## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.1 Catalyst and Catalyst Load
The reusability of 20% (_w/w_) H\({}_{2}\)SO\({}_{4}\)/K10 catalyst was fairly good because, after three reaction cycles, only the partial deactivation of the catalyst was noticed, which was partly attributed to coke deposition and a loss of active acid sites. In a related study, the organic acids-treated montmorillonite clay performed better as a catalyst in acetylation than the untreated [87]. The organic acids used include methanesulfonic acid (MSA), p-toluenesulfonic acid (pTSA), and phenoldisulfonic acid (PDSA), and over 90% glycerol conversion was obtained in each case when compared with the untreated montmorillonite clay with only about 60% glycerol conversion. The selectivity towards triacetin was favoured with the organic acids-treated clay in the following order: PDSA > MSA > p-TSA > untreated clay. The performance was attributed to the enhancement of the catalyst's acidity and pore characteristics (increase in pore volume, especially around the acid sites) arisen from the organic acid's treatment. In another study, the modification of montmorillonite with metals enhanced the selectivity towards diacetin. The lanthanum cation-exchanged montmorillonite (La\({}^{3+}\)-mont), was found to be efficient with 98% glycerol conversion and selectivity of > 99% towards diacetin only. However, when the La\({}^{3+}\) was substituted with Ce\({}^{4+}\), Sc\({}^{3+}\), Ti\({}^{4+}\) and Fe\({}^{3+}\), selectivity towards monoacetin and triacetin improved at 120 \({}^{\circ}\)C, glycerol-to-acetic acid molar ratio of 1:2, and reaction time of 24 h [109] which is not cost effective. Metals and metal oxides mostly used in catalysis in recent times are multi-metals or multiphase oxides and are of transition metal origin. Because of the synergy of various components, the resultant metals and metal oxides catalysts usually exhibit Bronsted and Lewis acid sites, stability, regeneration capacity, and were found to be active over a wide range of temperatures [120,121]. Hence, researchers have also deployed these catalysts in glycerol acetylation with different findings. The use of Cu and Ni metals and Cu-Ni bimetals supported on \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) were compared with sulphuric acid-treated alumina (H\({}_{2}\)SO\({}_{4}\)/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) catalysts in the esterification of glycerol with acetic acid at 110 \({}^{\circ}\)C, atmospheric pressure, the molar ratio of 1:9, and the catalyst loading of 0.25 g [122]. The results revealed that the 2 M H\({}_{2}\)SO\({}_{4}\)/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) catalyst showed the best activity with 97% glycerol conversion and a selectivity of 27%, 49.9%, and 23.1% mono-, di-, and triacetin after a 5 h reaction time. The catalyst was also reported not to lose activity even after three consecutive runs. The performance of the 2 M H\({}_{2}\)SO\({}_{4}\)/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) catalyst was attributed to the high acid strength of 2.51 mmol g\({}^{-1}\) when compared to the other catalysts tested. In a similar use of bimetallic-supported catalyst, [PERSON] et al. [90] reported the use of Ag-Cu-doped rice husk silica-alumina (RHS-Al)-based biomass catalyst under similar reaction conditions to the glycerol-to-acetic acid molar ratio of 1:10, 110 \({}^{\circ}\)C and a catalyst load of 0.80 g. The results showed high catalytic activity with 97.5% glycerol conversion and selectivity towards mono-, di-, and triacetin (3.7, 58.4, and 37.9%) over 1 Ag-10 Cu/RHS-Al catalyst indicating that the bimetallic catalyst performed better than the single metal catalyst and was attributed to the synergistic effect between the metals. The use of several metal oxides (Bi\({}_{2}\)O\({}_{3}\), Sb\({}_{2}\)O\({}_{3}\), SnO\({}_{2}\), TiO\({}_{2}\), Nb\({}_{2}\)O\({}_{5}\) and Sb\({}_{2}\)O\({}_{5}\)) was also investigated at the glycerol-to-acetic acid molar ratio of 1:6 and a temperature of 120 \({}^{\circ}\)C. Only Sb\({}_{2}\)O\({}_{5}\) (antimony pentoxide) resulted in good activity and selectivity towards diacetin after 1 h [123].