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

Lokare, Kapil Shyam; Braun‐Cula, Beatrice; Limberg, Christian; Jorewitz, Marcel; Kelly, John T.; Asmis, Knut R.; Leach, Stephen; Baldauf, Carsten; Goikoetxea, Itziar; Sauer, Joachim

doi:10.1002/anie.201810130

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

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Baldauf, Carsten
Theory, Fritz Haber Institute, Max Planck Society;

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Citation

Lokare, K. S., Braun‐Cula, B., Limberg, C., Jorewitz, M., Kelly, J. T., Asmis, K. R., et al. (2019). Structure and Reactivity of Al−O(H)−Al Moieties in Siloxide Frameworks: Solution and Gas‐Phase Model Studies. Angewandte Chemie International Edition, 58(3), 902-906. doi:10.1002/anie.201810130.

Cite as: https://hdl.handle.net/21.11116/0000-0002-CAB5-E

Abstract

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(OSiPh₂O₃)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.