Selective epoxidation of ethylene

The selective epoxidation of ethylene by hydrogen peroxide ia a 1,4-dioxane solvent ia the presence of an arsenic catalyst is claimed. No solvent degradation is observed. Ethylene oxide is the only significant product detected. The catalyst used may be either elemental arsenic, an arsenic compound, or both. 

Supported silver is used for the activation of the O2 in ambient air for the gas-phase selective epoxidation of ethylene (equation 121). ... 

If we apply the "6/7" rule (see Sachtler (17) for explanation) typically cited as evidence for the role of molecular O2 in selective epoxidation of ethylene for the case of butadiene epoxidation, we would not expect selectivity for epoxybutene to exceed "11/12", or 91.7%. In fact, selectivities of 93-96% are typically seen at all reaction conditions. Selectivities of 97-98% are observed at differential conditions and lower reaction temperatures. Therefore, based only upon the observed selectivities to epoxybutene, dissociatively-adsorbed oxygen is clearly the active oxygen in butadiene epoxidation. Further, the kinetic model, which has been derived from the kinetic plots in Figure 5 has been used to very satisfactorily fit a wide variety of reaction data from several different reactor formats, assumes dissociatively-adsorbed oxygen at both promoted and unpromoted Ag sites. The oxygen incorporated into epoxybutene is dissociatively-adsorbed oxygen, not molecular oxygen. 

C. Karavasilis, S. Bebelis, and C.G. Vayenas, Selectivity Maximization of Ethylene Epoxidation via NEMCA with Zirconia and (3"-Al203 Solid Electrolytes, Ionics 1, 85-91 (1995). 

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). 

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. 

FIGURE 21. Selected geometrical parameters of the transition structures for the epoxidation of ethylene with peroxyformic acid calculated at the QCISD/6-31G, CCD/6-31G (in parentheses), B3LYP/6-31G (in square brackets) and MP2(FC)/6-31G (in curly brackets) levels... 

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... 

The recent dramatic increase in the price of petroleum feedstocks has made the search for high selectivities more urgent. Several new processes based on carbon monoxide sources are currently competing with older oxidation processes.103,104 The more straightforward synthesis of acetic acid from methanol carbonylation (Monsanto process) has made the Wacker process obsolete for the manufacture of acetaldehyde, which used to be one of the main acetic acid precursors. Several new methods for the synthesis of ethylene glycol have also recently emerged and will compete with the epoxidation of ethylene, which is not sufficiently selective. The direct synthesis of ethylene... 

The shift from air-based, once-through processes to oxygen-based recycle processes, and the corresponding change from reactant-lean to oxidant-lean processes. This not only considerably reduces the emissions and makes purge streams more concentrated and hence more easily combusted but also may lead to improved selectivity and productivity. Examples are the oxychlorination of ethylene to 1,2-dichloroethane and the epoxidation of ethylene. 

Several profound theoretical and experimental studies performed on the laboratory scale have been reported which focus on the use of various configurations of membrane reactors as a reactant distributor in order to improve selectivity-conversion performances. In particular, several industrially relevant partial oxidations have been investigated, including the oxidative coupling of methane [56], the oxidative dehydrogenations of propane [57], butane [58], methanol [59, 60], the epoxidation of ethylene [61], and the oxidation of butane to maleic anhydride [62]. 

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. 

In industry many selective oxidations are carried out in a homogeneously catalyzed process. Heterogeneous catalysts are also applied in a number of processes, e.g. total combustion for emission control, oxidative coupling of methane, the synthesis of maleic acid from butanes, the epoxidation of ethylene. Here we focus upon heterogeneous catalysis and of the many examples we have selected one. We will illustrate the characteristics of catalytic oxidation on the basis of the epoxidation of ethylene. It has been chosen because it illustrates well the underlying chemistry in many selective oxidation processes. 

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 ... 

In early proposals the species responsible for epoxidation was identified as the adsorbed molecular oxygen, Ag 02(ads)> while combustion was attributed to monoatomic Ag O(ads) (Equations 14-16). The oxidation step envisages the transfer of one atom of molecularly adsorbed oxygen to the double bond, while the other remains adsorbed on silver. The consumption of the latter by the total oxidation of ethylene restores the site vacancies necessary for the continuation of catalysis. Up to a maximum of six oxygen atoms are required for the combustion of one ethylene molecule. Thus, the combination of the reactions (Equation 14) and (Equation 15) predicts that the maximum attainable selectivity in the epoxidation of ethylene is 6/7, i.e., 85.7% (Equation 16). A lower selectivity should normally be expected because some monoatomic oxygen independently formed by dissociative adsorption (Equation 13) raises the level of ethylene combustion above that predicted by Equation 16. 

Discussion Point DP2 Silver is unique among metal catalysts for its high selectivity in the epoxidation of ethylene. Rationalize this result on the basis of the different properties of surface metal-oxygen bonds and of the different adsorption behaviour of hydrocarbons on transition and non-transition metals. 

The catalyst-hydroperoxide complexes are more stable for the metals of Group B than for those of Group A. Epoxidation of the olefins proceeds more easily and more selectively. " They are particularly significant in the industrially important epoxidation of propylene. Reference should be made to the very selective epoxidation of cyclohexene with a Mo complex and to publications relating to the epoxidation of ethylene, hexene-1, and octene-1 with a Cr complex. 

The epoxidation of ethylene on silver is an important industrial process in which the practical yield is lower than 20%, with a selectivity of about 80%. It is remarkable that only Ag selectively catalyzes this reaction. The mechanism of epoxidation originally proposed by Twigg [49] for this reaction can be written as... 

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... 

In an equally unambiguous paper. Grant and Lambert showed that it was atomically held oxygen which was responsible for the epoxidation of ethylene and that the molecularly held species, though present, was merely a spectator [24]. The intermediate responsible for isomerisation was, therefore, CH2-CH2-O-Ag, and Grant and Lambert stated as much in their paper [24]. They produced the reaction mechanism in their paper, in which they proposed that selective oxidation results from an electrophilic attack by an O(a) on the olefinic n-bond. ... 

The authors demonstrated that it was the more weakly held aj state which was the more selective in the epoxidation of ethylene in two experiments. They first desorbed most of the ai state by heating a catalyst dosed with both the ai and 7.2 states to 503 K for 10 min in He, leaving the 72 state unaffected and the ai state reduced to 30% of its original value (Fig. 7.4). 

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