Advantages and Characteristics of Porous Molecular Sieve Performance! Molecular sieves are a type of aluminosilicate with a crystalline structure, consisting of ordered porous materials with TO4 (in most cases, T represents Al or Si) tetrahedra as the basic structural units connected by oxygen bridge bonds to form a certain topological network structure. The pore size is approximately 0.3-2 nm, making it a typical microporous material. Due to its unique crystal pore structure, adjustable acidity, and good hydrothermal stability, it has special “shape selective” catalytic and adsorption separation properties, and has a wide range of applications in various fields such as petrochemicals, fine chemicals, and environmental protection.

The microporous structure of molecular sieve

However, the microporous pore structure of molecular sieves can to some extent hinder the diffusion of large molecules to the active center. Taking ZSM-5 as an example, as shown in Figure 1, the straight cylindrical pores are 0.53 nm * 0.56 nm, and the “Z” shaped transverse pores are 0.51 nm * 0.55 nm. When encountering large molecule reactants such as phenol, it is difficult for the reactant molecules to enter the narrow 0.55 nm micropores and reach the active center, resulting in a decrease in catalytic performance. At the same time, microporous channels can easily lead to carbon deposition, shortening the service life of molecular sieve catalysts. Therefore, one of the hot topics in the research of molecular sieve catalysts today is how to reduce the adverse effects of microporous diffusion resistance on mass transfer and improve the utilization efficiency of catalysts

In microporous molecular sieves, intragranular diffusion is the rate controlling step. And the diffusion rate is more than ten orders of magnitude lower than molecular diffusion and Nusselt diffusion (see Figure 2). Mesoporous molecular sieves such as MCM and SBA have a large specific surface area, adsorption capacity, and pore size. The pore diffusion is mainly by Nusselt diffusion. And the diffusion rate is at least 7-8 orders of magnitude higher than intragranular diffusion. However, their acidity is weak and their hydrothermal stability is poor. But this limits their industrial applications. In order to solve the above problems, researchers have developed a multi-stage porous molecular sieve. It combines the advantages of mesoporous and microporous molecular sieves. The presence of multi-stage pores changes the pure intragranular diffusion behavior of reactant molecules to a synergistic effect of Nusselt diffusion and intragranular diffusion. It increases their diffusion rate by at least 2 to 3 orders of magnitude.

2、 How to construct a multi-stage porous molecular sieve?

At present, according to the different order of introducing multi-level pores. The synthesis methods of multi-level pore molecular sieves can be into two categories: “bottom-up” and “top-down”.
A. From bottom to top
The “bottom-up” method refers to the simultaneous introduction of microporous and mesoporous structures in a reaction system to produce multi-stage porous molecular sieves. It mainly includes hard template method, soft template method, and template free self-assembly method.
【 Rest assured product 】 Zhongci porous molecular sieve performance advantages and characteristics! The hard template method refers to the addition of a solid hard template in a hydrothermal synthesis system. And the secondary growth of molecular sieve crystals on the outer surface or within the pores of the hard template to produce a multi-stage porous molecular sieve. Carbon templates are the most common hard templates due to their diversity and versatility. This mainly includes carbon black, carbon nanoparticles, carbon nanotubes, ordered mesoporous carbon, and sucrose.
The soft template method is a commonly used method in the “bottom-up” approach. This involves the self-assembly of mesoporous molecular sieves through the interaction between soft templates and silicon-based species during the crystallization process. It has the advantages of simple operation and controllable mesoporous pore size. Typical soft templates include organosilane, surfactants, and water-soluble cationic polymers.
Template free self-assembly is an emerging “bottom-up” synthesis method. The absence of templates here refers to the fact. That mesoporous template agents are not required during the synthesis process, but microporous structure directing agents are still needed. This method achieves the synthesis of multi-stage porous molecular sieves by self-assembly of nanocrystals to form stable mesoporous aggregates.

B. From top to bottom

The “top-down” method is to introduce mesopores through post-treatment of existing molecular sieves. Mainly including molecular sieve skeleton desilication method, desilication recrystallization method, dealumination method, and recrystallization method using surfactants as templates. The desilication method is a commonly used approach to introduce mesoporous structures by selectively removing silicon atoms from zeolite crystals through alkaline treatment of molecular sieves. The pore connectivity between the micropores and mesopores of the desilication molecular sieve is good. Greatly improving the mass transfer performance. The desilication recrystallization method increases the recrystallization operation compared to the desilication method. The dealumination method is a commonly used approach to remove some of the aluminum in the zeolite framework through calcination, hydrothermal treatment, acid treatment. The synthesis of multi-stage porous molecular sieves by recrystallization method refers to the dissolution and recrystallization of microporous molecular sieves under mild conditions (such as low concentration alkaline solution, low temperature, short time) containing cationic surfactants. The recrystallized nano molecular sieves are re deposited as independent mesoporous molecular sieve phases on the surface of the molecular sieve to form microporous mesoporous composite molecular sieves.

C. Comparison of advantages and disadvantages

【 Rest assured product 】 Zhongci porous molecular sieve performance advantages and characteristics! Compared with conventional microporous molecular sieves, multi-stage porous molecular sieves have the advantages of good diffusion and mass transfer performance, strong acidity, high thermal stability, and slow carbon deposition and deactivation rate. And it has become a hot topic in molecular sieve research. The “top-down” method has the advantages of simple synthesis, economy, and environmental protection, but it requires precise optimization of synthesis conditions and has poor reproducibility. The catalytic performance of multi-stage porous molecular sieves synthesized by the “bottom-up” method is good. But the template synthesis process is complex, the cost of template agents is high, and the removal of template agents can cause environmental pollution.