# On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol
## 3 Results
### The Morphology and Structure of Different Catalysts  
The crystal structure of the catalysts was characterized by XRD, as shown in Figure 2. The peak positions of MgO, Al\({}_{2}\)O\({}_{3}\) and ZnO correspond to their respective standard cards. From the analysis, it can be concluded that MgO has a cubic crystal system, \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) has a cubic spinel structure, and ZnO has a fibrillar zinctite structure, all of which were successfully prepared. From the XRD pattern of the Mg-Al-Zn catalyst, peaks of MgO appear at around 36.8\({}^{\circ}\) and 44.0\({}^{\circ}\), while the characteristic peak of ZnO appears at around 62.9\({}^{\circ}\). Additionally, each peak position shows characteristic peaks of MgAl\({}_{2}\)O\({}_{4}\). Based on the XRD pattern analysis of the Mg-Al-Zn catalyst, it can be concluded that MgAl\({}_{2}\)O\({}_{4}\) has a cubic spinel structure (311 crystal plane), with an estimated grain size is approximately 25 nm. MgO has a cubic structure (200 crystal plane), and its estimated grain size is approximately 15 nm. ZnO has a hexagonal wurtzite structure (101 crystal plane), with an estimated grain size of approximately 30 nm. Phase analysis reveals that the positions and intensities of the XRD diffraction peaks for these four catalysts show no significant changes before and after use, indicating that the catalysts maintain excellent structural stability during the reaction. However, it can be seen from the results in Figure 1 that the catalytic performance of MgO has continuously decreased during use, and the catalytic activity of the Mg-Al-Zn catalyst has also changed significantly from the first to the second use. This indicates that the change in the activity of the catalyst during its use has no direct relationship with the crystal structure of the catalyst. From the SEM images in Figure 3, it can be observed that the Mg-Al-Zn catalyst exhibits loosely distributed crystal particles with relatively uniform sizes. This is similar to the SEM characterization of Mg-Al-Zn composite oxides described in other published articles [35]. After the Mg-Al-Zn catalyst was used 10 times, the SEM characterization of the catalyst showed that there was no obvious change in its microstructure, and the particle size remained basically the same. This is consistent with the BET characterization in Table 2 and the analysis results of the XRD pattern in Figure 2, indicating that the change  
Figure 2: XRD patterns of the catalysts before and after use for MgO (**A**), Al\({}_{2}\)O\({}_{3}\) (**B**), ZnO (**C**), and Mg-Al-Zn (**D**). (a: after used, 10C\({}^{a}\): the catalyst after 10 cycles) (test conditions: a 2\(\uptheta\) range of 5–85\({}^{\circ}\) and a scanning rate of 10\({}^{\circ}\)/min). The blue lines represent the standard cards of the crystal images corresponding to each oxide. in activity of the Mg-Al-Zn catalyst from the first to the second use is independent of the microstructure of the solid. The N\({}_{2}\) adsorption- desorption isotherms of MgO, Al\({}_{2}\)O\({}_{3}\), Mg-Al-Zn catalysts, and the Mg-Al-Zn catalyst after use are shown in Figure 4, and their structural properties are listed in Table 2. Based on the data in the table, it can be concluded that the pore size distribution of MgO is concentrated around 9-13 nm, while that of Al\({}_{2}\)O\({}_{3}\) is around 4-8 nm. The pore size distribution of the Mg-Al-Zn catalyst is entirely concentrated around 2-5 nm. Compared with the BET characterization of Mg-Al-Zn composite oxides in other literature [35], the specific surface area of the Mg-Al-Zn catalyst in this paper is relatively slightly larger, but the average pore size of this catalyst is smaller. Moreover, the specific surface area and pore size of the catalyst after 10 cycles of use show little difference compared to the unused catalyst, indicating that the pores of the catalyst are not blocked.