Structures and Preparation Methods for Commonly Used Silicon Sources

1 Structures and Preparation Methods for Commonly Used Silicon Sources 

Many zeolites were crystallized from strongly basic solution containing silicate and aluminate. In strongly basic solution, silicate exists as polymerized-state polysilicates with a variable distribution of molecular weight. Aluminate reacted with polysilicate to form various zeolite structures via condensation reaction under hydrothermal or solvothermal conditions. Therefore, in the study of the synthetic chemistry of zeolites, one of the key issues is to understand the existing state and reactivity of the polymerized silicate. 

Studying the polymerization state of the silicate in the solution during the formation of zeolites is a very important and complex issue. After years of study, it has been found that some correlations exist between the polymerization state of the silicate in the solution and the structure type of zeolites, while the polymerization state and its distribution could be affected by many factors such as alkalinity, nature of the cation, organic templates, temperature, and pressure. These parameters could partially determine the condensation rate between polymerized silicate and aluminate, which will finally affect the formation of zeolite structure. 

The following sections describe, as an example, the Na20-Al203-SiC 2-H20 four-component crystallization field diagram. 

Most syntheses of zeolites were conducted in strongly basic media. The silicate solution containing alkali metal cations such as Na+ or K+ is one of the main silicon sources in the synthesis of zeolites. The concentration of the basic species in the starting material for the synthesis of zeolites is usually about 0.5 5.5 mol/L. Within this concentration range, aluminate and silicate ions with various polymerization degrees existed in the system. The state and distribution of these ions could be affected by many factors. When the temperature and the concentration of SiC 2 were kept constant, the acid-base 

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