Fig. 7 Single-factor optimization of one-pot synthesis of DMC catalyzed by PILs-XSS/Na2CO3 : ( A ) catalyst dosage, ( B ) reaction pressure, ( C ) reaction temperature, ( D ) reaction time, ( E ) methanol / PO molar ratio, ( F ) Na2CO3 dosage
Reaction temperature. The effects of reaction temperature on PO conversion and DMC yield are shown in Fig. 7( C ). Other conditions are as follows: the optimum amount of catalyst PILs-XSS is 2.5 wt %, and the pressure is 1.5 MPa, and the undetermined reaction time is 3 h. The molar ratio of methanol to PO is 4: 1, and 5 wt % Na2CO3. It can be seen that with the increase in temperature, the conversion rate of PO did not change, and the yield of DMC increased continuously until 100 °C reached the highest 50.4 %. It indicated that the reaction between PO and CO2 had reached the highest conversion rate of 98.8 % at 90 °C. Therefore, 100 °C is the best temperature for this reaction.
Reaction time. The effect of reaction time on this reaction is shown in figure 7( D ). Other conditions are as follows : catalyst PILs-XSS dosage 2.5wt %, 1.5 MPa, 100 °C, methanol / PO molar ratio 4 : 1, 5wt % Na2CO3. In the reaction time of 2-6h, the conversion of PO first increased and then remained unchanged, reaching the highest 98.8 % at 3h, DMC yield reached the highest 53.7 % at4h, continuing to extend the reaction time, and DMC yield remained unchanged. Therefore, the optimal reaction time was 4h.
The molar ratio of methanol to PO. As shown in figure 7( E ), the other conditions are catalyst PILs-XSS dosage 2.5wt %, 1.5MPa, 100 °C, reaction time 4h, 5wt % Na2CO3. The reaction process was studied by changing the molar ratio of methanol to PO. It was found that the conversion of PO remained unchanged, indicating that the first step of the ring-opening reaction was not affected by the amount of methanol when the amount of PO and CO2 was constant. With the increase in methanol / PO molar ratio, the yield of DMC first increased and then stabilized. the The yield of DMC reached the highest 53.7 %, when the molar ratio was 3: 1. Therefore, the optima molar ratio of l methanol to PO was 3: 1.
Na2CO3 dosage. The effect of Na2CO3 as cocatalyst on the reaction is shown in Fig.7 ( F ). The optimized conditions are as follows: the amount of catalyst PILs-XSS 2.5 wt %, 1.5 MPa, 100 °C, reaction time 4 h, and methanol / PO molar ratio 3: 1. We found that the conversion rate of PO increased with the increase of Na2CO3 content, which was a special phenomenon, indicating that Na2CO3played a certain role in promoting the catalytic reaction of CO2. When the dosage was 3 wt %, the yield of DMC reached the highest 53.7 %. Therefore, the best dosage of Na2CO3 is 3wt %.
In summary, the optimum conditions were as follows: catalyst PILs-XSS 2.5wt %, CO2 pressure 1.5MPa, reaction temperature 100 °C, reaction time 4h, methanol / PO molar ratio 3: 1, Na2CO3 dosage 3wt %. Under these conditions, the conversion rate of PO reached 98.8 % and the yield of DMC was 53.7 %. Of course, after exploration, the first step of the addition reaction was rapid and the conversion rate was high. The transesterification reaction took a long time in the series and the conversion rate was relatively low, but it had little effect on the overall reaction.
3.3 Comparison of catalytic activity of the same series of PILs/Na2CO3
Table 2 Comparison of catalytic activity for DMC synthesis