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(OSiPh₂ O)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 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.