Molecular sieve manufacturer explains the nucleation and growth process of molecular sieves
The synthesis of molecular sieves is a well-known complex process. The crystallization rate, types of formed products, and their particle properties (morphology, morphology, and grain size distribution) depend on a large number of parameters. These parameters include crystallization conditions (temperature, stirring, seed and gel aging) and composition dependent parameters (pH value, water content, proportion between skeleton constituent elements, template concentration and ionic strength). A typical molecular sieve synthesis will involve the following steps:
① The mixture of amorphous reactants includes alkaline media, which will form heterogeneous partial reaction phases, known as the initial amorphous phase. The amorphous phase has properties from gel to colloid, and is synthesized in the so-called "transparent solution".
② Heat the reaction mixture (above 100 ° C) in a metal high-pressure vessel and under self generated pressure. Before the reaction, the reaction mixture can be aged for a period of time (several hours to several days).
③ A sub equilibrium solution phase formed as a "secondary amorphous phase". There is evidence to suggest that due to the structural effect of cations in solution, short-range ordered structures are generated during this stage.
④ After the induction period, crystal nuclei are formed. The induction time is related to the simple and systematic definition given in the equation. The relaxation time (tr) is the time required for steps ① to ③ to occur, i.e. to form a metastable amorphous solid, while tn and tg have the same meaning.
⑤ When amorphous solids are consumed, the molecular sieve material grows.
These steps are clear for the synthesis of a large number of molecular sieves, but in many cases they may be difficult to distinguish. This may be due to overlapping steps or excessive difficulties in synthesis experiments.
The nucleation of molecular sieves is a complex problem, as it means the initial amorphous or random structure is transformed into a crystal skeleton. As previously observed, a short-range ordered structure appeared during the formation of the secondary amorphous phase. Then, the random number of structured regions can reach the size of crystal nuclei and begin to grow into macroscopic crystals.
In the past, people used traditional nucleation theories to study the nucleation of molecular sieves, such as calculating the nucleation rate as the reciprocal of the induction period. However, there are important differences between the crystalline and condensed phases of molecular sieves, and one of the differences is that molecular sieves have a higher internal surface area.
The process of molecular sieve nucleation is difficult to study and analyze due to the difficulty of experiments in in-situ measurement. Research obtains relevant information through the use of particle size distribution and mathematical models to infer growth and nucleation rates. Other literature studies have examined the effects of aging and seed formation on the nucleation of molecular sieves. The aging of the initial solution has been shown to have an impact on the final crystal distribution, so it can provide valuable information about the nucleation mechanism, which has been confirmed in experiments on A-type zeolite and silicalite. The use of crystal seeds can be used to distinguish between primary nucleation and secondary nucleation.
The mechanism of nucleation process has also been widely discussed. It is widely accepted that the process of gradual sequencing within gel is carried out through the reversible process of "breaking" and re forming of chemical bonds in the framework catalyzed by hydroxyl ions. Cations and organic structural directing agents also play a crucial role in the nucleation process: in the preferred geometric shapes, they are surrounded by metal oxide species through electrostatic and van der Waals interactions. Hunan Providence New Materials Co., Ltd is a professional molecular sieve manufacturer
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