Ethylene epoxidation silver catalyst

Key Words Ethylene epoxidation, Silver catalyst, Gold catalyst, Au-Ag catalyst. 2008 Elsevier B.V. 

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 In the past 40 years, the ethylene oxide process based on chlorohydria has been replaced by the dkect oxidation of ethylene over silver catalysts.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. 

While chlorine is a poison for the ammonia synthesis over iron, it serves as a promoter in the epoxidation of ethylene over silver catalysts, where it increases the selectivity to ethylene oxide at the cost of the undesired total combustion to C02. In this case an interesting correlation was observed between the AgCl27Cl ratio from SIMS, which reflects the extent to which silver is chlorinated, and the selectivity towards ethylene oxide [16]. In both examples, the molecular clusters reveal which elements are in contact in the surface region of the catalyst. 

Silver is an important metallic catalyst for the selective oxidation of ethylene. The silver catalyst is used to selectively convert ethylene to ethylene epoxide, an important intermediate for antifreeze. Whereas the epoxidation of ethylene proceeds with high selectivity on oxidic silver phases, metallic silver surfaces give only total oxidation of ethylene. Electron-deficient O is created on oxidized silver surfaces and this readily inserts into the electron-rich ethylene bond. 

It was traditionally believed that the ethylene epoxidation network was triangular with both ethylene and ethylene oxide contributing to CO2 (Fig, 1,5, Scheme 1). Cant and Elall studied the epoxidation of deuterium-labeled and unlabeled ethylene on silver catalysts and found an inverse isotope effect in... 

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

Lefort Heterogeneous epoxidation of ethylene on silver catalyst... 

The catalytic route offers an excellent alternative for the production of ethylene epoxide. Silver is the best catalyst for this reaction. The reaction proceeds via the following elementary steps ... 

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. 

It seems that silver is a unique epoxidation catalyst for ethylene. All other catalysts are relatively ineffective, and the reaction to ethylene is limited among lower olefins. Propylene and butylenes do not form epoxides through this route. ... 

Ethylene oxide is an important intermediate for ethylene glycol (antifreeze) and for plastics, plasticizers, and many other products [R.A. van Santen and H.P.C.E. Kui-pers, Adv. Catal. 35 (1987) 265]. In Chapter 1 we explained that the replacement of the traditional manufacturing process - which generated 1.5 mole of byproducts per 1 mole of epoxide - by a catalytic route based on silver catalysts is a major success story with respect to clean chemistry (Fig. 9.16). 

The relative increase Ar /r Q in the rates of epoxidation (i=l) and combustion (i=2) is proportional to A/S, where A is the electrolyte surface area and S is the surface area of the silver catalyst electrode. Thus with a reactor having a low value of S (reactive oxygen uptake Q =.4 10 7 mol O2) a threefold increase in ethylene oxide yield was observed with a corresponding 20% increase in selectivity. 

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... 

Optimal Distribution of Silver Catalyst for Epoxidation of Ethylene... 

The optimal distribution of silver catalyst in a-Al203 pellets is investigated experimentally for the ethylene epoxidation reaction network, using a novel single-pellet reactor. Previous theoretical work suggests that a Dirac-delta type distribution of the catalyst is optimal. This distribution is approximated in practice by a step-distribution of narrow width. The effect of the location and width of the active layer on the conversion of ethylene and the selectivity to ethylene oxide, for various ethylene feed concentrations and reaction temperatures, is discussed. The results clearly demonstrate that for optimum selectivity, the silver catalyst should be placed in a thin layer at the external surface of the pellet. 

Propene epoxidation. Stoukides and Vayenas have studied the epoxi-dation of propene over silver catalysts.71 73 A Langmuir-Hishelwood type model was used to explain the results of work performed between 290 and 400°C.71 As with the work on ethylene oxidation, two types of oxygen were proposed to be involved, molecular and atomic oxygen responsible for partial and total oxidation respectively. 

Kenson and Lapkin [173] measured the rate of isomerization of the epoxide to acetaldehyde on a supported silver catalyst and found the rate of epoxide conversion REO 3.9 X 10 4 PE0 at 200°C with an activation energy of 9.8 kcal mol-1. Acetaldehyde may be an intermediate in the combustion of ethylene oxide, but, as such, it is unimportant because it is rapidly oxidized. 

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... 

Epoxides (oxiranes) are three-membered cyclic ethers. The simplest and commercially most important example is ethylene oxide, manufactured from ethylene, air, and a silver catalyst. In the laboratory, epoxides are most commonly prepared from alkenes and organic peroxy acids. 

Although the chlorohydrin route is still used to convert propylene to propylene oxide, a more efficient air epoxidation of ethylene in the presence of a silver catalyst is used and involves a direct oxidation method (Fig. 1). 

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 ... 

Is the Epoxidation of Olefins Other than Ethylene Feasible on Silver Catalysts ... 

Ethylene oxide is manufactured by oxidizing ethylene with air or oxygen in the presence of a silver catalyst Alkenes furnish hydroperoxides when oxidized by oxygen in the presence of catalysts like salts of cobalt and manganese the hydroperoxides are transformed to a number of products, including epoxides. Only in a few cases, such as oxidation of 1-phenylcyclooctene, have moderate yields of epoxides been obtained during autoxidation. ... 

Table 1 Intrinsic Rate of Ethylene Epoxidation and Selectivity Toward Ethylene Oxide as a Eunction of Mean Silver Particle Size for ETnpromoted Ag/Al203 Supported Catalysts... 

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