Generation of Broensted and Lewis acid sites on the surface of silica by addition of dopant cations
Pyridine adsorption was used to study the acidic properties of silica doped with the following cations: Sc/sup 3 +/, Mg/sup 2 +/, Fe/sup 2 +/, Fe/sup 3 +/, Zn/sup 2 +/, Al/sup 3 +/, and Ga/sup 3 +/. All samples were exposed to pyridine at 423 K and subsequently evacuated at successively higher temperatures to 723 K. Infrared spectra of the adsorbed pyridine indicated that all of these cations generated Lewis acid sites. This can be explained by the presence of coordinatively unsaturated dopant cations on the surface of silica, in accord with a model based on Pauling's electrostatic bond strength rules. The infrared frequency of the 19b band of pyridine adsorbed on these Lewis acid sites was found to increase with increasing electronegativity of the dopant cation. It is suggested that both of these quantities are related to the strength of the Lewis acid sites. Broensted acid sites were also observed by infrared spectroscopy for Sc/sup 3 +/, Al/sup 3 +/, and Ga/sup 3 +/ on silica. These dopant cations are believed to be bonded tetrahedrally on the surface of silica, generating bridging hydroxyl groups between the dopant cation and Si/sup 4 +/. As for zeolite catalysts, the proton associated with these groups and required for charge neutrality is the Broensted acid site. Finally, Broensted acid sites can also be generated on silica by highly electronegative anions, such as HPO/sub 4//sup 2 -/, which generate Broensted acidity in a different manner. 55 references.
- Research Organization:
- Univ. of Wisconsin, Madison
- DOE Contract Number:
- FG02-84ER13183
- OSTI ID:
- 5442161
- Journal Information:
- J. Catal.; (United States), Vol. 105:2
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
02 PETROLEUM
SILICA
PH VALUE
SURFACE PROPERTIES
ADSORPTION
ALUMINIUM
CATALYSTS
CATALYTIC CRACKING
DOPED MATERIALS
ELECTRONEGATIVITY
GALLIUM
INFRARED SPECTRA
IRON
LEWIS ACIDS
MAGNESIUM
PYRIDINE
SCANDIUM
ZINC
ALKALINE EARTH METALS
AZINES
CHALCOGENIDES
CHEMICAL REACTIONS
CRACKING
DECOMPOSITION
ELEMENTS
HETEROCYCLIC COMPOUNDS
HYDROGEN COMPOUNDS
INORGANIC ACIDS
MATERIALS
METALS
MINERALS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PYRIDINES
PYROLYSIS
SILICON COMPOUNDS
SILICON OXIDES
SORPTION
SPECTRA
THERMOCHEMICAL PROCESSES
TRANSITION ELEMENTS
400201* - Chemical & Physicochemical Properties
020400 - Petroleum- Processing