Liquid Phase Transition Mechanism And Bidirectional

Liquid phase transition mechanism
The liquid phase transition mechanism was first proposed by Kerr and Ciric, almost at the same time as the solid phase transition mechanism. They believe that the nucleation and growth of zeolite molecular sieve crystals are carried out directly in the solution, the initial gel slowly dissolves into the solution, and the active species aluminosilicate ions are generated, and then condensation occurs, slowly forming the structural units required for zeolite molecular sieves, and then further generating zeolite molecular sieves.
First, after the raw materials required for zeolite molecular sieves are mixed, the main species silicate and aluminate polymerize to form an initial aluminosilicate gel. This aluminosilicate gel is formed rapidly under high concentration conditions, so it has a high degree of disorder, but this aluminosilicate gel may contain some primary structural units, such as four-membered rings, six-membered rings, etc. At the same time, a dissolution equilibrium is established between this gel and the liquid phase. In addition, the solubility product of aluminosilicate ions is closely related to the structure and temperature of the gel. As the crystallization temperature changes, a new gel and solution equilibrium is established between this gel and the liquid phase. Secondly, the increase in the concentration of polysilicate and aluminate in the liquid phase leads to the formation of crystal nuclei, followed by the growth of zeolite molecular sieve crystals. During the nucleation and crystal growth of zeolite molecular sieves, polysilicate and aluminate ions in the liquid phase are consumed, causing the continued dissolution of the silica-alumina gel. Since the solubility of zeolite crystals is less than that of amorphous gel, the final result is the complete dissolution of the gel and the complete growth of zeolite molecular sieve crystals.
Zhdanov's experiment showed that the growth rate of zeolite molecular sieve crystals is closely related to the concentration of polysilicate and aluminate ions in the liquid phase, and the concentration of each component in the liquid phase is constantly changing during the crystallization process. These experimental results support the liquid phase transition mechanism. The most favorable proof of the liquid phase transition mechanism is the direct crystallization of zeolite molecular sieves from the liquid phase. Koizumi et al. directly synthesized zeolite molecular sieves such as SOD, GIS, and FAU from clarified solutions. 
Two-phase transition mechanism
When people were still arguing about whether zeolite molecular sieve crystallization was through liquid phase transition mechanism or solid phase transition mechanism, scientists proposed the two-phase transition mechanism after the 1980s. The two-phase transition mechanism believes that liquid phase transition and solid phase transition exist simultaneously in the crystallization process of zeolite molecular sieve, which can occur in two crystallization reaction systems separately or in one system at the same time.
Gabelica et al. confirmed the existence of the two-phase transition mechanism from their research on the crystallization of ZSM-5 molecular sieve and Na Y zeolite. Iton et al. first applied small angle neutron scattering technology to study the crystallization process of ZSM-5 molecular sieve, and found that the crystallization of ZSM-5 zeolite molecular sieve followed different mechanisms when using different silicon sources. Therefore, it is concluded that even if the same type of zeolite molecular sieve is used, its growth mechanism is different under different crystallization conditions.