Measuring Bronsted Acid Site O-H Distances in Zeolites HY and HZSM-5 with Low-Temperature ~(17)O-~1H Double Resonance MAS NMR Spectroscopy

摘要

Bronsted acid sites play a key role in controlling the catalytic performances of acidic catalysts. A determination of the structure of the acid site, in particular, the O-H bond length, is important for the understanding of acid strength. The apparent O-H distances in zeolite HY and HZSM-5 extracted from ~(17)O-~1H rotational echo double resonance (REDOR) NMR data acquired at room temperature are noticeably longer than those extracted from ab initio calculations, clue to the presence of some restricted motions at room temperature, such as zeolite framework vibrations and O-H librational motion. Our ~(17)O-~1H REDOR NMR results for zeolite HY and HZSM-5 obtained at a lower temperature of 183 K, where some of these motions are frozen out, are presented here. By comparing the line shapes obtained by simulation with the SIMPSON package with the experimental data, an O-H distance of ～0.97-0.98 A was obtained for zeolite HY, which is consistent with the previous ab initio calculation results. The results demonstrate that low-temperature REDOR NMR can provide more accurate estimates of the O-H distance, which should prove useful in structure-function correlations and as a method to explore the effects of hydrogen bonding.