## 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 most common reactants used in the conversion of glycerol to acetin include acetic acid and acetic anhydride [126; 134; 135]. However, it has been reported that acetic acid is, thermodynamically, resistant to acetylation due to the formation of a tetrahedral intermediate. The formation of water (in situ), may also deactivate the catalyst and reverse the reaction [41; 110]. Nevertheless, acetic acid is still preferred because it is cheaply available and less toxic despite the positive Gibbs free energy exhibited. Acetylation with acetic anhydride proceeds quickly to the product, attributed to the ease of formation of acylium ion [80]. However, the use of acetic anhydride is of grave concern because of its application in the production of narcotics. Currently, it is banned in some countries [114]. It was also reported that the rate of reaction of glycerol with acetic anhydride is much higher than with acetic acid, and because of its speed, it is much more difficult to handle [67; 80]. Several journal articles also reported the use of other carboxylic acids, such as oleic, levulinic, lauric, propionic, and butanoic acids in acetylation [58; 84; 143; 148]. Most of these carboxylic acids have shown good glycerol conversion and selectivity to monoacetin but limit the selectivity to higher acetins (diacetin and triacetin). The limitation was attributed to their bulky structure. The performance of these acetylating agents in the reaction can also be influenced by their molar ratios. In terms of the stoichiometric equation, 3 mol of acetic acid is needed to react completely with 1 mol of glycerol to produce monoacetin and subsequently to diacetin and triacetin as the final product [60]. However, excess acetic acid shifts the equilibrium towards higher diacetin and triacetin, respectively. An increase in the molar ratio of acetic acid to glycerol led to an increase in glycerol conversion [110]. In addition, 96.88% glycerol conversion and selectivity of 51.90, 45.27, and 2.11% monoacetin, diacetin, and triacetin were obtained at an acetic acid to glycerol molar ratio of 6 over a 20% sulphated silica catalyst. When the molar ratio of 3 was used over the same catalyst, the glycerol conversion reduced to 90.12% with a selectivity of 58.83, 37.10, and 3.24% monoacetin, diacetin, and triacetin, respectively. However, when 10% sulphated silica catalyst was used, monoacetin selectivity was favoured with 81.12%, while diacetin was 18.32%, and only traces of triacetin were observed at a molar ratio of 3. The difference in the catalyst performance was attributed to the acid density and the availability of the acetylating agent. That is, a catalyst with a higher acid density favours higher acetin (di- and triacetin) formation. A similar observation was reported with a crude glycerol-derived carbon catalyst [114]. At an acetic acid to glycerol molar ratio of 1, over 70% of monoacetin was obtained with 55% glycerol conversion. However, when the molar ratio was varied from 3 to 9, glycerol conversion increased to 100% with a high production of di- and triacetin (46%, 43%) and low production of monoacetin (11%). When the molar ratio was \(>\) 4, the triacetin production decreased, and it was attributed to the likely effect of excess acetic acid leading to mass transfer resistance. This resistance limits the sufficient interaction or contact between the glycerol and the catalyst. [PERSON] et al. [13] reported the esterification of glycerol with oleic acid over a mixture of silicotungstic acid-ionic liquid catalyst at 100 \({}^{\circ}\)C for 8 h. High glycerol conversion (96.4%) was achieved with 96% selectivity towards monoacetin. Only 4% of diacetin was reported. A good yield of glycerol ester was also reported by [PERSON] et al. [148] using the same reactants (glycerol and oleic acid) but with a different catalyst made of methyl ester sulfonic acid. [PERSON] et al.