Silver, catalyst selective epoxidation

The epoxidation of non-allylic, or kinetically-hindered, olefins can be carried out using supported silver catalysts. While epoxidation does occur for unpromoted catalysts, the strength of olefin epoxide adsorption leads to low activity and selectivity, as well as irreversible catalyst fouling. The additon of certain alkali metal salts, such as CsCI, lowers the desorption energy of the olefin epoxide, permitting dramatic increases in activity, selectivity, and catalyst lifetime. In the case of butadiene, the addition of an optimum level of CsCI increases activity and selectivity from approximately 1 % butadiene conversion and 50% selectivity for epoxybutene to 15% conversion and 95% selectivity, respectively. 

The selective oxidation is catalyzed by silver, which is the only good catalyst. Other olefins are not converted selectively to the epoxides in the presence of silver. However, propylene epoxidation is appHed commercially the catalysts are either molybdenum complexes in solution or soHd Ti02—Si02 (see... 

The relative increase Ar /r Q in the rates of epoxidation (i=l) and combustion (i=2) is proportional to A/S, where A is the electrolyte surface area and S is the surface area of the silver catalyst electrode. Thus with a reactor having a low value of S (reactive oxygen uptake Q =.4 10 7 mol O2) a threefold increase in ethylene oxide yield was observed with a corresponding 20% increase in selectivity. 

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. 

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 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 gas-phase oxidation of ethylene to ethylene oxide over a supported silver catalyst was discovered in 1933 and is a commercially important industrial process. Using either air or oxygen, the ethylene oxide is produced with 75% selectivity at elevated temperatures (ca. 250 °C). Low yields of epoxides are obtained with propylene and higher alkenes so that other metal-based catalysts are used. A silver-dioxygen complex of ethylene has been implicated as the active reagent.222... 

A fundamental issue in selective oxidation is the activation of C—H bonds that is always required for ODH (oxidative dehydrogenation) and oxo-functionalization and is detrimental for epoxidation. A particular case is silver [70] as catalyst, which can achieve highly selective epoxidation of ethene as well as highly selective dehydrogenation of methanol to formaldehyde although it is notably in both cases only the same metallic catalyst. We will return to this case in the next section, which deals with the multiplicity of active oxygen species. 

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. 

Oxidation of ethene on silver catalysts to yield ethene oxide is a good example of an industrial catalytic process with a high selectivity. In order to confirm a possible correlation between the catalysts affinity towards oxygen and their activity in ethene epoxidation, a heat-flow microcalorimeter equipped with a pulse flow reactor has been used to study the reaction of oxygen at 473 K with a series of silica-supported silver catalysts [71]. At 473 K, adsorption of oxygen at the surface of silver is a fast process incorporation of oxygen into deeper metal layers, though present, is a slow process. 

The results of the oxidation of C2—C5 olefins over copper(i) oxide, silver, and gold catalysts are summarized in Table 1. We have excluded data from studies where additives have been deliberately included in the catalyst, or process gas stream, in order to improve the performance. Where several studies have been carried out we have quoted the best selectively obtained. While copper(i) oxide and gold give unsaturated aldehydes as the major product of partial oxidation, silver gives the epoxide. Copperfii) oxide is not a selective catalyst for olefin oxidation. The difference in behaviour between copper(i) and copper(ii) oxides is in line with the general trend in oxide catalysis. The selective catalysts tend to be those with either a full or an empty tZ-shell, i.e. the oxides of Groups IVA, VA, and VIA, and IB, IIB, IVB, VB, and VIB. ... 

ARCO patents describe various approaches for the development of catalysts able to selectively epoxidize olefins other than ethene with oxygen. In all the catalyst formulations claimed, the main active component is supported silver, doped with various components [31a,b]. In the earlier patents, the best results reported were propene conversion 4.5%, selectivity for PO 59-61%, with a catalyst composition of 54% Ag, 2% K, 0.5% Mo, supported over calcium carbonate. Molybdenum was used to increase the selectivity (but the addition of Mo also caused a decrease in propene conversion). 

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... 

J. R. Monnier, The selective epoxidation of non-allylic olefins over supported silver catalysts, in R. K. Grasselli, S. T. Oyama, A. M. Gaffney, J. E. Lyons (Eds.), 3rd World Congress on Oxidation Catalysis, Elsevier, Amsterdam, 1997, Stud. Surf. Sci. Catal. 110, (1997)135. 

A. Lange de Oliveira, A. Wolf, E. Schueth, Highly selective propene epoxidation with hydrogen/ oxygen mixtures over titania-supported silver catalysts, Catal. Lett. 73 (2001) 157. 

The Selective Epoxidation of Non-Allylic Olefins Over Supported Silver Catalysts... 

US patent 5,763,630 claims silver catalysts supported on other alkaline earth metal compounds than carbonates, such as calcium titanate, tribasic calcium phosphate, calcium molybdate, or calcium fluoride, as well as the magnesium and strontium analogues. Such supports provide significantly higher selectivity to the desired epoxide than would be expected from the performance of related materials. Selectivities are lower than those reported in the original Union Carbide patent. 

Olefin epoxidation is not only important in the manufacture of bulk chemicals, e. g. ethylene and propylene oxides, but is also a widely used transformation in the fine-chemicals industry [1], Ethylene oxide is manufactured by vapor-phase oxidation of ethylene, with air or oxygen, over a supported silver catalyst [2], This method is not generally applicable as olefins containing allylic or other reactive C-H bonds give complex mixtures of products with low epoxide selectivity. The method has recently been extended to some other olefins that do not contain reactive allylic C-H bonds, e. g. butadiene, styrene, norbornene, and tert-butyl ethylene [3]. Some of these products, e. g. butadiene monoepoxide and styrene oxide, have potential applications as fine chemicals/intermediates. 

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. 

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