Mechanism ethanol dehydrogenation

The mechanism of the reaction is very complex since several reaction intermediates can be formed. In light of the above studies, it was claimed that the acetaldehyde, formed by ethanol dehydrogenation, is easily decarbonylated to form CH4 and CO, while the ethylene produced by dehydration was steam-reformed to Cj (very fast reactions). Rh/CeOg catalysts were also found to be highly active... 

Pichat P, Mozzanega MN, Courbon H. Investigation of the mechanism of the photocatalytic alcohol dehydrogenation over Pt/Ti02 by using poisons and labelled ethanol. J Chem Soc Faraday Trans I 1987 83 697-704. 

Fig. 15. Deactivation of Fe2(MoQ4)3 and a mechanical mixture (50 50) of Fe2(Mo04>3 + a-Sb2C>4 during the oxidative dehydrogenation of ethanol to acetaldehyde. The amount of Fe2(MoC>4)3 is identical (200 mg) in both experiments. T = 350°C ethanol/02/N2 2/1/20. Gas flow rate 50 ml/min.

The most important contribution in the field of simultaneous dehydrogenation, condensation, and dehydration made by Russian chemists is the synthesis of butadiene from ethanol over a double oxide catalyst by the method of Lebedev. Much has been published on this process. Lebedev s interest in rubber synthesis began with his researches on conversions of dienes in 1908 and his method of synthesis of butadiene was reported in 1927. An experimental synthetic rubber plant was founded for research in the field and the studies on the mechanism of formation of butadiene and of polymerization were continued after Lebedev s death by his students (103,104,105,188,190,378). A survey of the properties and methods of preparation of butadiene was published by Petrov (289). 

The work of Constable and Palmer 17 regarding the mechanism of catalyst action when alcohol is decomposed in the presence of copper catalysts, has contributed valuable information from a physical-chemical standpoint on the manner in which catalysts act. In the presence of their catalysts, the rate of dehydrogenation of ethanol, propanol, and butanol was the same notwithstanding that the length of the hydrocarbon chain had doubled in the series of alcohols. With isopropanol the velocity was five times that of the others. All primary alcohols, however, contain tile — CHsOH group at the end of the hydrocarbon chain and to explain the action of these alcohols these workers have undertaken to show that the primary alcohols are adsorbed by the catalyst surface with the — CH.. OH group in contact with the surface and the hydrocarbon chains perpendicular to that surface. Such an orientation would then make all of the alcohols alike as far as the catalyst surface was concerned and, hence, make their... 

Although ethanol is dehydrogenated to acetaldehyde in the presence of zinc oxide at temperatures of 300° to 400° C. and atmospheric pressure, no aldehyde results when the reaction is conducted under sufficient hydrogen pressure. Instead a complex mixture including esters of acetic, butyric, and caproic acids and alcohols up to and higher than octyl is formed.01 Condensation reactions of acetaldehyde are used to account for the formation of these compounds but no definite proof has as yet been advanced to establish the mechanisms. 

Rottenberg and Baertschi have carried out oxidation experiments in the presence of the isotope Ethanol was oxidized to acetic acid in two experiments. In the first, the oxygen used was labeled with and, in the second, the solvent water contained H20 . In the first case, only 5% of the was found in the acetic acid and, in the second, 80 to 90% of the 0 was removed from the water. Although these results could be explained by the dehydrogenation mechanism, they are not conclusive, since the isotope distribution can also be explained through isotope exchange of the intermediate acetaldehyde, as it is known that this exchange is very rapid. 

In an alternate ketonization mechanism, it was proposed that an alcohol is dehydrogenated to an aldehydic surface intermediate, which can be rapidly oxidized to an adsorbed carboxylate, even on reduced Ce02. The adsorbed carboxylates then condense to form ketone, CO2, and hydroxy, presumably according to Scheme 1.20.34-35,48 these experiments were performed using ethanol as the starting material, except for Idriss et al., who used acetaldehyde. The temperatures for ketonization ranged from 490-770 K. The proposed reactions can be partly written as follows (a = adsorbed) ... 

Dehydrogenation of 9a,10a- or 9)S,10j5-oestr-4-en-3-ones occurred rapidly with palladium in ethanol, giving phenolic products with retention of configuration at C-9 (Scheme 6). Various mechanisms were considered, but were rejected... 

As a practical example of the interpretation of kinetic rate data to distinguish between more than one reaction mechanism, consider the dehydrogenation of ethanol (i.e., CH3CH2OH, [A]) to acetaldehyde (i.e., CH3CHO, [C]) and hydrogen (i.e., H2, [D]). The reversible chemical reaction that occurs on a catalytic... 

If hydrogen volatilizes immediately upon dehydrogenation of ethanol, and H2 does not occupy an active site on the catalytic surface, then the following three-step mechanism is more appropriate ... 

It is believed that this reaction involves the following consecutive steps dehydrogenation of ethanol to acetaldehyde, condensation of acetaldehyde into acetaldol, dehydration of aldol product to crotonaldehyde, and deoxygenation of crotonaldehyde. This mechanism is illustrated here. 

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