Fourier transform infrared quantification of acid sites in solids: probe molecules in action

Heterogeneous catalysis plays a crucial role in enabling cleaner and more efficient chemical processes; its performance is largely governed by the nature and strength of active sites. Among these, Brønsted acid sites (BAS) in zeolites are especially important for industrial and emerging sustainable applications. This review discusses recent advances in using Fourier transform infrared spectroscopy with probe molecules to quantitatively assess BAS in zeolites. We highlight a practical and updated workflow emphasizing the determination of reliable molar extinction coefficients for adsorbed probe molecules (carbon monoxide, acetonitrile, ammonia, and pyridine), a key factor often overlooked in acidity measurements. By consolidating methodological developments and best practices, this review provides guidance for improving the accuracy and comparability of acidity quantification in zeolites.