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

Deuterated pyridine (pyridine-d5) is one of the NMR probe molecules widely used for determination of acid strength of solid catalysts. However, the correlation between the 1H chemical shift of adsorbed pyridine-d5 and the Brønsted acid strength of solid acids has rarely been investigated. Here, an 8T zeolite model with different Si-H bond lengths is used to represent the Brønsted acid sites with different strengths (from weak, strong, to superacid) and to predict the pyridine adsorption structure as well as the 1H chemical shift. The theoretical calculation suggests that a smaller 1H chemical shift of the pyridinium ions on the solid acids indicates a stronger acid strength. On the basis of the results of theoretical calculations, a linear correlation between the pyridine-d5 1H chemical shift and the proton affinity (PA) of the Brønsted acid site has been derived. In combination with the available 1H MAS NMR experimental data, we conclude that pyridine-d5 can be used as a scale to characterize the solid acid strength.

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Solid-state 13 C NMR experiments and quantum chemical Density Functional Theory (DFT) calculations of acetone adsorption were used to study the location of protons in anhydrous 12-tungstophosphoric acid (HPW), the mobility of the isolated and hydrated acidic protons, and the acid strength heterogeneity of the anhydrous hydroxyl groups. This study presents the first direct NMR experimental evidence that there are two types of isolated protons with different acid strengths in the anhydrous Keggin HPW. Rotational Echo DOuble Resonance (REDOR) NMR experiments combined with quantum chemical DFT calculations demonstrated that acidic protons in anhydrous HPW are localized on both bridging (O c) and terminal (Od) atoms of the Keggin unit. The CP/MAS NMR experiments revealed that the isolated acidic protons are immobile, but hydrated acidic protons are highly mobile at room temperature. The isotropic chemical shift of the adsorbed acetone suggested that the acid strength of the H(H 2O)n + species in partially hydrated HPW is comparable to that of a zeolite, while the acidity of an isolated proton is much stronger than that of a zeolite. Isolated protons on the bridging oxygen atoms of anhydrous HPW are nearly superacidic.

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Using trimethylphosphine (TMP) as a probe molecule, acid sites in a series of Al-MCM-41 materials with Si/Al ratios ranging from 16 to 80 were investigated by various solid-state NMR techniques including 29 Si, 31 P, 27 Al, 1 H magic-angle spinning (MAS), and some double-or triple-resonance methods such as 31 P f 1 H cross polarization (CP), 1 H/ 27 Al, and 1 H/ 31 P/ 27 Al TRAPDOR (transfer of population in double resonance). By means of the 1 H/ 27 Al TRAPDOR technique, which has the ability to establish a correlation between 1 H and 27 Al, the hydroxyl groups associated with Al could be discriminated from the silanol groups. Two signals at 3.5 and 1.9 ppm were observed in the 1 H/ 27 Al TRAPDOR spectra; they are likely due to the bridging hydroxyl (SiOHAl) and aluminum hydroxyl groups, respectively. 31 P MAS NMR strongly supports the formation of zeolitelike Bro ¨nsted acid sites in the mesoporous material after the incorporation of aluminum. The concentration of the Bro ¨nsted sites determined from 31 P MAS NMR is about 2 orders of magnitude lower than that of total SiOH groups, which renders their direct observation by 1 H MAS NMR spectroscopy difficult. However, these sites can be well studied by 31 P f 1 H CP/MAS and various triple-resonance TRAPDOR NMR techniques, which provide a more detailed description of the interactions between TMP and the different hydroxyl groups in the Al-MCM-41 materials. Although the acid strength of the Bro ¨nsted sites in the Al-MCM-41 materials is lower than that of microporous zeolites such as HY, these sites can protonate TMP molecules. Some of the framework T-site aluminum atoms are not associated with the bridging hydroxyl groups and are probably present as Al-OH. No Lewis acid sites were found in our samples, though the 27 Al MAS spectra show the existence of five-coordinate extraframework aluminum.