## 4 Solketal Synthesis
### Catalyst Study
#### 4.2.1 Recent Advances on Catalysts
\begin{tabular}{c c c c c c c} \hline \hline  & & & & & & \\ \hline
**Author** & **Catalyst** & **Additive (mmol/gad)** & **Reaction** & **Reactions** & **Results (\%)** & **Reusability** & **Ref.** \\ \hline Vannucci, 2020 & Sulfated zirconium oxide & & & & & \\  & 0.3 wt\% (glycerol) & & 0.09 & T = 313 X & X\({}_{gly}\approx 80\%\) & A 16\% activity loss after four cycles. & [91] \\  & & & & & \\  & & & & & \\ \hline Salistyio, 2020 & \begin{tabular}{c} Baseline F300 \\ 1 wt\% (glycerol) \\ \end{tabular} & N.A.1 & \begin{tabular}{c} GA = 14 \\ T = 323 K \\ \end{tabular} & X\({}_{gly}\approx 84.3\%\) & N.A.2 & [92] \\  & & & & \(t=60\) min & & \\ \hline Da Silva, 2020 & \begin{tabular}{c} Tan (II) silicotungstate acid salt \\ 0.01 molv\%.2 \\ \end{tabular} & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline Podolean, 2020 & \begin{tabular}{c} Germanosilicate zeolite \\ 5 wt\% (glycerol) \\ \end{tabular} & N.A.1 & \begin{tabular}{c} GA = 15 \\ T = 298 K \\ \end{tabular} & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline Li, 2020 & \begin{tabular}{c} Zirconium organophosphate \\ 5 wt\% 2 \\ \end{tabular} & & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline Hussein, 2020 & \begin{tabular}{c} Gallosilicate \\ 10 mg \\ \end{tabular} & & & & & & \\  & & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline Vivian, 2021 & \
\begin{tabular}{c} Gallosilicate \\ 32 wt\% (glycerol) \\ \end{tabular} & N.A.1 &
\begin{tabular}{c} GA = 14 \\ T = 323 K \\ \end{tabular} & X\({}_{gly}\approx 43\%\) & Four cycles without significant activity & \\  & & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline \hline \end{tabular}   
* Not available.2 Not stated whether in attraction to glycerol or to total reactions.3 Boosted Activity. Iron (III) complexes homogeneous catalysts were investigated recently due to their potential to be recovered and their low cost, desirable characteristics when considered on the industrial scale. Esposito et al. tested several types of iron (III) catalysts in an apparatus where water was removed by molecular sieves to shift the reaction equilibrium towards product formation and concluded that FeCl\({}_{3}\)(1-NO\({}_{2}\)) had the better performance, achieving almost total glycerol conversion and 100% selectivity to solketal [73,98]. Da Silva et al. compared different transition metal salts and found the best result for Fe(NO\({}_{3}\))\({}_{3}\)-9H\({}_{2}\)O, also converting almost all of the glycerol and achieving selectivity of 95%. Then, the authors compared the conversions of Bronsted acid catalysts (pTSA and H\({}_{2}\)SO\({}_{4}\)) and confirmed that the iron (III) salt is more efficient probably because besides polarizing acetone's carbonyl group, it releases H\({}^{+}\) ions in the reaction medium, therefore acting like Lewis and Bronsted acid simultaneously [74]. Another common conclusion of the articles is the high catalyst efficiency, once a small catalyst load leads to great conversion values [73,74]. Ionic Liquids (ILs), as iron (III) catalysts, combine the advantages of homo- and heterogeneous catalysts, although its major drawback is usually the high cost. Ji et al. avoided this problem by exploring the tributyl (tetradecyl)phosphonium p-toluenesulfonate (TPTT), an inexpensive IL. The researcher obtained high solketal yield (85.9%), also with a high reaction rate, because the reaction was not impaired by internal mass transfer resistance; however, he had to use excessive acetone, probably to diminish external mass transfer resistance. The results are in line with the ones obtained previously by Gui et al. (Xgly = 96%) with 1-(4-sulfonylbutyl) triphenylphosphonium methanesulfonate. The second study used almost half of the catalyst load, but more than double the acetone to glycerol ratio. The ILs of both studies were recovered and used in several cycles without losing the catalytic activity, an important characteristic for the industrial application [84,85].