Alumina pillared clays: An investigation of the catalytic properties for biphenyl alkylation
Alumina pillared clays are a relatively new class of microporous solid acids. The authors have selected the isopropylation of biphenyl as a test reaction for pillared clay catalysts. The study included clays of various charge densities, charge localization and extent of layer stacking. For comparison purposes, the performance of an acid washed montmorillonite and two zeolites were also investigated. Under liquid phase conditions at 250[degrees]C, the reaction was under diffusion control. The activities and selectivities of the catalysts were mainly dependent on their pore structure. The mesoporous volume was a critical factor of catalytic activity. This was demonstrated by a study of a series of highly mesoporous pillared clays with similar acidity, obtained by surfactant modification. When isopropanol was used as an alkylating agent instead of propene, hydrophobic alumina pillared montmorillonites were the most efficient catalysts. Catalytic activity was strongly correlated to acidity for alumina pillared clays of similar pore structure. The effect of calcination on the clay layer was studied by [sup 27]Al, [sup 29]Si and [sup 19]F MAS-NMR, FTIR spectroscopy and by mass spectrometric analysis of volatile by-products. Between 30[degrees]C and 500[degrees]C, specific lattice fluorine atoms adjacent to charged sites in the octahedral layers are replaced by hydroxyl groups. The Broensted acidity of hydroxyl groups is enhanced by the near-neighbor fluorine at neutral sites in the octahedral sheet. Between 350[degrees]C and 500[degrees]C, a second process occurs that causes dehydroxylation of the layers, and this results in a sharp decrease in the acidity and catalytic activity of the pillared clay. A computational study showed that the thermodynamic orientation was meta, para, whereas the kinetic orientation favored the ortho isomer. Very microporous catalysts favored the thermodynamic distribution whereas mesoporous catalysts favored the kinetic distribution.
- Research Organization:
- Michigan State Univ., East Lansing, MI (United States)
- OSTI ID:
- 7302298
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400201* -- Chemical & Physicochemical Properties
ALKYLATION
ALUMINIUM COMPOUNDS
ALUMINIUM OXIDES
AROMATICS
BIPHENYL
CALCINATION
CATALYTIC EFFECTS
CHALCOGENIDES
CHEMICAL REACTIONS
CLAYS
DECOMPOSITION
HYDROCARBONS
MICROSTRUCTURE
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PORE STRUCTURE
PYROLYSIS
THERMOCHEMICAL PROCESSES