The wide application of zeolite molecular sieve materials (such as adsorption separation, ion exchange, catalysis) is closely related to their structural characteristics. For example, the adsorption and separation performance 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, as well as the passability of its pore channels; The shape selectivity exhibited during the catalytic process is related to the pore size and orientation of the molecular sieve, while the pore size or cage structure of the intermediate and final products in the catalytic reaction are related to the molecular sieve. Therefore, the structure of molecular sieves is a fundamental issue in studying molecular sieve materials.
Structural unit
Firstly, conduct research on the simplest basic structural units. Generally speaking, zeolite molecular sieves are formed by stacking tetrahedra through shared vertices, so one tetrahedron is a primary structural unit (TO4 tetrahedron). For example, for silicalite-1 zeolite molecular sieve, its primary structural unit is silicon oxygen tetrahedra ([SiO4] 0), and this tetrahedral structural unit exhibits electrical neutrality. These silicon oxygen tetrahedra are connected by shared oxygen atoms to form a zeolite molecular sieve with MFI structure. In synthesis, template agents and adsorbed water are present in its pores. Of course, when aluminum is present in the synthesis system, there are two types of tetrahedra: silicon oxygen tetrahedra ([SiO4] 0) and aluminum oxygen tetrahedra ([AlO4] -), and aluminum oxygen tetrahedra have a negative charge. By assembling and synthesizing molecular sieves with MFI structure of silicon and aluminum, this structure itself carries a certain negative charge, so it must be balanced by additional cations to make it ultimately electrically neutral as a whole. And the phosphorus aluminum molecular sieve is strictly composed of alternating phosphorus oxygen tetrahedra ([PO4]+) and aluminum oxygen tetrahedra ([AlO4] -), with an electrically neutral skeleton. Of course, in the connection between primary structural units, the Lowenstein rule must be followed: in the silicon aluminum skeleton structure, aluminum cannot be adjacent to each other; In the phosphate skeleton structure, such as SAPO-34, aluminum cannot be adjacent to divalent or trivalent metal atoms, and phosphorus cannot be connected to silicon or phosphorus.
secondary building unit
The skeleton structure of molecular sieves is formed by finite or infinite connections of primary structural units. Finite structural units, such as secondary structural units, typically refer to multicomponent ring structures composed of TO4 tetrahedra that share fixed-point oxygen atoms and are connected in different ways. Common ring structures include four membered rings, five membered rings, six membered rings, double four membered rings, and double six membered rings. What has been discovered now are 18 types of secondary structural units. For example, the 4-4 secondary structural unit represents two quaternary rings, namely double quaternary rings. As we are familiar with A-type molecular sieves, they are formed by connecting SOD cages with double quaternary rings to form zeolite molecular sieves. Of course, the SBU we refer to is only a topological unit in the theoretical sense, in order to better understand and explain the structure of zeolite molecular sieves, and cannot be considered as a real species in the crystallization process of zeolite molecular sieves.
Cage shaped structural unit
There is a characteristic cage like structural unit in the skeleton of molecular sieves, which is described based on the multiple rings that determine their polyhedra. For example, the familiar SOD cage is composed of eight hexagonal rings and six quaternary rings, commonly abbreviated as 4668. Different molecular sieve skeletons will contain the same cage like structural units, in other words, the same cage like structural unit will form different types of molecular sieve skeleton structures through different connection methods. A classic example is the SOD cage.
The SOD zeolite molecular sieve is formed by the coplanar connection between the SOD cages; The SOD cages are connected by double quaternary rings to form LTA type molecular sieves; The SOD cages are connected by a double hexagonal ring to form FAU and EMT zeolite molecular sieves.
In addition, in the framework structure of zeolite molecular sieves, some characteristic chains, two-dimensional three connected network layers, and periodic structural units (PBUs) are often found. The five most common chain like structures are Pentasil chain, double zigzag chain, double zigzag chain, double axle chain, and short column stone 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 framework structure of MFI is composed of Pentasil chains. The parallel stacking of two-dimensional three connected network layers forms a three-dimensional four connected skeleton structure by interconnecting the three connected vertices oriented vertically. For example, the GIS type skeleton structure is composed of a 4.82 two-dimensional network layer structure connected vertically.