## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.3 Reaction Time  
For any reaction to reach equilibrium, the period of contact of the reactants with the catalyst plays an important role. Reports in the literature also showed the influence of time in glycerol acetylation. [PERSON] and [PERSON] [110] reported that in glycerol acetylation with acetic acid at a molar ratio of 1:3, 50 \({}^{\circ}\)C, and 200 mg catalyst load (10% sulphated silica), monoacetin was favoured at a lower reaction time (2 h). However, higher acetins were favoured as the reaction proceeds with time, attributable to the continuous conversion of monoacetin to diacetin and triacetin. The glycerol conversion increased from about 60% in 2 h to almost 100% in 6 h. The effect of time in glycerol conversion was also reported by [PERSON] et al. [114] over glycerol-based carbon catalyst and concluded that the glycerol conversion increased with an increase in reaction time from 1 to 3 h. In 1 h, about 70% glycerol  
Figure 10: Influence of temperature on glycerol conversion (GC) and selectivity to monoacetin (MA), diacetin (DA) and triacetin (TA) using different catalysts at various reaction conditions (A = amberlyst-15 (Dry), glycerol (G):acetic acid (AAc) 1:9, catalyst load (CL) = 2.645 g, t = 5 h [83]; B = propyl-SO\({}_{3}\)H-functionalized mesoporous silica SBA-15 (SSBA), G:AAc 1:3, CL = 50 mg, t = 3 h [92]; C = SO\({}_{4}\)\({}^{2}\)–12 ZrKIL-2, G:AAc 1:10, CL = 0.1 g, t = 2 h [105]; D = CsPWA, G:AAc 1:8, CL = 7 wt%G, t = 2 h [89]; E = 1 Ag-10 Cu-rice husk silica–Al\({}_{2}\)O\({}_{3}\), G:AAc 1:10, CL = 80 mg, t = 5 h [90]. conversion was achieved with high monoacetin selectivity, but in 3 h, the glycerol conversion increased to almost 100% with higher di- and triacetin selectivity but low monoacetin selectivity. [PERSON] et al. [126] also reported the effect of the reaction time on the esterification of glycerol with acetic acid (1:4) in the presence of 5 wt % glycerol-based carbon catalyst at 115 \({}^{\circ}\)C for various time of 1 to 4 h. Results indicate a complete glycerol conversion within 1 h with a selectivity of 22% monoacetin, 67% diacetin, and 11% triacetin, respectively. When the reaction time was increased up to 4 h, the triacetin increased to 25%, with a corresponding slight decrease in mono- and diacetin. Similar findings were reported by [PERSON] et al. [82] over the 3% yttrium on mesoporous silicate material support (3%/SBA-3), where the glycerol conversion was 62% after 0.5 h, and the conversion was completed after 1 h. The selectivity to monoacetin decreased, while the diacetin increased with time. Additional findings showed that triacetin commenced after 0.5 h into the reaction, which was attributed to the direct acetylation of monoacetin, and after 1.5 h, some of the produced diacetin also transformed into triacetin, hence the observed decrease in diacetin. The authors concluded that the complete glycerol conversion with combined selectivity of di- and triacetin of 89% was achieved. The biochar catalyst also showed excellent activity with the increased reaction time to a maximum of 4 h, which in turn improved the selectivity towards di- and triacetin with a concurrent decrease in the monoacetin selectivity [125]. The use of sulfonated carbonized willow caths by [PERSON] et al. [113] also showed a similar trend. The glycerol conversion increased rapidly to 100% with the increased reaction time from 0.25 to 2 h. The extension of time up to 7 h led to a slight decrease in glycerol conversion, which the authors attributed to the drop in glycerol concentration and the accumulation of water as the by-product. However, the selectivity to higher acetin improved significantly with time. In 7 h, the combined selectivity of di- and triacetin reached 95.3%. The catalyst performance was attributed to the acid property and the nature of the surface arisen from the sulfonation, which might have provided good access to the active sites for the hydrophilic glycerol. Similar findings have been reported by several researchers despite the use of different catalysts [89; 102; 133; 144].