LAUSR

Coordinatively unsaturated sites (CUS) present a key feature of alumina based catalysts as they are believed to act as Lewis-acid sites in heterogeneously catalyzed reactions. In the present study, the direct observation of active species on a fluoride-doped aluminum oxide catalyst is demonstrated. This new fluoride-doped aluminum oxide exhibits strong Lewis-acid sites and superior catalytic activity as compared to γ-Al2O3. To emphasize the labile state of Lewis-acid sites, two distinctive states of the catalysts surface are addressed using 1H–27Al cross-polarization (CP) MAS NMR. On the one hand, the highly dehydrated and active state after calcination at 700 °C and on the other hand the rehydrated and catalytically inactive surface (produced by contact to air) are probed. These experiments revealed the presence of significant amounts of coordinatively unsaturated sites in the form of 4- and 5-fold coordinated Al-sites on the highly dehydrated surface. In contrast to this, the rehydrated sample exhibited...