## 4 Modified Steel slag as Catalysts
### Transesterification
Therefore, the catalytic activity and the number of recyclable uses of catalysts are important reference indices. Modified steel slag enhanced the anti-leaching ability of the active center, making it reusable and stable. Secondly, the S-CaMgAlMO catalyst showed good catalytic performance. Under the conditions of 3 wt% catalyst, DMC and glycerin molar ratio of 1:3, reaction temperature of 75 \({}^{\circ}\)C, and reaction time of 90 min, the conversion rate of glycerol was 98.3% and the GC yield was as high as 96.2%. After several cycles, the conversion rate of glycerol and the yield of GC can still reach \(>\) 90%. S-CaMgAlMO also had the advantages of mild reaction conditions and low production costs, and showed great potential in the transesterification reaction. [PERSON] et al. [28] also used the modified steel slag catalyst of the ladle furnace (LF) for the transesterification reaction. They prepared the NaOH-LF catalyst by alkali modification and high temperature calcination of steel slag. During the calcination process, Na\({}^{+}\) can uniformly diffuse into the calcium oxide structure to form NaO, thus increasing the basicity of steel slag. Experimental research showed that, compared with unmodified steel slag catalyst, the catalytic performance of NaOH-LF catalyst was enhanced, and weak active sites became stronger with good reusability and stability. Under the best conditions, a glycerol conversion rate of 99% and a GC yield of 96.59% were achieved. In addition to catalyzing GC with dimethyl carbonate, glycerol can also produce hydrogen under other catalytic conditions. [PERSON] et al. [73] used titanium oxide doped alumina extracted from aluminum foil waste as a catalyst to perform photocatalytic reactions on glycerol under the condition of light. Under the optimal conditions, the yield could reach 4.7 mmol H\({}_{2}\) gTiO\({}_{2}\)\({}^{-1}\)h\({}^{-1}\) through light irradiation for 180 min. Compared with the similar catalyst prepared with industrial alumina, the new catalyst has higher photocatalytic activity. [PERSON] et al. [74] carried out the photo reforming experiment on glycerol for hydrogen production after physical mixing of CuO and TiO\({}_{2}\). The results showed that 88 mmol g\({}^{-1}\) hydrogen production products could be obtained after a 5 h reaction with the catalyst. [PERSON] et al. [75] prepared Al\({}_{3}\)O\({}_{3}\), CeO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\), MgO-Al\({}_{2}\)O\({}_{3}\), and La\({}_{2}\)O\({}_{3}\)-Al\({}_{2}\)O\({}_{3}\) catalysts using Al\({}_{2}\)O\({}_{3}\) as support for hydrogen production from steam reforming of glycerol. All the catalysts showed good stability during the reaction, and the glycerol conversion rate reached about 90%. The hydrogen yield can reach 2.4-2.9 mol H\({}_{2}\) mol\({}^{-1}\). In the previous section, we introduced the application of steel slag in the field of photocatalysis and found that the modified steel slag as a photocatalyst has potential, combined with the composition of steel slag. We can determine that the steel slag has a great potential in the field of glycerol photo reforming hydrogen production. [PERSON] et al. [76] used Fe/Mg-bearing metallurgical waste (UGSO) as raw material for the first time to prepare CaO-UGSO10/NiO mixed sorbents-catalyst material, and tested the performance of glycerol adsorption enhanced steam reforming. After 30 min of reaction, hydrogen with 95% purity was obtained, and the catalyst showed good stability. Therefore, steel slag has certain research potential in the field of hydrogen production from glycerol, both in terms of preparation cost and environmental governance. According to the above, modified steel slag catalyst has been studied extensively in the field of GC preparation by transesterification reaction, and it shows good catalytic performance.