Figure 1 Synthetic route of polymer ionic liquids
2.2.1 Synthesis of ionic liquid monomer
1 - vinyl imidazole, 1,2 - dibromoethane, and N, N ′ -methylene bisacrylamide were mixed in a ratio of 2: 1: 1 with anhydrous ethanol as solvent. The mixture was reacted at 80 °C for 36 hours.
2.2.2 Synthesis of Polymeric Ionic Liquids
The obtained ionic liquid monomer was cooled to room temperature, and the initiator 2,2 - azobisisobutyronitrile with 0.5 % mass fraction of mixture was added. The initiator and ionic liquid monomer were mixed evenly. The petroleum ether was added to the three-necks flask with magnetic stirring at 65 °C. Then, the mixture was added to the three-necks flask, and nitrogen was filled in the reaction process. After two hours, the temperature was raised to 75 °C, and the temperature was raised to 85 °C in the next two hours, The polymerization was continued for six hours at 85 °C. After the reaction, yellow spherical particles were obtained by precipitation, filtration, washing, and drying. The diameter was about 4 mm. The polymerization ionic liquid catalyst was prepared and named PILs-XSS.
Catalytic characterization\
Nicolet6700 / FT was used to analyze the functional groups of catalysts. The surface area of the catalyst was measured by Brunauer-Emmett-Teller ( Brunauer-Emmett-Teller ) method using Quantachrome ( NOVA2000 ). The pore size distribution is measured by BJH ( Barrett-Joyner-Halenda ) method. The TEM ( transmission electron microscopy ) instrument model running at 200 kV was JEM-2100 to observe the surface of the catalyst. Thermogravimetric analysis ( TG ) was performed using the STD-2960 instrument in an N2 atmosphere with a heating rate ( 10 °C / min ) from 25 °C to 800 °C.
2.4 Catalytic Synthesis and Product Analysis of DMC
One-pot synthesis of DMC was carried out in a rust steel reactor equipped with a heating outer layer and a thermometer with magnetic stirring. In the typical reaction, 0.15 mol propylene oxide, 0.4 mol methanol, 0.3 g PILs-XSS, and 0.3 g Na2CO3 were added to the reactor. At room temperature, the air in the reactor was removed by CO2, and then the reactor was pressurized to 15 bar by CO2, heated to 100 °C for 3 h. The pressure of the reactor was measured by a pressure gauge equipped with a pressure-reducing valve. After the reaction, the reactor was quickly cooled to 15 °C in an ice water bath, and the supernatant was taken to the centrifuge tube. The internal standard method was used for detection by GC-FID. The products were detected by gas chromatography-mass spectrometry, and the by-products were propylene carbonate, 1,2-propanediol, and propylene glycol methyl ether.
3 Results and discussion
3.1 Characterization of catalysts
3.1.1 FT-IR analysis
To verify the reaction and polymerization of 1,2-dibromoethane, N,N’-methylenebisacrylamide, and 1-vinylimidazole, the four samples were identified by FT-IR in the range of 500-4000cm-1. structure. As shown in Figure 2, first, the characteristic peak at 1670cm-1 (C=O vibration) [25], a special functional group in N,N’-methylenebisacrylamide. 2850cm-1 and 3040cm-1 (saturated C-H and unsaturated C-H) [26], 1020cm-1 is the C-N characteristic peak vibration in the imidazole ring [27], 590cm-1 is the C-Br vibration peak [28]. Tthe above The analysis results preliminarily judged that PILs-XSS was synthesized successfully.