OSDA-free and steam-assisted synthesis of PHI type zeolite showing a unique CO2 adsorption behaviour

Abstract

The best way to synthesize industrially useful zeolites is to avoid the use of costly organic structure-directing agent (OSDA) and the generation of synthetic wastewater. Recently, steam-assisted conversion methods have provided a new route for synthesizing zeolites under environment friendly conditions. In this study, the synthetic conditions for OSDA-free interzeolite conversion of FAU zeolite to CHA, phillipsite (PHI), and ANA zeolites were investigated. Importantly, green synthesis of the pure PHI phase was successfully achieved for the first time, by the OSDA-free steam-assisted interzeolite conversion method. More importantly, the amount of CO₂ adsorbed on PHI zeolite increased by ion exchange of alkaline cations from Na⁺ to K⁺, Rb⁺, and Cs⁺. Of particular interest is that K⁺-, Rb⁺-, and Cs⁺-exchanged PHI zeolites show unusual and sigmoid CO₂ adsorption isotherms. Remarkably, as the radius of the alkaline cation increases, the sharp increase in the sigmoid adsorption branch shifts to higher pressures.