## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.1 Catalyst and Catalyst Load
The CaSn(OH)\({}_{6}\) catalyst performed much better when compared to other hydroxy stannates and metal oxides like MgSn(OH)\({}_{6}\), ZnS(OH)\({}_{6}\), SrSn(OH)\({}_{6}\), Ca(OH)\({}_{2}\), CaO and MgO. The high glycerol conversion (78.2%) was attributed to the high basicity exhibited by the CaSn(OH)\({}_{6}\) catalyst contrary to the expectation. The authors also reported that this transesterification method is cost-effective. The use of acid-functionalized ionic liquids as a catalyst in glycerol acetylation has been reported to be of high catalytic activity [136; 137; 138]. A \(>\)95% yield of triacetin was obtained in glycerol acetylation with acetic acid over 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogen sulphate ([HSO\({}_{3}\)-pmim][HSO\({}_{4}\)]) as the catalyst after 6 h of reaction time at 120 \({}^{\circ}\)C [136]. The catalyst was reused ten times, and yet the activity remained at \(>\)91%. A similar performance was reported by [PERSON] et al. [137], but the selectivity to diacetin (51.4%) was the main product after 0.5 h at 100 \({}^{\circ}\)C and 0.5 wt% catalyst load. Similarly, [PERSON] et al. [138] reported a range of nitrogen-based Bronsted-acidic ionic liquids (alkyl-pyrrolidone and alkylamine cations) with reasonable activity. N-methyl-2-pyrrolidinium hydrogen sulphate [H-NMP][HSO\({}_{4}\)] was found to be the most active and cost-effective catalyst with \(>\)99% glycerol conversion and 42.3% selectivity towards triacetin, and \(>\)95% of combined di- and triacetin selectivity within 1 h. Non-porous materials such as hydroxylated magnesium fluoride were deployed in glycerol acetylation with acetic acid (1:3) at 100 \({}^{\circ}\)C, and the result indicates \(>\)90% glycerol conversion with diacetin as the major product (\(\approx\)60%) followed by triacetin (\(\approx\)30%) after 22 h reaction time, unlike the uncatalyzed reaction that produced only 30% glycerol conversion with monoacetin as the major product (\(>\)90%). However, the reaction time was reduced from 4 h to 0.5 h when non-conventional activation methods, namely microwave or ultrasound irradiation, were used [139]. The authors attributed the excellent performance of the catalyst to the density of acid sites on the external surface of the catalyst, while the selectivity was influenced by the nature of acid sites (Lewis and Bronsted). Findings from the above catalytic studies showed that an increase in catalyst loading to a certain level (amount) improved glycerol conversion, especially to diacetin and triacetin, due to the increased number of available and accessible active acid site in the reaction mixture. Additional findings also revealed that the presence of the Bronsted acid site on the catalyst promotes optimal glycerol acetylation. The effect on monoacetin production is minimal, and this can be seen in the pictorial representation in Figure 9. The performance of some selected catalysts in recent studies is summarized in Table 2.  
Figure 8: Keggin structure of heteropoly acids (HPA) catalyst [XM\({}_{12}\)O\({}_{40}\)]\({}^{n-}\)[118].