Metal oxide transfer

It has been shown that the cleavage of the olefin double bond can be avoided using a two-reactor system ((6.18) and (6.19)) in which a metal oxide transfers one lattice oxygen to an olefin with formation of the olefin oxide and is then re-oxidized using dioxygen in a separate reactor [116]. A single reactor can be used which is flushed with the olefin to get the epoxide and then with the correct amount of O2 to regenerate the oxide. 

Metal oxide electrodes have been coated with a monolayer of this same diaminosilane (Table 3, No. 5) by contacting the electrodes with a benzene solution of the silane at room temperature (30). Electroactive moieties attached to such silane-treated electrodes undergo electron-transfer reactions with the underlying metal oxide (31). Dye molecules attached to sdylated electrodes absorb light coincident with the absorption spectmm of the dye, which is a first step toward simple production of photoelectrochemical devices (32) (see Photovoltaic cells). 

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

Metal Cleaning. Citric acid, partially neutralized to - pH 3.5 with ammonia or triethanolamine, is used to clean metal oxides from the water side of steam boilers and nuclear reactors with a two-step single fill operation (104—122). The resulting surface is clean and passivated. This process has a low corrosion rate and is used for both pre-operational mill scale removal and operational cleaning to restore heat-transfer efficiency. 

Calcium carbonate makes up the largest amount of deposit in many cooling water systems (Fig. 4.16) and can be easily detected by effervescence when exposed to acid. Deposits are usually heavily stratified, reflecting changes in water chemistry, heat transfer, and flow. Corrosion may be slight beneath heavy accumulations of fairly pure calcium carbonate, as such layers can inhibit some forms of corrosion. When nearly pure, calcium carbonate is white. However, calcium carbonates are often intermixed with silt, metal oxides, and precipitates, leading to severe underdeposit attack. 

In this process ethylene, dissolved in a liquid hydrocarbon such as cyclohexane, is polymerised by a supported metal oxide catalyst at about 130-160°C and at about 200-500 Ibf/in (1.4-3.5 MPa) pressure. The solvent serves to dissolve polymer as it is formed and as a heat transfer medium but is otherwise inert. 

A substance (usually liquid) employed to remove surface metal oxides in preparation for soldering, brazing or other metal fusion techniques. Also, the rate of energy transfer across a given surface area. 

As was noted by Jones (ref. 12) the success of a metal bromide as a catalyst for alkylaromatic autoxidations depends on the ability of the metal to transfer rapidly and efficiently oxidizing power from various autoxidation intermediates onto bromide ion in a manner which generates Br-. The fact that no free bromine is observable in this system is consistent with rapid reaction of intermediate bromine atoms with the substrate. Inhibition of the reaction by cupric salts can be explained by the rapid removal of Br2 or ArCH2- via one-electron oxidation by Cu (Fig. 10). 

Another industrially important reaction of propylene, related to the one above, is its partial oxidation in the presence of ammonia, resulting in acrylonitrile, H2C=CHCN. This ammoxidation reaction is also catalyzed by mixed metal oxide catalysts, such as bismuth-molybdate or iron antimonate, to which a large number of promoters is added (Fig. 9.19). Being strongly exothermic, ammoxidation is carried out in a fluidized-bed reactor to enable sufficient heat transfer and temperature control (400-500 °C). 

Two samples, one (Sample C) with Cr Oj, the other (Sample A) without any catalytic metal oxide, showed no release at all below 530°C. Judging from the pilot plant experience with FcjOj-containing catalysts, these two are not expected to be able to Action as SO transfer catalysts. 

Certain three-dimensional electrodes, also known as slurry or fluidized-bed electrodes, are sometimes used as well in order to have a strongly enhanced working surface area. Electrodes of this type consist of fine particles of the electrode material (metal, oxide, carbon, or other) kept in suspension in the electrolyte solution by intense mixing or gas bubbling. A certain potential difference is applied to the system between an inert feeder elecnode and an auxiliary electrode that are immersed into the suspension. By charge transfer, the particles of electrode material constantly hitting the feeder electrode acquire its potential (fully or at least in part), so that a desired electrochemical reaction may occur at their surface. In this reaction, the particles lose their charge but reacquire it in subsequent encounters with the feeder electrode. 

The pore size of the catalyst plays an important role as the reactants and the products must be able to lit inside the catalyst to take full advantage of the total surface area available. The pore size of metal oxides are sufficiently large (>2 nm) to facihtate the mass transfer into and from the catalyst pores. This compensates for their lower acidity compared to other sohd acids. Table 33.1 gives an overview of the tested catalysts, showing their pros/cons with respect to the fatty acid esterification reaction. 

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