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 CO2 was generally initiated by nucleophilic attack of the alcoholate(OCH2CH2BrNBu4) at the carbon atom of CO2, and weak interaction between the central metal ion of SalenAlX and the lone pairs of one oxygen atom of CO2. It resulted in the insertion of CO2 to Al-O bond of Salen(X)AL-OCH2CH2BrN(n-Bu)(4) or SalenAl-OCH2CH2X 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. (C) 2002 Elsevier Science B.V. All rights reserved.