Probing brønsted acid sites in zeolite HY with low temperature ^17O MAS NMR spectroscopy

Brønsted acid sites play a key role in controlling the catalytic performances of acidic catalysts. A determination of the structure of the acid site is fundamental to the understanding acid strength in these systems. The O-H distances in zeolite HY and HZSM-5 extracted from ¹⁷O-¹H REDOR NMR data acquired at room temperature are noticeably longer than the results from calculations due to the presence of some restricted motions at room temperature, such as zeolite framework vibrations and O-H librational motion. We present here our ¹⁷-¹H REDOR NMR results of zeolite HY at a lower temperature of 183 K, where some of these motions are frozen out. By comparing the line shapes obtained from simulations performed with the SIMPSON package with the experimental data, an O-H distance of about 0.970.98 A was obtained, which is consistent with the previous _ab-initio calculation results. The results indicate that low temperature REDOR NMR spectroscopy can provide estimates of the O-H distance, which should prove useful in understanding zeolite structure and acidity.