Ethylene oxidation on silver

M. Stoukides, and C.G. Vayenas, Solid Electrolyte Aided Study of the Oxidation of Ethylene Oxide on Silver, J. Catal. 64, 18-28 (1980). 

Especially for the low temperature water gas shift reaction the mechanistic scheme, proposed here, seems to correspond to the three different adsorbed oxygen species, proposed by Kobaya-shi (13) for the ethylene oxidation on silver, whereas the importance of some surface complexes of CO - 1 0 type has been revealed (14) by analysing steady state data. 

In industrial applications the achievement of higher activity and selectivity is of course desirable. However, beyond a certain point, they are not the driving forces for extensive research. For instance, current processes for epoxidation of ethylene to ethylene oxide on silver catalysts are so optimized that further increases in selectivity could upset the heat-balance of the process. Amoco s phthalic acid and maleic anhydride processes are similarly well energy-integrated (7). Rather than incremental improvements in performance, forces driving commercial research have been... 

The first thorough investigation of the mechanism of ethylene oxidation on silver surfaces was undertaken by Twigg,1171 who passed a mixture of air and ethylene at 200-350° over fine glasB wool ooated with metallic silver and obtained ethylene oxide, carbon dioxide, and water vapor. The reaction appeared to oonsist of two independent overall processes, which could be depicted separately as shown in Eq. (09) and (100). Of the two reactants, only oxygen was actually o... 

Although the main routes to propylene oxide formation are not based on direct catalytic oxidation of propylene, the direct epox-idation of propylene on silver would be financially preferable if high yield and selectivity to propylene oxide could be achieved. Similarly to ethylene oxidation on silver part of the undesirable byproduct CO2 comes from the secondary oxidation of propylene oxide (2,3). The kinetics of the secondary silver catalyzed oxidation of propylene oxide to CO2 and H2O have been studied by very few investigators (2). 

The first thorou d investigation of the mechanism of ethylene oxidation on silver surfacee was undertaken by who paaeed... 

C. Stegelmarm, P. Stoltze, Isotope effect and selectivity promotion in ethylene oxidation on silver, J. Catal. 232 (2005) 444. 

The oxidation of ethylene oxide on silver yields carbon dioxide and water, but the amounts of these are not equivalent to C2H40 consumption. Twigg believes that this is accounted for by an adsorbed organic residue formed on the catalyst surface. Ethylene also detected in oxidation products was thought to be formed by ethylene oxide decomposition to ethylene and adsorbed oxygen. 

Ozgiilsen et. al. [24] and Chen et. el. [25] conducted numerical optimization of ethylene oxidation on silver in a CSTR. In both papers, substantial improvement in ethylene oxide yield was found, but at different parameters of cyclic regimes. This difference results from the fact that numerical techniques often identify some local maxima. Unfortunately, these papers do not contain interpretation of the results from the reaction mechanism point of view. 

Figure 98. Desorption spectra following adsorption of ethylene oxide on silver at 110 K up to an exposure of 13.3 L [378] A) 0.2-4.1 L B) 2.1 - 13.3 L C) Relation between surface coverage and exposure of ethylene oxide...

Stoukides, M., and S. Pavlou, Ethylene Oxidation on Silver Catalysts Effect of Ethylene Oxide and of External Transfer Limitations, Chem. Eng. Commun. 44 (1986) 53-74. 

For many years ethylene chlorohydrin was manufactured on a large iadustrial scale as a precursor to ethylene oxide, but this process has been almost completely displaced by the direct oxidation of ethylene to ethylene oxide over silver catalysts. However, siace other commercially important epoxides such as propylene oxide and epichlorohydrin cannot be made by direct oxidation of the parent olefin, chlorohydrin iatermediates are stiU important ia the manufacture of these products. 

Similarly, another accident occurred when metallic silver came into contact with aziridine. According to the authors of the report, the accident was interpreted by the formation of an aziridine silver derivative. Comparing this behaviour with the one of ethylene oxide when silver is present, a danger which is of the same nature is demonstrated. The interpretation that had been given at the time was based on the presence of acetylene in ethylene oxide, whose silver derivatives are very sensitive explosives. It may be that acetylene traces were present in aziridine although none of the authors mentioned such as possibility as far as we know. 

Mechanisms There is a derth of knowledge about the mechanisms operative in selective oxidation reactions. The only exceptions are the reactions of ethylene to ethylene oxide on supported silver catalysts and of propylene to acrolein on bismuth molybdate type catalysts. For the latter, it is well established through isotopic labeling experiments that a symmetric allyl radical is an intermediate in the reaction and that its formation is rate-determining. Many studies simply extrapolate the results substantiated for this case to other reactions. New ideas on mechanisms are presented by Oyama, et oL, Parmaliana, et aL, and Laszlo. 

The use of equation (3.2) to study the behaviour of catalysts is known as solid electrolyte potentiometry (SEP). Wagner38 was the first to put forward the idea of using SEP to study catalysts under working conditions. Vayenas and Saltsburg were the first to apply the technique to the fundamental study of a catalytic reaction for the case of the oxidation of sulfur dioxide.39 Since then the technique has been widely used, with particular success in the study of periodic and oscillatory phenomena for such reactions as the oxidation of carbon monoxide on platinum, hydrogen on nickel, ethylene on platinum and propylene oxide on silver. 

A simple Langmuir-Hinshelwood model explains quantitatively the steady-state behavior (4) but it fails to explain the oscillatory phenomena that were observed. The origin of the limit cycles is not clear. Rate oscillations have not been reported previously for silver catalyzed oxidations. Oxidation of ethylene, propylene and ethylene oxide on the same silver surface and under the same temperature, space velocity and air-fuel ratio conditions did not give rise to oscillations. It thus appears that the oscillations are related specifically to the nature of chemisorbed propylene oxide. This is also supported by the lack of any correlation between the limits of oscillatory behavior and the surface oxygen activity as opposed to the isothermal oscillations of the platinum catalyzed ethylene oxidation where the SEP measurements showed that periodic phenomena occur only between specific values of the surface oxygen activity (6,9). 

The gas phase oxidation of ethylene to ethylene oxide over silver catalysts has been studied extensively.49 la-c It has been suggested that epoxide formation involves transfer of oxygen from a silver-oxygen complex to the olefin on the catalyst surface.4913 Silver-on-silica also catalyzes the liquid phase oxidation of cumene to cumene hydroperoxide. A mechanism that involved insertion of coordinated oxygen into a C—H bond was proposed630 ... 

Ethylene oxide may isomerize to acetaldehyde which is readily oxidized on silver to C02 and H20. 

The oxidation and adsorption of ethylene oxide on a silver catalyst is stronger in the presence of acetaldehyde, and the former hinders the formation of carbon dioxide from acetaldehyde. The latter fact is probably due to interaction between acetaldehyde and ethylene oxide at the surface. A conjugated oxidation of this kind is often encountered with homogeneous reactions. 

It may be seen from comparison of results on ethylene oxidation over silver and vanadium pentoxide that with both catalysts the oxidation of unsaturated hydrocarbons will proceed by the same mechanism. C02 generation is not accelerated in the presence of aldehydes and these cannot be intermediates in ethylene combustion. When aldehydes are introduced into the reactant mixture, the ratio of ethylene oxide to C02 formation rates undergoes a change, due to strong adsorption of aldehydes on the catalyst surface. Ethylene oxide will form on silver and is in fact absent on vanadium oxides. It was shown experimentally that the absence of acetaldehyde and formaldehyde in the products of oxidation over silver, and the low absolute content of these substances for vanadium oxides is due to the fact that they are not formed at all, or formed at a low rate, and not to their oxidation or decomposition. 

The following scheme of ethylene oxidation over silver may be proposed on the basis of experimental information available ... 

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. 

Thus the binding of oxygen and hydrocarbons on the platinum surface is different from that for silver. Due to the great strength of oxygen-hydrocarbon bonds, the formation of ethylene oxide on platinum seems to be scarcely probable. It was found, moreover, that the propene oxidation reaction passed into the gas phase even at 70°. 

Ethylene oxidation on a porous silver catalyst proceeds over a wide temperature range in the transient internal region (190-260°). The selectivity of ethylene oxidation at 190-250° is the same inside the grain and at its external surface. 

Large tonnages of ethylene oxide and glycol were also produced by this route, for use in anti-freeze, until the direct oxidation of ethylene to ethylene oxide, using silver catalysts, was developed on a commercial basis. 

There is a bewildering amount of information on the kinetics of ethylene oxidation over silver. Static, flow, differential, and recycle systems have been used with various catalysts, feed ratios, and additives. From this work two conclusions are clear (1) it is not possible to cover all conditions with a simple rate law and (2) attempts to determine mechanisms by means of kinetics are quite risky. Earlier studies have been reviewed by Margolis (f), Sampson and Shooter (2), and Dixon and Longfield (S7). The classic paper was that of Twigg (34), who used mainly a static system. By now it is evident that, although Twigg s results contain much of value, they are not directly applicable to flow systems using modem catalysts. Under practical conditions the... 

The mechanism of ethylene oxidation over silver is not certain. On the basis of kinetic and adsorption data, a number of workers have proposed reaction between gas phase ethylene and adsorbed oxygen. It is more likely, however, that adsorbed ethylene reacts with adsorbed oxygen. This conclusion has been reached by Nault and co-workers (60), Belousov and Bubanik (11), and others. Although ethylene is not strongly adsorbed on bare silver, adsorption on oxygen-covered silver may be another matter. Crucial questions in the mechanism are the nature of the reactive adsorbed ethylene and oxygen species. 

Example 2.2-2 Kinetics of Ethylene Oxidation on a Supported Silver Catalyst... 

Gallezot, P., Tretjak, S., Christidis, Y., Mattioda, G., and Schouteeten, A., Oxidative Dehydrogenation of Ethylene Glycol on Silver Catalyst , presented at the 13th North American Meeting of the Catalysis Society, Pittsburgh, 1993. 

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