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Structure and Reactivity of Al−O(H)−Al Moieties in Siloxide Frameworks: Solution and Gas‐Phase Model Studies
Author(s) -
Lokare Kapil Shyam,
BraunCula Beatrice,
Limberg Christian,
Jorewitz Marcel,
Kelly John T.,
Asmis Knut R.,
Leach Stephen,
Baldauf Carsten,
Goikoetxea Itziar,
Sauer Joachim
Publication year - 2019
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201810130
Subject(s) - deprotonation , chemistry , reactivity (psychology) , isomerization , solvent , protonation , medicinal chemistry , crystallography , phase (matter) , stereochemistry , catalysis , organic chemistry , medicine , ion , alternative medicine , pathology
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.