Abstract
A new tool to study the interaction of aromatic molecules with the acid sites in zeolites is described yielding both geometric and electronic information simultaneously. A Fano resonance (or two-centre/two-electron transition state) produces a charge-transfer (CT) satellite peak that is observed in the Al K edge XANES of zeolite H-Beta when aromatic hydrocarbon molecules are adsorbed on its acid sites. The Fano resonance is exclusively observed in the electron yield spectrum. The Fano resonance can be recognized as a broad, featureless satellite peak at a somewhat higher energy than the edge. During the 1s-3p transition, a CT occurs from π orbitals on the aromatic molecule to the T2 (3p) orbital on the Al transferring charge from the benzene to the aluminium. The diffuse character of this latter orbital results in an overlap with the π* of the aromatics and enables the CT. FEFF8 FMS and DOS calculations on a large cluster representing the zeolite with and without benzene and in the presence and absence of the core hole effect confirm the satellite interpretation.
The Fano resonance satellite is not observed in the fluorescence detection mode, because its inherent excitation probability is small, but its intensity is magnified in the electron yield detection mode because of shake-off of the extra Al valence electrons. The data indicate that the adsorbates do not alter the symmetry of the tetrahedral Al sites as indicated by both fluorescence and electron yield.
The adsorption of 1-hexene on zeolite H-Beta is investigated and only after a heat treatment of 400 °C is the Fano transition visible. This indicates that aromatic coke has formed on the acid site at this temperature. In addition, some transformation from tetrahedral Al sites into octahedral sites, or at least some geometric distortion of a small amount of tetrahedral Al sites, is now observed in the fluorescence spectrum, which might be induced by the coking process.
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