Fig. 6 ( A ) 1HNMR spectra of the reaction system ; ( B ) 13CNMR spectra of the reaction system ; ( C ) Determination of carbon position at each position in the13CNMR spectrum.
From Fig. 6, we can see that DMC was detected in the product, which proved that the catalyst prepared by us is effective by the one-pot method with CO2, methanol, and propylene oxide. At the same time, there were also by-products. The by-products were propylene carbonate, 1,2 - propanediol, and propylene glycol methyl ether.
The next step was process optimization. The effects of pressure, time, alcohol/alkyl ratio, temperature, catalyst dosage, and other variables on the yield of DMC were investigated.
3.2.2 Single-factor experiment of one-pot synthesis of DMC
In this experiment, the effects of single factors such as the amount of catalyst PILs-XSS ( 1 wt % – 3 wt % ), the amount of cocatalyst Na2CO3 ( 1 wt % – 5 wt % ), the pressure of CO2 ( 0.9 MPa – 1.7 MPa ), the reaction temperature ( 90 °C – 110 °C ), the reaction time ( 2 h – 6 h ), and the molar ratio of methanol to propylene oxide ( 2: 1 – 4: 1 ) on the conversion of propylene oxide and the yield of dimethyl carbonate were studied.
Amount of catalyst PILs-XSS. Figure 7( A ) shows the effect of PILs-XSS on the conversion of propylene oxide and the yield of DMC. The reaction conditions were as follows: reaction time 3 h, 1.0 MPa, reaction temperature 95 °C, methanol / PO molar ratio 4: 1, and 5 wt % Na2CO3, respectively. When the amount of catalyst PILs-XSS increased from 1 wt % to 3 wt %, the conversion rate of PO and the yield of DMC increased continuously. When the amount was 2.5 wt %, the conversion rate of PO reached 86.8 % and the yield of DMC reached 41.2 %. When the catalyst dosage was increased again, the PO conversion and DMC yield did not increase, indicating that the catalyst PILs-XSS had the highest catalytic effect at this time. Therefore, the optimal dosage of PILs-XSS was 2.5 wt % for this reaction.
Pressure. The CO2 pressure had a decisive influence on the reaction. As shown in Fig. 7( B ), the pressure gradually increased from 0.9 MPa to 1.7 MPa. The conditions were as follows: the amount of catalyst PILs-XSS was 2.5 wt %, the reaction time was 3 h, the reaction temperature was 95 °C, and the molar ratio of methanol to PO was 4: 1, and the amount of Na2CO3 was 5 wt %. Under the pressure of 1.5 MPa, the conversion rate of PO reached the highest 98.8 % and the yield of DMC reached the highest 47.1 %. So the best pressure condition is 1.5MPa.