Synthesis Mechanism Of Zeolite Molecular Sieves

In-depth research on the formation and growth mechanism of zeolite molecular sieves will help people better design and synthesize new zeolite molecular sieve topological structures, expand new routes for the synthesis of zeolite molecular sieve materials, and develop new properties and new uses of zeolite molecular sieve materials. Although zeolite molecular sieves have been developed for many years, there has been no real conclusion on its synthesis mechanism. Studying the crystallization mechanism of molecular sieves is of great theoretical significance and also has practical guiding significance for the synthesis of new zeolite molecular sieves. At present, the most representative mechanisms are solid phase transformation mechanism (Solid hydrogel Transformation mechanism), liquid phase transformation mechanism (Solution-mediated Transport mechanism) and dual phase transformation mechanism.
Solid phase transformation mechanism
The solid phase transformation mechanism was first proposed by Flanigen and Breck, and it is also the earliest proposed zeolite molecular sieve crystallization mechanism. They believe that:
During the entire crystallization process of zeolite molecular sieve, only the gel solid phase itself is produced under hydrothermal conditions, and then the structure of the aluminosilicate skeleton is directly rearranged, which leads to the nucleation and crystal growth of the zeolite molecular sieve. In the crystallization process of zeolite molecular sieve, there is neither dissolution of the gel solid phase nor direct participation of the liquid phase in the nucleation and crystal growth of the zeolite molecular sieve.
First, after the various raw materials required for the synthesis of zeolite molecular sieve are mixed, the main species silicate and aluminate polymerize to form an initial aluminosilicate gel. At the same time, although the liquid phase between the gels is also produced, the liquid phase does not participate in the crystallization and nucleation process. Secondly, the initial aluminosilicate gel formed is continuously depolymerized and rearranged under the action of OH- ions, thereby forming the primary structural units required for the crystallization of certain zeolites. Finally, these primary structural units are further rearranged around the hydrated cations to form polyhedrons, which are further polymerized, connected, and formed into zeolite molecular sieve crystals.
In the 1970s, Dutch scientists Mcnicol et al. used molecular spectroscopy to track the entire crystallization process of LTA zeolite molecular sieves, thus providing sufficient experimental evidence for the solid phase transition mechanism. In the 1990s, the proposed synthesis method of dry gel conversion also added an example to the solid phase transition mechanism. In addition, the proposed solid phase solvent-free synthesis method developed in recent years also provides corresponding evidence for the solid phase transition mechanism to a certain extent.