## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.1 Catalyst and Catalyst Load
Glycerol conversion was 96.8% while selectivity towards mono-, di- and triacetin were 33.2, 54.2 and 12.6%, respectively. Incidentally, it was Sb\({}_{2}\)O\({}_{5}\) that showed strong Bronsted acid sites was able to protonate adsorbed pyridine which might have influenced selectivity towards diacetin. The Sb\({}_{2}\)O\({}_{5}\) catalyst was found to be active even  
Figure 4: Representative structure of montmorillonite clay [119]. after six reaction cycles. More recently, [PERSON] et al. [107] synthesized a diatomic-loaded H\({}_{2}\)SO\({}_{4}\)/TiO\({}_{2}\) catalyst and was used in the esterification reaction of glycerol with oleic acid. In a 6 h reaction time, 59.6% diacetin was obtained at a much higher temperature of 210 \({}^{\circ}\)C, low catalyst load (0.1%), and low molar ratio of glycerol-to-oleic acid (1:2). The reusability of the catalyst was tested five times. The diacetin obtained was about 50% in the first three cycles but decreased in the fourth cycle by 12.5%, and 10% in the fifth cycle. The author attributed the results to the blockage of active sites by the bulky triacetin and the leaching of active sites from the catalyst. Recently, [PERSON] et al. [108] deployed unsulphated and sulphated CeO\({}_{2}\)-ZrO\({}_{2}\) mixed oxide catalysts in glycerol acetylation with acetic acid and reported that the sulphated CeO\({}_{2}\)-ZrO\({}_{2}\) mixed oxide catalyst exhibited better performance when compared with the unsulphated catalyst. Within 3 h reaction time, 99.1% of glycerol conversion was achieved with 57.28% and 21.26% selectivity towards diacetin and triacetin at 100 \({}^{\circ}\)C. The catalyst was reused in three consecutive cycles without a significant loss of activity. Mesoporous Silica and Functionalized Silica  
Mesoporous silica is an ordered porous network material with SiO\({}_{4}\) tetrahedral as the building block, illustrated in Figure 5. It is made of weak Bronsted acid oxides and hence has a low catalytic value. However, because the surface is amenable to functionalization and other modifications, resulting in a highly stable material with a high surface area and large pore size, it is currently receiving significant attention as catalyst support in various reactions to enhance the reactivity and selectivity of products, especially of large molecules like fatty acids and their esters [59; 117; 124]. [PERSON] and [PERSON] [110] reported the influence of a sulphated silica catalyst, prepared with the sol-gel technique with different sulphate loading percentages (5, 10, 15, and 20%) in the esterification of glycerol with acetic acid at a molar ratio of 1:6, at a temperature of 50 \({}^{\circ}\)C and with 0.2 g catalyst load in 6 h. The results revealed that the catalytic activity increased with an increase in the acidic properties, which in turn was found to increase with the increased sulphuric acid loading. The 20% sulphated silica exhibited the highest catalytic activity with 96.88% glycerol conversion and a selectivity of 51.9, 45.27, and 2.11% towards mono-, di-, and triacetin. The authors further reported a strong correlation between the textural properties (surface area and pore volume) and the sulphur content of the catalysts. The decrease in both the surface area and pore volume was found to correlate with an increase in sulphur content, attributable to the blockage of pores by the active non-porous species. The authors further studied the effect of catalyst loading on the reaction, and the results revealed a slight increase in glycerol conversion with an increase in sulphated silica catalyst loading from 0.2 to 0.8 g. The selectivity to monoacetin decreased with an increase in catalyst loading, while the selectivity to diacetin and triacetin improved marginally. [PERSON] et al.