Synthesis of NaA zeolite via the mesoscale reorganization of submolten salt depolymerized kaolin: A mechanistic study

Abstract

Deep understanding of the formation mechanism is crucial to the rational design and controllable synthesis of zeolites with desired pore architecture and acidity. Herein, we take the synthesis of NaA zeolite from a submolten salt depolymerized kaolin (SMS-K) as an example to elucidate the formation mechanism of NaA zeolite. By using SMS-K as sole Si and Al sources, a highly pure and crystalline NaA zeolite with superior ion exchange ability was synthesized. By using various ex-situ and in-situ characterizations and studying the crystallization kinetics, a plausible formation mechanism of NaA zeolite via the mesoscale reorganization of SMS-K was proposed. The results indicate that the reorganization of SMS-K into NaA zeolite follows the solid-phase transformation route. Upon being mixed with H2O, SMS-K that is mainly composed of silicate monomers and chain/ring microstructures is in-situ transformed rapidly into mesoscale intermediates containing double four-membered rings and β cages, sufficient stirring during aging generates more mesoscale intermediates. During the subsequent crystallization process, the mesoscale intermediates are further self-assembled into NaA nanoparticles when theirs amount reaches a maximum, and finally the nanoparticles grow into well-shaped cubic NaA zeolite particles with smooth surfaces through layer spreading. Our work provides the theoretical foundation for the design and green synthesis of zeolites directly from the natural minerals.