Structure and Reactivity of Al−O(H)−Al Moieties in Siloxide Frameworks: Solution and Gas‐Phase Model Studies

Even though aluminas and aluminosilicates have found widespread application, a consistent molecular understanding of their surface heterogeneity and the behavior of defects resulting from hydroxylation/dehydroxylation remains unclear. Here, we study the well‐defined molecular model compound, [Al 3 (μ 2 ‐OH) 3 (THF) 3 (PhSi(OSiPh 2 O) 3 ) 2 ], 1 , to gain insight into the acid–base reactivity of cyclic trinuclear Al 3 (μ 2 ‐OH) 3 moieties at the atomic level. We find that, like zeolites, they are sufficiently acidic to catalyze the isomerization of olefins. DFT and gas phase vibrational spectroscopy on solvent‐free and deprotonated 1 show that the six‐membered ring structure of its Al 3 (μ 2 ‐OH) 3 core is unstable with respect to deprotonation of one of its hydroxy groups and rearranges into two edge‐sharing four‐membered rings. This renders Al IV −O(H)−Al IV units strong acid sites, and all results together suggest that their acidity is similar to that of zeolitic Si IV −O(H)−Al IV groups.