FT-Raman spectroscopic study of the active sites on silica-supported ZnCl2 catalysts

Laser Raman spectroscopy has been used to study ZnCl2 Lewis acid catalyst adsorbed on acid-treated clays and porous silicas ('clayzic' and 'silizic'). Shifts in the Zn-Cl stretching frequency have been interpreted in terms of the environment of adsorbed ZnCl2. The active catalyst has been identified as adsorbed anhydrous ZnCl2, which exhibits a Raman spectrum indistinguishable from that of the bulk crystalline salt. Clayzic/silizic poisoning by polar reactant molecules has also been investigated. Adsorbed benzene has been shown to have no effect on the ZnCl2 spectrum, whereas anisole dramatically changes the Zn-Cl stretching band, suggesting strong coordination. This is consistent with the low activity of these catalysts in the presence of polar compounds of this type.