Acidic and catalytic properties of aluminated mordenite zeolite: effect of extraframework aluminium

Abstract

The nature of the extraframework aluminium generated in dealuminated mordenite by alumination with AlCl₃ and its effect on acidity and catalytic activity have been studied. A series of mordenites with framework Si/Al atomic ratios between 11 and 220 was obtained by alumination of dealuminated mordenite and was characterized using atomic absorption spectrophotometry, IR spectroscopy, MAS NMR and reaction studies.

Alumination below 873 K preferentially introduced Al atoms into the extraframework sites instead of into the framework sites, while alumination at 873 K and higher temperatures led to the incorporation of more Al into the framework. In all cases a significant portion of the extraframework Al species was not ion-exchangeable with an aqueous solution of NH₄NO₃ and was unobservable by ²⁷Al MAS NMR, suggesting its lack of symmetry. IR studies with NH₃ and pyridine probes demonstrated that the extraframework Al species acted as Lewis acid sites. The mordenite aluminated at 873 K showed a comparable specific activity to the parent and dealuminated mordenite for toluene disproportionation and o-xylene coversion as measured by turn-over frequency based on the number of Brønsted acid sites. Those aluminated below 873 K, however, exhibited enhanced specific activity for both reactions. The enhancement is explained by the Lewis acidic extraframework Al exhibiting electron-withdrawing properties and thus intensifying the Brønsted acidity of the framework Al.