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

Abstract

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.