasite zeolites have been determined from 2H-NMR spin-lattice relaxation experiments. At low temperature, a rotation around 3-fold axis of the ammonia molecule is detected. At high temperature, an isotropic motion of ammonia is evidence. Moreover, an enhancement of the ammonia mobility from Na-X to N-Yd has been observed and explained.

Catalytic tuning of the olefin epoxidation with hydrogen peroxide and Faujasite Y occluded Mn bipyridine complexes

A series of cation (e.g. Li, Na, K) exchanged [Mn(Bpy) 2 ] 2+ -Y catalysts was prepared and tested in alkene oxidation reactions with hydrogen peroxide. The activity and selectivity undergoes marked influence from pre-ion exchange. Higher epoxide yields could be obtained for Li-pre-exchanged [Mn(Bpy) 2 ] 2+ -LiY. The Li + cation promotes hydrogen peroxide activation at the active site, resulting in a higher cyclohexene epoxidation rate.

Zeolite Effects in Organic Catalysis

New catalytic technology developed during the 1980s in Japan [1, 2], USA [3], China [4] and Europe [5] has been reviewed recently. Table 6.1 contains the reported catalytic technology extracted from this work which makes use of zeolite molecular sieves. It is clear that most of the technological developments have been made in the area of the refining of oil fractions (Table 6.1.I). Strong acidity in a shape-selective environment in which the zeolite pore and cage walls exert stereoselective constraints on the transition states and/or on the movement of products and reagents, together with intense hydrogen transfer reactions, are at the basis of this specific behavior (vide infra).