## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.1 Catalyst and Catalyst Load
At the optimum conditions of 105 \({}^{\circ}\)C, the glycerol-to-acetic acid molar ratio of 1.9, 0.5 g catalyst loading, and 4 h reaction time in the first step, the glycerol conversion was almost 100% with a selectivity of 25.9% towards triacetin with the amberlyst-35 catalyst. The catalyst performance was attributed to its high acid capacity when compared to the rest of the catalysts used. With the addition of acetic anhydride in the second step, the selectivity of triacetin improved to almost 100% within 15 min. Amberlyst-35 was found to be the most excellent catalyst for the conversion as no significant deactivation occurred after being reused five times. Even when the reaction was extended to 12 h, the catalyst was found to be very stable. The same authors also investigated the influence of the catalyst load at a fixed molar ratio of glycerol-to-acetic acid of 1:6, 105 \({}^{\circ}\)C, and 4 h reaction time. The glycerol conversion was almost 100% with the presence of 0.25 g of the catalyst. While without the catalyst, the conversion was 73.6%. The selectivity to monoacetin decreased with an increase in catalyst amount up to 0.5 g, while the di- and triacetin increased. However, no appreciable change was noticed in the selectivity of mono-, di-, and triacetin when the catalyst load was further increased to 1.0 g. The authors concluded that much of the catalyst added did not substantially improve its activity. However, it has been established by several researchers that ion exchange resins are not thermally stable. They become easily deactivated above 150 \({}^{\circ}\)C [59; 98]. Figure 7: Structure of ion exchange resin catalysts: (**a**) perfluorinated resinsulfonic acid (Nafion) (**b**) sulfonated polystyrene (Amberlyst) [128; 129]. Heteropoly acids (HPA) are complex compounds that exhibit very strong Bronsted acidity but with low specific surface area and low thermal stability [103; 130]. They also exhibit homogeneous behaviour (highly soluble in polar media) in most reactions. However, with intense research, the protons of HPA can now be substituted by incorporating metal ions, especially transition metals. The Keggin structure of heteropoly acids (HPA) is illustrated in Figure 8. Other materials such as silica, carbon, zeolite, resin, metal oxides, etc., have also been used to immobilized HPA as support, thereby improving their surface area, thermal stability, as well as reduce their solubility and leaching in reaction medium [59; 71; 102; 118]. The modifications have greatly improved the catalytic activity of HPA and have been used in glycerol acetylation. [PERSON] et al. [131] reported the complete crude glycerol conversion (100%) and selectivity of 6.4%, 61.3%, and 32.3% towards mono-, di-, and triacetin, respectively, at 120 \({}^{\circ}\)C in 4 h over a zirconia-supported H\({}_{4}\)SiW\({}_{12}\)O\({}_{40}\) (HSiW/ZrO\({}_{2}\)) catalyst. There was no catalyst deactivation noticed even after four reaction cycles and was attributed to good surface Bronsted acid site and hydrothermal stability. Similarly, 100% glycerol conversion was achieved within situ-generated supported silicomolybic heteropolyanions from the sol-gel synthesized MoO\({}_{3}\)/SiO\({}_{2}\) catalyst. As the mol% of MoO\({}_{3}\) supported on SiO\({}_{2}\) increased, the conversion remained the same, but the selectivity varies. 1 mol% MoO\({}_{3}\) gave selectivity of 56, 28, and 17% mono-, di-, and triacetin. A 10 mol% MoO\({}_{3}\) produced a selectivity of 36, 31, and 33% mono-, di-, and triacetin, respectively. While 20 mol% MoO\({}_{3}\) produced 17, 33, and 50% mono-, di-, and triacetin, respectively, in 8 h reaction time, at 100 \({}^{\circ}\)C, with a glycerol-to-acetic acid molar ratio of 1:10 and a catalyst load of 10 wt% with respect to glycerol. However, 76% selectivity towards triacetin was obtained when the temperature and reaction time were increased to 118 \({}^{\circ}\)C and 20 h, respectively [132]. [PERSON] et al.