Detection of Brønsted acid sites in zeolite HY with high-field 17O-MAS-NMR techniques.

The acidity and unique porous structures of zeolites play an important role in controlling the activity and selectivity of many zeolite-based catalysts. Although (27)Al, (29)Si and (1)H NMR spectroscopy represent standard analytical tools with which to study these materials, (17)O-NMR investigations are much less routine, owing to the very low natural abundance of (17)O (0.037%), its relatively low resonant frequency and its large quadrupole moment. (17)O-NMR resonances from framework oxygen sites in a variety of zeolites have been detected, but the (17)O-NMR resonance from oxygen directly bound to the Brønsted acid site (Si-O(H)-Al) has remained elusive. Here we report the direct observation of this resonance in dehydrated zeolite HY, by using high magnetic-field strengths. (17)O-(1)H double-resonance NMR experiments are used to prove unambiguously that the (17)O signal arises from O nearby H atoms. A large quadrupolar coupling constant, the measure of the local distortion of this site, of 6.6 MHz is determined, which is similar to that obtained in ab initio calculations of zeolite HY-like clusters; this value drops to 5 MHz on acetone binding. The results presented in this paper open up methods for characterizing zeolite acidity and investigating H(+)-sorbent interactions.