Ethylene over silver

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 In the past 40 years, the ethylene oxide process based on chlorohydria has been replaced by the dkect oxidation of ethylene over silver catalysts.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

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... 

Ethylene epoxidation. Stoukides and Vayenas studied the epoxidation of ethylene over silver between 250 and 450°C at atmospheric pressure.70 Three reactions occurred,... 

Heterogeneous oxidative processes operate at high temperatures (250-450 6C) and are useful for the synthesis of acrolein and acrylic acid from propylene over bismuth molybdate catalysts, the synthesis of maleic and phthalic anhydrides from the oxidation of benzene (or C4 compounds) and naphthalene (or o-xylene) respectively over vanadium oxide,101 arid the synthesis of ethylene oxide from ethylene over silver catalysts.102... 

C.-F. Mao, M. A. Vannice, High surface area a-aluminas III. Oxidation of ethylene over silver dispersed on high surface area a-alumrna, Appl. Catal. A Gen. 122 (1995) 61. 

N. Tories, X. E. Verikios, The oxidation of ethylene over silver-based alloy catalysts Silver-gold alloys, J. Catal. 123 (1987) 161. 

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... 

The production of ethylene oxide from ethylene over silver is an important industrial catalytic process, and its success depends on achieving a high selectivity for C2H40 rather than for C02, the principal by-product. Figure 21 shows the effect of silver particle size on both TOF and selectivity defined as (ethylene reacted to C2H40)/(total ethylene reacted). 

It is interesting that for the epoxidation of ethylene over silver it is the single-crystal surface that gives the highest TOF. At 217°C, Campbell (328, 329) has found that TOF = 2 s for the Ag (110) surface and about 1 s for the Ag (111) surface (330). The activity and selectivity of the single-crystal surfaces is modified by chlorine (330) and by cesium (331), for example, in much the same way as are those of supported silver catalysts. 

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. 

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 catalytic epoxidation of ethylene on silver has been studied extensively over the last thirty years. The literature in this area is very broad and has been reviewed by several authors (2>2 3). In recent years considerable progress has been made towards a satisfactory understanding of the mechanism of this important and complex catalytic system. 

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. 

As an example consider the oxidation of ethylene over metallic silver at 200 to 300°C. Let M represent the site of adsorption on the metal surface it can be a single Ag atom or a group of two, three, or four Ag... 

Oxidizing ethylene over a clean silver surface gives a selectivity of 45 to 50%, but by suitably moderating the surface with traces of adsorbed chloride the selectivity rises to 75 to 80% (16). The mechanism of this improvement is obscure, but a shift from Case V to Case I coupling can explain it. Undoubtedly the presence of chloride ions on the surface can change the types or relative amounts of adsorbed oxygen species. 

Ethylene oxide is prepared industrially by the vapor phase oxidation of ethylene over a supported silver catalyst at elevated temperatures.49la c Application of this reaction to higher olefins results in complete oxidation of the olefin to carbon dioxide and water. In general, autoxidations of olefins are notoriously unselective because of the many competing reactions of the intermediate peroxy radicals in these systems. 

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 ... 

Kobayashi (22) performed computer simulations via Eq. (4) as applied to his differential fixed-bed reactor. The model gas-phase reaction X Y is considered to pass in series through elementary steps to adsorbed X, an adsorbed intermediate in, adsorbed Y, to give finally Y. The forward and backward rate parameters were adjusted to simulate various mechanisms with their rate-determining steps. The shapes of the response curves for Y for typical mechanisms arc classified as instantaneous, monotonic, overshoot, S shaped, false start, and complex. This paper is a good so urce of ideas for the interpretation of transient responses. These ideas are illustrated by application to the oxidation of ethylene over a silver catalyst (23). The response curves last more than 100 min because the temperature is only 91°C and the bed contains 261 g of catalyst the flow rate is 160 ml/min. 

J. T. Jankowiak, M. A. Barteau, Ethylene epoxidation over silver and copper-silver bimetallic catalysts 1. Kinetics and selectivity, /. Catal. 236 (2005) 366. 

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

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