Introduction
The selective oxidation, or epoxidation, to produce ethylene oxide takes place between adsorbed ethylene and adsorbed oxygen. Neglecting the form of adsorbed oxygen (molecular or atomic), the basic reaction stoichiometry can be described as
C2H4 + 1/2O2 -> C2H4O
Since ethylene oxide is such a reactive species, due to its highly strained 3-member ring structure, care must be taken to stop the oxidation after the ethylene oxide is formed and not to further oxidize the product to carbon dioxide and water. In fact, care must also be taken to prevent the total oxidation, or combustion, of ethylene to carbon dioxide and water.
C2H4O + 2 1/2O2 -> 2CO2 + 2H2O
C2H4 + 3O2 -> 2CO2 + 2H2O
Two reaction mechanisms have been proposed for the production of ethylene oxide. One mechanism involves the adsorption of molecular oxygen as the active species in the epoxidation reaction, while the other involves the adsorption of atomic oxygen as the active species. Both of these mechanisms seek to explain how different types of adsorbed oxygen species may affect the selectivity of the reaction.
The molecular mechanism states that molecular oxygen is active in epoxidation, while atomic oxygen is active in combustion. So, according to this mechanism, it would be advantageous to utilize inhibitors to prevent the adsorption of atomic oxygen. The atomic mechanism states that atomic oxygen is the active species in both epoxidation and combustion, while molecular oxygen plays no part. According to this mechanism, it would desirable to use promoters or inhibitors to activate the atomic oxygen towards epoxidation rather than combustion.
Industrial processes do utilize inhibitors and promoters to improve selectivity. Chlorine is adsorbed onto the silver as an inhibitor, and alkali metals, such as cesium, are dispersed in the bulk of the silver as promoters. The roles of these promoters and inhibitors, as well as evidence supporting each of these mechanisms, are discussed under "Catalytic Mechanisms." The importance of improving the selectivity of this process is due to the wide-spread demand for ethylene oxide and its derivatives.
Although ethylene oxide is useful for some applications itself, most of the ethylene oxide produced is converted into other derivatives, particularly ethylene glycol. Derivatives of ethylene oxide include di-, tri-, and polyethylene glycol, as well as monoethylene glycol. These derivatives are used in a variety of applications, such as engine antifreeze, heat transfer fluids, synthetic (polyester) fibers, solvents, and plasticizers. Ethylene oxide itself is used in disinfecting, sterilizing, and fumigating applications. The world-wide capacity for ethylene oxide production in 1995 was over 11 million tons.