The Brønsted/Lewis acid synergy in dealuminated HY zeolite has been studied using solid-state NMR and density function theory (DFT) calculation. The 1H double quantum magic-angle spinning (DQ-MAS) NMR results have revealed, for the first time, the detailed spatial proximities of Lewis and Brønsted acid sites. The results from 13C NMR of adsorbed acetone as well as DFT calculation demonstrated that the Brønsted/Lewis acid synergy considerably enhanced the Brønsted acid strength of dealuminated HY zeolite. Two types of Brønsted 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 Brønsted/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 Brønsted acid sites. All these findings are expected to be important in understanding the roles of Lewis acid and its synergy with the Brønsted acid in numerous zeolite-mediated hydrocarbon reactions.

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The shielding tensor, which describes the anisotropic chemical shift, plays an important role in nuclear magnetic resonance (NMR) because it can extract valuable information about the molecular dynamics and the local electronic structures in solids. The tensor, which is second rank, can be expressed by three independent principal components in its principal axis system (PAS). These components can be easily converted into chemical zhift anisotropies, as three principal components, i.e. a,,, dZ2 and d,, , with conventionally d, > 6,, > 6,, .The average of the three components is the isotropic chemical shift do :(1)The other two necessary parameters are the chemical shift anisotropy, which we denote here as Ad, and the asymmetric parameter 9. They can be written as follows:6 0 = @ l l + 6 2 2 + 8 3 3 ) 6 2 2 -6 3 3The isotropic chemical shift of a dilute nucleus, such as 13C, in a solid is usually obtained from its highresolution NMR spectrum, which can be routinely yielded by the magic angle spinning (MAS) and heteronuclear dipolar decoupling techniques combined with cross-polarization (CP). ' p 3