title: Nature and structure of aluminum surface sites grafted on silica from a combination of high-field aluminum-27 solid-state NMR spectroscopy and first-principles calculations.
abstract: The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.
authors.affiliation: Université de Lyon, CNRS, Institut de Chimie de Lyon, Ecole Normale Supérieure de Lyon, 46 allée d'Italie, F-69364 Lyon Cedex 07, France.
authors.forename: Rachel Nathaniel
authors.initials: RN
authors.lastname: Kerber
authors.forename: Anthony
authors.initials: A
authors.lastname: Kermagoret
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Callens
authors.forename: Pierre
authors.initials: P
authors.lastname: Florian
authors.forename: Dominique
authors.initials: D
authors.lastname: Massiot
authors.forename: Anne
authors.initials: A
authors.lastname: Lesage
authors.forename: Christophe
authors.initials: C
authors.lastname: Copéret
authors.forename: Françoise
authors.initials: F
authors.lastname: Delbecq
authors.forename: Xavier
authors.initials: X
authors.lastname: Rozanska
authors.forename: Philippe
authors.initials: P
authors.lastname: Sautet