Characteristics of ZSM-5 Molecular Sieve

Characteristics of ZSM-5 Molecular Sieve

2.1 Thermal stability

The thermal stability of ZSM-5 zeolite is very high. This is caused by the structurally stable five-membered ring in the framework and the high silicon-to-aluminum ratio. For example, after calcining the sample at about 850 ℃ for 2 hours, its crystal structure does not change. Even the high temperature of 1100 ℃. To date, ZSM-5 is one of the best known zeolites for thermal temperature characterization. It is therefore particularly suitable for use in high temperature processes. For example, it is used as a hydrocarbon cracking catalyst, which can withstand the high temperature of the regenerant.

2.2 Acid resistance

ZSM-5 zeolite has good acid resistance, it can resist various acids except hydrofluoric acid.

2.3 Water vapor stability

When other zeolites are heated by water vapor, their structure is generally destroyed, resulting in irreversible deactivation. The Mobil company uses ZSM-5 as a catalyst for methanol conversion (water is one of the main products). This indicates that ZSM-5 has good stability to water vapor. The crystallinity of HZSM-5 was about 70% of the fresh catalyst after treating HZSM-5 and H stone with steam with a partial pressure of 22 mmHg at 540 °C for 24 hours, but under the same conditions, the framework of HY zeolite was almost completely destroyed. destroy.

2.4 Hydrophobicity

ZSM-5 has a high silicon to aluminum ratio and its surface charge density is small. Water is a highly polar molecule, so it is not easy to be adsorbed by ZSM-5. Although the diameter of water molecules is smaller than that of n-hexane, the amount of ZSM-5 to n-hexane is generally greater than that of water.

2.5 Not easy to deposit carbon

The effective shape, size and curvature of the orifice of ZSM-5 prevent the formation and accumulation of bulky condensates. At the same time, there are no cavities (cages) larger than the pores in the ZSM-5 framework, so the formation of condensed molecules from side reactions is limited. Thus, the possibility of carbon deposition on the ZSM-5 catalyst is reduced. ZSM-5 forms a barrier to the entry of alkyl aromatic hydrocarbons into the pores, so it cannot continue to react in the smaller pores during the reaction, and finally polycondenses to form coke. Therefore, the carbon deposition rate of ZSM-5 is much slower than that of Y-type and mordenite, and the difference is almost two orders of magnitude. The carbon capacity of ZSM-5 zeolite is also higher.

2.6 Excellent shape selectivity

With zeolite molecular sieve as catalyst, only molecules smaller than crystal pore can enter and exit the catalytic reaction, which is controlled by the size of zeolite crystal pore. Zeolite catalyst shows great selectivity to the size and shape of reactant and product molecules. The pore system composed of ten-membered rings of ZSM-5 zeolite has a medium pore diameter, which makes it have good shape selectivity.