## 4 Future Perspectives
### Eggshell Waste in Catalytic Applications  
The use of eggshell as catalyst systems are outlined across four categories, as illustrated in Figure 10:  
1. Biodiesel production: The search for biodiesel as alternatives to conventional fossil fuels is supported by the increasing rise of global warming and energy crises. Biodiesels are produced by transesterification of triglycerides with methanol using catalysts at various conditions (reaction time, type and ratio of starting material, and catalyst loading), but it is reported that the role the calcium oxide content and catalyst surface area are very important in catalytic activity. A commonly used heterogeneous catalyst is CaO, which can be obtained from different sources such as eggshell or ashes [167]. It was reported that 95% biodiesel yield was obtained when the calcination of ES is performed above 800 \({}^{\circ}\)C [188], while a yield of 90% and reusability of the catalyst up to six times without significant loss in activity [123]. In 2010, investigation on the use of quail and chicken eggshell for the production of biodiesel and the quail eggshell was reported to provide better catalytic activity [189]. Another study reported a yield of 100% biodiesel from used cooking oil [190].
2. Hydrogen gas synthesis: A cleaner alternative fuel that yields less pollution is desirable because CO\({}_{2}\) is a major greenhouse gas released through anthropogenic activities. Thus, H\({}_{2}\) is receiving greater attention and its production through gasification is a research topic of interest. Gasification of carbonaceous material can be significantly improved using catalyst [191]. The addition of eggshell as catalyst suppressed the production of CO\({}_{2}\), due to adsorption by CaO, which also promotes H\({}_{2}\) generation by the water gas shift reaction [192].
3. Industrial chemical production: Less toxic chemicals such as dimethyl carbonate, oximes, and glycerol oligomers used in the methylation reaction and other organic synthesis are replacing the more toxic ones like dimethyl halides and dimethyl sulfate. Successful dimethyl carbonate synthesis was performed using calcined eggshell as the catalyst. Transesterification of propylene carbonate and methanol was done, where 75% DMC yield was obtained. It was reported that ESP showed similar activity to pure CaO.
4. Synthesis of bioactive compounds: Bioactive compounds are used in cosmetics, pigments, and biodegradable agrochemicals. The use of catalysts based on eggshell to synthesize bioactive compounds like chromenes, pyran derivatives, and aromatic aldehydes were reported previously. ES have been used in the synthesis of 2-aminochronemes and pyrano[4,3-b]pyrans. These compounds possess antiviral, anticarcinogenic, and antifungal activities [193; 194].  
The utility of eggshells in photodegradation of organic pollutants such as dyes share features that are important in various aspects of catalysis, such as the key role of adsorption in the case of photocatalytic degradation of dye. The photodegradation process is illustrated in Figure 11 for methylene blue and toluidine blue, which involves the role of dye adsorption onto calcium oxide derived from ES waste. The role of adsorption-based processes are further revealed in Section 4.2.