Exposing active sites in zeolites with fluoropyridines as NMR probe molecules

Active sites in zeolites play a crucial role in catalysis, but their complete identification and characterisation remains a challenge due to structural variety and complexity. Solid-state NMR spectroscopy, a powerful tool for probing zeolite structures, often faces limitations in detecting and interpreting their acidic properties owing to the quadrupolar nature of ubiquitous metal dopants. Here, we use fluorinated pyridine as a probe molecule to identify active sites in zeolites HY and HZSM-5, two industrially-important solid acid catalysts, providing valuable insights into their activity using a cost-effective and highly-sensitive approach. ¹⁹F magic angle spinning (MAS) NMR is shown to be very responsive to the binding environment of the fluoropyridine and ¹⁹F–²⁷Al polarisation transfer illuminates hard-to-detect penta-coordinated Al^V Brønsted acid sites. Additionally, preferential binding of fluoropyridines to distorted framework-associated Al^IV and Al^V sites distinguished by large quadrupolar coupling constants is observed, shedding light on catalytic properties. Our results also emphasize the critical role of low-temperature (100 K) NMR methods in revealing these interactions. This work not only advances the understanding of active sites in zeolites, highlighting the importance of detailed characterization, but also underscores the broader applicability of fluorinated probe molecules for surface characterisation, offering a valuable platform for the chemistry community.