## 2 Acetin (Glycerol Esters or Acetyl Glycerol)
### Influence of Reaction Parameters on the Acetylation of Glycerol
#### 2.2.1 Catalyst and Catalyst Load
[82] reported a much higher selectivity towards diacetin and triacetin with 34 and 55%, when a 3% yttrium supported on mesoporous silicate material (SBA-3) prepared via hydrothermal method was deployed as a catalyst at a glycerol-to-acetic acid molar ratio of 1:4 and a temperature of 110 \({}^{\circ}\)C. A 100% glycerol conversion was achieved in 2.5 h. The catalyst was found to be stable after being reused in three reaction cycles. The high triacetin produced was attributed to the strong acidity, and the high surface area with the large pore size, which makes the diffusion of both the substrates and products possible. However, in the fourth cycle, the glycerol conversion and triacetin were reduced to 80% and 50, respectively, due to mass transfer limitations. The authors further reported that only 44% glycerol conversion was achieved with a catalyst load of 0.05 g. However, when the catalyst load was increased to 0.20 g, the glycerol conversion increased to 100%. The subsequent increase in the catalyst load did not improve the conversion, an indication of reaching the optimum limit. In a similar vein, [PERSON] et al. [73] also reported the excellent performance of SBA-15-functionalized with proplysulfonic group (Pr-SO\({}_{3}\)H-SBA-15) as a catalyst but at a higher temperature of 120 \({}^{\circ}\)C and a glycerol-to-acetic acid mole ratio of 1:6. In comparison with zeolites (H-ZSM-5 and H-Beta), the Pr-SO\({}_{3}\)H-SBA-15 catalyst exhibited a 96% glycerol conversion and a selectivity of 13, 55, and 32% mono-, di- and triacetin in 2.5 h reaction time. The good performance of the Pr-SO\({}_{3}\)H-SBA-15 catalyst was attributed to adequate balance between the amount of acid site and its distribution on the surface of the mesoporous structure. Unlike earlier studies, the Pr-SO\({}_{3}\)H-SBA-15 catalyst could not maintain the activity in consecutive cycles, even with treatment in between. In a comparable study, [PERSON] et al. [92] deployed the synthesized solid acid catalysts, namely propyl-SO\({}_{3}\)H functionalized amorphous silica (SAS), propyl-SO\({}_{3}\)H-functionalized mesoporous silica SBA-15 (SSBA), titania-doped silica (hexagonal mesoporous silica, HMS) (Ti10 HMS), sulphated zirconia (SZ and calcined SZ-470) and niobic acid nanoparticles (Nb\({}_{2}\)O\({}_{5}\)-nH\({}_{2}\)O) in the esterification reaction of glycerol with acetic acid in a molar ratio of 1:3 at 105 \({}^{\circ}\)C. The SAS and SSBA catalysts showed the best results. Though the complete glycerol conversion (100%) was achieved within 1 h, the triacetin of 49% and 33% were achieved after 3 h with SAS and SSBA, respectively. Both catalysts were found to be very stable, with no leaching of the sulfonic groups. The excellent activities of the two catalysts were attributed to their acid capacities, including the strength and the surface density of the acid site. The use of carbon-based catalysts in acetylation has been reported as well. Several literature articles have indicated the synthesis of a functionalized catalyst from biomass materials via carbonization and sulfonation, but only few reported their application in glycerol acetylation. [PERSON] et al. [114] reported the use of crude glycerol as the precursor material in carbon-based catalyst synthesis and was found to be active in glycerol acetylation. They also reported that an increase in catalyst load from 1 to 2 wt% increased glycerol conversion (>90%) and remained relatively constant even when the catalyst load was increased up to 4 wt%. This observation was attributed to active site saturation. An increased catalyst load favours the selectivity to di- and triacetin due to increased active sites, while a lower catalyst load favours monoacetin with decreased glycerol conversion. The authors also observed that the synthesized carbon-based catalyst was reused seven times, and the glycerol conversion, as well as the product selectivity, remained constant, indicating high stability.