Ethylene oxide molecular oxygen

Oxidation of organic substrates with molecular oxygen as the oxygen source and catalyzed by metal surfaces is industrially very important reactions. E.g. is ethylene oxide is produced in about 1 x 10 ° kg/year on a silver surface with ethylene and molecular oxygen as reactants, phthalic anhydride and maleic anhydride are produced in about 2 x 109 and 4 x 108 kg/year on a vanadyl pyrophosphate surface with o-xylene and n-butane, respectively, as substrates and molecular oxygen as the oxygen donor (ref. 1). 

Recently, a new process for the conversion of ethylene to ethylene glycol has been developed.2243 Oxidation of ethylene by molecular oxygen in acetic acid, in the presence of a manganese acetate-potassium iodide catalyst, gives ethylene glycol diacetate in 98% selectivity ... 

Another recent patent describes a multistep process for the production of ethylene glycol in near-critical or supercritical CO2 (Bhise, 1983). In this instance CO2 is first used as a solvent and then used as a reactant. Normally, ethylene oxide is produced by the vapor phase oxidation of ethylene with molecular oxygen over a supported silver catalyst. In conventional ethylene glycol processing, an effluent stream containing the ethylene oxide is scrubbed with water to recover the ethylene oxide. The ethylene oxide is then recovered for hydrolysis to ethylene glycol. 

Hayes 130) proposed in 1959 a scheme for ethylene oxidation over silver. He believed that ethylene oxide might be yielded only by interaction between ethylene and atomic oxygen, while carbon dioxide and water would be formed by a reaction of ethylene with molecular oxygen. If this were consistent with reality, the marked selectivity of silver with respect to ethylene oxidation to ethylene oxide would be inexplicable. Atomic oxygen is present on the surface of other metals as well, for instance on platinum and palladium, but no ethylene oxide is formed by reaction of these with ethylene. 

Structure. The straiued configuration of ethylene oxide has been a subject for bonding and molecular orbital studies. Valence bond and early molecular orbital studies have been reviewed (28). Intermediate neglect of differential overlap (INDO) and localized molecular orbital (LMO) calculations have also been performed (29—31). The LMO bond density maps show that the bond density is strongly polarized toward the oxygen atom (30). Maximum bond density hes outside of the CCO triangle, as suggested by the bent bonds of valence—bond theory (32). The H-nmr spectmm of ethylene oxide is consistent with these calculations (33). 

Both end groups can be determined quantitatively. A second side reaction is the transacetalization. Here a poly(oxymethylene) cation attacks an oxygen of a poly(oxymethylene) chain with formation of an oxonium ion that decomposes. Through continued cleavage and recombination of poly(oxymethylene) chains one obtains polymers which are chemically and molecularly largely homogeneous. For the case of a trioxane/ethylene oxide copolymer the following reaction scheme can be formulated ... 

The abstraction reaction appears to be very common and the preceding evidence shows that it occurs with Olc, O , 03, and 03 ions but the reaction with O- is particularly fast. An exception to this, which at the same time provides strong evidence for the participation of a molecular oxygen species, is the selective oxidation of ethylene over silver catalysts to form... 

Propene epoxidation. Stoukides and Vayenas have studied the epoxi-dation of propene over silver catalysts.71 73 A Langmuir-Hishelwood type model was used to explain the results of work performed between 290 and 400°C.71 As with the work on ethylene oxidation, two types of oxygen were proposed to be involved, molecular and atomic oxygen responsible for partial and total oxidation respectively. 

The first reaction is stoichiometric and would be of little value since palladium is expensive except that, in the presence of Cu2+, palladium metal is oxidized back to Pd2+ before it precipitates (Eq. 15.178). The Cu1 produced is reoxidized by molecular oxygen. The mechanism for conversion of ethylene to acetaldehyde has been extensively studied and the intermediates shown in the above cycle are now accepted by most chemists.183... 

Discovered by Phillips in 1894,382 the oxidation of ethylene to acetaldehyde by palladium(ll) salts in an aqueous solution was developed into a commercial process about 60 years later by Smidt and coworkers at Wacker Chemie.383,384 These researchers succeeded in transforming this stoichiometric oxidation by a precious metal (equation 150) into a catalytic reaction through the reoxidation of the resulting Pd° by molecular oxygen in the presence of copper salts (equations 151-152). 

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