FLUORINE-MEDIATED ACIDITY OF ALUMINA-PILLARED FLUOROHECTORITE

Structural fluorine atoms in the 2:1 layered silicate framework of fluorohectorite have a profound effect on the acidity of alumina-pillared derivatives prepared by intecalation of Al13 polycations and subsequent calcination at elevated temperatures. The alumina-pillared clay formed by calcination at 350°C exhibits greatly benhanced catalytic activity for propylene alkylation of biphenyl, relative to nonfluorinated smectite hosts. However, calcination of the Al13 intercalate at 500°C results in a relatively inactive clay with greatly diminished NH3 and pyridine chemisorption properties. The effect of the calcination process on the clay layer structure was carefully studied by 27Al, 29Si, and 19F MAS-NMR and FTIR spectroscopic methods and by mass spectrometric analysis of volatile by-products. It was shown that between 30 and 500°C, specific lattice fluorine atoms adjacent to charged sites in the octahedral sheet of the layers are replaced by hydroxyl groups. At calcination temperatures below 350°C the Brønsted acidity of protonated hydroxyl groups in the layers is enhanced by the electron-withdrawing effect of near-neighbor fluorine atoms at neutral sites in the octahedral sheet. A mechanism for fluorine hydrolysis is proposed in which gallery water molecules, formed by the dehydroxylation of the alumina pillars, migrate to hexagonal oxygen cavities above the reactive fluorine positions. Between 350 and 500°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. The relationships between structure and reactivitysuggest that the acid-catalytic activity of an alumina-pillared clay can be mediated by controlling the relative amounts of hydroxyl groups at charged octahedral sites and fluorine atoms at neutral octahedral sites in the host clay. © 1993 Academic Press, Inc.