Conversion of CO₂ to Carbonates Catalyzed by Ionic Liquids under Mild Conditions

The utilization of CO2 as a raw material is always a hot topic in both academic and international industry. The synthesis of five-membered cyclic carbonates by cycloaddition of CO2 with epoxides is one of the typical feasible routes. Ionic liquids (ILs) have been widely reported in the cycloaddition reaction of CO2, because of the unique features such as high thermal and chemical stability, easy recyclability, and tunable properties.

Although most of the ILs catalyst systems show a good yield to produce carbonates, high energy inputs such as high temperatures and/or high CO2 pressures are always needed. For energy saving reasons, we designed a series of functional ILs and used them to catalyze the cycloaddition reaction of CO2 with epoxides under mild conditions.

The major work and results for this dissertation are summarized as follows: 1. Carboxylic acid-based ionic liquids (CAILs) were designed and synthesized. They displayed temperature-dependent dissolution−precipitation transitions in propylene carbonate (PC) by controlling the chain length of the carboxyl group.

It was found that it could be attributed to the temperature-controlled hydrogen-bond formation/broken between the CAILs components and PC by NMR investigations and DFT calculations. Such a unique phase behaviour was successfully utilized for the cycloaddition reaction of CO2 with epoxides. Due to the activation of epoxide assisted by the hydrogen bond, the optimal ILs could show a 92% yield of PC within 20 min and quickly precipitate out from a homogeneous system at room temperature for easy recycling.

The reversible phase transition phenomenon supplied an efficient way to combine activity and recovery of homogeneous catalysts, which is a benefit for energy-saving and industrial applications. 2. An efficient 1,8-diazabicyclo- [5.4.0] undec-7-ene (DBU) based bifunctional protic ionic liquids (DBPILs) was easily prepared by acid-base reaction at room temperature.

DBPILs that composed by alkoxy anion, protic acid and nucleophilic groups exhibited high activity for the coupling reaction of CO2 and epoxide under mild conditions. As a metal free catalyst, the best DBPILs showed a 92 % yield of products within 6 hours at 30 oC and 1bar CO2 without any solvents and co-catalysts.

And it could afford carbonates in good yields with broad epoxide substrate scope and CO2 from simulated flue gas (15% CO2/85% N2). IR spectrum and DFT studies were carried out to invest the mechanism of the cycloaddition reaction. The results showed that the DBPILs could activate both CO2 and epoxide by the alkoxy anion and powerful hydrogen-bonding, which was well consistence with experiments. 3.

DBU based ionic liquids containing aluminium (DILA) catalyst was synthesized based on dissociating feature of proton from DBPILs. DILA was used as a single-component and solvent-free catalyst for the conversion of CO2 to cyclic carbonates with epoxide at mild conditions. DILA showed a catalytic activity of 94% within 6 h in catalyzing the reactions of CO2 with epoxides at 30 oC and 1 bar CO2, because of the efficiently activating epoxides by the aluminum ions.

The effect parameters such as temperature, reaction times and catalyst dose were investigated. Furthermore, DILA was found suitable for a broad epoxide substrate scope and could be easily recycled at least four times without obviously loss of activity. 4. A series of cross-linked poly (vinylimidazole/butyl acrylate) ionic liquids membranes (PVBILs) were prepared and used as highly efficient heterogeneous catalysts for fixation of CO2 with epoxide at mild conditions.

PVBILs could show comparable activities in the coupling of epoxide and CO2 with the traditional homogenous catalysts, because of the activity of CO2 and “homogenous-surroundings” caused by swellable properties. Swelling behaviour was studied and found to be consistent with the experimental results of the catalytic activity.

The dependence of conversion yield on catalyst dose, reaction temperature and time were investigated. The best PVBILs could show a 90% yield of carbonate within 24 hours at 50 oC and 1 bar of CO2. 5. A series of metalloporphyrin ionic liquids (MPILs) that combine the advantages of ionic liquid and porphyrin were designed and synthesized by one step method.

The structure and surface appearance of these BPILs were investigated. These BPILs catalysts have multiple active sites and a good capacity of visible light absorption due to the advantages of functional ionic liquids and metal porphyrin part, thus can reduce the energy inputs greatly by using the visible light.

They were successfully used as a photocatalyst in cycloaddition reaction of CO2 with epoxide to produce carbonate at room temperature and 1 bar pressure under irradiation of a visible light.