Modification of mordenite acidity by isomorphous substitution of trivalent cations in the framework sites using the atom-planting method

Abstract

Aluminum, gallium and antimony cations were incorporated into the framework of highly siliceous mordenite to modify the acid strength of Bronsted acid sites by the ‘atom-planting method’ with the corresponding metal chloride vapors at elevated temperatures. The framework incorporation of metal cations was confirmed by IR, MAS NMR and catalytic reactions. The optimum temperature to achieve a maximum incorporation of metal 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 3620cm⁻¹ and a characteristic band at 159 ppm in the ⁷¹Ga MAS NMR spectrum. The IR band owing to bridging Si(OH)Sb groups 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 of adsorbed pyridine and the activity for toluene disproportionation and cumene alkylation with 2-propanol was in the order Si(OH)Al>Si(OH)Ga> > Si(OH)Sb. The weakest acid sites, Si(OH)Sb, inactive for the toluene disproportionation, showed a significant activity for the cumene alkylation with higher para-selectivity.