## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.1 Catalyst and Catalyst Load
[89] studied various solid catalysts, namely cesium phosphothugstate (CsPWA), amberlyst-15, H-beta, sulphated zirconia (SZ), and montmorillonite K-10, in glycerol acetylation or esterification with acidic anhydride and acetic acid. The reaction of glycerol with acidic anhydride was carried out under a milder condition of 1:3 mole ratio, 30 \({}^{\circ}\)C and a catalyst load of 1 wt% with respect to glycerol when compared with that of acetic acid, a 1:8 mole ratio, 85 \({}^{\circ}\)C and catalyst load of 7 wt%. The activity of the catalysts in both reactions is in the order SZ \(<\) H-beta \(<\) K10 \(<\) A-15 \(<\) CsPWA. The CsPWA catalyst exhibited the highest glycerol conversion of \(>\)98% in both reactions with combined di- and triacetin selectivity of 99.1% with acetic anhydride and 75% with acetic acid. The high glycerol conversion was attributed to the presence of high Bronsted acidity while that of selectivity was found to depend on the nature of the active sites on the catalyst. The authors went further to study the effect of the CsPWA catalyst load on the glycerol esterification with acetic acid. The result showed increased glycerol conversion from 56 to 98% when the catalyst load was increased from 3 to 7 wt% in a reaction time of 2 h. The selectivity to di- and triacetin also increased from 31 and 0 to 59 and 16%, respectively. This observation was attributed to the presence of more active sites with a high catalyst load. However, when the catalyst amount was increased further to 9 wt%, no appreciable change was observed, and the authors concluded that the maximum glycerol conversion and product selectivity were attained with the catalyst load of 7 wt%. The CsPWA catalyst exhibited good reusability with minimal decreased activity after three reaction cycles. The activity of CsPWA in glycerol esterification was also confirmed by [PERSON] et al. [121], but the maximum glycerol conversion and esters yield were obtained at a 5 wt% catalyst load. Most of the earlier works prefer the use of lower catalyst loads irrespective of the catalyst used, and the pattern of the results was similar [98; 102; 133]. Polymeric material and polyvinylpyrrolidone (PVP) were separately impregnated with phosphothungstic, silicotungstic, and phosphomolybdic acids, and each used as a catalyst in the conversion of glycerol to acetin using acetic acid as the acetylating reagent [104]. It was observed that the PVP impregnated with phosphothungstic acid (PVP-DTP) exhibited the highest selectivity to triacetin (34%), and 100% glycerol conversion was also achieved at the optimum conditions. The performance was attributed to its high acidity. However, the catalyst was found to be highly hydrophobic. Even when the authors compared the PVP-DTP catalyst with the commercial montmorillonite KSFO catalyst under the same conditions, the former performed better. Others  
The use of a magnetic solid acid catalyst (Fe-Sn-Ti(SO42)-400) in glycerol acetylation with acetic anhydride was reported by [PERSON] et al. [134]. A 100% glycerol conversion and 99.0% selectivity to triacetin were achieved at a molar ratio of 1:6, at 80 \({}^{\circ}\)C, and within 30 min. The catalyst was reused three times, and the activity remained unchanged. The authors attributed the activity of the catalyst to the high surface area and pore volume. However, when the acetic acid was used as the acetylating agent under similar reaction conditions, the glycerol conversion reduced drastically to 32.3%, while the only product was monoacetin with a selectivity of 100%. However, when the reaction temperature increased to 140 \({}^{\circ}\)C, the glycerol conversion improved up to 75.7% with monoacetin selectivity of 90.3%. Meanwhile, [PERSON] et al. [135] showed that acetin could also be produced simultaneously with biodiesel by reacting glycerol with methyl acetate over Ca and Sn-mixed metal hydroxides catalysts.