Molecular sieve crystals have a uniform pore structure, the size of the pore size is equivalent to that of ordinary molecules; they have a large surface area. Moreover, the surface is very high; the cations that balance the negative charge of the framework can be ion exchanged; some metals with catalytic activity can also be exchanged into the crystal, and then reduced to the element state with a high degree of dispersion; at the same time, the stability of the molecular sieve framework structure is high. These structural properties make molecular sieve not an excellent adsorbent, but also an effective catalyst and catalyst carrier.

Molecular sieve is a kind of solid acid, which can provide high thermal stability, catalytic activity and selectivity in many acid-catalyzed reactions, and has been widely used in oil refining and petrochemical industries. For example, catalytic cracking, hydrocracking, isomerization, reforming, disproportionation, and transalkylation reactions.

Molecular sieve structure

Molecular sieve crystals have a uniform pore structure, a large surface area, and high surface properties, and the stability of the molecular sieve framework structure is also high. These structural properties make molecular sieve not a good adsorbent, but a good catalyst and catalytic carrier. The catalytic reaction inside the zeolite molecular sieve structure began in the laboratory of Mobil Company in the late 1950s. This discovery marked the beginning of molecular sieve research. Since the molecular sieve structure has uniform small pores, the selectivity of the catalyst reaction often depends on the size of the molecules and the pore size. This selectivity is called shape-selective catalytic selectivity. There are four different forms of shape-selective selective catalysis.

1. The shape-selective catalysis of the reactant. For some reactive molecules in the reaction mixture, only molecules with a diameter smaller than the inner pore diameter can enter the inner pores and carry out catalytic reactions at the catalyst site. The shape-selective catalysis of reactants has been applied in many aspects in the oil refining industry, such as molecular sieve dewaxing of oil products and hydrocracking of heavy oil.

2. The shape-selective catalysis of the product. Some molecules in the product mixture are too large and diffuse out of the inner pores of the molecular sieve catalyst. These undiffused macromolecules or isomers with small heterogeneous composition diffuse out or crack Into smaller molecules, and even continuously cracked, dehydrogenated, and finally deposited in the pores and pores in the form of carbon, resulting in deactivation of the catalyst.

3. Shape-selective catalysis restricted by the transition state Some reactions require a relatively large space to form the corresponding transition state, which constitutes the shape-selective catalysis restricted by the transition state. ZSM-5 catalysts are commonly used in this transition state selective catalytic reaction. It can be used to catalyze the isomerization reaction of low-molecular-weight hydrocarbons, cracking reactions, and two transalkylation reactions. ZSM-5 catalyst can prevent coking and has a longer life than other molecular sieves or amorphous catalysts, which is very beneficial to industrial production.

4. Shape-selective catalysis for molecular traffic control In molecular sieves with two different shapes and sizes of pores, the reactant molecules can easily enter the active part of the catalyst through one pore to carry out the catalytic reaction, while the product molecules are from another A channel diffuses out to minimize reverse diffusion, thereby increasing the reaction rate. The practical value of shape-selective selective catalysis is to use it to characterize the difference in pore structure. This kind of catalysis is also widely used in oil refining process and petrochemical industry.