Honeycomb zeolite molecular sieve Catalytic mechanism

Zeolite molecular sieve In various acid catalytic reactions, it can provide high activity and unusual selectivity, and most reactions are caused by the acidity of molecular sieves. Acidity and its acid strength distribution are important parameters of molecular sieves. The research shows that the B acid in the molecular sieve comes from the skeleton tetrahedral aluminum, while the L acid mainly comes from the non skeleton hexahedral aluminum, so the content and distribution of Al in the molecular sieve are closely related to the surface acidic substances of the molecular sieve. Therefore, secondary hydrothermal treatment such as dealumination and aluminum supplementation of the molecular sieve can be used to obtain the molecular sieve with an ideal silicon aluminum ratio. In addition, the acidity of molecular sieves is also affected by substituted metal ions. The hydrolysis of multivalent metal ions leads to the redistribution of acid centers on the catalyst surface.

1. Sieving effect

It includes reactant shape selection and product shape selection. Shape selective catalysis of reactants has been applied in many aspects in oil refining industry, such as molecular sieve dewaxing of oil products, heavy oil hydrocracking, etc. Product shape selection has a significant effect on changing product distribution and improving the efficiency of target products, but there is also a harmful side. These undissolved macromolecules, or isomers with smaller linear degree, diffuse out, or break down into smaller molecules, or even do not

The catalyst was deactivated after cracking and dehydrogenation, and then deposited in the form of carbon in the pores and orifices.

2. Coulomb field effect

The ionic properties of the inner surface of the crystal and the electrostatic field interaction strength between the zeolite and the adsorbed molecules depend on the silicon aluminum ratio of the zeolite and the cation exchanged. The nano exchanged zeolite with low Si/Al ratio has a strong electrostatic field, while the hydrogen zeolite with high Si/Al ratio does not.

3. Configuration diffusion

This occurs when the structural size of the catalyst is close to the molecular size. In this diffusion mode, a small change in molecular size will cause a significant change in its diffusion coefficient.

4. Spatial adaptability or filtered selectivity

Some reactions can form corresponding transition state only because the inner diameter or the cage cavity has a large space, or they will be limited, making the reaction impossible. ZSM-5 catalyst is used for this transition state selective catalytic reaction.

5. Walkthrough control

Some zeolites (HZSM-5 molecular sieves) have cross channels with different channel sizes. Because small channels can only pass through small molecules, while large channels can pass through large and small molecules.