Aluminum coordination and Lewis acidity in transition aluminas
- Univ. of Wisconsin, Milwaukee (United States)
Besides the resonance lines attributable to tetrahedrally (Al[sup IV]) and octahedrally (Al[sup VI]) coordinated aluminum, the MAS NMR[sup 27]Al spectra of transition aluminas obtained from finely divided precursors gibbsite and boehmite have a line which can be assigned to pentacoordinated Al(Al[sup v]). This line which is very intense en ex-boehmite aluminas, as compared to ex-gibbsite aluminas, is also observed in the nonframework alumina moieties in acid and dealuminated zeolites. The isotropic chemical shifts of the Al[sup VI], Al[sup V], and Al[sup IV] resonance lines are 11.4, 38.3 and 65.6 ppm, respectively, in a field of 11.77 T. To detect electron acceptor sites (or Lewis acid centers), an EPR molecular probe has been adsorbed and the EPR spectra have been recorded before and after introduction of molecular oxygen. With dimethylaniline (DMA) the formation of a radical cation is observed and its concentration is larger on aluminas with Al[sup V]. Molecular O[sub 2], which is adsorbed on DMA-treated aluminas, acts as an electron scavenger picking up the electron transferred from DMA to the surface. The surface density in electron acceptor sites with electron affinity [approximately][minus]7.1 eV is, at the best, on the order of 0.4 [times] 10[sup 12] spins/cm[sup 2]. These findings suggest either that strongly distorted shells of pentacoordinated Al are potential Lewis acid centers or that they generate such sites upon thermal activation. Other types of coordination, if sufficiently distorted, may play a similar role.
- DOE Contract Number:
- FG02-90ER14130
- OSTI ID:
- 7017344
- Journal Information:
- Journal of Catalysis; (United States), Journal Name: Journal of Catalysis; (United States) Vol. 133:2; ISSN 0021-9517; ISSN JCTLA5
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400201* -- Chemical & Physicochemical Properties
ADSORPTION
ALUMINIUM 27
ALUMINIUM COMPOUNDS
ALUMINIUM ISOTOPES
ALUMINIUM OXIDES
AMINES
AROMATICS
CATALYSTS
CHALCOGENIDES
CHEMICAL SHIFT
ELECTRON SPIN RESONANCE
ELEMENTS
HYDROGEN COMPOUNDS
INORGANIC ACIDS
ISOTOPES
LEWIS ACIDS
LIGHT NUCLEI
MAGNETIC RESONANCE
NONMETALS
NUCLEAR MAGNETIC RESONANCE
NUCLEI
ODD-EVEN NUCLEI
ORGANIC COMPOUNDS
OXIDES
OXYGEN
OXYGEN COMPOUNDS
PH VALUE
RESONANCE
SORPTION
STABLE ISOTOPES
SURFACE PROPERTIES
VALENCE