Exposing active sites in zeolites with fluoropyridines as NMR probe molecules

Abstract

Active sites in zeolites are crucial for catalysis, but their identification remains challenging due to structural complexity. Solid-state NMR, a key tool for studying zeolites, can struggle with directly detecting acidic properties due to the quadrupolar nature of ubiquitous metal dopants. Here, we use fluorinated pyridine as a probe to identify active sites in HY and HZSM-5, two important solid acid catalysts. ¹⁹F NMR effectively detects binding environments, while ¹⁹F–²⁷Al polarization transfer reveals elusive penta-coordinated Al^V Brønsted acid sites and shows that the dominant active sites are distorted framework-associated Al^IV and Al^V sites with large quadrupolar coupling constants, shedding light on catalytic properties. Low-temperature (100 K) NMR proves essential for capturing these interactions. This work enhances the understanding of zeolite active sites and highlights the broad applicability of fluorinated probe molecules for surface characterization, offering a cost-effective, highly sensitive approach for catalytic studies.