CYCLIC CARBONATE SYNTHESIS BY CARBON DIOXIDE FIXATION USING THE scCO2-IONIC LIQUID REACTION SYSTEM

INTRODUCTION From the stand of the protection of the environment, the development of environmentally benign process using chemical fixation of CO2, which is one of the greenhouse-effect gases, has been drawing much interest in industrial chemistry, because there are a lot of possibilities that carbon dioxide can be used as a safe and cheap C1 component to produce useful organic compounds. However, most of the chemical fixaion have taken place at high temperatures and long reaciton time is necessary, because of the low reactibity of CO2. For caron dioxide utilizaion, supercritidal CO2 (scCO2) is attractive not only as a conventional organic solvent but also as a good substrate. Organic reactions under scCO2 conditions are expected to be promoted even at rather milder conditions due to enhanced diffusivity and the disappearance of the gas-liquid phase boundary peculiar to the supercritidal state. The chemical fixaiton of CO2 has thus neen attempted under supercritical conditions. Cyclic carbonate synthesis from epoxide with CO2 is well-known synthesis since 1967. This reaction looks eivironmentally benign process compared to that usign poisonous phosgene, and to synthesize the cyclic carbonates effectively, quite numerous homogeneous catalytic process have so far been reported. However, this process has disadvantages such as long reaction time more than several hours and high reaction temperature more than 100 degree. Hence solid catalysts that possess high thermostability and easy catalyst-product separation leading to their recycling look promising, but the time taken to achieve the reactions in the presence of their catalysts was more than 6 h even at higher temperatures than 140 C, and so the activity was not so high. Recently, we have demonstrated a remarkable promotion of the CO2 chemical fixation in supercritical CO2, especially at the near-critical pressure in the presence of small amount of DMF even without any catalysts. This is due to the function of DMF as a scCO2 soluble acid-base catalyst. However, a reaction time longer than 12 hours was required for the CO2 fixation to obtain cyclic carbonate, because of the poor catalytic activity of DMF. For the sake of further improvement in the activity and selectivity, we attempted to apply scCO2-ionic liquid (IL) biphasic system to the CO2 fixation, because IL can be used as a prominent acidbase catalyst as well as a suitable reaction media. Although in gaseous CO2 below 5 MPa, the introduction of IL into propylene carbonate synthesis from propylene oxide led to high