Journal of the American Chemical Society, Vol.129, No.36, 11161-11171, 2007
Bronsted/Lewis acid synergy in dealuminated HY zeolite: A combined solid-state NMR and theoretical calculation study
The Bronsted/Lewis acid synergy in dealuminated HY zeolite has been studied using solid-state NMR and density function theory (DFT) calculation. The H-1 double quantum magic-angle spinning (DQ-MAS) NMR results have revealed, for the first time, the detailed spatial proximities of Lewis and Bronsted acid sites. The results from C-13 NMR of adsorbed acetone as well as DFT calculation demonstrated that the Bronsted/Lewis acid synergy considerably enhanced the Bronsted acid strength of dealuminated HY zeolite. Two types of Bronsted acid sites (with enhanced acidity) in close proximity to extra-framework aluminum (EFAL) species were identified in the dealuminated HY zeolite. The NMR and DFT calculation results further revealed the detailed structures of EFAL species and the mechanism of Bronsted/Lewis acid synergy. Extra-framework Al(OH)(3) and Al(OH)(2+) species in the supercage cage and Al(OH)(2+) species in the sodalite cage are the preferred Lewis acid sites. Moreover, it is the coordination of the EFAL species to the oxygen atom nearest the framework aluminum that leads to the enhanced acidity of dealuminated HY zeolite though there is no direct interaction (such as the hydrogen-bonding) between the EFAL species and the Bronsted acid sites. All these findings are expected to be important in understanding the roles of Lewis acid and its synergy with the Bronsted acid in numerous zeolite-mediated hydrocarbon reactions.