Catalytic performance of molecular sieves

Catalytic Performance of Molecular Sieves

Molecular sieve crystals have a uniform pore structure, with pore sizes comparable to those of common molecules. They possess a large surface area and high surface polarity. The cations that balance the negative charge of the framework can undergo ion exchange. Some metals with catalytic activity can also be introduced into the crystal through exchange and then reduced to the elemental state with a high degree of dispersion. Meanwhile, the framework structure of molecular sieves has high stability. These structural properties enable molecular sieves to not only serve as excellent adsorbents but also as effective catalysts and catalyst carriers.
Molecular sieves are solid acids. In many acid - catalyzed reactions, they can provide high thermal stability, catalytic activity, and selectivity, and thus have been widely used in the refining and petrochemical industries. For example, they are used in catalytic cracking, hydrocracking, isomerization, reforming, disproportionation, and alkyl transfer reactions.

Molecular Sieve Structure

Molecular sieve crystals have a uniform pore structure, a large surface area, and high surface polarity. The stability of the molecular sieve framework structure is also very high. These structural properties make molecular sieves not only excellent adsorbents but also good catalysts and catalytic carriers. The study of catalytic reactions inside the structure of zeolite molecular sieves began in the late 1950s in the laboratories of Mobil Company. This discovery marked the beginning of molecular sieve research. Due to the uniform small internal pores in the molecular sieve structure, the selectivity of catalytic reactions often depends on the size of the molecules and the pore diameter. This selectivity is called shape - selective catalytic selectivity. There are four different forms of shape - selective catalysis.

1. Reactant Shape - Selective Catalysis: Among the reactant mixtures, only those reactive molecules with diameters smaller than the internal pore diameter can enter the internal pores and undergo catalytic reactions at the catalytic sites. Reactant shape - selective catalysis has found various applications in the refining industry, such as molecular sieve dewaxing of oil products and heavy oil hydrocracking.

2. Product Shape - Selective Catalysis: Some molecules in the product mixture are too large to diffuse out of the internal pores of the molecular sieve catalyst. These undiffused large molecules either isomerize into smaller - sized isomers and then diffuse out or crack into smaller molecules. Eventually, through continuous cracking and dehydrogenation, they are deposited as carbon inside and at the entrances of the pores, leading to catalyst deactivation.

3. Transition - State - Restricted Shape - Selective Catalysis: Some reactions require a relatively large space to form the corresponding transition states, which constitutes transition - state - restricted shape - selective catalysis. The ZSM - 5 catalyst is commonly used in such transition - state - selective catalytic reactions. It can be used to catalyze the isomerization of low - molecular - weight hydrocarbons, cracking reactions, and alkyl transfer reactions of xylene. The ZSM - 5 catalyst can prevent coking and has a longer lifespan than other molecular sieves or amorphous catalysts, which is very beneficial for industrial production.

4. Molecular Traffic Control Shape - Selective Catalysis: In molecular sieves with two different shapes and sizes of channels, reactant molecules can easily enter the active sites of the catalyst through one type of channel to undergo catalytic reactions, while product molecules diffuse out through the other channel. This arrangement minimizes reverse diffusion as much as possible, thereby increasing the reaction rate. The practical value of shape - selective catalysis lies in the utilization of the differences in the pore structure. This type of catalysis also has wide applications in the refining process and petrochemical industry.