Fig. 8 Study on the reuse of PILs-XSS catalyst. The reaction conditions were as follows: catalyst PILs-XSS dosage 2.5 wt %, CO2pressure 1.5 MPa, reaction temperature 100 °C, reaction time 4 h, methanol / PO molar ratio 3: 1, and ester exchange catalyst Na2CO3 dosage 3 wt %.
3.4 Effect of Na2CO3 on conversion
For transesterification catalysts, the strength of the alkali determines the efficiency of transesterification, but too strong an alkali will lead to side reactions. For example, strong alkalis such as NaOH and KOH cannot be used as transesterification catalysts in this study. They are not only easily contaminated by CO2, but also can directly react with methanol. Catalysts with relatively strong basicity can only be used. Sodium methoxide is a strong base and does not react with CO2 and methanol. The basicity is much stronger than that of Na2CO3. Under the same conditions, the transesterification catalyst is replaced by sodium methoxide. The yield of DMC does not increase, but the conversion of PO is reduced to 86.7 %. In this regard, we carried out the corresponding research. After comparing several common transesterification catalysts, we found that carbonate had a certain promoting effect on CO2 and propylene oxide.
4. Conclusions
In summary, PILs-XSS/Na2CO3 has been used as a catalyst for the one-pot synthesis of DMC from methanol, propylene oxide, and carbon dioxide. The catalyst is synthesized by suspension polymerization. The polymerization ionic liquid catalyst PILs-XSS has a certain pore size, specific surface area, and certain heat resistance by characterization.
The optimal reaction conditions were as follows: the dosage of PILs-XSS was 2.5 wt %, the CO2 pressure was 1.5 MPa, the reaction temperature was 100 °C, the reaction time was 4 h, the molar ratio of methanol to propylene oxide (PO) was 3: 1, and the dosage of cocatalyst Na2CO3 was 3 wt %. Under the optimum conditions, the conversion of PO was up to 98.8 % and the yield of DMC reached 53.7 %. In addition, the combined catalyst PILs-Na2CO3 was easily recovered by filtration in the reaction system. After five times of repeated use, the activity and the shape of the catalyst were almost unchanged, which meant that the catalyst had amazing mechanical strength. It has a spherical shape and good repeatability, which is a special breakthrough for the continuous production of DMC. This catalyst(PILs-XSS) is a very promising catalyst in for CO2 development.