## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.2 Temperature  
Temperature is an important factor that influences the equilibrium of the reaction, which also has a significant impact on the rate of reaction, glycerol conversion, and product selectivity. It is generally reported that the acetylation of glycerol with acetic acid is mainly an endothermic reaction process, and hence requires a large amount of heat [61]. However, since the reaction is catalyst driven, care is taken to avoid the deactivation of thermally unstable catalysts by using moderate temperatures [80]. A good number of researchers have reported the influence of temperature in glycerol acetylation, and their findings are reviewed below. [PERSON] and [PERSON] [110] varied the temperature from 50 \({}^{\circ}\)C to 110 \({}^{\circ}\)C in the esterification of glycerol with acetic acid in a stirred batch reactor equipped with a reflux system, and the results showed a slight increase in glycerol conversion with an increase in temperature. The authors further reported that diacetin and triacetin were the major products at high temperature while monoacetin was the predominant product at a lower temperature. In a similar study, using a carbon-synthesized catalyst from crude glycerol, [PERSON] et al. [114] reported that at 100 \({}^{\circ}\)C, the glycerol conversion was almost 100% with the corresponding selectivity of 50.97%, 45.98%, and 3% towards mono-, di-, and triacetin, respectively. However, when the temperature increased to 105, 110, 115, and 120 \({}^{\circ}\)C, the monoacetin selectivity decreased, while there was a corresponding increase in diacetin at 105 and 110 \({}^{\circ}\)C, respectively. However, at 115 and 120 \({}^{\circ}\)C, the triacetin selectivity increased while diacetin and monoacetin continued to decline. The highest combined selectivity of the di and triacetin of 88% was obtained at 110 \({}^{\circ}\)C, with a 2 wt% catalyst load, the glycerol-to-acetic acid molar ratio of 1:3, and in 3 h. The authors explained that the increased temperature enhanced the endothermic process, which further promoted the deprotonation of the second and third hydroxyl groups of glycerol over the active sites of the catalyst, hence promoting the formation of di- and triacetin. Similar results were reported by [PERSON] et al. [75]. The di- and triacetin selectivity of 54 and 16% were obtained when the temperature was increased up to 120 \({}^{\circ}\)C over a sulfonated carbonized rice husk catalyst. However, the maximum glycerol conversion was achieved at 150 \({}^{\circ}\)C and subsequently decreased when the temperature was further increased to 180 \({}^{\circ}\)C. This observation was attributed to the non-availability of the acetic acid \(>\) 150 \({}^{\circ}\)C due to vaporization. It was concluded that the suitable temperature to improve the esterification of glycerol with acetic acid to di- and triacetin was 120 \({}^{\circ}\)C. [PERSON] et al. [92]  
Figure 9: Influence of various catalyst loads on glycerol conversion (GC) and selectivity to monoacetin (MA), diacetin (DA) and triacetin (TA) using different catalysts at various reaction conditions (A = SSBA, glycerol (G):acetic acid (AAc) 1:3, T = 105 \({}^{\circ}\)C, t = 3 h [92]; B = glycerol-based carbon, G:AAc 1:4, T = 115 \({}^{\circ}\)C, t = 1 h [126]; C = cesium phosphothungstate (CsPWA), G:AAc 1:8, T = 85 \({}^{\circ}\)C, t = 2 h [89]; D = 2 M H\({}_{2}\)SO\({}_{4}\)/Al\({}_{2}\)O\({}_{3}\), G:AAc 1:9, T = 110 \({}^{\circ}\)C, t = 5 h [106]; E = glycerol-based carbon, G:AAc 1:3, T = 110 \({}^{\circ}\)C, t = 3 h [114]. reported that at 60 \({}^{\circ}\)C, the glycerol conversion was 44% in 3 h with monoacetin as the main product. However, when the temperature was increased to 105 \({}^{\circ}\)C, the glycerol conversion was complete with 33% triacetin after 3 h of the reaction. Further increase in the temperature up to 120 \({}^{\circ}\)C did not change both the glycerol conversion and selectivity towards triacetin, contrary to the findings of [PERSON] et al.