Greetings to all my fellow Hivians! Hope you guys are doing great. Today, I shall be discussing about one of a group of minerals with high cation exchange property that helps to improve soil fertility and water retention of plants in agricultural systems. I shall go to their uses and benefits later. I am specifically talking about zeolites in this post. Zeolites are three dimensional crystalline aluminosilicates with pores of molecular dimensions. They contain alkali and alkaline earth metals and shows lability towards ion exchange and reversible dehydration. The general formula of zeolite is quite complex and is Mx/n[(AlO2)x(SiO2)y].mH2O. Here, M is the metal or hydrogen ion occupying valency n in the exchangeable cationic sites of the zeolite framework.
The structural feature of a zeolite molecule consists of a 3D tetrahedral framework having each oxygen atom shared by two tetrahedra. The fundamental units are AlO2 and SiO2 that shares oxygen ions to form tetrahedral AlO4 and SiO4 which serves as the building blocks for zeolite unit cell. If all the tetrahedron units contains silicon then the framework would be neutral. Substitution of aluminium for selection creates a charge imbalance. Since silicon (Si) ion has ‘+4’ and aluminium (Al) has ‘+3’ charge, thus there is a overall negative charge on the aluminosilicate frame work. There comes the cationic charge of the metal or the hydrogen ion which balances the negative charge on the aluminosilicate framework. Zeolites are to some extent similar to feldspar minerals but the catch is zeolites have larger cavities than those possessed by feldspar and they generally have water molecules.
Polymerisation of AlO4 and SiO4 tetrahedral units forms the aluminosilicates which in turns forms sheet like polyhedral. The polyhedral could vary form cubes, hexagonal prism to truncated octahedron. These 3D tertiary building blocks are then arranged to form superstructures inside which pores and supercages exists. A window size aperture is present in the supercage like structure that can block entry of sufficiently large molecules. This blocking of larger molecules is knows as the sieve effect. The pores on the other side are oriented in one, two or three directions leading to the formation if 1D, 2D, 3D structures.
Zeolites are classified based on their pore diameter and ring size. Zeolites 3A, 4A, 5A and erionite that contains 8 number of rings have pore diameter in the range if 3-5 Å. ZSM 5 and Modenite with 10 and 8 number of rings respectively have pore diameter again in the range of 3-5 Å. Faujasite X and Faujasite Y both have 12 number of rings and they have larger pore diameter in the range of 7-8 Å. Aluminophosphates (ALPOs) on the other hand have comparatively larger pore size containing 12 rings to about 10 Å while 18 ring ALPOs have a pore diameter of about 10-15 Å.
Preparations of Zeolites
Zeolites are primarily synthesized by crystallization process at a temperature range of 90-180°C and 1-10 atm pressure. The pH is maintained above 10. Reactive forms of silicon, aluminium, sodium and other organic templates are used during crystallization. The typical time for crystallization process varies from 16-36 hours. The seed crystals are added to the reactor to initiate the crystallization process. Organic templates are added to facilitate the formation of pores and supercages. A typical organic template is an organic amine or alkyl ammonium compound. The crystallization of the gels around the template molecular mold produces the porous network. Various zeolites are synthesised by varying the conditions such as temperature, pH, crystallization duration, order of mixing and amount of Si, Al, Na and water. We shall now discuss the preparations of two of the very important zeolites i.e., ZSM-5 and NaX zeolite.
Preparation of ZSM-5 zeolite
To prepare ZSM-5 firstly a hydrogel is prepared. This hydrogel can be prepared by addition of a mixture of aluminium nitrate and colloidal silica to a stirred mixture of tetrapropyl ammonium bromide and sodium hydroxide solution. Then this hydrogel is transferred to a stainless steel autoclave with a teflon lining and is placed in an oven for appropriate period. After completion of the crystallization, the autoclave is cooled down and the samples are washed off with water and then again dried at 120°C for 24 hours. In the final step, the sample is calcinated at 500°C for 16 hours to remove the organic base occluded in the zeolite network. Protonation of the compound is being done in a solution of HCl at room temperature for 24 hours and then dried again at 393K.
Preparation of NaX zeolite
In the case of NaX zeolite, sodium silicate and sodium aluminate are prepared separately. Silica gel and aluminium isopropoxide is used as the starting material for silica and alumina respectively. A mixture of silica gel, sodium hydroxide and deionised water is added to a beaker and stirred until the solids are completely dissolved to give the sodium silicate. Simultaneously, sodium aluminate is prepared by adding aluminium isopropoxide, sodium hydroxide and distilled water and stirred below 80°C until the solids are dissolved to form a clear gel. Then the mixture is cooled at rooms temperature. Finally, the two prepared solutions are mixed with each other with additional amount of water. The final mixture is stirred until a homogeneous solution and then placed in an oven for 24 hours at 90°C. After 24 hours, the mixture is cooled at room temperature giving the white zeolite crystals. The crystals are washed thoroughly with water, filtered and air dried.
We shall discuss the properties and applications of zeolites in my next posts to come. I am wrapping up my post for today and will catch you guys soon :)