Introduction
In recent years, the global temperature has been rising, and the melting of bipolar icebergs has appeared in front of us. CO2 is considered to be the main reason for these problems. Despite the corresponding measures taken by most countries around the world, CO2 emissions are increasing year by year [2-3]. Therefore, it is a cutting-edge remedy to achieve carbon neutralization and capture and utilize the emission of CO2 [4–8].
The special properties of dimethyl carbonate ( DMC ) determine its wide range of applications. Compared with traditional chemical products, DMC has low toxicity, and high oxygen content makes it appear in fuel oil additives and can be used as a green solvent. With the rapid development of lithium batteries, DMC can be used as an electrolyte for battery additives. Therefore, the wide application of DMC in the chemical industry can be determined. There are many methods for the production of DMC. Some have achieved large-scale applications, such as the phosgene method, but they have obvious shortcomings, such as the generation of harmful substances. Conversion of CO2 to DMC is a typical carbon neutralization pathway [ 14-17 ]. Since CO2 is abundant, non-toxic, environmentally friendly, and cheap, the direct synthesis of DMC from CO2 and methanol is a front-end reaction system, but the reaction system absorbs more energy, so it is greatly affected by the conversion rate, and the effect is difficult to make people satisfied [18]. On this basis, the method of introducing an alkylene oxide (ethylene oxide or propylene oxide) for a two-step reaction can largely solve this problem, thereby improving the methanol conversion rate and the yield of DMC. One-pot synthesis of DMC involves two steps: ring addition and transesterification. Cycloaddition is the reaction of CO2 with propylene oxide to generate propylene carbonate ( PC ), and transesterification is the transesterification of methanol with propylene carbonate to synthesize DMC and 1,2 - propanediol. Propylene glycol methyl ether as a by-product is produced by dehydration of 1,2-propanediol and methanol. Methanol is excessive. The consumption of 1,2-propanediol in the product has a certain role in promoting the yield of DMC.
The screening of catalysts is the most important part of the successful activation of CO2. CO2 is extremely stable, and electrophilicity is an important way to activate it. In the past, the catalyst is divided into homogeneous and heterogeneous two parts to study. Homogeneous catalysts are mainly ionic liquids for the direct synthesis of DMC from methanol and CO2. For example, Ashif H. Tamboli et al. [20] synthesized a 10 wt % chitosan/ionic liquid (IL)-catalyst system with hydroxyl and amine groups as nucleophilic and carbon dioxide absorption centers, with a methanol conversion rate of 16.90% and DMC selectivity 98.72%, the catalyst has good catalytic activity, selectivity, and stability, but requires higher pressure to improve the conversion rate and yield, which is a challenge for large-scale production. Heterogeneous catalysts include many: alkali metals, transition metal hydroxides, and supported catalysts [21], such as Ming Liu et al. [22] using K2CO3NaBr-ZnO catalyst system, methanol, propylene oxide, and CO2 one-pot synthesis DMC. The addition of Zn powder improves the selectivity of DMC, but the final selectivity is only 40.2%, which is unfavorable for production. awad Al-Darwish et al. [23] synthesized cerium oxide (CeO2) with three different shapes (nano-octahedron, nanocube, nanorod) by hydrothermal method, and activated carbonylation reaction of CO2 and methanol to dimethyl carbonate. ester (DMC), the results show that the shape of the nanostructured ceria affects the yield of DMC, and the catalytic effect is general.
Ionic liquids are commonly used in green production in recent years. For the activated CO2 reaction, alkali metal halides use the nucleophilic attack of halide ions to perform the first addition reaction [24]. Carrying halide ions in ionic liquids should also have the same effect [25], but for homogeneous reactions, ionic liquid catalysts are too troublesome to recycle and reuse. The emergence of polymeric ionic liquids solves this dispute. It not only inherited the strong catalytic activity, thermal stability, and chemical stability of ionic liquids, but also added the advantages of inorganic catalysts, such as certain mechanical strength, modifiability, and corrosion resistance.
The purpose of this study is to activate CO2 by one-pot method, conduct a cycloaddition reaction with propylene oxide, and then transesterify to synthesize DMC. The nucleophilic attack of halide ions in the polymerized ionic liquid is used to complete the first step, and the second step uses sodium carbonate as a catalyst for the transesterification of propylene carbonate and methanol. As we know, there are no similar papers in the literature, the polymer ionic liquid catalyst can be easily separated after the reaction. Results from material characterization and reaction testing allowed us to better understand the catalyst’s effect on the final conversion of CO2 to DMC. The resulting products are analyzed by gas chromatography (GC) for detailed optimization of the influencing factors in the catalytic process.
Experimental
2.1 Material
1 - vinylimidazole ( 99.0 % ), 1,2 - dibromoethane ( 99.0 % ), 1,3 - dibromopropane ( 99.0 % ), 1,4 - dibromobutane ( 99.0 % ), 1,5 - dibromopentane ( 99.0 % ), 1,6 - dibromohexane ( 99.0 % ), N, N ′ -methylenebisacrylamide, sodium carbonate ( 98 % ), 2. 2 ’ -azobisisobutyronitrile ( AIBN, 98.0 % ), anhydrous ethanol ( AR, ≥ 99.7 % ), ethyl acetate ( AR, ≥ 99.5 ) methanol ( AR, ≥ 99.5 % ), propylene carbonate ( GC, > 99.0 % ), dimethyl carbonate ( GC, > 99.0 % ), 1,2-propanediol ( GC, ≥ 99.5 % ), propylene glycol methyl ether ( GC, ≥ 99.5 % ), propylene oxide ( GC, ≥ 99.5 % ) were purchased from ALADDIN-E.com. Solvents (methanol, ethanol, etc.) were obtained from Tianjin Chemical Reagent Co. Ltd.
2.2 Catalyst preparation
The preparation of polymeric ionic liquid catalysts is shown in Fig. 1.