Decomposition of ethanol

Could be the activation energy for the decomposition of ethanol to CH4, CO and H2 rather than ethanol reforming into H2-rich syngas. 

The most exciting application of bond order indices concerns the description of chemical reactions involving the simultaneous change of several bonds. An example is the unimolecular decomposition of ethanol, which can happen at high temperature or IR multiphoton excitation of the molecule. Out of the possible dissociation channels, the lowest barrier characterizes the concerted water loss of the molecule, yielding ethene and H20 [30]. 

As previous studies had suggested that the selective oxidation of ethane might occur through the formation and further reaction of ethoxide, it seemed useful to investigate the effects of these molybdate catalysts in the decomposition of ethanol. The decomposition of ethanol at 603 K yielded acetaldehyde (64-69%), ethane (25-26%), ethylene (3-5%) and small amounts of methane and CO. A decay in catalytic activity was observed for all catalysts. At the steady state, neither the activity nor the selectivity differed significantly for these molybdates. 

Fig. 1. Decomposition of ethanol on copper according to Constable (2) and decomposition of nitrogen oxide on cupric oxide according to Cremer and Marschall (3) log A plotted versus Al .

It is sometimes a source of confusion that different catalysts can effect different courses of reaction on the same molecule. A good example of this is the decomposition of ethanol, which when metal-catalysed undergoes... 

The following results were obtained at 600 K for the decomposition of ethanol on an alumina (A1203) surface,... 

There is very scanty experimental evidence for the postulation of such reactions these are likely to be only of minor significance under the usual experimental conditions. It must be mentioned, however, that a minor product, presumably ethanol, was observed by Dexter and Trenwith. In addition, Collin and Delplace found ethylene, in significant amount, at pressures of 100 micron, which came (they assumed) from the decomposition of ethanol. 

An important characteristic of a catalyst is its effect on selectivity when several reactions are possible. A good illustration is the decomposition of ethanol. Thermal decomposition gives water, acetaldehyde, ethylene, and hydrogen. If, however, ethanol vapor is suitably contacted with alumina particles, ethylene and water are the only products. In contrast, dehydrogenation to acetaldehyde is virtually the sole reaction when ethanol is reacted over a copper catalyst. 

Processes of this sort have been classified under the general name of pyrogenic decomposition and may be differentiated into two types (1) those which take place under the action of heat alone, and (2) those which take place under the action of heat in the presence of a catalyst. Tn the former case the product of the reaction frequently consists of a very complex mixture, the character of which is determined by the temperature, pressure and time of contact.8 In the latter case, the course of the reaction may in certain instances be so controlled as to favor the formation of a single product. The procedure may be varied by passing the vapor of the substance through a tube or chamber the walls of which arc themselves inactive but into which an appropriate catalyst has been introduced. This latter modification of the reaction has been made the. subject of careful investigation by Ipatiew, who was indeed the first to call attention to the definite quantitative differences in the amounts of (a) acetaldehyde and (b) ethylene which resulted from the pyrogenic decomposition of ethanol under the action of specific catalysts. 

The activity of alumina as a catalyst for alcohol dehydration varies considerably with the method of production of the active material. With hydrated alumina on pumice, ground, screened, and heated at 300° Goris20 concluded tliat the aldehyde reaction was predominant up to 450° C., below which the ethylene reaction was relatively unimportant but increased rapidly at higher temperature. Several other workers have failed to obtain appreciable decomposition of ethanol to ethylene over alumina at temperatures below 270° C.30 In contrast to this, Moser obtained yields of 50 to 60 per cent of ethylene over alumina at 250° to 300° C.31 Other... 

Fig. 4.—Composition of the gases from the decomposition of ethanol in the presence of supported copper - chromium catalysts."...

These latter calculations give a value of K = 0.00376 for the hydration of ethylene or K = 266 for the dehydration of ethanol at 450° C. If this value represents more nearly the correct equilibrium, then the conversion of ethylene to ethanol under the above conditions would be decreased still further to a value of approximately 40 per cent. At atmospheric pressure and this temperature the decomposition of ethanol is practically complete at equilibrium. 

The oxidation catalyst lowers the activation energy for the total oxidation reaction and efficiently converts ethanol to carbon dioxide and water at low temperatures (reaction 1). However, if the selectivity for this reaction is low, byproducts wilt be formed. One example is acetaldehyde, which can be formed by partial oxidation of unbumed ethanol or by decomposition of unbumed ethanol (reactions 2 and 3). Considering that there is a 100% excess of air in our experiments, the decomposition of ethanol is not so likely to occur. The oxidative dehydration is a more plausible reaction in this case. 

Direct plasma-catalytic decomposition of ethanol with formation of syngas can be achieved in non-eqnilibrinm conditions at low levels of power and gas temperature as a mostly gas-phase process ... 

Figure 5.20. Effect of hydrogen peroxide on the ozonolytic decomposition of ethanol in water. Open circles, H2O2 present at SkIO"" M closed circles, HgOg absent. From Namba and Nakayama (1982). Reprinted by permission of the Chemical Society of Japan.

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