Do you know Crystallization Mechanism of Molecular Sieves?
The needs of practical human activities and the development of application fields continuously drive the development of zeolite molecular sieves. This progress has taken us from natural zeolite to synthetic zeolite, from low-silica zeolite to high-silica zeolite, and from silica-alumina molecular sieve to phosphorus-aluminum molecular sieve. Additionally, advancements have led to ultra-large micropores, mesoporous materials, inorganic porous frameworks, MOFs, and emerging macroporous materials. These developments effectively increase yield, reduce synthesis costs, and minimize environmental pollution. It is hoped that this paper provides powerful help for further development of zeolite molecular sieves.
Mechanism of liquid phase transition
Anten Chemical believes that molecular sieve crystal nuclei are formed in the liquid phase or on the interface of the gel. The growth of these nuclei consumes the silicate hydration ions in the solution. The solution provides the soluble structural units required for the growth of the zeolite molecular sieve crystal. During the melting process, the consumption of liquid phase components leads to the continued dissolution of the gel’s solid phase.
After the raw materials are mixed, the initial aluminosilicate gel is formed, which is disordered but may contain some simple primary structural units such as four-membered rings and six-membered rings. As the gel and liquid phase reach dissolution equilibrium, the solubility product of aluminosilicate ions depends on the structure and temperature of the gel. When the temperature rises to crystallize, a new equilibrium is established between the gel and the solution. An increase in the concentration of aluminosilicates in the liquid phase leads to the formation of crystal nuclei, followed by crystal growth.
During the nucleation and crystal growth of molecular sieves, silicate ions in the liquid phase are consumed, resulting in the continuous dissolution of the amorphous gel. Eventually, the gel is entirely dissolved, and the zeolite molecular sieve crystal grows to its full size.
Mechanism of solid phase transition
According to the solid phase theory, the gel’s solid phase does not dissolve, and the liquid phase does not directly participate in the nucleation of molecular sieves and crystal growth during the crystallization process. When the raw materials are mixed, the silicate and aluminate groups polymerize to form an initial aluminosilicate gel.
Although an intergel liquid phase is generated, this liquid phase does not participate in the crystallization process and remains constant throughout. The initial gel is depolymerized and rearranged under the action of OH ions to form the primary structural units required by certain zeolite molecular sieves. These primary structural units rearrange themselves around the hydrated cations to form polyhedrons. The polyhedrons then aggregate and connect to form zeolite crystals.
Two-phase transition mechanism
Two-phase transition mechanism, it is believed that both the solid-phase mechanism and the liquid-phase mechanism of molecular sieve crystallization exist, and they can occur in two systems respectively, or in the same system. For example, Gabelica found that with different reactant ratios and reaction conditions, both solid phase transition and liquid phase transition may occur in the ZSM-5 synthesis system.
The research on the formation mechanism of molecular sieves has made considerable progress, but it is still in development and needs to be confirmed by further research. if you have any question about crystallization mechanism of molecular sieves,please contact with us.
Reference
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