MODIFICATION OF MORDENITE ACIDITY BY ISOMORPHOUS SUBSTITUTION OF TRIVALENT CATIONS IN THE FRAMEWORK SITES USING THE ATOM-PLANTING METHOD

Aluminum, gallium and antimony cations were incorporated into the fram ework of highly siliceous mordenite to modify the acid strength of Bro nsted acid sites by the 'atom-planting method' with the corresponding metal chloride vapors at elevated temperatures. The framework incorpor ation of metal cations was confirmed by IR, MAS NMR and catalytic reac tions. The optimum temperature to achieve a maximum incorporation of m etal cations into the mordenite framework was demonstrated to be 873 K . Tetrahedral Ga cations exhibited an IR band owing to acidic bridging Si(OH)Ga groups at 3620 cm(-1) and a characteristic band at 159 ppm i n the Ga-71 MAS NMR spectrum. The IR band owing to bridging Si(OH)Sb g roups was observed at 3663 cm(-1). The acid strength of Si(OH)Me (Me = Al, Ga and Sb) groups estimated from their IR frequency, IR spectra o f adsorbed pyridine and the activity for toluene disproportionation an d cumene alkylation with 2-propanol was in the order Si(OH)Al>Si(OH)Ga much greater than Si(OH)Sb. The weakest acid sites, Si(OH)Sb, inactiv e for the toluene disproportionation, showed a significant activity fo r the cumene alkylation with higher para-selectivity. (C) 1997 Elsevie r Science B.V.