Silica and its forms

    Since decades ago, studies on the development of silica have been studied by many researchers. Silica, which initially was only known to us as a raw material for making glass, and ceramics is now starting to be recognized to be a material with various applications.

    Researchers from around the world have flocked to research silica. Starting from silica from commercial materials, even those sourced from natural materials. Processing silica from natural materials in Indonesia is a promising thing. Because we know silica in Indonesia is abundant. In addition, silica has also been developed into various morphologies and sizes, according to the application. The following is an example of silica processing from geothermal sludge that has been developed by Muljani et al. (2014), a lecturer at UPN "Veteran" East Java (Figure 1). Mesoporous silica gels were successfully produced from geothermal sludge by alkali extraction followed by acidification. The silica in the geothermal sludge was dissolved by NaOH solution to produce a sodium silicate solution, which was then reacted with HCl or tartaric acid to produce silica gels. The effects of silica concentration and pH on the silica gel properties were investigated. In addition, an improved method was proposed by incorporating two-step aging. The first aging step, which was conducted at pH 10, was used to induce Ostwald ripening to increase the size of the primary particles, and the second step was used to strengthen the gel network. Decreasing the silica concentration by diluting the as-prepared sodium silicate solution tended to increase the surface area and pore volume of the prepared silica gels. The silica gels produced by tartaric acid possessed higher surface area and pore volume than those by HCl. The pore diameter for all cases was greater than 5 nm, indicating that the silica gels were mesoporous. In full, the paper can be traced via the following link (https://www.sciencedirect.com/science/article/abs/pii/S0921883114001502).

                    Figure 1. processing of geothermal sludge into mesoporous silica 

                    (https://www.sciencedirect.com/science/article/abs/pii/S0921883114001502) 

    In addition, the processing of silica from natural materials had also been carried out by Fauziyah et al. (2019). Silica was extracted from local beach sand. By using a simple process, silica was obtained with three polymorphs (quartz, amorphous, and cristobalite) (Figure 2). This difference in structure will greatly determine the properties of the silica. Please visit (https://onlinelibrary.wiley.com/doi/abs/10.1002/app.47372 and https://link.springer.com/article/10.1007/s12206-017-0703-2) for detail.

                                        Figure 2. SEM images from polymorphs silica 

                                (https://onlinelibrary.wiley.com/doi/abs/10.1002/app.47372 and                                                                    https://link.springer.com/article/10.1007/s12206-017-0703-2)

    In addition, the use of silica had also been carried out by Nugraha et al. (2021) as a mesoporous silica-based material. Indonesian kaolin was successfully transformed into aluminosilicates (ALS) via two-step hydrothermal synthesis using different structure-directing agents (SDA). The effects of structure, porosity and catalytic activity of ALS were determined for deoxygenation of bio-oil into green diesel. The first hydrothermal step used silicalite and tetrapropyl ammonium hydroxide (TPAOH) as SDA, followed by the addition of cetyltrimethyl ammonium bromide (CTAB) in the second hydrothermal step to induce mesopores. Silicalite formed ZSM-5 with hierarchical structures meanwhile TPAOH produced ZSM-5 mainly with microporosity. ZSM-5 framework was rapidly formed when using TPAOH preventing the formation of mesostructure in the second crystallization processes (Figure 3). In the absence of SDA, the aluminosilicate was characterized as mesoporous Al-MCM-41. At similar Si/Al ratios, Al-MCM-41 exhibited high surface area, mesopore volume and surface acidities than ZSM-5, consequently enhancing the conversion and selectivity of deoxygenation reaction towards long-chain (C11-C18) green diesel hydrocarbon. The complete paper can be accessed at the following link (https://www.sciencedirect.com/science/article/abs/pii/S1387181121000433).

Figure 3. Micro/mesopore structures of aluminosilicates from Indonesian kaolin.

Based on the brief description above, our view of silica will be wider and open up possible applications that can be carried out in the future.