Oxidative alkali promotion

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. 

Figure 6.11 comes from the classical promotion literature and refers to CO oxidation on Pt(lll) promoted with Li.83 As with every alkali promoter,... 

This simple concept has already found some practical applications The idea to use supported alkali-promoted noble metal catalysts for NO reduction,3,4 even under mildly oxidizing conditions,5 came as a direct consequence of electrochemical promotion studies utilizing both YSZ (Chapter 8) and p"-Al203 (Chapter 9), which showed clearly the electrophi-licity of the NO reduction reaction even in presence of coadsorbed O. This dictated the use of a judiciously chosen alkali promoter coverage to enhance both the rate and selectivity under realistic operating conditions on conventional supported catalysts. 

The mass fraction f2 increases strongly in the order Li-, Na-, K-, and Cs-oxide/ carbonate. However, the increase of the growth probability a2 is the same for all alkali promoters. The growth probability a and, consequently, mechanism 1 are not affected. Therefore, it is necessary to modify only mechanism 2 of the novel hypothesis for alkali ions to take part in the catalytic cycle.13... 

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

Supported Rhodium Catalysts Alkali Promoters on Metal Surfaces Cobalt-Molybdenum Sulfide Hydrodesulfurization Catalysts Chromium Oxide Polymerization Catalysts... 

Oxidative Coupling of Methane over Alkali-Promoted Simple Molybdate Catalysts... 

Most mechanistic studies have focused on elucidation of the role of alkali promoters. The addition of Li+ to MgO has been shown to decrease the surface area and to increase both methane conversion and selective C2 production.338,339 As was mentioned, however, besides this surface-catalyzed process, a homogeneous route also exists to the formation of methyl radicals.340-342 The surface active species on lithium-doped catalysts is assumed to be the lithium cation stabilized by an anion vacancy. The methyl radicals are considered to be produced by the interaction of methane with O- of the [Li+0-] center330,343 [Eq. (3.32)]. This is supported by the direct correlations between the concentration of [Li+0 ] and the concentration of CH3 and the methane conversion, respectively. The active sites then are regenerated by dehydration [Eq. (3.33)] and subsequent oxidation with molecular oxygen [Eq. (3.34)] ... 

In water and in. hydrophilic solvents nitrourea dearranges rapidly into cyanic acid and nitroamide. Alkalis promote the reaction. If an aqueous solution of nitrourea is warmed, bubbles of nitrous oxide begin to come off at about 60°. If it is allowed to stand over night at room temperature, the nitrourea disappears completely and the liquid is found to be a solution of cyanic acid. Indeed, nitrourea is equivalent to cyanic acid for purposes of synthesis. It reacts with alcohols to form carbamic esters (urethanes) and with primary and second amines to form mono-and unsym-di-substituted ureas. 

This alcohol can be reacted with methanol in the presence of a catalyst to produce methyl-r-butyl ether. Although it is currently cheaper to make Ao-butyl alcohol from Ao-butcne (Ao-butylene), it can be synthesized from syngas with alkali-promoted zinc oxide catalysts at temperatures above 400°C (750°F). 

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