# Transforming Petrochemical Processes: Cutting-Edge Advances in Kaolin Catalyst Fabrication
## 5 Modification Processes of Kaolin: Enhancing Industrial Applicability
### Acid-Base Chemical Treatment
#### 5.3.1 Acid Activation  
Acid-activated kaolinite is widely utilized in adsorption and ion exchange due to its enhanced properties post-activation, but also for increasing the catalytic properties and adsorption capacity due to the acid activation process improves the acidity, surface area, pore size, and volume of kaolinite [46; 47]. This treatment leads to dealumination, the removal of mineral impurities, the disaggregation of kaolinite particles, and the dissolution of the external layers, altering both the structure and chemical composition of the material [48]. As a result, acid-activated kaolinite is an excellent precursor for synthesizing solid acid catalysts used for catalytic cracking in petrochemical processes [49]. The activation process not only increases the porosity of kaolinite but also enhances its acid centers and surface area, making it suitable as an inorganic host for intercalation and exfoliation. Acid activation involves the protonation of aluminol (AlOH) groups by hydrogen ions from the acidic medium, which leads to dealumination and an increase in the Si/ Al ratio of the synthesized materials. This process facilitates absorption by increasing the cation exchange capacity (CEC), pore volume, and surface area [50; 51], thereby enhancing the material's ability to physically absorb water. However, high concentrations of acid can reduce the structural and coordinated water in kaolinite, which is reflected in increased endothermic peaks during recrystallization and dehydrogenylation. Regarding solubility, several factors influence this parameter, such as the type of acid used, kaolinite-to-acid ratio, operating temperature, leaching period, and particle size. Generally, kaolinite solubility increases with higher acid concentrations and longer leaching periods, although excessive leaching can decrease the surface area [52]. Inorganic acids are more effective than organic acids in generating new surface acid sites, although they may cause the kaolinite structure to collapse due to excessive leaching of the octahedral layer. Conversely, organic acids preserve the structure of kaolinite better but are less effective at generating new acid sites. Specific surface area and pore volume increase with higher molarity of the activator and longer activation times. Under the same conditions, kaolinite solubility in HCl is less than in H2 SO4, with the order of solubility and subsequent increase in surface area as follows: CH3 COOH < H3 PO4 < HCl < H2 SO4 < HClO4 < HNO3 [48; 52; 53]. Recent interest has focused on synthesizing solid acid zeolites from kaolinite for the petrochemical industry, as the process is cost-effective and produces zeolites with enhanced pore size and structure compared to conventional zeolites [17; 54; 55]. Both organic and inorganic acids can chemically activate kaolinite, highlighting its potential as a precursor for zeolite synthesis in converting heavy molecules in petrochemical processes [56]. The process requires higher calcination temperatures (550 to 950 \({}^{\circ}\)C) to effectively activate kaolinite and obtain metakaolin due to the strong hydrogen bonds between its layers, which make the material resistant to chemical attack. The reactions involved in the dealumination process are as follows (Equations (6) and (7)):  
\[3\text{H}_{2}\text{SO}_{4}+\text{Al}_{2}\text{Si}_{2}\text{O}_{5}(\text{OH}) _{4}\rightarrow\text{Al}_{2}(\text{SO}_{4})_{3}+2\text{H}_{4}\text{SiO}_{4} +\text{H}_{2}\text{O} \tag{6}\]  
\[3\text{SO}_{3}+\text{Al}_{2}\text{O}_{3}\rightarrow\text{Al}_{2}(\text{SO}_{4 })_{3} \tag{7}\]  
Research has shown that kaolin's strong passivity makes it less susceptible to the effects of acid treatment alone. However, metakaolin, which is produced by heating kaolin, shows increased reactivity when subjected to acid treatment. Calcined kaolin clay, when converted to metakaolin and subsequently treated with hydrochloric acid (HCl) under reflux conditions, can achieve high surface areas, even with minimal alumina removal.