A variable temperature infrared spectroscopy study of CaA zeolite dehydration and carbonate formation

Variable temperature diffuse reflection infrared spectroscopy is used to monitor the dehydration of calcium enriched Linde type A zeolite (CaA). Infrared spectrum changes indicate that the energy imparted to the zeolite by heating causes water desorption and also facilitates reactions between water and zeolite framework, yielding SiOHAl acid sites. Water also reacts with ambient carbon dioxide producing carbonic acid, which readily dissociates to form carbonate. Three types of carbonate species are reported. The most thermally stable carbonate is coordinated to Ca²⁺ as a monodentate ligand. A second carbonate-containing species may be simultaneously interacting with Ca²⁺ and the zeolite framework. The third type of carbonate is characterized by infrared absorption consistent with a highly symmetric configuration, suggesting weak interactions with the local environment. Temperature-dependent infrared spectrum variations suggest that protons from carbonic acid dissociations react with framework and Ca(OH)⁺ moieties, altering internal CaA pore structure configurations.