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　　Molecular sieves refer to a class of substances with uniform micropores and the pore size is equivalent to the general molecular size. Commonly used molecular sieves are crystalline silicates or aluminosilicates, which are connected by oxygen bridges to form pores and cavities of molecular size (usually 0.3~2 nm). . In addition, phosphoaluminate molecular sieves have been discovered, and the framework elements (such as silicon, aluminum, and phosphorus) of molecular sieves can also be replaced by other elements (heteroelements, such as iron, boron, copper, manganese, etc.).

　　 Classification of molecular sieves:

　　According to the framework elements, it is divided into: silicon aluminum (including silicon aluminum phosphorus SAPO, the same below), phosphorus aluminum, and skeleton heteroatoms;

　　 Divided according to the pore size: microporous, mesoporous, macroporous molecular sieves;

　　 According to the topological structure (determined by the ratio of raw materials), it is divided into: A, X, Y;

　　 Divided by generation: the first generation (synthetic zeolite), the second generation (ZSM), the third generation (SAPO), etc.;

　　 Silica sol provides silicon source for silicon aluminum and silicon aluminum phosphorus molecular sieves. In addition, silica sol acts as a binder in the process of forming the original molecular sieve powder.

　　The properties and functions of molecular sieves mainly depend on the composition of the framework elements and the pore structure. Silico-alumina molecular sieve is a kind of aluminosilicate compound with cubic lattice, and its varieties reach dozens of kinds. Molecular sieves have a large specific surface area of ​​300～1000m2/g. The inner crystal surface is highly polarized. It is a kind of high-efficiency adsorbent and a kind of solid acid. The surface has a high acid concentration and acid strength, which can cause normal carbon. Ionic catalytic reaction. When the metal ions in the composition are exchanged with other ions in the solution, the pore size can be adjusted to change its adsorption properties and catalytic properties, thereby preparing molecular sieve catalysts with different properties.

　　The advantages of molecular sieve:

　　1. It has thermal and hydrothermal stability. The results of high-temperature and long-term calcination and steam treatment show that silica-alumina molecular sieve has excellent thermal and hydrothermal stability.

　　2. It has ion exchange and catalytic performance from medium strong acid to strong acid-the introduction of Si can obtain a negatively charged framework, especially with exchangeable cations and sufficient strong acid centers (with proton acidity) generated on the surface.

　　3. It has high selectivity and a wide range of applications and development prospects-it can be used as adsorbents, catalysts and catalyst carriers, and is widely used in petroleum and chemical industries. It can be used in almost all hydrocarbon reactions, such as catalytic reforming, catalytic cracking, hydrofining, hydroisomerization, dehydrocyclization, aromatics disproportionation and MTO.

　　4. It has thermal and hydrothermal stability-the results of high-temperature and long-term roasting and steam treatment show that silica-alumina molecular sieve has excellent thermal and hydrothermal stability.

　　5. It has ion exchange and catalytic performance from medium strong acid to strong acid-the introduction of Si can obtain a negative skeleton, especially with exchangeable cations and sufficient strong acid centers (with proton acidity) on the surface.

　　 6. It has high selectivity and a wide range of applications and development prospects-can be used as adsorbents, catalysts and catalyst carriers, widely used in petroleum and chemical fields. It can be used in almost all hydrocarbon reactions, such as catalytic reforming, catalytic cracking, hydrofining, hydroisomerization, dehydrocyclization, aromatics disproportionation and MTO.