Selectivity ethylene oxidation over supported silver

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. 

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 first set of reactions is the mainstay of the petrochemical industry 1 outstanding examples are the oxidation of propene to propenal (acrolein) catalysed by bismuth molybdate, and of ethene to oxirane (ethylene oxide) catalysed by silver. In general these processes work at high but not perfect selectivity, the catalysts having been fine-tuned by inclusion of promoters to secure optimum performance. An especially important reaction is the oxidation of ethene in the presence of acetic (ethanoic) acid to form vinyl acetate (ethenyl ethanoate) catalysed by supported palladium-gold catalysts this is treated in Section 8.4. Oxidation reactions are very exothermic, and special precautions have to be taken to avoid the catalyst over-heating. 

Fig. 21. Turnover frequency TOF and selectivity (fraction of ethylene reacted that forms ethylene oxide) for the partial oxidation of ethylene over supported silver catalysts, presented as a function of fraction exposed FE and mean particle size d (see Table XIV for details of the studies).

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. 

Successful examples of selective oxidation catalysis in industry include the conversions of ethylene to ethylene oxide and of methanol to formaldehyde, both on silver catalysts. Ethylene oxide, with an annual worldwide production capacity over 11 million tons, is an important intermediate for the production of glycols (antifreeze agents), ethoxylates (additives in washing powder), cosmetics, polyester fibers, and pharmaceuticals. The partial oxidation of ethylene to ethylene oxide is carried out on silver metal particles supported on o -Al203 or SiC and promoted by alkaline earth or alkali metals. Trace amounts of ethylene dichloride are also fed continuously into the reactor to suppress deep oxidation. Selectivities of about 75-85% are typical nowadays for this process. Formaldehyde, with a production capacity of... 

Olefin epoxidation is not only important in the manufacture of bulk chemicals, e. g. ethylene and propylene oxides, but is also a widely used transformation in the fine-chemicals industry [1], Ethylene oxide is manufactured by vapor-phase oxidation of ethylene, with air or oxygen, over a supported silver catalyst [2], This method is not generally applicable as olefins containing allylic or other reactive C-H bonds give complex mixtures of products with low epoxide selectivity. The method has recently been extended to some other olefins that do not contain reactive allylic C-H bonds, e. g. butadiene, styrene, norbornene, and tert-butyl ethylene [3]. Some of these products, e. g. butadiene monoepoxide and styrene oxide, have potential applications as fine chemicals/intermediates. 

An ethylene oxide plant at Chemische Werke Hills is illustrated in a review article on hydrocarbon oxidation by Broich 19). It is stated that ethylene oxidation is at 240-260° over a supported silver catalyst using 4 % ethylene and 7 % oxygen (added as air) with the balance inert gas. The pressure is 90-150 psig, conversion per pass is 34-40%, and selectivity to ethylene oxide about 60 mole %. The production rate is 300 gm of oxide per liter of catalyst per hour. 

Finally, titanium silicates have also been extensively investigated for the epoxida-tion of olefins. The reaction of ethylene over a silver-supported catalyst to ethylene oxide is one of the few large-scale industrial oxidation reactions with molecular oxygen as the oxidant. Numerous studies have shown TS-1 to be effective at selectively forming propylene oxide (PO) from propylene using hydrogen peroxide as the oxidant. This is a more environmentally friendly route to PO than the currently used chlorhydrin route, and it is likely that this process will see commercialization in the near future. 

The catalyst consists of silver supported on alumina and, while it is reasonably selective, appreciable amounts of CO2 and H2O 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... 

The chemical reactivity of the catalyst support may make important contributions to the catalytic chemistry of the material. We noted earlier that the catalyst support contains acidic and basic hydroxyls. The chemical nature of these hydroxyls will be described in detail in Chapter 5. Whereas the number of basic hydroxyls dominates in alumina, the few highly acidic hydroxyl groups also present on the alumina surface can also dramatically affect catalytic reactions. An example is the selective oxidation of ethylene catalyzed by silver supported by alumina. The epoxide, which is produced by the catalytic reaction of oxygen and ethylene over Ag, can be isomerized to acetaldehyde via the acidic protons present on the surface of the alumina support. The acetaldehyde can then be rapidly oxidized over Ag to COg and H2O. This total combustion reaction system is an example of bifunctional catalysis. This example provides an opportunity to describe the role of promoting compounds added in small amounts to a catalyst to enhance its selectivity or activity by altering the properties of the catalyst support. To suppress the total combustion reaction of ethylene, alkali metal ions such as Cs+ or K+ are typically added to the catalyst support. The alkali metal ions can exchange with the acidic support protons, thus suppressing the isomerization reaction of epoxide to acetaldehyde. This decreases the total combustion and improves the overall catalytic selectivity. 

A mixture of ethylene, oxygen and a diluent (e.g., carbon dioxide or nitrogen) is passed over a silver catalyst at about 250°C and 20 atmospheres. Although other materials have been mentioned, silver appears to be the only catalyst used in commercial-scale ethylene oxide production. Commercial catalysts are prepared by depositing metallic silver on supports such as alumina and silicon carbide. Such catalysts give a selectivity of about 70%, i.e., 70% of the ethylene consumed is converted to ethylene oxide and 30% to carbon dioxide ... 

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