Hydrogenation of acetaldehyde to ethanol

Most authors consider acetaldehyde as the primary product of methanol hydro-carbonylation which, depending on the reaction conditions and catalyst system, can be hydrogenated to yield ethanol. The potential of cobalt hydrocarbonyl to reduce aldehydes to alcohols in a homogeneous process in the presence of syngas, was recognized by Wender et al, in 1950 [78]. A mechanism according to Equations (29) and (30) was proposed involving an ethoxy cobalt imermediate. 

TI1C dihydride complex (8) was suggested for hydrogen activation [4, 79] and also the hydroxyethyl complex (9) was taken into account as an intermediate [1,80]. 

It is remarkable that the rate is inversely dependent upon carbon monoxide pressure, which has led to (he assumption of unsaturated cobalt intermediates. 

The addition of increasing amounts of iodine promoters accelerates the hydrocarbonylation of methanol, but at the same time detioriates the hydrogenation ability of the cobalt catalysis. To obtain a high ethanol selectivity under these conditions, catalysts active for hydrogenation in the presence of iodine have to be added. Ruthenium compounds have been proved to be most suitable, as was mentioned earlier. Althou no detailed studies on the ruthenium intermediates involved are available, it is well known that aliphatic aldehydes 

Tlic product distribution observed in methanol homologation can be deduced from the reaction steps catalyzed by metal cmnplexes or acids 5]. Side products such as alkanes can be explained by reductive elimination steps, as diown in Equation (31). 

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Ethyl acetate is obtained from methyl acetate if the reductive carbonyiation is carried out with a catalyst capable of in situ hydrogenation of acetaldehyde to ethanol. The reaction sequence is ... 

The conversions of acetic anhydride to acetaldehyde and of ethylidene diacetate to ethyl acetate both involve hydrogenolysis of C—O bonds, whereas the hydrogenation of acetaldehyde to ethanol involves 0=0 reduction. An appropriate choice of hydrogenolysis versus hydrogenation catalyst functions should enable discrimination between the reaction pathways and the development of highly selective processes to both ethyl acetate and propionic acid, respectively. However, a clear disadvantage common to both is the requirement for recycling of stoichiometric quantities of acetic acid (see the next section). 

The cocatalyst catalyzes the hydrogenation of acetaldehyde to ethanol (see the mechanism of olefin hydrogenation but replace the olefin C=C bond by the C=0 aldehyde bond). Same ref as 16.5. 

The reduction of acetaldehyde to ethanol could be explained by its chemisorption near rhodium particles and the action of spill over hydrogen. On Rh/La 0, Bell has observed that at low residence times acetaldehyde is the primary product whereas at longer residence times the formation of ethanol becomes the dominant process. They concluded that this pattern is characteristic of the sequential reaction process ... 

Under forcing conditions, temperatures > 200°, it is possible to hydrogenate acetaldehyde to ethanol, as it is formed, with a cobalt l2-PPh3 catalyst. Yields of ethanol up to 70% have been reported. Better results are obtained by adding Ru to the Co catalyst. Here, Ru serves as a hydrogenation catalyst for the conversion of acetaldehyde to ethanol. For example, with a catalyst consisting of Co Ru.T = 1 5 4, at 140°C and 24.1 MPa (H2 C0 = 2 1) the ethanol selectivity is 86% . 

The stereochemical aspects of many enzyme-catalyzed reactions have been determined. The enzyme alcohol dehydrogenase catalyzes the oxidation of ethanol to acetaldehyde by removing the pro-R hydrogen (abbreviated as H ). When the same enzyme catalyzes the reduction of acetaldehyde to ethanol, hydrogen is transferred to the Re face. 

Reduction. Acetaldehyde is readily reduced to ethanol (qv). Suitable catalysts for vapor-phase hydrogenation of acetaldehyde are supported nickel (42) and copper oxide (43). The kinetics of the hydrogenation of acetaldehyde over a commercial nickel catalyst have been studied (44). 

Other Methods of Preparation. In addition to the direct hydration process, the sulfuric acid process, and fermentation routes to manufacture ethanol, several other processes have been suggested. These include the hydration of ethylene by dilute acids, the hydrolysis of ethyl esters other than sulfates, the hydrogenation of acetaldehyde, and the use of synthesis gas. None of these methods has been successfilUy implemented on a commercial scale, but the route from synthesis gas has received a great deal of attention since the 1974 oil embargo. 

Hydrogenation of Acetaldehyde. Acetaldehyde made from acetylene can be hydrogenated to ethanol with the aid of a supported nickel catalyst at 150°C (156). A large excess of hydrogen containing 0.3% of oxygen is recommended to reduce the formation of ethyl ether. Anhydrous ethanol has also been made by hydrogenating acetaldehyde over a copper-on-pumice catalyst (157). 

In a similar manner, ethanol can be oxidized by the dichromate ion to form a compound called acetaldehyde, CHaCHO. The molecular structure of acetaldehyde, which is similar to that of formaldehyde, is shown at the bottom in Figure 18-6. We see that the molecule is structurally similar to formaldehyde. The methyl group, —CH3, replaces one of the hydrogens of formaldehyde. The balanced equation for the formation of acetaldehyde from ethanol is... 

Nicotinamide-dependent enzymes operate in a highly stereospecific manner. This phenomenon was first demonstrated for alcohol dehydrogenase which catalyzes the direct and stereospecific transfer of the pro-(R) hydrogen at C-l of ethanol to the re face of NAD+, or, in the reverse direction, the pro-(R) hydrogen of NADH to the re face of acetaldehyde (equation 2) (B-71MI11001, B-79MI11000). Many other nicotinamide-dependent... 

In contrast to the acetaldehyde decarbonylation, reactions with ethanol over Rh (111) did not lead to formation of methane but rather to an oxametallocycle via methyl hydrogen abstraction. These data suggest that ethanol formed over supported rhodium catalysts may not be due to hydrogenation of acetaldehyde. This study shows how surface science studies of model catalysts and surfaces can be used to extract information about reaction mechanisms since the nature of surface intermediates can often be identified by methods such as temperature programmed desorption and high resolution electron energy loss spectroscopy. 

Workers at Argonne Laboratory have recently discovered a catalytic method for the homologation of methanol to ethanol 122-124). The workers consider the mechanism to involve initial formation of acetaldehyde, followed by its hydrogenation. 

The dehydrogenation of ethanol over copper catalysts is not complete at 300° C. when moderate times of contact are used but if the temperature is raised to 350° C. or higher, secondary reactions become more and more evident. At temperatures above 350° C., copper catalysts begin to activate the decomposition of acetaldehyde to methane and carbon monoxide, to induce polymerization of the aldehyde, to cause dehydration processes to set in, to cause hydrogenation of the ethylene, and, in general, to promote secondary decompositions and condensations which complicate the product and destroy the activity of the catalyst. Hence, for the production of aldehydes and ketones it is desirable to use moderate temperatures of about 300° C. and to obtain maximum yields from the decomposition rather than maximum decomposition of alcohol per pass over the catalyst. 

Although alcohol has been produced by the hydrogenation of acetaldehyde obtained from the hydration of acetylene, this source is relatively unimportant ordinarily. It does, however, furnish a means for the synthesis of ethanol from such sources of carbon as calcium carbide, methane, the carbon arc, etc., which might become of importaice during periods of war, or in locations where very cheap electric power is available. Experiments on a technical scale78 in Switzerland have shown the process to be successful but at a cost too high to make the process competitive. 

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