What is Molecular Sieve?
There is a natural aluminosilicate in nature, which has the functions of sieving molecules, adsorption, ion exchange and catalysis. This natural substance is called zeolite. According to the structure of zeolite, synthetic zeolite is called molecular sieve. The general formula of the chemical composition of molecular sieve is: (M)2/nO·Al2O3·xSiO2·pH2O, M represents metal ion (usually Na in artificial synthesis); n represents the valence of metal ion; x represents the number of moles of SiO2, also known as the ratio of silicon to aluminum; p represents the number of moles of water. The most basic structure of molecular sieve framework is SiO4 and AlO4 tetrahedron, which constitute the primary structural unit. The primary structural units are combined by the shared oxygen atoms to form a crystal of a three-dimensional network structure, that is, the framework structure of the molecular sieve. The skeleton structure is connected to each other through oxygen bridges to form multi-element rings, and various multi-element rings are connected to each other through oxygen bridges to form polyhedrons with three-dimensional space. These polyhedrons are hollow cages, so they are also called cages. The cage of molecular sieve is the main structural unit that constitutes a three-dimensional space polyhedron. These structural units are repeatedly connected to form molecular sieves with pores and channels. The orifice is the part where the cavity is connected to the outside or other cavities. Whether various crystals or fluid molecules can enter into the zeolite crystal is controlled by the effective pore size of the main orifice. A channel is a channel inside a zeolite formed by interconnected pore orifices. The chemical composition, pore size and channel size of molecular sieves mainly determine the physical and chemical properties of molecular sieves.
Figure 1. The main building blocks of some molecular sieves
What are the Application of Molecular Sieve?
- Molecular Sieve as Absorbent Material: The adsorption of molecular sieve is a physical change process. The pore size of the molecular sieve is uniform, and when the molecular dynamics diameter is smaller than the pore size of the molecular sieve, it enters the inside of the pores and is adsorbed. Therefore, molecular sieve is like sieves for gas and liquid molecules, and whether they are adsorbed can be determined according to the size of the molecules. Due to the strong polarity in the pores of the molecular sieve, it can interact strongly with molecules containing polar groups on the surface, or induce strong adsorption by polarizing the polarizable molecules. Such polar or easily polarized molecules are easily adsorbed by polar molecular sieve to achieve selective adsorption characteristics.
- Molecular Sieve as Ion Exchange Material: In molecular sieve ion exchange materials, ion exchange generally refers to the exchange of compensatory cations outside the molecular sieve framework. The compensating ions outside the molecular sieve framework are generally protons, alkali metals or alkaline earth metals, which are easily ion-exchanged into metal ion molecular sieve of various valence states in the aqueous solution of metal salts. In aqueous solution, due to the different ion selectivity of different molecular sieves, different ion exchange properties can be exhibited.
- Molecular Sieve as Catalytic Material: Molecular sieve has a unique regular crystal structure, each of which has a certain size and shape of the pore structure, and has a large specific surface area. Most of the molecular sieve has strong acid centers on the surface, and there is a strong Coulomb field in the pores for polarization. These properties make it an excellent catalyst. When molecular sieve is used as a catalyst or catalyst carrier, the progress of the catalytic reaction is controlled by the size of the pores of the molecular sieve. Both the size and shape of the pores can play a selective role in the catalytic reaction. Under general reaction conditions, molecular sieve plays a leading role in the direction of the reaction and exhibit shape-selective catalytic performance, which makes molecular sieve has strong vitality as a new catalytic material.
- Prasenjit Saha. Molecular Sieve Zeolites [J]. Trans. Indian Ceram. Soc., 2014, 36, 99-123.