## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.1 Catalyst and Catalyst Load
Similarly, sulfonated carbon material derived from _Pongamia pinnata_ de-oiled seed cake was synthesized and used for the same reaction but with different acetylating agents (lauric and oleic acids) [101]. The catalyst was found to perform better when compared to zeolites H-Y, H-ZSM-5, and liquid H\({}_{2}\)SO\({}_{4}\). Glycerol conversion ranged from 80 to 95% with monoacetin as the major product (70-80%) within 24 h. The authors also reported that the sulfonated catalyst increased the conversion of glycerol, which was subsequently found to depend on the catalyst load. Up to 96% glycerol conversion was obtained at 7.5 wt% catalyst load. However, 5 wt% was considered to be the optimum load because it was not significantly different from the former. The performance of the sulfonated catalyst was found to correlate with its acid density, pore size, and the dimensions of the reacting molecules. The authors also confirmed the reusability of the sulfonated carbon catalyst in three successive esterification cycles with oleic acid. The results indicated that the catalyst was deactivated by only 14% but retained the active site (-SO\({}_{3}\)H) by almost 90% after the third reaction cycle with a slight improvement in the selectivity of monoacetin, which was the desired product. The slight loss was attributed to the possible restricted access to the active sites due to adsorbed carbonaceous impurities on the surface of the catalyst, and the high stability was due to the stability of the C(sp\({}^{2}\))-SO\({}_{3}\)H bond. Similarly, [PERSON] et al. [115] deployed the same catalyst but with acetic anhydride. Additionally, 100% triacetin selectivity was achieved in  
Figure 5: The tetrahedral network structure of SiO\({}_{4}\). 20-50 min with both mesoporous sulfonated carbon and zeolite H-Y catalysts. The authors further revealed that the selectivity to triacetin was influenced mainly by the pore structure and surface acid site density of the catalysts. Sulfonated carbon catalyst made from the mixture of carbonized rice husk and sulphuric acid was reported by [PERSON] et al. [75]. After 5 h of reaction, 90% glycerol conversion and selectivity of 11, 52, and 37% towards mono, di, and triacetin were obtained, respectively. The improved selectivity towards di- and triacetin was attributed to the high total acidity (5.8 mmol-g\({}^{-1}\)). Surprisingly, non-functionalized biochar (Karanja seed shells) was also able to catalyse the esterification of glycerol with acetic acid at 120 \({}^{\circ}\)C, at a molar ratio of 1:5, a catalyst load of 0.2 g, and \(>\)80% glycerol conversion was reported after 1 h. However, the major product's selectivity was monoacetin. The reaction was also attributed to moderate to strong acidic sites exhibited by the biochar despite the non-functionalization [125]. The use of a two-step reaction system with a sulfonic acid-functionalized glycerol-based carbon catalyst produced 100% triacetin [126]. In the first step, acetic acid was used, and in the second step, acetic anhydride was added to complete the reaction. The developed catalyst was found to be highly stable and recyclable. The authors further studied the effect of the catalyst loading (5-20 wt%) on the reaction and revealed that the catalyst load of 5 wt% led to 100% glycerol conversion with 22, 67, and 11% mono-, di- and triacetin selectivity after 1 h, respectively, and when compared with the control, the selectivity was 42, 54 and 4%, for mono-, di- and triacetin, respectively, which indicates the slow pace of the esterification reaction without the catalyst. However, when the catalyst load was increased to 20 wt%, the glycerol conversion increased to 100%, and the selectivity to monoacetin decreased while the di- and triacetin increased simultaneously. The trend was attributed to catalyst acidity, which enhanced the surface interactions of the glycerol with acetic acid.