FT-IR evidence of two distinct protonic sites in BEA zeolite: Consequences on cationic exchange and on acido-basic properties in the presence of cesium

Two acidic H-BEA zeolites having similar Al content but different structural and textural properties were used to prepare two series of BEA samples containing increasing Cs amounts, added by exchange in solution and then impregnation. Whatever the zeolite, the introduction of Cs up to a Cs/Al ratio close to 1 leads to solely exchanged Cs⁺ ions, indicating that almost all Al are framework atoms giving cationic exchange capacity. At higher Cs contents, Cs-overloading takes place, giving Cs-oxide-like species with strong basicity on which carbonates are formed upon adsorption of CO₂, as followed by Fourier transform infrared (FT-IR) spectroscopy. In spite of such comparable behaviors, the two parent BEAs act in a strongly different manner toward cationic exchange in solution. This is related to the presence, in the most defective sample, of a distinct type of protons able to protonate NH ₃ but significantly less acidic than protons on regular tetrahedrally coordinated framework sites, as is shown by FT-IR spectroscopy in the presence of CO. These distinct protons are not sensitive to exchange in aqueous solution, but they are replaced by Cs⁺ ions via solid-state exchange in dehydrated conditions. The nature, environment, and strength of the two distinct Br0nsted acid sites, as well as the behavior of the related framework Al sites upon cationic exchange and hydration/dehydration treatments, are described. Consequences on Lewis acidity are discussed.