# Green Production of Glycerol Ketals with a Clay-Based Heterogeneous Acid Catalyst
## 3 Results and Discussion
### Glycerol Ketalization Kinetic Studies
#### 3.3.1 System 1: Ketalization with Acetone. Preliminary Analysis  
Effect of Catalyst Concentration  
The effect of catalyst dosage on solketal yield was investigated by varying acid activated clay AC concentration, selecting the values 25, 50 and 100 g/L and keeping constant the other reaction parameters: 6/1 acetone/glycerol molar ratio at room temperature 25 \({}^{\circ}\)C for 24 h. The yields to the ketal and the initial overall reaction rate are displayed in Figure 9.  
In absence of the catalyst, no solketal formation was observed. Solketal 1 yields were increased significantly and reached from 66 to 71% in the presence of 25 to 100 g/L of catalyst, values that are notably higher than the one obtained in totally solventless conditions (biphasic system at zero time evolving towards a monophasic one) in the same acetone/glycerol molar ratio and temperature conditions: ca. 50% [17]. In the work by [PERSON] et al. [39], several strong acid exchange resins were assessed for the ketalization of glycerol and acetone, with Lanxess Lewatit GF101 being the most active one. It has an acidity of 5.11 mmol H\({}^{+}\)/g and the catalyst concentrations studied were 5 and 10 g/L, attaining initial reaction rates of about \(2.15\times 10^{-4}\) mol/(L-s) with a catalyst load of 5 g/L at 30 \({}^{\circ}\)C. In this case, the acid-activated smectite AC, at 50 g/L and 25 \({}^{\circ}\)C, with the same acetone to glycerol molar ratio, leads to an initial reaction rate value of \(5.1\times 10^{-4}\) mol/(L-s). If the initial reaction rates, in terms of TOF (turnover frequency), are calculated, referring to the real concentration of strong acid sites per gram of solid, the values are \(2.15\times 10^{-4}\) mol\({}_{\text{ketal}}\)/(L-mmolH\({}^{+}\)) for the benchmark biphasic system and the strong acid resin, and \(1.56\times 10^{-2}\) mol\({}_{\text{ketal}}\)/(L-mmolH\({}^{+}\)) for the monophasic system and the acid-treated clay here reported. These results indicate an apparent TOF of the clay AC under study \(\sim\)75 times higher. The small particle diameter of the clay--50 to 100 \(\upmu\)m--means that is probable that no internal mass transfer hindrance exists, as was the case with the acid resin used of the reference kinetic study (approx. 100 \(\upmu\)m). Therefore, although it remains to be studied, the difference should be allocated to the different nature of the support surface; for example, a less hygroscopic nature of the clay in comparison to the resin would lead to a more adequate distribution of reactants and products on the support surface. This hypothesis could explain the evident shift of the equilibrium towards the products when using clay AC: more water is expelled from the clay surface compared to strong acid exchange resins. Apart from the comparison to strong acid exchange resins, the evolution of the reaction rate value with the concentration of the catalyst is linear, as expected for Bronsted acid catalysts with a good contact between the active sites and the reagents molecules. Therefore, the partial order to be considered in any rate equation for the catalyst concentration should be one (first order). The effect of the reaction temperature was investigated at three different, and moderate, temperatures from 25 \({}^{\circ}\)C to 35 \({}^{\circ}\)C as shown in Figure 10.  
Figure 9: Effect of catalyst amount on the yield of solketal 1 in terms of the yield to the ketal 1 at 24 h and the initial reaction rate (reaction conditions: acetone/glycerol molar ratio, 6:1; temperature, 25 \({}^{\circ}\)C). The yields at equilibrium of the ketal 1 or solketal were slightly lower as the temperature decreased, showing that temperature shifts minimally from the equilibrium position towards the products. Thus, the process is slightly endothermal or even thermoneutral, considering the experimental error. Again, the yield at equilibrium is approximately 70%.