# Green Production of Glycerol Ketals with a Clay-Based Heterogeneous Acid Catalyst
## 2 Materials and Methods
### Methods
#### 2.2.4. Ketalization Reactions  
The reactions were carried out in glass reactor at atmospheric pressure. Typically, the glycerol (10 mmol), an adequate amount of ketone from 30 to 90 mmol (acetone, butanone also known as methylethylketone or MEK, or cyclohexanone) and 2 to 4 mL isopropanol were mixed in a 25 mL round bottom flask till a clear homogeneous phase was obtained. After reaching the adequate temperature (25 to 50 \({}^{\circ}\)C, depending on the system under study), a certain mass of the acid-activated clay AC was added (0.25 to 3 g, depending on the studied system) and stirred with a cylindrical magnetic stirrer to avoid catalyst attrition. The reactions of interest are shown in Figure 2.  
Due to the low solubility of glycerol in the pure ketone, isopropanol was used as a cosolvent in the minimal amount needed to obtain a homogeneous system (2.1 mL for acetone, 3.5 mL for MEK and 4 mL for cyclohexanone--higher amounts for more hydrophobic ketone). The reactions were monitored online with ReactIR 10 equipment (Mettler-Toledo, Columbia, SC, USA), based on FTIR spectroscopy, that is able to provide all the organic chemical species that are present in the solution as the reaction is being performed [36]. The FTIR spectrometer uses a mercury-cadmium telluride (MCT) detector that is cooled by liquid nitrogen and measurements are optically taken using a diamond-tipped probe with a 1.5 m long fiber-optic conduit. Data acquisition was performed from 2000 to 650 cm\({}^{-1}\) with a 4 cm\({}^{-1}\) nominal resolution. In Figure 3, an example of a reaction of acetone (60 mmol) and glycerol (10 mmol), using isopropanol as cosolvent (2.1 mL) is shown (System 1). In contrast, a blank using the aforementioned volume of isopropanol and the said molar amount of acetone is also presented. The signal at 1711 cm\({}^{-1}\) wavenumber is selected in this case and in the other runs as it is an isolated  
Figure 2: Reactions of acid-catalyzed of glycerol ketalization: (**a**) general ketalization; (**b**) System 1: ketalization of glycerol with acetone to 2,2-dimethyl-1,3-dioxolane-4-methanol (solketal or ketal 1); (**c**) System 2: ketalization of glycerol with MEK to 2-ethyl-2-methyl-1,3-dioxolane-4-methanol (ketal 2); (**d**) System 3: ketalization of glycerol with cyclohexanone to [1,4] dioxaspiro[4,5]decane-2-methanol (ketal 3). peak in all FTIR spectra that can be used to follow the disappearance of any ketone (with some shift depending on the ketone under study). This peak is due to the C = O stretch and its isolation can be easily perceived in Figure 4, where, as an example, all relevant FTIR spectra for the main components of system 1, with the exception of water, are displayed. From Figure 3, it is evident that, while acetone disappears due to its reaction with glycerol, a polyol, no reaction is detected with isopropanol, probably due to the instability of the potential ketal and the steric hindrance due to the two methyl groups of the isopropanol molecule. The most important variables affecting the reaction rate and final product yield (based on the ketone signal at time t compared to ketone signal at time 0, considering that all ketone was transformed to its corresponding ketal in a 1 to 1 molar relationship) were studied: temperature, molar ratio of reactants and catalyst concentration. Finally, the stability of the catalyst was assessed by repetitive  
Figure 4: FTIR spectra in the 650–2000 cm\({}^{-1}\) region for all relevant components of system 1 with the exception of water and isopropanol, which was added as a solvent to create and maintain a monophasic system during the ketalization process. Figure 3: Time course as detected by Fourier Transform Infrared Spectroscopy (FTIR) at 1711 cm\({}^{-1}\) wavenumber for the reaction of glycerol and acetone to solketal and water using isopropanol as solvent (system 1, in red dashed line) together with negative blank to show no reaction between isopropanol and acetone (in green solid line). reaction cycles.