The Structure and Properties of Molecular Sieves

The Structure and Properties of Molecular Sieves

①　Control of grain size and shape

The pore size of most zeolite molecular sieves is less than 1 nm. When small molecular organic compounds react in the zeolite pores, the diffusion will be limited to a certain extent, which will affect the pore utilization and catalytic performance. Reducing the grain size and changing the shape of the grain is the means to improve the molecular diffusion performance and the utilization rate of the pore channels. The diffusion path of the small grain or nano molecular sieve is shorter than that of the large grain molecular sieve, the utilization rate of the pore channel will be greatly improved, and the catalytic activity will also be reduced. There is improvement.

②　Multi-level pore compound

Most of the mesoporous materials reported so far have shortcomings such as poor thermal stability, lack of surface acid centers with a certain strength, and easy loss of acid centers. The main reason is that although the above materials have ordered mesoporous channels, their The skeleton is an amorphous structure. Although zeolite molecular sieves have good structural stability and strong acid centers, they are limited by molecular diffusion, which affects their catalytic activity and selectivity. The microporous and mesoporous or macroporous hierarchical porous composites are expected to combine the advantages of both and exert their advantages in practical applications. Hierarchical pore zeolite molecular sieves are expected to be used in some larger molecular catalytic reactions and liquid-phase catalytic reactions.

③　Co-crystal molecular sieve

The catalytic nature of co-crystalline molecular sieves is actually the fine adjustment of pores and acidity, which is a means to improve the performance of catalysts. The catalytic performance of crystalline molecular sieves has been greatly improved. For example, when ZSM-5/ZSM-11 (MFI/MEL) co-crystalline molecular sieves are used in MTG reaction, gasoline components can be adjusted in a wide range.

④　Surface modification of it and improvement of their hydrothermal stability

Thermal stability and hydrothermal stability are one of the important properties of molecular sieve catalysts to be investigated. Many industrial catalytic reactions require high thermal stability of catalysts, especially hydrothermal stability.

It is often the key to determine the life of catalysts and the selection of reaction processes.

Taking the catalytic cracking reaction of CTE as an example, because the reaction is carried out under the condition of steam, improving the hydrothermal stability of the catalyst is the key to the development of CTE catalysts.

The results show that the stability of the active center of the catalytic material under water vapor can be improved by assembling and modifying the catalytic active center of the porous material with phosphorus oxide compounds and introducing framework heteroatoms.