Catalysis of Organic..

448Propylene Oxidation to POSupercritical media, in general, have the potential to increase reaction rates, toenhance the selectivity of chemical reactions and to facilitate relatively easyseparations of reactants, products, and catalysts after reaction (3). However reactionsinvolving CO 2 and water are typically conducted as biphasic processes, with theorganic substrate dissolved mostly in the CO 2 -rich phase and the water-solublecatalysts and/or oxidant dissolved in the aqueous phase. Such systems suffer frominter-phase mass-transfer limitations (4).In this project, a ‘green” method for the epoxidation of propylene is developedby employing CO 2 expanded liquid solvents (CXLs) in the absence of a catalyst.Ternary CXLs are homogeneous mixtures of dense CO 2 , an organic solvent such asacetonitrile or methanol, and water. The oxidant is formed in situ through thereaction of carbon dioxide and hydrogen peroxide to produce peroxycarbonic acid asshown in Figure 1. A base will be used to control the acidity of the solution duringreaction between the peroxycarbonic acid and substrate propylene. By tuning thesolvent properties of the CXL containing CH 3 CN/H 2 O 2 /H 2 O with dense-phase CO 2both substrate and oxidant will be present in a homogeneous reaction mixture insidethe view cell reactor.CO 2 + H 2 O 2HO C OPeroxycarbonic AcidFigure 1. In situ formation of oxidant for the epoxidation of propylene in CXLs.OOHRichardson and co-workers (5). showed that epoxidation of alkene can achievedusing the percarbonate ion which is formed by reaction of H 2 O 2 and sodiumcarbonate under basic conditions. Beckman and his group (2, 6). also reported thelow conversion of propylene to propylene oxide using percarbonate formed viareaction of CO 2 , H 2 O and H 2 O 2 . Presumably, the reaction between H 2 O 2 and CO 2yields a peroxycarbonic acid species, i.e. a compound in which the peroxy group ofhydrogen peroxide binds to the carbon of carbon dioxide (7). According to this view,the peroxycarbonic acid transfers oxygen to the olefin.Building on earlier work in these laboratories (8) we have overcome the typical masstransfer limitations of phase transfer catalysis for propylene oxidation by the use of3-component liquid phases based on CO 2 expanded liquids (CXLs). For theapplication to oxidations by aqueous H 2 O 2 , the organic component of the CXL ischosen because it is miscible with both dense CO 2 and water. In this way,homogeneous systems are produced which decrease mass-transfer limitations andintensify chemical reactions. Previous reports using CXL systems have shown thatthey enhance the oxidation of the substrate and improve the selectivity at moderatereaction temperatures and pressures (3, 8, 9).