Silver catalyst preparation

The experimental apparatus and the silver catalyst preparation and characterization procedure is described in detail elsewhere (10). The porous catalyst film had a superficial surface area of 2 cm2 and could adsorb approximately (2 +. 5) 10-b moles O2 as determined by oxygen chemisorption followed by titration with ethylene (10). The reactor had a volume of 30 cm3and over the range of flowrates used behaved as a well mixed reactor (10, 11). Further experimental details are given in references (10) and (11). 

Table 2 Characterization data of gold and silver catalysts prepared with carbohydrates...

Specific Activilies oj Silver Catalysts Prepared in Different Ways"... 

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. 

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. 

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. 

Similarly, a catalytic route to indigo was developed by Mitsui Toatsu Chemicals (Inoue et al, 1994) to replace the traditional process, which dates back to the nineteenth century (see earlier), and has a low atom efficiency/high E factor (Fig. 2.15). Indole is prepared by vapour-phase reaction of ethylene glycol with aniline in the presence of a supported silver catalyst. The indole is selectively oxidised to indigo with an alkyl hydroperoxide in the presence of a homogeneous molybdenum catalyst. 

Various carbon-based catalysts were tested in the investigated air gas-diffusion electrodes pure active carbon [6], active carbon promoted with silver [7] or with both silver and nickel. Catalysts prepared by pyrolysis of active carbon impregnated with a solution of the compound Co-tetramethoxyphenylporphyrine (CoTMPP) are also studied [8],... 

The silver catalyst was prepared by reducing silver oxide. The silver oxide used was prepared by adding a solution of potasium hydroxide to an aqueous solution of silver nitrate. A small amount of 0.3% K SO solution was added to the silver oxide powder as a promoter and, after mixing, was dried at 105°C for 24hr in a dark room. This was coated on a-A O of 20-42 mesh in the presence of a small amount of ethanol until the sample reached a size of 12-14 mesh. After the ethanol in the silver oxide powder had been completely vaporized in air at room temperature, the sample was reduced in a reactor with a flow of for 12 hr at 50°C and successively for 12 hr at 100°C. The composition of the catalyst so prepared was 206.0 g-Ag, 1.132 g-K S0 / 5 3.5 g-A O. The BET surface area was 0.3 m / g-A.g. HThe constant activity of this catalyst was obtained by flowing the mixture of 5% C H, 20% 0 and 75% He at 91°C for 48 hr. 1... 

In this part, we prepared and studied the Ag/Si02 catalyst by one-step and two-step sol-gel methods. The results show that the Ag/Si02 catalyst prepared here is one kind of bulk material which has a high surface area. The Ag/Si02 catalyst is made up with functional component of Ag or silver oxide in 20 to 30 nm and carrier Si02. Moreover, we found that the different preparation methods have great effect on crystal structure of the samples. The structure of the sample prepared by the one-step method is always a single crystal structure. And the structure of the sample prepared by the two-step method is always a mixed crystal structure. 

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. 

O-Glycosylation. Traditional glycosylation catalysts are silver or mercury salts. Recently silver zeolite2 has been recommended as the catalyst for preparation of 1,2-cw-glycosides. The thallium zeolite is useful when the glycosyl bromide is unstable in the presence of silver catalysts.1 Example ... 

In this study they condensed the a-glycosyl bromide (243) with the disaccharide (245) in the presence of silver carbonate — silver perchlorate to give the a-linked tri-saccharide (241) in 58% yield or the bromide (244) with the disaccharide (246) in the presence of the mixed silver catalysts to give the a-linked trisaccharide (242) in 63 % yield. In the earlier approach, the preparation of the P-chloride (237) required a previous treatment of the a-bromide with tetraethylammonium chloride under carefully controlled conditions. 

Aza complexes, with mono-Cp Ti(IV), 4, 417 Aza-crown-substituted ruthenocenes, preparation, 6, 635-636 Aza-Diels-Alder reactions, via silver catalysts, 9, 567... 

Cyclohexyldienyl complexes, with Ti(IV), 4, 327 Cyclohexylisocyanides, with gold(I) halides, 2, 281 Cyclohexylphosphine, for semiconductor growth, 12, 9 Cyclohexyl selenides, preparation, 9, 480 Cyclohydrocarbonylation alkenes, 11, 515 alkynes, 11, 522 dienes, 11, 522 overview, 11, 511-555 for ring expansion, 11, 527 Cycloisomerizations, via silver catalysts, 9, 558 Cyclomanganation, product types, 5, 777-778 Cyclometallated azobenzenes, liquid crystals, 12, 251 Cyclometallated complexes for OLEDs... 

Catalysts prepared by the wash-coating method were first used to check the reproduction of the measured values. For this reason, six elementary metal salts (platinum, zirconium, molybdenum, nickel, silver, and rhodium) were dissolved and impregnated onto a titer-plate. The catalysts were pre-reduced inside the reactor with 5% hydrogen in 95% nitrogen at 250 °C. The results were recorded first before the pre-reduction and then after the pre-reduction. The repeated measurements indicated good reproducibility in both cases. The conversion of methane with the rhodium catalyst is better after the pre-reduction. Methane conversion after 18 h runtime was still stable. 

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