Silver catalysts, direct

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

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. 

Ethylene oxide [75-21-8] was first prepared in 1859 by Wurt2 from 2-chloroethanol (ethylene chlorohydrin) and aqueous potassium hydroxide (1). He later attempted to produce ethylene oxide by direct oxidation but did not succeed (2). Many other researchers were also unsuccesshil (3—6). In 1931, Lefort achieved direct oxidation of ethylene to ethylene oxide using a silver catalyst (7,8). Although early manufacture of ethylene oxide was accompHshed by the chlorohydrin process, the direct oxidation process has been used almost exclusively since 1940. Today about 9.6 x 10 t of ethylene oxide are produced each year worldwide. The primary use for ethylene oxide is in the manufacture of derivatives such as ethylene glycol, surfactants, and ethanolamines. 

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. 

Industrially it is now made by direct gas-phase oxidation of HCN with O2 (over a silver catalyst), or with CI2 (over activated charcoal), or NO2 (over CaO glass). (CN)2 is fairly stable in H2O, EtOH and Et20 but slowly decomposes in solution to give HCN, HNCO, (H2N)2C0 and H2NC(0)C(0)NH2 (oxamide). Alkaline solutions yield CN and (OCN) (cf. halogens). 

Several reactions have been demonstrated using microreactors. One of the potentially more important is the direct synthesis of MIC from oxygen and methyl formamide over a silver catalyst. Dupont have demonstrated this process using a microreactor cell similar to that described above in which the two reactants are mixed, then heated to 300 °C in a separate layer and subsequently passed through another tube coated with the silver catalyst. The estimated capacity of a single cell with tube diameters of a few millimetres is 18 tpa. 

Scheme 10.14 gives some other examples of Wolff rearrangement reactions. Entries 1 and 2 are reactions carried out under the classical silver ion catalysis conditions. Entry 3 is an example of a thermolysis. Entries 4 to 7 are ring contractions done under photolytic conditions. Entry 8, done using a silver catalyst, was a step in the synthesis of macbecin, an antitumor antibiotic. Entry 9, a step in the synthesis of a drug candidate, illustrates direct formation of an amide by trapping the ketene intermediate with an amine. 

When oxygen is pumped to the catalyst the activity of oxygen on the silver catalyst-electrode increases considerably because of the applied voltage. It thus becomes possible to at least partly oxidize the silver catalyst electrode. In a previous communication it has been shown that the phenomenon involves surface rather than bulk oxidation of the silver crystallites (17). The present results establish the direct dependence of the change in the rates of epoxidation and combustion Ari and Ar2 on the cell overvoltage (Equations 2,3, and 5) which is directly related to the surface oxygen activity. 

A method of considerable industrial importance for the large-scale preparation of ethylene oxide is direct oxidation of ethylene at elevated temperatures over a suitably prepared metallic silver catalyst. Although the reaction may be written aa indicated in Eq. (09), in actual practice only about half the ethylene is converted into ethylene oxide, the remainder being oxidized further to carbon dioxide and water. In spite of this seeming disadvantage, catalytic oxidation appears at present to bo economically competitive with chlorohydrin formation aa a means for the commercial production of ethylene oxide.MM Unfortunately, other olefins, such as propylene and mo-butylene for example, apparently give only carbon dioxide and water under the usual oxidation conditions,1310 so that until now the patent hu balance ethylene oxide has been the only representative accessible by tins route. 

Although the chlorohydrin route is still used to convert propylene to propylene oxide, a more efficient air epoxidation of ethylene in the presence of a silver catalyst is used and involves a direct oxidation method (Fig. 1). 

Hu and coworkers have examined N-H (and O-H and S-H) insertions in the presence of silver salts as well as with copper or rhodium catalysts with styryl diazoacetates (Schemes 8.18 and 8.19, Tables 8.9 and 8.10).47 Two possible products (114/117 and 115/118) were obtained that are derived from either direct insertion or insertion with net transposition (Schemes 8.18 and 8.19). Silver and copper salts tended to favor transposition (Table 8.9, entries 2-5 Table 8.10, entries 2-9), whereas rhodium favored direct insertion (Table 8.9, entry 1 Table 8.10, entry 1). The selectivity differences between the two products were again rationalized in terms of two mechanistic pathways. In the case of rhodium-based catalysts, it was proposed that the reaction occurs via a metallocarbene, whereas with copper and silver catalysts the reaction was interpreted as proceeding by Lewis acid activation. 

The developers of new processes have found it at times more expedient to set up their own separate entities to supply the catalysts they were advocating. Allied-Signal s subsidiary UOP did so for its platforming Houdry for its catalytic cracking Ralph Landau for the silver catalyst used for direct ethylene oxidation, which was marketed by Halcon SD and subsequently taken over by Denka, then by Bayer and Phillips Chemical for its polyolefin catalysts, sold through its subsidiary, Catalyst Resources. 

As was pointed out in the discussion of the Grignard method, a larger part of the chlorobenzene molecule appears in the finished silicone product than is true of the methyl chloride molecule. At the same price per pound for raw materials, the basic material cost for phenyl silicone therefore would be less than that for methyl silicone. The difference is accentuated by the fact that chlorobenzene is produced in very large volume at low cost, so that it becomes an inexpensive source of phenyl groups for phenyl silicone. On the other hand, the factors which act to increase the relative cost of phenyl silicone by the direct method are (1) the cost of recovering the silver catalyst, and (2) the possible uneconomical disposition of the hydrochloric acid, which cannot easily be recirculated in the process. 

The industrial production of ethylene oxide is based on the direct oxidation of ethylene in the gas phase on a silver catalyst in cooled, tubular reactors. For a large excess of ethylene the reaction scheme can be simplified to ... 

Description In the direct oxidation process, ethylene and oxygen are mixed with recycle gas and passed through a multi-tubular catalytic reactor (1) to selectively produce EO. A special silver catalyst (high-selectivity catalyst) is used it has been improved significantly over the years. Methane is used as ballast gas. Heat generated by... 

Despite much research the direct oxidation of propylene on silver catalysts remains an unselective reaction yielding primarily carbon oxides and a range of... 

The direct oxidation of propylene on silver catalysts has been intensively investigated, but has failed to provide results with commercial potential. Selectivities are generally too low and the isolation of propylene oxide is complicated by the presence of many by-products. The best reported selectivities are in the range 50-60% for less than 9% propylene conversion. The relatively low selectivity arises from the high temperature necessary for the silver catalysts, the radical nature of molecular oxygen, as well as the allylic hydrogens in propylene. Thus alternative routes have been studied based on the use of oxidants able to act heterolytically under mild conditions. Hypochlorous acid (chlorine+water) and organic hydroperoxides fulfill these requirements and their use has led to the introduction of the chlorohydrin (Box 2) and the hydroperoxide processes, both currently employed commercially. 

Organohalosilanes are industrially produced by direct synthesis from silicon and alkyl- or aryl-halides in the presence of copper or silver catalysts using a process developed by Rochow and Muller in 1941/42. 

For the simultaneous detection of chromium(III) and (VI) after both species have been separated on an ion exchanger, Somerset [153] suggested an oxidation of chro-mium(III) to chromium(VI). The latter can be determined directly via its absorption at 365 nm. Such an oxidation is possible by using potassium peroxodisulfate with a silver catalyst. A complete conversion within a short time is ensured by raising the reaction temperature to about 80 °C. 

Several companies are working on the direct oxidation of propene for instance Lyondell is operating a pilot plant in Newtown Square, PA, and intends to commercialize the technology by 2010. Shell Chemical is also working on a direct route to PO production, based on variations of the gold and silver catalysts it uses to make ethene oxide. 

D. Sullivan, P. Hooks, M. Mier, J. W. van Hal, X. K. Zhang, Effect of support and preparation on silver-based direct olefin epoxidation catalyst. Top. Catal. 38 (2006) 303. 

A number of other processes have been patented for the direct oxidation of ethanol to acetic acid. One of the earliest of these was described by Walter,88 who passed mixtures of ethanol, air, and steam over different catalysts. Since this early disclosure many catalysts have been patented for use at various temperatures. Thus, carbon in the form of coke, etc.,81 is claimed to be effective with air-ethanol mixtures at 150° to 300° C., copper compounds at 270° C.,80 metallic vanadates at 250° to 300° C.,80 and silver catalysts at temperatures 380° to 440° C.,87 to low red heat. Hunt claims the formation of acids from a mixture of isopropanol, olefins, air, and steam in the presence of copper compounds at 200° C.88... 

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