Synthesis of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture in the presence of bifunctional catalyst

Abstract

Ethylene carbonate was rapidly synthesized from supercritical carbon dioxide/ethylene oxide mixture by using as catalyst the system of tetradentate schiff-base aluminum complexes (designated as SalenAlX) coupling with a quaternary ammonium or phosphonium salt. The high rate of reaction was attributed to rapid diffusion and high miscibility of ethylene oxide in supercritical carbon dioxide under employed conditions. Various reaction periods present different formation rate of ethylene carbonate, mainly due to the existence of phase change during the reaction. The synergistic effect of the binary catalyst for ring-opening of ethylene oxide results from nucleophilicity of highly reactive anions of quaternary salts and the electrophilic interaction of SalenAlX with ethylene oxide. The activation of CO 2 was generally initiated by nucleophilic attack of the alcoholate(OCH 2CH 2BrNBu 4) at the carbon atom of CO 2, and weak interaction between the central metal ion of SalenAlX and the lone pairs of one oxygen atom of CO 2. It resulted in the insertion of CO 2 to AlO bond of Salen(X)AlOCH 2CH 2BrN( n-Bu) 4 or SalenAlOCH 2CH 2X to form linear carbonate which was transformed into ethylene carbonate by intramolecular substitution of halides. The experimental results demonstrate that supercritical carbon dioxide could be used as not only an environmentally benign solvent but also a carbon precursor in synthesis.