Understanding the Structure of Cationic Sites in Alkali Metal-Grafted USY Zeolites

This contribution describes the structure of cationic sites in alkali metal-grafted high-silica USY zeolites, a novel class of zeolite base catalysts, through the combination of computational studies and experimental evidence. Based on density functional theory calculations, we investigate the geometry various defects that coordinate sodium or potassium, and their interaction with adsorbed CO. The predicted CO-stretching frequencies are verified experimentally by infrared spectroscopy, which indicates the existence of a distribution of different site geometries rather than a single site type. The corresponding bands lie in the range of 2180–2160 and 2170–2145 cm^–1 for Na⁺ and K⁺ cations, respectively. While CO primarily interacts with the grafted metal cation (effect from the bottom), the CO stretching frequency is strongly modulated by neighboring silanol groups (effect from the top). Based on a good agreement of experimental and theoretical results, the deprotonation of silanol groups to silanolates followed by an ion exchange appears to be a realistic mechanism for alkali metal incorporation in silica-rich USY zeolites. The developed insights on the cation site structure represent an important step to understand the catalytic performance of these basic zeolites in aldol-type reactions, and pave the way for future studies investigating transition states to resolve their role in catalysis