Mechanism Of Ethylene Epoxidation Research Articles

Ethylene Electrooxidation into Ethylene Oxide on Nanostructured Ag/GDC Electrocatalysts

Ethylene oxide (EO) is nowadays a chemical intermediate of paramount importance. As one of the highest volume petrochemicals, it serves as a precursor for the further production of added value molecules, including plastics, polyester and ethylene glycol. EO is industrially produced by the partial oxidation (epoxidation) of ethylene with air (or oxygen) on Ag/α-Al2O3 catalysts at low reaction temperatures (around 220 oC) and high pressures (10-30 bar). Silver is the only one known active heterogeneous catalyst for the epoxidation of ethylene. The epoxidation is in competition with the combustion side reactions of both ethylene and EO, leading to the production of CO2 and H2O. The mechanism of the epoxidation and, in particular the nature of the involved oxygen species are under debate.We have designed by reactive magnetron sputtering a solid oxide electrochemical cell based on nanostructured Ag/Gadolinia-Doped Ceria (a well-known O2- conductor) catalyst. We found an unprecedent linear increase of the selectivity towards EO (Fig. 1) and of the ethylene conversion. These results demonstrate that EO can be electrocatalytically produced in a solid oxide electrochemical cell between 220 °C and 300 °C on a nanostructured Ag/GDC electrocatalyst prepared by reactive magnetron sputtering. Despite low values of Faradaic efficiencies, the EO selectivity and the ethylene conversion can be increased at 220 °C with the application of small positive currents (< 1 mA) that correspond to an energy consumption of around ~ 1 mW/cm². This proof-of-concept opens the way for a new environmentally-friendly route (without utilization of chlorinated hydrocarbons) for the epoxidation of ethylene in a solid oxide cell via direct electrooxidation of ethylene. These results also underline the key role of O2- species in the mechanism of ethylene epoxidation on Ag. Figure 1

The detailed kinetics and mechanism of ethylene epoxidation on an oxidised Ag/α-Al 2O 3 catalyst

Selectivity in the epoxidation of ethylene over Ag/α-Al 2O 3 catalysts has been shown to be a function of the Ag–O bond strength. The weaker the Ag–O bond is, the more selective it is. Promotion by Cl atoms, which are held under the surface of the Ag, is effected by their weakening the Ag–O bond. Promotion by Cs is effected by its being adsorbed on the Ag sites which form the stronger Ag–O bond, so preventing the formation of the unselective Ag–O bond. The reaction proceeds by the ethylene adsorbing on surface O atoms, forming two types of Ag–O–CH 2–CH 2 intermediates, whose fate is a function of the Ag–O bond strength. The Ag–O–CH 2–CH 2 intermediate formed on the area of the Ag surface having the weaker Ag–O bond (the Ag(1 1 1) surface) has a greater probability of cyclising to form EO than being involved in unselective H abstraction reactions. Conversely, the Ag–O–CH 2–CH 2 intermediate formed on the area of the Ag surface having the stronger Ag–O bond (a stepped surface) will have a greater probability of being involved in unselective H abstraction reactions by the more electronegative surface O atoms than cyclising to form EO. The unselective reaction on both surfaces is a collective reaction of the sea of surface O atoms forming H bonds of different strength to the adsorbed intermediate. The same adsorbed O atom is therefore involved in both the selective and unselective reactions, the extent of which it is involved in each is determined by the Ag–O bond strength. This is an entirely new concept in the partial oxidation of ethylene to ethylene oxide.

Density functional theory study of the ethylene epoxidation over Ti-substituted silicalite (TS-1)

The mechanism of ethylene epoxidation with hydrogen peroxide over Ti-substituted silicalite (TS-1) catalyst was investigated by using both the cluster and embedded cluster approaches at the B3LYP/6-31G(d) level of theory. The complete catalytic cycle was determined. The epoxidation of ethylene consists of three steps. First, the chemisorption of H2O2 at the Ti active site forms the oxygen donating TiOOH species and then the transfer of an oxygen atom from the TiOOH species to the adsorbed ethylene. The final step is the dehydration of the TiOH species to regenerate to active center. The oxygen atom transfer step was found to be the rate-limiting step with the zero-point energy corrected barrier of 17.0 kcal/mol using the embedded cluster model at B3LYP/6-31G(d) level of theory, which is in agreement with the experimental estimate of about 16.7 kcal/mol. Regeneration of the active center by dehydration of the TiOOH species was found to have a rather small barrier and the overall process is exothermic. Our results also show that inclusion of the effects of the zeolite crystal framework is crucial for obtaining quantitative energetic information. For instance, the Madelung potential increases the barrier of the oxygen atom transfer step by 5.0 kcal/mol.

Density functional study of ethylene oxidation on an Ag(111) surface

The mechanism of ethylene epoxidation on Ag surfaces has been investigated using the density functional method and Ag n clusters (n = 3 to 10) modeling the Ag(111) surface. The adsorption energy of O2 to the Ag clusters was strongly dependent on the HOMO level of the cluster, and the clusters with higher HOMO levels afforded larger O2 adsorption energies. The energetics was investigated for both the molecular and atomic oxygen epoxidation mechanisms. For the atomic oxygen mechanism, epoxidation was found to proceed without an activation energy, whereas a small amount of activation energy (about 5 kcal/mol) was calculated for the molecular oxygen mechanism.

Non-Faradaic Electrochemical Modification of Catalytic Activity: X. Ethylene Epoxidation on Ag Deposited on Stabilized ZrO2in the Presence of Chlorine Moderators

The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA), orin situcontrolled promotion was investigated during ethylene epoxidation on Ag films deposited on yttria-stabilized zirconia at temperatures from 240 to 300°C and 500 kPa total pressure in the presence of chlorinated hydrocarbon moderators. It was found that the rates of epoxidation and complex oxidation change by a factor of 230 as the catalyst potential and work function are varied by 0.6 V and 0.6 eV, respectively. The change in the total reaction rate is typically a factor of 100 higher than the rateI/2Fof electrochemical supply or removal of promoting oxide ions. The selectivity to ethylene oxide can be varied between 0 and 78% by varying the catalyst potential and dichloroethane partial pressure. At low catalyst potentials acetaldehyde becomes the main product with a selectivity up to 55%. The observed behavior is discussed on the basis of previous NEMCA studies and of the prevailing ideas about the mechanism of ethylene epoxidation.

In SituControlled Promotion of Catalyst Surfaces via NEMCA: The Effect of Na on the Ag-Catalyzed Ethylene Epoxidation in the Presence of Chlorine Moderators

The effect of non-Faradaic electrochemical modification of catalytic activity, orin situcontrolled promotion, was investigated during ethylene epoxidation on Ag films deposited on β′′-Al2O3, a Na+conductor, at temperatures 240 to 280°C and 500 kPa total pressure in the presence of chlorinated hydrocarbon moderators. The β′′-Al2O3support permits controlled and reversible potentiostatic introduction of various levels of sodium on the Ag catalyst surface. It was found that sodium coverages up to 0.03 enhance the rate of epoxidation without affecting significantly the rate of complete oxidation. The promotion index of sodium for ethylene epoxidation is up to 40. Maximum selectivity to ethylene oxide (88%) is obtained for θNa=0.03 and 1 ppm dichloroethane. The promoting and synergistic action of sodium and chlorine is discussed on the basis of previousin situcontrolled promotion studies and the prevailing ideas about the mechanism of ethylene epoxidation.

Non-faradaic electrochemical modification of catalytic activity 6. Ethylene epoxidation on Ag deposited on stabilized ZrO 2

It was found that the catalytic activity and selectivity of polycrystalline Ag for the epoxidation and complete oxidation of C 2H 4 can be affected in a pronounced and reversible manner by electrochemically supplying or removing oxygen ions O 2− to or from the Ag catalyst surface via ZrO 2 (8 mol% Y 2O 3), an O 2− conducting solid electrolyte. The steady-state changes in the catalytic rates of formation of C 2H 4O and CO 2 are typically 5 to 50 times larger than the rate of O 2− transport to or from the catalyst surface; i.e., the reaction exhibits the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA). Under wide ranges of experimental conditions, the catalytic rates of C 2H 4O and CO 2 formation depend exponentially on catalyst potential and work function. The corresponding activation energies are linearly related to catalyst potential and work function with slopes close to −1. The selectivity to C 2H 4O can be very significantly decreased, but only moderately increased relative to open-circuit conditions. The observed phenomena are discussed and interpreted within the framework of previous NEMCA studies, some independent earlier in situ XPS observations, and the currently prevailing ideas regarding the mechanism of ethylene epoxidation.

Ethylene oxidation over silver catalysts: A study of mechanism using nitrous oxide and isotopically labeled oxygen

The mechanism of ethylene epoxidation over a silver sponge catalyst under steady-state conditions has been probed by comparison of nitrous oxide and oxygen as oxidants. Under noncompetitive conditions the reaction with oxygen is about five times as fast and more than twice as selective. When both are present oxygen inhibits participation by nitrous oxide and the ratio of rates is much larger. Reaction has been carried out with a mixture of 18O 2 and N 2 16O. The aim was to see if 18O was preferentially incorporated into ethylene and 16O into carbon dioxide, as would be expected if diatomic adsorbed oxygen is the source of the epoxide. The data indicate that this does not occur and hence that ethylene oxide and carbon dioxide are both derived from oxygen atoms. However, it is very difficult to exclude artifacts since the minimal involvement of nitrous oxide makes the isotope analyses difficult. No isotopic mixing occurred between 18O 2 and N 2 16O; neither is it observable when ethylene is oxidized by 16O 2/ 18O 2 mixtures. It is detectable when ethylene is absent but the rate is lower than that of ethylene oxidation under similar conditions. Studies have been made using the same silver catalyst promoted by inclusion of dichloroethane in the feed. This raises selectivity to 80% when oxygen is used but the rate is much lower. The chlorine-moderated catalysts have no measurable activity for ethylene oxidation by nitrous oxide. This difference in behavior between the two oxidants can be correlated with their dissociation rates in the absence of ethylene. Surface chlorine sufficient to depress isotope mixing between 16O 2 and 18O 2 by a factor of 7 relative to unpromoted silver renders dissociation of nitrous oxide undetectable. Dissociation of nitrous oxide is also inhibited by adsorbed oxygen even when surface chlorine is present. It is concluded that the rate of ethylene oxidation is related to the rates of dissociation of the respective oxidants. Ethylene oxidation can be faster than oxidant dissociation in the absence of ethylene because reaction with ethylene prevents buildup of inhibiting surface oxygen atoms.

The silver-catalysed decomposition of N 2O and the catalytic oxidation of ethylene by N 2O over Ag(111) and [formula omitted

Single-crystal measurements show that N 2O decomposition is significantly activated on atomically clean Ag(111). The presence of preadsorbed atomic oxygen accelerates the process, as do small amounts of dissolved oxygen; larger amounts of dissolved oxygen have an opposite effect. Isotope distribution data show that the reaction tends to occur preferentially in the vicinity of preexisting surface oxygen atoms. Atomic oxygen deposited by N 2O decomposition is shown to be active for ethylene epoxidation in three different types of experiment, in each of which the effect of gaseous oxygen was negligible. These results provide strong support for the “atomic oxygen” mechanism of ethylene epoxidation.

The mechanism of ethylene epoxidation

The mechanism of the epoxidation of ethylene with oxygen over silver catalysts has been examined. This study was prompted by the notion that if one wishes to increase the selectivity of epoxide formation one should know the phase of adsorbed oxygen that reacts to give epoxide. Experiments were undertaken (a) to determine whether adsorbed atomic oxygen is incorporated into ethylene to give epoxide, and (b) to establish the relative rates of reaction of molecular oxygen impinging from the gas phase and of preadsorbed atomic oxygen. Experiments were performed in a high-vacuum recirculation system designed to allow volumetric absorption studies as well as kinetic studies using a mass spectrometer to be made. The catalyst consisted of silver powder precovered with chemisorbed 16O such that it gave epoxide upon reaction with ethylene. It was contacted with ethylene/ 18O 2 gas mixtures of different ratios. It is shown that under conditions where oxygen isotope equilibration in the gas phase is slow, ethylene initially reacts more rapidly with preadsorbed oxygen atoms than with molecular oxygen adsorbed in the precursor state ( Po 2 < 20 Torr; 1 Torr = 133.3 N m −2) if O ads Ag s ∼ 1. If in the begining only subsurface oxygen atoms are present, this subsurface oxygen and gas-phase oxygen are incorporated into ethylene at about equal rates. It is concluded that ethylene reacts with atomic oxygen to give epoxide either directly, or indirectly, after recombination of the oxygen atoms.

On the Mechanism of Ethylene Epoxidation

Abstract The epoxidation of olefins, while of considerable economic importance, forms a challenging problem to fundamental research in catalysis. When surveying the vast quantity of published material [1-6] two facts appear relevant.