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, [Al3(μ2‐OH)3(THF)3(PhSi(OSiPh2O)3)2], 1, to gain insight into the acid–base reactivity of cyclic trinuclear Al3(μ2‐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 Al3(μ2‐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 AlIV−O(H)−AlIV units strong acid sites, and all results together suggest that their acidity is similar to that of zeolitic SiIV−O(H)−AlIV groups. Al‐OH‐Al groups in molecular aluminosiloxides can catalyze olefin isomerizations like solid catalysts and, as for the solids, the reactivity of these sites is blocked by pyridine as a base. Upon deprotonation, the Al3(μ2‐OH)3 core rearranges, rendering the acidity of the AlIV‐O(H)‐AlIV units comparable to that of bridging SiIV‐O(H)‐AlIV entities in zeolites. Thus, these units may also contribute significantly to the catalytic activity of zeolites.