# Transforming Petrochemical Processes: Cutting-Edge Advances in Kaolin Catalyst Fabrication
## 5 Modification Processes of Kaolin: Enhancing Industrial Applicability
### Calcination Process  
Kaolin containing an attached water molecule is classified as hydrous. The chemical formula of hydrous kaolin is ((Al2O3)(2 SiO2)(2H2O)), and this form of kaolin, also referred to as washed kaolin, is extracted from raw kaolin in the earth's crust through a process commonly known as levigation. The water molecule can be removed from hydrous clay through calcination, a process involving heating the clay to high temperatures, which enhances its color. This process produces calcined clay. By varying the calcination temperature, the following distinct phases can be achieved (Figure 7): metakaolin (550-900 \({}^{\circ}\)C), spinel (925-1050 \({}^{\circ}\)C), or mullite (>1050 \({}^{\circ}\)C) [38].  
Endothermic dehydroxylation, or dehydration, is the process by which disordered metakaolin (Al2 Si2O7) is formed through the removal of structural hydroxyl groups and the emission of water vapor [39; 40]. The dehydroxylation of hydroxyl groups in kaolinite begins at 800 \({}^{\circ}\)C, leading to the disintegration of its crystal lattice and resulting in an amorphous structure of metakaolin enriched with active alumina and silica. This dehy  
Figure 6: Schematic representation of different modification processes of kaolin.  
Figure 7: Phase transition of hydrous kaolin to mullite via calcination temperature adjustment.  
droxylation process is activated when kaolin undergoes thermal treatment at 800 \({}^{\circ}\)C and can persist up to 900 \({}^{\circ}\)C, as detailed in Equation (1).  
\[2\text{Al}_{2}\text{Si}_{2}\text{O}_{5}(\text{OH})_{4}\to 2\text{Al}_{2} \text{Si}_{2}\text{O}_{7}+4\text{H}_{2}\text{O} \tag{1}\]  
This induces the breakdown of the crystal lattice structure, resulting in the formation of an amorphous metakaolin structure, which is rich in both active silica and alumina. Research indicates that metakaolin is not merely a mixture of amorphous alumina (Al2O3) and silica (SiO2) but rather has a complex amorphous structure with hexagonal layers and some degree of long-range order that is not fully crystalline [41]. Further heating to temperatures between 925 and 950 \({}^{\circ}\)C transforms metakaolin into a phase known as aluminum-silicon spinel (Si3 Al4O12), also referred to as a gamma-alumina-type structure. This transformation is represented by the following reaction (Equation (2)):  
\[2\text{Al}_{2}\text{Si}_{2}\text{O}_{7}\to\text{Si}_{3}\text{Al}_{4}\text{O}_{1 2}+\text{SiO}_{2} \tag{2}\]  
The calcination temperature significantly affects the environment of the aluminum atoms within the metakaolin, leading to variations in the material's structure and properties, including its mechanical strength and thermal conductivity [42]. During calcination at 1050 \({}^{\circ}\)C, the spinel phase (Si3 Al4O12) nucleates and subsequently transforms into mullite (3 Al2O3-2 SiO2) and highly crystalline cristobalite (SiO2), as illustrated by the following reaction (Equation (3)):  
\[3\text{Si}_{3}\text{Al}_{4}\text{O}_{12}\to 2\text{Si}_{2}\text{Al}_{6} \text{O}_{13}+5\text{SiO}_{2} \tag{3}\]