## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.1 Catalyst and Catalyst Load
The reusability and stability of the catalyst were found to be excellent even after five reaction cycles at a 1:4 glycerol-to-acetic acid molar ratio, at 115 \({}^{\circ}\)C and with a 1 h reaction time. No deactivation and leaching under the reaction conditions were observed. Synthesized ordered mesoporous carbon via hard template method and its modification with concentrated sulphuric or 4-aminobenzenesulfonic acid were tested in glycerol acetylation [116]. Both showed good activity with the material modified via diazonium cation exhibiting high glycerol conversion and selectivity to triacetin under similar conditions. Figure 6 is an illustration of a sulfonated carbon-based catalyst. Ion-exchange resins are usually co-polymers of polystyrene and divinylbenzene with exchangeable ions or functional groups. They exist commonly as Amberylsts, Nafion, Dowex-50W, and sulfonic acid resin. Figure 7 shows the structure of some resin catalysts. Several reports indicate the deployment of ion exchange resins in its original and functionalized forms as well as catalyst support in glycerol acetylation. [PERSON] et al. [99] compared the use of acidic anhydride and acetic acid in glycerol acetylation over amberlyst-15. A 100% glycerol conversion was achieved with both acetylating agents but with varying selectivity to mono-, di-, and triacetin. The conditions of the reaction with the acetic anhydride were much milder when compared with that of acetic acid. The reaction of glycerol with acetic anhydride was at a molar ratio of 1:3, a temperature of 60 \({}^{\circ}\)C, and a catalyst load of 5 wt%  
Figure 6: Schematic structure of a sulfonated carbon-based catalyst [127]. glycerol, which led to 100% glycerol conversion with a selectivity of 0.7, 1.3, and 98.1% mono, di, and triacetin in 2 h. While the reaction of glycerol with acetic acid produced 100% glycerol conversion with a selectivity of 3.5, 8.7, and 87.8% for mono, di, and triacetin, respectively, this was at a higher molar ratio of 1:6 and a higher temperature of 120 \({}^{\circ}\)C with the same catalyst load and time. Similar results were reported by [PERSON] et al. [88] using acetic anhydride but at a lower reaction time of just 20 min. [PERSON] et al. [41] also used amberlyst-15 catalyst in comparison with several other solid catalysts including silica-alumina, zeolite (HUSY), dodecamolybdophospheric acids supported on Nb\({}_{2}\)O\({}_{5}\) (HPMo/Nb\({}_{2}\)O\({}_{5}\)) and mesoporous SBA-15 (HPMo/SBA-15), sulphated ceria-zirconia (SCZ), proplysulfonic acid functionalized SBA-15 (PrSO\({}_{3}\)H-SBA-15), sulfonic acid functionalized SBA-15 (SO\({}_{3}\)H-SBA-15) and microcrystalline cellulose (SO\({}_{3}\)H-Cell) catalysts in glycerol acetylation with acetic acid at a molar ratio of 1:6, a temperature of 80 \({}^{\circ}\)C and the catalyst load of 5 wt% with respect to glycerol. After a long reaction time of 8 h, PrSO\({}_{3}\)H-SBA-15, amberlyst-15, and SO\({}_{3}\)H-SBA-15 catalysts exhibited superior performance in glycerol conversion and selectivity to higher acetins. However, the SO\({}_{3}\)H-SBA-15 catalyst showed significant deactivation due to the loss of acidity. The excellent performance by the PrSO\({}_{3}\)H-SBA-15 and amberlyst-15 catalysts could not be entirely due to the strength of Bronsted acid because the sulphated ceria-zirconia (SCZ) exhibited the higher Bronsted acid strength (2.9 mmol-g\({}^{-1}\)) compared to PrSO\({}_{3}\)H-SBA-15 (1.2 mmol-g\({}^{-1}\)). However, its catalytic activity in acetylation reaction was inferior. The authors suggested that in addition to PrSO\({}_{3}\)H-SBA-15 and amberlyst-15 catalysts exhibiting moderate to high Bronsted acid strength (1.2 and 4.9 mmol-g\({}^{-1}\)), they also have the configuration and spatial arrangement of their surface acid moieties that contributed to their excellent activities, unlike the other catalysts. The two-step acetylation of glycerol with acetic acid and acidic anhydride over amberlyst-35 resin catalyst was compared with zeolite (HY and HZSM-5) catalysts [80].