Properties of zeolite molecular sieves
1. Adsorption performance
The adsorption of zeolite molecular sieve is a physical change process. The main reason for adsorption is a "surface force" generated by molecular gravity on the solid surface. When the fluid flows through, some molecules in the fluid collide with the adsorbent surface due to irregular movement, resulting in molecular concentration on the surface, reducing the number of such molecules in the fluid, so as to achieve the purpose of separation and removal.
Since there is no chemical change in adsorption, as long as we try to drive away the molecules concentrated on the surface, zeolite molecular sieve will have adsorption capacity again. This process is the reverse process of adsorption, which is called analysis or regeneration.
Because the pore diameter of zeolite molecular sieve is uniform, it can easily enter the crystal cavity and be adsorbed only when the molecular dynamics diameter is less than the pore diameter of zeolite molecular sieve. Therefore, zeolite molecular sieve is like a sieve for gas and liquid molecules, and whether it is adsorbed is determined according to the size of molecules.
Due to the strong polarity in the crystal cavity of zeolite molecular sieve, it can have a strong effect with the molecules containing polar groups on the surface of zeolite molecular sieve, or induce the polarization of polarizable molecules to produce strong adsorption.
This polar or easily polarized molecule is easily adsorbed by polar zeolite molecular sieve, which reflects another adsorption selectivity of zeolite molecular sieve.
2. Ion exchange performance
Generally speaking, ion exchange refers to the exchange of compensation cations outside the sieve frame of zeolite molecules. The compensation ions outside the zeolite molecular sieve frame are generally protons and alkali metals or alkaline earth metals. They are easily exchanged into metal ion zeolite molecular sieves of various valence states in the aqueous solution of metal salts.
Ions are easy to migrate under certain conditions, such as aqueous solution or high temperature. In aqueous solution, due to the different ion selectivity of zeolite molecular sieve, it can show different ion exchange properties. The hydrothermal ion exchange reaction between metal cations and zeolite is a free diffusion process. The diffusion rate restricts the exchange reaction rate.
The pore size of zeolite molecular sieve can be changed by ion exchange, so as to change its performance and achieve the purpose of shape selective adsorption and separation of mixture.
After ion exchange, the number, size and position of cations in zeolite molecular sieve change. For example, the number of cations in zeolite molecular sieve decreases after the exchange of high valence cations with low valence cations, which often leads to the vacancy of position and the increase of pore size; However, when the ions with larger radius exchange the ions with smaller radius, the holes are easy to be blocked and the effective pore size is reduced.
3. Catalytic performance
Zeolite molecular sieves have a unique regular crystal structure, each of which has a certain size and shape of pore structure, and has a large specific surface area.
Most zeolite molecular sieves have strong acid centers on the surface, and there is a strong Coulomb field in the crystal pores for polarization. These characteristics make it an excellent catalyst.
Heterogeneous catalytic reaction is carried out on solid catalyst, and the catalytic activity is related to the crystal pore size of the catalyst. When zeolite molecular sieve is used as catalyst or catalyst carrier, the catalytic reaction is controlled by the crystal pore size of zeolite molecular sieve. The size and shape of crystal pores and channels can play a selective role in the catalytic reaction. Under general reaction conditions, zeolite molecular sieve plays a leading role in the reaction direction and presents shape selective catalytic performance, which makes zeolite molecular sieve have strong vitality as a new catalytic material.