# On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol
## 3 Results
### Alkainity of Different Catalysts  
According to the CO\({}_{2}\)-TPD results shown in Figure 5, it can be inferred that MgO has a weak desorption peak at 250-450 \({}^{\circ}\)C. Its alkali strength is stronger than that of MgO obtained under other preparation conditions [20], but the number of alkali sites is relatively small. However, the alkaline strength of ZnO, Al\({}_{2}\)O\({}_{3}\), and Mg-Al-Zn catalysts is relatively weak. Under the test conditions of CO\({}_{2}\)-TPD, no desorption peak occurred, indicating that there was no adsorption or the amount of adsorbed CO\({}_{2}\) was extremely small. This shows that the catalyst was less affected by CO\({}_{2}\)[21]. Therefore, in order to determine the CO\({}_{2}\) adsorption and desorption on the solid surface of the catalyst more accurately, the DRIFTS method was employed to characterize the adsorption and desorption process. Figure 6 presents the infrared spectra recorded during the CO\({}_{2}\) adsorption and desorption processes for the pre-treated Mg-Al-Zn catalyst. Gas-phase CO\({}_{2}\) peaks are observed at 2360 cm\({}^{-1}\) and 2295 cm\({}^{-1}\)[36], attributed to the introduction of CO\({}_{2}\) gas. Concurrently, characteristic peaks for carbonate and bicarbonate species appear at 1516 cm\({}^{-1}\) and 1681 cm\({}^{-1}\)[37], respectively, indicating that CO\({}_{2}\) was adsorbed onto the catalyst surface, forming carbonate and bicarbonate. At 25 min, the CO\({}_{2}\) gas flow was terminated and replaced with Ar gas, resulting in a sharp decline in the CO\({}_{2}\) gas peaks. Subsequently, the temperature began to rise. The CO\({}_{2}\) gas peaks were still detectable at this stage. Observing the carbonate and bicarbonate peak intensities revealed that during the heating \begin{table}
\begin{tabular}{c c c c} \hline
**Catalyst** & **S\({}_{\text{BET}}\) (m\({}^{2}\)/g)** & **Pore Volume (cm\({}^{3}\)/g)** & **Pore Size (nm)** \\ \hline MgO & 81.76 & 0.51 & 14.92 \\ Al\({}_{2}\)O\({}_{3}\) & 186.90 & 0.43 & 6.08 \\ ZnO & 0.61 & - & - \\ Mg-Al-Zn & 160.97 & 0.26 & 7.51 \\ Mg-Al-Zn-10C\({}^{\text{a}}\) & 163.94 & 0.25 & 7.87 \\ \hline \end{tabular} The catalyst sample was first degassed under vacuum at 120 \({}^{\circ}\)C for 6 h; subsequently, N\({}_{2}\) adsorption-desorption isotherms were measured at 77 K using liquid nitrogen.  
\end{table} 
Table 2: Textural and Structural Properties of Catalysts. Figure 5: CO\({}_{2}\)-TPD profiles of different catalysts. (Approximately 60 mg of the sample was pretreated at 500 \({}^{\circ}\)C for 1 h, cooled to 40 \({}^{\circ}\)C, then exposed to CO\({}_{2}\) at a flow rate of 30 mL/min. Helium purging was performed until the baseline stabilized, followed by heating to 750 \({}^{\circ}\)C for testing 10 \({}^{\circ}\)C/min). process, these species decomposed to CO\({}_{2}\), leading to CO\({}_{2}\) desorption. The DRIFTS results demonstrate that the Mg-Al-Zn catalyst adsorbs CO\({}_{2}\). To determine the alkali strength and alkalinity on the solid catalyst surface, measurements were conducted using the Hammett indicator method. The MgO solid catalyst caused bromothymol blue to change from yellow to blue, phenolphthalein from colorless to red, and alizarin yellow R from yellow to red, but it did not induce a color change in 2,4-dinitronaniline. Therefore, it can be concluded that the highest alkali strength on the surface of the MgO solid catalyst lies within the range of 11.0 to 15.0. On the other hand, the Mg-Al-Zn solid catalyst caused bromothymol blue to change from yellow to blue and phenolphthalein from colorless to red, but it did not alter the color of alizarin yellow R. Thus, the highest alkali strength on the surface of the Mg-Al-Zn solid catalyst is estimated to be between 9.3 and 11.0. This indicates that the alkali strength of the Mg-Al-Zn catalyst is slightly weaker than that of MgO. In addition, the highest alkali strength on the surfaces of Al\({}_{2}\)O\({}_{3}\) and ZnO solid catalysts is both between 7.2 and 9.3.