## 4 Modified Steel slag as Catalysts
### Transesterification  
Renewable energy has been one of the hot topics in the field of energy research. Among renewable energy sources, biodiesel has attracted much attention from researchers due to its similar properties to petroleum diesel: non-toxic and harmless, higher flash point, higher lubricity, and renewable advantages [59]. Biodiesel is catalyzed by the transesterification of triglyceride and methanol, so the preparation of highly efficient, recyclable, and low-cost catalysts plays an important role in the yield of biodiesel [60]. So far, a variety of solid catalysts have been developed for transesterification reactions, such as Ca/Al/Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\)/CaO, and basic zeolite [61; 62; 63]. [PERSON] et al. [64] prepared a heterogeneous CaO/Ag nano-catalyst and applied it to the transesterification of soybean oil to prepare biodiesel. The yield of biodiesel was optimized by response surface methodology (RSM), and the final yield of biodiesel could reach over 90%. Under the same reaction conditions, the yield of biodiesel catalyzed by CaO can also reach more than 85%, and all of them have good cycling performance. [PERSON] et al. [65] prepared a novel magnetic mesoporous polyphase basic catalyst (Fe@C-Sr) for the transesterification reaction to produce biodiesel. The catalyst has the characteristics of rich source, easy separation, environmentally friendly, and high catalytic efficiency, and its conversion rate is still 91.93% after four cycles. [PERSON] et al. [66] prepared a magnetic Fe\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) composite catalyst for the simultaneous transesterification and esterification of low-cost oil into biodiesel. Under suitable reaction conditions, a 93.3% oil conversion rate and complete conversion rate of free fatty acids can be achieved, and the catalyst can be reused many times without obvious catalytic activity. Through the summary and analysis of the above studies, we can see that when preparing catalytic biodiesel catalysts, cost, efficiency, recyclability, and the impact on the environment should  
Figure 9: Nickel and calcium cementitious materials-based catalyst for the MB dye photocatalytic degradation experiments. be considered at the same time, and steel slag contains Fe\({}_{2}\)O\({}_{3}\), Fe\({}_{3}\)O\({}_{4}\), CaO, and SiO\({}_{2}\) and other active oxides. First of all, the application potential of steel slag in this field can be analyzed from its composition. Steel slag, as a kind of rich metallurgical solid waste, will greatly reduce the cost and pollution of solid waste when it is used as a catalyst for the esterification of biodiesel. Therefore, steel slag has great potential in this field. As a by-product of biodiesel [67], glycerol carbonate (GC) produced through transesterification reaction has high utilization value, such as a cement composite aggregate, lithium-ion battery carrier, and fuel additive [68]. Catalysts play a very important role in the conversion of glycerol. Nowadays, the main catalysts commonly used for transesterification are double-layer hydrol calcite based on calcium oxide, magnesium oxide, Ca or Mg, and alkaline alumina materials [69; 70; 71]. These catalysts have high catalytic performance for GC yield. Therefore, the advantages of steel slag containing many metal oxides and for a low cost can be taken advantage of, and it is considered to be a kind of transesterification catalytic material with broad prospects. [PERSON] et al. [72] used modified steel slag to catalyze the transesterification reaction of dimethyl carbonate (DMC) and glycerol to synthesize glycerol carbonate (GC). They processed the steel slag through acid modification, co-precipitation, high-temperature calcination, and alkali modification to obtain the S-CaMgAlMO catalyst. The highly basic O\({}^{2}\)- and Ca-O in the metal oxides of S-CaMgAlMO catalyst split hydrogen and oxygen on the glycerol molecule, then react with DMC to form the target product GC, and finally separate from the metal part of the catalyst.