FT-RAMAN SPECTROSCOPIC STUDY OF THE ACTIVE-SITES ON SILICA-SUPPORTED ZNCL2 CATALYSTS

Laser Raman spectroscopy has been used to study ZnCl2 Lewis acid catal yst adsorbed on acid-treated clays and porous silicas ('clayzic' and ' silizic'). Shifts in the Zn-Cl stretching frequency have been interpre ted in terms of the environment of adsorbed ZnCl2. The active catalyst has been identified as adsorbed anhydrous ZnCl2, which exhibits a Ram an spectrum indistinguishable from that of the bulk crystalline salt. Clayzic/silizic poisoning by polar reactant molecules has also been in vestigated. Adsorbed benzene has been shown to have no effect on the Z nCl2 spectrum, whereas anisole dramatically changes the Zn-Cl stretchi ng band, suggesting strong coordination. This is consistent with the l ow activity of these catalysts in the presence of polar compounds of t his type.