Specific modification of the external surface of ZSM-5 zeolites by 12-tungstosilicic acid

Abstract

A medium-pore zeolite ZSM-5 has been modified by impregnation with an aqueous solution of 12-tungstosilicic acid. Adsorption measurements, IR spectroscopy and catalytic probe reactions (ethylbenzene conversion and m-xylene isomerization) show that the Keggin unit [SiW₁₂O₄₀]^4– cannot penetrate the ZSM-5 pore system but merely becomes deposited on the external zeolite surface. Thermal decomposition of the acid yields WO₃ at high loading accompanied by a loss of acid sites in the channel system. This is not exclusively caused by further dealumination but has partially to be attributed to site blocking by WO₃. Obviously, the decomposition of the acid involves mobile species of smaller size which are able to migrate into the interior of the zeolite.