Selective oxidation of ethylene

A good example is provided by the selective oxidation of ethylene to ethylene epoxide, an important intermediate towards ethylene glycol (antifreeze) and various polyethers and polyurethanes (Fig. 1.6). 

The abstraction reaction appears to be very common and the preceding evidence shows that it occurs with Olc, O , 03, and 03 ions but the reaction with O- is particularly fast. An exception to this, which at the same time provides strong evidence for the participation of a molecular oxygen species, is the selective oxidation of ethylene over silver catalysts to form... 

Selective oxidation of ethylene to acetaldehyde was carried out over carbon-supported Pd and Pt membrane catalysts.1322 The concept of supported liquid-phase catalysis was also successfully applied in the Wacker oxidation.1323 The Wacker reaction can be performed in alcohol-supercritical C02.1324 C02 as cosolvent accelerates reaction rates and remarkably affects the selectivity towards methyl ketone in the presence of an alcohol. 

A third group contains those metal catalysts which do not form specific crystal phases in an oxidized state. The common types of oxygen on the surface are then 02 (adsorbed) and O (adsorbed) which generally do not lead to selective oxidation. One of the exceptions is silver, which very probably catalyses the selective oxidation of ethylene by providing 02 on the surface. However, an active role of surface oxides, which may be formed particularly by the action of promotors, is not excluded. 

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. 

Ethylene epoxide (EO) is an important intermediate in the chemical industry and the mechanism of its formation has been studied in detail [94-98]. For the industrial aspects see Chapter 2. EO is produced by the selective oxidation of ethylene with oxygen ... 

Fig. 5.13. Proposed scheme for selective oxidation of ethylene by preadsorbed molecular oxygen.

In addition, silver is used on a very large scale in industry as an catalyst for the selective oxidation of ethylene to ethylene oxide ( an epoxide ) . The selectivity of silver catalysts can be enhanced by addition of trace amounts of alkali... 

It is a truism, bordering on a cliche, in heterogeneous catalysis to state that Ag is unique in its ability to epoxidise ethylene to ethylene oxide (EO). While there is no disagreement about the veracity of this truism, there is considerable disagreement and debate as to the means by which it effects this reaction. The overall reaction is superficially simple, comprising only three reactions (1) the selective oxidation of ethylene to EO (reaction 7.1), (2) the unselective oxidation of ethylene to CO2 and H2O (reaction 7.2) and (7.3) the over-oxidation of EO to COo and H2O (reaction 7.3). 

The authors therefore concluded that subsurface oxygen was not essential for the selective oxidation of ethylene on an Ag/7.-AI2O3 catalyst. Its presence had no effect on the activity or selectivity of the surface O atoms [3]. 

S. Rojluechai, Selective oxidation of ethylene qver supported ag and bimetallic Au-Ag catalysis, Ph.D. Dissertation, Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand, 2006. 

In the selective oxidation of ethylene over silver catalyst, the further oxidation of the product, ethylene oxide, EO, decreases the overall selectivity of the reaction. The isomerization of EO to acetaldehyde, AcH, has therefore been investigated8 over a single crystal Ag(lll) surface, which is an efficient catalyst for the isomerization EO -> CH3CHO, between 300 and 500 K and at pressures of up to 2 torr. Below about 410 K the rate of isomerization of the adsorbed reactant is rate limiting at higher temperatures adsorption becomes rate controlling. The dissolved oxygen, C)(d), enhances and the preadsorbed... 

Campbell (144, 145) has also studied Cs promotion of the selective oxidation of ethylene over Ag(lll) at low conversions. Under medium-pressure reaction conditions, the Cs is stabilized as a surface cesium oxide of approximate stoichiometry Cs O = 1 3, where bonding of the oxygen atoms both to the surface Ag atoms and to at least one Cs atom is... 

The selective oxidation of ethylene to acetaldehyde with Pd VCu° chloride solutions has attained major industrial importance (Wacker process). This reaction can be regarded as an oxidative olefin substimtion (oxypalladation). Once again the in-... 

The selective oxidation of ethylene to ethylene oxide (EO) is performed on supported silver catalysts at temperatures of250—280 °C, and a pressure of roughly 20 bar. In this process, it is necessary to avoid secondary reactions of EO. Typical industrial catalysts may contain 8—15 wt% silver dispersed on low surface area (X-AI2O3 (0.5-1.3 m g ) with a porosity of about 0.2—0.7 cm g In addition, the catalyst may contain several promoters in varying amounts (ppm by weight) 500—1200 ppm alkali metal (mostly cesium), 5-300 ppm of sulfur as cesium or ammonium sulfate, 10-300 ppm offluorine as ammonium fluoride, or alkaft metal fluoride (427). 

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. 

Vinyl acetate is synthesized via the selective oxidation of ethylene and acetic acid over supported Pd and PdAu catalysts via the reaction... 

A fuel cell reactor was designed for oxidation of ethylene with O2, as shown in Fig. 1. The cell was assembled using a membrane anode prepared from Pd-black supported carbon, a membrane cathode prepared from Pt-black supported carbon, and an electrolyte membrane of H3PO4/ Si02-wool. Selective oxidation of ethylene to acetaldehyde with a 95 % selectivity was performed using ethylene-02 fuel cell reaction at 373 K. Electrocatalysis of the Pd/C anode for the partial oxidation of ethylene is essential [1,4]. 

One method used to probe the crystallite interior involves an appropriate chemical reaction. It is assumed that the noncrystalline region is severed from the crystalline one by the reaction. An example of this method is the selective oxidation of ethylene copolymers.(80-83) It is presumed that the crystalline core remains behind. The residue can then be analyzed by several different methods. The problem here, as well as with other chemical methods, is in establishing the reactivity, and thus the contribution, of the interfacial region. The concentration of B units in this region will be relatively high. Therefore, if this region is not completely removed by the... 

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