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CaX-type molecular sieve

A synthetic hydrated aluminosilicate (zeolite) or a natural zeolite that functions as a molecular sieve. Its general chemical formula is (M′2M)O·Al2O3·xSiO2·yH2O, where M′ and M represent monovalent and divalent cations, such as K+, Na+, Ca2+, and Ba2+. Structurally, it features numerous uniform‑sized channels and well‑ordered cavities; molecular sieves with different pore sizes can separate molecules of varying sizes and shapes. Depending on the SiO2/Al2O3 molar ratio, molecular sieves with distinct pore dimensions are obtained. Common types include: 3A (potassium A‑type), 4A (sodium A‑type), 5A (calcium A‑type), 10Z (calcium Z‑type), 13Z (sodium Z‑type), Y (sodium Y‑type), and sodium mordenite, among others. These materials exhibit high adsorption capacity, strong selectivity, and excellent thermal stability. They are widely used in organic and petrochemical industries and serve as outstanding desiccants for gas dehydration. Moreover, they are increasingly recognized for their utility in exhaust‑gas purification.

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X-type molecular sieve

Keywords: Zeolite molecular sieve

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A synthetic hydrated aluminosilicate (zeolite) or a natural zeolite that functions as a molecular sieve. Its general chemical formula is (M′2M)O·Al2O3·xSiO2·yH2O, where M′ and M represent monovalent and divalent cations, such as K+, Na+, Ca2+, and Ba2+. Structurally, it features numerous uniform‑sized channels and well‑ordered cavities; molecular sieves with different pore sizes can separate molecules of varying sizes and shapes. Depending on the SiO2/Al2O3 molar ratio, molecular sieves with distinct pore dimensions are obtained. Common types include: 3A (potassium A‑type), 4A (sodium A‑type), 5A (calcium A‑type), 10Z (calcium Z‑type), 13Z (sodium Z‑type), Y (sodium Y‑type), and sodium mordenite, among others. It exhibits high adsorption capacity, strong selectivity, and excellent thermal stability. Widely employed in organic and petrochemical industries, it also serves as an outstanding adsorbent for gas dehydration and is increasingly recognized for its role in exhaust‑gas purification.

Introduction

In nature, there exist naturally occurring aluminosilicates that exhibit molecular sieving, adsorption, ion exchange, and catalytic properties. These natural materials are known as zeolites, while their synthetic counterparts are also referred to as molecular sieves. The general chemical formula of a molecular sieve is: (M)2/nO·Al2O3·xSiO2·pH2O, where M denotes a metal cation—typically sodium in synthetic materials—n represents the valence of the metal cation, x indicates the molar ratio of SiO2, also called the silicon-to-aluminum ratio, and p specifies the molar amount of water. The fundamental structural units of a molecular sieve’s framework are SiO4 and AlO4 tetrahedra, which link via shared oxygen atoms to form a three-dimensional crystalline network. This structural arrangement gives rise to uniform, molecular‑scale pores and channels.

Because the AlO4 tetrahedron carries a negative charge, it can bind cations such as sodium to achieve electrical neutrality. In aqueous solutions, Na⁺ readily exchanges with other cations. Most molecular sieve catalysts are exchanged with polyvalent metal cations or protons; owing to their acidity and size-selective pore structure, molecular sieves can serve as either catalysts or supports. High-silica zeolites exhibit strong affinity for organic groups, whereas low-silica zeolites, with their Lewis and Brønsted acid properties, display hydrophilicity. Silicon and aluminum atoms are linked by oxygen atoms to form oxygen rings, and the size of these rings determines the pore diameter of the zeolite. Each oxygen ring typically contains 4 to 12 oxygen atoms. Zeolites commonly exhibiting molecular sieve behavior include eight-membered rings (0.4–0.5 nm), ten-membered rings (0.5–0.6 nm), and twelve-membered rings (0.7–0.9 nm).

Molecular sieves with a twelve-membered oxygen ring include Y-type zeolites (x = 3.1–6.0) and mordenite (x = 9–11). The former can serve as a cracking catalyst or a bifunctional catalyst, while the latter is suitable as a catalyst for the disproportionation of toluene.

Ten-membered oxygen‑containing rings are found in certain ZSM‑series molecular sieves, such as ZSM‑5 and ZSM‑11.

Eight-membered oxygen‑containing rings are found in A‑type molecular sieves (x = 2), T‑type molecular sieves, and ZSM‑34, among others. These materials possess very small pores, allowing only linear hydrocarbons to enter their narrow channels. Catalysts that use molecular sieves as the catalytically active component or the primary active phase are referred to as molecular sieve catalysts. Molecular sieves exhibit ion‑exchange properties, uniform pore dimensions on the molecular scale, excellent acid catalytic activity, and superior thermal and hydrothermal stability. They can be formulated into catalysts with high activity and high selectivity for a wide range of reactions.

Molecular sieves are a class of crystalline aluminosilicates.

Molecular formula: Na2O·Al2O3·2.0SiO2·4.5H2O

■ Selective adsorption based on molecular size and shape, whereby only molecules smaller than the molecular sieve’s pore diameter are adsorbed.
■ For small polar molecules and unsaturated molecules, the material exhibits selective adsorption; the greater the polarity and the higher the degree of unsaturation, the stronger the selectivity.
■ It exhibits strong water‑absorption capacity. Even under elevated temperatures, high gas velocities, and low moisture levels, it maintains a remarkably high water‑adsorption capacity. It can be used to produce molecular sieves in the 3A, 4A, 5A (oxygen production), 5A (desulfurization), 10X, 13X, and CU‑13X series, available in both pelletized and granular forms. These materials are characterized by high mechanical strength, excellent thermal stability, and superior selective adsorption performance. They find extensive applications in the chemical, petrochemical, and natural gas industries.

Molecular sieve

It exhibits high adsorption capacity, strong selectivity, and excellent thermal stability. It is widely used in organic and petrochemical industries and serves as an outstanding adsorbent for gas dehydration.

BX-type molecular sieve

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BX-type molecular sieve

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