## 4 Solketal Synthesis
### Recent Advances on the Continuous Process  
Among the continuous process strategies studied, the fixed-bed reactor stands-out owing to its relatively simple operation with mild conditions, the use of heterogeneous catalysts, and the possibility of coupling other processes (as downstream distillation separation), or unit operations (as adsorption or membrane permeation). Most of the recent studies still relied on ion exchange resins and hierarchical zeolites. The latest developments, and tests on conventional continuous processes are summarized in Table 3.  
Figure 8: Mechanism of reaction of Bronsted acid catalysts. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Author** & **Catalyst** & **Catalyst Loading** & **Reaction Conditions** & **Results** & **Ref** \\ \hline Fixed Bed & & & & & \\ \hline \multirow{4}{*}{Dmitriev, 2016} & KU–2–8 & & & G:A = 1:5 & & \\  & & KU–2–8 & & T = 308 K & & \\  & Cation–Exchange & \multirow{2}{*}{205 g} & P = N.A. & \multirow{2}{*}{Xgly = 65\%} & \multirow{2}{*}{[105]} \\  & & Resin & & Solv. = Ethanol & & \\  & & & Space time = 1.45 h & & \\ \hline \multirow{4}{*}{Oliveira, 2016} & & & G:A = 1:20 & & \\  & & & T = 323 K & & \\  & & & P = 1 bar & & \\  & & & Solv. = DMF & & \\  & & & Space time = 0.2 h & & \\ \hline \multirow{4}{*}{Guidi, 2016} & & & G:A = 1:4 & \multirow{4}{*}{Xgly = 95\%} & \multirow{4}{*}{[107]} \\  & & & T = 298 K; 373 K & & \\  & & & P = 10 bar & & \\  & & AlF3-3H2O & 0.067 gcat/ggly & Solv. = Methanol & \multirow{4}{*}{Xgly = 80\%} & \multirow{4}{*}{[106]} \\  & & WHSV \({}^{1}\) = 2 h\({}^{-1}\) & & \\ \hline \multirow{4}{*}{Konwar, 2018} & & & G:A = 1:8 & & \\  & & & T = 313 K & & \\  & & macro/mesoporous & \multirow{2}{*}{0.5 g} & P = 1 bar & \multirow{2}{*}{Xgly = 92\%} & \multirow{2}{*}{[108]} \\  & & solid protonic acids & & Solv. = Ethanol & \\  & & & Space time = 0.25 h & & \\ \hline \multirow{4}{*}{Cornejo, 2019} & & & G:A = – & & \\  & & & T = 323 K & & \\  & & & P = High pressure \({}^{2}\) & \multirow{2}{*}{Xgly = 91\%} & \multirow{2}{*}{[71]} \\  & & & Solvent Free & & \\  & & & Space time = - & & \\ \hline \multirow{4}{*}{Dominguez-Barroso, 2019} & & & G:A = 1:8 & & \\  & & carbon-based & \multirow{2}{*}{0.345 gcat/ggly} & T = 330 K & \multirow{2}{*}{Xgly = 99\%} & \multirow{2}{*}{[109]} \\  & functionalized with & & Solvent Free & & \\  & & & WHSV \({}^{1}\) = 2.9 h\({}^{-1}\) & & \\ \hline \multirow{4}{*}{Kowalska-Kus, 2019} & ZSM5(P) & & G:A = 1:3 & \multirow{4}{*}{Xgly = 90\%} & \multirow{4}{*}{[110]} \\  & ZSM5(H) & & T = 323 K & & \\  & & & P = 1 bar & & \\  & & & Solv. = Methanol & \multirow{2}{*}{Xgly = 90\%} & \multirow{4}{*}{[110]} \\  & & Beta(H) & & WHSV \({}^{1}\) = 3.4 h\({}^{-1}\) &Nanda et al. and Moreira et al. proposed a similar system for the solketal synthesis in a fixed-bed reactor, both at 313 K, and an acetone to glycerol molar ratio of 2; the first, with the ion-exchange resin Amberlyst-36, achieved a conversion of 74%, and the latter, with Amberlyst-35, achieved a conversion of 81%. It is important to emphasize that these conversion values were obtained under transient operation [11, 51]. Additionally, Nanda et al. achieved a conversion of 94% using optimized conditions and an acetone to glycerol molar ratio of 4, like Guidi et al., who used the same resin under similar reaction conditions [107, 51]. Oliveira et al. also proposed an analogous system, but with the use of Amberlyst-15, and achieved excellent conversion, but the excessive acetone was a problem that could not be overcome [106]. Kowalska-Kus et al. compared the catalytic performance of three zeolites (H-Beta, H-Mordenite, and H-ZSM5), also in a fixed-bed reactor, with methanol as the solvent in mild conditions, and found conversion about 88%, with an acetone to glycerol molar ratio of 3. The H-ZSM5 needed a desilication and acid pre-treatment to enlarge the pores and achieve an acceptable conversion. Moreover, the zeolites presented a loss in their activities over time [110].