STRUCTURES OF ALUMINUM-CHLORIDE GRAFTED ON SILICA SURFACE

The structures of silica-grafted aluminum chloride catalysts were investigated by solid-state NMR measurements, together with their catalytic properties in the alkylation of benzene with cyclohexene. Catalyst samples prepared by contacting aluminum trichloride vapor with a silica gel surface at temperatures between 140 and 250 degrees C were found to be active for the alkylation, while other samples prepared at higher temperatures were inactive. The grafted aluminum chloride species were classified into two structural types, corresponding to monomeric and dimeric aluminum chloride. The combination of Al-27 and Si-29 NMR experiments elucidates important structural features of these aluminum chloride/silica systems. The active catalysts were found to have a dimeric aluminum chloride structure, with chemical shifts of 85 and 42 ppm in the Al-27 magic-angle spinning NMR spectra. Those peaks correspond to aluminum species coordinated to chloride and/or oxygen with coordination numbers of 4 and 5, respectively. It is speculated that the catalytically active species is the product of reaction of the aluminum trichloride dimer with one hydroxyl on the silica surface.