# On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol
## 4 Conclusions  
Due to the hydrolysis of DMC with reagents or water in the atmosphere to produce CO\({}_{2}\)[38], then CO\({}_{2}\) and H\({}_{2}\)O react with solid alkalis to form carbonates [21; 33]; both the alkali strength and the number of alkali sites of the catalyst will decrease, resulting in a decline in the catalytic activity of the catalyst. In this study, through the determination of the alkali strength and the analysis of catalytic activity of MgO, Al\({}_{2}\)O\({}_{3}\), ZnO, and Mg-Al-Zn catalysts, it was proved that when the alkali strength is greater than 9.3, the transesterification reaction between DMC and ethanol can achieve a certain catalytic effect. When the surface alkali strength of MgO is greater than 11.0, it reacts with CO\({}_{2}\)[33] to form carbonates, resulting in a decrease in catalytic activity. The alkaline strength of the solid surface of the Mg-Al-Zn catalyst is less than 11.0 and is less affected by CO\({}_{2}\). Furthermore, according to the conclusion drawn in Section 3.3, the surface alkali strength of the Mg-Al-Zn catalyst is greater than 9.3; therefore, it has good catalytic activity. The alkaline sites of  
Figure 7: Relationship between surface alkalinity and DMC conversion rate for Mg-Al-Zn and MgO. (Reaction conditions: DMC/C\({}_{2}\)H\({}_{5}\)OH = 1, catalyst dosage: 3.0 wt.%, reaction temperature = 78 \({}^{\circ}\)C, reaction time: 3 h. The alkalinity was determined by the Hammet indicator titration method. The experiment was repeated three times for each data point. The error bar is the error range of the data point obtained based on the calculated standard deviation, and the upper and lower horizontal lines represent the upper and lower limits of the 95% confidence interval of the mean). the Mg-Al-Zn catalyst are less affected by CO\({}_{2}\), so after the first use, the alkalinity of the catalyst slightly decreases. During the second use, the number of alkali sites on the solid surface remains basically unchanged. Therefore, the Mg-Al-Zn catalytic activity can remain stable, and the catalyst can be reused multiple times. We can conclude that the catalytic system of the transesterification reaction between DMC and ethanol requires a solid alkali catalyst with an alkali strength greater than 9.3. If the alkali strength on the surface of the catalyst is greater than 11.0, it will gradually deactivate with an increase in the number of uses. The surface alkali content of solid catalysts is directly related to the stability of their catalytic performance. For the transesterification reaction between DMC and ethanol, when the alkali strength of the solid catalyst is between 9.3 and 11.0, a better catalytic effects and stability can be achieved. Similar situations may also exist in other transesterification reactions; as long as the appropriate range of the alkali strength of the catalyst is explored, a catalyst with stable catalytic activity can be prepared [22]. Through a series of characterizations in this study, it was confirmed that when the surface alkali strength of the solid catalyst is between 9.3 and 11.0, it can stably catalyze the transesterification reaction system of DMC and ethanol. The Mg-Al-Zn ternary composite oxide catalyst meets the above conditions (9.3 \(<\) H\({}_{0}\)\(<\) 11.0) and demonstrates notable cycling stability as a solid catalyst. Under the conditions of a reaction temperature of 78 \({}^{\circ}\)C, a reaction time of 3 h, DMC/C\({}_{2}\)H\({}_{5}\)OH = 1, and a catalyst amount of 3.0 wt.%, the conversion rate of DMC of the Mg-Al-Zn catalyst remained stable at around 34.00% during 10 cycles of use. When the catalyst load increased to 7.0 wt.%, the conversion rate of DMC reached 56.53%. Compared with the existing industrial catalysts (such as CaO [22] and ILs [13, 14]), the catalytic activities of Mg-Al-Zn catalysts may be lower than those of CaO and ILs. However, the stability of many existing industrial catalysts is difficult to maintain, and there will be labor costs and catalyst costs when replacing the catalysts.