Catalytic Mechanisms
Silver is an effective catalyst for the epoxidation of ethylene due to its good ability to adsorb oxygen. The form of adsorbed oxygen, however, is different in two possible reaction mechanisms. One mechanism proposes that silver adsorbs oxygen molecularly, while the other mechanism proposes that an oxygen molecule dissociates into atoms on adsorption. The roles of chlorine-containing inhibitors and alkali-metal promoters are also explained differently in each mechanism.
In the molecular (or dioxygen) adsorption model, one molecule of oxygen adsorbs on the silver surface and reacts with one adsorbed ethylene molecule, producing one ethylene oxide molecule and one adsorbed oxygen atom. This remaining oxygen atom participates in a combustion reaction with ethylene. Chlorine inhibitors serve to block active sites from adsorbing atomic oxygen, thereby increasing selectivity. The role of alkali-metal promoters is not fully understood.
Since six oxygen atoms are required for the total oxidation of one ethylene molecule, six ethylene oxide molecules are produced for every molecule of ethylene that is combusted. Thus, the selectivity to ethylene oxide is 6/7, or 85.7% (assuming that neither ethylene nor ethylene oxide are combusted by any other pathways). This mechanism seems to be supported by industrial operations, since the maximum selectivity obtained in an industrial setting is approximately 80% [5]. Mechanistic research has shown, however, that the epoxidation of ethylene performed in the presence of atomic oxygen is not affected by the concentration of adsorbed molecular oxygen. The epoxidation can also be performed with N2O, which is a source of atomic oxygen only [2]. Figure 2 shows mass spectra obtained from a temperature-programmed reactor (TPR) during ethylene epoxidation performed under two sets of circumstances. The epoxidation was performed with both molecular oxygen and atomic oxygen present and with only atomic oxygen present. Molecular oxygen is represented by the short, broad peak of the O+2 signal, and atomic oxygen is represented by the tall, sharp peak. Ethylene oxide production, represented by the CHO+ signal, is independent of molecular oxygen concentration.
Figure 2. Multimass TPR spectra with molecular oxygen present in the left-hand sample and absent in the right-hand sample. [3]
The atomic oxygen adsorption mechanism explains the epoxidation of ethylene in terms of the electronic properties of the reactants. Atomic oxygen is the reactive species in both epoxidation and combustion; molecularly adsorbed oxygen is inactive. Epoxidation occurs when one oxygen atom reacts with the double bond of one adsorbed molecule of ethylene. Combustion occurs when one oxygen atom abstracts the slightly acidic hydrogen atom of one adsorbed molecule of ethylene, which results in complete oxidation. Similarly, one oxygen atom can react with and completely oxidize a newly produced ethylene oxide molecule.
The presence of inhibitors can increase selectivity by altering the electronic properties of the adsorbed oxygen atom and activating it for epoxidation. Electron-withdrawing species, such as adsorbed chlorine, act to decrease the electron density of the oxygen atom. The electron-deficient oxygen favors an attack on the electron-rich double bond of ethylene. Clean silver catalyst samples have been activated by annealing in the presence of oxygen, and this procedure dissolves atomic oxygen in the bulk of the silver [2]. This bulk-dissolved oxygen also acts as an electron-withdrawing species, further activating the surface atomic oxygen. Figure 3 is a representation of how electron-withdrawing species affect reaction selectivity. The top reaction takes place without electron-withdrawing species and produces combustion products; the bottom reaction takes place with electron-withdrawing species (in this figure, dissolved oxygen) and produces epoxidation products.
Figure 3. Surface structures active in combustion (top) and epoxidation (bottom). [1]
Although alkali-metal promoters are electron-donating species, their presence also increases the selectivity of the epoxidation. Electron donors, such as cesium, might be thought to increase oxygen electron density, thereby favoring combustion. But research has found that cesium promoters do not affect the electronic properties of the oxygen, but they instead affect the electronic properties of the ethylene [4]. Cesium prevents both ethylene and ethylene oxide from isomerizing to an isomer that is more active to combustion.
Most of the references sited in this paper favor the atomic oxygen mechanism over the molecular oxygen mechanism. Although the molecular oxygen mechanism presents a selectivity barrier that seems to be encountered in industrial practice, the research supporting the atomic oxygen mechanism is more persuasive. The particularly conclusive results are the successful epoxidations performed in the absence of molecular oxygen and the necessity of dissolved oxygen in the silver bulk to activate the catalyst to epoxidation.