STRUCTURES OF ALUMINUM-CHLORIDE GRAFTED ON SILICA SURFACE

The structures of silica-grafted aluminum chloride catalysts were inve stigated by solid-state NMR measurements, together with their catalyti c properties in the alkylation of benzene with cyclohexene. Catalyst s amples 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 highe r temperatures were inactive. The grafted aluminum chloride species we re classified into two structural types, corresponding to monomeric an d dimeric aluminum chloride. The combination of Al-27 and Si-29 NMR ex periments elucidates important structural features of these aluminum c hloride/silica systems. The active catalysts were found to have a dime ric 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 coordi nation numbers of 4 and 5, respectively. It is speculated that the cat alytically active species is the product of reaction of the aluminum t richloride dimer with one hydroxyl on the silica surface.