## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.4 The Reactants and Their Molar Ratios
[80] also reported the influence of acetic acid to glycerol molar ratio (2:1, 3:1, 6:1, 9:1) at fixed conditions of 105 \({}^{\circ}\)C, 0.5 g catalyst load and in 4 h over amherlyst-35 catalyst. The results indicate a higher conversion of glycerol and a higher selectivity towards triacetin with an increased amount of acetic acid. This observation was attributed to the improvement in the acid strength with the availability of more acetic acid, which subsequently improved the catalytic activity. In the work of [PERSON] et al. [140], it was reported that based on the stoichiometric calculation, three moles of acetic acid requires one mole glycerol to produce one mole of triacetin. However, in the experimental work, the increase in the molar ratio of acetic acid to glycerol (3, 4, 5, and 6) increased the glycerol conversion by 0.29% for each addition of 1 mol acetic acid. The maximum glycerol conversion of 98.50% was achieved. The increase in molar ratio also affected the selectivity as indicated by the decreased monoacetin and increased di- and triacetin, respectively. The influence of the acetic acidto glycerol molar ratio in esterification was also evaluated by [PERSON] et al. [75] over a sulfonated carbonized rice husk catalyst. The results indicate an increase in the formation of di- and triacetin with a simultaneous decrease in monoacetin as the molar ratio increases. The authors coined that the observation was due to the continuous conversion of monoacetin to the higher acetins; however, the glycerol conversion was not significantly affected until at a molar ratio of 5. The glycerol conversion was 90%, and a further increase in the molar ratio to 9 did not significantly improve the conversion. However, the selectivity to di- and triacetin improved greatly, but for economic and environmental reasons, the authors felt that there was no need to use a high volume of acetic acid given the amount of products produced. Similarly, a report by [PERSON] et al. [126] showed a marginal improvement of diacetin from 67 to 73% and triacetin from 11 to 20% when the molar ratio of acetic acid to glycerol was varied from 4:1 to 10:1 over 5 wt% of a glycerol-based carbon catalyst at 115 \({}^{\circ}\)C for 1 h. However, when the reaction time was increased to 4 h, the triacetin was increased to 40% at a molar ratio of 10:1. This finding is different from the findings of [PERSON] et al. [125], who used unfunctionalized biochar as a catalyst. About 75% glycerol conversion was obtained at a mole ratio of 3:1. The conversion increased up to 89% when the molar ratio was varied to 5:1 but at a higher temperature of 120 \({}^{\circ}\)C. The improvement was also observed in the combined di- and triacetin selectivity (60%). Further variation in the molar ratio did not produce any appreciable change in the glycerol conversion. The difference in the above performance may be due to the nature of the catalyst and the different operating temperatures used. Similarly, the higher conversion of glycerol (99%) was obtained when glycerol was esterified with lauric acid (3:1) but at a higher temperature of 180 \({}^{\circ}\)C in 2 h over layered double hydroxide (LDH, Mg-Al-CO\({}_{3}\)) catalyst with a selectivity of approximately 58, 30 and 12% for mono-, di- and tri-acetins, respectively [143]. The use of different carboxylic acids, namely acetic acid, propionic acid, and 1-butanoic acid as esterification agents in glycerol acetylation over silver-exchanged phosphotungstic acid (Ag\({}_{1}\)PW) catalyst (1 wt% catalyst load) at 120 \({}^{\circ}\)C, with a 1:10 molar ratio (glycerol to carboxylic acids), was reported by [PERSON] et al. [84]. In less than 20 min, acetic acid showed 96.8% glycerol conversion with 48.4, 46.4, and 5.2% selectivity towards mono-, di-, and triacetin, propionic acid achieved 70.9% glycerol conversion with 55, 43.1, and 1.9% selectivity towards mono-, di- and triacetin, respectively, while 1-butanoic acid produced 64.3% glycerol conversion with 53.9, 46 and 0.1% selectivity towards mono-, di-, and triacetin, respectively.