Jan L. Lievens

Cation site energies in dehydrated hexagonal faujaste (EMT)

Abstract The Na-ion distribution in dehydrated EMT-type zeolites was used as a probe for the characterization of the site-energy differences at the two different hexagonal-prism types using the combination of an (exact) statistical thermodynamical model and an X-ray powder diffraction study (Rietveld refinement). Compared to FAU-type zeolites, regions with a higher AI content most probably exist, coinciding with the twin planes, while the more FAU-like hexagonal prisms show a cation occupancy more typical for a high-Si material. An extensive materials characterization showed that the sample prepared in a crown-ether medium still contained about 25% FAU domains.

Cation site energies in dehydrated KFI-type zeolites : d-NaKFI, d-NaHKFI, and d-KKFI

Abstract The Na- and K-ion distribution in dehydrated KFI-type zeolites ( Si Ai = 3.3) was used as a probe for the computation of the site-energy differences at the hexagonal prism, the puckered (Na and K), and flat eight-rings (K only) using an (exact) statistical thermodynamical model and an X-ray powder diffraction study (Rietveld refinement). The site stability sequence was found to be (center of the hexagonal prism) Si Ai > 3).

Comparing homogeneous and heterogeneous acid catalysts by a density-functional based sensitivity analysis of their interaction with carbenium ions

Abstract An attempt has been made to develop a quantitative comparison of the properties of homogeneous and heterogeneous catalyst systems. The bond formation, and its projected covalent character, of a carbenium ion with the surface of zeolites and with water molecules in solution served as a basis to collect quantitative data. These data are the global electronegativity and hardness and the local softness (reactivity parameter of atoms-in-molecules) as these can be calculated for solids and molecules in the EEM formalism. A consistent model could be developed for a quantitative and qualitative comparison. It shows that generally, hydrated carbenium ions remain more ionic than those interacting with the surface of silicates or zeolites.

Verringerung des NOx‐Gehalts in Abgasen aus Magermixmotoren durch NO2‐Reduktion an silberhaltigen Zeolithkatalysatoren

Schwefelbestandigkeit sowie hohe Lebensdauerund Aktivitat zeichnen die silberhaltigen Zeolithkatalysatoren mit Bronsted-Saurezentren aus, die zur Reduktion von NO2 zu N2 in Gegenwart von Kohlenwasserstoffen dienen [Gl. (1)]. Dem postulierten Reaktionsmechanismus zufolge fuhrt die Stickstoffbildung uber intermediar entstehende Nitro-, Amino- und Diazoniumverbindungen.

CATION DISTRIBUTION AS A STRUCTURAL-CHEMICAL PROBE

The cations generate their own site energy by polarising the framework. This polarisation is easiest for the sites in the open parts of the framework (channels, cages) and for Si-rich frameworks (as opposed to materials with a high degree of Al and Ga isomorphous framework substitution). This has been experimentally demonstrated by determining the site-energy differences for FAU, EMT and KFI-type structures with varying cation loading and Si/AI (Si/Ga) ratio. This is consistent with the direct computation of the global “hardness” and local “softness” of zeolite-type frameworks. With the support of an “exact” statistical thermodynamical model for the cation distribution in zeolite structures with hexagonal prisms, the experimentally determined cation distribution can be used as a unique structural chemical probe for the direct measurement of the site energies, and the parameters influencing these, such as the Si/AI ratio, AI-zoning at the level of the unit cell, site geometry, substitution of Ga, … etc.

ZEOLITE INTRINSIC FRAMEWORK REACTIVITY AND SENSITIVITY ANALYSIS OF FRAMEWORK PERTURBATIONS

ABSTRACT The intrinsic electronic properties of 30 zeolite structure types with hypothetical SiO2 composition are estimated by the Electronegativity Equalization Method (EEM). The structure-dependent average (global) softness and the connectivity dependent local softnesses (Fukui functions) for all crystallographically different atom types are calculated. It is shown how these quantities depend on the structural features of the crystal and some general rules are formulated. Also some framework perturbations (by cations) are studied, the results of which are rationalized in terms of the rules derived. Finally, the electron donor capacity of the structural oxygens, and its relation with the hardness/softness concepts, is commented upon.