Catalytic formation of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture with tetradentate Schiff-base complexes as catalyst

Various tetradentate Schiff-base aluminum complexes (designated as SalenA1X) were prepared and used as catalysts for the synthesis of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture. Their catalytic activities can be markedly enhanced in the presence of a Lewis base or quaternary salt. With SalenAlCl/n-Bu4NBr as catalyst, the formation rate of ethylene carbonate is up to 2220 turnovers/h and about two times that under 4.0 MPa CO2 constant pressure at 110 degreesC. The high rate of reaction may be attributed to rapid diffusion and high miscibility of ethylene oxide in supercritical carbon dioxide under employed conditions. Axial X- group and substitution on the aromatic rings of SalenA1X also affect catalytic properties of these aluminum complexes. Compared to SalenA1X, some other metal-Salen complexes, alone or combined with a Lewis base as co-catalyst, show very little catalytic activity towards the reaction under employed conditions. However, with a quaternary salt as co-catalyst, these metal-Salen complexes all exhibit catalytic activities, which are in the following order: SalenCrCl > SalenCo > SalenNi > SalenMg, SalenCu, SalenZn. (C) 2002 Elsevier Science B.V. All rights reserved.