Catalytic Mechanism of Molecular Sieve Catalysts
Molecular sieves have clear pore distribution, extremely high internal surface area (600m2/s), good thermal stability (1000℃), and adjustable acid site centers. The acidity of molecular sieves mainly comes from the three-coordinated aluminum atoms and aluminum ions (AlO)+ on the framework and in the pores. The OH group on the molecular sieve HY obtained by ion exchange shows the acid site center, and the aluminum ion outside the framework will strengthen the acid site to form the L acid site center. Multivalent cations such as Ca2+, Mg2+, La3+ can display acid sites after exchange. The reduction of transition metal ions such as Cu2+ and Ag+ can also form acid sites. Generally speaking, the higher the Al/Si ratio, the higher the specific activity of the OH group. Modulation of the acidity of molecular sieves can introduce protons by direct exchange with dilute hydrochloric acid. Because of this approach, the molecular sieve framework is often dealuminated. So NaY becomes NH4Y, and then becomes HY.
(1) Molecular sieves have the properties of shape-selective catalysis
Because there are uniform small inner pores in the molecular sieve structure, when the molecular dimensions of the reactants and products are close to the pore diameters in the crystal, the selectivity of the catalytic reaction often depends on the corresponding sizes of the molecules and the pore diameters. This selectivity is called shape-selective catalysis. There are two mechanisms leading to the shape-selective selectivity. One is caused by the difference in the diffusion coefficients of the molecules participating in the reaction in the pore cavity, which is called mass transfer selectivity; the other is caused by the spatial limitation of the transition state of the catalytic reaction. called transition state selectivity. There are 4 forms of shape-selective catalysis:
Reactant Shape Selective Catalysis
When some reactive molecules in the reaction mixture are too large to diffuse into the catalyst pores, only those molecules with a diameter smaller than the inner pore diameter can enter the inner pores and react in the catalytically active part.
Shape-selective catalysis of products
When some molecules in the product mixture are too large to diffuse out of the inner pore window of the molecular sieve catalyst, the shape-selective selectivity of the product is formed.
In some reactions, the reactant molecules and product molecules are not limited by the diffusion of the catalyst window aperture, but the corresponding transition state can be formed only because the inner pore or cage cavity has a large space, otherwise the reaction cannot be carried out due to restrictions. On the contrary, some reactions that only require transition states with less space are not subject to this restriction, which constitutes shape-selective catalysis with restricted transition states. ZSM-5 is often used in this transition-state selective catalytic reaction, and the biggest advantage is to prevent coking. Because ZSM-5 has smaller inner pores than other molecular sieves, it is not conducive to the formation of large transition states required for the polymerization of coke-generated precursors. Therefore, it has a longer life than other molecular sieves and amorphous catalysts.
Shape-selective catalysis for molecular traffic control
In molecular sieves with two different shapes, sizes and pore channels, the reactant molecules can easily enter the active site of the catalyst through one pore channel to carry out the catalytic reaction, while the product molecules diffuse out from the other pore channel, reducing as much as possible. Reverse diffusion, increasing the reaction rate from the surface. This molecular traffic-controlled catalytic reaction is a special form of shape-selective selectivity, which is called molecular traffic-controlled shape-selective catalysis.
(2) Shape-selective modulation
It can poison the active center of the outer surface; modify the size of the entrance of the window hole.
The greatest practical value of shape-selective catalysis is to use it to characterize the difference in pore structure, and it is one of the methods to distinguish acidic molecular sieves. Shape-selective catalysis has been widely used in oil refining process and petroleum industrial production, such as molecular sieve dewaxing, shape-selective isomerization, shape-selective reforming, methanol synthesis of gasoline, methanol to ethylene, and shape-selective alkylation of aromatics.