Mixed-alkali effect in the thermal collapse and melting kinetics of Zeolite X

Abstract The thermal stability of zeolites underlies their many applications. Structural collapse usually occurs through a series of reactions, involving an initial low-density amorphous state (LDA), a high-density melt (HDA), and, eventually, a dense recrystallization product. The LDA–HDA transition resembles a glass melting process; the HDA phase can sometimes be quenched into a glass. Using isothermal as well as nonisothermal in situ observation methodology on the faujasitic zeolite X and its potassium-for-sodium ion-exchanged derivatives, we find a pronounced mixed-alkali effect in the characteristic nonisothermal transition temperatures as well as in the isothermal reaction timescale of the LDA–HDA transition. The latter is taken as being directly related to the viscosity of the liquid into which the zeolite framework is melting; the apparent viscosity associated with this melting process was found to reach as high as 1015 Pa.s at 740°C. The observed mixed-alkali effect is similar to the one observed in the dynamics of common oxide glasses and their corresponding melts.