Two-step hydrothermal synthesis and conversion mechanism of zeolite X from stellerite zeolite

We investigated the conversion mechanism of stellerite zeolite to zeolite X under two-step hydrothermal conditions. To elucidate the conversion mechanism, solid products were separated from the mixtures at different crystallization times and characterized by XRD, FESEM, FT-IR, Raman, solid-state NMR, XRF, and TEM. The results indicate that in this reaction process, the Si, Al, and Na in the gel solid phases were continuously dissolved and transformed into the gel–liquid-phase. When the concentration of each component reached supersaturation in the gel–liquid-phase, Si, Al, and Na were transferred to the surface of the gel–solid-phase, and nucleation and crystallization occurred on the surface. Abundant nuclei were formed during the second hour of the crystallization. As the crystallization time increased, the nuclei rapidly grew into zeolite X crystals, and the relative crystallinity of zeolite X reached a maximum when the crystallization time reached 4 h. These phenomena indicate that the formation mechanism of zeolite X is a liquid-phase conversion mechanism.