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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.
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Richter et al. (1991) studied this question.
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