The ethylene epoxidation reaction

02 AND C2H4 ADSORPTION AT LOW MILLER Ag SURFACES 3.1. The ethylene epoxidation reaction 

Two di-oxygen moieties are present on Ag(l 1 0) and on Ag(l 0 0), vibrating at 80meV and 84meV respectively [54, 55], On Ag(l 1 0) the latter species is unstable above 100 K, while on Ag(l 00) both moieties desorb at 150 K. Different 02 moieties were found also by theory [56, 57]. The two observed species correspond most probably to 02 sitting either in the fourfold hollows of Ag(l 0 0) or in the troughs of Ag(l 1 0) and to 02 at bridge sites (on both surfaces), respectively. STM measurements showed that the admolecules may order in chains or in c(4 x 4) islands on Ag(l 1 0) [58] and Ag(l 0 0) [59], respectively. 

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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... 

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,... 

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. 

Stereochemistry of the Epoxidation Reaction. The stereochemistry of the ethylene epoxidation reaction has been studied using cis- and trans- dideuterio-ethylene. The retention of configuration was found to be either 70% or about 55%, implying that at least partial rotation is possible at some... 

In this study, the effect of support material on the ethylene epoxidation reaction over Au and Au-Ag/HSAy-Al203 catalysts was determined. Ti02 and Ce02, as reducible oxide supports, were also investigated, in comparison with the HSA alumina support. 

Figure 9.4 illustrates the effect of Au loading of the 13.2 wt.% Ag/7-AI2O3 catalyst on the ethylene epoxidation reaction by plotting Au loading versus the normalized turnover number (the turnover number ratio of Au-Ag catalyst to Ag catalyst). The normalized turnover number of ethylene oxide increased slightly with... 

Table 9.2 compares the selectivity over all studied catalysts. The ethylene oxide selectivity of either the Ag/y-AhOa or Au-Ag/y-AhOa catalysts drastically decreased with increasing reaction temperature. This is because the total oxidation of ethylene is favorable at high temperatures. The addition of an appropriate amount (<0.63 wt.%) of Au to the 13.2 wt.% Ag catalyst (which exists in separate Au particles on Ag particles) was found to promote the ethylene epoxidation reaction by weakening the Ag-O bond in the reaction temperature range of 493-528 K [18,19]. When the Au loading was higher than 0.63 wt.%, the activity of the ethylene epoxidation decreased because of the formation of an Au-Ag alloy which resulted in a decrease in the adsorption capacity of molecular oxygen [18]. 

SERS is important also in the study of catalysis. Here silver itself is of interest such as for the ethylene epoxidation reaction." Moskovits et showed the usefulness of SERS for the study... 

Ethylenic compounds when oxidised with perbenzoic acid or perphthalic acid in chloroform solution yield epoxides (or oxiranes). This Is sometimes known as the Prileschajew epoxidation reaction. Thus pyrene affords styrene oxide (or 2-plienyloxirane) ... 

EXPLORING THE CATALYTIC ACTIVITY OF A NOBLE METAL THE Ag CATALYZED ETHYLENE EPOXIDATION REACTION... 

Because of its outstanding ceramic properties, a-A Ch has been the subject of numerous experimental (529-533) and theoretical studies (534-541). This solid is also used extensively as a support for thin films and for the catalysts used in the industrially important ethylene epoxidation reaction... 

Ethylene epoxidation reaction experiments over all studied catalysts were conducted in a differential flow reacfor, which was operafed at a constant pressure of 3.6 MPa and different reaction temperatures. The tubular reactor having 10-mm internal diameter was placed in a furnace equipped wifh a temperafure confroller. T)q)ically, 30 mg of a catalysf sample was placed inside the Pyrex tube reactor and secured with Pyrex glass wool plugs. The packed catalyst was initially pretreated... 

Poisons can he used to preferentially block a non-favourable pathway. Such selective poisons are often called reaction modifiers. An example of such a system is ethylene epoxidation catalysis. Above a certain coverage of chlorine the ethylene combustion reaction is severely deactivated, whereas the effect on the selective route is less. As soon as the EDC is introduced... 

In early quantum chemical studies that aimed to elucidate the reaction mechanism [89,90],semi-empirical methods were employed, which did not allow a quantitative prediction of activation barriers. A major breakthrough was achieved in 1998, when Rosch and co-workers [91] reported a density functional theory (DFT) study on the mechanism of the ethylene epoxidation with mono- and diperoxo complexes, [Re02(02)Me] and [Re0(02)2Me], derived from methyltrioxorhenium(VII) (MTO). 

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