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Molecular sieve manufacturer explains the structure of molecular sieve in detail

The wide application of zeolite molecular sieve materials (for example: adsorption separation, ion exchange, catalysis) is inseparable from its structural characteristics. For example, the performance of adsorption and separation depends on the size of the pores and pore volume of the molecular sieve; the ion exchange performance depends on the number and position of cations in the molecular sieve and the permissibility of the pores; the shape selectivity shown in the catalytic process and the size of the pores of the molecular sieve , Trends are related, and the intermediate products and final products in the catalytic reaction are related to the pore dimension of the molecular sieve or its cage structure. Therefore, the structure of molecular sieves is a basic problem in the study of molecular sieve materials. Hunan Providence New Materials Co., Ltd is a professional molecular sieve manufacturer which can provide USY zeolite, NaY zeolite, REY zeolite, ZSM-5 zeolite, welcome to consult.

Structural units
Start with the research from the simplest basic structural unit. Generally speaking, zeolite molecular sieves are obtained by stacking tetrahedrons by sharing vertices, so a tetrahedron is a primary structural unit (TO4 tetrahedron). For example: for silicalite-1 zeolite molecular sieve, its primary structural unit is silicon-oxygen tetrahedron ([Si O4]0), and this tetrahedral structural unit is electrically neutral. These silicon-oxygen tetrahedrons share oxygen atoms. Connected to form a zeolite molecular sieve with MFI structure. In the synthesis, the templating agent and adsorbed water are present in its pores. Of course, when there is aluminum in the synthesis system, there are two kinds of tetrahedrons: silicon-oxygen tetrahedron ([Si O4]0) and aluminum-oxygen tetrahedron ([Al O4]-), and aluminum-oxygen tetrahedron There is a negative charge. By assembling and synthesizing a silicon-aluminum molecular sieve with an MFI structure, this structure itself has a certain negative charge, so it must be balanced by additional cations to make the whole finally appear electrically neutral. . Phosphorus aluminum molecular sieves are composed of phosphorus oxygen tetrahedra ([PO4]+) and aluminum oxygen tetrahedra ([Al O4]-) strictly alternately, and the skeleton is electrically neutral. Of course, in the connection between the primary structural unit and the primary structural unit, the Lowenstein rule must be observed: in the silicon-aluminum framework structure, aluminum and aluminum cannot be adjacent; in the phosphate framework structure, such as SAPO-34, aluminum cannot be combined with each other. Valence or trivalent metal atoms are adjacent, and phosphorus cannot be connected to silicon or phosphorus.

Secondary structural unit
The framework structure of molecular sieve is formed by finite or infinite connection of primary structural units. Limited structural units, such as secondary structural units, usually refer to multiple ring structures composed of TO4 tetrahedrons through the common use of fixed-point oxygen atoms to form different connection modes. Common ring structures such as four-membered and five-membered rings , Six-membered ring, double four-membered ring and double six-membered ring. Now discovered are 18 types of secondary structural units. For example, the 4-4 secondary structural unit represents two four-membered rings, that is, double four-membered rings. As we are familiar with the A-type molecular sieve, it is through the connection between the SOD cage and the double four-membered ring to form a zeolite molecular sieve. Of course, the SBU we are talking about is only a topological unit in the theoretical sense, to better understand and explain the structure of the zeolite molecular sieve. It cannot be regarded as the real species in the crystallization process of the zeolite molecular sieve.

Cage structure unit
There is a characteristic cage-like structural unit in the framework of molecular sieve, and the cage-like structural unit is described according to the multi-element ring that determines their polyhedron. For example, the familiar SOD cage is composed of eight six-membered rings and six four-membered rings, generally abbreviated as 4668. Different molecular sieve frameworks will contain the same cage structure unit. In other words, the same cage structure unit will form different molecular sieve framework structure types through different connection methods. A classic example is the SOD cage.
The SOD cages are connected through their own coplanar connections to form SOD zeolite molecular sieves; the SOD cages are connected through double four-membered rings to form LTA-type molecular sieves; SOD cages are connected through double six-membered rings to form FAU and EMT zeolite molecular sieve.
In addition, in the framework structure of zeolite molecular sieve, some characteristic chains and two-dimensional three-connected network structure and periodic structural units (PBU) are often found. Our five most common chain structures are Pentasil chain, double zigzag chain, double zigzag chain, double shaft chain and short pillar chain. The Pentasil chain composed of cages shared by edges is a characteristic chain of the high silica zeolite molecular sieve family. The most representative, the skeleton structure of MFI is composed of Pentasil chains. The two-dimensional three-connected network layers stacked in parallel are connected to each other through the three-connected vertices oriented up and down to form a three-dimensional four-connected skeleton structure. For example, the GIS type skeleton structure is composed of a 4.82 two-dimensional network layer structure connected up and down.

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