## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.4 The Reactants and Their Molar Ratios
The very low selectivity towards triacetin could be attributed to steric hindrance arising from the structures of the carboxylic acids (acetic acid, propionic acid, and 1-butanoic acid) because the trend indicates decreased glycerol esterification with increasing chain length. The influence of the acetic acid to glycerol molar ratio (4:1, 6:1, and 8:1) was investigated at a fixed reaction temperature of 110 \({}^{\circ}\)C over 3% Y/SBA-3 by [PERSON] et al. [82]. The best result was obtained with a molar ratio of 4:1, which resulted in the complete conversion of glycerol with the corresponding selectivity of 11% monoacetin, 34% diacetin, and 55% triacetin, respectively. The good selectivity towards triacetin at a 4:1 molar ratio was also attributed to the excellent textural characteristics, the crystal phase stability of the catalyst, and the availability of the acetylating agent. It was a general expectation that excess acetylating agents could shorten the time required to reach the reaction equilibrium and enhance the formation of higher acetins due to consecutive acetylation reactions, but that was not the case. The authors attributed it to the hydrolysis effect of the co-produced water. Contrary to the above finding, [PERSON] et al. [132] showed higher glycerol conversion at a higher molar ratio of acetic acid to glycerol. The molar ratio of 3, 5, and 10 resulted in a glycerol conversion of 67, 91, and 100%, respectively, after 8 h of reaction using a supported silicomolybic heteropolyanions catalyst at 10 wt% with respect to glycerol. The selectivity to mono-, di-, and triacetin also varies with the molar ratio. At a molar ratio of 3, monoacetin was favoured with 81% selectivity, while the selectivity to di and triacetin was 16 and 3%, respectively. However, at a molar ratio of 10, triacetin was favoured with 50% selectivity, followed by diacetin with 33% while the monoacetin decreased to 17%. This result was similar to the earlier work reported by [PERSON] et al. [126], confirming the conversion of monoacetin to the higher acetins (di- and triacetin) in line with the earlier stated mechanism of the reaction. [PERSON] et al. [89] also reported that the molar ratio of acetic acid-to-glycerol of 8 produced the highest glycerol conversion of 98% with the CsPWA catalyst. When a molar ratio of 10 was used, no noticeable change in the conversion was reported. The selectivity of the desired product also followed the same pattern. The molar ratio of 4 and 6 did not produce much of di- (35% each) and triacetin (0 and 6%) until at a molar ratio of 8 (59% diacetin and 16% triacetin), which showed that high quantity of acetic acid increased its accessibility to glycerol. This finding was corroborated by [PERSON] et al. [121] with a maximum glycerol conversion of 98.2% at an acetic acid to glycerol molar ratio of 9 over a similar catalyst. Several literature articles previously reported a similar pattern of increased glycerol conversion with a variant molar ratio [34; 83; 102; 133; 145]. Figure 12 shows the pictorial representation of some selected findings of the influence of molar ratios of the reactants on the glycerol conversion and selectivity towards mono-, di-, and triacetin as reported by some recent studies. Figure 12: Influence of glycerol to acetylating agent (G:AA) molar ratios on glycerol conversion (GC) and selectivity to monoacetin (MA), diacetin (DA) and triacetin (TA) using different catalysts at various reaction conditions (A = 3% Y/SBA, AAc = acetic acid, T = 100 \({}^{\circ}\)C, CL = 0.2 g, t = 3 h [82]; B = 20 mol%MnO\({}_{3}\)/SiO\({}_{2}\), AAc, T = 100 \({}^{\circ}\)C, CL = 10 wt%G, t = 8 h [132]; C = CsPWA, AAc, T = 85 \({}^{\circ}\)C, CL = 7 wt%Gly, t = 2 h [89]; D = CsPWA, AAh = acetic anhydride, T = 30 \({}^{\circ}\)C, CL = 4 wt%G, t = 2 h [89]; E = 1 Ag-10 Cu-rice husk silica–Al\({}_{2}\)O\({}_{3}\), AAc, T = 110 \({}^{\circ}\)C, CL = 80 mg, t = 5 h [90].