Zeolite molecular sieve is a type of aluminosilicate crystal with framework structure.Cation ions and water molecules of the crystal has high freedom of moving in the framework. They can do exchange of cation ions and reversible dehydration.
The chemical composition of molecular sieve as below,
M2/nO. Al2O3. xSiO2. yH2O
M is metal ion ,
n is the valence of M ,
x is the molecule number of SiO2.,It is also the molecule ratio of SiO2/Al2O3
y is the molecule number of water
Above chemical formula could be changed to below formula
Mp/n[(AlO2)p()q] yH2O
P is the molecule number of AlO2
q is the molecule number of SiO2
M,n,y is as same as above.
From above chemical formula, we can know that each Alumina atoms and Silica atoms has two oxygen atoms. If the chemical valence of the metal atoms M is 1, then the atoms number of M is equal to Alumina atoms. If n is 2, then the atoms number of M is half of alumina atoms. The difference of serious type of molecular sieve is firstly the difference of the chemical composition. The important difference of chemical composition is the difference of the molecular ratio of silicon and alumina.
Zeolite A silicon/alumina 1.5-2
Zeolite X silicon/alumina 2.1-3.0
Zeolite Y silicon/alumina 3.1-6.0
When the value of x is different, the anti-acid ability, thermal stability and catalyzing activity of the molecular sieve is different. Normally, the bigger, the higher.
Structural matrix of the molecular sieve– tetrahedron
The most basic structure unit of molecular sieve is the tetrahedron of silicon oxygen and alumina oxygen. Silicon is +4, Oxygen is -2, so the tetrahedron of SiO4 can be showed as plan (2-1 a). Actually the four chemical bonds of the silicon atoms become angle to each other in space. So it can be showed as drawing 2-1(b) , the black dot is Si, and the big circle around it is Oxygen atom. We can know that the volume of Oxygen atoms is bigger than Silicon atoms.
In the tetrahedron of AlO4, Al is +3, so this tetrahedron has negative charge. In drawing 2-1, the metallic ion is to keep electrical neutral.
The Si and Al atoms is usually show as T, the distance of T-O and O-O is different.
Si-O = 1.61 A
Al-O = 1.75A
OSi-OSi =2.63 A
OAl-OAl = 2.86A
The structure of molecular sieve of type A
The space group of type A molecular sieve is Pm3m. The parameter of unit cell a0 is 12.29A. Its structure consists of three different component are zeolite frame,cation to balance the skeleton charge and absorbing molecules.
The frame structure of type A molecular sieve
The frame structure of type A is similar to the NaCl crystal structure. In crystal of NaCl,the sodium and chloride ions are located on the eight top angles of the cube. If we use β cage to replace all sodium and chloride ions and connected the next two β cage with oxygen bridge through four-membered ring, it will be the frame structure of type A molecular sieve.
Eight β cages connected each other can get a new cage, we call it α cage which is a flat-cut cubic octahedron formed with 12 four-membered ring,8 hexatomic ring and 6 octatomic ring. It has 26 sides and 48 top angles totally. α cage is larger than β cages.The average diameter of α cage is 11.4 A. The cavity volume is 760 A3.Its saturated capacity is equal to 25 water molecule or 19-20 NH3 or 12 CH3OH or 4 C4H10.
Type A molecular sieve with same cation has some change of aperture of molecule.In crystal frame structure Si:Al=1,as it can note became Al-O-Al bond,tetrahedra of SiO4 and AlO4 cross each other.Due to the cross distribution of the silicon oxygen tetrahedra and aluminium oxygen tetrahedra and the different size of the tetrahedras, type A molecular sieve frame structure hasn’t ideal symmetry.
Though type A zeolite frame is rigid relatively,it can also be reversed.At hydration,the pond of Si-O and Al-O has some increase because water and oxygen frame generate H pond.
The chemical structure of molecular sieve is very complex.We need keep studying and get more new information about it.
