## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.2 Temperature
[75] as indicated above, which may be due to the catalyst used (sulfonic acid-functionalized amorphous silica (SAS) and mesoporous silica (SSBA)). [PERSON] et al. [101] reported the effect of temperature on the esterification of glycerol with lauric acid and oleic acid. Results revealed that as the temperature increased from 100 to 150 \({}^{\circ}\)C, the rate of esterification increased for both agents with decreased selectivity towards monoacetin and increased selectivity towards di- and triacetin due to a secondary reaction. By optimizing the esterification reaction of glycerol with acetic acid, the influence of temperature was investigated by [PERSON] et al. [80] over the amberlyst-35 catalyst. The temperature range investigated was from 95 to 115 \({}^{\circ}\)C at a fixed acetic acid to glycerol molar ratio of 6, with a catalyst weight of 0.5 g. Almost 100% glycerol conversion was achieved after 4 h. The selectivity to mono- and diacetin decreased (55.5 to 47.3% and 28.0 to 24.3%) while triacetin increased (16.5 to 28.3%) with an increase in temperature. This performance was attributed to a consecutive esterification reaction. Similarly, [PERSON] et al. [140] reported an increased glycerol conversion with increased temperature (100-120 \({}^{\circ}\)C) over a sulphuric acid catalyst, however, above 118 \({}^{\circ}\)C, the conversion reduced due to the evaporation of the acetylating agent (acetic acid). The increased temperature also increased the selectivity to mono-, di-, and triacetin. The authors further reported that for every 5 \({}^{\circ}\)C rise in temperature, the triacetin selectivity increased. The increase from 1.96% to a maximum of 13.69% at 115 \({}^{\circ}\)C was recorded. However, a further increase in the temperature reduced triacetin to 12.93%, which was attributed to the evaporation of acetic acid. The use of iron oxide supported on mesoporous aluminosilicate (Fe/Al-SBA-15) catalyst at 100 \({}^{\circ}\)C did not show any glycerol conversion even after 8 h of reaction until at 120 \({}^{\circ}\)C when the quantitative conversion of glycerol was achieved (>99%) with a selectivity of 71% diacetin and 28% triacetin. Further increase in temperature to 140 \({}^{\circ}\)C led to a slight improvement of diacetin (80%), and a reduction of triacetin (20%) which was attributed to the low thermal stability of the triacetin. However, the acetylating agent used was levulinic acid (1:4) in a continuous stirring (1200 rpm) reactor. The formation of diacetin as the major product could also be explained based on the lower activity of the secondary hydroxyl group, thereby preventing further reaction to form triacetin. Meanwhile, monoacetin was not observed in the product due to the equal activity of the two terminal hydroxyl groups. In another study, [PERSON] et al. [82] also reported a substantial increase in glycerol conversion from 65 to 100% with increased temperature from 90 to 110 \({}^{\circ}\)C. Further increase in temperature did not improve the conversion. The selectivity to triacetin was found to depend on temperature. At lower temperatures (<90 \({}^{\circ}\)C), only about 7% of triacetin was produced, but when it was increased to 110 \({}^{\circ}\)C the triacetin increased to 55%. Further increase in temperature to 120 \({}^{\circ}\)C does not affect the triacetin selectivity. Similar findings have been reported by [PERSON] et al. [141] and [PERSON] et al. [142]. However, contrary to the above, [PERSON] et al. [125] revealed that increasing the temperature from 80 to 120 \({}^{\circ}\)C increased the glycerol conversion from 25 to 85%. However, further increase to 140 \({}^{\circ}\)C did not show any appreciable enhancement. The selectivity to mono-, di-, and triacetin followed the same pattern as reported previously. Di- and triacetin increased continuously with an increase in temperature, while monoacetin decreased, and this observation was related to the increased activity of the biochar.