Oxidation silver catalysts

Ethylene oxidation to ethylene oxide. Silver catalyst placed in the pores of the tube (8wt% Ag). 

Nitrogen and hydrogen can be determined simultaneously by burning in dry carbon dioxide in the presence of a copper-copper oxide-silver catalyst, when water and nitrogen are liberated and subsequently determined . 

From Ethyl Alcohol. Some acetaldehyde is produced commercially by the catalytic oxidation of ethyl alcohol. The oxidation is carried out by passing alcohol vapors and preheated air over a silver catalyst at 480°C (98). 

Most of the world s commercial formaldehyde is manufactured from methanol and air either by a process using a silver catalyst or one using a metal oxide catalyst. Reactor feed to the former is on the methanol-rich side of a flammable mixture and virtually complete reaction of oxygen is obtained conversely, feed to the metal oxide catalyst is lean in methanol and almost complete conversion of methanol is achieved. 

Silver Catalyst Process. In early formaldehyde plants methanol was oxidized over a copper catalyst, but this has been almost completely replaced with silver (75). The silver-catalyzed reactions occur at essentially atmospheric pressure and 600 to 650°C (76) and can be represented by two simultaneous reactions ... 

Oxidation of a glycol can lead to a variety of products. Periodic acid quantitatively cleaves 1,2-glycols to aldehydes and is used as an analysis method for glycols (12,13). The oxidation of propylene glycol over Pd/C modified with Pb, Bi, or Te forms a mixture of lactic acid, hydroxyacetone, and pymvic acid (14). Air oxidation of propylene glycol using an electrolytic crystalline silver catalyst yields pymvic aldehyde. 

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). 

Propylene oxide is also produced in Hquid-phase homogeneous oxidation reactions using various molybdenum-containing catalysts (209,210), cuprous oxide (211), rhenium compounds (212), or an organomonovalent gold(I) complex (213). Whereas gas-phase oxidation of propylene on silver catalysts results primarily in propylene oxide, water, and carbon dioxide as products, the Hquid-phase oxidation of propylene results in an array of oxidation products, such as propylene oxide, acrolein, propylene glycol, acetone, acetaldehyde, and others. 

Cyclohexane can be dehydrogenated to benzene very cleanly under the same conditions with the same copper-silver catalyst, as can 2-propanol to acetone. These catalysts almost certainly act by virtue of an oxide layer on the metal. 

Dehydrochlorination to Epoxides. The most useful chemical reaction of chlorohydrins is dehydrochlotination to form epoxides (oxkanes). This reaction was first described by Wurtz in 1859 (12) in which ethylene chlorohydria and propylene chlorohydria were treated with aqueous potassium hydroxide [1310-58-3] to form ethylene oxide and propylene oxide, respectively. For many years both of these epoxides were produced industrially by the dehydrochlotination reaction. In the past 40 years, the ethylene oxide process based on chlorohydria has been replaced by the dkect oxidation of ethylene over silver catalysts. However, such epoxides as propylene oxide (qv) and epichl orohydrin are stiU manufactured by processes that involve chlorohydria intermediates. 

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. 

Cyanuric acid can also be prepared from HNCO (100). Isocyanic acid [75-13-8] can be synthesized directiy by oxidation of HCN over a silver catalyst (101) or by reaction of H2, CO, and NO (60—75% yield) over palladium or iridium catalysts at 280—450°C (102). Ammonium cyanate and urea are by-products of the latter reaction. 

The reaction is carried out over a supported metallic silver catalyst at 250—300°C and 1—2 MPa (10—20 bar). A few parts per million (ppm) of 1,2-dichloroethane are added to the ethylene to inhibit further oxidation to carbon dioxide and water. This results ia chlorine generation, which deactivates the surface of the catalyst. Chem Systems of the United States has developed a process that produces ethylene glycol monoacetate as an iatermediate, which on thermal decomposition yields ethylene oxide [75-21-8]. 

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). 

The diazo ketone 3, when treated with silver oxide as catalyst, decomposes into ketocarbene 5 and dinitrogen Na. This decomposition reaction can also be achieved by heating or by irradiation with uv-light. The ketocarbene undergoes a Wolff rearrangement to give a ketene 6 ... 

The main route to ethylene oxide is oxygen or air oxidation of ethylene over a silver catalyst. The reaction is exothermic heat control is important ... 

Formaldehyde, produced by dehydrogenation of methanol, is used almost exclusively in die syndiesis of phenolic resins (Fig. 7.2). Iron oxide, molybdenum oxide, or silver catalysts are typically used for preparing formaldehyde. Air is a safe source of oxygen for this oxidation process. 

Ethylene is currently converted to ethylene oxide with a selectivity of more than 80% under commercial conditions. Typical operating conditions are temperatures in the range 470 to 600 K with total pressures of 1 to 3 Mpa. In order to attain high selectivity to ethylene oxide (>80%), alkali promoters (e.g Rb or Cs) are added to the silver catalyst and ppm levels of chlorinated hydrocarbons (moderators) are added to the gas phase. Recently the addition of Re to the metal and of ppm levels of NOx to the gas phase has been found to further enhance the selectivity to ethylene oxide. 

Methanol oxidation on Ag polycrystalline films interfaced with YSZ at 500°C has been in investigated by Hong et al.52 The kinetic data in open and closed circuit conditions showed significant enhancement in the rate of C02 production under cathodic polarization of the silver catalyst-electrode. Similarly to CH3OH oxidation on Pt,50 the reaction exhibits electrophilic behavior for negative potentials. However, no enhancement of HCHO production rate was observed (Figure 8.48). The rate enhancement ratio of C02 production was up to 2.1, while the faradaic efficiencies for the reaction products defined from... 

C05-0092. Ethylene oxide is produced industrially from the reaction of ethylene with oxygen at atmospheric pressure and 280 °C, in the presence of a silver catalyst ... 

The catalytic route, however, is simple and clean, although it does produce a small amount of CO2. Using silver, promoted by small amounts of chlorine, as the catalyst, ethylene oxide is formed directly from C2H4 and O2 at a selectivity of around 90%, with about 10% of the ethylene ending up as CO2. Nowadays all production facilities for ethylene oxide use catalysts. 

Ethylene oxide is an important intermediate for ethylene glycol (antifreeze) and for plastics, plasticizers, and many other products [R.A. van Santen and H.P.C.E. Kui-pers, Adv. Catal. 35 (1987) 265]. In Chapter 1 we explained that the replacement of the traditional manufacturing process - which generated 1.5 mole of byproducts per 1 mole of epoxide - by a catalytic route based on silver catalysts is a major success story with respect to clean chemistry (Fig. 9.16). 

GP 4] [R 11] For methanol oxidation over sputtered silver catalyst, conversion is higher when using micro channels of smaller diameter (8.5 vol.-% methanol balance oxygen 510 °C 4—27 ms sHghfly > 1 atm) [72]. For two channels of the same width, but different depths (70 pm, 130 pm), concentration differences of nearly 10% at the same residence time were detected, all other parameters being equal. 

Both processes - referring to the non-substituted and substituted methanol reactant- utilize elemental silver catalyst by means of oxidative dehydrogenation. Production is carried out in a pan-like reactor with a 2 cm thick catalyst layer placed on a gas-permeable plate. A selectivity of 95% is obtained at nearly complete conversion. This performance is achieved independent of the size of the reactor, so both at laboratory and production scale, with diameters of 5 cm and 7 m respectively. 

Methyl isocyanate is obtained by oxidation of methylformamide over a silver catalyst [71]. 

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