Ethylene epoxidation selectivity

S. Linic, M. A. Barteau, Control of ethylene epoxidation selectivity by surface oxametallacycles, /. Am. Chem. Soc. 125 (2003) 4034. 

Despite the poisoning action of Cl for oxygen dissociative adsorption on Ag, it is used as moderator in the ethylene epoxidation reaction in order to attain high selectivity to ethylene oxide. The presence of Cl adatoms in this... 

In summary one can view the ethylene epoxidation system as one where selectivity maximization requires the coexistence of the following two adsorption reactant states ... 

The commonly held view of the uniqueness of Ag for ethylene epoxidation may soon change in view both of the propene epoxidation work of Haruta and coworkers on Au/Ti02 catalysts upon cofeeding H2 123 and also in view of the recent demonstration by Lambert and coworkers124 126 that Cu(lll) and Cu(110) surfaces are both extremely efficient in the epoxidation of styrene and butadiene to the corresponding epoxides. In fact Cu was found to be more selective than Ag under UHV conditions with selectivities approaching 100%.124-126 The epoxidation mechanism appears to be rather similar with that on Ag as both systems involve O-assisted alkene adsorption and it remains to be seen if appropriately promoted Cu124 126 can maintain its spectacular selectivity under process conditions. 

Figure 4.42. Ethylene epoxidation on Ag/p"-Al203.101 Steady-state effect of catalyst potential on the selectivity to ethylene oxide at various levels of gas-phase dichloroethane (a) and 3-dimensional representation of the effect of dichloroethane concentration, catalyst potential and corresponding Na coverage on the selectivity to ethylene oxide (b).101 Reprinted with permission from Academic Press.

Figure 8.38. Steady state effect of current on the increase in the rates of ethylene epoxidation (rj) and deep oxidation to CO2 (r2) of C2H4 on Ag and comparison with the rate Go2=I/4F of electrochemical oxygen supply42 pC2H4=l-6 kPa, pO2=10 kPa, T=400°C intrinsic (1=0) selectivity 0.5, Reprinted with permission from Academic Press.

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). 

The epoxidation of C2H4 on Ag/p"-Al203 was investigated22 at temperatures 250° to 300°C and high pressure (5 bar) in the presence of C2H4CI2 moderators in order to simulate industrial practice.22 It was found that technologically important ethylene oxide selectivity values (Sc2H40ss 8%) can... 

Figure 9,11. Ethylene epoxidation on Ag/fT-AhC Transient effect of a negative applied current (Na supply to the catalyst) on catalyst potential, Na coverage and selectivity to ethylene oxide22 Conditions as in Fig. 9.10. Reprinted with permission from Academic Press.

Electrochemical promotion, or non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) came as a rather unexpected discovery in 1980 when with my student Mike Stoukides at MIT we were trying to influence in situ the rate and selectivity of ethylene epoxidation by fixing the oxygen activity on a Ag catalyst film deposited on a ceramic O2 conductor via electrical potential application between the catalyst and a counter electrode. 

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). 

Modifying the selectivity for a particular product is a more challenging task. To understand why Ag is the most selective catalyst for ethylene epoxidation, an highly important reaction practiced industrially for decades, Linic et al. performed detailed spectroscopic and kinetic isotope experiments and DFT calculations, and they concluded that the selectivity between the partial and total oxidation of ethylene on Ag(l 11) is controlled by the relative stability of two different transition states (TS s) that are both accessible to a common oxametallacycle intermediate One results in the closure of the epoxide ring and ethylene oxide (EO), while the other leads to acetaldehyde (AC) via intra-molecular H shift and eventually combustion. The authors... 

Linic and co-workers provided two additional examples of modifying selectivity for the ethylene epoxidation reaction. They demonstrated the promotional effect of Cs on EO formation using DFT calculations Cs atoms increase AEa by up to 0.2 eV vs. Ag only, via an induced electric field that interacts with the different dipoles of the two TS s." More recently Christopher et al. synthesized and tested (100) facet-dominated Ag nanowire catalysts, based on the DFT results that AEa is ca. 0.1 eV larger on Ag(lOO) than on Ag(lll), because of the extra elongation of the Ag-adsorbate bonds required to form the TS to AC on Ag(lOO). The Ag nanowire catalysts were indeed more selective than conventional Ag catalysts, in which Ag particles mainly exposes the (111) facet." Incidentally,... 

Transient response techniques are used to investigate the activation of silver powder for ethylene epoxidation at vacuum and atmospheric pressures. Results indicate that the activation process is qualitatively the same in both pressure regimes. Numerical simulation of the process indicates that activation involves the concurrent incorporation of oxygen into surface and subsurface sites. The reaction selectivity parallels the incorporation of oxygen into the subsurface. 

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. 

The ethylene epoxidation reaction network, occurring in a Dirac-type catalyst, has previously been studied theoretically (7-8). Both studies showed that the selectivity to eAylene oxide is maximized when the catalyst is located at the external surface of the pellet, i.e. for an egg-shell type catalyst. A systematic experimental investigation of the performance of such catalysts for this industrially important reaction network has recently been reported (9). A summary of this work, as well as some new results, are presented in this paper. 

Indeed Barteau and co-workers have already made important progress on the issue of epoxidation selectivity (see [60] for example). They have concluded that the branching of the oxametallacycle into ethylene oxide or acetaldehyde is the key factor controlling selectivity, as shown in Scheme IV. 

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 ... 

Linic S, Jankowiak J, Barteau MA (2004) Selectivity driven design of bimetallic ethylene epoxidation catalysts from first-principles. J Catal 226 245... 

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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