## 1 Introduction
\begin{table}
\begin{tabular}{c c} \hline \hline
**Feedstocks** & **Countries** \\ \hline Soybean oil (_Glycine max_) & America, Brazil, Argentina \\ \hline Rapeseed oil (_Brassica napus L._) & Germany, Australia, Argentina, France, Canada \\ \hline Linseed oil (_Linum visitatissimum_) and & Spain \\ Olive oil (_Olea europaea_) & Spain \\ \hline Sunflower oil (_Helianthus annuus_) & France, Italy \\ \hline Castor oil (_Ricinus communis_) & Brazil \\ \hline Guang pi (_Cornus wilsoniana_) & China \\ \hline Palm oil (_Elaes guinensis_) and & Indonesia, Malaysia, Colombia, Thailand \\ coconut oil (_Cocus nucifera_) & Australia, Ireland, Canada \\ \hline Animals fat/Beef tallow & Australia, Ireland, Canada \\ \hline Jojoba oil (_Simmondsia chinensis_) & Mexico, Southwest and Central America \\ \hline \hline \end{tabular}  
* Jatropha oil (_J. curcas_),
* Karanja seed oil (_Pongamia pinnata_),
* Neem oil (_Azadirachta indica_),
* Rubber seed oil (_Hewa brasiliensis_),
* Mahua oil (_Madluca indica_),
* Algae oil (_Cyanobacteria_)  
* Non-edible oil feedstocks recently receiving attention.  
\end{table} 
Table 1: Common feedstocks of biodiesel production by different countries [23; 24; 25; 26]. Figure 2: Annual total biodiesel production for the last ten (10) years. Data obtained from REN21 [21]. Crude glycerol, due to its impurities (methanol, inorganic salts, fatty acids, and water), has limited applications and is usually targeted at non-sensitive areas such as polymer and energy, including fuel [11; 12; 27; 28], unlike the refined glycerol that has applications in cosmetics, pharmaceuticals, and food, as stated previously. However, crude glycerol can be purified to improve its quality to over 95% via different processes outlined in Figure 3. The purification process is not economical due to the cost involved. The lowest cost was put at 0.149 USD/Kg [13]. Given the low commercial value of the produced glycerol, its surplus, and possible environmental issues, efforts are ongoing to convert it to high-value products, which is expected to improve its commercial viability, biodiesel economics, and eliminate the perceived environmental concern of the surplus. The conversion of glycerol to the high valued products is usually done through various catalytic reactions such as hydrogenolysis [29; 30], reforming [31; 32], dehydration [33; 34], carboxylation [35; 36], oxidation [37; 38], oligomerization [39; 40], esterification or acetylation [41; 42], etherification [43; 44], acetalization [45; 46], pyrolysis and gasification [47; 48]. A good number of literature reviews are available to show the research activities in most of the reactions above [7; 9; 34; 49; 50; 51; 52]. One of the high valued products receiving attention by researchers and industrialists in recent times is oxygenated fuel additives. These additives are usually blended with gasoline, diesel, and or biodiesel to improve their cold flow properties, viscosity, octane number, thermal stability, reduce gum formation and particulate emission [53; 54; 55]. The oxygenated fuel additives from glycerol include glycerol esters or acetyl glycerol (acetin), glycerol ethers, and glycerol formal (solketals and acetals) [3; 55; 56; 57; 58]. Scientific literature articles have revealed few reviews on the synthesis of acetin in recent years [59; 60; 61], unlike other oxygenated fuel additives, namely ethers, solketals, and acetals [62; 63; 64; 65; 66; 67; 68; 22; 69]. Thus, this paper reviews the recent research findings on acetin production from glycerol with a particular focus on the heterogeneous catalysts used and how they influenced the reaction. Unlike earlier reviews, this review further emphasized the conditions in which the catalysts were used with detailed findings documented to serve as an appropriate guide for subsequent studies in this area. This review also presents some of the findings in pictorial form for easy understanding, unlike earlier reviews. Figure 3: Crude glycerol purification process. * Matter organic non-glycerol.