Ethylene, catalytic oxidation moderators

Electrochemical promotion can be used to modify significantly the product selectivity, of catalytic oxidation reactions. An example is presented in Fig. 5 which shows the effect of catalyst potential and corresponding work function change on the selectivity to ethylene oxide (Fig. 5a) and acetaldehyde (Fig. 5b) of ethylene oxidation on AgA SZ at various levels of gas phase chlorinated hydrocarbon moderators [31] (The third, unde.sirable, product is CO2). As shown in the Figure a 500 mV decrease in catalyst potential causes the Ag surface to change from selective (up to 70%) ethylene oxide production to selective (up to 55%) acetaldehyde production. The same study [31] has shown that the total rate of ethylene oxidation varies by a factor of 200 upon varying the catalyst potential. (Fig. 6)... 

The use of liquid membranes for controlling chemical reactions such as that just discussed has been proposed for a number of other systems. This type of application, in which liquid membranes are used as heterogeneous catalysts or as reaction moderators, is an area that deserves more study. Ollis et al. and Wolytdc and Ollis studied liquid membranes as heterogeneous catalyst systems using the catalytic oxidation of ethylene to acetaldehyde (Wacker process) as a model. This process entails the following three... 

The [Os3(CO)io( t-H)( t-OSi)]surface catalyzes the isomerization and hydrogenation of olefins. When the hydrogenation of ethylene is carried out at 90 °C the trinuclear framework of the initial cluster remains intact in all the proposed elementary steps of the catalytic cycle [133]. However, at higher reaction temperatures the stability of the [Os3(CO)io( t-H)( t-OSi)]sujface depends on the nature of the reactant molecule. It is moderately active in the isomerization of 1-butene at 115 °C but decomposes under reaction conditions to form surface oxidized osmium species that have a higher activity [134]. 

The data in Table 1 summarize catalytic activities for epoxidation of a variety of olefins over an unpromoted 5%Ag/Al203 catalyst. These data illustrate the preferential reactivity at the allylic position relative to addition of oxygen across the C=C bond. While the selectivity to ethylene oxide is typical for an unpromoted catalyst, the selectivities to propylene oxide and butylene oxides are non-existent for propylene, 1-butene, and 2-butene, respectively. In addition to small amounts of the selective allylic oxidation products (acrolein in the case of propylene and butadiene in the case of 1-butene), the only products are those of combustion. However, the results for butadiene reveal it is possible to epoxidize this non-allylic olefin at moderate selectivity and activity. What is not obvious from Table 1 is the short-lived nature of this activity. After 2-3 hours of reaction time, activity and selectivity typically decreased to approximately <1% conversion of C4H6 and approximately 50-75% selectivity to epoxybutene. A typical chromatogram of the activity of an... 

Comparison of the kinetics for the most feasible stereochemical pathways of the alternative la—>4 and lb 2 routes clearly shows that the octadienediyl-Ni" complex is preferably generated via the ethylene-assisted coupling of two -butadienes along lb—>2. Thus, the thermodynamically favorable [Ni (ix -butadiene)2(ethylene)] form of lb also represents the catalytically active species for oxidative coupling. The [Ni (ix -s/ ,ri (C )>A-cis,-octadienediyl)-(ethylene)] species 2 is almost exclusively formed in a thermoneutral process (AG= -0.1 kcal mol 1) that requires a moderate activation free-energy of 12.8 kcal mol This indicates the oxidative coupling as a facile, reversible step. 

The redox reactions of metallo phthalocyanine electrodes cause a selective and efficient catalysis for many photochemical and electrochemical reactions at moderate potentials [3,57,70,161,171,172]. The electrons are transported by electronically conductive polymers and also by electron exchange reaction in the oxidizable or reducible groups present in the incorporated polymer. Phthalocyanine and their derivatives have been studied for the catalytic behaviour in the oxidation of acetaldehyde ethylene acetate, iodide and other aldehydes such as benzaldehyde and acrolein [3,57,70,120,231-235]. In the latter cases pyridine was used as an activator. The oxidation reactions are found to follow through a peroxy intermediate. The effectiveness of the catalyst for the reaction depends on the purity, uniformity, stability and turnover number of the phthalocyanine polymer. Oxidative polymerization of phenols to polyoxyphenylenes and thiols to sulphides has also been reported [57,70,120,236-238]. 

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