# Green Production of Glycerol Ketals with a Clay-Based Heterogeneous Acid Catalyst
## 3 Results and Discussion  
### Catalyst Selection  
The smectite clay under study was contacted with HCl for different lengths of time, from 0.5 to 20 h, and several key features were measured to select the most adequate contacting time. In Table 1, the yields after 24 h of reaction to solketal (ketal 1) and the ketal from cyclohexanone ([1,4] dioxaspiro[4,5]decane-2-methanol or ketal 3), together with the specific surface (S\({}_{\text{BET}}\)) and the Bronsted acidity are displayed, while Table 2 depicts the mineralogical data of these clays treated during different times with HCl 3 N. Finally, Figure 5 shows the initial reaction rate for the obtention of ketals 1 and 3 with fresh and used catalysts for the two reaction systems (systems 1 and 3). Activity and stability tests were performed using acetone and cyclohexanone as ketones to obtain the corresponding ketals 1 and 3. Common conditions were the molar ratio ketone/glycerol, set at 6, and the catalyst concentration in all cases, set at 50 g/L. Due to the lower volatility of cyclohexanone, 40 \({}^{\circ}\)C was the temperature chosen for this reacting system, while 25 \({}^{\circ}\)C were selected for acetone reactions. Therefore, the differences of initial reaction rate can be ascribed, at least partially, to the reaction temperature. Values of this variable, in Figure 5, indicate that the best contact time values are 0.5 and 1 h, with a decrease in the activity for higher contact times. Stability was similar for all contact times. The difference comes between the two reacting systems: much more activity is maintained for the acetone system or system 1, probably because glycerol can be much easily washed in a more polar liquid reacting phase. It is interesting to observe that almost double isopropanol is needed when using cyclohexanone to obtain a homogeneous reacting system, with an increased probability that non-reacted glycerol (about a 20% of the initial one) adsorbs onto the tested clays, resulting in the subsequent deactivation by fouling [37]. However, as indicated in Table 1, after 24 h of reaction, the yield to the corresponding ketal is higher for cyclohexanone, and for the 0.5 and 1 h contact time catalysts with both ketones. Probably, the more stable structure of ketal 3 in comparison to ketal 1 (solketal) can be the reason for a higher yield of the former. Moreover, its molecular structure of ketal 1 probably sterically hinders the addition of a water molecule needed for the hydrolysis, so this later reaction, the reverse reaction, proceeds slowly \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Contacting Time** & **24 h Yield to** & **24 h Yield to** & **S\({}_{\text{BET}}\)** & **Brönsted Acidity** \\
**[h]** & **Ketal 1 [\%]** & **Ketal 3 [\%]** & **[m2/g]** & **[mmol H\({}^{+}\)/gl**]** \\ \hline
0.5 & 68.2 \(\pm\) 2.8 & 83.4 \(\pm\) 4.3 & 125.4 & \((56\pm 3.5)\times 10^{-4}\) \\
1 & 69.3 \(\pm\) 3.5 & 85.3 \(\pm\) 5.1 & 186.6 & \((65\pm 5.1)\times 10^{-4}\) \\
2 & 68.1 \(\pm\) 3.2 & 83.6 \(\pm\) 3.8 & 225.4 & \((51\pm 4.8)\times 10^{-4}\) \\
7 & 66.4 \(\pm\) 3.4 & 81.8 \(\pm\) 4.2 & 195.3 & \((38\pm 2.4)\times 10^{-4}\) \\
20 & 63.3 \(\pm\) 3.4 & 78.4 \(\pm\) 4.4 & 170.4 & \((19\pm 2.1)\times 10^{-4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key parameters of the catalysts obtained at several contacting time HCl 3 N-smectite. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Contact Time** & **MgO** & **C as CO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **K\({}_{2}\)O** & **Fe as FeO** \\
**Time [h]** & **[molar \%]** & **[molar \%]** & **[molar \%]** & **[molar \%]** & **[molar \%]** & **[molar \%]** \\ \hline
0 & 1.3 & 54.5 & 11.9 & 25.4 & 0.7 & 1.2 \\
0.5 & 0.8 & 36.9 & 5.1 & 55.6 & 0.6 & 0.9 \\
1 & 0.3 & 37.3 & 4.7 & 55.3 & 0.8 & 1.2 \\
2 & - & 36.6 & 4.2 & 54.1 & 0.7 & 1.1 \\
7 & - & 38.6 & 2.5 & 58.4 & 0.5 & - \\
20 & - & 37.2 & 2.3 & 59.6 & 0.9 & - \\ \hline \hline \end{tabular}
\end{table}