Silver ethylene oxidation over

Catalyst Selectivity. Selectivity is the property of a catalyst that determines what fraction of a reactant will be converted to a particular product under specified conditions. A catalyst designer must find ways to obtain optimum selectivity from any particular catalyst. For example, in the oxidation of ethylene to ethylene oxide over metallic silver supported on alumina, ethylene is converted both to ethylene oxide and to carbon dioxide and water. In addition, some of the ethylene oxide formed is lost to complete oxidation to carbon dioxide and water. The selectivity to ethylene oxide in this example is defined as the molar fraction of the ethylene converted to ethylene oxide as opposed to carbon dioxide. 

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. 

Ethylene can be oxidized to ethylene oxide over a silver-alumina catalyst. Experimental data were obtained at 260 C and atmospheric pressure (Wan, Ind Eng Chem 45 234, 1951). Selected data are tabulated. Inlet... 

The gas-phase oxidation of ethylene to ethylene oxide over a supported silver catalyst was discovered in 1933 and is a commercially important industrial process. Using either air or oxygen, the ethylene oxide is produced with 75% selectivity at elevated temperatures (ca. 250 °C). Low yields of epoxides are obtained with propylene and higher alkenes so that other metal-based catalysts are used. A silver-dioxygen complex of ethylene has been implicated as the active reagent.222... 

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 ... 

Al-Saleh et al. [Chem. Eng. J., 37 (1988) 35] performed a kinetic study of ethylene oxidation over a silver supported on alumina catalyst in a Betty reactor. At temperatures between 513-553 K and a pressure of 21.5 atm, the observed reaction rates (calculated using the CSTR material balance) were independent of the impeller rotation speed in the range 350-1000 rpm (revolutions per minute). A summary of the data is ... 

C. Stegelmann, N. C. Schiodt, C. T. Campbell, P. Stoltze, Microkinetic modeling of ethylene oxidation over silver, /. Catal. 221 (2004) 630. 

S. R. Seyedmonir, J. K. Plischke, M. A. Varmice, H. W. Young, Ethylene oxidation over small silver crystallites, J. Catal. 123 (1990) 534. 

Supports with small specific surface area are reported to be effective for a formation of ethylene oxide over silver catalysts (ref. 19). This case may be due to suppression of a complete oxidation. However, in this reaction oxygen consumption is almost complete and CO, formation does not increase remarkably with the surface area (Fig. 5). Moreover, the C2 compounds yield does not much... 

F. Kinetics of Ethylene Oxidation to Ethylene Oxide over Silver. 474... 

Fig. 4. Scheme of ethylene oxidation to ethylene oxide over silver, according to Twigg. 

Todes and Adrianova (113) studied the kinetics of ethylene oxidation over pumice-supported silver and suggested that ethylene was converted to ethylene oxide, which yielded C02 and H20. In other words two consecutive reactions took place... 

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. 

Temkin et al. (159) studied the kinetics of ethylene oxidation over a stationary silver surface. It was shown by means of the flow-circulating method that the rate of ethylene oxide and carbon dioxide formation was proportional to ethylene concentration in the gas phase, and that there was inhibition with reaction products. 

Halogen species can also be important bonding modifiers, because they are powerful electron acceptors. Indeed, they are used as promoters in several catalytic processes (for example, ethylene oxidation to ethylene oxide over silver, or during partial oxidation of methane). Nevertheless, their molecular and atomic chemisorption behavior has been studied less and therefore is not as well understood as the role of coadsorbed alkali-metal ions. 

The oxidation of ethylene to ethylene oxide over silver was first published in a patent to Lefort in 1931 (S,9). Since that time many studies of the reaction have been made, and important industrial processes have been developed. Much private research has not been published. Many patents have been issued. Recently a number of new publications have appeared, mainly from academic and government laboratories. In the available information there is much that is conflicting or dubious. In many experiments it is likely that unsuspected impurities played a major role, for silver catalysts have low surface areas and are often significantly moderated by minor amounts of impurities, either from the preparation or from the gaseous reactants. Nevertheless, the main facts are clear. The catalyst is metallic silver and its surface should be moderated with a very small amount of a halogen or similar electronegative material for optimum selectivity. The support or carrier plays a small role it should be inert and of rather low surface area. 

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. 

Dehydrochlorination to Epoxides. The most useful chemical reaction of chlorohydrins is dehydrochlotination to form epoxides (oxkanes). This reaction was first described by Wurtz in 1859 (12) in which ethylene chlorohydria and propylene chlorohydria were treated with aqueous potassium hydroxide [1310-58-3] to form ethylene oxide and propylene oxide, respectively. For many years both of these epoxides were produced industrially by the dehydrochlotination reaction. In the past 40 years, the ethylene oxide process based on chlorohydria has been replaced by the dkect oxidation of ethylene over silver catalysts. However, such epoxides as propylene oxide (qv) and epichl orohydrin are stiU manufactured by processes that involve chlorohydria intermediates. 

The reaction is carried out over a supported metallic silver catalyst at 250—300°C and 1—2 MPa (10—20 bar). A few parts per million (ppm) of 1,2-dichloroethane are added to the ethylene to inhibit further oxidation to carbon dioxide and water. This results ia chlorine generation, which deactivates the surface of the catalyst. Chem Systems of the United States has developed a process that produces ethylene glycol monoacetate as an iatermediate, which on thermal decomposition yields ethylene oxide [75-21-8]. 

Unsteady-State Direct Oxidation Process. Periodic iatermption of the feeds can be used to reduce the sharp temperature gradients associated with the conventional oxidation of ethylene over a silver catalyst (209). Steady and periodic operation of a packed-bed reactor has been iavestigated for the production of ethylene oxide (210). By periodically varyiag the inlet feed concentration of ethylene or oxygen, or both, considerable improvements ia the selectivity to ethylene oxide were claimed. 

The main route to ethylene oxide is oxygen or air oxidation of ethylene over a silver catalyst. The reaction is exothermic heat control is important ... 

Ethylene oxide, the simplest epoxide, is an intermediate in the manufacture of both ethylene glycol, used for automobile antifreeze, and polyester polymers. More than 4 million tons of ethylene oxide is produced each year in the United States by air oxidation of ethylene over a silver oxide catalyst at 300 °C. This process is not useful for other epoxides, however, and is of little value in the laboratory. Note that the name ethylene oxide is not a systematic one because the -ene ending implies the presence of a double bond in the molecule. The name is frequently used, however, because ethylene oxide is derived pom ethylene by addition of an oxygen atom. Other simple epoxides are named similarly. The systematic name for ethylene oxide is 1,2-epoxyethane. 

The catalyst consists of silver supported on alumina and, while it is reasonably specific, appreciable amounts of C02 and H20 are also formed. Over the range of interest, the yield of ethylene oxide is relatively constant so that for present purposes, we may regard the reaction stoichiometry as... 

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